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What kind of processes are "top-down" and "bottom-up" processes in the context of processing visual information in brain?

Context:

...There are discrete areas in the visual cortex dedicated to identifying specific aspects of vision – vertical or horizontal lines, colour or movement, for example. These elements are brought together to ensure that the total image arrives consistently in space and time. And even while our brains are swiftly adjusting to focus on the scene before us, images established in memory form feedback loops to confirm that we recognise what we have seen before, mediating prior knowledge into perception. As we acquire this personal knowledge then, we impose ‘top-down’ concepts on to ‘bottom-up’ perceptions. When we look at a cluttered scene in front of our eyes we may receive only partial information but our brains automatically fill in the gaps to make sense of things.

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Top-down knowledge is knowing what a target is. When detecting an object, the rate at which you can detect it (measured via reaction time, or 'RT') is partially dependent on how fast you can identify it based on its sensory features.

Bottom-up knowledge is integrating recognition of the features, or the dimensions of an object, such as its color, orientation, and visibility. Neighbouring locations containing items with similar features tend to inhibit each other, whereas neighbouring locations containing different features tend to activate each other, thus lending to the saliency of an object. Think of how salient a bright red stop sign is in the dark, and consider why that may be important to ensure the safety of nighttime drivers.

Once top-down and bottom-up systems have calculated their outputs, they are summed together in a spatially-guided 'activation map', thus lending itself to perception.

Both are important for object recognition and reaction time. For example, you must use top-down processing while driving to recognize that a streetlight is a streetlight. You then use bottom-up processing to recognize if the light is red, green, or yellow. Both are important mechanisms of perceptions to use, so that you can act appropriately and avoid an accident on the road.

If you are curious and want to read more, this paper examines both top-down and bottom-up mechanisms in visual search, and may prove useful to you.

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