Chapter 6

1. Chapter 6 Vision

2. VISION Sensory receptor • A specialized neuron that detects a particular category of physical events. Sensory transduction • The process by which sensory stimuli are transduced into slow, graded receptor potentials.

3. Figure 6.1 The Electromagnetic Spectrum

4. Figure 6.3 The Human Eye

5. Figure 6.5 Details of Retinal Circuitry Adapted from Dowling, J. E., and Boycott, B. B. Proceedings of the Royal Society of London, B, 1966, 166, 80–111.

6. Connections Between Eye and Brain Dorsal lateral geniculate nucleus (LGN) • A group of cell bodies within the lateral geniculate body of the thalamus; receives inputs from the retina and projects to the primary visual cortex. • Magnocellular layer • transmits information necessary for the perception of form, movement, depth, and small differences in brightness to the primary visual cortex. • Parvocellular layer • transmits information necessary for perception of color and fine details to the primary visual cortex. • Koniocellular sublayer • transmits information from short wavelength (“blue”) cones to the primary visual cortex.

7. Figure 6.6 Lateral Geniculate Nucleus (LGN). The photomicrograph shows a section through the lateral geniculate nucleus and striate cortexofa rhesus monkey (cresyl violet stain Layers 1 and 2 are the magnocellular layers; layers 3–6 are the parvocellular layers. The koniocellular sublayers are found ventral to each of the parvocellular and magnocellular layers.

8. Figure 6.7The Primary Visual Pathway

9. Figure 6.8 Central Versus Peripheral Acuity. Ganglion cells in the fovea receive input from a smaller number of photoreceptors than in the periphery and hence provide more acute visual information.

10. Figure 6.9 ON and OFF Ganglion Cells. The figure shows responses of ON and OFF ganglion cells to stimuli presented in the center or the surround of the receptive field.

11. CODING FOR COLOR Objects in our environment selectively absorb some wavelengths of light and reflect others, which, to our eyes, gives them different colors. The retinas of humans, apes, Old World monkeys, and one species of New World monkey contain three different types of cones, which provides them (and us) with the most elaborate form of color vision.

12. Trichromatic Theory of Color Vision In 1802 Thomas Young, a British physicist and physician, proposed that the eye detected different colors because it contained three types of receptors, each sensitive to a single hue. Investigators have studied the absorption characteristics of individual photoreceptors, determining the amount of light of different wavelengths that is absorbed by the photopigments. These characteristics are controlled by the particular opsin a photoreceptor contains; different opsins absorb particular wavelengths more readily.

13. Trichromatic Theory of Color Vision The peak sensitivities of the three types of cones are approximately 420 nm (blue-violet), 530 nm (green), and 560 nm (yellow-green). For convenience the short-, medium-, and long-wavelength cones are traditionally called “blue,” “green,” and “red” cones, respectively. The relative number of “red” and “green” cones varies considerable from person to person.

14. Color Blindness Protanopia Aninherited form of defective color vision in which red and green hues are confused; “red” cones are filled with “green” cone opsin. Deuteranopia An inherited form of defective color vision in which red and green hues are confused; “green” cones are filled with “red” cone opsin. Tritanopia An inherited form of defective color vision in which hues with short wavelengths are confused; “blue” cones are either lacking or faulty.

15. Opponent-Process Theort At the level of the retinal ganglion cell, the three-color code gets translated into an opponent-color system. Thus, the retina contains two kinds of color-sensitive ganglion cells: red-green cells and yellow-blue cells. Some color-sensitive ganglion cells respond in a center-surround fashion.

16. Figure 6.11 Receptive Fields of Color-Sensitive Ganglion Cells. Whena portion of the receptive field is illuminated with the color shown, the cell’s rate of firing increases. When a portion is illuminated with the complementary color, the cell’s rate of firing decreases.

17. Figure 6.12 The Six Layers of the Striate Cortex. This photomicrograph of a small section of striate cortex shows the six principal layers. The letter W refers to the white matter that underlies the visual cortex; beneath the white matter is layer VI of the striate cortex on the opposite side of the gyrus.

18. Figure 6.13 Orientation Sensitivity. Anorientation-sensitive neuron in the striate cortex will become active only when a line of a particular orientation appears within its receptive field. For example, the neuron depicted in this figure responds best to a bar that is vertically oriented.

19. Analysis of Visual Information: Role of the Striate Cortex Simple cell An orientation-sensitive neuron in the striate cortex whose receptive field is organized in an opponent fashion. Complex cell A neuron in the visual cortex that responds to the presence of a line segment with a particular orientation located within its receptive field, especially when the line moves perpendicularly to its orientation. Hypercomplexcell A neuron in the visual cortex that responds to the presence of a line segment with a particular orientation that ends at a particular point within the cell’s receptive field.

20. Figure 6.14 Types of Orientation-Sensitive Neurons. The figure illustrates the response characteristics of three types of orientation-sensitiveneurons in the primary visual cortex: (a) simple cell, (b) complex cell, and (c) hypercomplex cell.

21. Depth Perception Retinal Disparity: Stereopsis Most neurons in the striate cortex are binocular—that is, they respond to visual stimulation of either eye. Many of these binocular cells, especially those found in a layer that receives information from the magnocellular system, have response patterns that appear to contribute to the perception of depth. In most cases the cells respond most vigorously when each eye sees a stimulus in a slightly different location.

22. Color In the striate cortex, information from color-sensitive ganglion cells is transmitted, through the parvocellular and koniocellular layers of the LGN, to special cells grouped together in cytochrome oxidase (CO) blobs. Cytochrome oxidase (CO) blob • The central region of a module of the primary visual cortex, revealed by a stain for cytochrome oxidase; contains wavelength-sensitive neurons; part of the parvocellular system.

23. Modular Organization of the Striate Cortex Most investigators believe that the brain is organized in modules, which probably range in size from a hundred thousand to a few million neurons. Each module receives information from other modules, performs some calculations, and then passes the results to other modules. The striate cortex is divided into approximately 2500 modules, each approximately 0.5 × 0.7 mm and containing approximately 150,000 neurons. The neurons in each module are devoted to the analysis of various features contained in one very small portion of the visual field.

24. Modular Organization of the Striate Cortex Collectively, these modules receive information from the entire visual field, the individual modules serving like the tiles in a mosaic mural. The modules actually consist of two segments, each surrounding a CO blob. Neurons located within the blobs have a special function…they are sensitive to color Outside the CO blob, neurons show sensitivity to orientation, movement, spatial frequency, and binocular disparity, but most do not respond to color.

25. Figure 6.18 One Module of the Primary Visual Cortex

26. Analysis of Visual Information: Role of the Visual Association Cortex Two Streams of Visual Analysis Visual information received from the striate cortex is analyzed in the visual association cortex. Neurons in the striate cortex send axons to the extrastriate cortex, the region of the visual association cortex that surrounds the striate cortex. Extrastriate cortex A region of visual association cortex; receives fibers from the striate cortex and from the superior colliculi and projects to the inferior temporal cortex.

27. Analysis of Visual Information: Role of the Visual Association Cortex Two Streams of Visual Analysis Dorsal stream A system of interconnected regions of visual cortex involved in the perception of spatial location, beginning with the striate cortex and ending with the posterior parietal cortex. Ventral stream A system of interconnected regions of visual cortex involved in the perception of form, beginning with the striate cortex and ending with the inferior temporal cortex

28. Figure 6.22 The Human Visual System. The figure shows the human visual system from the eye to the two streams of visual association cortex.

29. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Color As we saw earlier, neurons within the CO blobs in the striate cortex respond to colors. Like the ganglion cells in the retina (and the parvocellular and koniocellular neurons in the LGN), these neurons respond in opponent fashion. This information is analyzed by the regions of the visual association cortex that constitute the ventral stream.

30. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Form In primates the recognition of visual patterns and identification of particular objects take place in the inferior temporal cortex, located on the ventral part of the temporal lobe. This region of the visual association cortex is located at the end of the ventral stream. It is here that analyses of form and color are put together and perceptions of three-dimensional objects and backgrounds are achieved.

31. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Form Visual agnosia Deficits in visual form perception in the absence of blindness; caused by brain damage. Prosopagnosia Failure to recognize particular people by the sight of their faces. Fusiform face area (FFA) A region of the visual association cortex located in the inferior temporal; involved in perception of faces. Extrastriate body area (EBA) A region of the visual association cortex located in the lateral occipitotemporal cortex; involved in perception of the human body and body parts other than faces.

32. Figure 6.25 Perception of Faces and Bodies. The fusiform face area (FFA) and the extrastriate body area (EBA) were activated by images of faces, headless bodies, body parts, and assorted objects.

33. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Form Parahippocampal place area (PPA) A region of the medial temporal cortex; involved in perception of particular places (“scenes”).

34. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Movement We need to know not only what things are, but also where they are and if they are moving, where they are going. Without the ability to perceive the direction and velocity of movement of objects, we would have no way to predict where they will be. We would be unable to catch them (or avoid letting them catch us).

35. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Movement One of the regions of the extrastriate cortex—area V5, also known as area MT, for medial temporal—contains neurons that respond to movement. Damage to this region severely disrupts a monkey’s ability to perceive moving stimuli. Area V5 receives input directly from the striate cortex and from several regions of the extrastriate cortex. It also receives input from the superior colliculus, which is involved in visual reflexes, including reflexive control of eye movements.

36. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Movement A region adjacent to area V5 (sometimes called V5a but more often referred to as MST, for medial superior temporal) receives information about movement from V5 and performs a further analysis. MST neurons respond to complex patterns of movement, including radial, circular, and spiral motion.

37. Figure 6.27 Location of Visual Area V5. The location of visual area V5 (also called MT/MST or MT+) in the human brain was identified by a stain that showed the presence of a dense projection of thick, heavily myelinatedaxons. LOS = lateral occipital sulcus, IOS = inferior occipital sulcus.

38. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Movement Akinetopsia Inability to perceive movement, caused by damage to area V5 of the visual association cortex.

39. Analysis of Visual Information: Role of the Visual Association Cortex Perception of Spatial Location Intraparietal sulcus (IPS) The end of the dorsal stream of the visual association cortex; involved in perception of location, visual attention, and control of eye and hand movements.