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Retinal image Cortical stimulation

Figure 51-5

Pattern of excitation that occurs in the visual cortex in response to a retinal image of a dark cross.

of each line or border—that is, whether it is vertical or horizontal or lies at some degree of inclination. This is believed to result from linear organizations of mutually inhibiting cells that excite second-order neurons when inhibition occurs all along a line of cells where there is a contrast edge. Thus, for each such orientation of a line, specific neuronal cells are stimulated. A line oriented in a different direction excites a different set of cells. These neuronal cells are called simple cells. They are found mainly in layer IV of the primary visual cortex.

Detection of Line Orientation When a Line Is Displaced Laterally or Vertically in the Visual Field—"Complex" Cells. As the visual signal progresses farther away from layer IV, some neurons respond to lines that are oriented in the same direction but are not position-specific. That is, even if a line is displaced moderate distances laterally or vertically in the field, the same few neurons will still be stimulated if the line has the same direction. These cells are called complex cells.

Detection of Lines of Specific Lengths, Angles, or Other Shapes.

Some neurons in the outer layers of the primary visual columns, as well as neurons in some secondary visual areas, are stimulated only by lines or borders of specific lengths, by specific angulated shapes, or by images that have other characteristics. That is, these neurons detect still higher orders of information from the visual scene. Thus, as one goes farther into the analytical pathway of the visual cortex, progressively more characteristics of each visual scene are deciphered.

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