37 research outputs found
Bistable Percepts in the Brain: fMRI Contrasts Monocular Pattern Rivalry and Binocular Rivalry
The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1
The what and why of perceptual asymmetries in the visual domain
Perceptual asymmetry is one of the most important characteristics of our visual
functioning. We carefully reviewed the scientific literature in order to examine
such asymmetries, separating them into two major categories: within-visual field
asymmetries and between-visual field asymmetries. We explain these asymmetries
in terms of perceptual aspects or tasks, the what of the
asymmetries; and in terms of underlying mechanisms, the why of
the asymmetries. Tthe within-visual field asymmetries are fundamental to
orientation, motion direction, and spatial frequency processing. between-visual
field asymmetries have been reported for a wide range of perceptual phenomena.
foveal dominance over the periphery, in particular, has been prominent for
visual acuity, contrast sensitivity, and colour discrimination. Tthis also holds
true for object or face recognition and reading performance. upper-lower visual
field asymmetries in favour of the lower have been demonstrated for temporal and
contrast sensitivities, visual acuity, spatial resolution, orientation, hue and
motion processing. Iin contrast, the upper field advantages have been seen in
visual search, apparent size, and object recognition tasks. left-right visual
field asymmetries include the left field dominance in spatial (e.g.,
orientation) processing and the right field dominance in non-spatial (e.g.,
temporal) processing. left field is also better at low spatial frequency or
global and coordinate spatial processing, whereas the right field is better at
high spatial frequency or local and categorical spatial processing. All these
asymmetries have inborn neural/physiological origins, the primary
why, but can be also susceptible to visual experience, the
critical why (promotes or blocks the asymmetries by
altering neural functions)