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Fig. 1
Neisser's perceptual cycle
In Neisser's model, schemata represent the knowledge about our environment.
They are based on previous experiences and are located in the long term memory.
Neisser attributes them to generate certain expectations and emotions that steer our
attention in the further exploration of our environment. The exploratory process
consists, according to Neisser, in the transfer of sensory information (the stimulus)
into the short-term memory. In the exploratory process, the entirety of stimuli (the
stimulus environment) is compared to the schemata already known. Recognized
stimuli are given a meaning, whereas unrecognized stimuli will modify the sche-
mata, which will then in turn direct the exploratory process The differences in
schemata present in the human individual cause the same stimulus to provoke dif-
ferent reactions between subjects. Following Neisser's model, especially new
experiences (those that cause a modification of existing schemata) are likely to
generate a higher load in terms of processing requirements for the percepts.
The schemata therefore also control the attention that we pay toward stimuli. A
large number of studies have tried to identify and describe the strategy that is ac-
tually used in the human perceptual process. Pashler gives an overview and identi-
fies two main concepts of attention [10]: attention as based on exclusion (gating)
or based on capacity (resource) allocation. The first concept defines the mecha-
nism that reduces processing of irrelevant stimuli to be attended. It can be re-
garded as a filtering device that keeps out stimuli from the perceptual machinery
that performs the recognition. Attention is therefore identified with a purely exclu-
sionary mechanism. The second concept construes the limited processing resource
(rather than the filtering device) as attention. It suggests that when attention is
given to an item, it is perceptually analyzed. When attention is allocated to several
items, they are processed in parallel until the capacity limits are exceeded. In that
case, processing becomes less efficient.
Neither of the two concepts can be ruled out by the many investigations performed
up to now. Instead, assuming either, the gating or the resource interpretation, all
empirical results can be accounted for in some way or other. As a result, it must be
concluded that both capacity limits and perceptual gating characterize the human
