Illusory motion perception in blindsight

Abstract

Motion detection is typically spared in blindsight, which results from damage to the striate cortex (area V1) of the brain that is sufficient to eliminate conscious visual awareness and severely reduce sensitivity to luminance contrast, especially for high spatial and low temporal frequencies. Here we show that the discrimination of motion direction within cortically blind fields is not attributable to feature tracking (the detection of changes in position or shape), but is due instead to the detection of firstorder motion energy (spatiotemporal changes in luminance). The key to this finding was a version of the line motion illusion entailing reverse-phi motion in which opposing motion directions are simultaneously cued by motion energy and changes in stimulus shape. In forced-choice tests, a blindsighted test subject selected the direction cued by shape change when the stimulus was presented in his intact field, but reliably selected the direction cued by motion energy when the same stimulus was presented in his blind field, where relevant position information was either inaccessible or invalid. Motion energy has been characterized as objectless, so reliance on motion energy detection is consistent with impaired access to shape information in blindsight. The dissociation of motion direction by visual field (cortically blind vs. intact) provides evidence that two pathways from the retina to MT/V5 (the cortical area specialized for motion perception) are functionally distinct: the retinogeniculate pathway through V1 is specialized for feature-based motion perception, whereas the retinocollicular pathway, which bypasses V1, is specialized for detecting motion energy.superior colliculus | visual cortex | visual pathway | hemianopia B lindsight is a rare phenomenon caused by damage to the primary visual cortex (striate cortex, or V1) of the brain that is sufficient to eliminate conscious visual awareness (1, 2). Visual sensitivity to luminance contrast is reduced in the corresponding part of the visual field, most severely for high spatial and low temporal frequencies (3). The fact that it is not completely abolished for low spatial and high temporal frequencies accounts for many of the residual visual capacities that have been reported in blindsight, including, in forced-choice tests, the ability to detect and discriminate stimuli presented in the field defect (2, 4). Evidence obtained from monkeys with striate cortex lesions as well as from cortically blind patients suggests that these capacities are mediated by neural pathways that project from the retina to extrastriate visual cortex, which normally bypass the main visual pathway that projects from the retina to the striate cortex via the lateral geniculate nucleus (the retinogeniculate pathway) and involve the superior colliculus of the midbrain and the lateral geniculate and pulvinar nuclei of the thalamus (5-13).Blindsight is not merely normal vision without awareness. In addition to the loss of primary visual cortex, retrograde degeneration of relay neurons ...

The perception of object versus objectless motion

“…Given our results indicating the high-speed preference of motion perception based exclusively on spatiotemporal changes in luminance, it is conceivable that the collicular route to MT is the primary pathway for objectless motion perception. Supporting this possibility is evidence that hemianopes with unilateral damage to V1 can discriminate the direction of high-speed motion in their cortically blind hemifield, despite the absence of object/shape discrimination (Azzopardi & Cowey 2001;Barbur et al 1993), and further, that this direction discrimination is based on spatiotemporal changes in luminance rather than changes in shape (Azzopardi & Hock 2011).…”

“…Additional experiments have indicated that direction discrimination in a hemianope's cortically blind hemifield is based on the detection of spatiotemporal changes in luminance, irrespective of any other stimulus feature (Azzopardi & Hock 2011). These experiments were based on the line motion illusion (previously called polarized gamma motion), which occurs when a surface is present and another surface is presented adjacent to it, or both surfaces are present and one is then removed (Faubert & von Grünau 1995;Hikosaka et al 1993a, b;Kanizsa 1978).…”

“…1c and 1d). When this stimulus was presented in the hemianope's intact hemifield, Azzopardi and Hock (2011) found that direction judgments were determined by changes in shape. However, when the same stimulus was presented in the hemianope's cortically blind hemifield, where differences in shape could not be discriminated, direction judgments were determined by changes in the luminance profile of the line motion stimulus.…”

What is it like to have type-2 blindsight? Drawing inferences from residual function in type-1 blindsight

“…If the elements of the random dot kinematogram initially change in contrast whilst remaining stationary before the onset of motion then blindsight patients cannot even determine when motion starts, let alone discriminate its speed or direction. Recently Azzopardi and Hock (2011) showed that the responses of blindsight patients to motion are influenced by changes in the contrast polarity of stimuli. If we see a black bar against a grey background extend upwards we see motion in an upward direction whether the extension to the bar is black or white.…”

The case for characterising type-2 blindsight as a genuinely visual phenomenon

“…98). In addition, there is evidence that wavelength information, shape information, and other features of the stimulus do not feature as contents of the subjects' awareness (Azzopardi & Hock, 2011;Weiskrantz et al, 1995).…”