In all cases the OKN was interrupted by periods of fixation although no fixation point was provided. Variation of optokinetic efficiency with contrast That optokinetic efficiency varies with contrast has been known for some time. Unexpectedly, however, we have been unable to find other published records confirming the log-linear relationship between the efficiency of OKN and stimulus contrast in humans. The slow-phase eye velocity is increased when the stimuli are in the same direction and reduced when they are opposed. The analysis of variance showed that both interactive and additive changes occur.
Additive effects suggest independent processes.
We have been anxious to understand this apparent discrepancy. A contributory difference between the present and earlier studies may have been in the exact wording of the instructions. The apparently innocuous act of trying to fixate on a featureless black band seems to be adequate to quell the optokinetic response to second-order motion and argues against the reflexive nature of the response. This work was partially supported by NATO grant CRG 910076. L.R.H. is supported by operating grant no.