Multistable visual perception in Aging: an EEG-Study

• ¹ Institute of Psychology and Cognition Research, University of Bremen, Germany

Ambiguous figures make up a well-known class of visual phenomena in which an invariant stimulus pattern gives rise to at least two different perceptual alternatives. Such stimuli offer the possibility to investigate the internal organization of visual perception. The aim of the present study was to analyze age-related EEG-alterations during multistable perception. Recent ERP-studies imply alternated networks in elderly subjects [1]. Especially, a shift of the maximal EEG-power to frontal areas with increasing task complexity is notable [2]. According to these results, we assumed that networks with increased involvement of anterior activations might also be an important correlate of age-related changes during multistable visual perception. In our study we utilized three paradigms with different degrees of complexity: (1) a visual oddball-paradigm (using checker board stimuli) with a low degree of complexity, (2) the exogenous stroboscopic alternative motion, where reversals of perceived motion direction were induced externally, i.e. by a slight change of the visual stimulus, and (3) the endogenous stroboscopic alternative motion (SAM), a visual ambiguous pattern. Reversals of perceived motion directions were induced internally, and, therefore, the SAM had the highest degree of task complexity. Thirteen elderly healthy subjects (11 female, 2 male, mean age 62 +/- 3.82) and 10 young control subjects (8 female, 2 male, mean age 24.4 +/- 4.33) participated in our study. The EEG was recorded from frontal, central, parietal and occipital locations. The results confirm the alternated network hypothesis and imply possible compensation processes in the elderly group. The behavioural data showed no significant differences between old and young subjects in the oddball and the exogenous stroboscopic alternative motion paradigm, whereas the brain activity of the older persons differed from the younger subjects with increasing task complexity, predominantly for the SAM. While viewing the SAM older subjects showed their maximum EEG-response at frontal locations for all frequencies (delta, theta, alpha, gamma). A strong frontal activation has also been reported previously in younger subjects in the theta-band after omitted stimulation [3], and in the gamma band during multistable perception [4]. However, while the maximum delta response elicited during multistability occurs at right parietal locations in young adults [5, 6], the present study demonstrates a shift to frontal sites in elderly individuals. Of similar importance is the frontal shift of the alpha response, which is maximal over the visual cortex in young adults [7]. In conclusion, the shift of dominant EEG-activity from posterior to anterior locations in the delta- and alpha-band suggests that the brain network activated by multistable perception changes with age, and, thus, confirms the alternated network hypothesis.