The effect of context-dependent lightness on contrast detection and identification, and its neural correlates
Author
Karatok, Zahide Pamir
Advisor
Boyacı, Hüseyin
Date
2017-11Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Perceived contrast of a grating varies with its background (or mean) luminance:
of the two gratings with the same photometric contrast the one on higher luminance
background appears to have higher contrast. On the other hand, context
often causes a large perceived difference between equiluminant regions (e.g., simultaneous
brightness contrast). Does perceived contrast also vary with contextdependent
background lightness even when the luminance remains constant?
In this study, the effect of context-dependent lightness on contrast perception
was investigated using psychophysical and functional magnetic resonance imaging
(fMRI) methods. First, we measured appearance judgments of participants
and demonstrated that context-dependent lightness of background in
uences the
perceived contrast of rectified gratings. Perceived contrast of gratings superimposed
on equiluminant but perceptually lighter background is higher compared
to ones on perceptually darker backgrounds. However, this pattern is valid only
for incremental, not for decremental contrast.
Literature indicates a significant difference between visual processing near and
above threshold. Also, behaviorally it has been shown that appearance and
threshold tasks are mediated by different mechanisms. Therefore, here, we also
measured the effect of context-dependent lightness on contrast detection and
discrimination thresholds using a 2-IFC procedure. Results indicate that both
detection and discrimination thresholds are lower for the gratings superimposed
on perceptually lighter backgrounds. Differently from the appearance results, the
effect was observed both for incremental and decremental contrast.
In an fMRI study, we investigated whether activity in any brain region correlates
with background-lightness-dependent contrast perception. Although our
stimulus was physically identical, we observed difference in BOLD response within
pre-defined region of interests (ROIs) in different visual areas. Both for incremental
and decremental contrast, activation, especially in V1, was greater when the grating was superimposed on lighter background for all the contrast levels tested.
Variation in V1 activity with varying contrast links better with the detection
and discrimination thresholds than the appearance results. Therefore, this study
might offer a neural evidence for dissociation between the mechanisms underlying
detection (threshold) and identification (appearance) measures. However, the
relationship between the threshold and fMRI data does not really agree with the
previous findings in literature. These results indicate that the neural activation
caused by the detection mechanism may change depending on the absolute or
perceived value of the contrast.