The Effect of Stationary Patterns on Adaptation for Movement: Evidence for Inhibitory Interaction

Implications of motion detection for early non-linearities

Analysis of motion may be accomplished using the spatiotemporal variations produced when a spatially varying luminance waveform moves across linear receptive fields. Moving contrast-modulated patterns which consist of coarse-scale spatial variations in the contrast of fine-scale luminance patterns cannot be analysed in this way. The human visual system can analyse the motion of contrast-modulated patterns and this suggests it may contain mechanisms that use non-linear transformations. Non-linear transformation of contrast-modulated patterns would give rise to a component (a distortion product) that varies on the same spatial scale as the contrast variation; this can be analysed to extract motion. Is the non-linearity simply an inherent part of the transduction process or is it a characteristic of a mechanism specialized for the analysis of the motion of such patterns? Comparisons of the spatial and temporal limitations of motion discrimination using luminance and contrast-modulated patterns suggest that the mechanisms which analyse the two types of patterns are different, although recent physiological evidence suggests that they may have common elements.

Dynamics of adaptation to contrast

An investigation has been made into the temporal parameters with which the detection threshold for a sinusoidal grating changes during and after adaptation to the same grating at high contrast. Stationary high-spatial-frequency gratings and a phase-reversing low-spatial-frequency grating have been studied separately. It was found that the threshold continues to rise during adaptation for at least 6 min without sign of levelling off, and that full recovery from 6 min of adaptation can take more than 45 min. Intermittent adaptation and continuous adaptation for the same period produce similar effects. Single-phase and dual-phase exponential fits to the data are rejected, and it is concluded that the level of adaptation of the visual system to spatial contrast changes as a power function of time. However, recovery is not always monotonic, especially after adaptation to phase-reversing gratings. This may be due to inhibitory interactions between channels (in particular, those for pattern and movement information).