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| Figure 1 |
As an animal navigates its environment, it encounters a barrage of sensory
stimulation about which it must make decisions in real time. Some of the
most critical decisions concern ingestion: since tasting occurs when a
stimulus is already inside an animal's mouth, that stimulus necessarily
induces ingestive behavior if deemed palatable, and rejection behavior if
deemed toxic; that is, an animal must 'decide' (although the decision may
not be 'conscious') whether (and how much of) a substance on its tongue will
be consumed or expelled, on the basis of how nourishing or toxic it believes
the substance to be.
Underlying these behaviors in rats is a set of reciprocally interconnected
nuclei, including (in part) the solitary nucleus (NTS) and parabrachial
nuclei (PbN) in the brainstem, a region of small neurons in the
ventroposteriomedial nucleus of the thalamus (VPMpc), primary gustatory
(insular) cortex (GC) and the orbitofrontal area (OFC) anterior to it, the
hypothalamus, and the central and basolateral nuclei of the amygdalar (AMG)
complex (see Figure 2). While basic taste-related acceptance and rejection
behaviors can be performed using only brainstem circuitry (Spector 1995,
Travers et al 1987), gustatory forebrain regions are an integral part of
taste perception and learning. AMG neurons respond to tastant
administration in a palatability-dependent manner, such that similarly
palatable tastants evoke relatively similar overall magnitudes of activity
(the average firing rate across several seconds). This is particularly true
of central nucleus neurons (Nishijo et al 1998, Tabuchi et al 2002). In
addition, AMG lesions have been found to change the hedonic intensity with
which rats respond to tastants (Galaverna et al 1993, Touzani et al 1997).
In other preparations, AMG is believed to be involved in the attachment of
hedonic and motivational significance to percepts for purposes of learning
(Gallagher & Schoenbaum 1999, Ganaraja & Jeganathan 2000, LeDoux 2000). GC
neurons are also responsive to tastes (e.g., Katz et al 2001) and are
involved in and often necessary for the production of taste-related
behaviors, including the development of acceptability of new tastes (i.e.,
elimination of neophobia, Berman et al 2000, Berman et al 1998), and memory
for food rewards (Balleine & Dickinson 2000).
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| Figure 2 |
Figure 3 |
Given the tightness of the perception-action coupling and the re-entrant
nature of the neural connectivity, it makes sense that GC neurons should
produce dynamic patterns of activity in response to tastant administration.
This does, in fact, turn out to be the case (Katz, Simon, & Nicolelis,
2001). Figure 1 shows peri-stimulus time histograms (PSTHs) for two of many
GC neurons recorded from awake rats that produced taste-specific dynamics.
These dynamics allow us to break down the period following tastant
administration into three main epochs (see Figure 3): the early epoch is
somatosensory, the middle chemosensory, and the late a mix of chemosensory
and somatosensory (see the paper for more details). Preliminary evidence
suggests that the within-epoch dynamics may reflect the processing of
palatability, as the rat decides whether to swallow the liquid or to spit it
out.
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