In gustation, a single number-the average firing rate
across the three to five seconds following tastant delivery-has
often been viewed as the appropriate measure of a gustatory
neuron's response to tastant stimuli. There are reasons
both empirical and theoretical, however, to develop more
dynamic descriptions of the neural bases of gustatory
perception and learning.
In this talk, I will describe our initial examinations
of gustatory cortical (GC) single-unit and ensemble responses
to controlled delivery of tastant samples to awake rats.
The data demonstrate that GC single- neuron tastant responses
evolve across the 2.5 seconds following tastant application,
such that a single neuron may be maximally responsive
to different tastants at different times. When such responses
are accounted for, the percentage of GC neurons that are
modulated by tastants is seen to be three times that previously
reported. The observed single-unit dynamics reflect, in
part, multimodal inputs to GC.
Our analyses of GC firing rate modulations reveal three
separate "epochs" of gustatory responses, the first of
which is purely somatosensory, the second chemosensory,
and the third related to stimulus palatability (a process
that is itself partly somatosensory). Cross-correlations
among taste-specific assemblies of GC neurons make it
plain that firing patterns of GC neurons are also in part
governed by between-neuron interactions, however; in response
to tastants, assemblies of GC neurons go through coupled
progressions of firing rate changes. Gustatory perception
is thus a dynamic process, involving interactions at multiple
spatial and temporal scales.
The ensemble processing of tastants seems to develop
through post- stimulus time, as tastant responses are
first made more distinctive and then arranged into internally
generated categories. This process is plastic on a trial-by-trial
time scale, depending on an animal's experience with the
stimuli.
