Primary Visual Cortex is Active in Response to Stimulation of Phenomenally Blind Areas of the Visual Field in Patients with Cortical Blindness

Abstract

Geniculostriate pathway lesions disrupt vision in the contralesional visual hemifield. Loss of tissue may not be the only cause of cortical blindness from these lesions. Careful inspection of previous functional MRI (fMRI) studies of patients with visual field defects reveals numerous cases of blind areas of the visual field with a preserved V1 response to visual stimulation (‘blind voxels’). Here we describe the characteristics of blind voxels in stroke patients with homonymous visual field defects. We compared V1 activity (fMRI) with visual sensitivity (Humphrey perimetry) in 13 chronic stroke patients with cortical blindness. During fMRI, we presented 12 non-overlapping flickering wedges to map the retinotopic organization of visual cortex. The average number of blind voxels in V1 was significantly greater than zero (mean = 27.7, 95%, CI = 15.1–40.4 voxels) and was about one third of the average number of voxels activated for wedges presented in sighted areas of the visual field (‘sighted voxels’, mean = 96.7, 95%, CI = 81.3–112.2 voxels). These blind voxels were retinotopically organized. In 6 of the 13 patients we found that the number of blind voxels fell within the range of the number of sighted voxels. For example, in one subject, the number of voxels that responded to stimulation of a wedge presented in a blind area of the visual field ranged from 26–69 voxels, whereas this range was 20–274 voxels for wedges presented in sighted areas of the visual field. This observed dissociation between visual perception and V1 activity may be due to disordered information content in peri-lesional voxels such that the signal can no longer be read out as a visual percept. If the visual signal in blind voxels is merely disordered, it is possible that patients could recover some of their lost vision by learning a new read-out strategy.

Acknowledgement: F30EY027988 (CLS), R21NS076176 and R01NS089609 (BZM), Schmitt Program on Integrative Brain Research (BS and BZM), Research to Prevent Blindness to the Ophthalmology Department at the Flaum Eye Institute (ZRW), and P30EY001319 (BZM).