Abstract
Recently, Rousselet et al. reported a 1 ms/year delay in visual processing speed in a sample of healthy aged 62 subjects (Frontiers in Psychology 2010, 1:19). Here, we replicate this finding in an independent sample of 59 subjects and investigate the contribution of optical factors (pupil size and luminance) to the age-related slowdown and to individual differences in visual processing speed. We conducted two experiments. In experiment 1 we recorded EEG from subjects aged 18–79. Subjects viewed images of faces and phase scrambled noise textures under nine luminance conditions, ranging from 0.59 to 60.8 cd/m2. We manipulated luminance using neutral density filters. In experiment 2, 10 young subjects (age < 35) viewed similar stimuli through pinholes ranging from 1 to 5 mm. In both experiments, subjects were tested twice. We found a 1 ms/year slowdown in visual processing that was independent of luminance. Aging effects became visible around 125 ms post-stimulus and did not affect the onsets of the face-texture ERP differences. Furthermore, luminance modulated the entire ERP time-course from 60 to 500 ms. Luminance effects peaked in the N170 time window and were independent of age. Importantly, senile miosis and individual differences in pupil size did not account for aging differences and inter-subject variability in processing speed. The pinhole manipulation also failed to match the ERPs of old subjects to those of young subjects. Overall, our results strongly suggest that early ERPs to faces (<200 ms) are delayed by aging and that these delays are of cortical, rather than optical origin. Our results also demonstrate that even late ERPs to faces are modulated by low-level factors.
Highlights
One of the most prominent phenomena associated with aging is a progressive cognitive slowing
We wanted to determine if the event-related potentials (ERPs) of old observers can be matched to the ERPs of young observers tested at lower retinal illuminance levels
We found that aging prolongs peak latencies of the face-texture ERP differences at the average rate of 1.5 ms/year
Summary
One of the most prominent phenomena associated with aging is a progressive cognitive slowing. In keeping with findings in animals, fMRI studies in humans have described weaker differentiation between categorical responses in old subjects (Park et al, 2004; Voss et al, 2008) Such changes in tuning may lead to longer processing times, following a model of perceptual decision by accumulation of evidence in neuronal populations (Perrett and Ashbridge, 1998). There is more direct evidence from human ERP studies, showing delayed evoked responses to checkerboards (Sokol et al, 1981; Tobimatsu et al, 1993) These neural changes suggest an overall slowdown of perception with age (Rousselet et al, 2009, 2010), which in turn could affect higher cognitive functions, such as working memory (Gazzaley et al, 2008)
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