Light can be thought of in two ways: (1) as discrete particles of energy called photons that travel at about 300,000 kilometers (186,000 miles) per second; or (2) as waves of electromagnetic energy that are between 380 and 760 nanometers (billionths of a meter) in length (i.e., our visible spectrum). Why are waves of these lengths so special? Because the human eye responds to them .
Other species can see wavelengths of light outside of our visible spectrum; for example, rattlesnakes can see infrared rays (> 760 nm) and it has recently been discovered that rats have receptors that respond to ultraviolet rays (< 380 nm). Two important properties of light are wavelength and intensity (derived from the amplitude of the waveform). Wavelength plays an important role in the perception of color and intensity plays a role in the perception of brightness.
Light enters the eye through the cornea (the transparent outer covering of the eye; the white sclera is opague). The amount of light that gets passed this point is regulated by the size of the pupil - the hole in the middle of the iris which is a ring of muscles just behind the cornea. The size of the pupil changes in response to the amount of illumination. When the level of illumination is high the pupil constricts (closes) and the image that falls on the retina is sharper and there is greater depth of focus (i.e., a greater range of depths can be simultaneously kept in focus), thus resulting in higher acuity (ability to see the details of objects). When the level of illumination is low the pupil dilates to let in more light, thus increasing sensitivity (the ability to detect the presence of dimly lit objects).
There are two bands of muscles in the iris: sphincters and dilators. The sphincter muscles are innervated by cholinergic (muscarinic) fibers of the PNS; belladona alkaloids such as atropine block cholinergic activity which causes the sphincter muscles to relax, thus dilating the pupil.
Just behind the pupil is the lens which focuses light on the retina (the light-sensitive tissue layer that lines the inner portion of the eye). The natural shape of the lens is cylindrical (round). When we direct our gaze at a close object, the ciliary muscles contract resulting in less tension on the ligaments holding the lens in place; Thus, the lens assumes its natural cylindrical shape which increases its ability to refract (bend) light, thus bringing close objects into sharp focus. When we direct our gaze at a more distant object the cilliary muscles relax, putting more tension on the ligaments and flatening the lens. The process of adjustment for distance is called accomodation .
In humans, the movement of the two eyes is coordinated so that each point in the visual field is projected to corresponding points on your two retinas. This is accomplished when the eyes converge (turn slightly inward) to view a close object. The correspondence is never perfect because each eye views the world from a slightly different position. The visual system can use the difference in the position of images on the two retinas (called binocular disparity ) to construct a 3-D perception from two slightly different 2-D retinal images.
The retina contains 5 different layers of cells: receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells. Three of these cells (receptors, bipolar and ganglion cells) are responsible for vertical (back to front) communication. The two other cells (horizontal and amarcrine cells) are responsible for lateral communication (i.e., transmission across channels of sensory input).
Note that the retina seems to be inside out, in that light must go through 4 cell layers to reach the receptors. Once the receptors are activated, the neural message is transmitted forward to the retinal ganglion cells whose axons project on the surface of the retina.
There are two disadvantages to this arrangement: (1) light is distorted as it passes through retinal tissue, and (2) ganglion axons on the surface of the retina gather at an area called the optic disk

to exit the eye. Therefore, there are no receptors at the optic disk which is also called the blind spot . The visual system uses information from surrounding receptors to compensate for the blind spot by filling in the gaps in the retinal image. This phenomenon is called completion and demonstrates that the visual system does not always provide an accurate representation of the external world. The fovea is a tiny area in the center of the retina where there is a thinning of the ganglion cell layer. This thinning reduces the distortion of incoming light resulting in high-acuity vision.
The duplexity theory of vision asserts that the different types of receptors, rods and cones , form two kinds of visual systems. The cone system (or photopic system ) takes advantage of good lighting to provide high-acuity (fine-detailed) colored perceptions of the world. The rod system (or scotopic system ) functions in dim illumination to increase sensitivity at the expense of acuity and color vision.
The duplexity theory is supported by the fact that individuals who lack functional rods suffer night blindness but have normal vision under daylight conditions; in contrast, individuals without functional cones display day blindness characterized by a lack of color and detail vision but have normal vision under dim illumination.
The two systems differ in terms of their degree of convergence : several hundred rods converge on a single ganglion cell whereas it is not uncommon for a single ganglion cell to receive input from a few cones. Therefore, dim light simultaneously activates several rods which can summate to influence the firing of a retinal ganglion cell. However, the same dim light applied to a sheet of cones cannot summate to the same degree and therefore may not activate the retinal ganglion cells. Since the brain has no way of knowing which portion of rods was activated under dim illumination, acuity is poor. With more intense light, activation of cones provides a less ambiguous signal of the location of the stimulus that triggered the reaction.
Rods and cones also differ in their distribution across the retina. There are no rods in the fovea, only cones. At the boundary of the fovea the proportion of cones declines markedly and there is an increase in the number of rods, which reaches a maximum density at 20 degrees from the center of the fovea.
Since the nose blocks light from entering the peripheral portion of the retina (i.e., the temporal hemiretina ) there are less rods there then in the portion of retina close to the nose (i.e., the nasal hemiretina ).
In general, more intense light appears brighter. However, wavelength also has an effect on the perception of brightness. A graph of the relative brightness of lights of the same intensity but at different wavelengths is called a spectral sensitivity curve (see figure on page 196). Animals with both rods and cones have two spectral sensitivity curves: a photopic spectral sensitivity curve and a scotopic spectral sensitivity curve . Under scotopic conditions we are maximally sensitive to wavelengths of about 500 nm (blue-green range) and under photopic we are maximally sensitive to wavelengths of about 560 nm (green-yellow range).
The Purkinje effect refers to a phenomenon discovered by Purkinje in 1825 when he was walking through his garden. Before dusk, the red and yellow flowers appeared brightest, but at night these same flowers only appeared as shades of gray, and the blue flowers appeared as brighter shades of gray.
The fact that cones are crammed into the fovea does not mean that color and detail vision are restricted to central vision. The eye continually scans the visual field by making a series of brief fixations (about 3 every second) that are connected by very quick eye movements called saccades . The visual system adds together the foveal images from the preceding few fixations to produce a larger range of high-acuity color vision. This temporal integration also ensures that the world does not vanish every time you blink. The role of eye movements in vision was also

demonstrated with a projection device attached to a contact lens. Since the contact lens moves with the eye, this device enables the projection of a test stimulus on the same receptors. After a few seconds of viewing, the stabilized retinal image disappears and eye movements increase, presumably in an attempt to bring the image back. The reason the stimulus disappears is because the visual system responds best to change rather than to steady input.
Transduction refers to the conversion of one form of energy to another. So visual transduction refers to the conversion of light to neural signals, which is accomplished by rods and cones. In
1876 a red pigment was extracted from a frog retina which predominately contains rods. When the pigment (called rhodopsin ) was exposed to continuous light, it lost its color and its ability to absorb light; when returned to the dark, it regained its color and light-absorbing capacity. The degree to which rhodopsin absorbs lights of different wavelengths is related to the ability of humans to detect the presence of different wavelengths of light under scotopic conditions. In fact, the absorption spectrum of rhodopsin matches the scotopic spectral sensitivity curve (see figure on page 199). Rhodopsin is composed of two molecules: retinal and opsin . When rods are exposed to light, the threadlike opsin molecule is released from some of its points of contact with the retinal molecule and begins to straighten out. This chemical change causes the bleaching and induces a neural signal. If rods are continuously exposed to intense light, the opsin separates completely from the retinal molecule and the cell loses its ability to absorb light and generate signals. The bleaching of rhodopsin is one of the fastest chemical reactions ever recorder (it occurs in 200 femtoseconds or 200 x 10 to the -15 seconds) and it initiates a cascade of chemical events inside the rods. These chemical events close sodium ion channels which are normally open in the dark. This in turn hyperpolarizes the rods and reduces the number of neurotransmitter molecules that are continuously released from their terminals. Therefore, the transduction of light by rods is accomplished by inhibition rather than by excitation.
There are several pathways that process visual information, but the largest and most thoroughly studied is the retina-geniculate-striate pathway . Signals from the retina are projected to the lateral geniculate nuclei and then to the striate (primary visual) cortex.
The main thing to notice about this figure is that all signals from the left visual field end up in the right striate cortex , either ipsilaterally via the temporal hemiretina of the right eye or contralaterally via the nasal hemiretina of the left eye . The opposite is true of the right visual field. The lateral geniculate nucleus has six layers and each layer receives input from only one retina ( the book says “all parts of one retina” but the colors in the figure indicate that this should be restricted to one visual field) .
The lateral geniculate neurons project to the lower part of cortical layer IV, thus producing a characteristic stripe or striation when viewed in cross secion - hence the name striate cortex.
Each level of the retinal-geniculate-striate pathway is retinotopic - which means that two stimuli presented to adjacent areas of retina excite adjacent neurons at all levels of the system.
It should be noted that there is a disproportionately large representation of the fovea in striate cortex. So, although the fovea represents a small proportion of the retina, 25% of the striate cortex is devoted to its analysis. Also note that all information from the top of the visual field (red and green areas) ends up in the bottom portion of the striate cortex below the calcarine fissure , and the bottom portions of the visual field (blue and yellow or purpleish-orange) end up in the area of striate above the calcarine fissure.
What is a visual edge? It is a place where two different areas of a visual image meet. Thus the perception of an edge is really the perception of contrast between two adjacent areas of the visual field.

Mach bands - which are nonexistent stripes of brightness and darkness running adjacent to the edges of the real stripes. Mach bands enhance the contrast at each edge making the edges easier to see. This contrast enhancement occurs whenever we look at an edge, so our perception of an edge is better than the real thing .
The classic experiments on the physiological basis of contrast enhancement were conducted on the lateral eyes of the horseshoe crab, which have large receptors called ommatidia . The large axons of the ommatidium are interconnected by a lateral neural network called the lateral plexus .
If a single ommatidium is illuminated, it will fire at a rate that is proportional to the intensity of light striking it. Also, when a single ommatidium fires it inhibits its neighbors via the lateral plexus - this is called lateral inhibition or mutual inhibition .
Hubel and Wiesel studied single neurons of the cat and monkey visual systems. Their procedure involved implanting a microelectrode near a single neuron and then immobilizing the eye with carare. Next they used an adjustable lens to sharply focus a image on the retina and then mapped the neurons receptive field - the area of visual field within which it is possible for a visual stimulus to influence the firing of that cell. Since visual system neurons tend to be continually active any stimulus that increases or decreases the firing rate of the cell are within its receptive field. Finally, different test stimuli are presented within the neuron’s receptive field to identify the ones that influence the rate of firing. The electrode can then be advanced slightly until its tip is near another neuron and the process is repeated. Hubel and Wiesel used this procedure to study the different stages of processing along the retina-geniculate-striate pathway. They found that neuron’s at each of the 3 levels had round receptive fields. Because there is less convergence in foveal circuits, neurons that respond to stimuli in the foveal area have smaller receptive fields than those in the peripheral retina.
Most cells in the retina-geniculate-striate pathway respond in two different ways to a spot of light that briefly appears in its receptive field. On-firing is characterized by an increase in activity while the light is on and Off-firing is characterized by decreased firing while the light is on followed by a burst of activity when the light is turned off. Thus the receptive fields of these neurons fall into two categories: On-center cells respond to light shone in the center of the receptive field with onfiring, and respond to light shone in the periphery of their fields with off-firing - inhibition while the light is on and a burst of activity when the light is turned off. Off-center cells display the opposite pattern: off-firing in response to lights in the center of the receptive field and on-firing in response to lights in the periphery. Cells with On-center and Off-center concentric receptive fields are the most prevalent type of neuron found in the retina-geniculate-striate system.
As you may have guessed, these cells respond best to contrast (that’s why Pinel describes them in the section on “seeing edges”). So, the most effective way to influence their firing is to completely illuminate the entire center or the entire surround while leaving the other region completely unilluminated. Diffusely illuminating both regions of the receptive field has little effect.
Therefore, Hubel and Wiesel suggested that these neurons respond to the degree of brightness contrast between the two areas of their receptive fields.
Two independent channels flow through the lateral geniculate nucleus of the thalamus. One channel runs through the top 4 layers, called the parvocellular layers (or P layers) because the cells in these layers have small cell bodies ( parvo means small). The other channel runs through the bottom two layers, called the magnocellular layers (or M layers) because they are composed of neurons with large cell bodies ( mango means large). P-layer cells respond to color, fine pattern detail and stationary or slowly moving objects, whereas M-layer cells respond to movement.
Cones provide the major input to P-layer cells while rods provide the major input to M-layer cells.
P-layer and M-layer cells project to slightly different areas of lower layer IV in the striate cortex: Mchannel cells project to the top part whereas the P-channel cells project to the bottom.

With the exception of cells found in the lower part of layer IV, striate neurons tend to respond to straight edges instead of circular edges. The two categories of these cells are called simple and complex. Like lower layer IV neurons, simple cells have 1) ‘on’ or ‘off’ regions, 2) are unresponsive to diffuse light and 3) are all monocular. Unlike lower layer IV cells, simple cells 1) have straightline ‘on’ and ‘off’ regions, respond to lines of a particular orientation and position and
3) have receptive fields that are rectangular. Simple cells respond best to bars of light in a dark field, dark bars in a light field, or single edges between dark and light areas. Like simple cells, complex cells also 1) have rectangular receptive fields, 2) respond best to straight-line stimuli in a specific orientation and 3) are unresponsive to diffuse light. However they differ from simple cells in three ways: 1) they have larger receptive fields, 2) they do not have static ‘on’ and ‘off’ regions and 3) many (> 50%) are binocular - i.e., they respond to the preferred stimulus when presented to either eye. Following point 2, complex cells respond to straight-edge stimuli of a particular orientation regardless of its position within the receptive field. Furthermore, if you move a straight-line stimulus at the right orientation across its receptive field it responds continuously and demonstrates a preference for a particular direction of movement. Some binocular cells fire more robustly if the appropriate stimulus is presented simultaneously to both eyes, but in slightly different positions on the two retinas – retinal disparity . Also, binocular cells respond more robustly to stimulation of one eye than to the same stimulation of the other eye – ocular dominance.
On the basis of these findings Hubel and Wiesel proposed a hierarchical model of the monkey primary visual cortex where the complexity of the receptive fields at higher levels of processing is attributable to the convergence of inputs from preceeding levels. So, the center-surround neurons in the lower layer IV converge on simple cells, which in turn converge on complex cells.
If an electrode is lowered vertically into the striate cortex and the receptive fields of neurons along the path are mapped, you find 3 common characteristics of the cells within a column : 1) they have receptive fields in the same general area of the visual field (called the aggregate field ); 2) they all prefer straight lines in the same orientation; and 3) all monocular cells and binocular cells
(that display ocular dominance for one eye) are more responsive to stimulation of the same eye.
If an electrode is passed horizontally through the striate you find: 1) each successive cell has a receptive field that is in a slightly different location of the visual field; 2) each of these cells respond to lines of a slightly different orientation; and 3) successive cells alternate with respect to left- or right-eye dominance.
Hubel and Wiesel proposed that the primary visual cortex is divided into functionally independent columns that are responsible for analyzing input from one area of the visual field. Each cortical column can be further divided into two areas that display ocular dominance. Each functional half of a cortical column is subdivided into smaller columns that prefer lines of a particular orientation.
DeValois and DeValois have proposed an important qualification to Hubel and Wiesel’s theory.
They claim that the visual cortex operates on a code of spatial frequency instead of straight lines and edges. The evidence is that striate neurons respond even more robustly to sine-wave gratings placed at particular angles in their receptive fields. A sine-wave grating is a set of equally spaced parallel, alternating light and dark stripes. The spatial frequency theory is based on two principles: 1) that any visual stimulus can be represented by plotting the intensity of light along lines that run through the stimulus (e.g. the line that runs through the zebra scene on page
148); 2) that any light intensity curve, no matter how irregular, can be broken down into constituent sine waves by a mathematical procedure called Fourier analysis .
The theory is that each functional module of visual cortex performs a sort of Fourier analysis on the visual pattern in its receptive field; the neurons in each module are thought to respond to various frequencies and orientations of sine-wave gratings. The straight-edge stimuli that Hubel and Wiesel used can be translated into component sine-wave gratings of the same orientation.
Therefore, the research of spatial frequency detection by visual neurons extends and compliments their research rather than refuting it.

Achromatic colors include black, white and gray. Black is experienced when there is a lack of light; the perception of white is produced by an intense mixture of a wide range of wavelengths in roughly equal proportions and the perception of gray is produced by the same mixture at lower intensities. Chromatic colors or hues depend on the wavelengths of light that a stimulus or object reflects into the eye. It is rare that an object outside the laboratory will reflect a single wavelength. Most objects absorb the different wavelengths of light that strike them to varying degrees and reflect the rest. It is this mixture of wavelengths that an object reflects that influences our perception of its color.
The component theory of color vision (also referred to as the trichromatic theory ) was proposed by Young in 1802 and refined by Helmholtz in 1852. The theory states that there are 3 different kinds of color receptors (cones), each with a different spectral sensitivity, and the color of a particular stimulus is presumed to be encoded by the ratio of activity in the three receptors.
Young and Helmholtz observed that any color in the visible spectrum can be matched by mixing together 3 different wavelengths of light in different proportions. The fact that you need a minimum of 3 wavelengths to match every color suggested that there were 3 types of receptors.
Another theory of color vision was proposed by Hering in 1878. The opponent-process theory suggests that there are two different classes of cells that encode color and a third that encodes brightness. Each class of cells encode 2 complementary perceptions. For example, one class of cel ls signal red by changing its rate of firing in one direction (hyperpolarization) and signal red’s complement, green, by changing its firing rate in the opposite direction (hypopolarization).
Another class of cells signal blue and its complement yellow, in the same opponent fashion; and a class of brightness cells signal both black and white. Complementary colors are pairs of colors that produce white when combined.
Hering based his theory of color vision on several observations. First, complementary colors (blue and yellow or red and green) cannot exist together in the same color; so, there is no such thing as a yellowish blue or a greenish red. Second, the afterimage that is produced by staring at red is green or vice versa, and the afterimage produced by staring at blue is yellow or vice versa.
It turns out that both theories are correct. In the 1960’s the development of microspectrophotometry which is a technique for measureing the absorption spectrum of the photopigments contained in a single cone, confirmed the existence of 3 different types of receptors which supports component theory; the 3 cones are maximally sensitive to short, medium or long wavelengths of light. So, at the receptor level color coding operates on a purely component basis. However, at all other levels of the retina-geniculate-striate system there is evidence for opponent processes.
Neither theory can account for an important characteristic of color vision: color constancy - which is a tendency for an object to stay the same color despite major changes in the wavelengths of light that it reflects. This ability lies in the perception of contrast between adjacent areas of a visual display. Edwin Land (inventor of the polariod) demonstrated this in several laboratory experiments (e.g., the Mondrians viewed as different proportions of three wavelengths of light). Subjects would adjust the light from 3 projectors (short, medium and long wavelengths) till it appeared white. Then they were shown the Mondrians with one of the colors (blue) adjusted to reflect the same wavelengths just judged to be white. However, the subjects saw blue based on color constancy. The retinex theory proposes that color is determined by reflectance – the proportion of light of different wavelengths a surface reflects. Despite the fact that reflected light changes based on the overall illumination conditions, the amount of light absorbed and reflected by a surface is constant. Thus the visual system compares the light reflected by adjacent surfaces in at least 3 different wavelength bands.
If the perception of color depends on contrast then one may expect to find neurons that are responsive to color contrast. Such cells have been identified and are called dual-opponent color cells . They have a circul ar receptive field and respond with vigorous “on” firing when the center of the receptive field is illuminated with one wavelength, such as green, and the surround is

simultaneously illuminated with the complement wavelength red. The same cells display “off” firing when the pattern of illumination is reversed. Livingstone and Hubel have found that these cells have particularly high concentrations of the enzyme cytochrome oxidase , so when the visual cortex is stained for the enzyme peg-like columns can be seen. When the striate is cut parallel to the cortical layers the staining appears as blobs .
Mechanisms of Perception
Principles of Sensory System Organization
- primary sensory cortex receives most of its input directly from the thalamic relay nuclei.
- secondary sensory cortex receives most of its input from the primary sensory cortex of that system or from other areas of secondary sensory cortex.
- association cortex receives input from more than one sensory system and most of the input comes from secondary sensory cortex.
Hierarchical Organization
- This organization illustrates the first principle of sensory system organization, which is the same as the first principle previously mentioned for the sensorimotor system:
- hierarchical organization - the fact that specific levels can be ranked according to the specificity and complexity of their function. Each level of a sensory hierarchy receives its input from lower levels and adds another layer of analysis before passing it on up the hierarchy.
- Based on this principle one might predict that damage to different levels of a sensory system would have different effects. Destruction of a lower level (e.g., receptors) would completely eliminate the ability to detect the presence of stimuli - which is sometimes referred to as sensation. In contrast, destruction of a higher level (e.g., secondary sensory or association cortex) may disrupt more complex and specific fuctions. The higher-order process of integrating, recognizing and interpreting complete patterns of sensations is referred to as perception.
- e.g., Dr. P., the man who mistook his wife for a hat, had normal visual sensations (objects appeared as normal; e.g., his description of a glove), but could not recognize what the objects were.
- Although it is not possible to specify the point at which sensation becomes perception, many believe that the former is the result of subcortical processing and the latter cortical processing.
Functional Segregation
- It was once thought that all cortical areas of a sensory system, including primary, secondary and association areas, were functionally homogeneous (i.e., act together to perform the same function).
- However, recent research has shown that there is functional segregation at each level; that is, each level contains functionally distinct areas that specialize in different kinds of analysis.
Parallel Processing
- It was also once thought that different levels of a sensory hierarchy were connected in serial fashion; that is, information flows through one pathway encountering successive levels one at a time (e.g. like a string through a strand of beads).
- However, recent evidence has shown that sensory systems process information in parallel fashion over multiple pathways and in different ways simultaneously. This is called parallel processing.
Current Model of Sensory System Organization
- In the 60’s the sensory systems were believed to be hierarchical, functionally homogeneous, and serial. However, four decades of research have established that they are hierarchical, unctionally

segregated or heterogeneous, and parallel. Note that the difference involves only two attributes: functional segregation and parallel processing.
- If the current model, with functional segregation and parallel processing, is correct then how do our brains combine the information processed by multiple pathways to produce integrated perceptions?
- One idea is that these pathways might converge at the top of the hierarchy; however there is no area of cortex that receives input from all of the lower levels of a sensory system.
- Therefore it is likely that perceptions are a product of the combined activity of many cortical areas within a sensory system.
8.2 Cortical Mechanisms of Vision
- The primary visual (or striate) cortex is located in the posterior region of the occipital lobe, with much of it hidden from view in the longitudinal fissure.
- The secondary visual cortex is located in two areas: the prestriate cortex - which is a band of tissue in the occipital lobe that surrounds the striate cortex - and the inferotemporal cortex which is in the inferior part of the temporal lobe.
- There are several areas of association cortex that receive visual input, but the largest one
(discussed in the chapter) is the posterior parietal cortex.
- We have just described these cortical visual areas in their proper hierarchical sequence and as a general rule as you move up the hierarchy the neurons have larger receptive fields and the stimuli to which they respond are more specific and complex.
Scotomas and Completion
- Damage to the primary visual cortex in one hemisphere results in a scotoma (an area of blindness) in the contralateral visual field of both eyes (hemianopsia).
- The method used to determine the area of blindness is called perimetry . This test given to patients suspected to have damage in the visual cortex. The patient stares with one eye at a fixation point on a screen while a small dot of light is flashed in various areas. The patient presses a button when the dot is in view. This procedure results in a map of the visual field of each eye.
- In patients with damage to the primary visual cortex the center of the field of vision is frequently spared (macular sparing).
- We discussed the phenomenon called completion last week (with respect to the blind spot or optic disk)
- Scotoma patients frequently report seeing a whole figure even though part of the figure lies within the blind portion of the visual field. For example if a patient with hemianopsia focuses on a persons nose they might see a complete face even though half of it lies within the blind part of the visual field.
- Is the completion phenomenon based on some spared or residual visual function?
It could be in some cases but if you cover up the half of the face that lies within the scotoma they still see a complete face - suggesting that the visual system is filling in the gap just as it does with our natural blind spot.
- Karl Lashley, the esteemed physiological psychologist (who Pinel considered the father of the field in the first edition of his text) frequently experienced a large scotoma during migraine attacks and would cut off the head of the person he was talking to; the wallpaper in the background would fill in the gap of the persons missing head.
Blindsight: Seeing without Consciousness

- Patients with complete lesions of the primary visual cortex report total blindness yet can perform some visually guided tasts such as grabbing for an object or indicating the direction of an objects movement. This ability to perform some visual tasks without conscious awarness is known as blindsight.
- How can patients with damage to the primary visual cortex see features of visual stimuli - primarily their location, orientation, and movement - without being aware of it?
- Some subcortical visual signals bypass the primary visual cortex and go to the prestiate cortex and could mediate the spared visual function.
- Although in the second edition Pinel discribed one of these pathways that projects from the superior colliculi to the pulvinar nuclei of the thalamus, and from there directly to areas of the secondary visual cortex (collicular-pulvinar pathway); but then Pinel hedged this possibility by saying that it had not held up well against experimental tests. I guess this has resulted in the specifics being removed from the current edition but the idea is still there, so maybe there is some other pathway.
- Regardless of the basis of the spared visual abilities the fact that the patients are not consciously aware of visual stimuli suggests that the primary visual cortex is critical for conscious awareness.
Perception of Subjective Contours
- Perceived visual contours that do not exist are called subjective contours and demonstrate that our visual perceptions are often better than the physical reality of visual input.
- (Figure 8a) - You should see a circle in the top illustration and a triagle in the bottom one
(especially since I selected a circle and triangle with the computer program and then removed the contour or border).
- If you don’t see these shapes than I suggest getting an MRI because you might have damage to your visual cortex.
- What areas are important for perceiving subjective contours? Research shows that prestriate neurons and even primary visual cortical neurons respond when subjective contours of the appropriate orientation appear in their receptive field (Figure 8b).
Functional Segregation in Visual Cortex: The Dorsal and Ventral Streams*
- Secondary and association visual cortex is composed of many different functional areas; in the macaque 24 in secondary visual cortex and and 7 in association cortex with over 300 interconnecting pathways.
- Ungerleider and Mishkin (1982) were the first to point out that many of the parallel visual pathways can be divided into two general streams:
- The dorsal stream flows from the primary visual cortex to the dorsal prestriate and then to the posterior parietal cortex.
- The ventral stream flows from the primary visual cortex to the ventral prestriate and the inferotemporal cortex. then to
- Ungerleider and Mishkin proposed that the dorsal stream is involved in the perception of “where” objects are and the ventral stream is involved in the perception of “what” objects are.
- Some of the evidence supporti ng this “where” versus “what” theory comes from patients with damage to either posterior parietal cortex (dorsal stream) or the inferotemporal cortex. Patients with damage to the posterior parietal cortex have difficulty reaching accurately for objects that they have no difficulty identifying and patients with damage to the inferotemporal cortex have an opposite pattern - impaired object identification and spared reaching.
- Goodale and Milner have proposed an alternative theo ry: instead of “where vs what” they propose that the distinction should be “how vs what”. They stress that the difference isn’t in the kind of information the two streams deal with but rather the use to which the information is put.

The dorsal stream directs behavioral interactions with objects while the ventral stream mediates the conscious perception of objects
- The evidence for Goodale and Milner’s theory is that patients with dorsal damage who have difficulty picking up objects can often describe the size, location, shape, and movement of objects in great detail; whereas patients with ventral damage have difficulty describing the size, shape, location and movement of objects but show that these attributes have been perceived by their precision of reaching and grasping behavior.
- Case D.F., who had damage to the prestriate cortex (presumable the ventral “what” stream of both theories) couldn’t describe the details of the object very well but did demonstrate that size and shape information was perceived by changing her reaching and grasping movements in accordance with the dimensions of the object. How does this case contradict the “where vs what” theory of Ungerleider and Mishkin?
Visual Agnosia
- Agnosia is a failure of recognition that is not attributable to sensory, verbal, or intellectual impairment. It is usually specific to a particular sensory system.
- Visual agnosia is classified according to the category of visual material; prosopagnosia (faces), movement agnosia, object agnosia,or color agnosia.
- Some believe that prosopagnosia results from damage to an area of visual cortex that specializes in the recognition of faces. The supportive evidence is that they can recognize other test objects (e.g. a chair, pencil or door).
- However, this logic is flawed. Why? Prosopagnosics can recognize that a face is a face, so their is no reason why they should not be able to recognize a chair as a chair or a pencil as a pencil etc.
- They have a specific deficit in recognizing or distinguishing among similar members of a particular class of visual stimuli (e.g. the prosopagnosic farmer could no longer identify individual cows; and the prosopagnosic bird watcher could no longer identify particular types of birds).
- Histological findings suggest that prosopagnosics have damage to the inferior prestriate cortex and the adjoining portions of the inferotemporal cortex. Consistent with this evidence is the finding that individual neurons in the inferotemporal cortex respond selectively to faces.