The effects of adaptation to visual stimuli on the velocity of subsequent ocular following responses.
We examined the effect of prior adaptation to moving and flickering stimuli on the velocity of subsequent ocular following responses in man. Experiments consisted of two phases: an adaptation phase in which moving or flickering stimuli were presented while the eyes fixated a small spot and a test phase in which ocular following responses were free to occur. The effects resulting from prior adaptation were characterized by determining the mean initial eye velocities in the period 200-500 ms after the onset of the test stimulus. It was found that 8 s of prior exposure to a grating pattern moving at between 1.5 and 4 cycles.s-1 significantly reduced initial eye velocities in all subjects. Prior exposure to a flickering stimulus (temporal frequency 3.2 cycles.s-1) also attenuated the velocities of initial eye movements, but to a far lesser extent. These results suggest that a motion-dependent and a weaker flicker-dependent process have an adaptive influence on the generation of ocular following responses. Initial eye velocities were measured as a function of the contrast of the prior adapting gratings. The velocities were found to decrease with increasing adapting contrast. The reductions in eye velocity were well described by a decaying exponential function. The motion-dependent adaptive effect showed significant inter-ocular transfer and had the same temporal tuning when transferred (i.e. optimum adaptation at between 1.5 and 4 cycles.s-1). The flicker-dependent effect did not show inter-ocular transfer. There is a distinct similarity between the adaptive process that causes attenuation of ocular following velocities and the adaptive mechanism that induces perceptual motion after-effects. This similarity is discussed.