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Robert O. Duncan, Geoffrey M. Boynton; Cortical magnification factor in human primary visual cortex correlates with Vernier acuity thresholds. Journal of Vision 2002;2(7):129. doi: 10.1167/2.7.129.
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© ARVO (1962-2015); The Authors (2016-present)
We studied the relationship between visual acuity and areal cortical magnification factor (ACMF) 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 modification of the complex logarithmic transformation. 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. We discovered that across stimulus eccentricities and subjects, Vernier acuity thresholds are inversely proportional to the cortical area associated with the Vernier acuity task. At 3 degrees eccentricity, furthermore, we found a strong within-subject correlation (p=0.025; R=−0.58) between Vernier acuity threshold and ACMF; subjects with lower Vernier acuity thresholds have more area of V1 representing the stimulus.
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