Model Approximations to Visual Spatio-Temporal Sine-Wave Threshold Data

Tests of vision with sine-wave flicker go back at least fifty years to H. E. Ives.1 He determined flicker fusion frequencies with a number of wave shapes, including sinusoids. Spatial sinusoid test stimuli are more recent. The first to use them was probably Schade2 in the fifties. Soon after that Kelly3 suggested a stimulus which would simultaneously test the spatial and the temporal sine-wave response of vision. Such tests were implemented by Robson,4 Kelly, 5,6 and others. The special interest in the sine wave as a test stimulus stems from the ease with which one can extrapolate from its results. Provided a system is linear and time-invariant, Fourier analysis can be used to predict the system response to any input from its response to sinusoidal inputs. However, the visual system is neither linear nor time-invariant. Nevertheless, given a sufficiently constant adaptation state and input variations that result in small output variations/ linear theory can be used. * It is often incorrectly stated or implied that, for linearity, the input needs to be small. But consider the situation where a flickering light appears fused visually. The input may then swing between zero and many times the average luminance, yet the beh avior is linear. 1643