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Abstract:
Consciousness and attention are closely related concepts. In
this chapter, I will follow the common practice to use the term
consciousness to refer to what we can report having been aware of,
and the term attention to refer to the selection and maintenance of
a portion of information for privileged access, whatever its
consequences on consciousness. I will provide a case that seems to
illuminate, in an unexpected way, the neural substrate for the
distinction and relation between attention and consciousness in
vision.
It is well known that if one attends to a stationary point in
peripheral vision while maintaining fixation in central vision, the
peripheral point will fade from awareness within a few seconds.
This effect, known as Troxler fading (Troxler, 1804), has been
considered to reflect local adaptations in lower visual pathways
(Clarke and Belcher 1962, Millodisk 1967). Local adaptation here
means that the receptors or neurons responsible for detecting
stimulus-background edges by luminance or chromatic contrast cease
to respond to steady images. Because in Troxler fading the stimulus
is perceptually replaced by whatever in the surrounding area rather
than a black patch, it has been likened to the phenomena of
blindspot filling-in (Ramachandran 1992). Thus, the conscious
percept corresponding to the missing sensory input has been thought
to be somehow "fillled in" at higher visual centers by
interpolating from the surrounding visual areas. However, it is not
certain whether local adaptation is an accurate account of what
leads to the missing sensory input in the case of Troxler fading.
The following observation suggests that, unlike the blind spots,
the voluntary attention required to observe the fading may actually
be responsible, at least partly, for the fading.
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