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Using fMRI to compare cortical magnification factors in human V1 to visual acuity |
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Geoffrey M. Boynton |
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We studied the relationship between visual acuity and areal cortical magnification factor in human primary visual cortex (V1) by comparing Vernier acuity thresholds with retinotopic maps measured with fMRI. Vernier acuity thresholds were measured at eccentricities of 3, 6, 9 and 12 degrees in ten subjects using a staircase procedure and a 2-AFC paradigm. As expected, Vernier acuity thresholds increase with eccentricity in a roughly linear fashion. Area V1 was localized in the same observers by projecting fMRI responses to standard retinotopic mapping stimuli (expanding rings and rotating wedges) onto a computationally flattened representation of the each subject's occipital cortical surface. Next, the eccentricity dimension of these retinotopic maps was carefully measured using flickering checkerboards restricted to annuli of 1.5, 3, 6, 9 or 12 degrees. These annuli alternated with uniform gray fields every 20 seconds. Similarly, the polar angle dimension was measured with flickering wedges presented along the vertical and horizontal meridians in alternation. We quantified the topology of activity maps produced by these stimuli within each subject using a complex logarithmic transform. This provided an estimate of the area of cortex within V1 that represents a given patch of visual space. For each subject's cortical hemisphere, the area of V1 that represents the Vernier acuity stimulus was compared to Vernier acuity thresholds in that subject's contralateral visual hemifield. Our results show that the spatial offset of the Vernier acuity stimulus at threshold is associated with roughly the same distance in primary visual cortex, regardless of stimulus eccentricity. |
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