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David A. Westwood, Melvyn A. Goodale; Grasping remembered objects: Pinpointing the transition between on-line and off-line visuomotor control modes. Journal of Vision 2002;2(7):52. doi: 10.1167/2.7.52.
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© ARVO (1962-2015); The Authors (2016-present)
The scaling of grip aperture in visually guided prehension is largely insensitive to size-contrast illusions, but the same is not true of grasping movements initiated after a delay. When does the transition between veridical and illusory size-scaling occur? In experiment one (No-delay), participants (N=10) viewed a target object and an adjacent flanker object for 500 msec, and reached to grasp the target object in response to a subsequent auditory cue. Unpredictably, vision was occluded either at cueing or at movement onset. In experiment two (Delay), the target array was viewed for 500 msec followed by a 2.5 sec period of visual occlusion and then an auditory initiation cue. Unpredictably, vision was restored either at cueing (and withdrawn at movement onset), or not at all. In both experiments, peak grip aperture was insensitive to flanker size when vision was available between cueing and movement onset. When vision was unavailable at this time, peak grip aperture was modulated by flanker size in a direction consistent with a perceptual size-contrast effect. The magnitude of this effect was similar for both experiments. We propose two discrete modes of visuomotor control. On-line visuomotor control accesses a veridical representation of object size, and requires vision at the time of movement programming (i.e., between cueing and movement onset). Off-line control accesses a perceptual representation of object size, and is engaged when vision is unavailable at the time of movement programming. The transition from on-line to off-line control occurs within one reaction time (approx. 310 msec) or less of visual occlusion.
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