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Stanley Govenlock, Katrin Kliegl, Allison Sekuler, Patrick Bennett; Assessing the effect of aging on orientation selectivity of visual mechanisms with the steady state visually evoked potential. Journal of Vision 2008;8(6):424. doi: 10.1167/8.6.424.
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Orientation selectivity in V1 macaque neurons decreases with aging (Leventhal et al., 2003). However, using psychophysical masking techniques, we found no effect of aging on human orientation selectivity (Govenlock et al., Vision Res, submitted). One explanation for these discrepant results is that psychophysical thresholds may be mediated by a few highly-selective neurons whose tuning properties do not vary significantly with age. To test this idea, we measured the bandwidth of orientation-selective mechanisms using the steady-state visual evoked potential (ssVEP), which is an index of the activity of large populations of visual neurons. The ssVEP has been used previously to measure orientation tuning in younger adults (Regan & Regan, 1987) and infants (Candy et al., 2001), but not elderly adults. We recorded EEG while subjects viewed 2 superimposed Gabor patterns (contrast = 40%; SF = 1cpd). One Gabor was horizontal and the orientation of the other Gabor varied in different experimental conditions. The Gabors were counterphase-flickered at F1 (6.67 Hz) and F2 (8.57 Hz). Preliminary results from 5 young (mean age = 22) and 6 elderly (mean age = 69) subjects show that F1+F2 amplitude is tuned to the orientation offset of the two Gabors, just as Candy et al. found. The slope of the function relating F1+F2 amplitude to orientation offset does not differ with age [F(1,9) = 1.71; p = 0.22], which is inconsistent with the hypothesis that orientation selectivity broadens with age. However, the results were influenced significantly by one unusual younger subject whose ssVEP showed virtually no orientation tuning. With that subject removed, the amplitude vs. orientation offset function is significantly shallower in older subjects [F(1,8) = 5.37; p [[lt]]0.05], suggesting that orientation-selective mechanisms are more broadly tuned in older subjects.
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