Abstract

Recordings from human cones exhibit nonlinear response saturation at high intensities, which match the hyperbolic asymptote of the Naka-Rushton equation.¹ Psychophysical measurement of this saturation is made difficult by an adaptive gain control mechanism which holds the retinal response at the same operating point on the static nonlinearity. We, therefore, devised a paradigm to clamp the gain control at a fixed adaptation level, allowing direct measurement of the form of the nonlinear response saturation by increment and decrement thresholds. A 7′ test stimulus of 660 nm was viewed in the foveola with a large surround kept at a fixed level of 50,000 trolands (Td). The steady base level of the test stimulus was varied over about a ±1-log unit range but should have had a minimal effect on the local adaptation pool in the region of the test. Two-pulse measures of the response time constant verified constancy of pooled adaptation level over the base intensity range. Up and down test pulses of 65-ms duration were used to probe the static nonlinearity. Up/down asymmetries of up to a factor of 2 occurred at the highest base intensities, as expected from the Naka-Rushton function. A complete specification of the foveolar saturating nonlinearity was constructed from the up/down data.

© 1988 Optical Society of America