TACTILE SIMULTANEITY THRESHOLDS WITHIN AND BETWEEN HEMISPHERES - THE EFFECTS OF HEMISPACE

The effect of hemispace on lateralisation of temporal processing was i nvestigated by requiring simultaneity judgements of pairs of unimanual ly and bimanually presented tactile stimuli. The effects of lateral an d midline placement of hands on simultaneity thresholds were investiga ted. The first experiment (N = 16) showed that simultaneity thresholds were longer with bimanual than unimanual stimulation. Midline hand pl acement produced bimanual thresholds that were equivalent whether left or right hands were first stimulated. These two findings replicated e xperiments 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 s imultaneity was obtained when the left rather than the right hand rece ived the first stimulus. A second experiment (N = 30) replicated these results. A crossed-arms condition in the second experiment yielded eq uivalent thresholds for left versus right hand receiving the first sti mulus. The results of both experiments provide support for the hemisph eric equivalence model and indicate that a left hemispace disadvantage rather than left hemisphere specialisation for temporal processing co uld explain previous reports of simultaneity threshold asymmetries. Lo nger bimanual than unimanual simultaneity thresholds implicate transca llosal transmission time (IHTT) to compare the two signals. IHTT incre ased with lateral and crossed compared to midline hand placement, sugg esting that distance between the hands may be represented by increased IHTT. We conclude that temporal perceptions are affected by the proce ss of interhemispheric transmission.