Effects of orbital frontal and anterior cingulate lesions on object and spatial memory in rhesus monkeys.

Object memory processes, evaluated in rhesus monkeys by delayed nonmatching-to-sample with trial-unique stimuli and object reversal learning, were more severely impaired by orbital frontal than by anterior cingulate lesions. Spatial memory processes, assessed by spatial delayed response and spatial reversal learning, showed a weak trend in the opposite direction, though on these tasks neither lesion produced a serious loss. Comparison of the present results with those of earlier studies on the effects of various limbic system lesions suggests that object memory processes, including object recognition and object-reward association, are served by a circuit consisting mainly of the rhinal cortex, orbitofrontal cortex, and the magnocellular division of the medial dorsal thalamic nucleus. Although both the rhinal and orbitofrontal components of this circuit appear to participate in both functions, evidence from the present and earlier studies suggests that the orbitofrontal component is the more important one for associative memory, i.e. the formation across trials of associations between particular objects or classes of objects and reward, whereas the rhinal component is the more critical one for recognition memory, i.e. the storage and retrieval within trials of the representations of particular objects.