Topics and Speakers Vincent P. Ferrera, PhD
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Synopsis
The fact that the optic nerve contains about 1.2 million axons illustrates how complicated the human vision system is when compared to the display of a computer screen. This lecture offers an anatomical review of the visual system. Starting with an overview of the structure of the eye, Dr Vincent P. Ferrera then moves on to the cells that communicate with the two main thalamic pathways involved in vision, next describing the visual cortex, specifically the parietal and temporal pathways, and area MT.
The fovea, which is located in the center of the macula, is largely responsible for color vision. It is packed with photoreceptors known as cones. It can get much information from a small part of the visual field, while sacrificing information from the rest of the visual field. It has no rods, and therefore, it is not sensitive to dim light. The foveola, in contrast, is one degree wide and is packed with cones, being responsible for the acutest part of vision. The retina also deals with color containing four classes of photoreceptors: rods, blue cones, green cones, and red cones. When light enters the eye, it passes through a couple of cell layers and hits the photoreceptors at the back of the retina. The photoreceptors transmit the light information to bipolar neurons, known as ganglion cells, whose axons make up the optic nerve.
There are two major classes of ganglion cells: midget cells and parasol cells. Their difference lies in anatomy. The midget cells have very small dendritic arbors and very small receptive fields; whereas the parasol cells have large dendritic arbors and larger receptive fields. While the midget cells (responsible for the thalamic parvocellular or P pathway) prefer small, slow-moving stimuli, the parasol cells (responsible for the thalamic magnocellular or M pathway) prefer large, fast-moving stimuli. Where the midget cells are not sensitive to low contrast, the parasol cells are highly sensitive to low contrast. Finally, midget cells are more sensitive to color changes than parasol cells.
The physiological properties of the thalamic pathways involved in vision are very similar to the properties of the midget and parasol cells. Once the light enters the optic tract, its main target is the lateral geniculate nucleus (LGN), a six-layered structure where its layers are divided into to main pathways: the parvocellular pathway (or P pathway) and the magnocellular pathway (M pathway). The P pathway has small receptive fields, low contrast sensitivity, responds to slow temporal changes, and is color-opponent (mainly red-green). Meanwhile, the M pathway has larger receptive fields, high contrast sensitivity, responds to faster changes in luminance, but is not color-opponent. Each of the six layers of the LGN preserves the retinatopic structure of the visual field; in other words, the LGN contains a topographic representation of the visual field.
Dr Ferrera then moves on the different visual areas of the brain, of which 30 have been mapped in the rhesus monkey. It has been more difficult to map these 30 visual areas in the human being, but scientists know they are there because there are literally 30 different maps of the visual world in the human brain. (This point is explained in more detail via fMRI studies.) The main idea to take away from these different maps is that they are organized into two pathways: one pathway goes through the parietal cortex and the other goes through the temporal lobe. The parietal pathway has been nicknamed the "where" pathway since it answers the question, Where are things in my visual field?; whereas the temporal lobe pathway is nicknamed the "what" pathway since it answers the question, What are the things in my visual field?
But the most important area is the primary visual cortex, also known as V1. This is the first place where information from the two eyes comes together, specifically binocular information that allows you to do depth perception and respond to motion. V1 has three main characteristics: it is orientation selective (i.e., it does not respond to orthogonal orientation); it has a spatially localized receptive field; and it is direction selective. Dr Ferrera presents a video by Hubel and Wiesel, winners of the Nobel Prize for their work in the visual cortex, to illustrate this point.
The other area Dr Ferrera focuses on is the middle/medial temporal area or area MT. Area MT receives input from V1 and 90% of its neurons are motion selective, whereas only 25% of the neurons in V1 are motion selective. This means that the V1 motion selective neurons are projecting specifically into area MT, thereby providing specific input from direction selective neurons. MT then sends information to the parietal "where" pathway, where it gets integrated with areas involving eye movements, reaching movements, spatial attention, and spatial short term memory. An interesting fact is that a lesion in the parietal cortex can result in something called spatial neglect, in which the patient tends to ignore stimuli in the contralateral visual field. A humorous story involving the Italian film director Federico Fellini, who was afflicted with spatial neglect, illustrates this disorder.
Dr Ferrera concludes with a discussion of disorders that can affect the temporal "what" pathway. Lesions in these areas can cause strange disorders such as prosopagnosia (a type of visual agnosia), which is an inability to recognize faces, and the patient compensates by using clues for recognition, such as identifying a person by their clothing, their voice, and the way they talk. The lecture ends with the classic example of visual agnosia from Oliver Sacks's book The Man Who Mistook His Wife for a Hat, wherein a patient confuses his wife for a hat and tries to put her on his head.
