PSYC326_Chapter 6

Perception of color: extrastriate cortex - V4

**Color constancy: ensures that the perceived color of objects remains relatively constant under varying illumination conditions. Monkey: 1) Color-sensitive neurons in the CO blobs of the V1 send color-related information to the thin stripes in area V2 2) Neurons in V2 send information to an adjacent region of the extrastriate cortex, called V4. V4: respond selectively to colors, but their response characteristics are much more complex than those of neurons in V1 or V2. These neurons respond to a variety of wavelengths, not just the wavelengths that correspond to red, green, yellow, and blue. Responded to specific colors - some responded to colored bars of specific orientation. Area V4 seems to be involved in the analysis of form as well as color. Unusual secondary receptive field: a large region surrounding the primary field - When stimuli were presented in the secondary receptive field, the neuron did not respond. Stimuli presented there could suppress the neuron's response to a stimulus presented in the primary field. - These cells responded to particular wavelengths of light but subtracted out the amount of that wavelength that was present in the background.

Transduction: light current

1. All the positive charges cannot get into the photoreceptors and it polarises. Glutamate cannot be released into the synapse. - Graded potentials: IPSPs and EPSPs. No action potential 2. There is a disinhibition of the bipolar cell. Canceling out the negative results in an excitatory reaction - Graded potentials: IPSPs and EPSPs. No action potential 3. Bipolar cell can release its neurotransmitters on the ganglion cell. Bipolar cells will release glutamate which functions as excitatory on the ganglion receptors which will help them have an action potential 1. Resting potential is -40mV in a photoreceptor. When light comes on there is a slight delay and the reaction is a reduction in voltage to -70mV. When the light is turned off the photoreceptors go back to the resting potential 2. When the photoreceptors become slightly negative, the bipolar cells become excited (because glutamate is not being released which acts as an inhibitor) and they depolarise. When it becomes excited, it releases a neurotransmitter and can excite the ganglion cell.

Visual pathway: photoreceptors to V1

1. Begins in the photoreceptors of the retina which synapse with bipolar and retinal ganglion cells 2. The axons of the retinal ganglion cells send visual information to the rest of the brain by ascending through the optic nerves to the LGN in the thalamus 3. The neurons in the LGN send their axons through optic radiations to the primary visual cortex/V1 (the first cortical area devoted to vision)

Visual pathway: V1 onwards

1. From the V1, visual information is sent to a number of areas in the visual association cortex or V2. 2. Information about more complex aspects of visual processing may also travel on to additional cortical areas, such as V4 or V5.

Visual pathway: overview

1. Photoreceptors in Retina 2. Synapse with bipolar and retinal ganglion cells 3. Ascend through optic nerves to LGN 4. LGN sends axons to primary visual cortex (V1) 5. Visual Association Cortex (V2)

Perception of color: striate cortex

1. The retinal ganglion cells encode information about the relative amounts of light falling on the center and surround regions of their receptive field and about the wavelength of that light 2. This information is then relayed to the LGN, then on to the striate cortex. 3. The striate cortex performs additional processing of this information, which it then transmits to the extrastriate cortex

Retina in color perception: opponent-color system theory - detection and coding

A cell might be excited by red and inhibited by green in the center of their receptive field while showing the opposite response in the surrounding ring. Black-and-white detectors: ganglion cells that receive input from cones do not respond differentially to different wave- lengths but simply encode relative brightness in the center and surround. The response characteristics of retinal ganglion cells to light of different wavelengths are determined by the particular circuits that connect the three types of cones with the two types of ganglion cells. 1. Red light excites "red" cones, which causes the excitation of red-green ganglion cells 2. Green light excites "green" cones, which causes the inhibition of red-green cells. 3. Because the wavelength that produces the sensation of yellow is intermediate (wavelengths of red and green) it will stimulate both "red" and "green" cones equally. Yellow-blue ganglion cells are excited by both "red" and "green" cones -their rate of firing increases - Red-green ganglion cells are excited by red and inhibited by green - their firing rate does not change - The brain detects an increased firing rate from the axons of yellow-blue ganglion cells, which it interprets as yellow. Blue light inhibits the activity of yellow-blue ganglion cells

Vision: blindsight

A phenomenon in which cortical regions involved in conscious perception of visual stimuli are damaged, but other visual pathways that are not involved in conscious perception are intact. You can still things you are not aware of you. There are different levels that we experience things

Perception of form: extrastriate cortex - recognising categories - lateral occipital cortex (LOC)

A relatively large region of the ventral stream of the visual association cortex Responds to objects other than faces and bodies - The fact that neurons in the primate inferior temporal cortex respond to very specific complex shapes suggests that the development of the circuits responsible for detecting them must involve learning Categories of other objects, not faces - Once you pick it up, tactile information can help you recognise what is is Just as active in the child as it is in the adult

Perception of color: striate cortex - koniocellular system

Additional information from "blue" cones, which are much less numerous than "red" and "green" cones Does not provide information about fine details. The ventral stream receives approximately equal input from the magnocellular, parvocellular and koniocellular systems.

Visual pathway: visual association cortex (V2)

Also known as the extrastriate cortex because the region surrounds the striate cortex

The optic nerves

At the back of each eye, axons of the retinal ganglion cells bundle together to form the optic nerve The optic nerves convey information from the retina to a portion of the thalamus - dorsal lateral geniculate nucleus (LGN) The lens inverts the image of the world projected on the retina (and similarly reverses left and right) - Each hemisphere receives information from the contralateral half (opposite side) of the visual scene - the right hemisphere receives information from the left half of the visual field, and the left hemisphere receives information from the right - It is not correct to say that each hemisphere receives visual information solely from the contralateral eye

Central and peripheral vision: receptive fields - periphery

At the periphery of the retina many individual receptors converge on a single ganglion cell, bringing information from a relatively large area of the retina - relatively large area of the visual field - One ganglion cells are connected to several bipolar cells to several photoreceptors We are not sure what part of the retinal field this information is coming from because it is not very specific Less discriminating Receptive field: pretty large

Perception of form: extrastriate cortex - inferior temporal cortex TEO

Being able to learn new objects The receptive fields of neurons in area TEO are larger than those in area V4 Comes from primate literature Temporal/occipital lobe - Find V8 at the interception of these two lobes

Retina: ganglion cells

Bipolar cells form synapses with the ganglion cells Neurons axons travel through the optic nerves (the second cranial nerves) and carry visual information into the rest of the brain. - Goes quite a long distance to the brain itself An action potential is necessary to send information from the cell body to the axon If a cell is really large, it has a very long projection. It might be projecting a great distance or it has a lot of projection going out from it. You need a large cell to support the large amount of material - Motor cortex —> spinal cord: head to the big toe - Spinal cord —> muscles (big toe)

Transduction: photoreceptors - bipolar cells

Bipolar cells have two different responses to the presence of glutamate: 1. ON center bipolar cells are hyperpolarized by glutamate 2. OFF center bipolar cells are depolarized The ON and OFF center bipolar cells then synapse with ganglion cells and influence their rate of firing - Light shining on the photoreceptor can produce a variety of responses from bipolar and ganglion cells as the transduced message begins to make its way out of the retina and to the brain for additional visual processing

Sensation

Cells of the nervous system detect stimuli from the environment

Retina: fovea

Central region of the retina There is a little indentation in the back of the eye which is the fovea. Color-sensitive cones are the only type of photoreceptor found in fovea Mediates our most acute vision

Extrastriate cortex: pathways - dorsal and ventral in perception of form

Children study: children played with larger toys, were removed from the room and toys were replaced with smaller toys, and the children could not tell the difference. - Reflects incomplete maturation of connections between the dorsal and ventral streams - The ventral stream recognizes the identity of the objects, and the dorsal stream recognizes their size, but in the developing brain the information is not adequately shared between these two systems

Perception of color: extrastriate cortex - V8

Color discrimination: distinguish between colors A color-sensitive region that included the lingual and fusiform gyri Damage to this region disrupts color vision. - Some people with brain damage lose the ability to perceive shapes but can still perceive colors.

Perception of color: striate cortex - magnocellular system

Color-blind They are not able to detect fine details They can detect smaller contrasts between light and dark. Especially sensitive to movement Dorsal stream exclusively The ventral stream receives approximately equal input from the magnocellular, parvocellular and koniocellular systems.

Perception of form: extrastriate cortex - development aspects of recognition - cataract

Congenital cataracts (cataract present at birth): tested recognition of subtle differences between pairs of faces - Had been unable to see more than light and dark until they received eye surgery when they were 62-187 days old - The early visual deprivation resulted in a severe deficit, compared with the performance of control participants, in recognizing the facial differences. Cataracts in one eye: Because of the immaturity of the newborn brain, visual information received by one eye is transmitted only to the contralateral visual cortex. - People born with cataracts in their left eye would show a deficit in recognizing faces because the right fusiform gyrus is critical for facial recognition - People born with cataracts in the right eye would show normal discrimination

Perception

Conscious experience and interoperation of information from the sense

Visual processing: striate cortex (V1) - layers

Consists of six layers arranged in bands parallel to the surface. The fourth layer is further subdivided into sublayers

Central and peripheral vision: types of eye movements - vergence movements

Cooperative movements that keep both eyes fixed on the same target - keep the image of the target object on corresponding parts of the two retinas. Example: Your eyes will follow your fingers as they come closer together in front of your face. They will also rotate outwards when you move them away from one another

Perception of color: extrastriate cortex- damage to V8 - cerebral achromatopsia

Damage V8 Loss of color vision without disruption of visual acuity You can be color blind for two reasons: 1) You don't have the photoreceptors in the retina 2) You have the color receptors and they work fine but there is damage in V8 and it cannot discriminate between one color and the next. May not be able to imagine colors or remember the colors of objects they saw before their brain damage occurred Brain damage is unilateral: people will lose color vision in half of the visual field.

Perception of color: extrastriate cortex - V4 damage

Damage does not disrupt color constancy. Performance was impaired when the color of the overall illumination was changed. - Some region besides area V4 must be involved in color vision

Perception of form: extrastriate cortex - visual agnosia

Damage to the human lateral occipital cortex can cause a category of deficits known as visual agnosia. - Damage to the parts of the extrastriate cortex that contribute to the ventral stream. Case study: she could not identify common objects by sight, even though she had relatively normal visual acuity. She could still read, even small print, which indicates that reading involves different brain regions than object perception does. When holding objects she could not recognise she could immediately recognize it by touch and say what it is - she had not lost her memory for the object or simply forgotten how to say its name.

Extrastriate cortex: pathways

Dorsal stream Ventral stream The streams begin to diverge after area V2

Visual processing: lateral geniculate nucleus (LGN) - magnocellular layers

Dorsal stream - Rods Two inner layers - Larger than those in the outer four layers These cells project to the cortex and they also project to other sub cortex structures They are very fast Perception of form, movement, depth, peripheral vision, and small differences in brightness to primary visual cortex One of the first things we are born with that are working - Develops early Babies can recognise faces in general because of the magnocelluar - Gets an overall shape of the head and face

Receptor potentials

Electrical changes in the cells membrane potential from sensory receptors Receptor potentials affect the release of neurotransmitters and can modify the pattern of firing in neurons with which sensory receptors form synapses

Transduction

Energy from the environment is converted to a change in membrane potential in a neuron. It is a process that converts an external stimulus to an internal stimulus If we were to put a recording electrode (extracellular electrode) you would get a recording that shows that there is no immediate change in the ganglion cell and it takes some time for the ganglion cell to react - Has to wait for the bipolar cell and the photoreceptors to act - Extracellular recording so it shows spike: single unit recording

Extrastriate cortex

For us to perceive objects and entire visual scenes, the information from the V1 (visual perception) must be combined. That combination takes place in the extrastriate cortex. Structures: Regions V2 - V8 and others - Each contains one or more independent maps of the visual field - Each region is specialized - containing neurons that respond to particular features of visual information: orientation, movement, spatial frequency, retinal disparity, or color. Specialized to respond to features Arranged hierarchically

Perception of color: retinal ganglion cells - ON and OFF cells in frogs, cats, and primates

Frogs: contain three types of ganglion cells - ON cells: responded with an excitatory burst of action potentials when the retina was illuminated - OFF cells: responded when the light was turned off - ON/OFF cells: responded briefly when the light went on and again when it went off. Cats: receptive field consists of a roughly circular center, surrounded by a ring. Stimulation of the center or surrounding fields had contrary effects - ON cells: excited by light falling in the central field (center) and were inhibited by light falling in the surrounding field (surround) - OFF cells: excited by light falling in the surrounding field (surround) and were inhibited by light falling in the central field (center) - ON/OFF cells: briefly excited when light was turned on or off In primates: - Most of these ON/OFF cells project mainly to the superior colliculus, which is primarily involved in visual reflexes in response to moving or suddenly-appearing stimuli. These cells do not appear to play a direct role in form perception. - ON cells signal increases in illumination and OFF cells signal decreases, but both convey changes in illumination through an increased rate of firing action potentials. - ON cells and OFF cells signal different kinds of information about light and dark. Rebound effect: occurs when the light is turned off again. Neurons whose firing is inhibited while the light is on will show a brief burst of excitation when it is turned off. Neurons whose firing is increased will show a brief period of inhibition when the light is turned off

Perception of form: extrastriate cortex - development aspects of recognition - autism

Have difficulty developing typical social relations with other people. People with autism have a deficit in the ability to recognize faces and that looking at faces failed to activate the fusiform gyrus. - The lack of interest in other people, caused by the brain abnormalities responsible for autism spectrum disorder, resulted in a lack of motivation that normally promotes the acquisition of expertise in recognizing faces as a child grows up.

Perception of form: extrastriate cortex - parahippocampal place area (PPA)

Hippocampus and nearby regions of the medial temporal cortex are involved in spatial perception and memory Activated by the sight of scenes and backgrounds Scene recognition does not depend on recognition of particular objects found within the scene

Retina

Images are focused on the retina. The inner lining of the eye - Image is flipped upside down when looking at imagines The image causes changes in the electrical activity of millions of sensory receptors in the retina, which results in messages being sent through the optic nerves to the rest of the brain. The optical nerve and the retina are part of the central nervous system Fibers from the retina also take several other nonimage forming pathways

Transduction: dark current

In the dark the photoreceptors are slightly excited. When light hits the photoreceptors they become slightly inhibited. - In the dark there are channels that are slightly open that allows sodium to come into the cells - Because it is slightly less inhibited they are releasing neurotransmitters that are impacting the bipolar cells - The influence of these neurotransmitters on the bipolar cells is actually an inhibition. So normally, no signal goes into the brain Glutamate (Glu): it is known for being an excitatory neurotransmitter. The released glutamate by the photoreceptors acts as an inhibitor on the bipolar cells.

Visual pathway: primary visual cortex (V1)

In the occipital lobe Also called the striate cortex because it contains a dark-staining layer (striation) of cells. Each of the thousands of modules of the striate cortex sees only what is happening in one tiny part of the visual field.

Perception of form: extrastriate cortex - recognising faces - congenital prosopagnosia

Inability to recognize faces without having obvious damage to the FFA Anterior fusiform gyrus is smaller in people with congenital prosopagnosia Have decreased connectivity within the occipito-temporal cortex

Perception of form: extrastriate cortex - recognising faces - prosopagnosia

Inability to recognize particular faces. Patients with this disorder can recognize that they are looking at a face, but they cannot say whose face it is - They still remember who these people are and will usually recognize them when they hear the person's voice.

Perception of form: extrastriate cortex - inferior temporal cortex

Inferior temporal cortex: 1) Posterior area (LOC/TEO - temporal/occipital) - Visual discrimination 2) Anterior area (TE and IT) - Visual object recognition (TE = temporal) - Visual learning (IT = inferior temporal) - Damage to these regions causes severe deficits in visual discrimination **Learning specific objects These neurons respond best to three-dimensional objects. Respond poorly to simple stimuli such as spots, lines, or sine-wave gratings The development of the circuits responsible for detecting them must involve learning - Neurons in the inferior temporal cortex that respond specifically to objects that the monkeys have already seen many times but not to unfamiliar objects

Visual processing: striate cortex (V1) - cytochrome oxidase (CO) blobs

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. Found: 1. In layers 2, 3, and more faintly in 5 and 6 of the V1. Modules 1. Neurons located within the blobs have a special function: Most of them are sensitive to color 2. Outside the CO blob, in the interblob regions, neurons show sensitivity to orientation, movement, and binocular disparity, but most do not respond to color

Perception of form: striate cortex - low spatial frequency - magnocellular

Large areas of light and dark are represented by low frequencies. The most important visual information is that contained in low spatial frequencies When low-frequency information is removed: shapes of images are very difficult to perceive.

Perception of form: extrastriate cortex - development aspects of recognition

Lateral occipital complex: is the same in children and adults Left FFA does not reach its eventual size until adulthood. Ability to recognise faces is directly related to the expansion of the FFA - The ability to recognize faces is a learned skill that grows with experience. Newborn babies prefer to look at stimuli that resemble faces - the presence of prewired circuits that dispose babies to look at faces and learn to recognize them. The experience of seeing faces very early in life plays a critical role in the development of the skills necessary for recognizing them later in life.

Pathway of light

Light --> lens --> main part of the eye filled with vitreous humor (clear, gelatinous substance) --> retina Within the retina: must pass through the optic nerve, ganglion cells, bipolar cells, and reach the photoreceptor cells. Light must pass through the overlying layers. Once it reaches the photoreceptors --> bipolar cells --> ganglion cells --> optic nerve --> brain

Retina: photoreceptors - direct eye

Light hits the photoreceptors (developed from skin) directly —> neural ganglia Invertebrates, arthropods - Squid, octopus, spiders

Perception of form: extrastriate cortex - recognising faces - fusiform face area (FFA)

Located in the fusiform gyrus on the base of the temporal lobe Recognising faces - We recognize faces of people of our own race more accurately than faces of people of another race - people have more experience seeing members of their own race, which indicates that expertise does appear to play a role in face recognition Expert recognition - A farmer being able to recognise the faces of his cows; a bird expert can recognise different species of birds etc. Face-recognition circuits develop as a result of the experience we have of seeing people's faces. Because of the extensive experience we have of looking at faces, we can become expert at recognizing them

Retina: photoreceptors - cones

Maximally sensitive; color vision; acuity Provide most of the visual information about our environment - Daytime vision - In dim light: color blind and lack foveal vision Color vision - discriminate light of different wavelengths - Red, green, and blue cones. All colours that we see are combinations of these colours Found in fovea (central retina) Not many cones in the periphery

Mechano stimulation

Mechanical forces that are changing. Sensory stimulation to the understanding of it in the nervous system. Important for balance and hearing

Retina: photoreceptors - horizontal cells

Neuron in the retina that interconnects adjacent photoreceptors and the outer processes of the bipolar cells Transmit information in a direction parallel to the surface of the retina. They combine messages from adjacent photoreceptors.

Perception of form: striate cortex

Neurons in the primary visual cortex detected lines and edges

Retina in color perception: opponent-color system theory

Neurons respond specifically to pairs of primary colors, with red opposing green and blue opposing yellow Retina contains two kinds of color-sensitive ganglion cells: 1. Red-green 2. Blue-yellow What it explains: - Why we cannot perceive a reddish green or a bluish yellow: An axon that signals red or green (or yellow or blue) can either increase or decrease its rate of firing; it cannot do both at the same time.

Perception of color: striate cortex - CO blobs

Neurons within the CO blobs in the striate cortex respond differentially to colors. These neurons respond in opponent fashion - This information is analyzed by the regions of the extrastriate cortex that are a part of the ventral stream

Perception of color

Not all components of the visual system have the same amount of involvement in different visual functions.

Central and peripheral vision: fovea and central vision

One photoreceptor to one bipolar cell to one ganglion cells - More discriminating - Receptive field: very small The fovea contains approximately equal numbers of ganglion cells and cones - These receptor-to-axon relationships explain the fact that our foveal (central) vision is very acute but our peripheral vision is much less precise

Central and peripheral vision: types of eye movements - pursuit movement

Only by performing a pursuit movement can you make your eyes move more slowly

Visual processing: striate cortex (V1)

Orientation, movement, spatial frequency, retinal disparity, and color - Important for seeing how close or far away something is from you The neural circuitry within the visual cortex combines information from several sources in such a way as to detect features that are larger than the receptive field of a single ganglion cell or a single cell in the LGN. - Feature detectors: can help people understand what they are looking at i.e. is it a line? Is the line moving in a particular direction at a particular speed? Are the lines converging? What are the dimensions? - Neurons respond to specific features The striate cortex (V1), receives input from the LGN and is the first cortical region involved in combining visual information from several sources. - Primary visual cortex does not do a lot of color processing The retina can only detect dots of lights which will be combined to create images in the cortex Keeps information from the different eyes separate Highly organized structure - Contains neurons arranged in... 1) Layers 2) Cytochrome oxidase blobs 3) Modules. Look at it as a flat surface: contains a map of contralateral half of the visual field. - The map is disproportionate: a large portion is devoted to the analysis of information from the fovea - corresponds to a very small part of the visual field

Visual processing: lateral geniculate nucleus (LGN)

Part of the thalamus that relays visual information from the eye to the cerebral cortex. Receives information from both eyes Contains six layers of neurons. Each layer receives information from the retinal ganglion cells in the optic nerve of only one eye in a semialternating fashion Layers: 1, 4, 6 - Receive input from the contralateral (or opposite) eye Layers: 2, 3, 5 - Receive input from the ipsilateral (correlating) eye

Perception of color: extrastriate cortex - patient P.B.

Patient P. B. experienced damage to the extrastriate cortex. Structural and functional MRI data from the patient P. B. show activation in area V1 when correctly identifying colors, though he could not perceive the form or shape of the stimulus.

Vision: damage to the cortical visual system

People can use the nonimage forming pathways to guide hand movements toward an object even though they cannot see what they are reaching for.

Perception of form: extrastriate cortex - development aspects of recognition - William's syndrome on chromosome 7

People with this disorder usually show intellectual deficits and an intense interest in music. Generally very sociable, charming, and kind. - Show great interest in other people and spend a great deal of time looking closely at their faces. They are generally better at recognizing faces than people without the syndrome. The fusiform face area was enlarged in people with William's syndrome and that the size of the FFA was positively correlated with a person's ability to recognize faces.

Transduction: photopigments

Photopigments are responsible for the transduction of light energy into changes in membrane potential

Retina: photoreceptors - indirect eye

Photoreceptors are at the very back of the eye Light —> cornea —> pupil —> lens —> has to pass through the ganglion cells, bipolar cells to reach the photoreceptor cells —> then goes to the bipolar cells —> ganglion cells —> optic nerve —> brain Vertebrates - mammals

Retina: bipolar cells

Photoreceptors form synapses with bipolar cells No action potential because it is so small - The inside of the cell does need to become less negative but it does not need an action potential to release the neurotransmitter. A lot of processing can be done without an action potential.

Perception of color: extrastriate cortex - TEO

Plays a critical role in visual discrimination. Destroyed area TEO, leaving area V4 intact: severe impairment in color discrimination. No difficulty in discriminating shades of gray. The deficit was restricted to impaired color perception.

Perception of color: striate cortex - parvocellular system

Receives information only from "red" and "green" cones They are able to detect very fine details, but their response is slow and prolonged. The ventral stream receives approximately equal input from the magnocellular, parvocellular and koniocellular systems.

Perception of form: extrastriate cortex - laboratory animals

Recognition of visual patterns and identification of particular objects take place in the inferior temporal cortex (in the ventral part of the temporal lobe) Inferior temporal cortex: 1) posterior area (TEO) and anterior area (TE) Area V1 is concerned with the analysis of el- ementary aspects of information in very small regions of the visual field, and successive regions (V2 etc.) analyze more complex characteristics. The size of the receptive fields also grows as the hierarchy is ascended.

Retina: photoreceptors - overview

Rods: 120 million Cones: 6 million Dendrites of the photoreceptors communicates with the bipolar cell --> bipolar cells communicate with the ganglion cell The photoreceptors and bipolar cells are not very large and do not have to go very long distances - They do not have action potentials at all because they are so small - The inside of the cell does need to become less negative but it does not need an action potential to release the neurotransmitter. A lot of processing can be done without an action potential.

Retina: photoreceptors - rods

Sensitive to low intensity light; brightness; movement Do not detect different color, poor acuity Found in peripheral retina - There are some rods that look at the central image - Don't perceive color very well in the periphery - You can get general information about what is in the periphery There are more rods than cods

Perception of form: extrastriate cortex - recognising categories

Several regions of the ventral stream that are activated by the sight of particular categories of visual stimuli. Regions of the inferior temporal and lateral occipital cortex that are specifically activated by categories such as animals, tools, cars, flowers, letters and letter strings, faces, bodies, and scenes. - General-purpose regions contain circuits that can learn to recognize shapes that do not fall into these categories. There are few regions of the extrastriate cortex devoted to the analysis of specific categories of stimuli. - 3 regions showed the greatest activation to the sight of specific categories: faces, bodies, scences - Regions that responded to faces and body parts were adjacent to each other, as were those that responded to objects and scenes of places.

Perception of form: striate cortex - high spatial frequency - parvocellular

Small objects, details within a large object, and large objects with sharp edges provide a signal rich in high frequencies An image that is deficient in high-frequency information looks fuzzy and out of focus - This image still provides much information about forms and objects in the environment

Sensory receptors

Specialized neurons that detect a variety of physical events Many lack axons. A portion of their somatic membrane forms synapses with the dendrites of other neurons.

Sensory transduction

Stimuli are detected by sensory receptors that alter, through various processes, the membrane potentials of the cells Eye: how to take the physical stimulus (photons) and convert them into an electrical stimulus that it can understand The processing of light in different parts of the eye is different in the nervous system

Perception of color: color constancy

The appearance of the colors of objects remains much the same whether we observe them under artificial light, under an overcast sky, or at noon on a cloudless day. Our visual system does not simply respond according to the wavelength of the light reflected by objects in each part of the visual field, it compensates for the source of light - Compensation appears to be made by simultaneously comparing the color composition of each point in the visual field with the average color of the entire scene.

Retina: optic disk

The axons all come together in one location. Ganglion cells form optic nerve. Cranial nerve II - Where the axons conveying visual information gather together and leave the eye through the optic nerve Back of the eye

Perception of color: retinal ganglion cells - center-surround organization

The center-surround organization of retinal ganglion cell receptive fields enhances our ability to detect the outlines of objects even when the contrast between the object and the background is low.

Extrastriate cortex: hierarchically

The extrastriate regions are arranged hierarchically, beginning with the striate cortex Most of the information passes up the hierarchy; each region receives information from regions located beneath it in the hierarchy (closer to the striate cortex), analyzes the information, and passes the results on to "higher" regions (farther from the striate cortex) for further analysis - Some information goes the opposite way, but much less.

Retina in color perception: trichromatic theory

The eye detected different colors because it contained three types of receptors, each sensitive to a single hue. Problems: - Cannot explain why yellow is included in this group—why it is perceived as a pure color. - Some colors appear to blend, whereas others do not - Cannot explain why colors that are reddish green or bluish yellow are not seen

Vision: areas important for vision

The human brain contains several mechanisms involved in vision. The system of cortical structures for conscious perception of vision is responsible for our ability to perceive the world around us At least one separate nonimage forming pathway is devoted mainly to controlling eye movements and bringing our attention to sudden movements that occur off to the side of our field of vision.

Perception of form: striate cortex - spatial frequency

The image of a stimulus on the retina varies in size according to how close it is to the eye, the visual angle is generally used instead of the physical distance between adjacent cycles. The spatial frequency of a sine-wave grating is its variation in brightness measured in cycles per degree of visual angle. Most neurons in the striate cortex respond best when a sine-wave grating of a particular spatial frequency is placed in the appropriate part of the visual field Spatial frequency analysis plays a central role in visual perception - The information present in a scene can be represented very efficiently if it is first encoded in terms of spatial frequency - Sharp edges contain high spatial frequencies, so the transformation eliminates them.

Retina: retinotopic organisation

The mapping of visual input from the retina to neurons, particularly those neurons within the visual stream Retinotopic maps in cortical areas other than V1 are typically more complex, in the sense that adjacent points of the visual field are not always represented in adjacent regions of the same area An image that falls on a certain part of the retina, is perceived as a different area than those that are near it. - A map of the visual field. Localisation of function

Retina: optic disk - blind spot

The optic disk produces a blind spot because no receptors are located there. We do not normally perceive our blind spots, but their presence can be demonstrated.

The optic nerves: optic chiasm

The optic nerves join together at the base of the brain to form the X-shaped optic chiasm. Some information from the left eye will cross over and some of it will stay on the left - If you are focused at the very center: you can divided the visual field into the left or right. Eventually the information that is seen, this information is brought together in the brain - Experience the whole visual field as one thing

Perception of form: extrastriate cortex - inferior temporal cortex TE

The receptive fields of neurons in area TE are the largest of all, often encompassing the entire contralateral half of the visual field Animal literal Further object identification in primates. Object recognition

Central and peripheral vision: receptive fields

The visual system is the part of the visual field that an individual neuron "sees"— the place in which a visual stimulus must be located to produce a response in that neuron. The location of the receptive field of a particular neuron depends on the location of the photoreceptors that provides it with visual information - If a neuron receives information from photoreceptors located in the fovea, its receptive field will be at the point at which the eye is looking - If the neuron receives information from photoreceptors located in the periphery of the retina, its receptive field will be located off to one side. The receptive fields are perceived in all parts of the nervous system

Retina: optic disk - cranial nerves II

There are 12 cranial nerves - They have numbers to them, and are numbered from the closest to the front to the farthest away

Retina in color perception: trichromatic theory - photoreceptors

Three different types of photoreceptors (three different types of cones) are responsible for color vision. - Blue, green, and red

Central and peripheral vision: types of eye movements

To keep stimuli from the environment projecting to the retina, particularly the fovea, the eyes make three types of movements: vergence movements, saccadic movements, and pursuit movements.

Retina: bipolar cells - amacine cells

Transmit information in a direction parallel to the surface of the retina and thus combine messages from adjacent photoreceptors. Neuron in the retina that interconnects adjacent ganglion cells and the inner processes of the bipolar cells

Perception of form

Ventral stream Analysis of visual information that leads to the perception of form begins with 1) neurons in the striate cortex that are sensitive to spatial frequency. 2) These neurons send information to area V2 and then on to the ventral stream of the extrastriate cortex - Young infants cannot do these fine discriminations. They can only see the general shape - They cannot tell the differences between two individuals but they can see that there are two individuals Damage to the temporal lobes: cannot perceive forms. But they can perceive colors.

Perception of form: extrastriate cortex - extrastriate body area (EBA)

Ventral stream Specifically activated by photographs, silhouettes, or stick drawings of human bodies or body parts and not by control stimuli Greatest response to headless bodies and body parts

Visual processing: lateral geniculate nucleus (LGN) - parvocellular layers

Ventral stream - Cones Four outer layers Smaller than the center two layers - Ability to detect small details (acuity) Acuity, red, green, central vision Important for later development for being able to discriminate between one face and another

Visual processing: lateral geniculate nucleus (LGN) - koniocellular sublayers

Ventral stream Third set of neurons are found below each of the magnocellular and parvocellular layers. Fine detail, blue, central vision

Perception of color: retinal ganglion cells

We contain 3 different cones which provide us with color vision Ganglion cells normally produce action potentials at a relatively low rate. Then, when the level of illumination in the center of their receptive field increases or decreases the ganglion cells signal the change in illumination.

Extrastriate cortex: pathways - ventral

What pathway: size, shape, color, and texture of objects - What an object is and what colors it has Towards the bottom - Starts from the occipital cortex and goes towards the temporal lobes Ventral stream receives approximately equal input from the magnocellular and the parvocellular/koniocellular systems. Object identification Form of perception

Perception of color: retinal ganglion cells - dark and light

When monkeys received a drug that selectively blocked synaptic transmission in ON bipolar cells, the animals had difficulty detecting spots that were brighter than the background but had no difficulty detecting spots that were slightly darker than the background - Administering this drug completely blocked vision in very dim light, which is normally mediated by rods. Rod bipolar cells must all be of the ON type.

Transduction: photoreceptors

When the receptors experience darkness they respond by depolarizing. When they experience light, they respond by hyperpolarizing. Photoreceptors and bipolar cells do not produce action potentials - their release of neurotransmitter (glutamate) is regulated by their membrane potential - Depolarizations increase the release of neurotransmitter, and hyperpolarizations decrease it - In the dark, photoreceptors are depolarized and constantly release glutamate into synapses with bipolar cells - In the light, photoreceptors are hyperpolarized and less glutamate is released into synapses with bipolar cells.

Central and peripheral vision: types of eye movements - saccadic movements

When you scan a scene, your gaze does not roam slowly and steadily. Your eyes make jerky saccadic movements. You shift your gaze abruptly from one point to another You cannot consciously control the speed of movement between stops;

Extrastriate cortex: pathways - dorsal

Where pathway: navigation and skilled movements directed toward objects - Where the object is located and, if it is moving, its speed and direction of movement Occipital cortex and goes to the upper areas of the brain (partial lobes) Binocular vision goes into this particular vision. Some of the orientation of things - How do you interact with this particular object? Where is it in space? How is it oriented? Some axons conveying information received from the magnocellular system bypass area V2: They project from area V1 directly to area V5 (also called area MT for medial temporal), a region of the dorsal stream devoted to the analysis of movement. The dorsal stream receives mostly magnocellular input