Visual Brain: The Neural Basis of Visual Perception!

Visual Brain: The Neural Basis of Visual Perception!?!

Human Brain: Amazing Machine! Cerebral cortex! Highest level of all sensory integration Highest level of somatic motor control Memory, association and cognitive processes (higher processes) Control of neural activities over lower levels 14-15 billion neurons 150 billion glial cells 2.5 ft2 if unwrinkled Sulcus! or Fissure! Gyrus! Gray matter! White matter!

About 40% of the human cerebral cortex is specialized in visual! functions ( > 50% in monkeys)! Area 19" (V3)" Area 18" (V2)" The primary visual cortex (area 17 or V1)"

Neural Basis of Visual Sensations and Perception Neural Detectors & Network: Information Processing! (encoding & decoding)" Dorsal Stream of the! Extrastriate Visual Areas! Primary Visual Cortex! (Area 17, V1, Striate! Cortex)! Ventral Stream of the! Extrastriate Visual Areas! Network of Neurons! Neurons! Retina LGN Visual Cortex

Understanding how visual information is processed in the brain Three stages of neural processing! 1. Low-level processing: First-order! stimulus feature information (stimulus! contrast, orientation, spatial frequency,! temporal frequency, wavelength) is processed.!!! 2. Mid-level processing: Grouping! or linking of first-order stimulus information.!!! 3. High-level processing:! Decision-making mechanisms: Initiation of! perceptual responses, e.g., recognition of! an object in visual scenes.!!

Information processing by neurons of our visual brain! 1. Encoding of stimulus contrast and spatial frequency information (RGC & LGN)! 2. Detection and encoding of orientation and spatial frequency of lines and contours (V1)! 3. Encoding of angles or curvatures (V2, V4)! 4. Encoding the spatial relationships of orientated lines or contours (Linking)(V2, V4)! 5. Decoding of linked features Perception of face )(IT, beyond)!

Methods to investigate the neural mechanisms underlying visual functions! Microelectrode Recording! Optical Imaging! fmri! Center! x! Fixation! target! Receptive! Field (RF)! Surround! Individual neurons as:! fmri!! 1. Mapping units! (e.g., topographic map)! Retina! 2. Stimulus feature detectors! (e.g., stimulus orientation! detection & discrimination)! 3. Integrators of local features! for visual perception! (e.g., contour integration, object! discrimination, face recognition)! Computers! LGN! Visual! cortex! Logothetis! Probe! Action Potentials! Amplifier!

Visual Pathways Subcortical structures! that receive direct retinal! input! LGN Superior Colliculus Pretectal Area Area 17

Pulvinar Tail of! Caudate! Nucleus! Posterior Commissure! Hippocampus Cerebral Aqueduct & Mid-brain Reticular formation Medial Geniculate Nucleus (MGN) Lateral Geniculate Nucleus (LGN)

X! LGN Sorting out retinal output from the two eyes! L! R! N! T! P-cells! Parvocellular layers! Layers 3,4 5, & 6! Midget RGC! from! Mid-brain! Reticular! Formation! Right LGN! 1, 4, & 6 = Contralateral! K-cells! Intercalated! or interlaminar! layers! On-Off RGC! from! Layer VI! 2, 3, & 5 = Ipsilateral! Right Visual Cortex! M-cells! Magnocellular layers! Layrs 1 & 2! Parasol RGC!

Mapping the visual world! Coronal section! Parasagittal section of the GN! lateral! +8! Retinotopic organization! 0! 8! -8! -60! anterior! Coronal section! L-12! A8! Disproportionately large! representation of the central field!

Receptive Field of Retinal Ganglion Cells & LGN cells ON and OFF Pathways On/Off Contrast Color Spatial frequency + ON! Light! Off! ON! Light! Off! + = Excitatory Center - = Inhibitory surround Insensitive to stimulus orientation or direction of stimulus movement

Spatial Frequency Magno vs. Parvo M P M! P! Response P Contrast Sensitivity Response (spikes/sec) Contrast (%) M! P! Contrast Gain!! Contrast Threshold! M

Contrast Sensitivity! Contrast Sensitivity! Temporal Frequency Tuning! P cells! M cells! Temporal frequency (Hz)! Temporal frequency (Hz)! Visual Latency:! Integration time! 55 msec! 50 msec! Movshon et al, 2005!

P-cells! Color Opponency! ON-center! +! +! -! -! +! -! +! +!! -! OFF-center! -! +! -! +! +! -! -! +! M-cells! ON-center! +! -! + = stimulus on! - = stimulus off! OFF-center! -! +! +!

P M

M + L cones! S-Cone! OFF! ON! Konio-Cell in LGN!! ON-OFF! Ganglion! Cell!

Effects of tracer injections in V1! Layer I! CO blobs of! Layers II-III! Tracer injection in V1!

CO blobs! Tangential section through the upper layers of V1 stained for the mitochondrial enzyme CO. CO level -----> Long-term neural activity!

P rop e r t y P C e ll s M ce lls S pec tr a l s e l ec ti v it y Ye s No L um i n a nce c ont r a st ga i n L ower H i ghe r R e cep ti ve fi e l d s i ze Sm a ll e r L arg e r S pa ti a l r e s o l ut i on H i ghe r L ower R e t i n a l i npu t s M i dg e t ce ll P a r aso l ce ll Condu c t i on ve l oc it y L ower H i ghe r C e l l s i z e Sm a ll e r L arg e r R e spons e t o l i gh t s t ep s T oni c P has ic K cells Some (Blue-ON) High Larger Low Bistratifid Low Large/Varied Phasic L in e ar it y of sp a t ia l su mm a ti on L in e ar ( X) 75 % X, 25 % Y P a tt e r n v i s i on a t s c o t opi c l eve ls No Ye s Nu m be r o f c e ll s 1. 2 0. 15 T e m pora l r eso l u ti on L ower H i ghe r Con tr a s t s e ns iti v it y L ower H i ghe r L GN l a m i n a e 3, 4,5, 6 1, 2 X-like No? 0.1 Low Low? Intercalated

Functions of the LGN -Theories & Findings! 1. Simple relay of retinal signals! (traditional view)!! 2. Sorting of retinal information!! 3. Modulation or gating of retinal! signals (Kaplan & Shapley, 1996;! McCormick et al, 1997; Andolina et al,! 2007)!! Retinal signals! Cortical Feedback signals! Modulatory signals from! Reticular formation! Feed-forward & Feedback! Inhibitory signals!

Glutamate! Reticular formation! LGN! Perigeniculate neurons! GABA A&B! 30% (15%) Retina! 40% (40%) Inhibitory! 30% (45%) Cortex &! Brain stem! Glutamate! Brain-stem! (Mid-brain! Reticular! Formation)! Glutamate! GABA A! Ach!

Modulation of signals from retina! 1. Gain controls (PGN)!! Poor & distorted! transmission! Good & faithful! transmission! 2. Effects of arousal! (Brain stem RF)!

Neural Basis of Visual Perception Neural Detectors & Network" Dorsal Stream of the! Extrastriate Visual Areas! Primary Visual Cortex! (Area 17, V1, Striate! Cortex)! Ventral Stream of the! Extrastriate Visual Areas! Network of Neurons! Neuron! Retina LGN Visual Cortex Several millions! Several billions!

Functional Organization! (from simple detection/discrimination to perception)! Functional specializations of visual areas in the mature visual brain:!! (1) The detection and! discrimination of simple visual features depends on the spatiotemporal properties of V1 neurons.!! (2) More complex perceptual tasks, e.g., motion perception or contour integration, require specialized extrastriate mechanisms.!! Extrastriate Visual Areas!

Cytoarchitecture V1 From Zeki, 1993 V1/V2 border

Stettler & Gilbert, Neuron, 2006" The Rockefeller University" Dendritic spines -- Sites of synaptic interactions "

Major Neurons in the Visual Cortex! Pyramidal cells! (Meynert cell)! Stallate! cells! Stallate Cells (B)! Double Bouquet Cell (C)! Chanderlier cell (E)! Bitufted cell (F)! Basket cells (D & H)! Neurogliaform cells (G)! Excitatory (80%)! Inhibitory (20%)!

Pathways required for binocular vision in primates! R! Optic chiasm! Ocular dominance columns" X! L! Amplifier Fixation plane! Optic nerve! Optic tract! Lateral geniculate nucleus (LGN)! IVc Primary visual cortex (V1)! Higher order! visual areas! e.g., V2, V3, V4, &MT!

Pyramidal cells! I! II! III! IVA!!IVB! IVCα! IVCβ! V! VI! The Striate Cortex No direct LGN input! Stallate cells! To: LGN! K! P! M! To: extrastriate! To: Superior! From: LGN! Visual areas! Colliculus! 4A" I! II! III! IVA! IVB! IVCα! IVCβ! V! VI! 4Calpha" 4C beta" P-input! M-input" P-input!

Neurophysiology of V1 Neurons! 1. Topographic organization of V1" 2. Cell classification--simple vs. complex, & hypercomplex? " 3. Orientation tuning-feed forward vs. long-range signals" 4. Direction selectivity " 5. Spatial/Temporal frequency tuning" 6. Ocular dominance" 7. Functional organization (1) Ocular dominance columns" 8. Functional organization (2) Orientation columns"

Pathways required for binocular vision in primates! R! Optic chiasm! Ocular dominance columns" X! L! Amplifier Fixation plane! Optic nerve! Optic tract! Lateral geniculate nucleus (LGN)! IVc Primary visual cortex (V1)! Higher order! visual areas! e.g., V2, V3, V4, &MT!

Retinotopic Maps of Visual Cortex Calcarine Fissure!

Retinotopic Organization 2-deoxyglucose labelling method Visual Stimulus Monkey fixates the center of the spoke pattern. I.V. injection of radio-active sugar to label active cells in visual cortex lateral Flat mount of operculum processed for autoradiography 1st ring = 1 deg 2nd ring = 2.3 deg 3rd ring = 5 deg foveal representation medial V1 operculum! From Tootell et al., 1984

Logothetis, 2000! Operculum!

Ma (mm 2 /deg 2 )! Cortical Magnification! Eccentricity (degrees)!

Cortical magnification or reflection of retinal density?!

1. Topographic organization of V1" 2. Cell classification--simple vs. complex, & hypercomplex? " 3. Orientation tuning-feed forward vs. long-range signals" 4. Direction selectivity " 5. Spatial frequency tuning" 6. Ocular dominance" 7. Functional organization (1) Ocular dominance columns" 8. Functional organization (2) Orientation columns"

DeAngelis & Anzai, 2003!

Hubel and Wiesel s Simple Cell Model The classic Hierarchical Model Simple Cell LGN RFs Simple cells produced by converging inputs from a row of LGN that have the same RF polarity. From Hubel & Wiesel, 1962

DeAngelis & Anzai, 2003!

Hubel and Wiesel s Complex Cell Model The classic Hierarchical Model Simple RFs Complex Cell From Hubel & Wiesel, 1962 Complex cells are produced by the converging inputs from spatially offset simple cells that have the same preferred orientation.