Subsequent Experiments Concerning Visual Search Tasks example essay topic

... Theories of Visual Search 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 pre attentive stage. During the pre attentive 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 pre attentive 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 g eons as the T's. This illustrates that line orientations are important features in the pre attentive 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 'distract or' objects.

Her hypothesis was that if pre attentive 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 distr actors 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. 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 distr actors. 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 distr actors.

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 distr actors (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 distr actors. The target was stood out from the distr actors. In contrast, when the target lacked the line, the search time increased linearly with the number of distr actors. 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 distr actors. A curved line target stood out from straight line distr actors. 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 distr actors. In the remaining displays, the target was different from the distr actors 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. Arn i 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 distract or 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. Kristjannson 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.