It has recently been suggested that visual
working memory capacity may vary depending on the type
of material that has to be memorized. Here, we use a
delayed match-to-sample paradigm and event-related
potentials (ERP) to investigate the neural correlates that are
linked to these changes in capacity. A variable number of
stimuli (1–4) were presented in each visual hemifield.
Participants were required to selectively memorize the
stimuli presented in one hemifield. Following memorization,
a test stimulus was presented that had to be matched
against the memorized item(s). Two types of stimuli were
used: one set consisting of discretely different objects
(discrete stimuli) and one set consisting of more continuous
variations along a single dimension (continuous stimuli).
Behavioral results indicate that memory capacity was much
larger for the discrete stimuli, when compared with the
continuous stimuli. This behavioral effect correlated with
an increase in a contralateral negative slow wave ERP component that is known to be involved in memorization.
We therefore conclude that the larger working memory
capacity for discrete stimuli can be directly related to an
increase in activity in visual areas and propose that this
increase in visual activity is due to interactions with other, non-visual representations.