Effects of restricted lesions of the chick forebrain on the acquisition of filial preferences during imprinting.

The effects of placing bilateral lesions in that part of the chick brain (IMHV) which was previously been implicated in imprinting, was studied in young domestic chicks. Twenty-four dark-reared chicks were matched in pairs on the basis of their approach activity during a 30 min period of exposure to one of two visual imprinting stimuli. Both members of the chick pair were then anaesthetized and bilateral lesions were made by radio-frequency coagulation in the IMHV of one chick; the other chick served as a sham-operated control. On the following day each chick was exposed for 2.5 h to the imprinting stimulus to which it had previously been exposed. After training, the preferences of all chicks were measured by comparing their approach to the training stimulus with that to the second stimulus. Sham-operated chicks showed a strong preference for the training stimulus; lesioned chicks showed none. Subsequently the latency of each chick to approach and accurately peck a shiny rod was measured. The two groups of chicks did not differ significantly in this test of visuomotor coordination. The area of tissue damaged by the lesion was reconstructed: IMHV was severely damaged with relatively little damage to other areas of the brain.