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Stanley W. Govenlock, Allison B. Sekuler, Patrick J. Bennett; Assessing the effect of aging on spatial frequency selectivity selectivity of visual mechanisms with the steady state visually evoked potential (ssVEP). Journal of Vision 2010;10(7):481. doi: 10.1167/10.7.481.
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Electrophysiological studies suggest that the spatial frequency selectivity of V1 neurons declines with age (Leventhal et al., 2003; Zhang et al., 2008). However, no psychophysical evidence for such an age-related decline has been found in humans (Govenlock et al., Vision Res, submitted). Three possible explanations for this discrepancy exist: (1) psychophysical performance is determined by the few neurons that remain highly-selective throughout aging, rather than by a larger less-selective population of neurons; (2) compensatory mechanisms enhance tuning in older humans; or (3) human and macaque monkeys are differentially affected by age. To test hypothesis 1, we measured the bandwidth of spatial frequency-selective mechanisms using the steady-state visually evoked potential (ssVEP) (Regan & Regan, 1988; Peterzell & Norcia, 1997). Twelve younger (22 years) and 12 older (69 years) subjects viewed two superimposed, iso-oriented, high contrast Gabor patterns counter-phase flickering at 6.67 (F1) and 8.57 (F2) Hz. The spatial frequency of one Gabor was fixed at 1 cpd; the frequency of the other Gabor varied +/- 0.66 octaves around 1 cpd. The dependent variable -- the amplitude of the F1+F2 Hz component of the ssVEP -- was measured as a function of the spatial frequency difference (Δf) between the Gabors. In both age groups, F1+F2 amplitude was greatest when Δf was zero and declined as Δf increased. The full bandwidth (at half-amplitude) of the F1+F2 response was approximately 0.65 octaves in both age groups. Thus, spatial frequency selectivity, as indexed by the population response registered by the ssVEP, does not become more broadly tuned in older humans. These results do not support the hypothesis that the discrepancy between human and macaque results can be explained by performance being determined by a few highly-selective neurons.
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