Learning-dependent Changes in the Responses to Visual Stimuli of Neurons in a Recognition Memory System

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Abstract
When young chicks are trained by exposing them to a conspicuous object they learn its characteristics. The learning process is known as imprinting. In the present study neuronal activity in a region crucial for imprinting was shown to be affected by training and by the object on which the chicks had been trained. The region is the intermediate and medial part of the left hyperstriatum ventrale (left IMHV). No such effects were found in a visual projection area, the left hyperstriatum accessorium. Domestic chicks were imprinted on either a rotating red box (n=7 chicks) or a rotating blue box (n=8). When the chicks were approximately 48 h old they were anaesthetized and multiple-unit activity was recorded in simultaneous, single penetrations through each of the two regions. Records were also made from eight dark-reared chicks. Whilst recording, the red or blue box, placed in front of the contralateral eye, was switched on to give a total of 20 rotations, the interval between each rotation being 10 s. The alternative stimulus was then presented 20 times. Unit activity in the 3 s before and after stimulus onset was compared and the data for each of the 20 presentations were combined. In the left IMHV 18 out of a total of 115 recording sites (16%) responded significantly to the stimuli; in the left hyperstriatum accessorium 39 out of 126 recording sites (26%) did so. Measures of unit activity at each recording site were combined for a given penetration to provide a 'mean penetration response'. The response to the red box differed from the response to the blue box in the left IMHV of dark-reared chicks. After training with the blue box the response to both boxes was similar to the response to the blue box in dark-reared birds. After training with the red box the response to both boxes was similar to the response to the red box in dark-reared birds. No significant effects were found in the left hyperstriatum accessorium. The two training boxes were virtually identical apart from the differences in colour and brightness. Training appeared to stabilize the response of the visually naive left IMHV to the training stimulus whilst changing its response to the alternative, but similar stimulus. That is, one consequence of training is that the two stimuli are placed in the same category, and this neural change may provide a basis for stimulus generalization. The underlying neural system is modelled and a mechanism that allows such stimuli to be discriminated is proposed.