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Have you ever wondered how we are able to detect our friends in a overly crowded areas full of lots of different faces or find our favorite mouth watering chocolate bar stacked among other endless food products? These tasks are often accomplished by the brain carrying out attention and visual searches for particular objects within a given environment at a given time.  Some of these tasks can be more difficult than others and require more time in order to carry out certain tasks, but the question that needs to be addressed is why some tasks are more difficult than others?In this article I will attempt to explain some of the possibilities that may bring about these difference. One should note that these are only theories at the moment.


Studies have revealed there are two important mechanisms in place, these are used in combination with each other when most simple visual searches are carried out. Bottom up activation involves searching for items that grab our attention, these are usually objects which are unique to look at compared to the environment which they are. An example could be a red circle among a group of green circles or a single tick in a group of crosses. Top down activation is often used when we are searching for a specific object within an environment, this mechanism often takes place when the search is not simple. The attention of a subject will only be captured if something is unique or irrelevant when they are set up to use bottom up guidance. If however, you are searching for a particular image or something very specific and are relying on top down information, then something unique and irrelevant will not capture your attention. However, visual search may not always be as simple as this.


It is important to understand why our attention is often captured. For example, if I am to search for my small blue Daweoo Matiz in a car park, I may end up getting distracted if I saw a large yellow truck.  The yellow truck is this case is irrelevant and not necessary because i am looking for my car. I might have my attention captured by the truck for a number of reasons. This may be because the yellow truck is physcologically, larger and brighter in size and therefore easier to detect.  This also shows that i am using bottom up information to look for my car but it is not being particularly useful because its causing me to become distracted.  It is important to realise physically, there is a lot of information about the truck coming into my visual system because of its large size and brightness, this causes me to be increasing aware of the object.

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Dr Michael Proulx has focused his work on finding out if there are individual differences in the abilities to attend to information or if there is a greater capacity for inhibiting information that is distracting. He has looked at visual search system  in normal individuals and people suffering from autistic disorders .  Subjects who do not have the disorder are asked to look for a unique item such as a green bar among blue bars, the results showed their response times were fairly efficient.  However, when these same subjects were asked to look in conjunction with other items the response times generally increase as the number of items they looked within increased.  However with high functioning autistic subjects, he found  they often had response times faster for looking for a unique item but when given a conjunctive task was given, their response times were  just as fast. There has been a lot of debate in the literature about why this ability may arise.

One possibility could be that there is some sort of enhanced discriminate ability in their top down or bottom up mechanism.  This could result in the autistic individual having a greater sense for what they are searching for and can hone in on their target information.  Another theory suggests that maybe they are viewing the target as unique piece of information and are able to use bottom up information to guide them the location of the target.

When objects become similar, the level of bottom up information used by the brain changes. For example, if we are looking for green apples among green limes the amount of bottom up information used will be less than when we are looking for red apples among green limes.  This could mean that as the similarities of the objects increases, the task becomes harder and the use of bottom up guidance goes down.  

When we are making complex visual search, we use conjunction searches.  We are often looking for something unique but can often have a combination of different features such as size, shape and colour. These searches can use the top down activation to guide our attention to the objects we are after. There have been a number of theories proposed to explain how these searches may be performed. They suggest that the use of bottom up guidance is non existent because a number of features have to be joined together when making these searches, and cannot rely on just one feature which can only be done in simple searches.

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Ann Treisman proposed the theory of feature integration in visual search. Her theory suggests that attention is like a spotlight searching random locations, at each location subjects combine features such as size and colour to determine if they have found the target or not. If the target is not found then the attention spotlight moves to another location until the right target has been found. This theory has been modified to suggest that people can also search for a subset instead of attending to random sites, the subjects pay attention to things that match the colour, size, shape of the target objects.

Jeremy Wolfe proposed the guided search theory. He suggests that when top down information is available for a distinct objects, you may be able to detect it efficiently by using top down guidance instead of searching at random locations, this result in finding objects more quickly.