Chapter 6 - Chapter 6 Chapter Vision Introduction...

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Unformatted text preview: Chapter 6 Chapter Vision Introduction Introduction Sensory receptors – a specialized neuron that detects a particular category of physical events physical Sensory transduction – the process by which sensory stimuli are transduced into slow, graded receptor potentials receptor The Stimulus The The perceived color of light is determined by 3 The dimensions: dimensions: – Hue – the dominant wavelength – Saturation - purity – Brightness - intensity Anatomy of the visual system Anatomy Eyes – – Suspended in the orbits of Suspended the skull the – Held in place by 6 Held extraocular muscles extraocular – Retina – the neural tissue and photoreceptive cells located on the inner surface of the posterior position of the eye position – Fovea – the region of the retina that mediates the most acute vision; color sensitive cones constitute the only type of photoreceptor here photoreceptor Anatomy of the visual system Anatomy Eyes – Photoreceptor – one of the receptor cells of the retina; transduces photic energy into electrical potentials transduces Rod – sensitive to light of low intensity Cones – maximally sensitive to one of 3 different wavelengths of light and hence encodes color vision light – Optic disk – the location of the exit point from the retina of the fibers of the ganglion cells that form the optic nerve; responsible for the blind spot for – 3 types of movements: Vergence movements – the cooperative movements that keep both eyes fixed on the same target eyes Saccadic movements – rapid, jerky movements of the eyes used in scanning a visual scene scanning Pursuit movement – the movement made to maintain an image of a moving object on the fovea moving Photoreceptors Photoreceptors Each photoreceptor consists of Each an inner segment and an outer segment, which contains several hundred lamellae (thin lamellae plate of membrane) plate Visual info transduction Visual Photopigments are special molecules embedded in the lamellae (e.g. rhodopsin) and rhodopsin and consists of 2 parts: consists – Opsin – protein – Retinal – lipid When a molecule of rhodopsin When is exposed to light it breaks into its two parts, and this causes a change in the membrane potential of the photoreceptor, which changes the firing rate of glutamate the Visual info transduction Visual The membrane of photoreceptors is The different from others – the cation channels (sodium and calcium) are normally open normally In the dark these ion channels are In open, and so the photoreceptors continually release glutamate continually When light strikes the photopigment, When the G protein transducin is activated, transducin which then activate the enzyme phosphodiesterase which closes the phosphodiesterase ion channels; this lowers the rate of glutamate release glutamate Light hyperpolarizes the Light photoreceptor and then depolarizes the bipolar cell the Connections between eye and brain brain The axons of the retinal ganglion cells project through the optic The nerves, cross over through the optic chiasm to the dorsal lateral optic geniculate nucleus (LGN) geniculate The LGN consists of 6 layers of neurons: the inner 2 layers are called the magnocellular layers and the outer 4 are called the magnocellular parvocellular layers parvocellular The cells in the LGN project through the optic radiations to the The primary visual cortex (aka striate cortex) primary Diagram of visual pathways Coding of light and dark Coding The receptive field of a neuron in the visual system is the part of the The receptive visual field that an individual neuron “sees”, i.e. the part in which light must fall for the neuron to be stimulated light 2 major types of retinal ganglion cells, ON center and OFF center major cells cells – ON center cells are excited by light falling in the center of the field ON (center), and inhibited by light falling in the surrounding field (surround) (center), – OFF center cells are excited by light in the surround, and inhibited by OFF light in the center light Coding of light and dark Coding The center-surround organization of the retinal ganglion cells The enhances our ability to detect the outlines of objects enhances Coding of color Coding The retinas of humans, Old World monkeys and apes contain 3 The different types of cones which provide us with an elaborate form of color vision color All visible colors (for humans at least) can be mixed from the 3 main All colors: red (long), green (medium), and blue (short); due to the wavelengths absorbed by the 3 different cones (trichromatic theory) wavelengths Genetic defects can cause one or more of the 3 types of cones to Genetic not function properly, resulting in either protanopia, where red and protanopia where green are confused because the red cones respond to green; deuteranopia, where where red and green are confused also, but deuteranopia where because the green cones respond to red; or tritanopia, where they tritanopia where lack blue cones lack Anatomy of the striate cortex Anatomy Consists of 6 principle layers, arranged in bands parallel to the Consists surface surface The striate cortex of one hemisphere contains info from the The contralateral visual field contralateral Approx. 25% of the striate cortex is devoted to anlaysis of info from Approx. the fovea the Neurons in the visual cortex selectively respond to specific features Neurons of the visual world, not just to light of Orientation and Movement Orientation Simple cell – an orientation-sensitive neuron whose receptive field is organized in an opponent fashion organized Complex cell – a neuron that responds to the presence of a line segment with a particular orientation located within it s receptor field, especially when the line moves perpendicularly to its orientation orientation Hypercomplex cell – a neuron that responds to the presence of a line segment with a particular orientation that ends at a particluar point within the cell’s receptive field point Spatial Frequency Spatial Neurons in the primary visual cortex respond best to sine-wave Neurons gratings, which are a series of straight parallel bands varying gratings which continuously in brightness according to a sine-wave function, along a line perpendicular to their lengths line A sine-wave grating is designated by its spatial frequency, or the sine-wave spatial or relative width of the bands, measured in cycles per degree of visual angle angle The most important visual information is that contained in low spatial The frequencies frequencies Retinal disparity Retinal Binocular vision (i.e. from 2 eyes) provides a vivid perception of Binocular depth through the process of stereoscopic vision, or stereopsis stereopsis Most neurons in the primary visual cortex are binocular, they Most respond to info of either eye respond The cells respond most vigorously when each eye sees a stimulus The in a slightly different location, called retinal disparity retinal Color Color In the striate cortex, info from color-sensitive ganglion cells is In transmitted through to special cells grouped into cytochrome oxidase (CO) blobs oxidase Modular organization of the striate cortex cortex The striate cortex is divided into ~2500 modules, each containing The ~150,000 neurons ~150,000 The neurons in each module are devoted to the analysis of various The features contained in one very small portion of the visual field, and combine together to form a complete whole combine Info from the layers of the LGN project to the different layers of the Info modules modules The modules consist of 2 segments, each surrounding a CO blob – The neurons in the CO blobs are sensitive to color and low spatial The frequencies frequencies – The neurons outside of the blobs are sensitive to orientation, movement, The spatial freq, texture and binocular disparity spatial Role of the visual association cortex cortex Two streams of visual analysis – Visual info receive from the striate cortex is analyzed in the visual assc Visual cortex cortex – Neurons in the striate cortex project to the extrastriate cortex, which Neurons extrastriate which surrounds the visual assc cortex surrounds – The primate extrastriate cortex consists of several specialized regions The that respond to particular features of a visual stimulus that – Contains 2 streams of analysis: Dorsal stream – a system of interconnected regions of visual cortex involved in the perception of spatial location, beginning with the striate cortex and ending with the posterior parietal cortex ending Ventral stream – a system of interconnected regions of visual cortex involved in the perception of form, beginning in the striate cortex and ending in the inferior temporal cortex in Perception of color Perception Color constancy – the relatively constant appearance of the colors of objects under varying lighting conditions; the visual system compensates for the source of light when process visual information about colors about Achromatopsia – inability to discriminate among different hues; caused by damage to the visual assc cortex caused Analysis of form Analysis In primates the recognition of visual patterns and identification of particular objects In take place in the inferior temporal cortex, llocated at the end of the ventral stream on ocated inferior the ventral part of the temporal lobe the – – Analyses of form and color are put together here and perceptions of 3D objects and Analyses backgrounds are achieved backgrounds Consists of 2 major regions: TE and TEO Damage to the human visual assc cortex can result in a visual agnosia, which results Damage visual which in an inability to perceive or identify a visual stimulus in – – Apperceptive visual agnosia – failure to perceive objects, even though visual acuity is normal (e.g. cannot name an object by looking at it, but can if allowed to touch it) normal Prosopagnosia – failure to recognize particular people by the sight of their faces (i.e. can recognize by voice, hair color, etc.) recognize Fusiform face area – region of the extrastriate cortex located at the base of the brain; involved in perception of faces and other complex objects that require expertise to recognize recognize Associative visual agnosia – inability to identify objects that are perceived visually, even though the form of the perceived object can be drawn or matched with similar objects; appears to involve difficulty in transferring visual info to verbal mechanisms objects; Perception of movement Perception Area V5 of the extrastriate cortex contains neurons that respond to Area movement, and show directional sensitivity movement, An area adjacent to V5 receives info from V5 about movement and An respond to more complex movement features, such as radial, circular or spiral motion circular – Optic flow – the complex motion of points in the visual field caused by relative movement b/t the observer and env’t; provides info about the relative distance of objects from the observer and of the relative direction of movement direction Akinetopsia – inability to perceive movement, caused by damage to area V5 of the visual assc cortex area Perception of spatial location Perception 3 subareas of the extrastriate cortex send info through area V5 to subareas the parietal cortex, which is involved in spatial perception the Damage to this area disrupts performance on tasks that require Damage perceiving and remembering the location of objects perceiving Balint’s syndrome – caused by bilateral damage to the parietooccipital region; includes optic ataxia (difficulty in reaching for occipital optic objects under visual guidance), ocular apraxia (difficulty in visual ocular scanning), and simultanagnosia (difficulty in perceiving more than simultanagnosia one object at a time) one ...
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