The Visual System Flashcards

Question

Summary shinning light on an on-center and off-surround receptive field

Answer 1

These retinal ganglion cells have on-center-off-surround receptive fields, reflecting convergent input from multiple photoreceptors.

Answer 2

* It is important for the visual systems ability to detect edges and contrast. * The visual system is not that interested in uniform lighting (not interested in areas of the visual world where lighting is all the same). * The ganglion cells are tuned to be better at detecting contrast between light and dark regions then they are at detecting uniform illumination.

Answer 3

Photoreceptors also make excitatory connections with bipolar cells. In this case, the ganglion cells are off-centeron-surround.

Answer 4

* The difference is in these bipolar cells. * If the cell is on-center, it has a bipolar cell that is hyperpolarized by glutamate. * If the cell is off-center, it has a different kind of bipolar cell that is depolarized by glutamate.

Answer 5

* Every single cone in your fovea is connected to an on-bipolar cell, and an off-bipolar cell. So each cone is then connected to these two streams: one on-center, off-surround and one that is off-center, on-surroud. In the fovea, each cone forms the center of an oncentered and an off-centered pathway. * Why does it have this? this means that the output of this region of the retina is going to be very good at detecting increases in illumination and decreases in illumination. Sudden increase in illumination will cause an increase in firing in the on ganglion cell. Sudden decrease in illumination will cause an increase in firing in the off ganglion cell. * Every single cone in the fovea is connected to an on bipolar cell, an off bipolar cell and horizontal cells. So it has its own central pathway. And every single cone contributes to the suround receptive field of other cones. * The receptive field of each ganglion cell in the fovea is quite small. That makes sense because your fovea is where you have high precision vision. This precision is a result of the receptive fields of the ganglion cells being so small.

Answer 6

* As you start to move away from the fovea, the receptive fields of the ganglion cells become larger. Outside the fovea, more photoreceptors feed into a single ganglion cell, so the receptive fields are larger. * When you start moving outside the fovea, multiple photoreceptors are going to contribute to the center and then a whole bunch of photoreceptors will contribute to the surround. * They are collecting light from more photoreceptors, meaning they will be more sensitive to light. So periphery of your visual field is more sensitive to light then the center of your visual field.

Answer 7

* There more than a dozen distinct types of RGCs. Each type tiles the entire retina, creating multiple parallel labelled lines to the lateral geniculate nucleus. Each labelled line conveys a distinct type of visual information. * Different types of ganglion cells tile the retina - each type of ganglion cell carpets the entire retina. * somewhere between 15-20 different kinds of ganglion cells. * The significance of this; each type is like a seperate labelled line or a seperate channel that is conveying a particular type of info from the retina to the visual system.

Answer 8

The three best characterized channels from the retina originate with the midget ganglion cells (~ 70% of RGCs) parasol ganglion cells (~ 10% of RGCs) and bistratified ganglion cells (~ 8% of RGCs). * midget ganglion cells = P-pathway * parasol ganglion cells = M-pathway * bistratified cells = K-pathway Each one of these types of ganglion cells are conveying different aspects of the visual scene.

Answer 9

* The three types of ganglion cells are responsible for colour vision. * In daylight, each of these channels gets its main input from cones. The human retina contains three types of cones, called S, M and L. **Each type is tuned to respond best to a distinct range of wavelengths within the visible spectrum.** * You need all 3 cones to see different colours. * S responds to short wavelength, blue light * M responds to medium wavelength, green light * L responds to long wavelength, red light. * You need all 3 cones to see colour, colour blind = missing one of the cones (more common in men, usually missing red or green). * If you only had green cones, you would not be able to tell blue from any other colour.

Answer 10

* Brain is assessing the relative activation of the 3 labelled lines and uses this to intepret colour.

Answer 11

**Midget cells are telling us how much green versus red is in the scene.** * Midget cells are concentrated in the fovea. They convey the amount of red versus green in their receptive fields. * Their receptive fields are small, so they are good at resolving fine details, but not especially good at detecting rapid changes. * They synapse with parvocellular layers in the LGN (p-pathway) which make a major contribution to cortical systems involved in processing of color and form. * Start of a labelled line (p-pathway). * They have a very small receptive field (high precision of fovea). * Everything we have been talking about so far has been midget cells.

Answer 12

* A red-on-green-off cell will **increase firing** to a** red** light covering the entire receptive field. This is because the center is activated by the red light and the surround will not be inhibited much as it is sensitive to green light. * It will **decrease firing** in response to a **green** light covering its entire receptive field. There is strong inhibition because the center is not activated by green light and the surround is strongly sensitive to green light.

Answer 13

* White light is the whole visible spectrum - has red and green component * The same red-on-green-off cell will **increase firing** to **white light in the center**. * It will **decrease firing** in response to a white light in its surround. * So it also acts as an contrast detector.

Answer 14

1. detect the relative amount of green or red 2. detect contrast/edge detectors: allows you to see the boundaries of objects.

Answer 15

* Parasol cells contribute to the m pathway (another labelled line) that goes up all to way to the primary visual cortex. * Parasol cells are found near the central part of the fovea, but they are not so concentrated right in the fovea. * Parasol cells have **larger receptive fields** (because multiple cones contribute to center and multiple cones contribute to the surround). Not as precise. * Convey an **achromatic signal** (i.e. no color discrimination). * They have lower spatial resolution than midget cells, but better **sensitivity to rapid changes in illumination**. Detect rapid changes in illumination and movement. * They project to the magnocellular layers in the LGN (m-pathway) which contribute to cortical systems involved in **analysis of location and movement**.

Answer 16

* Parasol cells have larger receptive fields. * They cannot detect colour because both their center and surround get input from both red and green cones. The center and surround of the receptive field is made up of a mixture of red and green cones.

Answer 17

* Bistratified cells detect the amount of blue versus red/green (red + green = yellow). Relative amount of blue vs yellow. * They project to the koniocellular layers of the LGN (k pathway) and are involved in color perception. * They do not have a center-surround. They are connected to on bipolar cells and off bipolar cells.

Answer 18

* Bistratified cells have larger receptive fields. * Center and surround get input from both red and green cones. * The receptive field has green, red and blue cones. * If you shine blue light over the entire receptive field, it will activate the blue cones = excitation (firing of AP). * If you shine yellow light over the entire receptive field, it will activate the green and red cones = inhibition of firing.

Answer 19

* A single type of cone cannot distinguish between different wavelengths of light, e.g. M cones respond best to green light, but will also respond to wavelengths covering most of the visible spectrum. * The reason you can see colour is because you are getting ouput from the retina coming from all those different kinds of ganglion cells. * Ie, if someone only has green cones. All that the cone can do is absorb photons of light and then chance how much NT is being released. So, the cells downstream from this cone cannot tell wether the cone is being exposed to a dim green lught or a very bright red light (bright red light will activate the cone, even though the cone is less sensitive to red light). * This green cone is not very sensitive to red or blue light, but a bright red light or blue light will still activate the cone. * The neurons downstream can only tell that there is a change in the amount of neurotransmitter that is being released. So, a single cone cannot descriminate between different colours.

Answer 20

* Color is encoded in the retina by the relative output of the parasol, midget and bistratified cells. - Parasol cells = give a measure of the overall luminance and brightness - Midget cells = convey the amount of red vs green that is in the scene - Bistratified cells = convery the amount of blue vs yellow that is in the scene. * The brain is interpreting the relative activation of these different pathways (labelled lines) and based on this is identiying the colour. - If there is strong activation of red on center off suround ganglion cells and weak activation of these bistratified cells = red - If there is strong activation of the blue on center bistratified cells and weak activation of the midget cells than that is probably going to be blue.

Answer 21

* Information from multiple photoreceptors converges on individual retinal ganglion cells. * As a result of this convergence and the complex circuitry of the retina, the receptive fields of the retinal ganglion cells are more complex than the receptive fields of the photoreceptors. * Different types of RGCs tile the retina. Each cell type forms a separate channel conveying a specific type of visual information.

Answer 22

* The axons of retinal ganglion cells project as the optic nerves to the lateral geniculate nuclei. The fibers partially cross at the optic chiasm and continue as the optic tracts. Projections: * Nasal side of retina crosses over (red) * Temporal part of retina stays ipsilateral (blue) * Left side of the retina of both eyes goes to the right side of the LGN * Right side of the retina for both eyes goes to left side of the LGN.

Answer 23

* Midget and parasol ganglion cells project to six distinct layers in the lateral geniculate nucleus. * Retina is tilled with ganglion cells - each ganglion cell is telling your brain something about a small part of the visual field. * Retinotopic map is preserved to cortex like the somatosensory map.

Answer 24

* The output from the 2 eyes is going to stay seperate all the way to the cortex. The output from the midget and parasol cells also stay seperate all the way to the cortex. - m pathway: layers 1, 2 - p pathway: layers 4-6 - k-pathway: project in between these layers These 3 different labelled lines are stauing seperate from the thalamus all the way up to the cortex. * The outputs that are coming from the temporal part of the retina and will stay ipsilateral (red) will go to layers 2, 3 & 5. * The contralateral lateral geniculate is projecting to layers 1, 4 & 6. * Functionally what this means is that from the retina going through the lateral geniculate all the way up to the cerebral cortex, the output coming from the two eyes is going to stay seperate because each eye is projecting to different layers.

Answer 25

* Bistratified retinal ganglion cells project to LGN layers in between the prominent magnocellular and parvocellular layers. * They are very small and project to regions in between the prominent regions. * These regions, which are referred to as koniocellular layers, are sparsely populated with small neurons. * output from bistratified cells will also stay seperate all the way to cortex.

Answer 26

Lateral geniculate neurons faithfully relay inputs from retinal ganglion cells to primary visual cortex, without significant change in the receptive fields. * The receptive fields of the neurons in the thalamus hardly change at all. This means that if a ganglion cell fires some AP and this ganglion cell is makinga synapse with the LGN, the LGN neuron is pretty much going to do the same thing. * LGN neurons are basically relaying the input from the retina up to the cerabral cortex. They are not changing the receptive property. * Most of the neurons in the LGN have some kind of center surround receptive field just like the retinal ganglion cells.

Answer 27

* The output from the lateral geniculate nucleus projects to the primary visual cortex (V1), through the optic radiation. * If you were to have a left lesion to the bottom axons: the top half of the visual side is lost on the contralateral side of the brain. * Right top half of vision is lost, loses top 1/4 of visual field.

Answer 28

Projections from the lateral geniculate to V1 are vastly outnumbered by reciprocal projections coming back from the cortex.

Answer 29

~ 90% of retinal ganglion axons project to the lateral geniculate nucleus. The remaining 10% project to the superior colliculus, the pretectum and the hypothalamus. * Projections to the superior colliculus are involved in saccadic eye movements. * Projections to the pretectum are involved in pupillary reflexes. * Projections to the suprachiasmatic nucleus of the hypothalamus modulate circadian rhythms. The suprachiasmatic nucleus is the internal clock for circadian rhythms.

Answer 30

* Primary visual cortex comprises Brodmann’s area 17. * Most of this area is on the medial surface of the brain surrounding and within the calcarine sulcus.

Answer 31

Primary visual cortex contains a retinotopic map of the external visual field. Notice: 1. The area corresponding to the fovea takes up practically half of the primary visual cortex --> larger representation of high precision vision 2. Left primary cortex represents the right side of the visual space 3. Top half of cortex represents bottom half of visual space and vice versa.

Answer 32

Layer IV is the main recipient of afferent inputs from the thalamus. * Projections from the LGN terminate in layer IV of V1 * Layer IV in V1 is specifically large and has lots of sub layers.

Answer 33

Lateral geniculate neurons project mainly to layer 4C of primary visual cortex.

Answer 34

The M, P and K pathways are integrated in different layers of V1. Outputs from V1 form separate dorsal (where) and ventral (what) channels. There are 3 different labelled lines as they get to V1: 1. P-Pathway: midget cells (R/G) goes to sublayer in layer IV. 2. M-Pathway: parasol cells (colour blind) - larger recepti e field. Go to sublayers in layer IV. 3. K-Pathway: blue vs yellow. End in layers II and III in the blobs. This is an exception to the general rule, goes directly to layer 2/3 instead of going to IV first. All 3 of these pathways contribute to colour vision. All 3 of these pathways converge on the blobs where they are finally starting to be combined together. Another exception from general rule: in m-pathway, some of the axons from layer IV will go directly to the cortex (will bypass layers 2/3). The layers II and III send projections to cortex. Runs along the ventral surface of the cortex and ends in the ventral what pathway.

Answer 35

V1 neurons respond best to bars of light at a specific orientation. * Neurons in V1=more complex receptive fields. * Respond to bars of light (the beggining of edges) * The orientation of the bar of light matters as well as the direction of movement of the bar.

Answer 36

In 1981, Torsten Wiesel (left) and David Hubel (right) won the Nobel Prize for their discoveries concerning how brain neurons encode visual stimuli. Hubel went to grade school in Outremont, obtained his undergraduate (in math and physics) and medical degrees from McGill and studied neurology for three years at the Montreal Neurological Institute, before moving to the U.S. (Johns Hopkins and then Harvard) * present different orientations of bars of lights to cats and record their APs. * Fires AP when correct orientation and in receptive field. * edge detectors = inhibitory surround. * light off in the surround = fires AP.

Answer 37

* The elongated receptive fields of simple cells are built from convergent input from many layer 4 cells with roughly circular receptive fields * Receptive fields are getting more complex due to conversion.

Answer 38

* These orientation-selective neurons in V1 are organized into orientation-selective columns. All the neurons in the column will have the same response properties. * Discovered by Hubel and Weisel. * V1 is the brain region in which our understanding of cortical columns is the most developed. * The columns are arranged in a complex pattern of swirls. A cycle around a swirl contains neurons responding to the full range of orientations for a particular location in space. * Experiment: they used optical techniques to detect when neurons are active and they were able to colour code the activity. * All the neurons in 1 column have the same response properties (white lines are the boundaries of the columns). * We have a whole series of columns that make a pinwheel (circle) and if you go all the way around all the different columns in that circle, it corresponds to neurons that respond to bars of light of all the orientations going from this all the way to 180 degrees. * Taken together, you have neurons that respond to a small region of visual space to bars of light of all the possible orientations going around 180 degrees. * The whole pinwheel together is a functional unit - this single round structure is going to enable the little region of the retina to detect edges of all possible different orientations. * The primary visual cortex, each little pinwheel is mapping out all the possible orientations for a bar of light in that particular region of visual space. * Hypercolumn = entire pinwheel that includes all the possible orientations.

Answer 39

* The neurons in one single orientation-selective column were not all the same, they had 2 different types of neurons: - **Simple cells** are orientation-selective and have centersurround receptive fields. They have an on-center and off-surround. - **Complex cells** don’t have the inhibitory surround region in their receptive fields. Really good at detecting edges (all they need is an edge moving into their receptive field and they will start firing).

Answer 40

The receptive fields of complex cells result from integration of input from simple cells. * You have a bunch of simple cells converging and that can give you a complex cell with more complex receptive fields * Even in the primary visual field, we are buiding up this complexity, center-surround receptive fields are being assembled together to create simple cells, and then simple cells are being assembled together to create complex cells. * The ability of the primary visual cortex to detect edges is getting more and more integrated.

Answer 41

* Some V1 cells respond best to bars of light that are moving in one direction (a subset of the orientation selective neurons). * Within these orientation-selective neurons, you have a subset of direction selective neurons.

Answer 42

* Some V1 neurons are sensitive to bar length. * Prefer a bar of light but only if the bar of light only goes part way through the receptive field. * These neurons are really good for detecting the ends of edges.

Answer 43

* Blobs are found just in layers 2 and 3 (superficial layers). * Blobs are interspersed within the orientation columns. * Neurons in the blobs tend to be more responsive to color and less sensitive to stimulus orientation. * The functional significance of this is that the neurons in the blobs actually don't care that much about orientation. They do not have the same kind of orientation preference that the regions outside the blobs do. - the blobs are the starting point for colour processing in the cortex. - the interblob regions are the regions where you have the orientation selective neurons. Where simple cells and complex cells are located. * Blobs and interblobs are in essence columns: all the neurons in the blobs are going to prefer colours and all the neurons in the interblob are going to prefer certain orientations.

Answer 44

* Superimposed on the orientation columns and blobs are columns corresponding to alternating input from the ipsilateral and contralateral eyes. These are called ocular dominance columns. * Inputs coming into layer 4, are seperated into 2 different columns (one group of inputs coming from the ipsilateral eye and one group of inputs coming from the contralateral eye) - columns of neurons responding to either the contralateral eye or ispilateral eye. * Mixing together of the two eyes starts to occur in primary visual cortex, but at least all the way up to the primary visual cortex the inputs from the two eyes are segregated. * There are multiple different columns in primary visual cortex. So if you think about the cortex as a flat sheet but there are a whole bunch of different properties that have to be mapped onto that sheet. - we have to map on orientation selectivity - we have to map colour - we have to map motion (direction) - we have to map depth These are all organized as columns, so you really do not have one kind of column in the primary visual cortex. You have a whole bunch of different columns mashed together!! - Blobs and interblobs - ocular dominance columns - orientation selective columns - columns for motion and depth perception

Answer 45

Visual information flows from V1 through separate dorsal (where) and ventral (what) streams. * The dorsal stream encodes location and movement. From the occipital lobe to the parietal lobe. * The ventral stream encodes form and color. From the occipital lope to the ventral part of temporal lobe

Answer 46

dont need to know these

Answer 47

Area MT in the dorsal stream is involved in detection of motion.

Answer 48

Lesions in MT neurons in monkeys produce deficits in detecting global motion. * Using a screen that has dots on it and they are moving around.

Answer 49

Because V1 receptive fields are small (they are only seing a small part of the object/edges of object), they may give misleading information on direction of movement. This is referred to as the aperture problem. * Concerns the idea that receptive fields get larger as you move higher and higher up. * deceived about the direction of movement.

Answer 50

* Neurons in area MT integrate input from neurons with smaller receptive fields, so they can detect the overall direction of movement of an object. * The little receptive fields are being combined together to from larger receptive fields. It is therefore seing a large part of the object! * Importance of the receptive field getting larger, because as the receptive field gets larger, you can see more and more of the object and you can understand the global properties of the object like its overall direction of movement.

Answer 51

The dorsal stream contributes to multimodal sensory pathways from parietal cortex to premotor cortex that are involved in transforming perception into action.

Answer 52

The ventral stream is involved in perception of color, form and object recognition. * The ventral stream has specific regions that are involved in colour perception * Ventral stream is involved in your ability to see. * Lesions to specific stream that processes colour = no longer see colour.

Answer 53

* Neurons in the fusiform face area respond preferentially to faces (FFA = neurons that only respond to faces). * The fusiform gyrus is involved in the processing of faces * Processing of face is built up from dots, bars ... to complex image. * Will respond to the face ANYWHERE in visual space.

Answer 54

1. Recognizing faces is really important for us and also for monekys - so FFA has evolved to process faces in the primate. 2. Expert at recognizing something specific. Area that recognizes something you are an expert at (ie, birds, cars...) Involved in seing objects and recognizing objects which is the main function of ventral stream.

Answer 55

Neurons in the fusiform face area respond preferentially to faces.

Answer 56

Prosopagnosia: Loss of the ability to recognize familiar faces * Caused by lession to FFA * Vision is okay --> just cannot recognize who's face it is (themselves, family members). * Can recognize a face but not whos face.

Answer 57

* Clusters of neurons in localized regions respond preferentially to different types of visual objects. * Some areas in ventral stream are specialized for recognizing different things. * Point is that there is not just a face area. In the ventral stream, there seems to me a lot of different areas that are specialized for a lot of different things.

Answer 58

* Receptive fields get progressively larger at progressively higher levels in the visual hierarchy * Visual information is getting all the way up to the frontal lobes of the brain.

Answer 59

There are extensive short-range and long-range reciprocal connectivity low and high levels in visual processing. * Information is being fed forward to higher levels and at the same time there is feedback projections. * 2 way arrows: every connection has feedforward and feedback projections.

Answer 60

* Feedback connections may be involved in visual attention and in topdown anticipatory mechanisms that enable us to differentiate an object from the background. * Feedback connections enable us to use prior knowledge to organize information (ie: organize the visual info flowing up) * Decide what part of the visual field you attend to. * The idea is tha the info is flowing up (bottom-up) and the control is being done via top-down feedback projections.

Answer 61

According to Predictive Processing models of sensory perception, higher levels in the sensory hierarchy feedback perceptual expectations to lower levels. This feedback is compared with sensory input. Differences between the feedback expectation and the feedforward signal create a prediction error signal that is fed forward to update the predictive model at the next level in the heirarchy. * High levels build a simulation of what the world is like and then runing it --> Feed this back to lower levels. * Lower levels will compare the sensory information coming in to the existing visual model from the higher levels. If there is a discrepency between the two, then the new information is fed forward to update the model. What is different has to be fed forward to update.

Answer 62

* Man had stroke and has no primary visual cortex (V1) so it therefor blind. However, he is still able to avoid objects in his path! * Can also tell the direction of movements of objects * Can also tell the orientation of a slot and put a paper in it.

Answer 63

The main visual pathway responsible for conscious visual perception goes through the LGN to primary visual cortex. * ventral stream: conscious visual perception * dorsal stream: important for you ability to use visual information, location and movement of the objects to guide your own movements.

Answer 64

* There is an alternate pathway by which visual information gets up to the cerebral cortex. * An alternate route goes through the superior colliculus to regions of the parietal and frontal lobes involved in detecting motion and controlling eye movements. This alternate may be responsible for blindsight. * 10% of from optic nerve project to the superior colliculus. * Superior colliculus is involved in rapid eye movement. Superior colliculus is connected to regions in the parietal lobe and frontal lobe as well (involved in detecting the location of objects and detecting visual motion). * These connections **bypass** the **primary visual cortex** and bypass the **ventral stream** of visual processing. * In people with blindsight, who have lesions to the primary visual cortex (no conscious visual perception), but they are getting visual information that is reaching these higher order levels of the parietal lobe that are important for detecting location and motion - so they can still detect where things are and that things are moving. This suggests: 1. the dorsal stream is really not involved in conscious visual perception (ventral stream and V1 are more critical for conscious visual process). 2. A lot of the things you use your visual field for don't actually require a conscious visual perception at all. Suggests a lot of what your visual system is doing is below the level of concsiousness.

Answer 65

* Another unconscious pathway (bypasses the primary visual pathway and ends in amygdala) goes from the thalamus (LGN and pulvinar) to the amygdala. * This pathway is thought to be involved in rapid, unconscious emotional responses to visual stimuli. * Rapid direct visual pathways that go direct to the amygdala. Enables us to react emotionally. * This pathway enables the brain to detect threatening situations (situations that elicit fear before you become consciously aware).

Answer 66

Projections to the prefrontal cortex are involved in visual working memory.

Answer 67

**The binding problem:** How are the different components of vision (e.g. form, color, location, motion), spread out over disparate regions of cortex, bound together to form a unified, coherent percept. * When you look at the world. The visual world is unified (you perceive the visual world as being unified - all put together in a single perception). BUT visual processing is not unified. There are areas of the ventral stream that are processing color, other areas processing form of object, dorsal stream processing location, movement. Different anatomically seperated regions of the brain that are processing different aspects of the visual scene. * How does that happen? how does it get bound all together? We don't really know. one idea is that the neurons that represent the different aspects of the car (red, location, direction of movement...) , start firing AP in ways that synchronize their behaviour. Activity of these neurons becomes synchronized.

Answer 68

* Information destined for the cortex passes through the thalamus (the lateral geniculate nucleus). * Information is processed at multiple hierarchically organized levels (LGN, V1, the dorsal and ventral streams). Neurons at high levels in the hierarchy have more complex receptive fields than neurons at low levels in the hierarchy. * Information flows through multiple parallel pathways (e.g. the M, P and K pathways). Information flowing through the separate pathways is bound together, through a process that is not fully understood, to create a unified percept. * Perception of color is a combinatorial process. * Perception is an abstraction not a replication of the external world.

The Visual System Flashcards

(94 cards)