Abstract
When feeling objects, vision and touch, simultaneously provide information about size or shape. The purpose is to determine which representation we use for discriminating between objects which differ only in their visual and/or haptic size. We consider three respresentations: visual-alone, haptic-alone and combined visual-haptic. Integrating the information optimally, the combined visual-haptic percept is a weighted average of the two sizes. To measure discrimination performace subjects indicated the odd of three sequentially presented stimuli. These were horizontal bars, two of which were identical and had equal visual and haptic heights (standard). The third (odd) stimulus had a different visual and/or haptic height. If subjects used the visual-alone or haptic-alone representations, discrimination would occur whenever the visual or haptic height in the odd stimulus differed from the standard by more than the threshold. If subjects relied on the combined representation, no discrimination should occur when visual and haptic heights differ in opposite directions so that their weighted averages are equal and discrimination should be best when both these heights are either bigger or smaller then the standard. We found that discrimination was indeed most difficult when the weighted averages were equal. Thus, the combined visual-haptic percept is used for discrimination. However, if the conflict between visual and haptic heights became too large, discrimination improved, indicating that we also can access the visual-alone and haptic-alone representations.