Origins of anthropoid intelligence. III. Role of prefrontal system in delayed-alternation and spatial-reversal learning in a prosimian (Galago senegalensis).

A species of prosimian (bush baby, Galago senegalensis) was tested on delayed-alternation and spatial-reversal learning before and after ablation of prefrontal cortex. The results show that normal performance on the two behavioral tasks depend on different subdivisions of the MD-prefrontal system. Delayed alternation is disrupted by prefrontal lesions which cause degeneration in the lateral division of MD while spatial-reversal learning is disrupted by lesions causing degeneration of the medial division of MD. Therefore, the bush baby prefrontal system can be subdivided either on behavioral or anatomical grounds into at least two chief parts. Because of several similarities in the MD-prefrontal system of bush baby and monkey despite their remote common ancestry, it can be concluded that the differentiation of the MD-prefrontal system into distinct divisions and the involvement of this system in delayed alternation and spatial reversal are features probably as old as the order Primates itself. It can be further concluded that the further evolution of the anthropoid variety of prefrontal system beyond this common primate stage probably depended on selective pressure on abilities other than those measured here.