Visual processing is the interpretation of otherwise raw sensory data to produce visual perception.

The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different parts of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (Greek, for “crossing”) found at the base of the brain and allows us to coordinate information from both eyes.

Once in the brain, visual information is processed in several places. Its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except smell. In the thalamus, the magnocellular and parvocellular distinctions remain intact; there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (“where”) information. A second stream projects to the temporal lobe and carries both magnocellular (“where”) and parvocellular (“what”) information.

Another important visual route is a pathway from the retina to the superior colliculus in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN, a cluster of cells, is considered to be the body’s internal clock, which controls our circadian (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep/wake patterns and annual cycles.

There are two types of bottom-up processing that take place in visual processing: feature detection and parallel processing.

Parallel processing is the use of multiple pathways to convey information about the same stimulus. It starts at the level of the bipolar and ganglion cells in the eye, allowing information from different areas of the visual field to be processed in parallel. Through two types of ganglion cells, visual information is split into two pathways: one that detects and processes information about motion and one that is concerned with the form of stimuli (like shape and color). The motion and form pathways project to separate areas of the lateral geniculate nucleus (LGN) and visual cortex.

Once visual information reaches the visual cortex via parallel pathways, it is analyzed by feature detection. There are cells in the visual cortex of the brain that optimally respond to particular aspects of visual stimuli. These cells provide information concerning the most basic features of objects, which are integrated to produce a perception of the object as a whole. Feature detection is a type of serial processing where increasingly complex aspects of the stimulus are processed in sequence.

In perception of light by the eye, the proximal stimulus refers to physical stimulation that is available to be measured by an observer’s sensory apparatus. In the case of the eye the diistal Stimulus is any physical object or event in the external world that reflects light. This light or energy, called the proximal stimulus, is what excites the receptors on our eyes, leading to visual perception.

Practice Questions

Khan Academy

 

MCAT Official Prep (AAMC)

Section Bank P/S Section Passage 4 Question 24

Section Bank P/S Section Passage 8 Question 60

Sample Test P/S Section Passage 9 Question 48

Key Points

• The magnocellular pathway carries information about form, movement, depth, and differences in brightness; the parvocellular pathway carries information on color and fine detail.

• The optic chiasma allows us to coordinate information between both eyes and is produced by crossing optical information across the brain.

• Visual signals move from the visual cortex to either the parietal lobe or the temporal lobe.

• Some signals move to the thalamus, which sends the visual signals to the primary cortex.

• Visual signals can also travel from the retina to the superior colliculus, where eye movements are coordinated with auditory information.

• Visual signals can move from the retina to the suprachiasmatic nucleus (SCN), the body’s internal clock, which is involved in sleep/wake patterns and annual cycles.

• There are two types of bottom-up processing that take place in visual processing: feature detection and parallel processing.

• Through two types of ganglion cells, visual information is split into two pathways: one that detects and processes information about motion and one that is concerned with the form of stimuli (like shape and color). The motion and form pathways project to separate areas of the lateral geniculate nucleus (LGN) and visual cortex.

• There are cells in the visual cortex of the brain that optimally respond to particular aspects of visual stimuli. These cells provide information concerning the most basic features of objects, which are integrated to produce a perception of the object as a whole.

• In light processing, the distal Stimulus is any physical object or event in the external world that reflects light and the light itself is the proximal stimulus, is what excites the receptors on our eyes, leading to visual perception.

Key Terms

Superior colliculus: The primary area of the brain where eye movements are coordinated and integrated with auditory information.

Optic chiasma: found at the base of the brain and coordinates information from both eyes.

Suprachiasmatic nucleus: A cluster of cells that is considered to be the body’s internal clock, which controls our circadian (day-long) cycle.

Parallel processing: The use of multiple pathways to convey information about the same stimulus.

Feature detection: A type of serial processing where increasingly complex aspects of the stimulus are processed in sequence.