psyc381exam4studyguide

Dr. Gerald S. Hecht
Associate Professor of Psychology
College of Sciences
webmaster@psiwebsubr.org

PSYC 381 - Sensation & Perception Exam 4 Study Guide

Chemosensory Perception

taste and smell represent phylogenetically old sensory system

they are the sensory systems that let us detect & discriminate the molecules in our environment

these senses help to link the external environment with internal needs, e.g. hunger, thirst, etc.

are vivid emotionally & perceptually important nutritionally for regulating food intake

Distal Stimulus Properties

sweet

associated with organic molecules
sweetest compounds are the proteins thaumatin & monellin can be detected at concentration of 10-8 M

bitter

often associated with harmful stimuli
not know which chemical structures elicit the taste

sourness

acids elicit sour taste

saltiness

a molecule that breaks into its ions, the ions pass through a channel in the cell membrane

Initial Transduction at the Receptor

- several different events underlie the transduction of the 4 basic tastes
- bitterness & sweetness are signaled by second messengers
- saltiness & sourness are signaled by direct ion entry from the distal stimulus

Different Parts of the tongue are more sesnsitive to the different tastes:
tip = sweet
sides = sour (middle and rear) and salty (closer to the tip)
back and roof of mouth = bitter

Transduction Pathways Into the Brain (CNS)
The axons of all taste receptors form the Chorda Tympani Nerve—This is where it goes into the CNS for processing-- First to the NTS, then to the PBN, then to the Thalamus and finally to the Insular Gustatory Cortex.
How are the different tastes encoded?

labeled pathway theory -- nerve fiber responds to only one taste (specific pathway)

across-fiber pattern coding -- central neuron compares pattern of input from a group of fibers, each fiber is more sensitive to a particular taste but also responds to other taste stimuli

taste coding involves comparison of activity across groups of fibers responding preferentially to different tastes (contrast)
NOTE: The Second Synapse in the Brain (the PBN) or Parabrachial Nucleus of the Pons integrates information from taste, smell, and visceral and inner ear neurons. This is where we learn taste preferences and aversions through Pavlovian (Classical) Conditioning (learning):

Psychological Phenomena (Reflexive Responses & Pavlovian Learning)
I. REFLEXIVE RESPONSES TO TASTE STIMULI

Above is a rat “gaping” after tasting something bitter.
Below is a human infant showing the same “gape” (yucky) response to a bitter taste on the left and the reinforcing (yummy) response to a sweet taste on the right.

II. PAVLOVIAN LEARNING WITH TASTE STIMULI
Garcia's Conditioned Taste Aversion Paradigm
Here is the Outline for Garcia's Classic CTA Experiment (turning a “yummy” taste into a “yucky” taste by way of Pavlovian learing:

Phase 1 -- Rats are divided into the following three groups:

Group 1 -- Emitic drug will be given after Phase 2, rats get sick ½ hr. later.
Group 2 -- X-rays will be given after Phase 2, rats get sick ½ hr. later.
Group 3 -- Each lick during Phase 2 is followed by a painful electric shock..

Phase 2 -- All rats have ten minutes access to water.

The water has a slightly salty taste. Each lick produces a click and flash of light.

Phase 3 -- The next day (rats in groups 1 and 2 have recovered).

The rats are again offered water. The rats in each of the three main treatment groups are further subdivided into three groups, each offered water accompanied by only one of the three potential CSs -- salty taste, light flash, or tone. The results for the nine subgroups are:

	SALTY TASTE	FLASH	CLICK
1	Supress	Drink	Drink
2	Supress	Drink	Drink
3	Drink	Supress	Supress

Conclusion: Animals are biologically prepared to make some associations easier than others. Specifically we are prepared to associate tastes with illness rather than a visual stimulus such as a flash of light.