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

Abstract

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.

Acute Δ9-Tetrahydrocannabinol-Induced Deficits in Reversal Learning: Neural Correlates of Affective Inflexibility

“…Expression of IEG mRNA was also altered in several cortical and striatal regions as a composite result of test experience, but, although not assessed in the present investigation, it is likely that other regions, such as the parietal cortex (Fox et al, 2003), amygdala (Schoenbaum et al, 2000), and mediodorsal thalamic nucleus (Chudasama et al, 2001), additionally contributed to task performance.…”

Multiple Dopamine Receptor Subtypes in the Medial Prefrontal Cortex of the Rat Regulate Set-Shifting

“…In addition, these impairments cannot be explained by a disruption of simple discrimina- tion learning, because lesions or inactivations of the PFC do not impair the initial learning of either visual or response based discriminations (Ragozzino et al, 1999;Chudasama et al, 2001). With respect to the relative selectivity of the drugs used in the present study, eticlopride is a highly selective and potent D2 antagonist with 4300 times greater affinity for D2 vs D4 receptors, and does not appear to have any agonist actions on 5-HT receptors (Kohler et al, 1986;Seeman and Ulpian, 1988;Woodward et al, 1992;Durcan et al, 1995).…”

“…Previous studies have shown that manipulations of the PFC do not disrupt the initial learning of either a response or visual-cue-based discrimination (Ragozzino et al, 1999;Chudasama et al, 2001;Ragozzino, 2002). Rather, PFC manipulations only disrupt behavior when rats are required to shift from one strategy to another (Ragozzino et al, 1999;Ragozzino, 2002;Stefani et al, 2003).…”

A limited role for mediodorsal thalamus in devaluation tasks.

“…In strategy set shifting tasks, MD lesions increase perseverative responding based on the old rule rather than errors committed once responding has reached chance level (Block et al, 2007; but see Chudasama, Bussey, & Muir, 2001). This suggests that the role of MD is to reduce the impact of previous associations so that new associations can be formed, rather than in the formation of the new associations themselves.…”