Functional Properties of Neurons in the Anterior Bank of the Parieto-occipital Sulcus of the Macaque Monkey.

Extracellular recordings were made in the anterior bank of the parieto-occipital sulcus of two waking monkeys trained to perform fixation tasks in normal illumination or in complete darkness. Of the recorded neurons, 73% (251/343) were responsive to visual stimulation, but their overall organization did not conform to a simple, continuous retinotopic map. Most of the visual neurons showed a high degree of orientation and direction sensitivity, higher than that found in areas V1, V2 and V3A under the same experimental conditions. Whether they had a resolvable receptive field or not, the discharge rate of many neurons in the anterior bank of the parieto-occipital sulcus was influenced by oculomotor activity. The animals were required to execute pursuit or saccadic eye movements in darkness. Saccadic eye movements were found to influence 19% of the neurons tested (29/156); by contrast, pursuit eye movements were without effect (0/64). Saccade responses were direction-tuned and, in several cases, the neuronal discharge started before the onset of eye movement. The animals were also required to gaze, in darkness, at nine different positions on the screen they faced. Of the neurons tested, 59% (102/174) were affected by the direction of gaze. Higher discharge rates were generally observed when the animals looked towards the lower part of the field of view. Given the functional properties of its neurons, its connections with area V3A-where neural signals appropriate for building an objective map of the visual space are present (Galletti and Battaglini, 1989, J. Neurosci., 9, 1112-1125)-and its output to the visuomotor centres involved in the generation of saccades (frontal eye fields and superior colliculus), we infer that the cortex of the anterior bank of the parieto-occipital sulcus might be part of the network involved in the control of gaze in order to locate objects in visual space.