Like almost all other portions of the cerebral cortex, the primary visual cortex has six distinct layers, as shown in Figure 1. Also, as is true for the other sensory systems, the geniculocalcarine fibers terminate mainly in layer IV, but this layer is also organized into subdivisions. The rapidly conducted signals from the M retinal ganglion cells terminate in layer IVcα, and from there they are relayed vertically both outward toward the cortical surface and inward toward deeper levels.
Fig1. Six layers of the primary visual cortex. The connections shown on the left side of the figure originate in the magnocellular layers of the lateral geniculate nucleus (LGN) and transmit rapidly changing black and white visual signals. The pathways to the right originate in the parvocellular layers (layers III through VI) of the LGN; they transmit signals that depict accurate spatial detail, as well as color. Note especially the areas of the visual cortex called color blobs, which are necessary for detection of color.
The visual signals from the medium-sized optic nerve fibers, derived from the P ganglion cells in the retina, also terminate in layer IV, but at points different from the M signals. They terminate in layers IVa and IVcβ, the shallowest and deepest portions of layer IV, shown to the right in Figure 1. From there, these signals are trans mitted vertically both toward the surface of the cortex and to deeper layers. It is these P ganglion pathways that transmit the accurate point-to-point type of vision, as well as color vision.
Vertical Neuronal Columns in the Visual Cortex. The visual cortex is organized structurally into several million vertical columns of neuronal cells, with each column having a diameter of 30 to 50 micrometers. The same vertical columnar organization is found throughout the cerebral cortex for the other senses as well (and also in the motor and analytical cortical regions). Each column represents a functional unit. One can roughly calculate that each of the visual vertical columns has perhaps 1000 or more neurons.
After the optic signals terminate in layer IV, they are further processed as they spread both outward and inward along each vertical column unit. This processing is believed to decipher separate bits of visual information at successive stations along the pathway. The signals that pass outward to layers I, II, and III eventually transmit signals for short distances laterally in the cortex. Conversely, the signals that pass inward to layers V and VI excite neurons that transmit signals much greater distances.
“Color Blobs” in the Visual Cortex. Interspersed among the primary visual columns, as well as among the columns of some of the secondary visual areas, are special column-like areas called color blobs. They receive lateral signals from adjacent visual columns and are activated specifically by color signals. Therefore, these blobs are presumably the primary areas for deciphering color.
Interaction of Visual Signals from the Two Separate Eyes. Recall that visual signals from the two separate eyes are relayed through separate neuronal layers in the lateral geniculate nucleus. These signals remain separated from each other when they arrive in layer IV of the primary visual cortex. In fact, layer IV is interlaced with stripes of neuronal columns, with each stripe about 0.5 millimeter wide; the signals from one eye enter the columns of every other stripe, alternating with signals from the second eye. This cortical area deciphers whether the respective areas of the two visual images from the two separate eyes are “in register” with each other—that is, whether corresponding points from the two retinas fit with each other. In turn, the deciphered information is used to adjust the directional gaze of the separate eyes so that they will fuse with each other (i.e., be brought into “register”). The information observed about degree of register of images from the two eyes also allows a person to distinguish the distance of objects by the mechanism of stereopsis.
