Behavioural Braht Research, 48 (1991) 103-112 9 1992 Elsevier Science Publishers B.V. AIl rights reserved. 0166-4328/92/S05.00 BBR 01286 103 Research Reports Lesions of the fornix but not the amygdala impair the acquisition of concurrent discriminations by rats J.P. Aggleton, R.W. Kentridge and S. Sembi Department of Psycholog)'. Universityof Durham, Durham (UK) (Received 13 September 1991) (Revised version received and accepted 28 January 1992) Key words: Fornix; Hippocampus; Amygdala; Amnesia; Memory; Discrimination; Rat Rats with lesions in either the fornix, the amygdala, or both were compared with control animals on the acquisition of three different concurrent object discrimination tasks. In the first task the animals received one trial per day on each of six pairs of stimulus objects ('spaced' condition). In the second task the animals received four trials per day on each of six stimulus pairs ('standard' condition), and in the last task the animals received 36 trials on each oftwo stimulus pairs in just a single day ('massed' condition). Animals with fornical lesions were impaired on all three conditions. In contrast, the amygdala lesions only affected the 'massed' condition and then only when the animals had to select the 'non-preferred" stimulus. Although animals with combined amygdala and fornical lesions were impaired on all three conditions there was no evidence that their deficit was greater than that in the animals with lesions restricted to just the fornix. In view of the evidence that concurrent discrimination learning offers an appropriate test for anterograde amnesia these findings are seen as consistent with the notion that the hippocampus, but not the amygdala, is critically involved in the mnemonic processes disrupted by amnesia. INTRODUCTION It has consistently been found that humans with anterograde amnesia are impaired on the acquisition of concurrent visual discriminations. A clear deficit is found irrespective of whether the discriminative stimuli are three-dimensional objects 38, photographic slides 5, or computer-generated symbols ~3. Furthermore, this deficit appears to be independent of the type of feedback used to indicate correct responses '3"38. As impairments have been found in subjects with a variety of different brain pathologies, including Korsakoff's syndromeS,19.3 ~.3s, postencephalitis ~, and anoxia 3s, this task seems to provide a sensitive assay for both tem- poral lobe and diencephalic amnesia. These findings have helped lead to the suggestion that concurrent discrimination problems should be used as benchmark tests for the assessment of experimentally induced amnesias in non-human primates 42. In support Correspondence: J.P. Aggleton, Department of Psychology, University of Durham, South Road, Durham DH1 3LE, UK. of this proposal is evidence that medial temporal lesions, involving both the hippocampal formation and the amygdala, produce marked acquisition deficits in monkeys 10"~-42. In these concurrent learning studies the animals are presented with pairs of discriminative stinmli (typically between six and eight) several times a session over a number of sessions. Further experiments using this standard procedure have indicated that dam- age to the hippocampal formation may be sufficient to impair acquisition 21"2s'42"43, but that there may be important contributions from adjacent perirhinal and parahippocampal regions 44. While concurrent discrimination problems appear to provide a sensitive test for anterograde amnesia in pri- mates, remarkably little is known about the value of these tests for assessing memory impairments in rats. The present study therefore examined the performance of animals with lesions of the fornix, the amygdala, or both, on the postoperative acquisition of three different concurrent discrimination tasks. The effects of fornix transections were examined in the light of evidence of hippocampal involvement in this task in primates.