Tactile simultaneity thresholds within and between hemispheres: The effects of hemispace

The effect of hemispace on lateralisation of temporal processing was investigated by requiring simultaneity judgements of pairs of unimanually and bimanually presented tactile stimuli. The effects of lateral and midline placement of hands on simultaneity thresholds were investigated. The first experiment (N =16) showed that simultaneity thresholds were longer with bimanual than unimanual stimulation. Midline hand placement produced bimanual thresholds that were equivalent whether left or right hands were first stimulated. These two findings replicated experiments reported previously that supported an equivalence model of cerebral hemispheric temporal judgements (Clark & Geffen, 1990). With lateral hand placement, longer stimulus onset asynchrony to perceive simultaneity was obtained when the left rather than the right hand received the first stimulus. A second experiment (N = 30) replicated these results. A crossed-arms condition in the second experiment yielded equivalent thresholds for left versus right hand receiving the first stimulus. The results of both experiments provide support for the hemispheric equivalence model and indicate that a left hemispace disadvantage rather than left hemisphere specialisation for temporal processing could explain previous reports of simultaneity threshold asymmetries. Longer bimanual than unimanual simultaneity thresholds implicate transcallosal transmission time (IHTT) to compare the two signals. IHTT increased with lateral and crossed compared to midline hand placement, suggesting that distance between the hands may be represented by increased IHTT. We conclude that temporal perceptions are affected by the process of interhemispheric transmission.