Jacobus Dessing

Spatial updating across saccades during manual interception

We studied the effect of intervening saccades on the manual interception of a moving target. Previous studies suggest that stationary reach goals are coded and updated across saccades in gaze-centered coordinates, but whether this generalizes to interception is unknown. Subjects (n = 9) reached to manually intercept a moving target after it was rendered invisible. Subjects either fixated throughout the trial or made a saccade before reaching (both fixation points were in the range of -10° to 10°). Consistent with previous findings and our control experiment with stationary targets, the interception errors depended on the direction of the remembered moving goal relative to the new eye position, as if the target is coded and updated across the saccade in gaze-centered coordinates. However, our results were also more variable in that the interception errors for more than half of our subjects also depended on the goal direction relative to the initial gaze direction. This suggests that the feedforward transformations for interception differ from those for stationary targets. Our analyses show that the interception errors reflect a combination of biases in the (gaze-centered) representation of target motion and in the transformation of goal information into body-centered coordinates for action.

The effects of TMS over dorsolateral prefrontal cortex on trans-saccadic memory of multiple objects

Humans typically make several rapid eye movements (saccades) per second. It is thought that visual working memory can retain and spatially integrate three to four objects or features across each saccade but little is known about this neural mechanism. Previously we showed that transcranial magnetic stimulation (TMS) to the posterior parietal cortex and frontal eye fields degrade trans-saccadic memory of multiple object features (Prime, Vesia, & Crawford, 2008, Journal of Neuroscience, 28(27), 6938-6949; Prime, Vesia, & Crawford, 2010, Cerebral Cortex, 20(4), 759-772.). Here, we used a similar protocol to investigate whether dorsolateral prefrontal cortex (DLPFC), an area involved in spatial working memory, is also involved in trans-saccadic memory. Subjects were required to report changes in stimulus orientation with (saccade task) or without (fixation task) an eye movement in the intervening memory interval. We applied single-pulse TMS to left and right DLPFC during the memory delay, timed at three intervals to arrive approximately 100 ms before, 100 ms after, or at saccade onset. In the fixation task, left DLPFC TMS produced inconsistent results, whereas right DLPFC TMS disrupted performance at all three intervals (significantly for presaccadic TMS). In contrast, in the saccade task, TMS consistently facilitated performance (significantly for left DLPFC/perisaccadic TMS and right DLPFC/postsaccadic TMS) suggesting a dis-inhibition of trans-saccadic processing. These results are consistent with a neural circuit of trans-saccadic memory that overlaps and interacts with, but is partially separate from the circuit for visual working memory during sustained fixation.

Spatial biases in motion extrapolation for manual interception

The exact mechanisms by which humans control the manual interception of moving targets are currently unknown. Here, the authors explored the behaviors associated with the spatial control for manual interception. The examined task required controlling a cursor to intercept moving targets on a touch screen. They explored the effects of target motion direction, curvature and occlusion on manual interception. They observed occlusion-dependent spatial errors and arrival times for curved and diagonal trajectories (larger errors and earlier arrival of the finger at its final position with longer occlusion). These effects were particularly apparent for targets moving away from screen center at interception due to curve. In a follow-up experiment, the authors showed that the outward curve effects on spatial errors were absent because the associated trajectories appears to move toward positions that participants could expect the target to never reach. Their analyses also revealed occlusion-dependent spatial errors for diagonal trajectories, which is the well-known angle-of-approach effect. Follow-up experiments demonstrated that this effect was not due to the central initial cursor position acting as a visual reference point or the initial ocular pursuit. Most importantly, the angle-of-approach effect persisted in a judgment task. The authors thus concluded that this effect does not stem from online information-based modulations of movement speed, but from target information used to control aiming (i.e., movement direction). Moreover, processing for diagonal target motion appears to be biased toward straight downward.