GUSTATORY NEURAL CODING IN THE MONKEY CORTEX: ACID STIMULI.
Plata-Salam n, Carlos R., Thomas R. Scott and Virginia L. Smith-Swintosky.
Department of Psychology, University of Delaware, Newark, Delaware
19716.
APStracts 2:0121N, 1995.
SUMMARY AND CONCLUSIONS
1. We sought to define the gustatory neural code for acidic stimuli.
Therefore, we analyzed the responses of 44 single neurons in the insular
cortex of four alert cynomolgus macaques in response to the oral application
of four basic taste stimuli (glucose, NaCl, HCl and quinine HCl) and fruit
juice, and to a series of 20 additional acids. 2. Neurons responsive to
gustatory stimulation were encountered within a volume of 38.2 mm3 (3.5 mm A-P
x 2.1 mm M-L x 5.2 mm D-V). Taste cells constituted 81 (5.2%) of the 1552
neurons whose sensitivities were tested. Of these, the activity of 44 was
followed through at least one complete application of the stimulus series, and
those responses compose the data of this study. Nongustatory cells included
those responsive to mouth movements (36.3%), tactile stimulation within the
mouth (2.1%), visual approach of the taste stimulus (1.4%), and extension of
the tongue (0.1%). The functions of the remaining 54.8% were not determined.
3. The mean spontaneous discharge rate of these cortical taste cells was 3.0
spikes/s (range = 0.0 - 14.4 spikes/s). The mean breadth-of-tuning coefficient
was a moderate 0.72 (range = 0.26 -0.98). Most evoked activity was excitatory,
though inhibition was a prominent response option for four (9%) taste cells.
4. There was no evidence that taste cells with similar functional
characteristics were clustered within the cortex, i.e. there was no apparent
topographic organization of taste quality. 5. Thirty-four of the 44 cells were
divisible into three functional types, based on their response profiles to the
four basic stimuli used here. The types could be characterized as sugar-,
salt-, and acid-oriented. 6. A 2-dimensional taste space was generated from
correlations among the response profiles evoked by the stimulus array. The 21
acids formed a diffuse group that occupied the space through nearly the full
extent of both dimensions. The mean response profile for the acids was most
closely related to that evoked by water, then, with increasing distance, to
those representing quinine, NaCl and glucose. 7. Within the acid group, six
stimuli (propionic, nitric, ascorbic, butyric, acetic and formic acids)
generated activity that matched this group profile, and so occupied the center
of the space. Others lacked the typical relationship with NaCl (folic,
hydrochloric, sulfuric, benzoic and valeric acids), or with glucose (citric,
succinic, tartaric and lactic acids). Three acids (glycolic, pyruvic and
aspartic) showed a nearly equal affiliation with all four basic stimuli. The
remaining three diverged sharply from the main group: malic away from all non-
acids, implying a purely acid taste; glutamic and tannic away from acids,
implying a dominant quality other than acidity. 8. For the subset of stimuli
for which comparisons could be made, electrophysiological data from the
macaque is in accord with human psychophysical results.
Received 21 October 1994; accepted in final form 24 March 1995.
APS Manuscript Number J654-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 1 May 1995.