Incremental brightness thresholds (DI) were psychophysically determined at several background illumination intensities for three squirrel monkeys. Gross asymmetrical electrodes were then chronically implanted in the lateral geniculate nucleus of the same animals, and activity was recorded in stimulus conditions identical to behavioral testing. Overall activity, recorded through an integrating voltmeter, showed 1. a tendency to decrease as steady background illumination increased, and 2. an abrupt transient increase at both onset and offset to DI test flashes, directly proportional to test flash intensity. Background illumination, in proportion to its intensity, depressed response to a superimposed test flash. Test flashes below intensity DI at the various levels of background illumination produced no measurable response. The quantity DI was shown to be a function of the depressive or inhibitory effect of background illumination on the capacity of the system to respond to transient stimulation. A secondary determinant of DI appeared to be the amount of variability in ongoing neural activity upon which the DI flash is imposed.