1.1. A summary statistic encoding in peripheral vision

A significant loss of information occurs in peripheral vision, particularly due to visual crowding. Crowding refers to phenomena in which peripheral vision degrades in the presence of clutter. In a common example, an observer views a peripheral word, in which each letter is closely flanked by others. Observers might perceive the letters in the wrong order, or a confusing jumble of shapes made up of parts from multiple letters (Lettvin, Reference Lettvin1976). Lettvin (Reference Lettvin1976) suggested that the crowded letters “only [seem] to have a ‘statistical’ existence.” Move the letters farther apart, and at some critical spacing letter identification improves (Bouma, Reference Bouma1970). Building on these observations, several researchers have suggested that crowding occurs due to the representation of peripheral information in terms of statistics summarized over sizable pooling regions (Parkes, Lund, Angelucci, Solomon, & Morgan, Reference Parkes, Lund, Angelucci, Solomon and Morgan2001; Balas, Nakano, & Rosenholtz, Reference Balas, Nakano and Rosenholtz2009; Freeman & Simoncelli, Reference Freeman and Simoncelli2011). These pooling regions grow linearly with eccentricity, overlap, and sparsely tile the visual field (Freeman & Simoncelli, Reference Freeman and Simoncelli2011; Rosenholtz, Huang, & Ehinger, Reference Rosenholtz, Huang and Ehinger2012; Chaney, Fischer, & Whitney, Reference Chaney, Fischer and Whitney2014). Based on these intuitions, my lab has developed and tested a computational model of peripheral crowding, known as the Texture Tiling Model (TTM). The model measures a rich set of summary statistics derived from the texture perception model of Portilla and Simoncelli (Reference Portilla and Simoncelli2000), with a first stage computing V1-like responses to oriented, multiscale feature detectors, and the second stage measuring a large set of primarily second-order correlations of the responses of the first stage, computed over local pooling regions (Balas et al., Reference Balas, Nakano and Rosenholtz2009; Rosenholtz, Huang, Raj, Balas, & Ilie, Reference Rosenholtz, Huang, Raj, Balas and Ilie2012; see also Freeman & Simoncelli, Reference Freeman and Simoncelli2011). Figures 2 and 4 show model outputs to provide intuitions about predictions of the model.

Figure 2.

(A) Original image, fixation at center. (B) Two visualizations of the information available at a glance, according to our model of peripheral vision.

Figure 3.

Example stimulus patches from three classic search conditions (top row, target specified at top). Akin to the demonstration in Figure 2, here one can use the texture analysis/synthesis model of Portilla and Simoncelli to visualize the information encoded by the model summary statistics (bottom two rows). The summary statistics capture the presence (column 1, row 2) or absence (row 3) of a uniquely tilted line. However, the information is insufficient to correctly represent the conjunction of color and orientation, producing an illusory white vertical from a target absent patch (column 2, row 3). For T among L search, no clear T appears in the target-present synthesis (column 3, row 2), whereas several appear in the target absent (row 3); this ambiguity predicts difficult search.

TTM can in fact predict performance getting the gist of a scene. Given a stimulus and a fixation, TTM generates random images with the same pooled summary statistics as the original (Figure 2). These images provide visualizations of the consequences of encoding an image in this way. Aspects of the scene that one can consistently discern in these images will be readily available when fixating the specified location. Conversely, details not consistently apparent will be less available to peripheral vision. A summary statistic encoding in peripheral vision preserves a great deal of useful information for getting the gist of a scene. In Figure 2, the encoding suffices to identify a street scene, most likely a bus stop, people waiting, cars on the road, trees, and a building in the background. Ehinger and Rosenholtz (Reference Ehinger and Rosenholtz2016) demonstrated that this encoding quantitatively predicts performance on a range of scene perception tasks. Though not yet well studied, the model also seems promising for predicting the ease with which one can get the gist of a set, relative to reporting individual items of that set (Rosenholtz, Yu, & Keshvari, Reference Rosenholtz, Yu and Keshvari2019; though see Balas, Reference Balas2016).

A summary statistic encoding in peripheral vision also provides insight into a second anomaly: that different methods for distinguishing between automatic and attention-demanding tasks have not agreed on which tasks require attention. In particular, search and dual-task results have disagreed on this classification, as have attentional capture (Folk, Remington, & Johnston, Reference Folk, Remington and Johnston1992) and inattentional blindness experiments (Mack & Clarke, Reference Mack and Clarke2012; Mack & Rock, Inattentional blindness, Reference Mack and Rock1998). VanRullen et al. (Reference VanRullen, Reddy and Koch2004) provide a particularly useful table of search vs. dual-task results (Figure 3A).

Figure 4.

(A) Search and dual-task paradigms give different answers about what tasks do and do not require attention. Understanding peripheral vision resolves this conundrum by more parsimoniously explaining search, and (B) noting that search displays are often more crowded than dual-task displays. Tasks circled in gray (blue) are behaviorally or theoretically easy (hard) for peripheral vision in dual-task displays. Table adapted from VanRullen et al. (Reference VanRullen, Reddy and Koch2004).

Recall that in the traditional interpretation, easy or efficient search implies that observers can discriminate the target from distractors preattentively, automatically, and without selective attention (Treisman & Gelade, Reference Treisman and Gelade1980). Inefficient or difficult search, conversely, indicates that the target-distractor discrimination requires selective attention.

In dual-task experiments, an observer performs either a task in central vision, a task in the periphery, or both. Observers often perform worse when given two tasks instead of one. According to the experimental logic (VanRullen, Reddy, & Koch, Reference VanRullen, Reddy and Koch2004), easy dual tasks do not require selective attention; rather this processing happens automatically. Difficult dual tasks require attention.

Consequently, discriminations that lead to easy (hard) search tasks should also lead to easy (hard) dual tasks (upper left and lower right quadrants, Figure 3A). However, in the two remaining quadrants, the two experiments disagree on whether tasks require attention.

A summary statistic encoding in peripheral vision can make sense of these anomalous results. First, our model of visual crowding provides a more parsimonious account of critical visual search phenomena (Figure 4). Difficulty discriminating a crowded peripheral target from a distractor explains easy feature search, difficult conjunction search, and difficult search for a T among Ls (Rosenholtz, Huang, Raj, Balas, & Ilie, Reference Rosenholtz, Huang, Raj, Balas and Ilie2012); phenomena that originally led to feature integration theory. The model also predicts difficult search for a scene among scenes, and for a green-red bisected disk among red-green ones (Rosenholtz, Huang, & Ehinger, Reference Rosenholtz, Huang and Ehinger2012). In addition, it explains phenomena that defy easy explanation by feature integration theory, such as easy search for a cube among differently lit cubes compared to similar 2D conditions (Zhang, Huang, Yigit-Elliot, & Rosenholtz, Reference Zhang, Huang, Yigit-Elliot and Rosenholtz2015), and effects of subtle changes to the stimuli (Chang & Rosenholtz, Reference Chang and Rosenholtz2016). A summary statistic model of peripheral vision, in other words, collapses the two rows of Table 1; search may probe peripheral discriminability, not what tasks require attention.

Perhaps more surprising, peripheral vision allows us to collapse the columns of the table. A number of the easy (hard) dual tasks are easy (hard) for peripheral vision, based on either behavioral experiments or modeling (Rosenholtz, Huang, & Ehinger, Reference Rosenholtz, Huang and Ehinger2012). Dual task difficulty, then, may also depend more on the strengths and limitations of peripheral vision than on tasks that do or do not require attention.

One must ask why, if search and dual-task difficulty both probe peripheral vision, these tasks disagree on the off-diagonal of the table. Search displays contain considerably more clutter. This would impair scene search relative to the scene dual task (Figure 3B; Rosenholtz, Huang, & Ehinger, Reference Rosenholtz, Huang and Ehinger2012). A less cluttered dual-task display fundamentally changes the conjunction task; the observer only needs to identify the color and orientation of two uncrowded items (Braun & Julesz, Reference Braun and Julesz1998). Clutter in the cube search case had the opposite effect: the dense array of cubes aligned in a way that aided search (Rosenholtz, Huang, & Ehinger, Reference Rosenholtz, Huang and Ehinger2012).

A summary statistic encoding in peripheral vision resolves the anomaly that search and dual-task methods do not agree on what tasks require attention. Supposedly automatic tasks that did not require attention (Treisman, Reference Treisman2006) might simply have been inherently easy given the information available in peripheral vision. Looked at another way, this points to another sign of a Kuhnian crisis: The search and dual-task methods no longer lead to the answers promised by the paradigm. Search and dual-task experiments were seemingly fruitful, agreed-upon methods, with difficulty indicative of important aspects of the attentional mechanisms. Now it seems that one cannot easily interpret the results in that way.

This is not just a problem of methods. Rather, it necessitates rethinking a substantial amount of attention theory (Figure 5). Selection may not be early, and the information available preattentively may include more than basic feature maps. Feature binding may not require attention; the difficulty of both correctly binding features (Rosenholtz, Huang, Raj, Balas, & Ilie, Reference Rosenholtz, Huang, Raj, Balas and Ilie2012) and perceiving the details may arise from losses in peripheral vision rather than from inattention.

Figure 5.

Understanding peripheral vision necessitates rethinking a significant amount of attention theory, including preattentive processing and selective attention mechanisms as well as the need for attention for binding and to perceive details (red X’s).

More profoundly, we need to rethink selection itself. In order for the strengths and limitations of peripheral vision to predict search performance, observers must use peripheral vision to search for the target. This leaves room for considerably more parallel processing, albeit with reduced fidelity due to crowding. This does not sound much like selection, as commonly envisioned. Recent work has also called into question supposed physiological signs of sensory selection. When monkeys perform a discrimination task with one of several items in a neuron’s receptive field, the neuron responds as if only the target were present, as one would expect if the neuron selected the cued stimulus at the expense of the others (Desimone & Duncan, Reference Desimone and Duncan1995). However, if one inactivates the superior colliculus, thought to play a critical role in visual attention, the monkey behavior displays attentional deficits, while the attentional modulations in cortex persist, suggesting that the modulations may not demonstrate a causal mechanism for sensory selection (Krauzlis, Bollimunta, Arcizet, & Wang, Reference Krauzlis, Bollimunta, Arcizet and Wang2014).

One should also reconsider automaticity (which one might think of as the dual of selection). Evidence has not supported a distinction between tasks that do and do not require attention, nor “automatic” preattentive processing. Whether one notices a salient item or gets the gist of a scene — two common candidates for automatic processing — depends upon the difficulty of other simultaneous tasks (Joseph, Chun, & Nakayama, Reference Joseph, Chun and Nakayama1997; Rousselet, Thorpe, & Fabre-Thorpe, Reference Rousselet, Thorpe and Fabre-Thorpe2004; Matsukura, Brockmole, Boot, & Henderson, Reference Matsukura, Brockmole, Boot and Henderson2011; Cohen, Alvarez, & Nakayama, Reference Cohen, Alvarez and Nakayama2011; Mack & Clarke, Reference Mack and Clarke2012; Larson, Freeman, Ringer, & Loschky, Reference Larson, Freeman, Ringer and Loschky2014). In other words, these tasks are not automatic.

1.2. A paradigm shift and addressing the remaining crisis

One might reasonably call summary statistic encoding a paradigm shift, both for the science of visual attention and for vision more broadly. Researchers have long suggested that vision might use summary statistics to perform statistical tasks, such as texture discrimination and segmentation (Rosenholtz, Reference Rosenholtz and Wagemans2014). This summary statistic encoding, however, would underlie all visual tasks, including getting the gist of a scene, identifying a peripheral object, or searching for a target. The underlying computations make use of neural operations like those previously described – feature detectors, nonlinearities, and pooling operations. However, pooling over substantially larger areas to compute summary statistics makes available qualitatively different information (Figure 2). Vision looks fundamentally different from within this new paradigm. The encoding captures a great deal of useful information, while lacking the details necessary for certain tasks. This may at least partially explain such diverse phenomena as change blindness (Smith, Sharan, Park, Loschky, & Rosenholtz, under revision) and difficulty noting inconsistencies in an impossible figure. The richness of the information available across the field of view means that eye trackers likely tell us less than we thought about the attentional state of the observer and has led to re-examining the subjective richness of visual perception (Rosenholtz, Reference Rosenholtz2020; Cohen, Dennett, & Kanwisher, Reference Cohen, Dennett and Kanwisher2016). Research on summary statistic encoding has led to new proposals about what visual processing occurs in area V2 (Freeman, Ziemba, Heeger, Simoncelli, & Movshon, Reference Freeman, Ziemba, Heeger, Simoncelli and Movshon2013). It has as-yet-unrealized implications for all later visual processing, as vision scientists must re-envision how the visual system finds perceptually meaningful groups and computes properties such as 3D shape from the information available in the summary statistics. Researchers have even looked for evidence of similar mechanisms in auditory perception (McDermott & Simoncelli, Reference McDermott and Simoncelli2011).

Despite this paradigm shift, the field of attention remains in crisis. Summary statistics have been tacked onto attention theory with little change to old notions of selection, preattentive processing, automaticity, or what tasks require attention. By analogy, the Copernican paradigm shift may have put the sun at the center, but it maintained the ideas of circular orbits and unchanging celestial spheres, and as a result, the new theory needed even more epicycles than the old (Gingerich, Reference Gingerich and Beer1975). Sixty years passed before Kepler introduced elliptical orbits governed by physical laws. The remainder of this paper attempts to gain insights into, essentially, the epicycles, celestial spheres, and elliptical orbits of visual attention.

Kuhn suggested that new and old paradigms are incommensurable, meaning that scientists struggle to hold in their minds both the new and old ideas. This suggests that it helps, when looking for a new theory, to eliminate the old from one’s thinking as much as possible. For visual attention, we need not only rethink the theory. Given that established methods may have probed, for instance, peripheral vision rather than attention, we also need to reexamine the critical set of phenomena. One might better draw an analogy not to Copernicus – who faced general agreement on the relevant phenomena – but to Francis Bacon’s reasoning about the nature of heat (Bacon, Reference Bacon2015).

Bacon gathered “known instances which agree in the same nature… without premature speculation” into three lists: “Instances Agreeing in the Nature of Heat”, “Instances in Proximity Where the Nature of Heat is Absent” – examples like those on the first list, but lacking heat – and a “Table of Degrees of Comparison in Heat” – examples in which heat is present to varying degrees. By analogy, this paper re-examines phenomena attributed to attention, and which therefore seem likely to provide insights into its nature, and creates two lists of critical phenomena: The first contains phenomena that demonstrate clear evidence of additional capacity limits. If we abandoned attention theory, these phenomena would require explanation. The second contains phenomena in which human vision works well, seemingly without the same sorts of limits apparent in the first list. This list enumerates capabilities of the visual system that any viable theory must explain.

It can be difficult to reason about critical phenomena and a new theory when attention is already an overloaded term (Chun et al., Reference Chun, Golumb and Turk-Browne2011; Anderson, Reference Anderson2011; Hommel, et al., Reference Hommel, Chapman, Cisek, Neyedli, Song and Welsh2019), often used gratuitously (Anderson, Reference Anderson2011). Language can stand in the way. To help make a fresh start, for a year I banned the word “attention”. Lab members avoided or attempted to clarify terminology like “selection”, and I encouraged the group to be dissatisfied with explanations that relied on “available resources” without suggesting the nature of those resources. Given the difficulty measuring attention, I advised students not to blithely assume they knew a person’s attentional state.

Banning “attention” provides a couple of additional benefits. Examining whether a given phenomenon demonstrates attention while simultaneously rethinking attention poses a chicken-and-egg problem. Restricting the use of the word suggests instead asking, “If we were frugal in the use of ‘attention’, would we use it here?” Banning “attention” also encourages us to talk about phenomena with minimal assumptions about the nature of capacity limits and the mechanisms for dealing with those limits. Examining phenomena as agnostically as possible helped in developing seeds of alternative theories.

The following section presents vignettes about phenomena that my lab pondered in search of the critical phenomena. Of course, it does not provide an exhaustive literature review, and my lists of critical phenomena are no doubt incomplete. Rather, this paper showcases specific examples that I think provide important insights into the nature of attention and/or demonstrate the process of rethinking the old paradigm. It focuses on behavioral effects rather than physiology (for discussion of additional phenomena see Rosenholtz, Reference Rosenholtz2017; Rosenholtz, Reference Rosenholtz2020). The section “Thoughts and a proposal” asks what the critical phenomena might have in common, and what might be the associated capacity limit(s) and mechanism(s). I will discuss a couple of alternative theories.

First, a brief example: overt attention, in which one directs attention by directing one’s eyes. Terminology already exists for fixation, saccade planning, and so on; if one had to pay money every time one used “attention”, one might not use it here. In deciding what goes on the list of critical phenomena, we should also ask to what degree covert and overt attention point to similar limits and mechanisms.

Humans typically only point our eyes at one location at a time. In contrast, attention theories propose that covert attention can be divided or diffused. Overt and covert attention also seem to have different consequences. Fixating provides excellent information at the point of fixation, but puts much of the visual field in the periphery, subject to crowding, reduced acuity, etc. On the other hand, after many years of studying attention, I still do not feel like I have a clear understanding of the perceptual consequences of focusing covert attention, for either the attended or unattended information. If the limits of covert and overt attention differ, as do their consequences, then what might justify calling them both attention? Perhaps the similarity lies only in how one decides what to focus on next. Common factors likely come into play: bottom-up stimulus factors, top-down task demands, prior probabilities and other contextual knowledge, the information gathered so far, reward, and so on. Several papers rethinking attention point to the importance of studying priority maps (Hommel, et al., Reference Hommel, Chapman, Cisek, Neyedli, Song and Welsh2019) and Bayesian decision processes (Anderson, Reference Anderson2011). I certainly agree with the value of such topics, although I would question whether associating them with the overused “attention” adds clarity.

2.1. Attention as task

Some experiments ask the observer to “pay attention”. For example, one might cue the observer to attend to and make a judgment about a target while ignoring other items, e.g. (Lavie, Hirst, de Fockert, & Viding, Reference Lavie, Hirst, de Fockert and Viding2004). Or one might tell the observer that only non-targets will have a singleton color, so they should ignore those items during visual search (Theeuwes, Reference Theeuwes1992). Nonetheless, the ignored items often distract the observer, although this can come at a cost of as little as 20-40 ms (Theeuwes, Reference Theeuwes1992). Researchers often interpret the results in terms of what captures attention.

Selectively attending is the observer’s task. Experimenters describe a task in natural language, and the brain must convert that description to its internal instruction set, making use of existing mechanisms. The visual system could do its best to perform the nominal task even if it had no mechanism for selecting an individual item. One can imagine similar results even without capacity limits. The observer perceives the entire display. Maybe it takes a few milliseconds to remember that responding based on the distractor gives the wrong answer. Or why not spend 40 ms enjoying an unusual item?

The most interesting thing about attention-as-task experiments is the observer’s failure to process only the target. One might expect distraction by salient stimuli, supposedly preattentively processed. However, results also suggest processing of not-terribly-salient numbers or letters to the extent that their category can cause response conflict (Lavie et al., Reference Lavie, Hirst, de Fockert and Viding2004). Furthermore, researchers have suggested that distraction depends on whether saliency is task-relevant (Folk, Remington, & Johnston, Reference Folk, Remington and Johnston1992), on the perceptual task difficulty, and in a complex way on the difficulty of simultaneous cognitive tasks (Lavie et al., Reference Lavie, Hirst, de Fockert and Viding2004), suggesting that distraction does not merely arise from automatic processing. These results seem puzzling from the point of view of the attention paradigm; the main mechanism for getting around a bottleneck in vision is leaky, allowing other information to pass through? Of course, if selection were perfect, vision would never work, with disastrous consequences; how would you notice the approaching tiger when you were paying attention to picking berries?

To help us rethink attention, we should try to describe phenomena in a more agnostic way that does not rely on earlier concepts of attention or selection. We might say that the experimenter asks the observer to make a judgment about the target, making sure not to respond based on any other item. Nonetheless, the observer perceives, to some degree, the task-irrelevant items, and this can cause a modest degradation in performance at judging the target. Both the modest cost of distraction and the apparent failure to select only the target seem interesting critical phenomena (see also Hommel, et al., Reference Hommel, Chapman, Cisek, Neyedli, Song and Welsh2019).

2.2. Object-based attention

Another set of cueing experiments tests object-based attention (OBA). Figure 6A shows the basic methodology. Observers respond to a target about 10-20 ms faster when it appears on a cued object than when it appears on a different object, despite controlling for the distance between the invalid cue and the target (e.g. Egly, Driver, & Rafal, Reference Egly, Driver and Rafal1994; Francis & Thunell, Reference Francis and Thunell2022). This has been taken as evidence that the observer’s attention automatically spreads from the cue location to the entire cued object. At the time of the earliest OBA experiments, these phenomena required a significant shift in thinking about attention, since many models assumed that attention was directed to a location rather than an object (Kanwisher & Driver, Reference Kanwisher and Driver1992). Complicating this story, the same-object advantage applies more for horizontal objects than vertical (Al-Janabi & Greenberg, Reference Al-Janabi and Greenberg2016; Chen & Cave, Reference Chen and Cave2019; Francis & Thunell, Reference Francis and Thunell2022). Furthermore, when comparing two targets with no precue (Lamy & Egeth, Reference Lamy and Egeth2002), one less often finds a same-object advantage (Chen, Cave, Basu, Suresh, & Wiltshire, Reference Chen, Cave, Basu, Suresh and Wiltshire2020).

Figure 6.

Object-based attention. (A) Experimental methodology. The experimenter cues one end of an object. After a delay (here, 100 ms), a target randomly appears either at the same location, the opposite end of the same object, or the equidistant location on the other object. Results often show a small (∼10 ms) same-object benefit in the invalid conditions. (B) Object-based attention with a single object. Observers are faster to make judgments about the target (circle) when the object between it and the cue (+) is simple (left) than when it is complex (right). Based on stimuli from Chen et al., Reference Chen, Cave, Basu, Suresh and Wiltshire2020.

The OBA literature has complex and often seemingly conflicting results, with small effects; later work (Francis & Thunell, Reference Francis and Thunell2022) has called into question much of the literature, due to underpowered studies. (In this paper, any results citing Francis & Thunell, Reference Francis and Thunell2022, indicate OBA effects that replicated in their higher-powered study.) I nonetheless discuss OBA in the hope that it provides insights.

Peripheral vision has already explained several supposedly attentional phenomena; what about OBA? The horizontal-vertical asymmetry in OBA mirrors peripheral vision’s horizontal-vertical asymmetries (Strasburger, Rentschler, & Jüttner, Reference Strasburger, Rentschler and Jüttner2011): both acuity and crowding are worse at a given eccentricity in the vertical direction than in the horizontal direction. In addition, OBA experiments typically have more clutter between the cue and the different-object target than between the cue and the same-object target, which might make judging the latter easier due to crowding. In the two-rectangle stimuli in Figure 6A, for instance, the cue and same-object target have nothing but blank space between them, whereas the region between the cue and different-object target contains the edges of both objects. Crowding might not prohibit identification of the target but rather might necessitate additional time to integrate information from the stimulus, slowing identification.

However, drawing parallels between crowding and object-based attention presumes that the observer fixates on or near the cue. If they fixate the central “+”, as directed, all targets would appear diagonal relative to the fixation, one would not expect a horizontal-vertical asymmetry due to peripheral vision, and the same- and different-object targets would usually be equally crowded. However, the vast majority of object-based attention experiments have not used an eye tracker (including Francis & Thunell, Reference Francis and Thunell2022), so many of the classic results could have had a peripheral confound. Observers need only fixate nearer the cue on a small number of trials for crowding to explain the effects. The experimental design typically allows more than enough time for a saccade, and researchers typically find an OBA effect only under that condition (e.g. Egly et al., Reference Egly, Driver and Rafal1994; Francis & Thunell, Reference Francis and Thunell2022). Furthermore, the observer would benefit from breaking fixation; with cue validity as high as 75-80% (e.g. Egly et al., Reference Egly, Driver and Rafal1994; Francis & Thunell, Reference Francis and Thunell2022), fixating the cued location often means fixating the target, making the task easy. Comparison tasks may less often lead to OBA effects because with two task-relevant locations the observer less obviously benefits from breaking fixation.

More recent work questions object-based attention by putting target and cue on the same object and varying the complexity of the intervening object (Figure 6B; Chen et al., Reference Chen, Cave, Basu, Suresh and Wiltshire2020). Observers perform better when the object between cue and target is simple, and worse when it is complex, in line with a crowding explanation.

Taken together, the so-called object-based attention phenomena may instead derive from peripheral vision, with the small effects occurring because observers break fixation on a small fraction of trials. This is certainly not to say that all processing is location-based rather than object-based. One would expect some object-based effects from the proposal in Section 3.4. Rather, it is not clear at this point that one needs to include amongst the critical phenomena the automatic spread of attention from a cued location to the rest of the object.

2.3. Cued search, object recognition, and ideal observers

The cued search methodology provides another interesting cueing manipulation. The experimenter flags a subset of display items as potential targets. Observers search faster through this subset than through the complete set; performance often appears equivalent to searching through a display containing only the cued items (Grindley & Townsend, Reference Grindley and Townsend1968; Davis, Kramer, & Graham, Reference Davis, Kramer and Graham1983; Palmer, Ames, & Lindsey, Reference Palmer, Ames and Lindsey1993). In cases in which the presence of uncued items negatively impacts performance, researchers have suggested that lateral masking — a term previously used somewhat interchangeably with “crowding” — degrades search in the more cluttered display (Eriksen & Lappin, Reference Eriksen and Lappin1967; Eriksen & Rohrbaugh, Reference Eriksen and Rohrbaugh1970). Nonetheless, because cued search keeps the stimulus constant while varying the cue, the effects cannot be purely sensory.

James (Reference James1890) describes the “taking possession by the mind” of a subset of objects as happening at the expense of perception of others. Because the target always appears within the cued subset, these particular experiments do not provide evidence of any cost of attending to the subset. Nonetheless, Palmer et al. (Reference Palmer, Ames and Lindsey1993) suggest the results provide a clear example of attention: one attends to a subset of the items, as both evidenced by and leading to faster search times. (Zivony and Eimer, Reference Zivony and Eimer2021 have criticized this tendency to use an experimental result to infer both that attention occurred, and that attention was the cause of that result. Hommel et al., Reference Hommel, Chapman, Cisek, Neyedli, Song and Welsh2019, make a similar point.)

Attempting again to describe these results without reference to attention, one might say that cueing changes the priors for likely target locations (i.e. where one expects to find the target before getting any evidence from the stimulus itself), and that that effectively changes the task from “search all items for the target” to “search these items for the target.” Those changes matter: people perform better when they know more about where to search. This rephrasing raises the question of whether cueing affects decisional rather than perceptual processes.

Palmer et al. (Reference Palmer, Ames and Lindsey1993) asked whether better performance searching through a subset of items necessarily implies a limited capacity perceptual mechanism that samples information only from the cued subset. To do this, they utilized ideal observer analysis. An ideal observer is a model that performs a task optimally, given the information available and certain assumptions. Palmer et al. (Reference Palmer, Ames and Lindsey1993) showed that an unlimited capacity ideal observer explains their results, without need for a perceptual attention mechanism like early selection. Knowing which subset contains the target reduces errors by allowing decision processes to ignore false alarms from non-cued distractors, improving performance. Whereas my lab began by examining what phenomena peripheral vision explains, Palmer et al. (Reference Palmer, Ames and Lindsey1993) — and others, e.g., Anderson (Reference Anderson2011) — suggest the important step of examining what phenomena Bayesian decision theory can explain, i.e. to what degree any intelligent system would exhibit the behavior.

Other researchers have used similar experiments and modeling to argue that search results do demonstrate limited capacity (e.g. Palmer, Fencsik, Flusberg, Horowitz & Wolfe, Reference Palmer, Fencsik, Flusberg, Horowitz and Wolfe2011). The question becomes whether one can make sense of the conditions under which this occurs (and occurs without a sensory confound like peripheral crowding). One hint perhaps comes from Palmer et al.’s (Reference Palmer, Ames and Lindsey1993) observers, who complained about the difficulty of searching within a subset of four items. Other research has also pointed to limits to an observer’s ability to respond to an arbitrarily complex cue (e.g. Eriksen & Webb, Reference Eriksen and Webb1989; Gobell, Tseng, & Sperling, Reference Gobell, Tseng and Sperling2004; Franconeri, Alvarez, & Cavanagh, Reference Franconeri, Alvarez and Cavanagh2013). We should consider these limits as some of the critical phenomena when developing a new understanding of limited capacity and the associated mechanisms.

A related issue arises when pondering the role of attention in ordinary object recognition. Does one select dog-like features to recognize a dog? Modern statistical-learning-based classifiers would distinguish a dog from a cat by making an intelligent decision based on both dog-like and cat-like features. Dog-recognition mechanisms fundamentally involve not-dog features. The selection/attention terminology reduces clarity, as it implies particular mechanisms that allow higher-level processing of some features but not others. On the other hand, to the extent that the visual system performs sub-optimally at object recognition, in a way not explainable by purely sensory limits, such phenomena could inform our understanding of capacity limits and attention.

Both of these examples use “attention” to mean something like “use top-down knowledge to perform a task.” While that concept might deserve its own word, our goal here is to collect phenomena that demonstrate capacity limits and in doing so understand the special mechanisms for dealing with those limits. As such, we clearly should not include phenomena explainable by an unlimited-capacity ideal observer. The human visual system must make use of top-down information, and any viable theory of vision would include such a mechanism. There is no need to elevate such a mechanism by calling it “attention”.

2.4. Mental marking

Another set of cueing tasks falls in a category one might call mental marking (Figure 7). Loosely speaking, this refers to tasks in which the observer gives some sort of special status to cued locations, so as to make judgments about those locations. Huang and Pashler (Reference Huang and Pashler2007) enumerate a number of such tasks. Multiple object tracking (MOT) provides a classic example (Figure 7E). The observer views a set of items, with a subset of them cued. The cue disappears, and the observer must track the cued subset as the objects move, ultimately identifying the tracked objects. With no visible cue during the object motion, one might think of the observer as mentally marking the items to track. Many of the classic visual cognition tasks of Ullman (Reference Ullman and Ullman1996) fall into this category, as one can interpret them as asking the observer to mentally trace along a path (Figure 7A-C).

Figure 7.

Mental marking tasks. (A) Is there a path between the dot and “x”? (B) Do the two dots appear on the same line? (C) Does the dot lie within the closed curve? (D) What shape do the red dots form? (E) Multiple object tracking. (F) Bistable Necker cube. Reddish glow shows loci of attention described in the text.

Should we consider performance on mental marking tasks in our list of critical attentional phenomena? While most tasks examined in this paper aim to better perceive the details of the attended object(s), mental marking tasks instead typically have the goal of keeping track of a subset of location(s) and making judgments about those locations (Huang and Pashler, Reference Huang and Pashler2007, draw a similar distinction). If these tasks have different goals, they might also probe different limits and mechanisms. To examine this possibility, consider two kinds of evidence for limits in mental marking tasks: changes in the percept and performance limits.

2.5. Inattentional blindness

Inattentional blindness (IAB) experiments provide another explicit manipulation of attention. The observer performs a nominal task, and then must indicate whether they noticed an unexpected stimulus and/or make judgments about that stimulus. Often the experimenter also tests the observer in a dual-task condition, requiring them to both perform the original task and judge the now-expected stimulus (e.g., Mack and Rock, Reference Mack and Rock1998). Across a variety of stimuli and tasks, observers more easily notice an expected stimulus than an unexpected stimulus.

The usual interpretation (Mack & Rock, Reference Mack and Rock1998) presumes that the observer attends to the features and/or portion of the display relevant for the nominal task, and inattention to the unexpected stimulus renders them unable to perceive it. In the dual-task trials, the observer supposedly divides attention, making possible some perception of both stimuli. Inattentional blindness has been taken as evidence for little processing without attention. However, researchers have suggested other interpretations. Perhaps observers perceive the unexpected stimulus but do not become aware of it, and/or quickly forget (Wolfe, Reference Wolfe and Coltheart1999). This might explain reduced inattentional blindness for meaningful stimuli like one’s own name, as well as priming effects (Mack & Rock, Reference Mack and Rock1998). Some IAB paradigms attempt to overcome these issues by making the unexpected stimulus sufficiently shocking (a gorilla, a unicycling clown, money on a tree) to make forgetting unlikely; nonetheless, observers often fail to become aware of the unexpected stimulus (Simons & Chabris, Reference Simons and Chabris1999; Hyman, Boss, Wise, McKenzie, & Caggiano, Reference Hyman, Boss, Wise, McKenzie and Caggiano2010; Hyman, Sarb, & Wise-Swanson, Reference Hyman, Sarb and Wise-Swanson2014). Interestingly, in some studies, even though observers fail to report the unexpected stimulus, they move to avoid a collision, suggesting that perception occurs (Tractinsky & Shinar, Reference Tractinsky and Shinar2008; Hyman et al., Reference Hyman, Sarb and Wise-Swanson2014). Any viable explanation for IAB also needs to explain this perception without awareness.

Peripheral vision seems unlikely to explain the classic Mack and Rock (Reference Mack and Rock1998) experiments. Their primary task discriminating the lengths of two crossing lines likely encourages center fixation, and dual-task performance suggests the IAB is not purely perceptual. Peripheral vision may be one factor in the invisible gorilla phenomena (Simons & Chabris, Reference Simons and Chabris1999), or blindness to scene cut errors (Levin & Simons, Reference Levin and Simons1997), as examples with real-world stimuli have typically not tracked the observer’s gaze (Rosenholtz, Reference Rosenholtz2020).

In describing these results without reference to attention, one might say that the observer has a nominal task and performs poorly at a surprise task (noticing the unexpected stimulus). When the experimenter later informs the observer about the latter task, the observer can to some degree perform both that and the nominal task. On the other hand, observers can complete tasks such as safely navigating the world – moving to avoid a collision – even when not explicitly informed of those tasks.

Framed this way, inattentional blindness does not appear particularly surprising. Surely one can often perform a task better if one knows the task. Inattentional blindness was surprising in part because observers missed supposedly automatically processed stimuli. If one automatically processes salient items, then one should notice them even when engaged in another task, but this not the case for 25% of observers (Mack & Rock, Reference Mack and Rock1998). However, as previously noted, later evidence has not supported the notion of perception that occurs automatically and without requiring attention.

If no perception occurs automatically, then perhaps we should more usefully think of all perception as resulting from performing a task. If the visual system had no limits on the tasks it could simultaneously perform, then it would perform all tasks, and an observer would have no trouble noticing an unexpected item. It should come as no surprise that the visual system does have task limits. The visual system must constantly choose what task to do next among many options.

2.6. Dual task and multitasking in general

3.1. Is there an additional limit?

I began by being open to the possibility of no additional capacity limit with an attention-like or selection-like mechanism. In examining the need for an additional limit, and the nature of that limit, I have argued against including some supposedly attentional phenomena in our list of critical phenomena, at least to begin with. In some cases, peripheral vision may have been the dominant factor, rather than attention (e.g., search, scene perception, change blindness, and object-based attention). Other phenomena could be at least partially explained by an unlimited-capacity ideal observer (e.g. some cued search), and one need not posit an additional attentional mechanism. In a related point, I have argued against associating attention with mechanisms that any reasonable model of vision would include (use of top-down information and planning eye movements).

However, many phenomena seem to point to a possibly coherent additional limit. Box (left column) shows my conservative list. I have suggested that to synthesize these phenomena it helps to assume that all perception results from doing a task and that there are limits to task performance. If so, knowing the task should often matter (inattentional blindness, cued search), and dual-task performance would often be worse than single-task. Task limits could restrict the pattern of items observers can monitor or mentally mark, or the number of curves they can simultaneously follow. Visual search and change detection are likely subject to such task limits as well. Task limits are compatible with cognitive load phenomenology (e.g. Lavie et al., Reference Lavie, Hirst, de Fockert and Viding2004) meaning that task limits might apply to more than just visual processing. On the other hand, many studies have suggested that attention modestly changes appearance (e.g., increases apparent contrast or size), corresponding to similar changes in physiological response. These results seem harder to think of in terms of task limits and may point to different limits or mechanisms.

Box B.

Critical visual attention phenomena, including both evidence of additional capacity limits and rich perception in apparent contradiction to such limits.

The reader may be interested in comparing these two lists to those in other recent papers questioning attention (Anderson, Reference Anderson2011; Hommel, et al., Reference Hommel, Chapman, Cisek, Neyedli, Song and Welsh2019; Zivony & Eimer, Reference Zivony and Eimer2021). Can a single additional limit explain both vision’s failures and successes, and if so, what is its nature and associated mechanisms? The following subsections discuss two alternative proposals and indicate my current favorite.

3.2. Is inattention like looking away?

Contemporaneous work proposed that in the absence of attention, vision might only encode summary statistics (Rensink, Reference Rensink2000; Treisman, Reference Treisman2006; Oliva & Torralba, Reference Oliva and Torralba2006; Wolfe et al., Reference Wolfe, Vo, Evans and Greene2011). Arguably this misattributed summary statistic-like effects to inattention rather than peripheral vision. Nonetheless, the idea that covert and overt attentional mechanisms might share more than priority maps (Section 1.2) has a certain appeal, not least because of the success of summary statistics in making sense of diverse phenomena.

How might this work? Attention could narrow the pooling mechanisms that lead to crowding. Summarization would occur over a larger region when not attending. Some behavioral and physiological evidence seems consistent with receptive fields changing size, whether due to attention (Moran & Desimone, Reference Moran and Desimone1985; Desimone & Duncan, Reference Desimone and Duncan1995), or training (Chen, et al., Reference Chen, Shin, Millin, Song, Kwon and Tjan2019). Researchers have even proposed that the pooling mechanisms underlying peripheral crowding might vary depending upon the perceptual organization of the stimulus (Sayim, Westheimer, & Herzog, Reference Sayim, Westheimer and Herzog2010; Manassi, Sayim, & Herzog, Reference Manassi, Sayim and Herzog2012; Manassi, Lonchampt, Clarke, & Herzog, Reference Manassi, Lonchampt, Clarke and Herzog2016). Adjusting the size of the pooling regions need not require rewiring new receptive fields on the fly. Rather, attention could reweight receptive field inputs, or change the effective size through interactions between neighbors. In the latter case, attention might enable use of multiple overlapping receptive fields to better reason about the stimulus. The presence of a cat and a boat within a single receptive field (RF) could prohibit identifying the cat from that RF alone. Another RF might contain only the boat, leading to its identification. Together, the two receptive fields could explain away the boat features, effectively – but not actually – reducing the size of the first receptive field in order to identify the cat. Vision might be limited by the arrangement of receptive fields and the complexity of the computations that combine their information, in line with suggestions from Franconeri et al. (Reference Franconeri, Alvarez and Cavanagh2013).

Despite my early advocacy for this flexible pooling theory (Rosenholtz, Reference Rosenholtz2011; Rosenholtz, Huang, & Ehinger, Reference Rosenholtz, Huang and Ehinger2012), I became disillusioned for two main reasons. First, success of the theory depends greatly on the specifics regarding the size and layout of the pooling regions. Crowding experiments – with observers attending to the peripheral target – presumably probe the minimum pooling size. How much larger would pooling regions grow without attention, and at what cost to performance? Furthermore, Freeman and Simoncelli (Reference Freeman and Simoncelli2011) found that the same set of pooling regions predicted performance at both peripheral letter identification and a scene metamer task. This suggests that employing focused versus diffuse attention has minimal impact on the pooling. What happens to the overlap between neighboring pooling regions as they change size? This affects the available information (Chaney et al., Reference Chaney, Fischer and Whitney2014). The lack of details makes it hard to get intuitions. Inattention would degrade central vision, but otherwise its impact remains unclear.

Second, after a year of rethinking attention from the ground up, this theory felt like epicycles. A flexible pooling mechanism with better encoding at attended locations still essentially treats attention as spatial selection, gating the information available to some other mechanism that actually performs the task. This seems likely, yet again, to require additional mechanisms to address, for example, non-spatial selection of task-relevant features. If attention merely gates access to other processes that perform the task, do those latter processes themselves have capacity limits, as suggested by cognitive load effects?

Research into visual attention has long assumed that attention acts as a gate, but our critical phenomena do not obviously provide evidence for such a mechanism. A significant amount of processing appears to happen in parallel, and outside of the nominal task focus. This would seem surprising if the attention gates access to higher-level processing. Yet a working human visual system likely demands such parallelism and imperfect focus. Dual-task and cognitive load phenomena seem to suggest a gradation of available information rather than a gate. Nonetheless, the view of attention as a gate has persisted, even in papers challenging attention theory (Hommel, et al., Reference Hommel, Chapman, Cisek, Neyedli, Song and Welsh2019; Zivony & Eimer, Reference Zivony and Eimer2021). A flexible pooling theory takes a step away from attention as gate, allowing access to some information across the field of view regardless of task relevance. The pooling mechanisms actually change the encoding of the visual information, rather than merely gating access to higher-level processes. Having pondered this step, perhaps we should consider an attention theory without any gate.

3.3. If not a gate, then what?

Without a gate, one must explain why observers do not perceive all available information, and in what way attention mechanisms address limited capacity. Perhaps perception results from doing a task, and tasks have limits. Gate theories suggest that one chooses a location, object, or feature for further processing by some separate part of the visual system. Instead, perhaps one chooses a task. I find it natural to equate selecting a task and setting up the classifier or mechanism to perform that task. Selection and performing a task might be inseparable. Executive functions, rather than choosing what information to gate based on task demands, rewards, and so on, might instead choose what task to perform. The observer asks a question about the visual world – chooses and performs a task. They receive an answer: the percept.

The limit on tasks cannot simply be on the number of simultaneous tasks, nor the number of items processed, as both numbers depend on the specifics of the stimuli and nominal task (e.g. Alvarez & Franconeri, Reference Alvarez and Franconeri2007; Franconeri et al., Reference Franconeri, Alvarez and Cavanagh2013; VanRullen et al., Reference VanRullen, Reddy and Koch2004). Nor can the limit be on overall task difficulty, as dual-task experiments controlled for difficulty in the component tasks by varying display time (e.g. VanRullen et al., Reference VanRullen, Reddy and Koch2004).

3.4. A limit on decision complexity

Perhaps the limit applies to a specific kind of task difficulty. Easy classification tasks look essentially the same in some high-dimensional feature space. The point clouds corresponding to chairs and rabbits will be well separated, and a simple classifier can discriminate between them with a low error rate. Hard tasks, however, can be hard for different reasons. A simple linear classifier might discriminate between a coffee table and a dining room table but overlap between the point clouds might inherently lead to errors. Such a task is data-limited but simple. At the other extreme, one might perfectly distinguish between two classes, but only if one used a complex — e.g., high-dimensional or wiggly — classification boundary. Perhaps humans are limited in the complexity of our decision boundaries.

This would immediately explain the difficulty of many dual tasks relative to their component single tasks. If each single task is two-alternative forced-choice, the dual task must distinguish between four possibilities. The decision boundary will be inherently more complex, and may often encounter limits, leading to poorer performance.

Earlier papers discuss this proposal in more detail (Rosenholtz, Reference Rosenholtz2017; Rosenholtz, Reference Rosenholtz2020). We can currently only speculate about what might be the nature of a limit on decision complexity. It could be the number of dimensions or neurons used to perform a task; the curvature of the decision boundary; or the number of linear hyperplanes needed to approximate the ideal boundary. If optimal task performance exceeded that limit, one would have to simplify the task, leading to errors. More complexity might also require more effort. However, in many cases, for example without time constraints, one could next perform a different task, thus further probing the available information, and eventually piece together a reasonable approximation of the more complex task.

Unlike the flexible pooling proposal, one can make reasonable qualitative guesses about decision complexity even without nailing down the details. Presumably, the visual system has developed to make many natural, ecologically important tasks simple, such as getting the gist of a scene. Hard tasks simultaneously operating on multiple items might be complex, such as following multiple curves, monitoring a complex set of cued items, or using peripheral vision to find a target in visual search. Due to the unusual nature of the peripheral encoding, hard peripheral tasks might often be complex. Gestalt grouping of stimuli might make tasks simpler, and so on.

Are these ideas merely a repackaging of the concept of “late selection”? While selecting a task based on the output of completed sensory processing does indeed sound “late,” the two theories then diverge. In the classic late selection theory (as exemplified by Deutsch & Deutsch, Reference Deutsch and Deutsch1963), processing proceeds until meaning is extracted, at which point the system selects relevant parts for access to short-term memory and awareness. In contrast, in the theory proposed here, meaning comes from choosing the task. Moreover, classic late selection inefficiently identifies all objects only to discard much of this information. Instead, I propose that the visual system efficiently employs sensory processing to create a general-purpose representation, applicable to multiple tasks, from which it can selectively choose.

The concept of task selection aligns with the idea that the visual system resolves competition among potential interpretations of the current visual state, rather than competing sensory data (Krauzlis, Bollimunta, Arcizet, & Wang, Reference Krauzlis, Bollimunta, Arcizet and Wang2014). Additionally, we can relate this to proposals of attention for action. Neumann (Reference Neumann, Heuer and Sanders1987), for instance, emphasizes physical constraints: humans have a maximum of two hands and two legs, and can only express one word at a time. Consequently, the brain faces limits in generating simultaneous action plans; it must choose “what to do and how to do it.” However, the present theory primarily focuses on decision-making limits and tasks with less physical demand

3.5. Thinking about tasks

If perception always results from doing a task, then we need to rethink the nature of tasks. Tasks must not be limited to discriminations between named categories, like “bee” vs. “fly,” because perception is not. Consider a useful back-pocket task, which one might employ when initially free-viewing a scene: getting the gist. The precise nature of this gist has largely eluded vision science. We should ask what task might correspond to getting the gist or getting subjective awareness of a scene. Perhaps the visual system attempts to discriminate the scene from other possible scenes. Getting the gist might equate to approximately localizing the visual input in a high-dimensional representation space. The results of that gist-localization would provide a rich percept that is difficult to describe in words. When first viewing a scene, or getting the gist without performing any other task, the localization might be fairly precise; the observer would know a great deal about the scene (Figure 8A). That knowledge would suggest new tasks to probe the details. A difficult subsequent task, such as a fine-grained judgment of object pose, performed in conjunction with precise gist-localization might exceed the task-complexity limit. The observer would need to resort to less precise localization in the high dimensional space, leading to less understanding of the scene (Figure 8B), and in extreme cases to inattentional blindness. Back-pocket tasks would not be automatic, per se, but would depend upon the complexity of the rest of the task. Shifting the primary task from moment to moment, while continuing to gather whatever gist information one could, given complexity limits, would provide stability of the percept over time. The notion of more precise localization in a higher dimensional space can be thought of as a generalization of the idea of attention leading to more precise spatial location of receptive fields (Section 3.2).

Figure 8.

(A) Perhaps the gist of a scene is defined by the scenes the observer can discriminate from that scene (red) as opposed to those confused with it (gray). (B) When judging whether pedestrians are near (green outline) or far (yellow outline), the visual system might still get the gist, discriminating the current scene (gray) from others (red), albeit more coarsely than in (A).

If we choose to equate choosing and performing a task with attention, then all perception requires attention. The observer may be unaware of the visual system’s task, which may often differ from the nominal or conscious task. The observer may simplify a too-complex nominal task. Conversely, the observer may typically do more than the nominal task because of the benefits of perceiving one’s environment and not only the prioritized object. The brain may well be built to make ignoring everything but a single object more complex, requiring more effort. The percept resulting from performing the task might also be unconscious, for example in zombie behaviors.