Retinal mapping can distort to avoid the "impossible space" outside the visual field

Abstract

We recently found that direct stimulation of visual cortex using TMS could result in phosphenes that extended outside the visual field (Wu et al, VSS 2006). Brightness was perceived to emanate from positions inside the mouth, beneath the head, or even behind the ear. Though previous studies using retinal stimulation had failed to produce similar results (Hayhoe & Williams 1984, Cavanagh & Barton VSS 2001), we decided to revisit the use of retinal afterimages with a larger number of subjects and stronger stimuli. Vivid rod-based afterimages were induced in dark-adapted subjects using four 2400W flash tubes to illuminate a grid-shaped stimulus. Shortly after the flash, subjects made a large eye movement (e.g. downwards), causing parts of the stimulated retinal area to point in directions beyond the visual field (straight down, etc.). As in the TMS study, some (4/8) of the subjects reported that the afterimages extended well outside the natural field of view (below or behind the head). Only one subject reported a simple truncation of the afterimage at the edge of the visual field. The other (3/8) subjects reported that the afterimage was constrained within the natural visual field via compression along an anisotropic gradient. The compression was strongest near the impinged edge of the visual field, where the afterimage of the grid appeared as thin rectangles. Meanwhile, other regions of the grid remained veridically square. Two subjects also reported compensatory translation of the afterimage—the afterimage did not follow their eye movement completely, and they felt they were looking directly at a lower portion of the afterimage. This study confirms and elaborates on earlier findings that visual representations of light outside the visual field are possible, and that distortions and shifts in retinal mapping can occur when an image might otherwise extend beyond the natural visual field.