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either separately or simultaneously. Subjects
detected the increments in one modality without
interference from simultaneously monitoring the
other modality, and performance of detection
was comparable to that of only monitoring one
modality. Other studies, for example, Shiffrin
and Grantham (1974) and by Gescheider, Sager,
and Ruffolo (1975), also support these results for
presentations of short bimodal stimuli.
As the stimuli presented in the auditory and
the visual modalities were not contextually related
in the study of Eijkman and Vendrik (1965), they
constituted what could be called separate percep-
tual streams. Yet, detection of increments in the
duration of the same stimuli was showing marked
interference. This suggests that temporal judg-
ments might be processed by the same processing
system (the same cortical areas), a theory that is
further supported by the findings of Shams et al.
(2000, 2002) already discussed in the subsection
on visual dominance.
Interestingly, other studies combining auditory
and visual discrimination tasks showed modest
but considerable decrements in terms of perfor-
mance. This was observed when test subjects were
confronted with bimodal stimuli in comparison
to unimodal ones. To give an example, Tulving
and Lindsay (1967) presented test subjects with
tones and patches of light. Subjects were asked
to judge the intensity of either tone or light, and
results were compared to the bimodal judgment
of intensity of both stimuli.
All of these studies characteristically involve
magnitude judgments rather than categorical judg-
ments. Therefore, the performance of test subjects
in the bimodal case might have been limited by
the difficulty of maintaining a standard in memory
against which to judge the inputs, rather than by
the influence of a second modality itself.
the Perceptual cycle
Neisser's model of the Perceptual Cycle describes
perception as a setup of schemata, perceptual
exploration and stimulus environment (Farris,
2003). These elements influence each other in a
continuously updated circular process, see Figure
1. Thus, Neisser's model describes at a very ab-
stract level how the perception of the environment
is influenced by background knowledge, which in
turn is updated by the perceived stimuli.
In Neisser's model, schemata represent an
individual's knowledge about the environment.
Schemata are based on previous experiences and
Figure 1. The Perceptual Cycle after Neisser. (Adapted from Farris, 2003)
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