Anticipatory gain modulation in preparation for smooth pursuit eye movements

The neuronal basis of on-line visual control in smooth pursuit eye movements

Smooth pursuit eye movements allow us to maintain the image of a moving target on the fovea. Smooth pursuit consists of separate phases such as initiation and steady-state. These two phases are supported by different visual-motor mechanisms in cortical areas including the middle temporal (MT), the medial superior temporal (MST) areas and the frontal eye field (FEF). Retinal motion signals are responsible for beginning the process of pursuit initiation, whereas extraretinal signals play a role in maintaining tracking speed. Smooth pursuit often requires on-line gain adjustments during tracking in response to a sudden change in target motion. For example, a brief sinusoidal perturbation of target motion induces a corresponding perturbation of eye motion. Interestingly, the perturbation ocular response is enhanced when baseline pursuit velocity is higher, even though the stimulus frequency and amplitude are constant. This on-line gain control mechanism is not simply due to visually driven activity of cortical neurons. Visual and pursuit signals are primarily processed in cortical MT/MST and the magnitude of perturbation responses could be regulated by the internal gain parameter in FEF. Furthermore, the magnitude and the gain slope of perturbation responses are altered by smooth pursuit adaptation using repeated trials of a step-ramp tracking with two different velocities (double-velocity paradigm). Therefore, smooth pursuit adaptation, which is attributed to the cerebellar plasticity mechanism, could affect the on-line gain control mechanism.

Neural activity in the frontal pursuit area does not underlie pursuit target selection

The frontal pursuit area (FPA) contains neurons that are directionally selective for pursuit eye-movements. We found that FPA neurons discriminate target from distracter too late to account for pursuit directional selection. Rather, the timing of neuronal discrimination is linked to pursuit onset, suggesting a role in motor execution. We also found buildup of activity of FPA neurons prior to pursuit onset that correlated with eye acceleration. These results show that the FPA is unlikely to be involved in selection of initial pursuit direction, but could be involved in motor preparation by increasing pursuit gain prior to pursuit onset.

Spatial coincidence of intentional actions modulates an implicit visuomotor control

We investigated a visuomotor mechanism contributing to reach correction: the manual following response (MFR), which is a quick response to background visual motion that frequently occurs as a reafference when the body moves. Although several visual specificities of the MFR have been elucidated, the functional and computational mechanisms of its motor coordination remain unclear mainly because it involves complex relationships among gaze, reaching target, and visual stimuli. To directly explore how these factors interact in the MFR, we assessed the impact of spatial coincidences among gaze, arm reaching, and visual motion on the MFR. When gaze location was displaced from the reaching target with an identical visual motion kept on the retina, the amplitude of the MFR significantly decreased as displacement increased. A factorial manipulation of gaze, reaching-target, and visual motion locations showed that the response decrease is due to the spatial separation between gaze and reaching target but is not due to the spatial separation between visual motion and reaching target. Additionally, elimination of visual motion around the fovea attenuated the MFR. The effects of these spatial coincidences on the MFR are completely different from their effects on the perceptual mislocalization of targets caused by visual motion. Furthermore, we found clear differences between the modulation sensitivities of the MFR and the ocular following response to spatial mismatch between gaze and reaching locations. These results suggest that the MFR modulation observed in our experiment is not due to changes in visual interaction between target and visual motion or to modulation of motion sensitivity in early visual processing. Instead the motor command of the MFR appears to be modulated by the spatial relationship between gaze and reaching.

Trial-by-trial updating of the gain in preparation for smooth pursuit eye movement based on past experience in humans

To understand how the CNS uses past experiences to generate movements that accommodate minute-by-minute environmental changes, we studied the trial-by-trial updating of the gain for initiating smooth pursuit eye movements and how this relates to the history of previous trials. Ocular responses in humans elicited by a small perturbing motion presented 300 ms after appearance of a target were used as a measure of the gain of visuomotor transmission. After the perturbation, the target was either moved horizontally (pursuit trial) or remained in a stationary position (fixation trial). The trial sequence randomly included pursuit and fixation. The amplitude of the response to the perturbation was modulated in a trial-by-trial manner based on the immediately preceding trial, with preceding fixation and pursuit trials decreasing and increasing the gain, respectively. The effect of the previous trial was larger with shorter intertrial intervals, but did not diminish for at least 2,000 ms. A time-series analysis showed that the response amplitude was significantly correlated with the past few trials, with dynamics that could be approximated by a first-order linear system. The results suggest that the CNS integrates recent experiences to set the gain in preparation for upcoming tracking movements in a changing environment.

How do primates anticipate uncertain future events?

The timing of an upcoming event depends on two factors: its temporal position, proximal or distal with respect to the present moment, and the unavoidable stochastic variability around this temporal position. We searched for a general mechanism that could describe how these two factors influence the anticipation of an upcoming event in an oculomotor task. Monkeys were trained to pursue a moving target with their eyes. During a delay period inserted before target motion onset, anticipatory pursuit responses were frequently observed. We found that anticipatory movements were altered by the temporal position of the target. Increasing the timing uncertainty associated with the stimulus resulted in an increase in the width of the latency distribution of anticipatory pursuit. These results show that monkeys relied on an estimation of the changing probability of target motion onset as time elapsed during the delay to decide when to initiate an anticipatory smooth eye movement.

Anisotropies in the gain of smooth pursuit during two-dimensional tracking as probed by brief perturbations

Previous investigations suggest the gain of smooth pursuit is directionally anisotropic and is regulated in a task-dependent manner. Smooth pursuit is also known to be influenced by expectations concerning the target's motion, but the role of such expectations in modulating feedback gain is not known. In the present work, the gain of smooth pursuit was probed by applying brief perturbations to quasi-predictable two-dimensional target motion at multiple time points. The target initially moved in a straight line, then followed the circumference of a circle for distances ranging between 180 degrees and 270 degrees . Finally, the path reverted to linear motion. Perturbations consisted of a pulse of velocity 50 or 100 ms in duration, applied in one of eight possible directions. They were applied at the onset of the curve or after the target had traversed an arc of 45 degrees or 90 degrees . Pursuit gain was measured by computing the average amplitude of the response in smooth pursuit velocity over a 100 ms interval. To do so we used a coordinate system defined by the motion of the target at the onset of the perturbation, with directions tangential and normal to the path. Responses to the perturbations had two components: one that was modulated with the direction of the perturbation and one that was directionally nonspecific. For the directional response, on average the gain in the normal direction was slightly larger than the gain in the tangential direction, with a ratio ranging from 1.0 to 1.3. The directionally nonspecific response, which was more prominent for perturbations at curve onset or at 90 degrees , consisted of a transient decrease in pursuit speed. Perturbations applied at curve onset also delayed the tracking of the curved target motion.

Preparatory Gain Modulation of Visuomotor Transmission for Smooth Pursuit Eye Movements in Monkeys

It has been reported that the visuomotor processing underlying the initiation of smooth pursuit eye movement is modulated in relation to the recent experience of eye movements: the initial pursuit eye velocity is larger after experiencing repeated pursuits than saccades. To assess which parameters of the previously executed pursuits play an essential role in modulating the gain of visuomotor transmission, we recorded the ocular responses of monkeys to a brief perturbing motion of the tracking target injected before the start of the eye movements. First, we compared the perturbation responses among the blocks in which the duration of executing pursuit was varied. We found that the response amplitude increased with the increase of the pursuit duration and it reached a plateau level at 100–200 ms of the duration. Second, a comparison of the perturbation responses in the blocks in which target velocity was different showed a gradual increase of the response as a function of the required pursuit velocity. Third, when the animals repeatedly performed pursuits, the response amplitude gradually increased with increasing interval between the appearance of the target and the onset of perturbation. On the other hand, such an increase was not observed when the animals repeatedly performed saccades. These results suggest that before initiating eye movements, the pursuit system modulates the gain of visuomotor transmission so as to be closely related to the properties of the repeatedly experienced eye movements and this gain modulation is triggered by the target’s appearance.