The perception of illusory contours in young and older observers - an explorative study using psychophysics and EEG

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.