VISUAL PATHWAYS Valentin Dragoi Department of Neurobiology and Anatomy Parallel streams of visual processing Outline • Visual pathways: From retina to the cortex • Information ppgrocessing in visual cortex Outline • Visual pathways: From retina to the cortex • Information ppgrocessing in visual cortex VISUAL PATHWAY Retinal ggganglion cell axons leave the eye at the optic disc The retinal ganglion axons exit the eye to form the optic nerve. VISUAL PATHWAY OPTIC NERVE Consists of 3° afferent axons Axons of retinal ganglion cells OPTIC CHIASM Optifibiiific nerve fibers originating from the nasal retina that decussate in the optic chiasm Optic nerve fibers originating from the temporal retina do not decussate in the optic chiasm OPTIC TRACT Consists of 3° afferent axons from the nasal half of the contralateral eye & the temporal half of the ipsilateral eye RETINAL GANGLION CELL AXONS: OPTIC TRACT Optic Optic nerve tract Optic Hypothalamus: chiasm regulation of cidihthircadian rhythms Lateral geniculate nucleus Pretectum: reflex contlftrol of pup il and lens Optic radiation Superior colliculus: orienting the movements of head and eyes Striate cortex LATERAL GENICULATE NUCLEUS OF THE THALAMUS The visual system thalamic nucleus LGN MGN LGN organized into 6 cell layers 2 Magnocellular layers 4 Parvocellular layers with thin layers of kinocellular (dust-like) neurons interposed All LGN neurons • are monocular - respond to stimulation of one eye only • have concentric (ON/OFF or OFF/ON) receptive fields Type mLGN neurons in LGN magnocellular layers • synapse with Type M retinal ganglion axons • have large concentric receptive fields • are insensitive to color • sensitive to small changes in brightness levels (scotopic vision) • are rapidly-adapting (motion sensitive) Type pLGN neurons in LGN parvocellular layers • synapse with Type P retinal ganglion axons • have small concentric recepti ve fields ( high acuity ) • are sensitive to color (color sensitivity) • are not sensitive to small changes in brightness levels • are slowly-adapting (indicate the duration stimulus is “on”) Outline • Visual pathways: From retina to the cortex • Information ppgrocessing in visual cortex PRIMARY VISUAL CORTEX Primary visual cortex Calcarine fissure Cerebral cortex is differentiated into 6 horizontal layers COLOR SELECTIVITY SPECIALIZED PROCESSING : CHARACTERISTICS OF V1 BLOB (COLOR ) CELLS •coliilor sensitive - target ofkf kLGN axons • small concentric receptive fields • monocular - onlyyy one eye stimulates the blob cells • indifferent to stimulus orientation • indifferent to stimulus movement (slowly-adapting) BLOBS RETINA …. …. Striate cortex: V1 V1 BLOB CELL RESPONSES Orientation selectivity Stimulus Stimulus orientation presented Light b ar sti mul us projected on screen Recording from visual cortex Record 01 23 Time (sec) ORIENTATION SELECTIVITY SPECIALIZED PROCESSING : COMMON CHARACTERISTICS OF INTERBLOB CELLS •eldifildlongated receptive fields • insensitive to color • binocular w/ ocular dominance > one eye elicits a stronger response • most are sensitive to stimulus orientation > strongest response to particular orientation …. …. Interblob cells ORIENTATION SELECTIVITY SPECIALIZED PROCESSING : SHAPE/FORM & LOCATION V1 INTERBLOB CELLS •eldifildlongated receptive fields • insensitive to color • binocular w/ ocular dominance > one eye elicits a stronger response • sensitive to stimulus orientation > strongest response to particular orientation •insensitive to movement - target of pLGN axons > slowly-adapting response • location specific > stronggpest response in field center DIRECTION OF MOTION SPECIALIZED PROCESSING : MOTION SENSITIVE V1 INTERBLOB CELLS •eldifildlongated receptive fields • insensitive to color • binocular w/ ocular dominance > one eye elicits a stronger response • sensitive to stimulus orientation > strongest response to particular orientation • sensitive to MOVEMENT => target of mLGN axons DIRECTION OF MOTION SPECIALIZED PROCESSING : MOTION+LOCATION SENSITIVE V1 INTERBLOB CELLS •eldifildlongated receptive fields • insensitive to color • binocular w/ ocular dominance > one eye elicits a stronger response • sensitive to stimulus orientation > strongest response to particular orientation • most sensitive to motion => target of mLGN axons •some location specific DIRECTION OF MOTION SPECIALIZED PROCESSING : MOTION DIRECTION SENSITIVE V1 INTERBLOB CELLS •eldifildlongated receptive fields • insensitive to color • binocular w/ ocular dominance > one eye elicits a stronger response • sensitive to stimulus orientation > strongest response to particular orientation • most sensitive to motion - => target of mLGN axons •others direction specific ‘Ice-cube’ model of primary visual cortex R L R L R L R L functional domains Optical imagingof Single conditionmaps Orientationmap Cortical vasculature domains Stimulus computer Camera Li g ht g uide Amplifier Video data acquisition EXTRA-STRIATE AND ASSOCIATION AREAS NON-HUMAN PRIMATES VISUAL CORTICAL AREAS Posterior parietal lobe Middle & Medial Superior Temporal gyrus Striate (Visual) cortex Extrastriate cortex Inferior temporal gyrus HIGHER ORDER PROCESSING V1 SENDS MOST OF ITS AXONS TO EXTRASTRIATE CORTEX EXTRASTRIATE CORTEX (V2, V3 & V4) SEND AXONS TO ASSOCIATION CORTEX (TEMPORAL & PARIETAL AREAS) DORSAL STREAM: > SUPERIOR TEMPORAL & PARIETAL NECESSARY FOR •spatial orientation • binocular fusion • depth perception • spatial location • movement detection “THE WHERE” HIGHER ORDER PROCESSING V1 SENDS MOST OF ITS AXONS TO EXTRASTRIATE CORTEX EXTRASTRIATE CORTEX (V2, V3 & V4) SEND AXONS TO ASSOCIATION CORTEX (TEMPORAL & PARIETAL AREAS) VENTRAL STREAM: > INFERIOR “TEMPORAL” GYRUS IMPORTANT FOR SHAPE & COLOR • perception • discrimination • recognition • memory • learning “THE WHAT” Motion processing Shape processing in infero-temporal (IT) cortex Parallel streams of visual processing Spatial vision Lesion to MT abolishes the perception of motion X without affecting object Object vision vision X Lesion to V4 abolishes color vision without affecting spatial vision Suggested readings 1. Neuroscience book (Purves et al.), Chapter 12.