Abstract
BACKGROUND. Stroke-induced damage to the primary visual cortex (V1) causes immediate loss of luminance sensitivity in the contralateral hemifield when measured by automated perimetry, the clinical gold-standard. Unlike chronic stroke patients (>6 months post-stroke), we recently found (in a small sample) that some subacute patients <3 months post-stroke have preserved conscious motion discrimination within their perimetrically-defined blind-fields. Here, we standardized a method of defining visual preservation and used it to assess its incidence, location inside the blind-field, and quality in 32 subacute patients (23M/9F; mean±SEM: 57±2.3 yrs old) 65±5.3 days post-stroke. METHODS. Monocular Humphrey perimetry was used to identify binocular regions of clinical blindness (luminance sensitivity ≤12dB). Motion discrimination was mapped inside these regions using random dot stimuli (5° diameter, 500ms duration, black dots on mid-grey background). With eye-tracker enforced fixation, we mapped performance, starting at the vertical meridian, then moving laterally into the blind field in 1˚ steps. RESULTS. Preservation, defined as >75% correct performance and measurable direction difference (DD) or integration (DI) thresholds inside the blind-field, was found in 19/32 (~60%) participants. There were no significant differences in age or time since stroke for those with/without preservation. Preserved thresholds averaged 10.7±3.8° (DD) and 35.4±4.6% (DI) – similar to intact-field thresholds. Yet, mean perimetric luminance sensitivity at preserved locations was 2.9±0.3dB—well under the clinical definition of blindness. Preservation extended at least 1.6±0.5° from the nearest blind-field border, and as far as 10.4˚. Because we could not test the entire blind-field, these data may under-estimate preservation. CONCLUSIONS. A majority of subacute participants tested had preserved, near-normal direction discrimination and integration at one or more clinically-defined, blind-field locations. These results suggest a gradual, rather than sudden, loss of conscious motion perception after V1 damage, raising the question of what other visual modalities might show relative initial preservation, and over what time-course.