It may be debated whether individual neurons are "tuned" to react to only asingle tastant

It may be debated whether individual neurons are "tuned" to react to only a

single tastant such as salt or sugar--and therefore signal only one taste

quality--or whether the activity in a given neuron contributes to the neural

representation of more than one taste. Studies show that both peripheral and

(5) central gustatory neurons typically respond to more than one kind of stimulus,

and although each neuron is attuned most acutely to one tastant, it usually also

generates a reaction to others with dissimilar taste qualities. How then can the

brain represent various taste qualities if each neuron is receptive to many

different-tasting stimuli？

(10) Electrophysiological studies of gustatory sensory neurons, first performed

by Pfaffmann, demonstrated that peripheral neurons are not specifically

responsive to stimuli representing a single taste quality (which might be

symbolized by the pattern of activity across gustatory neurons because the

activity of any one cell was ambiguous) but instead record a spectrum of tastes.

(15) But in the 1970s and 1980s several scientists began to accumulate data indicating

that individual neurons are tuned maximally for one taste, and they interpreted

this as evidence that activity in a particular type of cell represented a given taste

quality--an idea they called the labeled-line hypothesis. According to this idea,

activity in neurons that experience the strongest reaction to sugar would signal

(20) "sweetness," activity in those that are most sensitive to acids would signal

"sourness", and so forth.

Smith later proved that the same cells that previous researchers had

interpreted as labeled lines actually defined the similarities and differences in

the patterns of activity across taste neurons, suggesting that the same neurons

(25) were responsible for taste-quality representation, whether they were viewed as

labeled lines or as critical parts of an across-neuron pattern. These investigators

further established that the neural distinction among stimuli of different

qualities depended on the simultaneous activation of different cell types, much

as with the function of color vision, but unlike auditory perception. These and

(30) other considerations have led us to favor the idea that the patterns of activity

are key to coding taste information.

Scientists now know that things that taste similarly evoke similar patterns

of activity across groups of taste neurons. Furthermore, we can compare these

patterns and use multivariate statistical analysis to plot the similarities in the

(35) patterns elicited by various tastants. Taste researchers have generated such

comparisons for gustatory stimuli from the neural responses of hamsters and

rats and these correspond very closely to similar plots generated in behavioral

experiments, from which we may infer which stimuli taste alike and which taste

different to animals. Such data show that the across-neuron patterns contain

(40) sufficient information for taste discrimination and this may be a reasonable

explanation for neural coding in taste, though researchers continue to debate

whether individual neuron types play a more significant role in taste coding than

they do in color vision. Scientists question whether taste is an analytic sense, in

which each quality is separate, or a synthetic sense like color vision, where

(45) combinations of colors produce a unique quality

A．illustrating the extent to which the physiological principles behind taste mirror those behind vision

B．contrasting the available evidence for the labeled-line and across-neuron theories of taste

C．describing the evolution of theories concerning the neurological mechanisms which determine taste

D．demonstrating the superiority of holistic models over component models in analyzing neuron function

E．showing that individual neurons play a negligible role in the physiological mechanisms of perception