Vision Chapter 10 PDF

Chapter 10: Vision The Visual Stimulus The Anatomy of the Visual System Coding of Light and Dark Coding of Color The Primary Visual Cortex Perception of Visual Information 1 The Visual Stimulus The Electromagnetic Spectrum. - Visibl...

Chapter 10: Vision The Visual Stimulus The Anatomy of the Visual System Coding of Light and Dark Coding of Color The Primary Visual Cortex Perception of Visual Information 1 The Visual Stimulus The Electromagnetic Spectrum. - Visible light is small portion of electromagnetic spectrum (380-760 nm) - human eye not sensitive to electromagnetic radiation outside this narrow range - honeybees can detect ultraviolet radiation from flowers that appear white to us - light behaves both as particle and as wave, focus on wave-like aspects of light 2 2 The Visual Stimulus Color, Purity, and Brightness. - light travels at constant speed (186,000 miles/sec) - therefore waves of a given wavelength of light strike surface at constant rate - longer wavelengths strike less frequently, shorter strike more frequently - wavelength encoded as color or hue 3 3  The Visual Stimulus Color, Purity, and Brightness. - if light contains fairly equal mixture of all wavelengths, it produces no sensation of hue, appears white - if light contains only one wavelength, it is saturated - if light contains differing degrees of saturation of mixture of wavelengths, dominant wavelength(s) will be perceived - magnitude or intensity of waves determines perception of brightness 4 4 The Anatomy of the Visual System The Human Eye. - light travels in straight lines - amount of light controlled by iris/pupil - dilation = sympathetic - constriction = parasympathetic - light focused on retina by lens (ciliary muscles) - inverted upside-down image extraocular muscle - fovea: contains only cones - mediates high acuity, color vision iris pupil fovea optic disk (blind spot) lens retina ciliary muscle optic nerve (retinal ganglion cell axons) 5 5 The Anatomy of the Visual System The Human Eye. Vergence Movements: - cooperative movements both eyes to fix on target (i.e. foveate) Saccadic Movements: - scanning movements where gaze is abruptly shifted from one point to next Pursuit Movements: - smooth tracking movements that keep object’s image fixed in place on fovea convergence divergence 6 6  The Anatomy of the Visual System The human eye. - 120,000,000 rods and 6,000,000 cones synapse on 1,000,000 bipolar cells - bipolar cells synapse on retinal ganglion cells - axons of retinal ganglion cells form optic nerve light light light 7 7 The Anatomy of the Visual System The Human Eye. - photoreceptors at back of retina light rods light light cones light RGCs bipolar photo- cells receptors 8 8 The Anatomy of the Visual System The Human Eye. - photoreceptors at back of retina - 120,000,000 rods (scotopic and mesopic vision) - 6,000,000 cones (mesopic and photopic vison) cone rod 9 9  The Anatomy of the Visual System Receptive fields. - low convergence in cone-fed circuits - high convergence in rod-fed circuits 10 10 The Anatomy of the Visual System Receptive fields. - high density of small, tightly-packed cones in fovea - high acuity vision in bright light conditions - high density of rods outside fovea - spatial summation in low light conditions with loss of precision and detail 11 11 The Anatomy of the Visual System Receptive fields. - high density of small, tightly-packed cones in fovea - high acuity vision in bright light conditions - high density of rods outside fovea - spatial summation in low light conditions with loss of precision and detail 12 12  The Anatomy of the Visual System The Human Eye. - optic disk: contains no photoreceptors, hence provides blind spot + 13 13 The Anatomy of the Visual System The Human Eye. - optic disk: contains no photoreceptors, hence provides blind spot + 14 14 The Anatomy of the Visual System The Human Eye. - axons of retinal ganglion cells form the optic nerve - problem: how do distal inputs arrive at optic disc at same time as proximal inputs? - solution: distal inputs have higher conduction velocities y x 15 15  The Anatomy of the Visual System The Primary Visual Pathway - axons of retinal ganglion cells form optic nerves (cranial nerves #2) - optic nerves join at ventral aspect of brain to form x-shaped optic chiasm - optic nerves reach dorsal lateral geniculate nucleus of the thalamus - neurons of dorsal lateral geniculate send axons to primary visual (striate) cortex 16 16 The Anatomy of the Visual System 4 Types of Retinal Ganglion Cells (RGCs). midget (P) cell Parvocellular (P cell or midget cell): - small cell bodies, small receptive fields, parasol (M) cell - originate mostly in fovea bistratified cell - highly sensitive to color and detail - synapse on parvocellular cells of LGN Magnocellular (M cell or parasol cell): - larger cell bodies, larger receptive fields - originate throughout retina, incl. periphery - not very sensitive to color and detail photosensitive RGC - respond best to moving stimuli - important for brightness & depth perception - synapse on magnocellular cells of LGN Bistratified Cells: - about 8-10% of RGCs - inputs from rods, cones, and amacrine cells - synapse on koniocellular (“small as dust”) cells of LGN Photosensitive Retinal Ganglion Cells: - about 1-3% of RGCs - giant cells containing melanopsin - sluggish, long-term responses - contribute to synchronization of daily rhythms, regulation of pupillary constriction.. 17 17 The Anatomy of the Visual System Parvocellular, Magnocellular, and Koniocellular Pathways. - parvocellular, magnocellular, and bistratified/koniocellular cells project in distinct pathways from retina to LGN and from LGN to PVC koniocellular primary visual (striate) cortex 18 18  The Anatomy of the Visual System Parvocellular, Magnocellular, and Koniocellular Pathways. - axons of cells from primary and secondary visual cortices project to temporal and parietal cortices - dorsal “where” pathway; ventral “what” pathway DORSAL STREAM (magnocellular, “where”) VENTRAL STREAM (parvocellular, “what”) 19 19 The Anatomy of the Visual System Additional Visual Pathways. - retino-suprachaismatic-hypothalamic pathway synchronizes diurnal activity cycles 20 20 The Anatomy of the Visual System Additional Visual Pathways. - retino-tectal pathway organizes coordination of eye movements, control of muscles of the iris (pupil size), control of ciliary muscles (lens shape for focus) 21 21  Coding of Light and Dark 3 Stages of Visual Information Processing - reception - transduction - coding light rods light cones light RGCs bipolar photo- cells receptors 22 22 Coding of Light and Dark 3 Stages of Visual Information Processing - 1) Reception: - each photoreceptor contains several hundred lamellae (thin membrane plates) - each lamella contains about 10 million molecules of photopigment - each molecule consists of 2 parts, opsin and 11-cis retinal - rods contain rhodopsin and cones contain iodopsin - the energy from a photon of light splits rhodopsin or iodopsin into its 2 parts 23 23 Coding of Light and Dark 3 Stages of Visual Information Processing - 2) Transduction: - at rest, cation (Na+ and Ca2+) channels are open - resting potential of photoreceptors is less polarized than in other neurons - photoreceptors continuously release neurotransmitter at rest (in dark) - when struck by photon, breakage of photopigment molecule frees opsin from retinal - rhodopsin in rods; iodopsin in cones; melanopsin in photosensitive RGCs - opsin interacts with G-proteins (transducins) and closes cation channels - hyperpolarization, and graded decrease in release of glutamate opsin dark light 24 24  Coding of Light and Dark 3 Stages of Visual Information Processing - 2) Transduction: in the dark in the light photopigment molecules photopigment molecules photopigments are inactive 2nd absorb photon and activate cGMP messengers Na+ channels are held transducin → open by cGMP phosphodiesterase → Na+ breaks down cGMP, so Na+ enters photoreceptor, Na+ channels close partially depolarizing it Na+ cannot enter photoreceptor, so receptor is hyperpolarized light photoreceptor continuously glutamate release is releases glutamate diminished glutamate 25 25 Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - photoreceptors input to retinal bipolar cells, which input to retinal ganglion cells - photoreceptors and bipolar cells do not produce action potentials - they produce graded potentials - retinal ganglion cells produce action potentials 26 26 Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - horizontal cells are interneurons that also receive input from photoreceptors, and extend distributed inhibitory inputs back to photoreceptors - horizontal cells provide lateral inhibition of surrounding cells - amacrine cells are many types of interneurons with diverse morphologies - most amacrine cell function not well characterized, but may play a role in direction selectivity, on-off… 27 27  Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - photoreceptors continuously release glutamate in the dark - turning a light on always decreases glutamate release from photoreceptors - glutamate hyperpolarizes some bipolar cells, depolarizes others (hard-wired) - therefore, turning on a light either depolarize or hyperpolarize a bipolar cell - bipolar cells also release glutamate, and glutamate always depolarizes RGCs - RGCs have on or off properties depending upon the class of bipolar cells that input - on/off RGCs respond to both onset and offset of light 28 28 Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - photoreceptors continuously release glutamate in the dark - turning a light on always decreases glutamate release from photoreceptors - glutamate hyperpolarizes some bipolar cells, depolarizes others (hard-wired) - therefore, turning on a light either depolarize or hyperpolarize a bipolar cell - bipolar cells also release glutamate, and glutamate always depolarizes RGCs - RGCs have on or off properties depending upon the class of bipolar cells that input mGluR6 mGluR6 AMPA AMPA kainate kainate retinal ganglion cell AMPA, NMDA, kainate AMPA, NMDA, kainate AMPA, NMDA, kainate 29 29 Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - receptive fields of cells in visual system - defined as area of visual field that neuron “sees” 30 30  Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - receptive field of RGC contains 2 concentric regions - RGCs fall into three categories (ON, OFF, and ON/OFF): + - ON-cells - excited when light shines in center - inhibited when light shines on outer area light in center action potentials light in outer area action potentials diffuse light action potentials 31 31 Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - receptive field of RGC contains 2 concentric regions - RGCs fall into three categories (ON, OFF, and ON/OFF): OFF-cells - inhibited when light shines in center - excited when light shines on outer area - + light in center action potentials light in outer area action potentials diffuse light action potentials 32 32 Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - receptive field of RGC contains 2 concentric regions - RGCs fall into three categories (ON, OFF, and ON/OFF): ON/OFF-cells - excited briefly when light intensity changes anywhere in receptive field ++ light in center action potentials light in outer area action potentials - ON/OFF cells project primarily to the superior colliculus, and do diffuse not appear to play a direct role light in object perception action potentials 33 33  Coding of Light and Dark 3 Stages of Visual Information Processing - 3) Coding: - Horizontal cells release GABA and when active, they inhibit surrounding cells 34 34 Coding of Light and Dark Coding of Light and Dark. - overlapping receptive fields provide contrast enhancement - overlapping ON-cells, light striking excitatory center of one field will strike inhibitory surround of neighbouring field - from bipolar cells onward in primary visual system, cells respond better to spatial and temporal contrast than to absolute light levels 35 35 Coding of Light and Dark Coding of Light and Dark. - overlapping receptive fields provide contrast enhancement - overlapping ON-cells, light striking excitatory center of one field will strike inhibitory surround of neighbouring field - from bipolar cells onward in primary visual system, cells respond better to spatial and temporal contrast than to absolute light levels What is out there - - - - What you see + + + + Mach bands 36 36  Coding of Light and Dark - Coding of Light and Dark. + - + 37 37 Coding of Color Cones and Color Vision. - black and white vision is adequate for most purposes - color vision is important in identifying ripeness, counteracting camouflage… - humans, old world monkeys and apes each have 3 types of cones (3 iodopsins) providing the second most elaborate color vision in the animal kingdom 38 38 Coding of Color Cones and Color Vision. - black and white vision is adequate for most purposes - color vision is important in identifying ripeness, counteracting camouflage… - humans, old world monkeys and apes each have 3 types of cones (3 iodopsins) providing the second most elaborate color vision in the animal kingdom 39 39  Coding of Color Cones and Color Vision. - black and white vision is adequate for most purposes - color vision is important in identifying ripeness, counteracting camouflage… - humans, old world monkeys and apes each have 3 types of cones (3 iodopsins) providing the second most elaborate color vision in the animal kingdom 40 40 Coding of Color Cones and Color Vision. - mixing of colored light differs from pigment mixing - mixing of red, yellow, blue pigments in equal amounts = muddy gray - mixing of red, yellow, blue lights in equal amounts = white light Trichromatic (Young-Helmholtz) Theory of Color Vision. - based upon observation that any color of light can be obtaining by mixing various amounts of red, yellow, blue - proposed that humans have 3 kinds of photoreceptors that work together to give perception of color pigments lights 41 41 Coding of Color Cones and Color Vision. - cones exhibit maximal responses at 440 (short), 530 (medium), or 560 (long) nm - determined by type of iodopsin in cone - each cone responds over a range of wavelengths 440 nm 530 nm 560nm medium wavelength cone short wavelength cone long wavelength cone 42 42  Coding of Color Opponent Process Theory of Color Vision. - based upon idea that some colors don’t blend, and upon negative afterimages (trichromatic theory cannot explain) x x 43 43 Coding of Color Opponent Process Theory of Color Vision. - 2 kinds of opponent color sensitivity in ganglion cells - roughly concentric spectrally-opponent cone-fed inputs - medium opposes long - short opposes medium/long X S on L on M on L+M off M off L off 44 44 Coding of Color Retinal Color-Coding, Long Wavelength Light. long wavelength light absorbed cones best by long wavelength cone 440 530 560 some inhibition long wavelength cone excites L-on/M-off RGC long wavelength cone inhibits M-on/L-off RGC S-on/L+M-off L-on/M-off M-on/L-off signals red 45 45  Coding of Color Retinal Color-Coding, Medium Wavelength Light. medium wavelength light absorbed cones best by medium wavelength cone 440 530 560 some inhibition medium wavelength cone excites M-on/L-off RGC medium wavelength cone inhibits L-on/M-off RGC S-on/L+M-off L-on/M-off M-on/L-off signals green 46 46 Coding of Color Retinal Color-Coding, Short Wavelength Light. short wavelength light absorbed cones best by short wavelength cone 440 530 560 short wavelength cone excites S-on/L+M-off RGC cancellation cancellation minor effects on L-on/M-off and on M-on/L-off cancel S-on/L+M-off L-on/M-off M-on/L-off signals blue 47 47 Coding of Color Retinal Color-Coding, Long/Medium Wavelength Light. long/medium light absorbed cones best by medium and long 440 530 560 wavelength cones med and long wavelength cones inhibit S-on/L+M-off RGC cancellation cancellation large effects on L-on/M-off and on M-on/L-off cancel S-on/L+M-off L-on/M-off M-on/L-off signals yellow 48 48  Coding of Color That Mantis Shrimp Again. 49 49 normal vision protanope deuteranope tritanope 50 50 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - location in the visual field - color - ocular dominance - orientation optic LGN nerve primary occipital lobe visual cortex 51 51  The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - location in the visual field - LGN functions as topographically organized relay station, provides topographically-organized input to PVC, known as retinotopic organization - finer mapping of foveal/central region - parallel processing of magno- and parvocellular inputs - for any cortical column, receptive fields of all cells have roughly the same retinotopic location, and these locations change systematically in nearby columns koniocellular ABCD ABCD lateral geniculate nucleus primary visual (striate) cortex 52 52 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - color - parvocellular circuits carry input from medium and long wavelength cones - koniocellular circuits carry information from short wavelength cones (always blue “on”, red/green “off”) koniocellular lateral geniculate nucleus primary visual (striate) cortex 53 53 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - color - parvocellular circuits carry input from medium and long wavelength cones - koniocellular circuits carry information from short wavelength cones (always blue “on”, red/green “off”) - color-sensitive cells in CO blobs of V1 send color information to thin stripes of V2 - area V4 is particularly rich in color-sensitive cells, important in color constancy 54 54  The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - color - parvocellular circuits carry input from medium and long wavelength cones - koniocellular circuits carry information from short wavelength cones (always blue “on”, red/green “off”) - color-sensitive cells in CO blobs of V1 send color information to thin stripes of V2 - area V4 is particularly rich in color-sensitive cells, important in color constancy 55 55 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - color - parvocellular circuits carry input from medium and long wavelength cones - koniocellular circuits carry information from short wavelength cones (always blue “on”, red/green “off”) - color-sensitive cells in CO blobs of V1 send color information to thin stripes of V2 - area V4 is particularly rich in color-sensitive cells, important in color constancy 56 56 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - color - parvocellular circuits carry input from medium and long wavelength cones - koniocellular circuits carry information from short wavelength cones (always blue “on”, red/green “off”) - color-sensitive cells in CO blobs of V1 send color information to thin stripes of V2 - area V4 is particularly rich in color-sensitive cells, important in color constancy 57 57  The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - interblob regions of V1 - interblob regions of V1 input to thick stripes and pale stripes to provide information about ocular dominance, orientation, movement… 58 58 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - ocular dominance - if electrode is advanced through interblob column of cortex (i.e. perpendicular to surface), all neurons encountered will similar response properties (e.g. same ocular dominance) - most cells in PVC are binocular, but respond more vigourously for one eye’s input relative to the other - if electrode is advanced tangentially across columns (i.e. parallel to surface), cellular properties change systematically - ocular dominance shifts back and forth, demonstrating ocular dominance columns 59 59 The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - orientation sensitivity - if electrode is advanced through interblob column of cortex (i.e. perpendicular to surface), all neurons encountered will have similar response properties (e.g. same orientation sensitivity) - some cells respond maximally to a vertical line - some cells respond maximally to a horizontal line - some cells respond maximally to an oblique line - all orientation sensitive cells have tuning curves - if electrode is advanced tangentially across columns (i.e. parallel to surface), cellular properties change systematically - orientation sensitivity shifts smoothly through a rotation, with occasional larger jumps stimulus stimulus stimulus action potentials action potentials action potentials time time time 60 60  The Primary Visual Cortex Primary Visual Cortex Represents Four Aspects of Visual Stimulus. - orientation sensitivity - some cells respond maximally to a vertical line - some cells respond maximally to a horizontal line - some cells respond maximally to an oblique line - all orientation sensitive cells have tuning curves stimulus on action potentials time 61 61 Perception of Visual Information Orientation Selectivity and Hierarchical Processing. stimulus - magnocellular inputs to area V1 of PVC provide information regarding orientation and movement action potentials - if a line is positioned in simple cortical cell’s receptive field, time and rotated around its center, the cell will only respond stimulus when the line is in a particular range of orientation - optimal orientation varies from one cortical neuron to another action potentials - some respond best to vertical orientation time - some respond best to horizontal orientation stimulus - some respond best to oblique orientation - all exhibit tuning curves - LGN concentric receptive fields input to simple cells of V1 action potentials time to determine orientation selectivity receptive fields of the retinal -+ receptive fields ganglioncells -+ of the lateral -+ lateral geniculate -+ geniculate cells -+ cells simple cell simple cell excitatory and -+ -+ is excited - is inhibited - inhibitory inputs cancel - -+ + + + - receptive field + simple cortical cell of the simple cortical cell 62 62 Perception of Visual Information Orientation Selectivity and Hierarchical Processing. - simple cells’ orientation-selective receptive fields input to complex cells of PVC with orientation selectivity tuning that has no center/surround organization - simple cells found in layer 4 which receive extensive direct input from LGN - complex cells found in layers 2+3, - - - - - - which receive input mostly from layer 4 - additional cells in area V2 respond to + + + + + + differing orientations within their receptive fields of the simple receptive fields to represent corners cortical cells receptive field of the complex simple cortical cell cortical cells complex cortical cell 63 63  Perception of Visual Information Spatial Frequency. - eyes are in constant motion, even when fixed on object in visual field - contrast between lighter and darker parts of visual stimulus as eyes move yield patterns of spatial frequency, including high and low frequency aspects 64 64 Perception of Visual Information Spatial Frequency. - eyes are in constant motion, even when fixed on object in visual field - contrast between lighter and darker parts of visual stimulus as eyes move yield patterns of spatial frequency, including high and low frequency aspects 65 65 Perception of Visual Information Two Streams of Visual Analysis. - dorsal stream = Where? - mostly magnocellular, important in identifying spatial location, and organizing movement toward objects - ventral stream = What? - mostly parvocellular, important in color vision, and in identifying forms and features of objects DORSAL STREAM (magnocellular, “where”) VENTRAL STREAM (parvocellular, “what”) 66 66  Perception of Visual Information Dorsal Stream: Where? - occipito-parietal, parietal cortex - specific pathways exist for processing, with bi-directional signaling and shortcuts - e.g. V1→V2 → V3 → MT → MST SC V3 V5/MT V1 V4 V2 67 67 Perception of Visual Information Motion Detection - simple motion and direction analyzed by cells in middle of temporal lobe (area MT or area V5) - these cells respond specifically to movement, in specific direction, at specific speed, regardless of other attributes of stimulus (size, brightness, color, shape...) - an opponent process! V3 V5/MT V1 V4 V2 68 68 Perception of Visual Information Motion Detection - simple motion and direction analyzed by cells in middle of temporal lobe (area MT or area V5) - these cells respond specifically to movement, in specific direction, at specific speed, regardless of other attributes of stimulus (size, brightness, color, shape...) - an opponent process! SC V3 V5/MT V1 V4 V2 69 69  Perception of Visual Information Motion Detection - adjacent area, medial superior temporal cortex (area MST) is important for analysis of complex motion (e.g. circular or spiral motion) - area MSTd important for optic flow - area MST also stabilizes visual image by comparing relative movements of objects in visual field - important to compensate for changes in images position on retina during head and eye movements 70 70 Perception of Visual Information Dorsal Stream: Where? - motion detection constructed in your brain 71 71 Perception of Visual Information Dorsal Stream: Where? - area at junction of temporal and parietal lobes stabilizes visual image - area MSTd important for optic flow 72 72  Perception of Visual Information Depth Perception. - analyzed by combination of monocular and binocular cues - monocular cues: - perspective - relative retinal size - loss of detail in distance - relative apparent movement as you move your head - binocular cues: - retinal disparity 73 73 Perception of Visual Information Ventral Stream: WHAT? - occipito-temporal, inferior-temporal and inferior frontal cortex - important in recognition of objects V5/MST V3 V4 V2 V1 74 74 Perception of Visual Information Ventral Stream: WHAT? - recognition becomes more complex at higher (more frontal) levels - posterior = general information about facial features, gender etc. - anterior = recognition of individual faces V5/MST V3 V4 V2 V1 75 75  Perception of Visual Information Two Streams of Visual Analysis. - Ventral Stream: WHAT? - specific regions are involved in recognition of specific classes of objects (e.g. fusiform cortex for facial recognition, extrastriate body area or EBA for body parts) - general-purpose regions exist that respond to broad variety of stimuli, and may learn to recognize shapes that do not fall into general categories (e.g. lateral occipital complex or LOC) 76 76 Perception of Visual Information Two Streams of Visual Analysis. - Ventral Stream: WHAT? - specific regions are involved in recognition of specific classes of objects (e.g. fusiform cortex for facial recognition, extrastriate body area or EBA for body parts) - general-purpose regions exist that respond to broad variety of stimuli, and may learn to recognize shapes that do not fall into general categories (e.g. lateral occipital complex or LOC) 77 77 Perception of Visual Information Two Streams of Visual Analysis. - Ventral Stream: WHAT? - specific regions are involved in recognition of specific classes of objects (e.g. fusiform cortex for facial recognition, extrastriate body area or EBA for body parts) - general-purpose regions exist that respond to broad variety of stimuli, and may learn to recognize shapes that do not fall into general categories (e.g. lateral occipital complex or LOC) 78 78  Perception of Visual Information The Retinogeniculocortical Pathway. > 50% of primate cortex implicated in visual processing and associated functions 79 79