Different integration of conflicting motion signals in perception and eye movements during object tracking

The processing of visual motion signals is pivotal to accurate sensorimotor control: to suc-cessfully intercept an object in motion the brain needs to take into account the object’s velocity together with neural and motor delays. Here we test the effect of a target’s motion on perception, saccade, and pursuit responses, collecting all three measures together on each trial. Motion information may have different effects on these three processes (Simoncini et al., 2012; Spering et al., 2011). Our comparison across the three different systems make use of a moving target that contains a second motion signal unrelated to its displacement (the double-drift stimulus). We presented peripheral moving apertures filled with noise that either drifted orthogonally to the aperture’s direction, dramatically affecting its perceived direction of motion (Tse and Hsieh, 2006; Lisi and Cavanagh, 2015; Shapiro et al., 2010), or that varied dynamically with no net motion. Participants were asked to saccade to and track these targets with their gaze as soon as they appeared (Rashbass, 1961) and then to report their direction. In the trials with internal motion, the target disappeared at saccade onset so that the first 100 ms or so of the post-saccadic pursuit response was driven uniquely by peripheral information gathered before saccade onset. This provided independent measures of perceptual, pursuit and saccadic responses to the double-drift stimulus on a trial-by-trial basis. There was nevertheless a strong dissociation between saccadic responses on one hand and and perceptual and pursuit responses on the other. We conclude that the dissociation between perception and saccadic eye movements in the localization of moving objects (Lisi and Cavanagh, 2015) does not involve different processing of motion signals but rather a difference in how motion signals are combined with target position. that pursuit speed was not influenced by the condition (double-drift vs. control), F (1 , 7)=0.66, p =0.44; nor by the direction of motion (inward vs. outward), F (1 , 7)=0.65, p =0.45; nor by the interaction between condition and direction, F (1 , 7)=2.41, p =0.16. This finding reveals that the eye movement behavior did not differ, in the first 100ms after saccade landings, between the control and double-drift conditions, despite the fact that the moving target was present only in the control condition (in the double-drift condition the screen was blank after the saccade). We then restricted the analysis to double-drift trials. We found that the direction of the pursuit was shifted on average by 25.76 ° (SD 3.74) from the physical direction of motion of the target, toward the direction of the internal motion. The size of the shift did not differ across inward vs. outward trials, t (7)=1.64, p =0.14. The deviations of direction measured in pursuit were not different from those measured in the perceptual responses, t (7)=1.13, p =0.29 (see Fig. 4C). Additionally, there was a tendency for a positive correlation across participants between pursuit and perceptual responses. The correlation was statistically significant with a one-tailed test, r (7)=0.63, p =0.04. We assessed also whether there were measurable correlations on a trial-by-trial basis. Correlations were quantified using a measure of circular dependence which has the same properties of the product moment correlation coefficient for linear variables, but is appropriate for angular variables (Jammalamadaka and SenGupta, 1988; Jammalamadaka and SenGupta, 2001). The correlation coefficients resulted in: or away from fixation. Participants were asked to saccade to these targets as soon as they appeared, and then track them with their gaze. Participants were instructed that in some trials the target would disappear upon saccade landing, in which case they would have been required to report its direction using a directional arrow. In these trials, early post-saccadic pursuit (open loop) still occurred so that the effect of the pre-saccadic illusory direction was seen in the post saccadic pursuit, enabling a trial-by-trial comparison of perception, and pursuit responses. We also estimated the direction and velocity of target motion taken into account by the saccadic system from the saccade landing errors. The results showed that motion