An Investigation of the Neurophysiological Correspondents of Learning and Memory in Two Forebrain Regions of the Day-Old Chick

Gigg, John (1991). An Investigation of the Neurophysiological Correspondents of Learning and Memory in Two Forebrain Regions of the Day-Old Chick. PhD thesis The Open University.

Abstract

Spontaneous bursting (5 or more spikes of 200-450mV amplitude at 400Hz) occurs in many areas of chick forebrain. Day-old chicks trained on a one-trial passive avoidance task show a bilateral increase of up to 350% in bursting following training in one of these areas: the intermediate medial hyperstriatum ventrale, or IMHV (Mason & Rose, 1987; 1988).

An investigation was carried out into the time course and lateralization of this change in bursting activity following the training of day-old chicks on a passive avoidance task. Chicks were trained to either avoid a bead coated with the bitter-tasting substance methylanthranilate (M-birds) or were trained to peck a water coated bead (W-birds). Bursting was recorded sequentially from the IMHV of both hemispheres at 8 time points over the period 1 to 9 hours post-test. The results indicate that there are significant differences in bursting activity recorded from M-birds only during the period 3-7hr posttest, when compared to W-birds. Between 6-7hr posttest there are significant differences in the burst firing patterns of the right IMHV of M-birds compared to the left. At other time points tested there are no significant differences between hemispheres. No between hemisphere differences are evident in W-birds.

Multi-unit recordings were made from the lobus parolfactorius (LPO), another forebrain structure to show changes in biochemistry and morphology following passive avoidance training. M-birds showed a higher incidence of bursting when compared to W-birds over the period 1-10hr posttest. No lateralization of bursting was seen in either group at any time posttest.

In a further experiment, chicks trained to avoid the methylanthranilate coated bead were subjected to subconvulsive electroshock 5min posttraining. This procedure was used to test whether the training-induced increase in bursting in the LPO was a direct correlate of memory formation for the task. This electroshock treatment produced two groups of birds: one group that avoided the bead (remembered the task) and another that pecked the bead (forgot the task). Multi-unit recordings from the LPO of these two groups revealed that the group that avoided the bead had a significantly higher mean burst-frequency when compared to the group that pecked the bead, indicating that increased bursting in the LPO following training is directly associated with recall for the task. These results are similar to those of Mason and Rose (1988) who showed that amnesia abolished a training-induced enhancement of bursting in the IMHV.

The effects of pretraining bilateral LPO lesions on IMHV bursting activity were examined. The IMHV of four groups of birds was recorded ftom following training: two groups of M-birds, one with LPO lesions the other with sham LPO lesions and two similarly treated groups of W-birds. A significant increase in overall IMHV bursting activity was observed in sham-lesioned M-birds when compared to sham-lesioned W-birds. However, no significant difference in bursting activity was seen between lesioned M-birds and lesioned W-birds. There was a trend towards a higher overall level of bursting in lesioned W-birds, when compared to sham-lesioned W-birds.

These results are discussed with reference to previous electrophysiological studies concerning the role of burst-firing patterns in models of learning and memory.

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