Visual receptive fields of striate cortex neurons in awake monkeys.

THE EXPERIMENTS of Lettvin and colleagues (25) and Hubel and Wiesel (19) have indicated that a neuron in the vertebrate visual system is sensitive to particular characteristics of the visual stimulus. These characteristics have been referred to as the “trigger features” of the stimulus (3) and include such factors as size, shape, orientation, color, and rate and direction of movement; the area of the visual field over which the stimulus is effective is referred to as the receptive field of the neuron (17). Receptive-field characteristics of visual system neurons have subsequently been determined in a wide range of animals (for references see 7, 16). In addition, Hubel and Wiesel (19-24, 30) have shown that there are changes in the characteristics of the stimulus needed to activate a neuron at successively higher levels of the visual system. In both cats and monkeys they observed a progressive change in the predominant type of receptive field from the circular receptive fields of retinal ganglion and lateral geniculate cells, through the elongated receptive fields of the simplest cortical cells, to the more complex fields of other cortical neurons. This understanding of the visual system has been derived entirely from experiments in which the animal was paralyzed and frequently also anesthetized. Such procedures have been designed to eliminate all eye movements, particularly the small eye movements of physiological nystagmus that are characteristic of normal vision (1). The purpose of the present experiments was to determine whether receptive fields of striate cortex neurons in the awake, behaving animal are similar to those in the anesthetized,