'Real-motion' cells in the primary visual cortex of macaque monkeys.

Extracellular recordings were carried out in the primary visual cortex of behaving macaque monkeys. Neurons were activated by moving a visual stimulus across their receptive fields during periods of steady fixation and by moving their receptive fields (by visual tracking) across a motionless visual stimulus, taking care that the velocities of stimulus and eye movements were the same. The total cell population (108 neurons) ws divided into 3 groups according to the cell sensitivity to visual stimulus orientation (non-oriented cell and oriented cells) and to the presence or absence of antagonistic areas in in the receptive fields (oriented cells with antagonistic areas). All the non-oriented cells (n = 14) showed almost the same response to visual stimulation both during steady fixation and during visual tracking. Out of a total number of 86 oriented cells, 77 turned out to be activated by the visual stimulation both during fixation and tracking. Eight oriented cells gave a very weak response or no response at all to visual stimulation during smooth pursuit eye movements and one neuron of the same group showed a greater response during visual tracking than during fixation. Six out of 8 oriented cells with antagonistic areas showed almost the same response to the two types of visual stimulation, while the remaining two neurons showed very weak responses during smooth pursuit eye movements. Our results show that a small percentage (about 10%) of striate neurons in macaque monkeys gave very different responses to the same physical stimulation at retinal level, according to the presence or absence of slow eye movements (smooth pursuit eye movements). The activity of these neurons seems to be related to the real movement of something in the visual world, in spite of the retinal image movement per se.