Chapter 6 Vision - Summary Bio Psychology PDF

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Chapter 6: VISION: VISUAL CODING, PROCESSING AND PERCEPTIONObjectives:  Outline the basic visual pathways from the eye to the cortex  Describe the structure of the human retina, and explain how the structure of the retina influences vision.  Explain how photoreceptors lead to colour perception  ...

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Chapter 6: VISION: VISUAL CODING, PROCESSING AND PERCEPTION Objectives:  Outline the basic visual pathways from the eye to the cortex  Describe the structure of the human retina, and explain how the structure of the retina influences vision.  Explain how photoreceptors lead to colour perception  Describe how receptive fields work  Describe how form, colour & motion are perceived in the brain  Identify 2 disorders of the visual system SENSATION VS. PERCEPTION Sensation  Sense organs gather information from environment  Requires sensory receptors, which can: 1. Detect (quality & quantity) 2. Discriminate  A dimension of the physical world is translated into neuronal code & then conducted to the brain Perception  Brain organizes & interprets sensations  Makes sensory information stable, meaningful & coherent  Influenced by prior knowledge & experience  No hard & fast line between sensation and perception; more of a continuum What Do We See?  Somehow a distorted and upside-down 2-D retinal image is transformed into the 3-D world we perceive.  Two types of research are needed to study vision. o Research probing the components of the visual system o Research assessing what we see  Light Enters the Eye and Reaches the Retina  No species can see in the dark, but some are capable of seeing when there is little light. o Humans see light between 380–760 nanometers. o Wavelength: perception of color o Intensity: perception of brightness The Pupil and the Lens  Light enters the eye through the pupil, whose size changes in response to illumination.  Sensitivity: the ability to see when light is dim (pupil large)  Acuity: the ability to see details (pupil small)  The lens focuses light on the retina and is located behind the pupil  Ciliary muscles alter the shape of the lens as needed.  Accommodation: the process of adjusting the lens to bring images into focus

The Retina  The retina is, in a sense, inside-out. o Light passes through several cell layers before reaching its receptors (which detect light)  Vertical pathway: receptors (rods and cones) > bipolar cells > retinal ganglion cells  Lateral Communication (refinement) o Horizontal cells o Amacrine cells

Rods and cones are activated by light ^^ explain 

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Blind spot: no receptors, where information exits the eye o The visual system uses information from cells around the blind spot for “completion,” filling in the blind spot. o Where axons of retinal ganglia leave the eye Fovea: high-acuity area at center of retina o Thinning of the ganglion cell layer reduces distortion due to cells between the pupil and the retina. o Rich in cones

The Optic Nerve

Cone and Rod Vision  Duplexity theory of vision: cones and rods mediate different kinds of vision. o Cones: photopic (daytime) vision  High-acuity color information in good lighting (colour & detail) o Rods: scotopic (nighttime) vision  High-sensitivity, allowing for low-acuity vision in dim light  but lacks detail and color  Peripheral vision  There is more convergence in the rod system, increasing sensitivity while decreasing acuity.  Only cones are found at the fovea. ^ top is phototopic: Only a few cones converge on each retinal ganglion cell to receive input from only a few cones

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Day: only 1 is connected, convergence is sacrificed, less likely to potentiate

, bottom is scotopic: - Several rods converge on a single retinal ganglion cell - Action potential, more likely for message – dim light A schematic representation of the convergence of cones and rods on retinal ganglion cells. There is a low degree of convergence in cone-fed pathways and a high degree of convergence in rod-fed pathways. Eye Movement  We continually scan the world with small and quick eye movements : saccades.  These bits of information are then integrated.  Stabilize retinal image; see nothing.  The visual system responds to change. VISION: VISUAL PATHWAYS From Retina to Primary Visual Cortex  The retinal-geniculate-striate pathways include about 90% of axons of retinal ganglion cells.  The left hemiretina of each eye (right visual field) connects to the left lateral geniculate nucleus (LGN); the right hemiretina (left visual field) connects to the right LGN.  Most LGN neurons then project to primary visual cortex (V1, striate cortex).  Left [Hemi]Retina (right visual field)left lateral geniculate nucleus (LGN)striate cortex  Right HemiRetina (left visual field)  right LGN  striate cortex    

Light from the LEFT side of the world strikes the RIGHT half of the retina and vice versa Information from the nasal half of each eye crosses to the contralateral hemisphere. Information from the temporal side of each eye goes to the ipsilateral hemisphere o (Only some fibers cross) Crosses at the optic chiasm o High in myelination for fast processing

Visual Pathways

Retinotopic Organization  Information received at adjacent portions of the retina remains adjacent in the striate cortex (retinotopic).  More cortex is devoted to areas of high acuity—like the disproportionate representation of sensitive body parts in somatosensory cortex.  About 25% of primary visual cortex is dedicated to input from the fovea. The M and P Channels Magnocellular Layers (M Layers)  Big cell bodies; bottom two layers of LGN  Particularly responsive to movement  Input mainly from rods Parvocellular Layers (P Layers)  Small cell bodies; top four layers of LGN  Color, detail, and still or slow objects  Input mainly from cones  

The channels project to slightly different areas in lower layer IV in striate cortex (V1); M neurons are just above the P neurons. The channels project to different parts of visual cortex beyond V1.

Seeing Edges  Edges are the most informative features of visual display as they define the extent and position of the various objects in it  Contrast Enhancement o Mach bands: nonexistent stripes the visual system creates for contrast enhancement due to lateral inhibition The Horseshoe Crab  Large photoreceptors – ommatidia  If a single ommatidium is illuminated, it fires at a rate proportional to the intensity of light striking it  When a receptor fires, it inhibits its neighbours via the lateral neural network: Lateral inhibition o Lateral inhibition is greatest when the receptor is most intensely illuminated, and the inhibition has greatest effect on the receptors immediate neighbours How Lateral Inhibition Creates Contrast Lateral Inhibition in Humans  Is the reduction of activity in one neuron due to inhibition by horizontal neurons  Responsible for heightening contrast in vision  The response of bipolar cells in the visual system depends upon the net result of excitatory and inhibitory messages it receives Receptive Fields  The part of the visual field that either excites or inhibits a cell in the visual system  For a receptor, the receptive field is the point in space from which light strikes it

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For other visual cells, receptive fields are derived from the visual field of cells that either excite or inhibit them Rods and cones are activated by light which activate other cells

Receptive Fields: Neurons Hubel and Wiesel looked at receptive fields in the retinal ganglion and the LGN in the cat. Similarities seen:  Receptive fields of foveal areas are smaller than those in the periphery (high acuity).  Neurons’ receptive fields are circular in shape.  Many neurons at each level had receptive fields with excitatory and inhibitory areas  Many cells have receptive fields with a center-surround organization: excitatory and inhibitory regions separated by a circular boundary.  Some cells are on-center and some are off-center. o Neurons respond to change

Receptive Fields: Simple and Complex Cortical Cells  From V1 (primary visual cortex) and beyond, neurons with circular receptive fields (as in retinal ganglion cells and LGN) are rare.  Most neurons in V1 are either: o Simple—receptive fields are rectangular with “on” and “off” regions—or o Complex—also rectangular, with larger receptive fields, and respond best to a particular stimulus anywhere in their receptive fields SIMPLE Rectangular “On” and “off” region Orientation and location sensitive All are monocular (one eye)

COMPLEX Rectangular Larger receptive fields Do not have static “on” and “off” regions Not location sensitive Motion sensitive Many are binocular (both eyes)

Contextual Influences in Visual Processing  From the retina  thalamus  lower level IV of visual cortex  simple cells  complex cells, the “preferences” of neurons became more complex o Neurons with simpler preferences converge onto neurons with more complex preferences.  Plasticity appears to be a fundamental property of visual cortex function. o E.g., receptive field properties depend on the scene in which the stimuli to its field are embedded. VISION: COLOUR AND PERCEPTION Seeing Color: Component and Opponent Processing  Most primates are trichromats; most other mammals are dichromats Component Theory (Trichromatic Theory)  Proposed by Young, refined by Helmholtz  Three types of receptors, each with a different spectral sensitivity Opponent-Process Theory was proposed by Hering.  Two different classes of cells encoding color, and another class encoding brightness  Each encodes two complementary color perceptions.  This theory accounts for color afterimages and colors that cannot appear together (reddish green or bluish yellow). Colour Vision: Trichromatic Theory  3 types of cones receptors o each maximally sensitive to one range of wavelengths: blue (short), green (medium), red (long)  Colours determined by comparing ratio of activation coming from each type o Most colours are a mix (such as orange)  Brightness determined by the number of cones activated  Can’t explain why yellow seems like a primary colour or why we see afterimages of complementary Colour Vision: Opponent Process  Receptors in visual system respond positively to one colour & negatively to that complementary colour  Colour perception depends on bipolar cells that make antagonistic responses to 3 pairs of colours: o red vs. green o yellow vs. blue o black vs. white  Use 4 categories, (not 3) to describe “basic” colours: red, green, blue, yellow Basal level slight inhibit bipolar Cell becomes fatigued – low level basal activation than usual – opposite in after image

Which theory is correct? Both are:  We have 3 types of colour receptors (cones), corresponding to the blue-green-red of trichromatic theory  We also have cells further along the visual pathway (i.e., bipolar cells) that respond in antagonistic ways to the pairs predicted in opponent process theory (e.g., turned on by red and switched off by green) Color Constancy and the Retinex Theory  Color constancy: color perception is not altered by varying reflected wavelengths. (E.g. shirt colour across the course of a day).  The brain looks at all colours, not just one to determine constancy  Land (1977) - Mondrians Retinex Theory  Retinex theory (Land): color is determined by the proportion of light of different wavelengths that a surface reflects.  Relative wavelengths are constant, so perception is constant. o Dual-opponent color cells are sensitive to color contrast. o Found in cortical “blobs” Cortical Mechanisms of Vision and Conscious Awareness  Flow of Visual Information o Thalamic relay neurons > o 1˚ visual cortex (striate) > o 2˚ visual cortex (prestriate) > o Visual association cortex  As visual information flows through hierarchy, receptive fields: o Become larger o Respond to more complex and specific stimuli

Damage to Primary Visual Cortex Scotomas o Areas of blindness in contralateral visual field due to damage to primary visual cortex o Can see normal but can’t perceive o Detected by perimetry test  Completion o Patients may be unaware of scotoma; missing details are supplied by “completion.”  Blindsight o Response to visual stimuli outside conscious awareness of “seeing” o Possible explanations of blindsight:  Islands of functional cells within scotoma  Direct connections between subcortical structures and secondary visual cortex; not available to conscious awareness  Asked to grab something they are not consciously aware of  Explanation: some functioning neurons still  Compensation: other pathways 

The completion of a migraine-induced scotoma as described by Karl Lashley (1941).

Visual Perception Our visual system detects basically 3 things: 1. Form/shape 2. Colour 3. Motion Functional Areas of Secondary and Association Visual Cortex  Neurons in each area respond to different visual cues, such as color, movement, or shape.  Lesions of each area results in specific deficits.  Retinotopically organized Dorsal and Ventral Streams  Dorsal stream: pathway from primary visual cortex to dorsal prestriate cortex to posterior parietal cortex o The “where” pathway (location and movement), or o Pathway for the control of behavior (e.g., reaching) o Damage = can’t reach for objects that they have no problems describing  Ventral stream: pathway from primary visual cortex to ventral prestriate cortex to inferotemporal cortex o The “what” pathway (color and shape), or o Pathway for the conscious perception of objects o Damage = aware of objects in space, but cannot describe them

The “where” versus “what” and the “control of behavior” versus “conscious perception” theories make different predictions. Agnosia: impairment in recognition of visually presented objects Prosopagnosia  Inability to distinguish among faces  associated with damage to the ventral stream between the occipital and temporal lobes.  Prosopagnosics may be able to recognize faces in the absence of conscious awareness. o Prosopagnosics have different skin conductance responses to familiar faces compared to unfamiliar faces, even though they reported not recognizing any of the faces. Fusiform gyrus is important for detecting faces. Part of ventral? Akinetopsia  Deficiency in the ability to see movement progress in a normal, smooth fashion o Movement appears like a strobe light  Can be induced by a high dose of certain antidepressants  Associated with damage to the middle temporal (MT) area of the cortex  Dorsal stream issue Summary  Five types of cells in the retina: receptors (cones and rods); bipolar cells; retinal ganglion cells, amacrine cells and horizontal cells Horizontal: contrast lateral inhibition Opponent: bipolar cells stimulated or inhibited  Cones = photopic vision; rods = scotopic vision/peripheral/low light  Visual information travels from the retinal ganglion cells > LGN of thalamus > striate cortex/primary visual cortex > secondary visual cortex > association areas 

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Colour vision o Trichromatic theory o Opponent Process theory Lateral inhibition o Contrast Receptive fields o Get larger and more complex as you move from retina – LGN – V1 and beyond

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Two streams of visual processing: 1. Ventral / temporal stream “What” 2. Dorsal / parietal stream “Where” Visual system detects 3 things: 1. Form / shape 2. Colour 3. Motion...