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Kurtz, Brigitta (2011): The perception of illusory contours in young and older observers - an explorative study using psychophysics and EEG. Dissertation, LMU München: Faculty of Medicine
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Abstract

A series of psychophysical and electrophysiological (EEG) experiments are presented which aimed at assessing the effect of normal brain aging on the perception of illusory contours (ICs). ICs were here considered as exemplary tasks for the process of visual binding, as required for the handling of complex visual situations such as car driving for example. Furthermore, ICs are often used in the context of clinical neuropsychological assessments. Since as yet IC perception has not been systematically studied in the elderly population, our data can provide a baseline measure. A total of 153 healthy paid volunteers, aged between 18 and 90 years, took part in two psychophysical and two electrophysiological experiments. In our first explorative behavioural paradigm we found that the time to identify ICs of the Kanizsa type (compared to control stimuli) increased steadily with advancing age, suggesting a gradual decline in visual binding capacities, which starts already from the age of 30 years on. The observed effect could not be explained by older people’s well documented deficits in processing stimuli at higher eccentricities (see for example Poggel & Strasburger 2004; Sekuler et al. 2000; Kosslyn et al. 1999), since it proved robust, also when we reduced stimulus size from 10° to 5° of visual angle in our second psychophysical experiment. In our first EEG experiment – a non-response paradigm – we did not replicate previous findings concerning an IC effect, i.e., a differential electrophysiological reaction between IC and control stimuli, which is considered to reflect the perception of the “Gestalt” (see for example Murray et al. 2004, 2002; Kruggel et al, 2001; Herrmann & Bosch 2001). The replication failed for both our young and older observers. A reduction of the stimulus size from 6° to 4° of visual angle in our second EEG experiment brought only a slight increase of differential activity in the young observers. Only when we induced a conscious processing of the presented shapes by introducing new target stimuli in the second part of this experiment, did we find a distinct IC effect in both subject groups, suggesting that the IC stimuli were perceived. This finding speaks for the importance of top-down influences in IC perception, an issue that still provokes considerable debate (Senkowski et al. 2005; Montaser-Kouhsari & Rajimehr 2004; Vuilleumier & Landis 1998; Gurnsey et al. 1996; Davis & Driver 1994; Pritchard & Warm 1983). Our findings suggest that the perception of ICs is weakened or delayed with advancing age, but that this deficit can be compensated for by cognitive strategies.