199 EFFECTS OF BRAIN LESIONS ON IMPRINTING IN THE DOMESTIC CHICK HORN, G., McCABE, B.J. AND ClPOLLA-NETO, J. University of Cambridge, Department of Zoology, Cambridge CB2, England The acquisition of a preference through imprinting is impaired if part (IMHV) of the hyperstriatum ventrale is destroyed bilaterally before chicks are exposed to a visually conspicuous (training) object 2. Bilateral destruction of IMHV 3 h after training impairs retention of this preference 3. Can preferen- ces be established through imprinting when only one IMHV is intact during train- ing? If so, is retention impaired when that IMHV is subsequently destroyed? The present study was designed to answer these questions. Chicks were hatched and reared in darkness to 12 h post hatch. The chicks were then anaesthetized and a lesion placed in (i)the left IMHV (n = 12 chicks), (ii) the right IMHV (n = 12), (iii) the left hyperstriatum accessorium (HA) (n = 12) or (iv) the right HA (n = 12). There were forty-eight sham-operated controls. Chicks were returned to a dark incubator and, 24 h after the operation, exposed to the training stimulus. Their approach to this stimulus and to a second, novel stimulus, was then mea- sured. All five groups of chicks preferred the training stimulus and the magni- tude of the preference was similar in all groups. To determine whether the IMHV region which had been intact during training was essential for retention, all chicks were anaesthetized for a second time, 20 h after the end of training. The remaining IMVH region was then destroyed. 28 h later the chicks' preferences were then measured. The sham-operated and HA lesioned chicks continued strong- ly to prefer the training stimulus, but the birds lesioned in IMVH showed no such preference, the retention of the IMHV chicks being significantly lower than that of the sham-operated chicks and also that of the HA chicks. We conclude firstly (from this study and earlier work) that at least one IMHV must be intact for the acquisition of a preference by imprinting and it does not matter which. Secondly if only one IMHV region is present during training it must also be present for retention. In a previous study we analysed the roles of the right and left IMHV in the retention of a preference which had been acquired when both of these regions I were present during training . The results of that study will be related to those of the present study in which only one IMHV was intact during training.