What Are Complex Cells?

S. Berger

The visual cortex, located toward the posterior side of the cerebrum, has many specialized cells that contribute toward the perception of visual stimuli. Some of these specialized neurons are known as complex cells. These cells are found in three areas of the largest part of the brain, called the cerebrum. The three areas are known as the primary and secondary visual cortices, and another nearby region called Brodmann area 19, all of which are located toward the back of the cerebrum.

The visual cortex is located towards the posterior side of the cerebrum.
The visual cortex is located towards the posterior side of the cerebrum.

Complex cells seem to play an important role in object recognition, particularly when located in Brodmann area 19. This area seems to be the point where visual signals branch off into the "what" and "where" pathways. The first pathway determines what objects are being looked at, and the latter determines where they are located in space. These specialized cells seem to help the brain decide what an object is by providing information on the lines that compose it.

In the visual system, complex cells are involved in recognizing boundaries and edges. Specific cells will respond to specific orientations of lines. A related cell type, known as simple cells, will also respond in this fashion.

Complex cells, unlike simple cells, do not require these lines to be at a specific point in space. They will respond to line orientations anywhere in their large receptive field, regardless of their exact location. Keeping with the need for specific orientations to activate, some of these cells fire when they detect directional movement.

Contrast between regions of light and darkness, as seen by the eye's visual cells, is needed for these cells to determine line orientation. Rods and cones send information about boundaries between light and darkness to simple cells, which interpret them as edges. Simple cells then tell complex cells about small portions of these edges, and complex cells sum this information together to create a larger conception of edges or outlines of shapes.

The way that the visual system is laid out involves several simple cells receiving visual input first. Lines in a certain orientation, and occurring in an exact place in a cell's receptive field, will cause it to fire. Receptive fields are the exact regions in which visual cells detect stimuli. For example, simple cells only respond to edges detected inside of their specific field. Stimuli outside of the receptive field will lead to inhibition by nearby cells.

After being activated by a given orientation, simple cells relay this information to a smaller number of complex cells through converging inputs. These cells integrate the information they receive, and send it to other visual areas for further processing. Other processes like visual memory and shape recognition may require input from these cells, depending on which area they are located in.

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