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Chapter 4-6

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Phillip Servos

Sensory Process & Perception Chapter 4-6 and 8 Notes CHAPTER 4  Light is reflected from an object into the eye o This light is focused to form an image of that object on the retina o Light, in a pattern that illuminates some receptors intensely and some dimly, is absorbed by the visual pigment molecules that pack the rod and cone outer segments o Chemical reactions in the outer segments transducer the light into electrical signals o As these electrical signals travel through the retina, they interact, excite, and inhibit, eventually reaching the ganglion cells, which because of this processing have center-surround receptive fields on the retina o After being processed by the retina these electrical signals are sent out the back of the eye in fibres of the optic nerve Following the Signals From Retina to Cortex The Visual System  Figure 4.1a, which is an overview of the visual system, pictures the pathway that the neural signals follow once they leave the retina  Most of the signals from the retina travel out of the eye in the optic nerve to the lateral geniculate nucleus (LGN) in the thalamus o From here, signals travel to the primary visual receiving area in the occipital lobe of the cortex o The visual receiving area is also called the striate cortex because of the white stripes (striate = striped) that are created within this area of cortex by nerve fibres that run through it o From the striate cortex, signals are transmitted along two pathways, one to the temporal lobe and the other to the parietal lobe (blow arrows) o Visual signals also reach areas in the frontal lobe of the brain  Figure 4.1b shows the visual system as seen from the underside of the brain o In addition to showing the pathway from eye to LGN cortex, this view also indicates the location of the superior colliculus, an area involved in controlling eye movements and other visual behaviours that receives about 10% of the fibres from the optic nerve o This view also shows how signals from half of each retina cross over to the opposite side of the brain Processing in the Lateral Geniculate Nucleus  What happens to the information that arrives at the lateral geniculate nucleus? Receptive Fields of LGN Neurons  Recording from neurons in the LGN shows that LGN neurons have the same center-surround configuration as retinal ganglion cells  Thus, neurons in the LGN, like neurons in the optic nerve, respond best to small spots of light on the retina  A major function of the LGN is apparently not to create new receptive field properties, but to regulate neural information as it flows from the retina to the visual cortex Information Flow in the Lateral Geniculate Nucleus  90% of the fibres in the optic nerve arrive at the LGN (the other 10% travel to the superior colliculus)  But these signals are not the only ones that arrive at the LGN o The LGN also receives signals from the cortex, from the brain stem, from other neurons in the thalamus (T), and from other neurons in the LGN (L). o Thus, the LGN receives information from many sources, including the cortex, and then sends its output to the cortex  Figure 4.2b indicates the amount of flow between the retina, LGN, and cortex  Notice that (1) the LGN receives more input back from the cortex than it receives from the retina; and (2) the smallest signal of all is from the LGN to the cortex o For every 10 nerve impulses the LGN receives from the retina, it sends only 4 to the cortex o This decrease in firing that occurs at the LGN is one reason for the suggestion that one of the purposes of the LGN is to regulate the neural information as it flows from the retina to the cortex  But the LGN not only regulates information flowing through it; it also organizes the information Organization by Left and Right Eyes  The lateral geniculate nucleus (LGN) is a bilateral structure, which means there is one LGN in the left hemisphere and one in the right hemisphere o Viewing one of these nuclei in cross section reveals six layers o Each layer receives signals from only one eye o Layers 2, 3, and 5 (red layers) receive signals from the ipsilateral eye, the eye on the same side of the body as the LGN o Layers 1, 4, and 6 (blue layers) receive signals from the contralateral eye, the eye on the opposite side of the body from the LGN o Thus, each eye sends half of its neurons to the LGN that is located on the left hemisphere of the brain and half to the LGN that is located in the right hemisphere o Because the signals from each eye are sorted into different layers, the information from the left and right eyes is kept separated in the LGN Organization as a Spatial Map  Figure 4.4  Retinotopic map – a map in which each point on the LGN corresponds to a point on the retina o We can determine what this map looks like by recording from neurons in the LGN  The correspondence between locations on the retina and locations on the LGN means that neurons entering the LGN are arranged so that fibres carrying signals from the same area of the retina end up in the same area of the LGN, each location on the LGN corresponds to a location on the retina, and neighbouring locations on the retina o Thus, the receptive fields of neurons that are near each other in the LGN, such as neurons A, B, and C, in layer 6, are adjacent to each other at A’, B’ and C’ on the retina  Retinotopic maps occur not only in layer 6, but in each of the other layers as well, and the maps of each of the layers line up with one another  One million ganglion cell fibres travel to each LGN, and on arriving there, each fibre goes to the correct LGN layer (remember that fibres from each eye go to different layers) and finds its way to a location next to other fibres that left from the same place on the retina Receptive Fields of Neurons in the Striate Cortex  More than 80% of the cortex responds to visual stimuli  Using the procedure described in Chapter 2 (pg 34) in which receptive fields are determined by flashing spots of light on the retina, Hubel and Wiesel found cells in the striate cortex with receptive fields that, like center-surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory areas o However, these areas are arranged side by side rather than in the centre-surround configuration (figure 4.6a) o Cells with these side by side receptive fields are called simple cortical cells  We can tell from the layout of the excitatory and inhibitory areas of the simple cell shown in figure 4.6a that a cell with this receptive field would respond best to vertical bars  The relationship between orientation and firing is indicated by a neuron’s orientation tuning curve, which is determined by measuring the responses of a simple cortical cell to bars with different orientations  Although Hubel and Wiesel were able to use small spots of light to map the receptive fields of simple cortical cells like the one in Figure 4.6, they found that many of the cells they encountered in the cortex refused to respond to small spots of light o As they inserted a glass slide containing a spot stimulus into their slide projector, a cortical neuron “went off like a machine gun” o The neuron, as it turned out, was responding not to the spot at the center of the slide that Hubel and Wiesel had planned to use as a stimulus, but to the image of the slide’s edge moving downward on the screen as the slide dropped into the projector o Upon realizing this, Hubel and Wiesel changed their stimuli from small spots to moving lines and were then able to find cells that responded to oriented moving bars o They discovered that many cortical neurons respond best to moving barlike stimuli with specific orientations o Complex cells, like simple cells, respond best to bars of a particular orientation  However, unlike simple cells, which respond to small spots of light or to stationary stimuli, most complex cells respond only when a correctly oriented bar of light moves across the entire receptive field  Another type of cell, called end-stopped cells, fire to moving lines of a specific length or to moving corners or angles  Hubel and Weisel’s finding that some neurons in the cortex respond only to oriented lines was an extremely important discovery because it indicates that neurons in the cortex do not simply respond to “light”; they respond to some patterns of light and not to others o Their discovery that neurons respond selectively to stationary and moving lines was an important step toward determining how neurons respond to more complex objects  Because simple, complex, and end-stopped cells fire in response to specific features of the stimulus, such as orientation or direction of movement, they are sometimes called feature detectors Table 4.1 Properties of Neurons in the Optic Nerve, LGN, and Cortex Type of Cell Characteristics of Receptive Field Optic nerve fiber (ganglion cell) Center-surround receptive field. Responds best to small spots, but will also respond to other stimuli Lateral geniculate Center-surround receptive fields very similar to the receptive field of a ganglion cell Simple cortical Excitatory and inhibitory areas arranged side by side. Responds best to bars of a particular orientation Complex cortical Responds best to movement of a correctly oriented bar across the receptive field. Many cells respond best to a particular direction of movement End-stopped cortical Responds to corners, angles, or bars of a particular length moving in a particular direction Do Feature Detectors Play a Role in Perception?  One way to establish a link between the firing of these neurons and perception is by using a psychophysical procedure called selective adaptation Selective Adaptation and Feature Detectors  When we view a stimulus with a specific property, neurons tuned to that property fire  The idea behind selective adaptation is that if the neurons fire for long enough, they become fatigued, or adapt o This adaptation causes two physiological effects:  (1) the neuron’s firing rate decreases, and  (2) the neuron fires less when that stimulus is immediately presented again o According to this idea, presenting a vertical line causes neurons that respond to vertical lines to respond, but as these presentations continue, these neurons eventually begin to fire less to vertical lines o Adaptation is selective because only the neurons that respond to verticals or near-verticals adapt, and other neurons do not Grating Stimuli and the Contrast Threshold  Grating stimuli are alternating bars  A grating’s contrast threshold is the difference in intensity at which the bars can just barely be seen  Figure 4.9  Figure 4.11a shows the results of a selective adaptation experiment in which the adapting stimulus was a vertically oriented grating o This graph indicates that adapting with the vertical grating caused a large increase in contrast threshold for the vertically oriented test grating o That is, the contrast of a vertical grating had to be increased for the person to see the bars o The important result of this experiment is that our psychophysical curve shows that adaptation selectively affects only some orientations, just as neurons selectively respond to only some orientations Selective Rearing and Feature Detectors  Further evidence that feature detectors are involved in perception is provided by selective rearing experiments  The idea behind selective rearing is that if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent  This follows from a phenomenon called neural plasticity or experience-dependent plasticity – the idea that the response properties of neurons can be shaped by perceptual experience o According to this idea, rearing an animal in an environment that contains only vertical lines should result in the animals visual system having neurons that respond predominantly to verticals Maps and Columns in the Striate Cortex  This organization, in which nearby points on a structure receive signals from nearby locations on the retina, also occurs in the striate cortex Maps in the Striate Cortex  As for the LGN experiment in Figure 4.5, recordings were made from neurons encountered as the electrode was inserted into the cortex, first neuron 1, then 2, and so on o Hubel and Wiesel found that the receptive filed of each neuron was displaced slightly on the retina, as indicated by the squares in Figure 4.13b, but that receptive fields of neurons close to each other along the electrode track had receptive fields that were close to each other on the retina  Retinotopic mapping indicates that information about objects near each other in the environment is processed by neurons near each other in the cortex  Cortical magnification factor – the apportioning the small fovea with a large area on the cortex  The cortical magnification factor in the human cortex has been determined using a technique called brain imaging, which makes it possible to create pictures of the brains activity  Robert Dougherty and coworkers used brain imaging to determine the magnification factor in the human visual cortex o Figure 4.17a shows the stimulus display viewed by the observer, who was in an fMRI scanner o The observer looked directly at the center of the screen, so the dot at the center fell on the fovea o During the experiment stimulus light was presented in two places:  (1) near the center (red area), which illuminated a small area near the fovea; and  (2) farther from the center (blue area), which illuminated an area in the peripheral retina  The connection between cortical area and acuity has been confirmed by Robert Duncan and Geoffry Boynton o They measured brain activation with the fMRI and visual acuity using a psychological task o The fMRI indicated that the magnification factor was not the same for all of their observers o Some people had more cortical space allotted to their foveas than other people, and those with more cortical space also had better acuity Columns in the Striate Cortex Location Columns  Hubel and Wiesel recorded from neurons along a perpendicular electrode track as shown in figure 4.19a, which shows a side view of the cortex o The receptive fields of neurons 1,2,3 and 4, indicated by the squares in figure 4.19b, are all located at about the same place on the retina o Hubel and Wiesel concluded from this result that the cortex is organized into location columns that are perpendicular to the surface of the cortex so that all of the neurons within a location column have their receptive fields at the same location on the retina Orientation Columns  Orientation column – a column in the visual cortex that contains neurons with the same orientation preference  Hubel and Wiesel also showed that adjacent columns have cells with slightly different preferred orientations o When they moved an electrode through the cortex obliquely, as was done for the LGN, so that the electrode cut across orientation columns, they found that the neurons preferred orientations changed in an orderly fashion, so a column of cells that respond best to 90 degrees is right next to the column of cells that respond best to 85 degrees Ocular Dominance Columns  Neurons in the cortex are also organized with respect to the eye which they respond best  About 80% of the neurons in the cortex respond to stimulation of both the left and right eyes  However most neurons respond better to one eye than to the other  This preferential response to one eye is called ocular dominance, and neurons with the same ocular dominance are organized into ocular dominance columns in the cortex o This means that each neuron encountered along a perpendicular electrode track responds best to the same eye Hypercolumns  Hubel and Wiesel proposed that all 3 types of columns could be combined into one larger unit called a hypercolumn  They thought of a hypercolumn as a “processing module” that processes information about any stimulus that falls within the location on the retina served by the hypercolumn, neurons within the hypercolumn that respond to the orientation will be activated  Research done since Hubel and Wiesel’s proposal of the ice-cube model has shown that the actual organization of the three kinds of columns is far more complex than the picture in figure 4.22 How is an Object Represented in the Striate Cortex?  Looking at the tree results in an image on the retina, which then results in a pattern of activation on the striate cortex that looks something like the tree because of the retinotopic map in the cortex  Notice, however, that the activation is distorted compared to the actual object  More space is allotted to the top of the tree, where the observer is looking, because the magnification factor allots more space on the cortex to the parts of the image that fall on the observer’s fovea  Since our trunk is oriented vertically, it will activate neurons within the vertical (90 degree) orientation within each hypercolumn, as shown in figure 4.25  Thus, a large stimulus, which stretches across the retina, will stimulate a number of different orientation columns, each in a location in the cortex that is separated from the other orientation columns  Therefore, our tree trunk has been translated into activity in a number of separated orientation columns, and this activity looks quite different from the shape of the stimulus, which is a single continuous bar Streams: Pathways for What, Where, and How  One of the most influential ideas to come out of this research is that there are pathways , or “streams”, that transmit information from the striate cortex to other areas in the brain o This idea was introduced in 1982, when Leslie Ungerleider and Mortimer Mishkin described experiments that distinguished two streams that served different functions Streams for Information About What and Where  Ungerleider and Mishkin used a technique called ablation (also called leisoning)  Ablation refers to the destruction or removal of tissue in the nervous system  They presented monkeys with two tasks o (1) an object discrimination problem o (2) a landmark discrimination problem  In the object discrimination problem, a monkey was shown one object, such as a rectangular solid, and was then presented with a two-choice task like the one shown in figure 4.26a, which included the “target object” (the rectangular solid) and another stimulus  If the monkey pushed aside target object, it received food reward hidden under object  The landmark discrimination problem is shown in figure 4.26b  Here, the monkeys task was to remove to food well cover that was closest to the tallest cylinder  In ablation part of experiment temporal lobe removed from some monkeys  After ablation, behavioural testing showed object discrimination problem difficult for these monkeys o This indicates that the pathway that reaches temporal lobes is responsible for determining object identity o They therefore called this pathway from the striate cortex to temporal lobe the what pathway  Other monkeys had parietal lobes removed o Landmark problem difficulty o Indicates pathway to parietal lobe responsible for object location o Therefore called pathway from striate cortex to parietal lobe the where pathway  The what and where pathways also called the ventral pathway (what) and dorsal pathway (where)  Dorsal refers to top (of brain) and ventral to lower  (1) the pathways are not totally separated but have connections between them; (2) signals flow not only “up” the pathway toward parietal and temporal lobes, but “back” as well Streams for Information About What and How  David Milner and Melvyn Goodale o What and where streams should be called what and how o Ventral stream – for perceiving objects o Dorsal stream – taking action o Evidence for the dorsal stream being involved in how to take action – discovery of neurons in partial cortex that respond to  (1) when a monkey looks at an object  (2) when it reaches toward object The Behaviour of Patient DF  Milner and Goodale  D.F o 34 year old woman o Damage to ventral pathway from carbon monoxide o One result – not able to match orientation of card held in hand to different orientations of a slot  Figure 4.29a  Was able to “mail” card through slot  Once started moving card toward slot, was able to rotate to match orientation of slot  Thus, o Performed poorly in static orientation-matching task but, o Did well as soon as action was involved  Interpreted this as: o One mechanism for judging orientation and, o Another for coordinating vision and action  Based on results, they suggested that ventral pathway still be called what pathway  But dorsal pathway should be called how pathway or action pathway The Behaviour of People Without Brain Damage  Rod and frame illusion o Two small lines inside titled squares appear slightly tilted in opposite direction even though they are parallel vertical lines  Richard Dyde and David Milner o Present observers with two tasks:  Matching task and a grasping task  Matching task – adjust matching stimulus until it appeared to match orientation of vertical rod in tilted square o Results: had to adjust matching stimulus 5 degrees from vertical  Grasping task – grasped a rod in the titled square between thumb and fore finger. Position was measured. o Results: observers positioned fingers appropriately for rods orientation  Rationale behind experiment:  Since 2 tasks involve different processing streams (matching = ventral, grasping = dorsal) they may be affected differently by presence of surrounding frames  Results support idea that perception and action are served by different mechanisms Modularity: Structures for Faces, Places, and Bodies  Found neurons that respond best to more complex stimuli o Keiji Tanaka  Evidence that neurons that respond to similar stimuli are often grouped together in one area of brain  Structure specialized to process info about particular type of stimulus is called a module Face Neurons in the Monkey’s IT Cortex  Edmund Rolls and Martin Tovee o Measured response of neurons in monkeys inferotemporal (IT) cortex o Presented face and nonface stimuli o Many neurons respond best to face Areas for Faces, Places, and Bodies in the Human Brain  Brain imaging o Used to identify areas that contain neurons that respond best to faces, scenes, and human bodies  Nancy Kanwisher o fMRI to determine brain activity in response to faces and objects o when subtracted response to other objects from face response, found activity remained in area called fusiform face area (FFA)  damage to temporal lobe causes prosopagnosia – difficulty recognizing faces of familiar people  parahippocampal place area (PPA) – activated by pictures of indoor and outdoor scenes o spatial layout important for this  extrastriate body area (EBA) – activated by pictures of bodies and parts of bodies Something to Consider: How do Neurons Become Specialized? Is Neural Selectivity Shaped by Evolution?  Theory of natural selection o Genetically based characteristics that enhance animals survival ability will be passed on to future generations How Neurons can be Shaped by Experience  Experience-dependent plasticity o First suggested by experiments with animals o Experience causes changes in how neurons are tuned in the human cortex CHAPTER 5 Why is it so Difficult to Design a Perceiving Machine? The Stimulus on the Receptors is Ambiguous  A particular image on the retina (or a computer vision machines sensors) can be created by many different objects is called inverse projection problem Objects can be hidden or blurred  Problem of hidden objects occurs anytime one object obscures a part of another object  People are also able to recognize objects that are not in sharp focus Objects Look Different from Different Viewpoints  Images of objects are continually changing depending on what angle they are viewed from  Ability to recognize an object seen from different viewpoints is called viewpoint invariance  Perception more complex than it seems  Gestalt Psychologists o Gestalt – whole configuration that cannot be described merely as the sum of its parts The Gestalt Approach ro Object Perception  Wilhelm Wundt o Approach to psychology called structuralism  Perceptions created by combining elements called sensations  Idea that perception is result of “adding up” sensations was disputed by Gestalt psychologists o Instead thought the whole differs from the sum of its parts  When two stimuli that are in slightly different positions are flashed one after another with the correct timing, movement is perceived between the two stimuli o This is an illusion called apparent movement because there is actually no movement in the display, just two stationary stimuli flashing on and off  Having rejected the idea that perception is built up of sensations, the Gestalt psychologists proposed a number of principles which they called laws of perceptual organization The Gestalt Laws of Perceptual Organization  Perceptual organization involves grouping of elements in an image to create larger objects  Six laws proposed to explain how perceptual grouping occurs Pragnanz  “good figure”  Law of pragnanz (law of good figure or law of simplicity) o Central law of gestalt psych o Every stimulus pattern is seen in such a way that the resulting structure is as simple as possible Similarity  Law of similarity: similar things appear to be grouped together  Grouping can occur because of colour, shape, size, or orientation o Also occurs for auditory stimuli Good Continuation  Law of good continuation: points that, when connected, result in straight or smoothly curving lines are seen as belonging together, and the lines tend to be seen in such a way as to follow the smoothest path Proximity (nearness)  Law of proximity, or nearness: things that are near eac
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