Influences of uniform and textured backgrounds on the impulse activity of neurons in area V1 of the alert macaque.

The influences of the visual background on the spontaneous and evoked activity of neurons in the striate cortex (V1) of the awake and behaving macaque were investigated using uniform (dark and bright) and textured (dynamic random-dot) large fields (10 degrees) centered on the receptive field of the cell under study. Rhesus monkeys were trained to fixate a small target while visual patterns were presented on monitor displays and the impulse activity of single cortical neurons recorded extracellularly with metal microelectrodes. The discharge rates of the ongoing, spontaneous activity of the vast majority of V1 neurons, as well as their responses to optimally adjusted bar stimuli, were not significantly influenced by the luminance of a uniform background. On the other hand, the activity of more than 50% of V1 neurons was clearly affected by a textured background. Comparison of the effects of a uniformly dark background and a background of dynamic random dots showed that the neuron's spontaneous discharge rate was typically higher in the presence of the textured background, while the evoked response was often reduced in amplitude or even suppressed. The opposite effects were observed in only a few neurons. These findings indicate that neurons in area V1 are highly sensitive to a textured background of dynamic random dots which exert on them an activating effect, chiefly by stimulation of the neuron's receptive field, with consequent increase in the ongoing discharge and a reduction of the dynamic range of impulse activity, leading to a reduction in the amplitude of the response evoked by a contrast stimulus.