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Visuomotor extrapolation

Published online by Cambridge University Press:  14 May 2008

David Whitney
David Whitney
Affiliation:
Department of Psychology, and The Center for Mind and Brain, University of California, Davis, CA 95618. dwhitney@ucdavis.eduhttp://mindbrain.ucdavis.edu/content/Labs/Whitney/
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Abstract

Accurate perception of moving objects would be useful; accurate visually guided action is crucial. Visual motion across the scene influences perceived object location and the trajectory of reaching movements to objects. In this commentary, I propose that the visual system assigns the position of any object based on the predominant motion present in the scene, and that this is used to guide reaching movements to compensate for delays in visuomotor processing.

Information

Type
Open Peer Commentary
Information
Behavioral and Brain Sciences , Volume 31 , Issue 2 , April 2008 , pp. 220 - 221
Copyright
Copyright ©Cambridge University Press 2008

Nijhawan's article provides evidence for compensation mechanisms in visual perception and visually guided action. Most of this evidence is drawn from the flash-lag effect, where a single object moves across the retina. There are several other illusions, some of which are briefly mentioned in the target article, which might also support Nijhawan's position (De Valois & De Valois Reference De Valois and De Valois1991; Hess Reference Hess1904; Matin et al. Reference Matin, Boff and Pola1976; Nishida & Johnston Reference Nishida and Johnston1999; Ramachandran & Anstis Reference Ramachandran and Anstis1990; Regan & Beverley Reference Regan and Beverley1984; Snowden Reference Snowden1998; Whitaker et al. Reference Whitaker, McGraw and Pearson1999; Whitney & Cavanagh Reference Whitney and Cavanagh2000). For a review of these illusions, see Whitney (Reference Whitney2002). The strongest support for compensation in the perceptual system (e.g., extrapolation) comes from the displacement of stationary edges by motion. For example, visual motion viewed through a static aperture causes the aperture to appear shifted in the direction of the motion (De Valois & De Valois 1991; Ramachandran & Anstis Reference Ramachandran and Anstis1990; Regan & Beverley Reference Regan and Beverley1984); the motion aftereffect is accompanied by a concurrent shift in the apparent position of a static test stimulus (Nishida & Johnston Reference Nishida and Johnston1999; Snowden Reference Snowden1998; Whitaker et al. Reference Whitaker, McGraw and Pearson1999); and, static flashed objects appear shifted in the direction of nearby motion (Whitney & Cavanagh Reference Whitney and Cavanagh2000). Whereas the flash-lag effect may be due to differential latencies for moving and flashed objects (Ogmen et al. 2004; Purushothaman et al. Reference Purushothaman, Patel, Bedell and Öğmen1998; Whitney & Murakami Reference Whitney and Murakami1998), these other mislocalizations of static edges by visual motion cannot be caused by temporal mechanisms such as differential latencies (Whitney Reference Whitney2002).

Although these mislocalizations of static edges by visual motion provide the strongest support for perceptual extrapolation (i.e., compensation for neural delays in the perceptual system), some of these illusions greatly complicate things: Several papers have shown that flashed objects appear shifted forward, in a direction consistent with any nearby visual motion, even when that motion is several degrees away from the flash. This has been called the “flash-drag” effect or the “flash-shift” effect (Durant & Johnston Reference Durant and Johnston2004; Eagleman & Sejnowski Reference Eagleman and Sejnowski2007; Watanabe et al. Reference Watanabe, Nijhawan and Shimojo2002; Reference Watanabe, Sato and Shimojo2003; Whitney 2006; Whitney & Cavanagh Reference Whitney and Cavanagh2000; Reference Whitney and Cavanagh2003). Because the flash is not moving, and it is distantly separated from the moving object, it does not immediately make sense why the flash should appear shifted (or extrapolated) in the direction of nearby motion. This result is somewhat difficult to reconcile with the notion of compensation for moving object positions, but it is not entirely incompatible. In fact, this flash-drag effect suggests that the sort of compensation that Nijhawan describes for a single moving object extends to all objects, and may be a far more pervasive and important mechanism than simply allowing us to perceptually extrapolate a baseball or other moving object's position.

In Nijhawan's article, the primary case that is considered is one in which a single moving object needs to be perceived or grasped. This is a relatively rare situation compared to what normally happens: usually, there is image motion across the entire retina, not just a single moving object. Normally the world is physically stationary, and it is we (our eyes, heads, or bodies) that move around; and it is our movement which generates retinal image motion. For example, when we reach to any object, we usually make an eye or head movement during or just before the reach. In this case, there is retinal motion of the scene and the target object. On account of delays in visual processing, delays in coordinate transformations, and other factors such as imperfect efference copy signals (Bridgeman Reference Bridgeman1995) – along with the fact that targets of reaching movements are coded in eye-centered coordinates (Buneo et al. Reference Buneo, Jarvis, Batista and Andersen2002; Crawford et al. Reference Crawford, Medendorp and Marotta2004; Henriques et al. Reference Henriques, Klier, Smith, Lowy and Crawford1998) – our visuomotor system faces a somewhat similar challenge to the one outlined by Nijhawan, but on a much grander scale. Because of these visuomotor delays, we should miss-direct nearly every reaching movement we make to virtually any object. Every time we reach toward our coffee cup, we should either hit the cup, knocking it over, or fall short of the cup – all because of sluggish visual and motor processing.

How does the visuomotor system avoid these errors? In a recent series of studies, we found that the visuomotor system samples motion across the visual field and then shifts the trajectory of the hand in the direction of that motion when reaching to any object in the scene (Whitney & Goodale Reference Whitney and Goodale2005; Whitney et al. Reference Whitney, Westwood and Goodale2003; Reference Whitney, Ellison, Rice, Arnold, Goodale, Walsh and Milner2007). This effect was recently called the manual following response (Gomi et al. Reference Gomi, Abekawa and Nishida2006; Saijo et al. Reference Saijo, Murakami, Nishida and Gomi2005) and reveals an adaptive mechanism: The visuomotor system uses retinal motion to gauge movements of the eye and body (probably because it is as fast or faster than using vestibular or proprioceptive cues), and then adjusts the trajectory of the reach based on this information to improve the accuracy of goal-directed action. In support of this, when subjects were passively rotated, the presence of background retinal image motion improved the accuracy of reaching movements compared to cases in which only static information, or nothing, was visible (Whitney et al. Reference Whitney, Westwood and Goodale2003). The manual following response is conceptually similar to the “flash-drag effect” described above, and it suggests that the visual and visuomotor systems use retinal image motion (the kind generated every time we move our eyes) to update/extrapolate/shift the representations of object position (causing objects to appear shifted in position) – and this allows us to guide our hand more accurately than would otherwise be possible.

This visuomotor extrapolation model has the advantage that it accounts for several psychophysical findings that are discrepant with the perceptual extrapolation model; and it also has the advantage that it explains accurate visuomotor behavior under the most common circumstances – where the world is stationary and we are moving.

References

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