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Abstract:
If a letter is contrasted against a background of dots by
slowly increasing dot density in its shape, naive observers will
relatively late segment figure from ground and perceive that
letter. Conversely, gradual contrast reduction from supra-threshold
levels will allow percept maintenance to much lower contrast levels
before it is lost. This temporal order effect manifests as
hysteresis when plotting the perceptual level as a function of
contrast for the increasing vs. decreasing change. We used
functional magnetic resonance imaging and statistical parametric
mapping to identify where brain activity is correlated with
perception. Methods. Data acquisition: 6 subjects; 2T MRI (Siemens
Vision, head coil), T2*-weighted echoplanar images. Data analysis:
modelled hemodynamic responses for areas displaying a perceptually
driven behaviour (general linear model as employed by SPM97),
analysis of time courses in percept-sensitive areas. Results. An
analysis probing signal intensity to parallel percept presence vs.
absence revealed activation in widespread areas involved in
processing categorical and spatial object features and in
attentional selection. Hysteresis in these foci was found when
expressing activity as a function of figure-ground contrast and
temporal order. Conclusion. Awareness of an object in the visual
field was disambiguated from physical stimulus properties
subserving figure-ground segmentation. Exploiting temporal order
effects, we show that over and above signal changes accounted for
by the stimuli, presence of a conscious percept yields an
additional response.
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