dc.contributor.advisor | Boyacı, Hüseyin | |
dc.contributor.author | Er, Görkem | |
dc.date.accessioned | 2018-10-08T11:59:41Z | |
dc.date.available | 2018-10-08T11:59:41Z | |
dc.date.copyright | 2018-09 | |
dc.date.issued | 2018-09 | |
dc.date.submitted | 2018-10-04 | |
dc.identifier.uri | http://hdl.handle.net/11693/48079 | |
dc.description | Cataloged from PDF version of article. | en_US |
dc.description | Thesis (M.S.): Bilkent University, Department of Neuroscience, İhsan Doğramacı Bilkent University, 2018. | en_US |
dc.description | Includes bibliographical references (leaves 48-57). | en_US |
dc.description.abstract | Behavioral experiments have demonstrated that observers' ability to discriminate
the drift direction of a grating improves as its size increases if the grating has a
low contrast, and deteriorates if it has a high contrast [1]. It has been proposed
that receptive field organization in middle temporal (MT+) visual area underlies
this counter-intuitive perceptual effect. Supporting evidence for this proposal has
been provided in literature [2]. However, previous studies have not unequivocally
showed that MT+ is the sole area whose activity underlies the perceptual effect.
Here, we investigate the activity patterns of primary visual cortex (V1) and
middle temporal (MT+) in response to drifting Gabor patches in differing size
and contrast levels to elucidate the neural region involved in size-contrast interaction
in motion perception. We first replicated the findings in the literature with
a behavioral experiment, where small or large (1.67 and 8.05 degrees of visual
angle) drifting gratings with either low (2%) or high (99%) contrast levels were
presented at the periphery (centered 9.06 degrees of visual angle to left and right
of fixation). We measured the duration thresholds (79%) for accurately discriminating
the drift direction of gratings for eleven participants using an adaptive
staircase and two-alternative forced choice (2AFC) design. In line with previous
literature, we observed that increasing the size of the low-contrast stimuli resulted
in decreased discrimination threshold, while for high-contrast stimuli, increasing
the size resulted in increased discrimination threshold.
In the second stage of the study, six observers participated in a block design
fMRI study with the same spatial configuration and contrast levels used in the
behavioral experiment. We first identified the region of interests (ROI) for visual
area V1 and MT+ separately for all participants. Then, we identified a "sub-ROI"
that corresponds to the region that was selectively responsive to the small sized
stimuli (1.67 degrees) using an independent localizer. With this setup, we allowed
for both large and small sized gratings to stimulate the sub-ROI throughout the
entire scan. Therefore, changes in Blood Oxygenated Level Dependent (BOLD)
response at the sub-ROI in response to large compared to small sized gratings
indicated the in
uence of the surrounding region to the center of the gratings.
In area MT+, we observed that increasing the size of the grating increases the
BOLD activity if the stimuli have low contrast, compared to high contrast. In
other words, surrounding region had a facilitative in
uence to the group of MT+
neurons encoding the center of the stimuli if the stimuli had low contrast. This
neuronal facilitation observed with the neuroimaging data explain the enhancement
of the performance with increasing the size of the low-contrasted stimuli
observed at the behavioral experiment. In V1, however, increasing the size of
the high-contrasted gratings increased the BOLD activity, compared to activity
evoked by increasing the size of the low-contrasted gratings. On the whole, we
show that center-surround interaction in V1 and MT were differentiated in response
to peripherally viewed drifting Gabor patches at differing contrast and
size levels, hence we provide further evidence that the perceptual size-contrast
interaction effect is likely to originates at cortical area MT+. | en_US |
dc.description.statementofresponsibility | by Görkem Er. | en_US |
dc.format.extent | xiii, 57 leaves : charts (some color) ; 30 cm | en_US |
dc.language.iso | English | en_US |
dc.rights | info:eu-repo/semantics/openAccess | en_US |
dc.subject | Visual Motion | en_US |
dc.subject | Spatial Suppression | en_US |
dc.subject | Surround Suppression | en_US |
dc.subject | Surround Facilitation | en_US |
dc.subject | Primary Visual Cortex | en_US |
dc.subject | Middle Ttemporal Region | en_US |
dc.subject | Contrast | en_US |
dc.subject | Size | en_US |
dc.title | The role of contrast and size in motion perception: behavioral and neuroimaging study of center-surround interactions in primary visual cortex (V1) and middle temporal area (MT+) | en_US |
dc.title.alternative | Kontrast ve büyüklüğün hareket algısındaki rolü: kortikal bölge V1 ve MT'deki çevre ve merkez etkileşimlerinin FMRI ve davranışsal deneylerle incelenmesi | en_US |
dc.type | Thesis | en_US |
dc.department | Graduate Program in Neuroscience | en_US |
dc.publisher | Bilkent University | en_US |
dc.description.degree | M.S. | en_US |
dc.identifier.itemid | B159106 | |