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This thesis investigates the functions of three midline cortical structures: medial frontal (MF), anterior cingulate (ANT), and posterior cingulate (POS) cortex. Although these regions have been little studied relative to other limbic structures such as the hippocampus, recent anatomical and behavioural evidence suggests that they may serve an equally important role in learning and memory. Initial investigations examined the effects of discrete, cytotoxic lesions of these regions on a specific type of learning: stimulus-response (S-R) or "habit" learning, using an automated conditional visual discrimination task. Secondly, a novel method was developed for testing rats on tasks in which multiple computer-graphic stimuli can be presented, the rat responding directly to these stimuli via a nose-poke to a touch-sensitive screen. Finally, the hypothesis was tested that the anterior cingulate cortex mediates stimulus-reward learning. The touchscreen apparatus was used to test animals using a sign-tracking (autoshaping) procedure. ANT lesioned animals were severely impaired on this task. It is concluded that the medial frontal, anterior cingulate and posterior cingulate cortices have dissociable functions in learning, memory and attention; specifically it is proposed that the anterior cingulate cortex mediates the learning and memory of stimulus-reward associations, the posterior cingulate is involved in the acquisition of S-R "habits", and the medial frontal cortex is involved in maintaining attention to relevant stimuli during learning. These suggestions are consistent with a theoretical framework which regards the brain as consisting of anatomically distinct systems which mediate dissociable forms of learning, memory, and attention. |
