HEMISPHERIC to work out whether global precedence

HEMISPHERIC DIFFERENCES AND PRIORITY
BETWEEN LOCAL AND GLOBAL PROCESSING

 

(ABSTRACT)

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Our research aims to work out
whether global precedence plays a major role in visual perception. It would
also indicate whether this means that the right hemisphere can process the
larger picture faster than the left hemisphere can process the finer details. The
experiment that we are conducting is a replication of a part of David Navon’s
global precedence investigation mentioned in his paper, (Navon, 1977).

In the
experiment, the subjects had to respond to a visual stimulus in the form of a
letter on a screen. Their reaction time was recorded and results were formed
from this. The results showed that the right hemisphere was slightly faster at
processing the global information which was represented by the faster reaction
time when both the local and global detail was corresponding. The findings have
been similar to the research of Navon who proved that the right hemisphere
processes global information faster than the local information.

 (INTRODUCTION)

Perception is commonly thought of as ability to interpret
information that sensory organs transmit as well as using prior knowledge to
form the basis of understanding. This also includes how a subject would respond
to the information received from the environment. The process of perceiving is
very complex and uses multiple specialised brain structures in order to form a
reaction to a certain stimulus. A major problem for psychologists is to
explicate how sensory inputs are somehow converted into perceptions of real
objects to “form the basis of perceptual experiences”, (McLeod, 2008).

Visual information is processed through the visual cortex
part of the brain, near the back end of the brain. The cerebral cortex is the
largest part of the brain and plays a key role in awareness, language, memory,
attention and many other processes, (Kandel, Schwartz, Jessell, 2000). When visual information is processed, the
stimulus enters through the optic nerve and the signal is sent down the
occipital lobe, located at the back of the cerebral cortex. Just before the
signal is received by the occipital lobe, the signal is taken through the
lateral geniculate nucleus which separates the data into a stream that contains
colours and fine details. 1

It is often assumed that the human
brain processes the global and local properties of visual stimuli in a
localised fashion, with the left hemisphere specialising in local detail, and
the right hemisphere specialising in global formations. 

Visual processing has been divided into two different
categories: top-down and bottom-up. It would make sense to combine both
theories together to form the basis of a conventional theory, however, over the
years, theorists have chosen to side with one or the other with some
emphasising on top-down processing and the others focusing on bottom-up
processing.

Gregory’s constructivists theory stated that perception is a
“constructive process which relies on top-down processing”, (Gregory, 1970).
Information that we acquire from the environment is often too ambiguous for us
to understand and comprehend, so we require higher cognitive knowledge or
experiences to make inferences about what we perceive. Top-down is the
reference to the use of the contextual information to understand what we
perceive. An example of this theory is the fact that we can understand
difficult handwriting within a sentence better than isolated words. 2

In contrast to Gregory’s concept, Gibson introduced the ‘direct
perception’ approach. (Gibson,
1972) His theory suggested that environmental data is all that is
necessary for visual perception and that there is no need for any other pieces
of information to perceive our surroundings. According to McLeod, Gibson’s
bottom-up system implies that perception “involves innate mechanisms forged by
evolution and that no learning is required” (McLeod, 2008).

 (EXISTING RESEARCH)

David Navon suggested that we perceive global features and
then progress towards a more detailed analysis of local features. He also
explored the relationship between local and global perception which is
mentioned in his report, (Navon,
1977). Navon’s experiment’s
aim was to test global precedence in perception by measuring the reaction time
of the global processing and comparing it to the reaction time of the local
processing. He did this by asking subjects to respond to images that consisted
of larger letters made up of smaller letters. Sometimes the smaller letters
would be correspondent to the larger letters and sometimes they would not.

A great number of different studies have been conducted, to
further improve our understanding of the complex procedure of visual
processing. An example of this is Rebecca Chamberlain’s research into artists
and observational drawings. Individuals with enhanced talents in visual art
have been shown to display heightened local visual processing skills. The aim
of Chamberlain’s study was to “assess whether local processing biases
associated with drawing ability result from a reduced ability to cohere local
stimuli into global forms, or an increased ability to disregard global aspects
of an image”, (Chamberlain,
McManus, Riley, Rankin, Brunswick, 2013). This helps reveals the brains
ability to prioritise the certain parts for local and global processing, and
also the reason as to why it would do so. It links into our investigation as it
also displays why artists with better local processing skills and dominance in
the right hemisphere would produce better, detailed pieces of artwork.

Another study, by Lamb and
Robertson, investigates whether the reaction time is the correct variable to
measure in order to understand the processing order and hemispheric dominance
of global and local information, (Lamb et al, 1989). The
experiments conducted by Lamb supported the statements by Navon which suggests
that the right hemisphere is dominant in global processing and the left
hemisphere dominant in the local processing. However, it also considers the
interference of global distractors on the processing of the local information.

This report by Lamb goes hand in
hand with another study by Shihui Han. Han’s report questions the brain’s
processing by examining “the neural mechanisms of functional asymmetry between hemispheres
in the processing of global and local information”, (Han et al,
2001). Shihui Han used functional magnetic resonance imaging (fMRI) to
view specific parts of the head and map the operational sections when
processing information. From this, he could confirm Navon’s study on the
function of both sides of the brain when processing visual stimuli.

The aim of the experiment we
conducted was to test the ability of people to recognise smaller letters which
formed a larger product; which would show whether the brain prioritises global
or local processes. This experiment would also demonstrate the repeatability of
Navon’s Experiment.

 

 

(METHOD)

(Participants)

For the experiment, 9 healthy
students (5 males and 4 females, aged 16/17) volunteered to participate in the
experiment from The West Bridgford 6th Form. Two sessions were
administered which consisted of a practice session and the main recorded test.

(Materials)

For the experiment, the congruency
of the stimuli shown to the participants was the independent variable and the
dependent variable was the accuracy and reaction time for the responses. The
accuracy and time was recorded by a device automatically using software onto a
spreadsheet for greater accuracy.

The device used to conduct the
experiment was a Lenovo ThinkPad laptop which was running PyschoPy (Version 2x).
The software used to conduct the experiment was a python program which was
linked to an Excel spreadsheet that recorded results. The spreadsheet was also
used to work out the averages of the large set of results.

In total, four different stimuli
were used in order to test the processing of local details. The characters H
and S were used in order to test this in congruent and incongruent forms.
However, the format of the stimuli changed randomly and included a large S and
H consisting of smaller formations of the same letters. Congruent forms
consisted of the larger letter consisting of the same smaller letter, meaning
that the global and local stimuli were the same. On the other hand, the
incongruent form comprised on different global and local stimuli, including
both S consisting of H and H consisting of S.

(Design)

Before the test, the subjects were given the opportunity to
practise through a series of preparation questions that allowed them to
familiarise themselves with the experiment. After this the test followed in the
format mentioned below.

A fixation cross appeared in the centre of the screen for 1
second, each time, in order to ensure that the subjects visual field was at the
centre of the screen and that they were prepared. Then, the stimulus appeared
for 200 milliseconds at a random point on the screen before being
backward-masked for 5 seconds. Backwards masking was used in order to ensure
that no trace of the image was left in the subject’s mind as well as on the screen.
The stimulus appeared in any of the 4 quadrants at random, with no hint or
indication as to where the next stimulus would appear. The subjects had 7
seconds to respond to the appearance and to press the corresponding button of
the smaller letters which formed the larger letter.

During the test, the subjects were exposed to the stimuli 64
times in total. The 2 different types of congruent and incongruent forms were
shown 16 times each. The position of the stimuli varied every time in order to
prevent the subject from devising a strategy and predict the positioning of the
stimuli as this would invalidate the research.

The subjects responded using the S and H keys on the
keyboard and were told that their reaction time would be recorded; but the
candidates were given a 7 second time limit to respond. This was done using an
Excel spreadsheet which also allowed us to work averages and produce visual
interpretations of the data in the form of a graph.

(Procedure)

The
experiment was taken in a regular school classroom, in the presence of 5 other
members for the first experiment and 4 other members for the second experiment.
The participant conducted the experiment on a table, with the laptop positioned
centrally, just below eye level but in a comfortable position for the user.

The short
practice session at the start of the experiment was conducted in a relaxed
environment where the participant was able to talk to others and ask for advice
on how to perform the test. The second part to the experiment was done with no
interference with the subject directly. However, the other members in the room
were free to have conversations amongst themselves. This, as well as other
factors would have had a effect on the outcome of the experiment and should
have been controlled for more accurate results.

(Data)

The results gathered from the experiment were recorded onto
a dedicated spreadsheet. The computer program on the laptop measured the type
of stimulus that was shown on the screen, the accuracy and the reaction time. The
standard deviation of the averages was calculated to allow us to compare the
stimuli and accuracy. The outliers in the experiment were included in the data
collection and the averages, despite the fact that they may have had a small
influence on the averages and the standard deviation.

(RESULTS)

From the data gathered in the experiment, the mean and
standard deviation was worked out. Also, P-values were calculated which is the
marginal significance in a hypothesis, showing the degree to which the results were
significant.

The results for the averaged accuracy are shown below in Figure 1; represented in the form of a bar chart and the reaction time is
also included in a table in Table 1.
The mean accuracy is greater for congruent stimuli when compared to incongruent
stimuli. For congruent stimuli, the percentage accuracy was 84.7% (SD +/-16.3%)
whereas, on the other hand, for incongruent stimuli, the percentage accuracy
was only 79.9% (SD +/- 18.5%). This meant there was a 4.8% greater accuracy for
the congruent similarity.

Table 1 –
Percentage accuracy of the responses as well as the average reaction time
(RT – milliseconds) for both the congruent and incongruent stimuli in the
form of a table.

 

 

 

 

Figure 1 –
Percentage accuracy of the responses for both the conflicting and
consistent stimuli in the form of a bar chart

 

 

 

 

 

When presented in the form of a bar chart in Figure 1, the variance seems to be
quite significant. However, the results achieved from a T-test, to calculate
the P-value, shows that the accuracy difference between the consistent and inconsistent
stimuli recognition is not substantial. The P-value (p = 0.528), as shown in Table 2 indicated to us that p>0.05,
revealing that the difference is not momentous.

On the other hand, the P-value of the reaction time (p =
0.015), shown in Table 2, between
the congruent and incongruent stimuli is less than the standard value of 0.05.
We can see from this that there is a significant difference for the reaction
time for the two stimuli. The average values obtained for the reaction time had
a difference of 62.3ms. As shown in Table
1, the incongruent stimuli had an average time of 635.5ms (SD +/- 119.1ms),
whereas for the congruent stimuli, it was faster at 573.2ms (SD +/- 136.2ms).

We also calculated the difference in accuracy and reaction
time between the left and right of the screen. This gave us an insight into the
idea of left or right dominance of the hemispheres. The variance was not
significant for both variables as p>0.05 and the difference between the
accuracy was only 2%; greater on the right-hand side (83.3%) of the screen when
compared to the left-hand side (81.3%). As for the reaction time, 613.6ms for
the left-hand side compared to the 595.1ms for the right. This was only a
marginal difference which would be expected on a small-scale experiment.

 

 

 

Table 2 – P-values
indicating the significance of the different variables. *Highlighted value
for the reaction time shows that the congruent and incongruent stimuli have
a significant effect on this dependent variable as it has a value below
0.05.

 

Figure 2 –
Reaction time (milliseconds) for both the conflicting and consistent
stimuli represented as a bar chart.

 

 

 

 

 

 

 

 

(Discussion)

Before the experiment, we collectively predicted that the
reaction time and the accuracy would be greater and faster for congruent
stimuli. This was due to the fact that it would be easier for the subjects to
process the global image first if it consisted of the same information on a
local level, and on the whole, it would decrease the time needed to process the
information.

Consequently, the results support our hypothesis and also
indicate to us that the results that Navon acquired were reproducible. As
mentioned in his paper, Navon’s data indicated that “global difference are more
frequently detected than local differences”, (Navon, 1977). The data collected by us also
supports this statement, nonetheless to a smaller extent. The reaction times
differed for the congruent and incongruent stimuli; the congruent
representations being faster of the two. The accuracy indicated the same
principle.

Nevertheless, the experiment had many limitations that had a
great influence on the results achieved. First of all, the procedure of
recognising letters consisting of smaller letters on a device not a common
everyday task. This means that it is not possible to use this research to form
a representation of the process of visualisation.  To improve the findings, we would have to
research common tasks that specify the visual processing of the brain.

The experiment was also conducted on a very small scale.
Using only 11 students may give us a small trend to form the basis of our
conclusion on, but it is not representative of a larger population. To improve
the accuracy of the experiment, it would have to be conducted on a much larger
scale. This would also allow us to find any anomalous values within the
results; which were omitted in this research.

 

 

 

References:

·       
Chamberlain,
McManus, Riley, Rankin, Brunswick. (2013) –
Local processing enhancements associated with superior observational drawing
are due to enhanced perceptual functioning, not weak central coherence, 66, (7), pg. 1448-1466.

·       
Gibson. (1972) – A theory of
direct visual perception, in J. Royce, W. Rozenboom (eds). The psychology of
knowing. New York: Gordon and Breach.  

·       
Gregory. (1970) – The intelligent
eye. London: Duckworth.

·       
Kandel, Eric R, Schwartz, James H, Jessell, Thomas M.
(2000) – Principles of Neural Science, (4), pg. 324.

·       
Lamb, Robertson, Knight. (1989) – Attention and
interference in the processing of global and local information: effect of
unilateral temporal-parietal junction lesions, 27, (4), pg. 471-483.

·       
Navon (1977) – Forest Before Trees: The Precedence of
Global Features in Visual Perception. Cognitive
Psychology, 9, (3), pg. 353-383.

·       
McLeod (2008) – Visual
Perception Theory – https://www.simplypsychology.org/perception-theories.html.

·       
1 https://www.brainhq.com/brain-resources/brain-facts-myths/how-vision-works.

·       
2 The Asymmetric Brain
Scholar’s Program Booklet – pg. 34.

 

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