The influence of hippocampus on habituation of the HSR task.

Roberta Wiediger, Fran McSweeney & Jay WrightPsychology Department - Washington State University

Introduction

The hippocampus is one of the most important structures in the mammalian brain linked to learning and memory storage.It has the ability to pass information from one neuron to another, to reconfigure and consolidate information thus exhibiting synaptic plasticity.

Habituation has been defined as a decrease in responsiveness to repeated presentation of a stimulus (Groves & Thompson, 1970).It is the simplest for of learning (Harris, 1943).The waning of a reflexive response to repeated stimulation (Staddon, 2001).

Characteristic of habituation McSweeney and Murphy (2000)

14 characteristicsE.g. Spontaneous recovery – the recovery of a habituated stimulus when that stimulus is not presented for a period of time.

The Present StudyThe purpose of this experiment was to determine the role of the hippocampus during habituation of the HSR task during a classical conditioning paradigm (introduction of a tone), by using hippocampectomized rats.

MethodSubjects – 32 male Sprague-Dawley ratsApparatus – wood pedestal

Tone – 74dBAir stimulus applied to the ear

SurgeriesHippocampectomyNeocortex control

Behavioral TestingHSR task

5-min adaptation period24 trial – 15s stimulus presentation (air)ITI of 15sIBM compatible computer signaled the intervalsGreen – 15s of air presentationRed – 15s of ITI

AssociationTone sessionSame procedure as standard habituation session

ButA tone was introduced 1s preceding air presentation

Groups4 groups of rats (8 per group)All were tested over 2 habituation sessions with a 24h ITI.Group 1 – standard habituationGroup 2 – tone habituationGroup 3 – hippocampectomized toneGroup 4 – neocortex control tone

Predictions1. Spontaneous recovery of the HSR was predicted for the standard habituation group after 24hISI but NOT for the tone habituation group.

2. Spontaneous recovery of the HSR was expected to occur in the hippocampectomized group but NOT for the neocortex group.

3. It was anticipated that the pattern of responding for the tone habituation group would be similar to the neocortex tone group.

Results

DiscussionThese data suggest that the tone is an important component for the HSR task to become associative

This implies that the hippocampus is indeed important for associative learning to take place.

However, the hippocampus is not important for the habituation of a non-associative task

ReferencesGroves, P. M., & Thompson, R. F. (1970). Habituation: A dual-process theory. Psychological Review, 77, 419-450.

Harris, J.D., (1943). Habituation response decrement in the intact organism. Psychological Bulletin, 40, 385-422.

Murphy, E. S., McSweeney, F. K., & Kowal, B. P. (2003). Within-session decrease in operant responding as a function of pre-session feedings. The Psychological Record, 53, 313-326.

Staddon, J. E. R., (2001). The new behaviorism. Philadelphia, PA: Taylor & Francis.

HSR VS TONE

3 trial blocks

0 2 4 6 8

Mea

n #

of H

SR

0

2

4

6

8

10

HSR Session 1 HSR Session 2 Tone Session 1 Tone Session 2

HSR session 1 = Mean 6.3 HSR

session 2 = Mean 5.2 HSR

Tone session 1 = Mean 6.6 HSR

session 2 = Mean 2.8 HSR

ToneHippocampectomy VS Cortex Control

3 Trial Blocks

0 2 4 6 8

Mea

n #

of H

SR

0

2

4

6

8

10

No Hip Tone Session 1 No Hip Tone Session 2 Tone Cortex Session 1 Tone Cortex Session 2

No hip session 1 = Mean 9.1 HSR

session 2 = Mean 7.8 HSR

Neocor session 1 = Mean 8.3 HSR

session 2 = Mean 4.5 HSR