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Abstract:
We investigated visuo - motor integration in grasping by studying haptic, visual, and cross-modal size discrimination. Our main interest concerned the effect of presentation time on discrimination performance and the differences between intramodal and cross-modal thresholds.
The experiments were conducted in a virtual environment in which two force-feedback devices (PHANToMTM) provided haptic information to the thumb and the index finger. Stereoscopically rendered objects were used for the visual presentation. In a two-interval forced-choice paradigm subjects had to determine which interval contained the larger object (we used cubes in all cases). Depending on condition, subjects either saw or felt each cube for a specified time. Feeling a cube required subjects to perform a two-finger grasp. The intramodal tasks were repeated with an appropriate mask between the two presentations; in these runs haptic masking consisted of randomly disturbing the finger span by the force-feedback devices. Intramodal thresholds (+/-4 visual - visual; +/-7 haptic - haptic) were significantly smaller than cross-modal thresholds (+/-13 visual - haptic; +/-14 haptic - visual). Gradually decreasing the presentation time in the intramodal conditions to less than 50 ms increased the thresholds monotonically, but significantly less so for the visual - visual condition (from +/-4 to +/-6) than for the haptic - haptic condition (from +/-7 to +/-20). We found no significant effect of masking on these thresholds.
Visuo - motor adaptation studies have shown that the coordinate transformation from vision to touch exhibits a considerable amount of plasticity. We hypothesise that the continuous recalibration of this transformation during the experiment constitutes the reason for the inflated cross-modal thresholds. Furthermore, we conclude that acquiring precise size information is much slower in the haptic modality than it is in vision.
