Ocular responses to brief motion of textured backgrounds during smooth pursuit in humans.

We studied the effects of horizontal smooth pursuit on the ocular responses to brief vertical perturbations of textured backgrounds in humans. When the subject was fixating on a stationary target, a brief vertical perturbation of the background elicited a small tracking response. When the subject was pursuing a target moving horizontally, the same background perturbation elicited a larger response: that is, the response to vertical background perturbations was enhanced during pursuit (pursuit-related enhancement). On the other hand, the dependencies of the ocular responses on spatial frequency, temporal frequency, and stimulus contrast were similar regardless of the ongoing behavior of the subject. We also found that a low-level energy-based mechanism underlies the ocular responses to vertical perturbations of the background during fixation and smooth pursuit. We conclude that the pursuit-related enhancement is independent of the properties of visual processing for cross-axis motion of backgrounds, which suggests that this enhancement results from uniform facilitation of the visual system and/or from facilitation of visuomotor transmission downstream of the visual processing.