Effects of selective thalamic and prelimbic cortex lesions on two types of visual discrimination and reversal learning.

The effects of excitotoxic lesions of the mediodorsal nucleus of the thalamus, the anterior thalamic nuclei and of the prelimbic cortex were examined on two tests of discrimination and reversal learning. In experiment 1A (visual discrimination and reversal), rats were required to discriminate two stimuli, and respond to the stimulus associated with reward (the S+ stimulus). There was no effect of lesion on acquisition of this task. However, when stimulus-reward contingencies were reversed, animals with lesions of the mediodorsal nucleus of the thalamus made significantly more errors than control animals or animals of other lesion groups. In experiment 1B (conditional discrimination), animals were required to learn a rule of the type 'If stimulus A then go left, if stimulus B then go right'. No main effect of lesion on acquisition was observed in this experiment. To test the generality of the reversal effect obtained in experiment 1A, a second cohort of animals with the same lesions was tested on acquisition of the visuospatial conditional task immediately postsurgery, followed by the reversal of the conditional rule (experiment 2). As in experiment 1B, no main effect of lesion group was observed during acquisition of the task. However, lesions of the mediodorsal nucleus of the thalamus resulted in a mild impairment according to number of sessions required to attain criterion performance of the task when the response rule was reversed. The results of the present study provide evidence for a role for the mediodorsal nucleus of the thalamus in new learning, particularly when stimulus-reward contingencies are reversed. Furthermore, they show that the functions of this thalamic nucleus can be dissociated from those of the anterior thalamus and the prelimbic cortex.

Dissociable effects of anterior and posterior cingulate cortex lesions on the acquisition of a conditional visual discrimination: facilitation of early learning vs. impairment of late learning.

Two experiments investigated the effects of quinolinic acid induced lesions of the anterior and posterior cingulate cortices on the acquisition and performance of a conditional visual discrimination (CVD) task, in which rats were required to learn a rule of the type: "If lights are flashing FAST, press the right lever; if SLOW press left". In Experiment 1, animals with lesions of the anterior cingulate cortex (ANT group) demonstrated a significant enhancement in learning during the early stages of task acquisition. Conversely, animals with lesions of the posterior cingulate cortex (POS group) were impaired in learning during the later stages of acquisition. There were no significant differences between the ANT and POS groups on the performance of the task when either variable inter-trial intervals or reduced stimulus durations were imposed. In Experiment 2, the specificity of the lesion effects for processes operative during the early and late stages of learning was tested. Animals were trained to a criterion of 70% correct choices on two consecutive sessions prior to lesioning, and subsequently allowed to continue to acquire the task to the mean asymptotic performance level of 85% correct choices on two consecutive sessions. Animals of the POS group were impaired in learning during this later stage of task acquisition, thus replicating the pattern of results obtained in Experiment 1. The animals in Experiment 2 were then tested following a 30-day retention interval and during extinction (removal of sucrose from the magazine). The extinction test revealed an impairment in the ability of animals in the ANT group to omit lever responses in the absence of reinforcement. These results indicate that the anterior and posterior cingulate cortices are functionally dissociable, and suggest that they may form part of complementary, but competing, learning and memory systems.

Dissociable effects of AMPA-induced lesions of the vertical limb diagonal band of Broca on performance of the 5-choice serial reaction time task and on acquisition of a conditional visual discrimination.

The aim of the present study was to investigate the role of the cholinergic innervation of the cingulate cortex in visual attentional function and acquisition of a visual conditional discrimination task. Following AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) lesions of the vertical limb diagonal band of Broca (VDB) which provides the main cholinergic projection to cingulate cortex, animals were not significantly impaired on the 5-choice serial reaction time task. This task, which provides a continuous performance test of visual attention, has previously been shown to be sensitive to AMPA lesions of the nucleus basalis magnocellularis (nbM). In contrast to the results obtained for visual attentional function, lesions of the VDB did significantly affect the acquisition of a visual conditional discrimination. While showing a significant facilitation in the early learning stage of acquiring this task animals with lesions of the VDB were significantly impaired during the late stages of learning this task. This late learning deficit was not the result of the animals being unable to learn the task due to the presence of the lesion throughout task acquisition as the results of a second experiment revealed that when animals were pre-trained to 70% accuracy on the task and then lesioned, the impairment in late learning was still apparent. In light of the results presented in the accompanying paper (Bussey et al., Behav. Brain Res., 1996), these results suggest that the early learning effects may be due to cholinergic denervation of the anterior cingulate cortex while the late learning effects may be due to denervation of the posterior cingulate cortex. Taken together with previous work indicating a role for the nbM cholinergic system in visual attentional function, these results suggest a role for the cholinergic innervation of the cingulate cortex in conditional learning but not for continuous attentional performance.