NEURAL MECHANISMS OF GUSTATORY CONTRAST

Project: Research project

Project Details

Description

Most sensory systems have a mechanism for comparison that, through
exaggeration of perceived stimulus difference, serves to facilitate
accurate detection of, and responding to, a range of stimuli. Such an
effect can be demonstrated in the gustatory system by varying the temporal
presentation of different concentrations of a normally preferred stimulus.
Specifically, when presented in separate sessions, rats will demonstrate a
concentration effect by licking more for a high than for a low
concentration of sucrose. When presentation of the two concentrations is
alternated within the same session, however, the concentration effect is
exaggerated 2-3 times. In behavioral studies this exaggeration in referred
to as a Simultaneous Contrast Effect. That is, licking for the high
concentration is enhanced (positive contrast) and licking for the low
concentration is suppressed (negative contrast) when rats are given the
opportunity to compare the two closely in time. This effect can be likened
to an"edge effect" in the visual system. Despite the likely contribution
of such comparison effects to detecting, coding, and responding to
gustatory stimuli, the behavioral and electrophysiological assessment of
taste processing has been limited to investigation during conditions
designed to prevent stimulus comparison. For example, behavioral
responsiveness to stimulus quality and intensity typically is evaluated
using random stimulus delivery in an effort to reduce the potential effects
of prior experience on ingestive behavior. During electrophysiological
experiments, a similar attempt is made to eliminate order effects by
presenting the stimuli in a rinse-stimulus-rinse sequence. The present
proposal seeks to establish the neural basis for this comparison process.
Preliminary data indicate that an intact brainstem is sufficient to mediate
the behavioral expression of Simultaneous Contrast Effects. Other
experiments, however, discount adaptation of the peripheral receptors as an
explanation of the phenomenon and rule out involvement of the pontine
parabrachial nuclei. This leaves the nucleus of the solitary tract (NST)
as a potential site for the neural mechanisms that produce athe effect. In
fact, our preliminary data demonstrate exaggerated responses to alternating
concentrations of sucrose, NaCl, or citric acid in NST units. This effect
occurs for sucrose even when the two concentrations are applied to separate
receptive fields, further discounting an adaptation interpretation of the
phenomenon. The focus of this proposal is to: (1) select the sucrose
concentration pair associated with the largest contrast effects in
behavior; (2) examine the effect of comparison and non-comparison
conditions (using the Simultaneous Contrast procedure) on gustatory coding
in NST units; (3) determine the parametric constraints on contrast effects
both behaviorally and electrophysiologically; and (4), extend pilot data
that rule out peripheral adaptation as the sole mechanism for Simultaneous
Contrast.
StatusFinished
Effective start/end date2/1/961/31/01

Funding

  • National Institute on Deafness and Other Communication Disorders
  • National Institute on Deafness and Other Communication Disorders: $114,568.00
  • National Institute on Deafness and Other Communication Disorders
  • National Institute on Deafness and Other Communication Disorders
  • National Institute on Deafness and Other Communication Disorders

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