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General Mechanisms of Human Visual Search and Eye Movement

In this article, you will learn about the two mechanisms that define human visual search and how they help in the understanding of eye tracking metrics.

Do you know that 50 to 80% of our brains is dedicated to visual processing? What we see plays a big part in how we interact with the world around us and studying how we process visual information can be valuable for several use cases, from medicine to marketing. In this article we are going to explore the basic concepts about visual search and eye movement.

Visual search is a type of task that envolves scanning the environment to look for a specific object or feature, namely the target, among other objects and features, called distractors. The human visual search is defined by two mechanisms: “bottom-up” and “top-down” search. By understanding how they work, you can understand how eye tracking metrics are interpreted.

Bottom-up search

Bottom-up search means that the visual search (the participants eye movement) is driven by the physical characteristics of the objects in front of a person. The very early eye movements and fixations (0-0.15 sec) after a person started looking at a stimulus (e.g., an image,website, video etc.) are being defined by the bottom-up characteristics of the objects looked at within that stimulus (objects can be any element in a stimulus, e.g., a face, a button, atext, a logo etc.). Objects that are especially “salient” are being noticed in bottom-up search. This can be because they are flashy, big, moving, colorful, very central in the screen or visually stand out compared to the other objects in some ways. If the object is a human face, it will also be noticed very quickly.

In this stage, there are very quick shifts of attention from one object to another and the processing is likely not very deep, meaning that the information containing the object are likely not being processed, understood and stored consciously. A quick, subconscious judgement is made whether or not an object is interesting or relevant to the person. Objects that are noticed as interesting or relevant will be consciously revisited later to extract its information, after most of the salient objects have been fixated. Non-relevant objects will likely not be revisited at all, or only very briefly, to confirm the subconscious judgement that they are indeed not relevant.

The later stages of visual search (>0.15 sec) are driven by top-down search.

Top-down search

Top-down search means that the visual search (the eye movement) is influenced by the cognitive processes of a person. A person’s unique knowledge, expectations and mental state come into play here. E.g., if a person wishes to exit a program on a computer, they might look in the top right corner because they expect to see a red “x” there, based on their knowledge and experience with some browsers. A person can also make the active decision to focus on certain objects in a stimulus.

In this stage, the processing happens consciously and more deeply than in bottom-up processing. That means that the information in the area in focus can be extracted and stored. People also make conscious judgements about the content, such as “this information is interesting” or “this information is relevant for my task completion”.

In general, eye tracking metrics can only be interpreted when the exact task and stimulus material used in that certain eye tracking study are kept in mind. The number of participants as well as the context in which they completed the study might also influence the interpretation of the data.

Now that you already know how human visual search and eye movement work, you can access this article about the main metrics used in eye tracking.

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November 3, 2021

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