The Role of Spared Calcarine Cortex and Lateral Occipital Cortex in the Responses of Human Hemianopes to Visual Motion

Abstract

Some patients, who are rendered perimetrically blind in one hemifield by cortical lesions, nevertheless exhibit residual visual capacities within their field defects. The neural mechanism that mediates the residual visual responses has remained the topic of considerable debate. One explanation posits the subcortical visual pathways that bypass the primary visual cortex and innervate the extrastriate visual areas as the substrate that underlies the residual vision. The other explanation is that small islands of the primary visual cortex remain intact and provide the signals for residual vision. We have performed behavioral and functional magnetic resonance imaging experiments to investigate the validity of the two explanations of residual vision. Our behavioral experiments indicated that of the seven hemianopes tested, two had the ability to discriminate the direction of a drifting grating. This residual visual response was shown with fMRI to be the result of spared islands of calcarine cortical activity in one of the hemianopes, whereas only lateral occipital activity was documented in the other patient. These results indicate that the underlying neural correlates of residual vision can vary between patients. Moreover, our study emphasizes the necessity of ruling out the presence of islands of preserved function and primary visual cortex before assigning residual visual capacities to the properties of visual pathways that bypass the primary visual cortex.

Training-induced cortical representation of a hemianopic hemifield

“…Neuroimaging studies have indicated residual responsiveness to blind hemifield stimulation in the extrastriate [1][2][3][4][5][6] and spared calcarine cortex, 7 but studies on improvements of residual vision using training have reported conflicting results. Training of visual functions is not a standard procedure after a local cerebral damage, whereas benefits of motor rehabilitation and reorganisation of the motor cortex are widely accepted.…”

Visualizing the blind brain: brain imaging of visual field defects from early recovery to rehabilitation techniques

“…Evidence for basic residual visual motion (Barbur et al, 1980; Weiskrantz, 1986; Schoenfeld et al, 2002; Morland et al, 2004), shape (Barbur et al, 1993; Stoerig and Cowey, 1997; Goebel et al, 2001), and color in the blind field (Stoerig, 1987; Barbur et al, 1998; for a review see Huxlin, 2008) has been reported in the literature consistently.…”

“…Some authors have postulated a direct relation between the preserved portion of striate areas and blindsight in the corresponding visual field (Fendrich et al, 2001; Morland et al, 2004). …”

“…Specifically Morland et al (2004) used fMRI with static, moving and flickering stimuli in seven hemianopes and one patient with a clearly spared region of the visual field in an otherwise blind hemifield. Their findings support the existence of small, spared active regions of V1 that mediate residual vision in some patients, consistent with the results of Raposo et al (2011).…”

Visual restoration in cortical blindness: Insights from natural and TMS-induced blindsight

“…These are not mutually exclusive; each may apply in some patients but not in others (Morland et al, 2004):…”