The cost of parallel processing in the human visual system
Date
2016
Authors
Rideaux, Reuben
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Abstract
Our environment is visually rich, containing a multitude of
objects that can be
defined by many different features, e.g. shape, colour, and
motion. To navigate and
interact with the environment, we must process this information
efficiently. The human
visual system can process information either serially or in
parallel. While there is a clear
timesaving benefit of parallel processing, its cost is less well
understood. Consequently,
the aim of this thesis is to address three key theoretical
questions underlying the cost of
parallel processing.
The first aim was to determine how the capacity of parallel
processing varies as a
function of the detail of information extraction. Previous
research has demonstrated that
brief presentations of five and six motion signals can be
differentiated; this suggests that
up to five signals can be simultaneously processed. However, it
is unclear how much
information is being extracted, i.e. whether observers are
extracting direction information
from all five signals. To examine this we presented observers
with multiple moving
objects and evaluated their parallel processing capacity as a
function of the information
required to perform the task. We found that the resolution of
parallel motion processing
varies as a function of the information that is extracted;
specifically, as information
extraction becomes more detailed, the capacity to process
multiple signals is reduced.
The second aim was to investigate whether there is a cost to the
fidelity of
information that is processed in parallel. Previous research
suggests that there may not be
a cost associated with parallel consolidation of information from
sensory to visual shortterm
memory (VSTM). Here we examined this by first determining that
motion direction,
and possibly orientation, can be consolidated in parallel, then
explicitly evaluating the
cost to the fidelity of information consolidated in parallel,
compared to serially. We found
that there is a twofold cost associated with parallel
consolidation: a reduction in resolution
of encoded items due to spreading of spatial attention, and an
increase in the likelihood
of consolidation failure due to interference between items.
The third aim was to examine whether the cost associated with
parallel processing
can ultimately explain its capacity. We extended our previous
findings regarding the cost
associated with parallel consolidation to examine whether the
capacity of parallel
consolidation results from biased competition, the same mechanism
proposed to account
for spatial attention and VSTM storage, as evidenced from the
interference between items
presented simultaneously. This was achieved by demonstrating that
parallel consolidation
performance is influenced by factors predicted by a biased
competition model.
Furthermore, we found evidence suggesting that the capacity may
be as high as three,
with increasingly poorer resolution and higher consolidation
failure-rates.
Together, these results demonstrate that a) parallel processing
is limited by the
complexity of information to be processed, b) there is a twofold
cost of processing
information in parallel, and c) that increasing the amount of
information processed in
parallel also increases this cost to the fidelity of the
information and ultimately leads to
the capacity of this process.
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Keywords
parallel processing, parallel consolidation, visual working memory, VWM, visual short-term memory, VSTM, motion, orientation, biased competition
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