THE EFFECTS OF LESIONS TO THE FORNIX AND DORSOMEDIAL THALAMUS ON CONCURRENT DISCRIMINATION-LEARNING BY RATS

Rats with lesions in either the fornix or the thalamic nucleus mediali s dorsalis were unimpaired on the acquisition of two object discrimina tion tasks. The same animals were then tested on a concurrent learning task in which various object discriminations were presented at differ ent rates during the same session. This arrangement was primarily desi gned to minimise any response bias effects. Animals were able to acqui re the various concurrent tasks, even when only one trial per day was given for a particular discrimination. It was found that fornix lesion s had little or no effect and only produced a mild impairment when the rate of stimulus presentation resembled that used in typical concurre nt tasks. Lesions of medialis dorsalis produced a more general, but ag ain mild, deficit. In a final task, the animals were trained to discri minate between two stimuli composed of common elements arranged in dif ferent spatial combinations. Fornix lesions impaired acquisition of th is spatial configural discrimination, while lesions of medialis dorsal is had a variable effect. The results indicate that fornix lesions can spare concurrent discrimination learning, and that any deficits may b e related to interference effects associated with common elements in t he stimuli. Lesions in medialis dorsalis appear to affect the initial learning of reward-based performance rules.