Even after light has entered the eye and has been registered by the retinal photoreceptors, the brain still has not “seen” anything. The message has to travel to the brain where it can be processed. However, things are a bit more complex than a simple telegram; the information travels to a number of places in the brain that work together to produce the seamless sense of vision.
For an understanding of what happens just before the message gets sent to the brain, please see “How the Eyes Tell the Brain What They “See.”
How the Visual Message Travels to the Brain via the Optic Nerve
The axons of retinal ganglion cells leave the eye in a location called the “optic disk.” This disk does not contain any photoreceptors, and thus, it cannot detect light, giving rise to the blind spot phenomenon. All of these ganglion cell axons bundle together to from the optic nerve, which travels toward the brain until it reaches the optic chiasm.
At the optic chiasm (see figure at the bottom of the page), some of the neurons cross to the other side and some continue on the same side. Whether or not a neuron crosses depends on which side of the visual field its message represents. When you focus on an object, you can see things to the left and to the right of the object. This represents the left and right visual fields, respectively. Information from the left visual field crosses over and continues on the right optic tract, and information from the right visual field crosses over and continues on the left optic tract.
Brain Areas that Process Visual Stimuli
Once the neurons cross in the optic chiasm, they continue on to a number of destinations. One of these destinations is the hypothalamus, a portion of the brain that largely oversees the activity of hormones in the body. It uses light information to regulate the circadian rhythm—the daily cycle of regular activity, including sleep and wakefulness. The suprachiasmatic nucleus, a structure in the hypothalamus, uses light information to tell the pineal gland when to secrete melatonin, the hormone that regulates the body’s circadian cycle.
The neurons also travel via the optic tract to the pretectum, which controls the muscles of the iris. The iris makes the pupil bigger or smaller depending on the amount of light in the environment.
The visual message also travels to the superior colliculus, a portion of the midbrain. Here, the information organizes head-eye coordination, allowing the eyes to stay focused on a target while the head moves.
Finally, the message travels to the primary visual cortex, the place that produces the sense of vision. To get there, the electrical signal travels through the optic tract until it gets to the lateral geniculate nucleus (LGN) in the thalamus. Even though each LGN is receiving input from both eyes (because some neurons crossed at the optic chiasm), the information from the left and right eyes are kept separate to allow the brain to keep objects in the proper visual field.
Once the electrical signal arrives at the LGN, the message is then relayed via optic radiation to the appropriate place in the primary visual cortex (also known as the striate cortex) in the occipital lobe.
The Striate Cortex and the Decoding of Visual Input
The striate cortex is laid out to carefully organize visual input. Each hemisphere processes a different visual field, and both hemispheres are separated top from bottom by the calcarine sulcus. The region above the calcarine sulcus processes the lower visual field, and the region below processes the upper visual field.
These regions are also separated by the radial position of the information in the visual field. Within the visual field are objects that can be seen by both eyes (bionocular position) and objects in the periphery that can only be seen by one eye (monocular position). The brain has designated regions for both binocular and monocular information. Finally, the information from neurons in the fovea (i.e. the place on the retina that processes objects being focused on) is processed in a disproportionately large area of the striate cortex called the “macula.”
How, exactly, the brain puts together all of this information into a complete picture is still a mystery, but scientists have a few clues. Certain cells in the striate cortex, for instance, have demonstrated the specific function of determining the orientation of shapes. There are also cells in the striate cortex that deal only with the perception of depth.
Visual Perception Outside the Striate Cortex
While the aptly named primary visual cortex is the site of primary visual perception, the information collected by the eye gets transferred to nearby regions for further processing. The extrastriate cortex, for example, encompasses regions like ventral 4 (V4) that detects color and the middle temporal area that detects movement.
Information from the striate cortex also gets sent to the parietal lobe via the dorsal pathway and to the temporal lobe via the ventral pathway. The parietal lobe has been shown to play a role in spatial perception, and the temporal lobe has demonstrated object recognition ability.
References
Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W., LaMantia, A., McNamara, J. O., & White, L. E. (Eds.). (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.
 |
 |
 |
