The lingering effects of an artificial blind spot.
BACKGROUND
When steady fixation is maintained on the centre of a large patch of texture, holes in the periphery of the texture rapidly fade from awareness, producing artificial scotomata (i.e., invisible areas of reduced vision, like the natural 'blind spot'). There has been considerable controversy about whether this apparent 'filling in' depends on a low-level or high-level visual process. Evidence for an active process is that when the texture around the scotomata is suddenly removed, phantasms of the texture appear within the previous scotomata.


METHODS
To see if these phantasms were equivalent to real low-level signals, we measured contrast discrimination for real dynamic texture patches presented on top of the phantasms.


RESULTS
Phantasm intensity varied with adapting contrast. Contrast discrimination depended on both (real) pedestal contrast and phantasm intensity, in a manner indicative of a common sensory threshold. The phantasms showed inter-ocular transfer, proving that their effects are cortical rather than retinal.


CONCLUSIONS
We show that this effect is consistent with a tonic spreading of the adapting texture into the scotomata, coupled with some overall loss of sensitivity. Our results support the view that 'filling in' happens at an early stage of visual processing, quite possibly in primary visual cortex (V1).
Morgan
Michael J
MJ
Henry Wellcome Vision Laboratories, Department of Optometry and Visual Science, City University, London, United Kingdom. m.morgan@city.ac.uk
McEwan
William
W
Solomon
Joshua
J
D000222:Adaptation, Physiological / Q000502:physiology; D015350:Contrast Sensitivity / Q000502:physiology*; D004192:Discrimination, Psychological / Q000502:physiology*; D005403:Fixation, Ocular / Q000502:physiology*; D006801:Humans; D007092:Imagination / Q000502:physiology; D009898:Optic Disk; D012607:Scotoma / Q000503:physiopathology*; D013997:Time Factors; D014793:Visual Cortex / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*; D055815:Young Adult