Effect of stimulus hemifield on free-field auditory saltation

Auditory saltation is the orderly misperception of the spatial location of repetitive click stimuli emitted from two successive locations when the inter-click intervals (ICIs) are sufficiently short. The clicks are perceived as originating not only from the actual source locations, but also from locations between them. In two tasks, the present experiment compared free-field auditory saltation for 90 degrees excursions centered in the frontal, rear, left and right acoustic hemifields, by measuring the ICI at which subjects report 50% illusion strength (subjective task) and the ICI at which subjects could not distinguish real motion from saltation (objective task). A comparison of the saltation illusion for excursions spanning the midline (i.e. for frontal or rear hemifields) with that for stimuli in the lateral hemifields (left or right) revealed that the illusion was weaker for the midline-straddling conditions (i.e. the illusion was restricted to shorter ICIs). This may reflect the contribution of two perceptual channels to the task in the midline conditions (as opposed to one in the lateral hemifield conditions), or the fact that the temporal dynamics of localization differ between the midline and lateral hemifield conditions. A subsidiary comparison of saltation supported in the left and right auditory hemifields, and therefore by the right and left auditory forebrains, revealed no difference.