Motion processing for saccadic eye movements in humans

Abstract

1. We studied the latencies and amplitudes of saccades to moving targets in normal human subjects. Targets underwent ramp or step-ramp motions. The goal was to determine how the saccadic system uses information about target velocity. 2. For simple ramp motion saccadic latency decreased as target speed increased. A threshold distance model, which assumes that the target has to move a minimum distance before saccadic processing starts, provided a good fit to the responses of all four subjects and explains discrepancies between previously published findings. 3. A double step experiment showed that target position may have some effect on saccadic amplitude when sampled approximately 70 ms before saccade onset, but it must be sampled at least 140 ms before onset for an accurate saccade to occur. 4. Saccades to simple ramp targets approximated the target position 55 ms before saccade onset. Based on our double step results, this is more compensation than possible by a simple position estimate and implies extrapolation of target motion by the saccadic system. The lack of complete compensation may be due to an underestimate of the target speed and/or of the saccadic latency. 5. A delayed-saccade paradigm resulted in saccades with a longer, constant latency and allowed longer viewing of target motion. These saccades accounted for all but approximately 20 ms of target motion, suggesting that with more processing time of target motion a better extrapolation may be generated. 6. In a step-ramp paradigm the target stepped in one direction, then moved smoothly in the opposite direction. Saccades in this paradigm could be made in either the direction of the step or in the direction of target motion: the direction and latency were determined solely by the time at which the target crossed the fixation point. This time must be calculated from target speed and position, implying that the saccadic system must use speed information to adjust latency or to cancel unnecessary saccades.

Visual cues that are effective for contextual saccade adaptation

“…Within this general theoretical framework, our observed failure of color and shape as contexts for saccade adaptation can be seen as the system having a very low, or nonexistent, prior for these features; conversely, making saccades to moving targets would have a strong existing prior for using motion direction and speed information in nonadaptation situations (Gellman and Carl 1991;de Brouwer et al 2002) and thus may be able to incorporate this information more readily into context-specific adaptations. Indeed, this was a rationale for our choice of task (see Introduction).…”

“…The saccade system extrapolates target motion when planning saccade amplitudes to moving targets (Gellman and Carl 1991). We conjectured that a context with a strong existing predictive element based on visual properties of the target might lend itself relatively strongly to contextual adaptation (somewhat analogously to the arm reach-ing for a glass example above).…”

Evidence for a retinal velocity memory underlying the direction of anticipatory smooth pursuit eye movements

“…Additionally, the pursuit system is known to share its eye-inhead position and velocity information with the saccadic system (Blohm et al 2003a(Blohm et al , 2005(Blohm et al , 2006de Brouwer et al 2001de Brouwer et al , 2002ade Brouwer et al , 2002bGellman and Carl 1991;Keller et al 1996;Keller and Johnsen 1990;Orban de Xivry et al 2006;Ron et al 1989aRon et al , 1989b, suggesting that these signals could also be accessible to the anticipatory pursuit system to be encoded in velocity memory. Therefore, the anticipatory pursuit system could have access to all of the extraretinal signals required for a 3D visuomotor transformation of velocity memory, meaning that a velocity memory could be coded in a spatial frame rather than a head-centered frame and so requires no updating across any extraretinal changes to produce spatially accurate anticipatory pursuit.…”

Saccadic Interception of a Moving Visual Target after a Spatiotemporal Perturbation

“…Yet the common observation that animals are able to intercept an object at the right place and time suggests that the brain can somehow estimate its current spatiotemporal coordinates (hic et nunc). The build-up of this neural estimate is also very rapid since saccadic eye movements can bring the image of a moving target onto the fovea within 150 -200 ms (Segraves et al, 1987;Gellman and Carl, 1991;Guan et al, 2005).…”

“…One signal would encode the location where the target is initially detected (snapshot) whereas the other would encode the subsequent target displacement on the basis of target motion signals (Keller and Johnsen, 1990;Gellman and Carl, 1991;Guan et al, 2005;Schreiber et al, 2006). These two signals would then be summed at the level of premotor neurons in the pontomedullary reticular formation (PMRF) to produce the appropriate saccadic motor commands.…”