Theories of Visual Search
A standard theory of visual search tasks assumes that when a person searches
for a target in an array of other items, memory is used in locating the target.
The following analysis of three articles shows that there is both strong
support for this highly respected theory and evidence that this theory may have
some flaws in reasoning.
In the article \"Features and Objects in Visual Processing,\" Anne
Treisman states that there are two theoretical levels of visual processing. In
the first level of visual processing, certain components of visual information
are processed instantly and unconsciously. A person does not have to
concentrate on individual parts of the scene. This stage of processing is
called the preattentive stage. During the preattentive stage, the light
received by the visual receptors is translated in to the lines, curves, colors
and textures of the objects. Within the brain, there are two distinct visual
areas that specialize in different areas of processing. The first area
processes lines, curves, color, and texture and other areas deal with movement.
After this initial processing occurs, another area of the brain processes the
more complex and distinct qualities of a scene. Then, all of the components of
the objects in the scene are recombined into whole objects. In order to support
the preattentive stage of visual processing, Treisman devises that parts of
objects that belong to the same object share similar attributes. Attributes
such as color, continuity of lines and curves that define the boundaries
between objects. She uses an experiment to determine which properties of a
visual stimulus make its boundaries stand out from other similar objects. The
properties of an object that make it stand out are used by the visual
processing system in distinguishing the object from ground. In actuality,
boundaries are conspicuous between components that are distinctive in basic
properties such as color, brightness and line orientation but not in the way
their properties are connected or grouped. In an experiment showing evidence of
this principle, subjects are shown a picture in which a region of T’s easily
distinguishes itself from an area of slanted T’s but not from a region of
backwards L’s that are constructed of the same geons as the T’s. This
illustrates that line orientations are important features in the preattentive
stage of visual processing and that the specific configurations or conjunctions
of lines are not. Next, the subjects are shown an array of red O’s and blue V’s
and red V’s and blue O’s. A clear boundary between the O’s And V’s is apparent.
This indicates that basic shape properties, such as line curvature, are
important. A boundary between blue letters and red letters is noticeable. This
condition implies that color is important in creating boundaries. However, a
boundary between red V’s and blue O’s and red O’s and blue V’s is not
conspicuous. Early visual processing involves individual properties but not
conjunctions of properties. These results allude to the fact that analysis of
parts and properties occurs before they are recombined into whole objects. If
these components are noticed before they are joined with objects, they exist on
their own. This fact leads to the possibility of errors in synthesis.
One of the experiments that Treisman uses to show the possibility of errors in
synthesis involves illusory conjunctions. In an illusory conjunction,
properties of two different objects appear to belong together. In this
experiment, subjects were shown a schematic that contained a dollar sign among
a series of S’s and slanted lines and a series pointed shapes that when
conjoined with the lines, form triangles. Then, the subjects were shown a
similar schematic that did not include a triangle. They reported the presence
of dollar signs even though no signs were present. They also saw dollar signs
when they were shown a diagram that contained only S’s and triangles. The
results of this experiment propose that early visual processing detects
features regardless of their location.
Treisman conducted other experiments to prove the existence of early visual
processing. In one of these experiments, she used visual search tasks in which
subjects were asked to locate a target object that was surrounded by other
\"distractor\" objects. Her hypothesis was that if preattentive
processing happens automatically across the complete visual field, a target
that is strikingly different from the objects in its vicinity will stick out.
If a target differs from the distractors in a basic property such as
orientation or curvature, the target is located in about the same time in a
series of a few items as in a series of several items. Thus, the time it takes
to find the target is not influenced by the number of disractors.
Conversely, if a target is distinguished only by a conjunction of properties or
if it is characterized by its specific combination of components, the time it
takes to find the target or rule out the presence of the target increases with
the number of distractors. Subjects involved in these experiments are forced to
focus on each item in the schematic in order to figure out how the properties
are conjoined. In a trial in which a target is not present, it takes subjects
longer to notice the lack of the target, because they have to analyze all of
the distractors. In subsequent experiments concerning visual search tasks,
subjects were presented a situation in which they had to locate a target
distinguished by its lack of a feature that is present in the other distractors(i.e.
a diagram of O’s and Q’s). In this case the discriminating factor between the
O’s and Q’s is that one object is a circle and the other is a circle
intersected by a line segment. Selected results from the experiment indicated a
significant difference in the search time based on whether the target was a Q
or an O. When the target was the contained the line, the search time was not
related to the number of distractors. The target was stood out from the
distractors. In contrast, when the target lacked the line, the search time
increased linearly with the number of distractors. The search time increased in
this condition because the items had to be scanned serially. The results of
this experiment defend the theory that a combined neural signal early in visual
processing conveys the presence but not the absence of a distinctive feature.
Treisman theorizes that this evidence illustrates possible feature maps. These
experiments dealt with simple properties of line orientation and curvature. A
tilted line was more distinguishable from vertical line distractors. A curved
line target stood out from straight line distractors. The converse targets did
not stand out.
In the second section of the article, Treisman discusses how focused attention
is necessary for combining features in a given location of a scene and for
making structured representations of objects and their spatial relationships
with other objects. A piece of evidence that shows that conjunctions need
attention comes from experiments in which subjects identified a target in a
display and commented on its location. In one type of display, a basic feature
distinguishes the target from the distractors. In the remaining displays, the
target was different from the distractors in the way its features were attached.
Treisman and her colleagues hypothesized that subjects would be able to
identify a target based on an individual feature. However, they would
incorrectly state the location of the feature.
In the last part of the Treisman study, an experiment was carried out to
explore the role of prior knowledge in the conjoining of properties. The study
focused on illusory conjunctions. Results of the study indicated that prior
knowledge and expectations help a person to use attention effectively to
combine features, but prior knowledge does not influence illusory exchanges of
features to make abnormal objects appear normal. In short, illusory
conjunctions seem to arise at a stage of visual processing that happens before
conjunctions that do not make sense are corrected in the late stages of visual
processing.
At the end of this article, Anne Treisman devises a model of visual processing.
The visual system commences by coding a certain number of simple and useful
properties in a series of feature maps. These maps preserve the spatial
relationships of the properties. However, spatial relationships are not readily
available to the late states of visual processing since this part of processing
occurs automatically. Focused attention is important in the late stages of
visual processing. In this stage the location of the objects is coded.
Attention uses this information , simultaneously selecting , via links to the
individual feature maps, all the features that are currently present in a
specific location. These are stored in a temporary file. The model now assumes
that the integrated information about the properties and structural relations
in each object file is compared with objects that are recognized due to prior
knowledge. Conscious awareness depends on representations that integrate
information about particular objects from the analyses of sensory features and
from prior knowledge and constantly refresh the information. When a significant
discontinuity in space or time occurs, the original file on an object is eliminated
if it no longer is relevant to the visual experience. The object then
disappears and the visual system commences processing of new visual stimuli.
An article that adds a new perspective to how visual systems operate is titled
\"Visual Search has no memory.\" The research in this article was
conducted by Todd. S. Horowitz and Jeremy Wolfe. A series of experiments were
run in which participants were asked to pick out the letter T among a screen of
L’s. In one version of the experiment, the researchers relocated the letters
several times a second, which was too rapid for the visual system to follow.
Remarkably, the subjects looking at scrambled screen found the constantly
\"drifting\" letter as quickly as those looking at stationary screens.
It seems that the brain does not waste any memory space storing information in
short-term visual tasks, responding instead to whatever happens to be seen from
moment to moment. Other aspects of memory compensate and keep a constant image
intact. In other words, people exhibit a kind of visual amnesia when doing
these kinds of searches. The visual system remains locked into an ongoing
present state of awareness, noting what it sees at any given moment but
retaining no record of what it looked at before or what it saw there. The
results of the experiment show that the visual system does not accumulate
information about object identity over time. These new findings somewhat
discredit a standard theory that states that observers use memory for locations
when searching for objects.
Most theories of visual search assume that when observers search a complex
display for a target, the display items are scanned systematically and the same
locations are not analyzed repeatedly. Arni Kristjannson discusses the flaws in
the research findings of Wolfe and Horowitz in the \"In Search of
Remembrance: Evidence for Memory in Visual Search.\" Kristjannson conducts
an experiments similar to those of Wolfe and Horowitz. In the first experiment,
there is a static condition and a relocation condition. In the relocation
condition, the target was moved to a position previously occupied by a
distractor several times per second. In the static condition the display items
remained in the same location in the series of frames shown. In both conditions
were randomly rotated in each successive frame. The purpose of using these two
conditions is to analyze whether location-based memory is used during visual
search since the only difference between the two conditions was that the
location of the target fluctuated with each successive frame in the relocation
condition. Results showed that when the target was reallocated, it took longer
for subjects to locate the target. These results strongly support the role of
memory in visual search tasks. Kristajannson points out some major divisions in
the results of this experiment and those of Horowitz and Wolfe. First,
locations were constantly reused in the present experiment. In Horowitz and
Wolfe’s experiment, relocated items appeared randomly anywhere within the
display, often appearing in locations that were previously lacked and item.
Also, there were no appearances of items in locations that were previously
blank in the present experiment, whereas in Wolfe’s experiment, items were
continuously popping up in blank locations. Lastly, the results of Horowitz and
Wolfe’s experiment may not even be related to the models of search involved in
the present visual search task.
There is a series of potential flaws in the experiments used in each of the
three articles to support theories of visual search. First, the subjects may be
overcome with boredom from the repetitiveness of the experiment and may give an
answer in order to complete the experiment as fast as possible. The subjects
may also become confused with the constant rotation of the targets in some
experiments and they may make up an answer. The methods of these experiment do
not take into the account the fact that some individuals perform better in
visual search tasks than others. Lastly, the physical, mental, and emotional
state of the subjects during the time of the experiment may influence the
results. For example, a person who is mentally stimulated during the experiment
may respond to stimuli at a faster rate that someone who is physically
exhausted. Also, individuals who perform well on visual search tasks are more
likely to volunteer to participate in visual task experiments. It is not known
whether or not these external factors have an effect on the results of these
experiments. However, the visual system seems to operate consistently
regardless of the intrusive factors.
Theories of Visual Search
A standard theory of visual search tasks assumes that when a person searches
for a target in an array of other items, memory is used in locating the target.
The following analysis of three articles shows that there is both strong
support for this highly respected theory and evidence that this theory may have
some flaws in reasoning.
In the article \"Features and Objects in Visual Processing,\" Anne
Treisman states that there are two theoretical levels of visual processing. In
the first level of visual processing, certain components of visual information
are processed instantly and unconsciously. A person does not have to
concentrate on individual parts of the scene. This stage of processing is
called the preattentive stage. During the preattentive stage, the light
received by the visual receptors is translated in to the lines, curves, colors
and textures of the objects. Within the brain, there are two distinct visual
areas that specialize in different areas of processing. The first area
processes lines, curves, color, and texture and other areas deal with movement.
After this initial processing occurs, another area of the brain processes the
more complex and distinct qualities of a scene. Then, all of the components of
the objects in the scene are recombined into whole objects. In order to support
the preattentive stage of visual processing, Treisman devises that parts of
objects that belong to the same object share similar attributes. Attributes
such as color, continuity of lines and curves that define the boundaries
between objects. She uses an experiment to determine which properties of a
visual stimulus make its boundaries stand out from other similar objects. The
properties of an object that make it stand out are used by the visual
processing system in distinguishing the object from ground. In actuality,
boundaries are conspicuous between components that are distinctive in basic
properties such as color, brightness and line orientation but not in the way
their properties are connected or grouped. In an experiment showing evidence of
this principle, subjects are shown a picture in which a region of T’s easily
distinguishes itself from an area of slanted T’s but not from a region of
backwards L’s that are constructed of the same geons as the T’s. This
illustrates that line orientations are important features in the preattentive
stage of visual processing and that the specific configurations or conjunctions
of lines are not. Next, the subjects are shown an array of red O’s and blue V’s
and red V’s and blue O’s. A clear boundary between the O’s And V’s is apparent.
This indicates that basic shape properties, such as line curvature, are
important. A boundary between blue letters and red letters is noticeable. This
condition implies that color is important in creating boundaries. However, a
boundary between red V’s and blue O’s and red O’s and blue V’s is not
conspicuous. Early visual processing involves individual properties but not
conjunctions of properties. These results allude to the fact that analysis of
parts and properties occurs before they are recombined into whole objects. If
these components are noticed before they are joined with objects, they exist on
their own. This fact leads to the possibility of errors in synthesis.
One of the experiments that Treisman uses to show the possibility of errors in
synthesis involves illusory conjunctions. In an illusory conjunction,
properties of two different objects appear to belong together. In this
experiment, subjects were shown a schematic that contained a dollar sign among
a series of S’s and slanted lines and a series pointed shapes that when conjoined
with the lines, form triangles. Then, the subjects were shown a similar
schematic that did not include a triangle. They reported the presence of dollar
signs even though no signs were present. They also saw dollar signs when they
were shown a diagram that contained only S’s and triangles. The results of this
experiment propose that early visual processing detects features regardless of
their location.
Treisman conducted other experiments to prove the existence of early visual
processing. In one of these experiments, she used visual search tasks in which
subjects were asked to locate a target object that was surrounded by other
\"distractor\" objects. Her hypothesis was that if preattentive
processing happens automatically across the complete visual field, a target
that is strikingly different from the objects in its vicinity will stick out.
If a target differs from the distractors in a basic property such as
orientation or curvature, the target is located in about the same time in a
series of a few items as in a series of several items. Thus, the time it takes
to find the target is not influenced by the number of disractors.
Conversely, if a target is distinguished only by a conjunction of properties or
if it is characterized by its specific combination of components, the time it
takes to find the target or rule out the presence of the target increases with
the number of distractors. Subjects involved in these experiments are forced to
focus on each item in the schematic in order to figure out how the properties
are conjoined. In a trial in which a target is not present, it takes subjects
longer to notice the lack of the target, because they have to analyze all of
the distractors. In subsequent experiments concerning visual search tasks,
subjects were presented a situation in which they had to locate a target
distinguished by its lack of a feature that is present in the other
distractors(i.e. a diagram of O’s and Q’s). In this case the discriminating
factor between the O’s and Q’s is that one object is a circle and the other is
a circle intersected by a line segment. Selected results from the experiment
indicated a significant difference in the search time based on whether the
target was a Q or an O. When the target was the contained the line, the search
time was not related to the number of distractors. The target was stood out
from the distractors. In contrast, when the target lacked the line, the search
time increased linearly with the number of distractors. The search time
increased in this condition because the items had to be scanned serially. The
results of this experiment defend the theory that a combined neural signal
early in visual processing conveys the presence but not the absence of a
distinctive feature. Treisman theorizes that this evidence illustrates possible
feature maps. These experiments dealt with simple properties of line
orientation and curvature. A tilted line was more distinguishable from vertical
line distractors. A curved line target stood out from straight line
distractors. The converse targets did not stand out.
In the second section of the article, Treisman discusses how focused attention
is necessary for combining features in a given location of a scene and for
making structured representations of objects and their spatial relationships
with other objects. A piece of evidence that shows that conjunctions need
attention comes from experiments in which subjects identified a target in a
display and commented on its location. In one type of display, a basic feature
distinguishes the target from the distractors. In the remaining displays, the
target was different from the distractors in the way its features were
attached. Treisman and her colleagues hypothesized that subjects would be able
to identify a target based on an individual feature. However, they would
incorrectly state the location of the feature.
In the last part of the Treisman study, an experiment was carried out to
explore the role of prior knowledge in the conjoining of properties. The study
focused on illusory conjunctions. Results of the study indicated that prior
knowledge and expectations help a person to use attention effectively to
combine features, but prior knowledge does not influence illusory exchanges of
features to make abnormal objects appear normal. In short, illusory
conjunctions seem to arise at a stage of visual processing that happens before
conjunctions that do not make sense are corrected in the late stages of visual
processing.
At the end of this article, Anne Treisman devises a model of visual processing.
The visual system commences by coding a certain number of simple and useful
properties in a series of feature maps. These maps preserve the spatial
relationships of the properties. However, spatial relationships are not readily
available to the late states of visual processing since this part of processing
occurs automatically. Focused attention is important in the late stages of
visual processing. In this stage the location of the objects is coded.
Attention uses this information , simultaneously selecting , via links to the
individual feature maps, all the features that are currently present in a
specific location. These are stored in a temporary file. The model now assumes
that the integrated information about the properties and structural relations
in each object file is compared with objects that are recognized due to prior
knowledge. Conscious awareness depends on representations that integrate
information about particular objects from the analyses of sensory features and
from prior knowledge and constantly refresh the information. When a significant
discontinuity in space or time occurs, the original file on an object is
eliminated if it no longer is relevant to the visual experience. The object
then disappears and the visual system commences processing of new visual
stimuli.
An article that adds a new perspective to how visual systems operate is titled
\"Visual Search has no memory.\" The research in this article was
conducted by Todd. S. Horowitz and Jeremy Wolfe. A series of experiments were run
in which participants were asked to pick out the letter T among a screen of
L’s. In one version of the experiment, the researchers relocated the letters
several times a second, which was too rapid for the visual system to follow.
Remarkably, the subjects looking at scrambled screen found the constantly
\"drifting\" letter as quickly as those looking at stationary
screens. It seems that the brain does not waste any memory space storing
information in short-term visual tasks, responding instead to whatever happens
to be seen from moment to moment. Other aspects of memory compensate and keep a
constant image intact. In other words, people exhibit a kind of visual amnesia
when doing these kinds of searches. The visual system remains locked into an
ongoing present state of awareness, noting what it sees at any given moment but
retaining no record of what it looked at before or what it saw there. The
results of the experiment show that the visual system does not accumulate
information about object identity over time. These new findings somewhat
discredit a standard theory that states that observers use memory for locations
when searching for objects.
Most theories of visual search assume that when observers search a complex
display for a target, the display items are scanned systematically and the same
locations are not analyzed repeatedly. Arni Kristjannson discusses the flaws in
the research findings of Wolfe and Horowitz in the \"In Search of
Remembrance: Evidence for Memory in Visual Search.\" Kristjannson conducts
an experiments similar to those of Wolfe and Horowitz. In the first experiment,
there is a static condition and a relocation condition. In the relocation
condition, the target was moved to a position previously occupied by a
distractor several times per second. In the static condition the display items
remained in the same location in the series of frames shown. In both conditions
were randomly rotated in each successive frame. The purpose of using these two
conditions is to analyze whether location-based memory is used during visual
search since the only difference between the two conditions was that the
location of the target fluctuated with each successive frame in the relocation
condition. Results showed that when the target was reallocated, it took longer
for subjects to locate the target. These results strongly support the role of
memory in visual search tasks. Kristajannson points out some major divisions in
the results of this experiment and those of Horowitz and Wolfe. First,
locations were constantly reused in the present experiment. In Horowitz and
Wolfe’s experiment, relocated items appeared randomly anywhere within the
display, often appearing in locations that were previously lacked and item.
Also, there were no appearances of items in locations that were previously
blank in the present experiment, whereas in Wolfe’s experiment, items were
continuously popping up in blank locations. Lastly, the results of Horowitz and
Wolfe’s experiment may not even be related to the models of search involved in
the present visual search task.
There is a series of potential flaws in the experiments used in each of the
three articles to support theories of visual search. First, the subjects may be
overcome with boredom from the repetitiveness of the experiment and may give an
answer in order to complete the experiment as fast as possible. The subjects
may also become confused with the constant rotation of the targets in some
experiments and they may make up an answer. The methods of these experiment do
not take into the account the fact that some individuals perform better in
visual search tasks than others. Lastly, the physical, mental, and emotional
state of the subjects during the time of the experiment may influence the
results. For example, a person who is mentally stimulated during the experiment
may respond to stimuli at a faster rate that someone who is physically
exhausted. Also, individuals who perform well on visual search tasks are more
likely to volunteer to participate in visual task experiments. It is not known
whether or not these external factors have an effect on the results of these
experiments. However, the visual system seems to operate consistently
regardless of the intrusive factors.