Abstract
BackgroundWhen 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.MethodologyTo 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.Principal FindingsPhantasm 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.ConclusionsWe 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).
Highlights
Peripheral visual stimuli appear to fade into the background with steady fixation [1,2]
The site of the sensory threshold must be in these neurones themselves, rather than earlier or later in the pathway, to account for our data
Our cross-eye adaptation experiment rules out an earlier site than V1, and a later site for the threshold would not be consistent with at least one neuroimaging study [14]
Summary
Peripheral visual stimuli appear to fade into the background with steady fixation [1,2]. This ‘Troxler fading’ could be due to fatigue in the mechanisms for detecting the stimuli, or to an active ‘fillingin’ process, mediated by lateral connections [3]. When the adapting texture is suddenly replaced by a blank field, phantasms appear where the holes had been [4]. We wondered whether filling-in occurred sufficiently early in the visual pathway for phantasms to affect signal processing in the same way as real signals. 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)
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