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Joost C. Dessing, J. Douglas Crawford, W. Pieter Medendorp; Testing the spatial reference frames used for manual interception. Journal of Vision 2010;10(7):1063. doi: 10.1167/10.7.1063.
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© ARVO (1962-2015); The Authors (2016-present)
While early cortical reach areas are known to represent earth-fixed movement goals in a dynamic, gaze-centered map, it is unknown whether the same spatial reference plays a role in the coding of moving targets for manual interception. We tested the role of gaze-dependent and gaze-independent reference frames in the coding of memorized moving targets, rendered invisible prior to a saccade that intervened before the reach. Gaze-centered coding would require the internal representation of the interception point (IP) to be actively updated across the saccade whereas gaze-independent coding would remain stable. Head-fixed subjects (n = 9) sat in complete darkness and fixated a visual fixation point (FP) presented on a screen in front of them. A target started moving for 2.1 s downward at 7 deg/s at various approach directions (-18, 0, +18 deg), after which it disappeared. Occluded targets passed fixation height in a range from -5 to +5 deg (relative to straight ahead); FP locations ranged from -10 to +10 deg. After a saccade (in saccade trials - as opposed to fixation trials) subjects reached out to intercept the occluded target at fixation height with their index finger (saccade trials). We analyzed the pointing errors using regression analyses. Both initial and final fixation direction, as well as IP relative to these gaze directions affected the pointing errors (fixation trials: R2 = 0.15-0.50; saccade trials: R2 = 0.15-0.62). Importantly, errors in the saccade trials reflected combined effects of fixation direction during target presentation and during the memory period. This suggests that a gaze-dependent representation of the IP is transformed into gaze-independent coordinates before the saccade, but that this transformation is not entirely finished at saccade onset. This explains why the pointing errors reflect a mixture of gaze-dependent and gaze-independent reference frames.
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