Relationship between learning characteristics and the properties of visual objects in rhesus macaques with bilateral removal of parietal cortex field 7.

Behavioral experiments were used in rhesus macaques with bilateral excision of field 7 of the lower parietal cortex to study the relationship between visual differentiation learning processes and a variety of stimulus properties. All animals showed significant differences associated with stimulus properties, which produced different types of learning curves. For each monkey, visual stimuli were divided into compact groups in terms of the "similarity" of their learning characteristics. Removal of field 7 had no effect on the process of learning visual image discrimination when this was based on properties such as color and geometrical shape, but worsened the learning characteristics when visual differentiation was based on spatial information, when the learning process became unstable, with increases in the numbers of peaks and troughs on the learning curve and a significant increase in the duration of the learning period. The time to a stable motor response also became significantly greater than for visual images distinguished by shape and color. It is suggested that during the process of learning visual discrimination, processing and extraction of image signs by the visual system for objects characterized by spatial relationships is accompanied by the formation of spatial distinguishing signs, this process involving neuronal structures in field 7 of the lower parietal cortex, which appears to be the main area determining visual-vestibular interactions. Increases in oscillations and in the difficulty of the learning process for differentiation on the basis of spatial information after removal of field 7 might be due to a transfer from one strategy to another, resulting from disruption of the mechanisms which evaluate body image and egocentric orientation on the basis of visual-vestibular interactions.