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P. Cavanagh, J. J. S. Barton; Lights from beyond the visual field are not seen. Journal of Vision 2001;1(3):129. doi: 10.1167/1.3.129.
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For each direction of gaze, some parts of the peripheral retina do not have a “line of sight” through the pupil to the outside world — the bony orbit of the eye or the nose gets in the way. Of course, these regions of the retina are not always occluded, as the world does swing into view for them for other directions of gaze. If an occluded region of the retina were somehow stimulated, it should correspond to a light from outside the visual field. Hayhoe and Williams (Perception, 1984, 13, 455–9) tested the visibility of such “impossible” lights with afterimages in the periphery. When a shift in gaze moved the afterimage outside the visual field, observers reported that it disappeared. Moreover, it reappeared whenever the direction of gaze brought the afterimage back into the normal visual field. We modified their technique by delivering light to a fixed location on the peripheral retina through the sclera. We fixed a single optical fibre to a 22 mm diameter scleral contact lens and inserted the lens into one observer's eye. A strobe light, connected to the single fibre, delivered repetitive pulses of light to the sclera. The retina underneath the illuminated sclera received transmitted light that was experienced as a bright, flickering spot in the far periphery on the side opposite to the stimulation. As the eye changed direction of gaze, the end point of the fibre remained more or less over the same location on the sclera and the light was experienced at different locations in space. At extreme directions of gaze that brought the apparent light source outside the visual field, the flickering light was no longer visible. This result supports Hayhoe and Williams' claim that the visibility of retinal stimulation is gated by eye position. Compared to the afterimage procedure, scleral light delivery has the advantage of providing a continuous, flickering source that does not fade. This gives us a new tool to explore vision in the far periphery and beyond.
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