Early vision, also known as preattentive vision, includes those mechanisms that subserve the first stages of visual processing. These mechanisms can operate in parallel across the visual field, and is believed to be used for detecting the most basic visual features, such as color, luminance, orientation, motion etc.

For instance, if you are asked to look for the red spot among these six green spots, it is clear that you don't have to scan the figure carefully to find the red spot. The red spot "pops out" at you, drawing your attention without any effort on your part.

You can also see that this ease of detection is not dependent on how many green spots surround the red one. Even if there are twenty green spots, the task is just as easy as when there are only six. No serial search is necessary.

Another example of basic visual features that can be processed by early vision mechanisms is oriented bars. This is illustrated in the next example. The horizontal bar can be detected "preattentively" from the vertical ones.

What about if two basic visual features, color and orientation, for example, were combined in a task? Find the red horizontal bar in the following figure. How does the difficulty of this task compare to the previous ones? Did the red bar pop-out at you? Or did you find yourself conducting a serial search for the bar in question?

Tasks that involve two or more visual features are called "conjunction search tasks". This qualitative perceptual difference suggest the involvement of visual mechanisms beyond that of early vision.

References:

Neisser, U. (1967). Cognitive Psychology, Englewoods Cliffs,NJ: Prentice-Hall.

Beck, J. (1982). Textural segmentation. In J.Beck (Ed.), Organization and Representation in Perception, pp.285-317, Hillsdale,NJ: Lawrence Erlbaum.

Treisman, A. (1982). Perceptual grouping and attention in visual search for features and for objects. Journal of Experimental Psychology: Human Perception and Performance, 8, 194-214.

Julesz, B. (1984). Toward an axiomatic theory of preattentive vision. In G.M. Edelman, W.E. Gall & M. Cowan (Eds.), Dynamic Aspects of Neocortical Function, pp. 585-611. New York: Neurosciences Research Foundation.