Common and differential effects of attentive fixation on the excitability of parietal and prestriate (V4) cortical visual neurons in the macaque monkey

The excitability of cortical neurons of prestriate area V4 and area PG of the inferior parietal lobule were examined using the method of single-neuron analysis in awake macaque monkeys. Levels of excitability were measured as the intensity of response to optimal visual stimuli placed in the most responsive region of the cell's receptive field. Physically and retinotopically identical stimuli were delivered during eye movement pauses under 3 conditions: during a no-task state in which the animal was awake and alert, but not receiving or expecting rewards or working in any task; between trials of the task state, the intertrial interval, while the animal awaited the appearance of a fixation target; and during the foreperiod of the task state, as the animal attentively fixated a small target light, waiting to detect its dimming in order to receive liquid reward. Experiments were carried out in 6 hemispheres of 4 monkeys; both V4 and PG were examined through the same chamber placements in 2 hemispheres. A total of 478 neurons in V4 and PG were identified as visual; quantitative studies were done on 146 in V4 and 54 in PG. We found in these experiments a common effect, a 3-4-fold facilitation of the responses of both V4 and PG visual neurons during the task state as compared to in the no-task state, and a differential effect, in that V4 neurons showed a similar 3-4-fold facilitation of responses to stimuli presented during the intertrial interval, whereas PG neuronal responses during this interval were similar to those evoked in the no-task state. We describe the functional properties of V4 neurons studied in the waking state. The findings are discussed in relation to the positions of these 2 areas in the occipitoparietal and occipitotemporal transcortical visual systems and to their respective roles in visuospatial perception and pattern recognition. They are also discussed with regard to the candidate neural mechanisms through which the changes in cortical neuronal excitability might be mediated.