We examined the biases in vection strength caused by motion direction (temporonasal vs. nasotemporal motion) and position of stimulus presentation (nasal and temporal semi-retinas) to investigate a subcortical contribution to vection. These biases have been identified for optokinetic nystagmus (OKN) and are acknowledged as evidence for a subcortical origin of OKN. In experiments, subjects monocularly observed hemi-field motion stimuli and made magnitude estimations. The results indicated significant directional and positional biases when luminance modulated gratings were used as stimuli. Vection was stronger with nasotemporal motions and nasal retina presentations, but there were no interactions between the two factors. However, these biases disappeared for second-order motion stimuli (contrast modulation), which are presumably processed by the cortex. In addition, when subjects were asked to make subjective ratings of motion impression, there was no significant difference in subjective strength between the stimuli that induced the strongest vection and weakest vection. These results, together, suggest the involvement of the subcortical pathway in vection induction.
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