2.1. Background

The origins of the idea that there are a few basic tastes predate modern science by at least two millennia. The evolution of the concept is rooted in ancient cultural and philosophical thought long before the advent of the scientific method. It also appears to be derived from the characteristics of our cognitive abilities. These may be the primary root reasons for the development of the idea of basic tastes, but it is clear that this model has never been construed as a scientific hypothesis to be tested.

2.2. Basic tastes from antiquity

In China during the early Han dynasty, the first two or three centuries B.C., two ways of organizing experience were formalized. These were the Yin–Yang dichotomy, and the five ways of wu xing (Unschuld Reference Unschuld1993). Prominent in the latter were the five tastes, sweet, sour, salty, bitter, and acrid. Both of these aspects of knowledge are evident in modern thought.

The Yin–Yang idea points to the importance of differences, meaning that a concept cannot exist without something different from it; “day” would not exist without “night,” “high tide” needs “low tide,” good and bad depend on each other, male and female, red depends on green, and there would be no “salty” if there were no other taste. This differentiation provides the logical basis for taste discrimination and its quantification in the across-fiber pattern theory (see Sect. 6.4 on Quantification).

2.3. Basic tastes and information theory

But why did the Chinese create the idea of five tastes? The reason is not immediately obvious. Perhaps it is clear from simple sensory experience that five tastes are distinct perceptual entities, like pigs, pineapples, and planets. Or it may be that the ancient Chinese had jars of spices on the table that included a sugar and a salt, a citrus juice, bitters, and something acrid (pepper?); they needed – and we need – words to communicate about these important substances. Still the main reason for the idea of five tastes may have been that the traditional Chinese method of organizing experiences, the “five ways” demanded by wu xing. But then where did these five ways come from?

By the 20th century the idea of basic tastes had not evolved much beyond asking how many there are. Boring (Reference Boring1942) stated that the general consensus was that there are four. Of relevance here, in an information theory approach, Miller (Reference Miller1956) showed that human memory limits the contents of facile categorizations to about seven members each. His research provides one very practical rationale both for the invention of the five ways of Chinese thought, and for our four or five basic tastes. Our limited ability to remember the contents of a category containing more than “the magical number seven plus or minus two” of unrelated items certainly could have influenced our understanding in the field of taste, as it does in many other aspects of our description of the world.

As examples of the ubiquity of such categorizations, we have seven days in the week, seven sins and seven virtues, and Newton's seven colors – all arbitrary man-made rubrics. The many geological time periods are packaged into groupings and subgroupings of seven or less each. The standard format of scientific papers also has about six sections – Introduction, Methods, Results, Discussion, Summary, and Conclusions – which is very useful. But why is this format so useful? We have not been able to deal with the 365 days of the year without dividing that number into small categories; four seasons of three months each, each month consisting of about four weeks, each week of seven days. The year is a natural phenomenon – once around the sun – but these other terms are not. Aristotle, the great classifier of all things into short lists, proposed seven tastes. These short lists seem to make good intuitive sense. But are there really seven seas and seven continents? And is the Mediterranean Sea really made up of seven mini-seas? Should we anticipate seven tastes?

3.1. Hanig sets the stage

The modern empirical history of basic tastes is herein taken rather arbitrarily as starting with a seminal experiment by Hanig (Reference Hanig1901). He demonstrated that the areas of sensitivity of the tongue to the basic tastes differ, with sweet largely in the front, bitter in the back, and salty and sour on the sides. Although their points of maximum sensitivity differ, their total distributions overlap greatly. It has been data of this sort that has supported the idea that we can sense only these “basic” tastes – the main assumption of the basic tastes doctrine. Subsequent research in taste largely followed his format. In the present article Hanig's work is used as a short-hand rubric for the idea of basic tastes.

An attempt to define the idea of basic tastes would be helpful here, although it will become clear throughout this article how nebulous and indefinable this idea is. But at this point, the idea of basic tastes may be put simply: The basic tastes are all we can sense, thus all tastes are reducible to only these few separate and distinctly different sensations.

3.2. Following Hanig

The idea of four basic tastes gave us a very convenient structure for research. The job then was just to find evidence for them. And we did.

3.3. Hanig omitted control groups

Unfortunately, Hanig's format did not provide the essential perspective of control tests demanded by science. If his findings were to have any relevance to the basic tastes model, it would have been necessary to learn if other tastants, for example KCl and CaCl₂, proline, and lysine, also had differentially distributed sensitivities across the tongue. If they were distributed in this way, the implications of his data for the basic tastes idea would have been nullified. No one has completed his research with the required control studies. Why not?

Since Hanig, there has been a routine omission of control groups in research using the basic tastes for its organization. This omission is notable in the three areas of research just noted: psychophysics, and receptor and neural organization. The effect of omitting control groups is to support the basic tastes model, rather than to question it. This format is not helpful in science.

4.1. Language

4.2. Our methods fit Hanig's orientation

Of course, all of our knowledge comes in terms of what we can measure, and thus what we can measure can determine how we think. What we measure depends on our techniques of measurement. Thus the techniques of measurement that are available can set the arena into which our discourse falls. These techniques include the biological and mathematical methods at hand. Then the characteristics of our language bias our understanding of the data.

5.1. Lack of controls

As mentioned in Section 3.3, Hanig set the stage for omitting control groups. This, in itself, disqualifies most data since then used to support the basic tastes position.

5.2. Undefended assertions

There is a general uncritical acceptance of the reality of basic tastes (Sect. 3.2.1). The many textbook statements that there are four basic tastes do not define what a basic taste is. They do not discuss the lack of the necessary underlying testable hypotheses or the absence of controls, although ad hoc reasons, such as control of food intake, may be offered in support of the idea. This procedure of only supporting the basic tastes position has become routine (Amrein & Bray Reference Amrein and Bray2003; McBurney Reference McBurney1974, Reference McBurney, Carterette and Friedman1978; McBurney & Gent Reference McBurney and Gent1979; Scott Reference Scott2004) instead of fairly testing it. When both sides of this issue are represented, the lack of support for the basic tastes position becomes evident (Schiffman & Erickson Reference Schiffman and Erickson1980).

By contrast, in the field of physics, some textbooks, and certainly discussions of the history of physics, point out where the idea of atoms came from, what data support that idea, how falsifications were attempted, and in what ways it is a heuristic idea. Could that be done in taste? Probably not within the present structure in this field.

5.3. Data are ignored

In order to follow Hanig, much data and logic contrary to the basic tastes position have had to be jettisoned.

5.4. Logic is ignored

The primary logical fault with the basic tastes position is that – as previously mentioned – it does not have a rational and testable definition. We do not know what “basic tastes” are, but that does not appear to be a topic of concern. This fault runs through the following discussion.

In addition to not addressing contrary data, articles supporting the basic taste idea do not address the logical strength of coding in populations (e.g., the across-fiber pattern theory) and the corresponding logical weaknesses of the labeled-line basic tastes position (Erickson Reference Erickson1968, Reference Erickson, Le Magnen and MacLeod1977, Reference Erickson1982a, Reference Erickson and Neff1982b, Reference Erickson1984b, Reference Erickson and Pfaff1985a, Reference Erickson2000, Reference Erickson2001; Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990; Erickson & Covey Reference Erickson and Covey1980; Erickson et al. Reference Erickson, Covey and Doetsch1980; Ishii & O'Mahoney Reference Ishii and O'Mahoney1987; O'Mahoney & Ishii Reference O'Mahony, Ishii, Kawamura and Kare1987; Schiffman & Erickson Reference Schiffman and Erickson1980, Reference Schiffman, Erickson, Simon and Roper1993). It is to be noted next that there is a general acceptance of some form of population coding throughout the field of neurobiology (see Erickson Reference Erickson2000, Reference Erickson2001).

The evidence for receptor or neuron types is held as a falsification of, or at least awkward for, across-fiber patterning (Scott & Giza Reference Scott and Giza2000). However, the original presentation of across-fiber patterning was in terms of a few receptor and neuron types (Erickson Reference Erickson and Zotterman1963). This model requires receptor types in nontopographic sensory systems such as color vision and taste (Erickson Reference Erickson1968). It is odd that groupings have been used as an argument against this theory (Smith & Scott Reference Smith and Scott2001).

The basic tastes assumption has provided an illogical format for neuron categorization. The fact that each taste neuron or receptor is more sensitive to one of the basic tastants than others (Frank Reference Frank1974, Reference Frank2000) is logically inescapable; how could a neuron not be more sensitive to one tastant than others? And this must be as true for any other set of tastants as for the four basics, qualifying any set of four stimuli as basics; but, again, these control studies of trying other tastants are absent. And in Frank's model the side-band responses to the non-“best” stimuli are considered to be noise, or the tastes of the basic stimuli must be assumed to be a combination of several tastes (Bartoshuk Reference Bartoshuk, Atkinson, Herrnstein, Lindzey and Luce1988) – both unparsimonious ad hoc assumptions apparently devised to support the basic tastes model. The fact that larger amounts of data, from more than the four basic stimuli, can be expressed along a continuum in the same form as Frank's plots (Erickson Reference Erickson, Kare and Maller1967; Scott & Erickson Reference Scott and Erickson1971) is not addressed in the basic tastes model.

Although both the rationale and control experiments are missing, Frank's data have been cited as strong support for the idea of labeled lines, and thus basic tastes. Other methods of obtaining data in support of neuron types or groupings have been undertaken, also without independent rationale and definition (e.g., Smith et al. Reference Smith, St. John and Boughter2000; St. John & Smith Reference St. John and Smith1999).

The similarity of the specific taste receptor data to the characteristics of color vision provides another area for an unbiased critique of the basic taste model. Adaptation, or the genetic absence, of the long-wavelength visual receptors, results in a strong diminution or absence of responsiveness to a range of “reds,” analogous to the taste effects seen in mice missing a taste receptor (Zhang et al. Reference Zhang, Hoon, Chandrashekar, Mueller, Cook, Wu, Zuker and Ryba2003; Zhao et al. Reference Zhao, Zhang, Hoon, Chandrashekar, Erlenbach, Ryba and Zuker2003). In this situation, responses of cells to other wavelengths may vary from not affected to very affected depending on their wavelength. The across-fiber pattern model would accommodate increases in saturation in the perception of shorter wavelength stimuli along with decreases in saturation in longer wavelengths. Similar effects in taste are evident, and not surprising. Resolution of this issue in taste would require close attention to the responses to systematically varied stimuli other than the basics (analogous to variations in wavelength), and a rationalization of the conclusions of specificity in taste with the contradictory historical findings.

The assumption that there are only four tastes has a strong but illogical effect on psychophysical data collection (Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990). The finding of “suppression” between tastes (Bartoshuk Reference Bartoshuk, Atkinson, Herrnstein, Lindzey and Luce1988) required the prior assumption of the existence of a few perceptually distinct basic tastes that cannot each be modified in mixtures. In that idea, only the basics should be sensed, so only changes in the strength of the basic tastes in the mixtures could be accepted as data. Any change in taste quality, such as synthesis of different tastes (Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990; Schiffman & Erickson Reference Schiffman and Erickson1980), was not admissible. If the subjects are only given the basics for use, then they can respond only in terms of the basics. But given other standards for use, they possibly would have used those – there was an omission of this key control group. But if the doctrine of basic tastes is not assumed, then mixtures of “basic” or other tastants could be understood to produce a changed across-fiber pattern with some similarity to, or correlation with, each of the components. A new and intermediate taste, no longer a basic taste, should be sensed, usually similar to both components, but not just suppressed versions of basics. For perspective, if the descriptions of colors are limited to only a few basic terms or color samples, then the ratings for colors intermediate to these “basic” colors are reduced (Erickson Reference Erickson, Le Magnen and MacLeod1977); these could be inappropriately considered “suppressed.”

So in addition to the prior assumption of basic tastes, the idea of suppression requires an additional assumption that these tastes are somehow related while also being distinct and separate. No additional assumptions are required in the across-fiber model, as the continuum requires that tastes be related. Parsimony has proven useful in science.

The lack of linking hypotheses is one of the most serious of the problems with the logic of basic tastes. This model encompasses four unrelated events at various levels of meaning. First, it holds that only four tastes are perceivable. This is extended without explanation to the neural level, where four different sets of sensory neurons each exclusively represent one of these tastes. It is also held for the receptors, and then receptor mechanisms, as well as for stimuli, without articulating the relationships between these levels.

5.5. Convenience

It may be that the most generally persuasive drawing power of the basic tastes model is that without a formal restraining definition, this model is very convenient in designing experiments, and it easily categorizes any data – especially from experiments cast in the four tastes format. The design is thus very straightforward and is supported by our words and techniques. And this procedure, as Hanig's, seems to validate the basic tastes idea without test. Arguments in favor of the basics position flow easily from this context, encouraging a lack of attention to contrary matters of data and logic. It is certainly hard to think of a simple model different from that provided by the basic tastes to guide our research and understanding – and against which the basic model might finally be tested. It is improbable that an ultimately successful science was ever built on convenience.

5.6. Formality and science

In the remainder of this article the term “formal” will refer to the use of data-based, rational and heuristic hypotheses that provide clear enough definitions of their terms to be potentially falsifiable. Hanig's work does not fit into this definition. The core orientation herein is that formal definitions are required for taste to qualify as a science, just as they are in physics. One aspect of this process is that it provides the essential groundwork for control experiments, as illustrated in Section 7. But first, some previous experiments with controls will be discussed.

6.1. Singularity of tastes

If there are only four (or a few more) basic tastes, then each should be sensed as being singular because each is not more than one taste (Erickson Reference Erickson2000; Erickson & Covey Reference Erickson and Covey1980; Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990; O'Mahoney et al. Reference O'Mahony, Atassi-Sheldon, Rothman and Murphy-Ellison1983). On the other hand, if each stimulus that was not basic were composed of several of the basics, it should be perceived as “more-than-one” taste. The across-fiber pattern model indicates that any tastant, basic or nonbasic, might be sensed as singular (Erickson Reference Erickson2000; Erickson & Covey Reference Erickson and Covey1980; Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990; O'Mahoney et al. Reference O'Mahony, Atassi-Sheldon, Rothman and Murphy-Ellison1983). This is testable. Previous studies (Erickson & Covey Reference Erickson and Covey1980) have demonstrated that the basic tastes are judged as only relatively singular (by around 80% of the subjects). This research also showed that the nonbasics are somewhat less singular than the basics (about 50% of the subjects reporting singularity), but not clearly “more-than-one” as would be predicted in the basic tastes model.

And certainly mixtures of tastes should contain more than one basic taste, and thus be rated as more-than-one. However, it was shown that mixtures of several tastants can be perceived as being relatively singular (Erickson Reference Erickson1982a; Erickson & Covey Reference Erickson and Covey1980; O'Mahoney et al. Reference O'Mahony, Atassi-Sheldon, Rothman and Murphy-Ellison1983; Schiffman & Erickson Reference Schiffman and Erickson1980). Indeed, mixtures of two basics can appear to be as singular as the individual basic components of that mixture; for example, the singularity of mixtures of HCl with either NaCl or Q₂SO₄ were equivalent to the singularity of those basics when tasted alone.

As controls on these methods (Erickson Reference Erickson1982a), individual tones were routinely rated as one stimulus, and multiple tones were rated as more-than-one; this complexity increased with the number of tones in the “mixture.” That is, the tonal stimulus appeared to be “analyzed” into its components, all components remaining clear and unchanged as predicted for tastes in the basic model. However, increasing the number of components of a taste mixture does not show this increase in complexity. Instead of being analyzed into its components, a new somewhat singular but nonbasic taste was “synthesized” in mixtures of two, three, or four components. Like taste, a nonbasic color resulting from the combination of two “basic” colors (Erickson Reference Erickson, Le Magnen and MacLeod1977; Erickson & Covey Reference Erickson and Covey1980), such as a blue-green, was also rated as singular – this being an example of the synthesis of a sensation different from either of the “basic” components (Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990). “Analysis” is predicted for topographic systems such as auditory tones, and location on the skin or retina. “Synthesis” is predicted for the nontopographic systems such as color, taste, the vestibular sense, kinesthesis, and olfaction. These unbiased methods could have falsified the across-fiber pattern theory, but instead supported it and falsified this aspect of the basic tastes position.

On the other hand, and in partial support of the basic tastes model, mixtures of sucrose or fructose with any one of the other three basic stimuli were rated as strongly less singular (around 39% to 64%) than either of the components alone (about 85% each for the sugars), suggesting that these sensations do indeed remain somewhat independent and singular. Also, when these sugars were mixed with any nonbasic stimulus, the combinations were rated as less singular than either component, an indication of semi-independence. Also, quinine behaved somewhat like the sugars in that, in mixtures with sucrose or NaCl, the singularity of the mixture was greatly reduced – as it should be in the basic tastes doctrine; however, the combination of QHCl with HCl remained singular. These data are suggestive of the receptor specificity claimed for sweet and bitter (Zhang et al. Reference Zhang, Hoon, Chandrashekar, Mueller, Cook, Wu, Zuker and Ryba2003). It was concluded that although NaCl, HCl, and QHCl were interrelated, sucrose might well stand further apart, perhaps not being on the same dimension as the others (Erickson & Covey Reference Erickson and Covey1980). That sucrose and quinine might lie outside the continua of the other tastants had been suggested (Erickson et al. Reference Erickson, Doetsch and Marshall1965). Further control studies could provide clarity here.

6.2. Tastes other than the basics

The singularity of nonbasic tastes and mixtures, just noted, is indicative of the synthesis of a different taste, this taste necessarily being something other than and in addition to the basics (Erickson Reference Erickson, Le Magnen and MacLeod1977; Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990; O'Mahoney et al. Reference O'Mahony, Atassi-Sheldon, Rothman and Murphy-Ellison1983; Schiffman & Erickson Reference Schiffman and Erickson1980).

The question of whether the basic tastes are the only tastes perceived in mixtures, as held in the basic tastes model, has also been tested (Erickson Reference Erickson1982a; Erickson et al. Reference Erickson, Priolo, Warwick and Schiffman1990). Subjects very rarely fail when asked to identify one of the four basic tastes alone. But when mixed with another basic taste, the failure rate in identifying a stimulus increased dramatically, indicating that that basic was not clearly and separately experienced in the mixture. In a control study, a simultaneous “mixture” tone did not generate this confusion in the identification of a given tone.

These findings together suggest that there are more than just the four basic tastes.

6.3. A continuum of tastes, not just four

As proposed by Henning (Reference Henning1916), many tastes comprise a continuum in the shape of a tetrahedron defined at its four corners by sweet, sour, salty, and bitter; this continuum is explicitly composed of many singular tastes, not just those tastes at the four corners. But the four corner-tastes were later held to be the only tastes possible, and called the “basics” (Bartoshuk Reference Bartoshuk, Atkinson, Herrnstein, Lindzey and Luce1988). Henning's continuum is a clear contradiction of the basic tastes position, while being a core aspect of the across-fiber pattern theory.

That multidimensional scaling solutions are possible indicates that the spaces are continuous. If the data were unrelated, solutions of low dimensionality would be impossible. This invalidates the basic tastes idea that the few tastes are independent of each other. Interrelatedness along a continuum is a core part of the across-fiber pattern model.

That taste is probably organized as a continuum also shows up in reports of neural activity. For example, Frank's data (1974, 2000) on the responses of individual neurons to the various basic tastes derives from a continuous organization presented as her baseline (Woolston & Erickson Reference Woolston and Erickson1979); she demonstrates that neurons will show decreasing responsiveness with increasing distance from the “best stimulus,” a “neural response function” along these continua (Doetsch & Erickson Reference Doetsch and Erickson1970; Erickson Reference Erickson, Kare and Maller1967, Reference Erickson1968, Reference Erickson, Schmitt and Worden1974, Reference Erickson and Masterton1978, 1982, Reference Erickson1984b, Reference Erickson1985b, Reference Erickson1986; Erickson et al. Reference Erickson, Doetsch and Marshall1965). This is contrary to the basic tastes position and in accord with the across-fiber pattern theory (Erickson Reference Erickson, Kare and Maller1967; Erickson et al. Reference Erickson, Doetsch and Marshall1965; Scott & Erickson Reference Scott and Erickson1971. But her data have been accepted as strong support for the basic tastes position instead. Why?

In this basic tastes orientation, the fact that many of these neurons respond best to stimuli other than the basics is not considered because the topic is restricted to basic stimuli. The desired results are on the whole mathematically assured once the best stimulus of these four is found; that the other three stimuli are less effective is therefore not surprising. However, this relatedness may be extended to provide the same coherent organization for at least 16 stimuli across a continuum when the requirement is dropped that only the four basic stimuli be considered (Doetsch & Erickson Reference Doetsch and Erickson1970; Erickson et al. Reference Erickson, Doetsch and Marshall1965, 1967, 1968; Erickson et al. Reference Erickson, Doetsch and Marshall1965; Scott & Erickson Reference Scott and Erickson1971). Plots similar in form to Frank's were obtained, but for many stimuli. These are the smooth, bell-shaped neural response functions that describe the responses of individual neurons along the taste continua, as they appear for color vision receptors and neurons. These much more extensive and powerful neural response functions are neither expected nor checked for in the quadripartite logic.

It is important to note that even Hanig's data are what would be expected from a continuum (Schiffman & Erickson Reference Schiffman and Erickson1980), as would Frank's groupings (Schiffman et al. Reference Schiffman, Orlandi, Erickson, Ordy and Brizzee1979; Woolston & Erickson Reference Woolston and Erickson1979).

6.4. Quantification of neural information

There has been no definitive attempt to quantify taste information in the quadripartite model. On the other hand, the across-fiber pattern theory has provided a testable method to quantify the differences or similarities between any stimuli, including tastants. The general idea for the representation of the differences in quality and intensity for any two discriminable tastants is that they must provide neural responses that are somehow different from each other. This is the meaning of the Chinese Yin/Yang differentiation required for our understanding of any concept; to exist, “sweetness” must have another taste different from it. People with only the “red” receptors can see no colors at all. So as to create the most bias-free measure (no prior assumption of what constitutes the difference, such as groupings) the different across-fiber patterns set up by two tastants are superimposed, and the absolute difference between them, say in impulses/second, is measured.

The quantification given in the across-fiber pattern model may be best presented first by example, and then formalized. The across-fiber patterns shown by Lemon and Smith (Reference Lemon and Smith2005) provide the first demonstration. In their Figure 3, the basis for the quantitative differences between two stimuli is given by the differences between the pairs of curves. The difference between sucrose and fructose is small, as would be the difference between KCl and HCl. The difference between HCl and sucrose is large. As absolute differences in impulses/time, these would be linear distances, and could be appropriately used for multidimensional scaling. Other examples that could be analyzed this way may be seen in the across-fiber patterns presented by Boughter and Smith (Reference Boughter and Smith1998), Boughter et al. (Reference Boughter, St. John and Smith1999), Dahl et al. (Reference Dahl, Erickson and Simon1997), Schiffman et al. (Reference Schiffman, Orlandi, Erickson, Ordy and Brizzee1979), Smith & Li (Reference Smith and Li1998, Reference Smith and Li2000), and Smith et al. Figure 2 (2000). This approach has been found to be a useful heuristic (Dahl et al. Reference Dahl, Erickson and Simon1997; Erickson Reference Erickson and Neff1982b, Reference Erickson1986; Ganchrow & Erickson Reference Ganchrow and Erickson1970; Schiffman et al. Reference Schiffman, Orlandi, Erickson, Ordy and Brizzee1979).

Figure 1.

Accounting for ten Comparison taste stimuli with basic and nonbasic Standard tastes; the experimental design and results. In the six panels, the first two, E 1–2, give the results from the two experimental “basic tastes” groups, and C 1–4 each give the results from one of the four “nonbasic tastes” control groups. In panels E 1 and E 2 the four basic words (sweet, sour, salty, and bitter) and stimuli (QHCl, HCl, NaCl, and sucrose) were used as Standards to describe the tastes of the ten Comparison stimuli. In the rest of the panels (C 1–4), from two to five stimuli not considered to be basic were used in the same role as Standards.

Figure 2.

The degree to which each nonbasic Standard stimulus accounted for the Comparison stimuli depends on how many Standards were used (from Fig. 1, C 1–4). The lower curve shows that each Standard's account was not constant but decreased (averages) as the number of Standards increased. The upper curve shows that the total amount accounted for by all Standards may be approaching an asymptote of somewhat less than 100% as the number of Standards increases. The upper curve equals the lower curve times the number of Standards.

Figure 3.

Comparison stimuli rated as “singular” or “morethan-one;” data from the basic tastes protocols (Fig. 1, E1, E2). The left column indicates how the ratings would appear for a hypothetical stimulus that was rated as singular by all subjects, and the right column illustrates the data for a stimulus that was always rated as more-than-one. No stimulus is completely either singular or more-than-one, the only two outcomes that would be predicted in the basic tastes model.

Stated in general terms, the total amount of activity set up by a given stimulus, here a tastant, is its neural mass. These are the areas under each across-fiber pattern curve discussed above. (Lashley [Reference Lashley1931] noted the importance of “neural mass” in the handling of high levels of information of importance to a species.) The absolute difference in impulses/second (or some other measure) between this profile and the profile set up by a second stimulus is called the neural mass difference (Erickson Reference Erickson1986). In the across-fiber pattern theory, the prediction is made that the perceived differences between two stimuli is a positive function of the amount of this neural mass difference. As an example, two sodium salts of similar concentration would produce small neural mass differences, and would thus be predicted to be poorly discriminable from each other.

The total neural mass produced by a stimulus is taken to be indicative of intensity. If the neural masses produced by two stimuli are different, that indicates a difference in intensity. Both intensity and quality give neural mass differences information about discrimination between stimuli. If it is desired to inspect only taste quality differences, leaving out intensity differences, the neural masses – representing intensity – can be brought to the same level by multiplication through one of them.

As examples, just noticeable differences in the color of a stimulus, or the location of a visual or somesthetic stimulus, would require the same neural mass differences. In the topographic systems (Erickson Reference Erickson1968), such as visual or somesthetic location, very many neurons are available; therefore they may be narrowly tuned and still evoke enough neural mass to differentially define the event. For usable events in nontopographic systems such as taste or color, few neurons are available, so to produce sufficient neural masses with their concomitant neural mass differences, they must be broadly tuned. This broad tuning would bring many neurons into play to provide the necessary neural mass (Erickson Reference Erickson1968, Figures 2 & 3).

7.1. A proposed control experiment

A model might be evaluated by the degrees to which it can encompass and predict large amounts of useful data. Such tests, described in Section 6, appear to controvert the basic tastes doctrine. An even stronger test would be to inspect the standard textbook definition of the basic tastes. Are the “basics” the only ones that can account for other tastes (Pfaffmann Reference Pfaffmann and Stevens1951, Reference Pfaffmann, Woodworth and Schlosberg1954), or might they be more efficient at this than other tastants or words? And are the basic tastes the only tastes perceivable in other tastants and mixtures (Bartoshuk Reference Bartoshuk, Atkinson, Herrnstein, Lindzey and Luce1988; Halpern Reference Halpern, Pashler and Yantis2002a)? The following sketch, an example extended from the control formats in Section 6, is presented as a tentative illustration of how such an experiment might be done, and the rich vein of information to be expected from such experiments.

7.2. Introduction to the illustration

The key aspect of this demonstration was that control groups composed of tastants not considered to be basics were included to compare with the performance of the basic tastes. If the few-basic tastes position is correct, then the experimental basic words and stimuli should account very strongly or completely for various other taste stimuli, and the four controls using nonbasic stimuli should completely fail, or nearly so. If there were no clear differences between the experimental and control groups in effectiveness in accounting for other stimuli, the basic tastes position in this definition would be considered falsified – and this is how the results came out.

7.4. Main results

The main issue was to compare the performance of the two experimental basic taste groups (Fig. 1, panels E1–2) with the four control nonbasic taste groups (C1–4). Contrary to the basic tastes idea, the data for the control groups overlap those of the experimental groups.

As examples of the data, the subjects rated the taste of NaCl (Fig. 1, E1, fourth column) as 0.3% sweet, 75.6% salty, 2.8% sour, and 5.8% bitter for a total of 84.5%, leaving 15.5% of its taste not accounted for. On the other hand, the four basic taste stimuli (E2) accounted for a total of 93.7% of the taste of NaCl.

Averaged over all the Comparison stimuli, the basic words (Fig. 1, E1) were relatively ineffective as Standards, accounting for 69% of the Comparison stimuli, and the basic stimuli (Fig. 1, E2) were relatively effective, accounting for 84%. Although the nonbasic Standards should not have accounted for the Comparison stimuli, they were competitive with the basics; the 2 nonbasic stimuli accounted for 62% of the Comparison stimuli (Fig. 1, C1), the 3 nonbasics for 81% (C2), the 4 nonbasics for 73% (C3), and the 5 nonbasics for 78% (C4).

To be noted is the overlap in effectiveness between the basic (E1–2) and nonbasic (C1–4) Standards, and the general increase in effectiveness as the number of nonbasic Standards was increased from two (C1) to five (C4).

Individual subjects occasionally accounted for more than 100% of a Comparison stimulus. For example, one subject rated KCl as 30% accounted for by lysine, 50% by tryptophan, and 50% by phenylalanine (total=130%), and another rated MSG as 30% lysine, 10% tryptophan, and 80% phenylalanine (120%). These aspects of the data are hidden within the averaged data in Figure 1, but were strong enough to appear in the averaged data as 109% of HCl accounted for in Figure 1, C 4. This is discussed further in “A Completion Effect,” Section 7.8.

7.5. This psychophysical definition of basic tastes was falsified

The definition that basic tastes (Fig. 1, E 1–2) will account for the tastes of a variety of taste stimuli, and that nonbasic controls (C 1–4) cannot, was falsified; each of the experimental and control groups accounted for considerable and overlapping proportions of the Comparison stimuli. This is illustrated in Figure 1 in that the columns have substantial height in both the experimental and control groups; no basic group (E1–2) achieves 100% in accounting for any Comparison stimulus, and no nonbasic group (C 1–4) completely fails with columns at 0%. For this finding of broad overlap no statistical assay was needed. If a rational statistical definition of basic tastes were offered, for example, if a reason was given why basic tastes should do better than nonbasics at p<.01, or that the basics should be at 100% and the nonbasics at 0%, those predictions might be testable. At any rate, it is doubtful that the basic words could be found more efficient than the 3, 4, or 5 nonbasic stimuli, as the latter have higher raw data averages.

The present format is different from Hang's (1901) and those in other psychophysical articles supporting this model that followed him, in that it used controls, and could have supported or falsified the basic tastes model.

7.6. An inconvenient truth

After the experiments it became disconcertingly clear that the subjects were biased towards the idea that the four basic tastes should account for the Comparison stimuli! We failed in our efforts to keep this issue balanced between the experimental and control groups, and so the data are probably skewed in favor of basic tastes. Many of the subjects commented on this issue in debriefing, such as “Aren't there just four tastes?” They reported that they had learned from textbooks, in middle school through college, that there are four basic tastes. This unexamined bias may be unavoidable in all psychophysical studies involving the basic tastes. The future researchers in the field of taste will be drawn from such students.

This bias could account in part for the use of basic words and stimuli herein, and the relative singularity of the basics, and would raise the issue of whether the use of basic words is learned, or a sensory verity. These questions might be approached experimentally, but the cultural “common truth” of basic tastes would be hard to null out.

7.7. Further tests of the basic tastes model

7.8. A “completion” effect

This failure to find fixed amounts of accounting for the Comparison stimuli by individual Standards is not in line with the basic tastes model. A reason for this failure may be provided by a look at the overall averages for the nonbasic Standards shown in Figure 2. The average decreasing effectiveness of each of the individual Standards as the number of Standards increases from 2 to 5, appears in the lower curve. On average, when two Standards were used they each accounted for about 31% of the taste of the Comparison stimuli; but with three Standards this decreased to 26% each, with four to 18% each, and decreased to 16% for each of five Standards. But this curve should be level if each nonbasic Standard was indeed made up of definite and fixed amounts of four basic tastes as held in the basic tastes doctrine.

The reason for this steady decline (lower curve, Fig. 2) may be seen in the totals accounted for with each set of Standards (top curve). The implication is that these totals may be rising towards “completion,” which would be close to 100%. With only two Standards, the total amount of a Comparison stimulus accounted for is 62%; this rises to 78% with three Standards, 72% with four, and 80% with five Standards. It might be concluded that completely accounting for the taste of a Comparison stimulus was an implicit objective for the subjects. To do this, they would have to reduce the value given for each Standard when there were more of them; otherwise, the total could quickly rise to more than 100%. If each Standard were held to the 31% seen for two Standards, then when 5 standards were used, 155% of the Comparison stimulus would be accounted for. This might be perceived by the subjects as an excess to be avoided, a potentially strong effect in previous research of this nature. Those controls were missing.

In summary, subjects in psychophysical procedures may take on two conflicting goals; to veridically report how well each Standard accounts for a Comparison stimulus, and to completely account for the Comparison stimulus with whatever Standards are available. The direction the subjects take in these kinds of experiments might depend heavily on the nature of the instructions given to them. Because the subjects were clearly aware of the idea and role of basic tastes before the experiment, their reports could be biased towards the completion usually required in studies of this sort (sect. 7.6). Support for this idea is seen in that subjects very rarely go over 100% in accounting for a Comparison stimulus (Fig. 1 C4, HCl). Further discussion and testable speculations on the “completion” effect are contained in the Appendix.

This problem may be summarized thus: If the subjects believe in four (or five etc.) basic tastes, and are given this number of stimuli to use, then they might for this reason complete the accountings with four (or five etc.) stimuli.

7.9. Singularity, or what is A taste?

As found in previous tests of singularity (sect. 6.1), neither were the basics singular (one taste) nor the nonbasics more-than-one (of the basic tastes), as required in the basic tastes doctrine. This was confirmed in the present data.

The perceived singularity of each of the 10 Comparison stimuli was determined in the basic words and stimuli controls (Fig. 1, E1 and E2) after the experiment proper. It is seen in Figure 3 that only 70% of the subjects found the basics NaCl, QHCl, and sucrose to be singular. The taste of the basic stimulus HCl was rated as less singular (43%) than the other basics, and also less singular than some of the nonbasic stimuli. The nonbasics were rated as singular to some degree; for example, KCl was rated as singular by 48% of the subjects. To the extent that the basic tastes model requires singularity (the model is not clear on this), these data falsify the model.

These are not statistical statements; as for Figure 1, the point of Figure 3 is not that one stimulus is statistically more singular than another, but that the data are intermingled between basic stimuli and the nonbasics. Also it seems clear, as nonstatistical statements, that the basics do not rest at complete singularity (left column) and the nonbasics do not rest at completely more-than-one (right column), as might be held in the basic tastes doctrine. It is hoped that this conclusion will be tested in future studies.

Some of the singular ratings of the basics may be accounted for by the fact that the subjects were biased towards the verity of basic tastes, as mentioned in Section 7.6.

7.10. Other comments on the experiment

It could be argued from the basic tastes model that the nonbasic stimuli could account for the Comparison stimuli because each was composed of set amounts of basic tastes. If this argument were made, it must equally be argued that the basic tastes might function because they were composed of various amounts of the tastes of the nonbasics. To remain unbiased, the argument cannot be made in one direction only, in support of the idea of basic tastes. The basic tastes position may not be assumed valid while testing its validity.

If words are not trustworthy vehicles of communication between what we perceive and what we report, should less than the expected basic tastes outcomes be taken very seriously? Might nonverbal methods, not using the basic words, give different, more valid results?

Were the protocols appropriate for the questions asked? It could be argued that the present procedures did not properly examine the basic taste model – but it must be admitted that the effort was made. This then raises the good question of how this idea might be more properly tested. This is not as clear as it should be. It is hoped that other more substantial and appropriate experimental protocols will be pursued.

The present data are not as forceful as the previous findings in other psychophysical studies in supporting the basics view. This may be because herein the subjects were not instructed on how to use the basics. That the data are as supportive as they are may result in part from the fact, mentioned in Section 7.6, that the subjects expected that their responses should be in terms of the basic tastes. This issue could be tested.

7.11. Do these data falsify the Basic Tastes model?

These data and discussion falsify the basic tastes model as it has been presented thus far. But, strictly speaking, without a clear rationale and definition of what “basic tastes” are, the model can neither be falsified nor supported. It is in a fatal limbo. This may be seen as a serious problem by some in the field of taste and by others from various fields, as well. It is hoped that a clear and testable underlying hypothesis and the resulting definitions will be elaborated and tested. These could lead to a clearer view of the nature of taste.

8.1. A definition of “theory”

In this article it is held as an essential part of investigating the basic tastes that there must be a rational, data-based hypothesis from which the basic terms are derived, and that makes the hypothesis and its terms testable. The term “formal” was used to introduce this idea in Section 5.5. The following quote from Young's theory of color vision, first presented in Reference Young1802 and Reference Young, Teevan and Birney1807, meets these requirements in brief, forceful and elegant fashion. Because it meets these requirements, Young's statement is classed as a “theory” in this article.

8.2. Young's theory

Young's is a model of scientific thinking in the neurosciences that has seldom, if ever, been equaled; perhaps it had something to do with the fact that he was a polymath linguist who knew the vagaries of words. His interpretation of the Rosetta stone, although incorrect in detail, showed that he understood both the “labeled-line” and “population” implications of the symbols encountered (Erickson Reference Erickson2001). Here the “labeled-line” idea means that each of a limited number of symbols of determinate meaning will represent an equally limited number of events; three color receptors could signal only three colors. “Cleopatra” would be represented by one dedicated symbol, as the $ sign. The “population” idea means that a limited number of symbols (individual letters), each of indeterminate meaning but taken in combination, can represent an enormous number of different events; three color receptors could represent a continuum of colors. “Cleopatra” would be spelled out.

In the 1802 passage we will quote, note that Young first presents the rationale for the hypothesis – its economic necessity, that there cannot be enough color-coded private “labeled lines” to have one for each perceivable color from each point on the retina. The words in parentheses are by the present writer. Then he proposes the testable solution to this problem: If each of a very few (three) receptor types are relatively responsive to many wavelengths, each different from the other two in the placement of this responsiveness along the continuum, an unequivocal code in the pattern of activity across these receptor types for each color will result. Illustrations of Young's theory are given by Helmholtz (Reference von Helmholtz and Southall1924) and are found in many other texts (relevant discussion is provided by Erickson Reference Erickson1968, Reference Erickson, Schmitt and Worden1974, Reference Erickson and Neff1982b, Reference Erickson2001; Erickson & Schiffman Reference Erickson, Schiffman, Gazzaniga and Blakemore1975). In the 1807 quote, Young states that his economical logic would give the receptor types the ability to represent very many different colors at each point on the retina, a continuum.

His hypothesis for the encoding of color is succinctly put in what are arguably the two most powerful sentences in the history of neurobiology.

8.3. The implications of Young's theory

Young's was an unusual mode of thinking, not common since, in that it did not depend on available techniques or words of problematic meaning. His idea was driven by a precisely defined logical problem, given a straightforward solution, and the complete theory can best be conceptualized graphically, without words. His entire comment sets in its most elemental form the rationale of across-fiber pattern coding throughout the brain, to provide the means for the immense power of the brain to handle extremely large amounts of information (Erickson Reference Erickson2001). This model is particularly applicable for the most complex of the brain's functions where the amount of information far outweighs the number of basic “labeled” neurons available (Erickson Reference Erickson2001). And it is nicely testable.

His idea solves these problems by letting the information remain in the population form from its “first representation” to its “destination,” unchanged in general across-fiber pattern format, such as from sensory input to behavioral output. Very few transforms would be required, even into the level of motor responses (e.g., the problem of the “binding” together of the various aspects of a stimulus is eliminated) (Erickson Reference Erickson, Schmitt and Worden1974, Reference Erickson and Masterton1978, Reference Erickson1982a, Reference Erickson1984b, Reference Erickson1986, Reference Erickson2000, Reference Erickson2001). The data in all neural systems, including taste, fit this model. But the solution to this problem would be impeded by linear gatherings of information reduced to bits traveling along many labeled axons and across many synapses to a final neuron labeled “grandmother,” which is the general tenor of the basic tastes position.

Young's theory not only predicted the general broad form of the response characteristics of the receptors and neurons across wavelengths (“neural response functions,” Doetsch & Erickson Reference Doetsch and Erickson1970; Erickson Reference Erickson1968, Reference Erickson2000, Reference Erickson2001; Erickson et al. Reference Erickson, Doetsch and Marshall1965), but also their broadly-tuned character at more central levels; at no place in the brain could they return to “labeled-lines,” as that would destroy Young's economic ability to represent large amounts of information.

This economic statement provides perhaps the most important, or only, rational and testable hypothesis of sensory coding, and neural functioning in general. Very many colors can then be represented in the across-fiber pattern of activity across only three receptor types, just as a 3-digit number (base 10) can represent 1000 different events, and 10¹¹, even more. This model thus presents the heuristic question of how many different levels of activity may be represented in the firing of a neuron, or a population of neurons. In this model, a relatively small number of neurons could hold massive amounts of information with redundancy, even in the presence of noise (Erickson Reference Erickson1985b, Reference Erickson2001).

But the simplicity of the model, which Young set in the context of three neurons, while making the idea altogether clear and generalizable to other more complex situations, also makes it easy to ignore when considering larger issues.

The across-fiber pattern idea of the neural representation of information closely parallels Young's theory. The rationale and successfully predicted data are of the same nature in both color vision and taste. The quotes we cited could well be used in taste, exchanging taste terms for visual terms.

8.4. Across-fiber patterning and Young

8.5. Across-fiber patterning and taste

The original datum driving the “across-fiber pattern” theory (Erickson Reference Erickson and Zotterman1963, Reference Erickson1968) was the broad responsiveness of individual taste neurons (Erickson Reference Erickson and Zotterman1963; Pfaffmann Reference Pfaffmann1941, Reference Pfaffmann and Stevens1951, Reference Pfaffmann, Woodworth and Schlosberg1954, Reference Pfaffmann1955). This has been elaborated extensively, both empirically and logically (e.g., Boughter et al. Reference Boughter, St. John and Smith1999; Erickson Reference Erickson2000; Scott & Giza Reference Scott and Giza2000; Smith et al. Reference Smith, St. John and Boughter2000). With this broad sensitivity, the stage is set for the encoding of many tastes with few neurons – and the taste neurons are notoriously few.