Visual Neurophysiology: Primary Visual Cortex Lecture 8 PDF

Visual Neurophysiology OPTOG 1675 Primary Visual Cortex Lecture 8  Learning Objectives The basic anatomy and histology of the primary visual cortex (V1) The circuitry of V1 – Input from LGN – Internal connections – Output to other cortical areas The bas...

Visual Neurophysiology OPTOG 1675 Primary Visual Cortex Lecture 8  Learning Objectives The basic anatomy and histology of the primary visual cortex (V1) The circuitry of V1 – Input from LGN – Internal connections – Output to other cortical areas The basic organizational and functional features of V1 – Receptive fields – Retinotopic mapping – Ocular dominance columns – Orientation columns – Blobs  Outline for Primary Visual Cortex 1. Anatomy and Histology 2. Circuitry of V1 (inputs from LGN) 3. Receptive fields of V1 neurons 4. Retinotopic mapping 5. Ocular dominance columns 6. Orientation columns 7. Blobs 8. Outputs to other visual cortex areas 1. Anatomy and Histology  Histological Techniques Nissl – Stains the rough endoplasmic reticulum – Visualizes cell bodies Golgi – Fills almost entire cell – Only stains occasional cells – Visualizes structure of isolated cells Weigert – Stains myelin – Visualizes axonal processes Cytochrome oxidase (CO) – CO is a mitochondrial enzyme involved in ATP production – Active cells express more of this enzyme – Visualizes cell bodies – Cells that have higher metabolism stain more darkly  Cortical Neuron Types Pyramidal neurons (E) – Pyramid-shaped cell body – Long apical dendrite that extends through several layers of cortex – Collects and integrates information from many layers Granular neurons (B) – Round cell body, much smaller than pyramidal neuron (they are drawn to the same scale) – Short, locally extending dendrites – Performs local integration and dissemination of information From: Nolte 2009  Structural Themes in Cortex Laminar – The cerebral neocortex (which we will call the cortex) is arranged in 6 layers Columnar – Cells are oriented perpendicularly to the layers – Cells generally interact with those above and below them – Pyramidal cells can span many layers Composed of regularly spaced units – modular – Modules can be distinguished functionally and anatomically From: Nolte 2009  Organization of Cortex The basic six-layered architecture is maintained throughout the cortex The detailed organization (e.g., thickness of layers, sizes of cells, connectivity, inputs) varies across the cortex, depending on function German neurologist, Korbinian Brodmann, subdivided the cortex into approximately 50 areas, based on histology in 1909 Brodmann’s area 17 is the primary visual cortex From: Brodmann, 1909  Visual Cortex Defined as the part of the cortex that processes what we visually perceive Includes the occipital lobe, and parts of the temporal and parietal lobes Many areas are involved within these lobes, and some contain From: Logothetis, 1999 their own representation of the entire visual field Primary visual cortex (V1) is the area that receives direct input from the LGN From: https://human-memory.net/visual-cortex/  Histology of Primary Visual Cortex In primary visual cortex, layer 4 is thicker than it is in adjacent areas and can be divided into sublayers Layer 4C has a higher density of cells than adjacent areas The dense Nissl (cell body) staining of layer 4C 2011 Webvision gives this part of cortex the name “striate”  Functional Histology of V1 Cytochrome oxidase (CO) staining demonstrates a functional columnar organization – Most obvious in layers 2 α and 3 β Organization is repeated across visual cortex Darkly stained areas are 2011 Webvision called “blobs” (arrows)  2. Circuitry of V1 – Layer 4 Receives inputs from the LGN Subdivided into layers 4A, 4B, 4Cα, 4Cβ – Sub-layer 4C Highest density of stellate neurons (striate appearance) 4Cα receives input from M cells of LGN and projects primarily to 4B 4Cβ receives input from P cells of LGN α and projects to layers 4A, 2, 3 β – Sub-layer 4B Large pyramidal cells (output) Receives input from 4Cα Projects to other visual cortex areas (MT, V2) – Sub-layer 4A Small granular neurons (local circuit) Receives input from 4Cβ 2011 Webvision Projects to layers 2 and 3  Other Layers Layer 1 – Intracortical interactions – Few neurons – Dense network of synapses between apical dendrites of pyramidal cells and various inputs Layer 2/3 – Processing and output – Receives input from the Konio (direct) and Parvo (indirect) sublayers of the LGN – Contains prominent “blob” and “interblob” (CO staining) areas – Projects to “higher” visual cortex sites Layer 5 – Processing and output – Receives input from nearly all other layers – Provides output to superior colliculus Layer 6 – Processing and feedback – Receives parvocellular input from LGN – Forms a neural loop with the LGN – reciprocal connections  Diagrammatic M and P LGN inputs project to layer 4 Other Areas of Visual Cortex K and P LGN inputs project to 1 Blobs 2 – K projects directly 3 – P projects via 4Cβ P LGN inputs also project to 6 Inter-blob areas M LGN inputs project to other Parasol Small Midget cortical areas via 4B Bistratified M and P LGN inputs have feedback to LGN through layer From: Tovee 2008 6  3. Receptive Fields of V1 Neurons Input to receptive fields – Monocular in layers 4Cα and 4Cβ – Bi-ocular, with varying degrees of preference for one eye in other layers – Binocular – integrate input from both eyes to produce new information Types of receptive fields – Center-surround – Simple – Complex – End-limited – Color opponent – Disparity sensitive (binocular)  Center-Surround Exclusively found in layer 4C – This sublayer receives direct input from the LGN Retains basic form of retinal and LGN receptive fields Produced by lateral inhibition in V1 Feedforward connections to V1 V1 horizontal connections Refined and expanded by Feedback connections to V1 higher order feedback From: Angelucci and Bressloff, 2006  Cortical Center Surround +D+ DD∂ DD∂ +++++ V1 Cell With Sharpened V1 Cell With More Retinal Ganglion Cell Center-surround Antagonism Localized Center Region  Simple Cell Primarily found in layers 4 and 6 – These layers receive direct input from LGN Best response is to a bar of a specific orientation and width – May respond best to light or dark bar – Excitatory area may be next to single inhibitory area or flanked by two inhibitory areas Combination of aligned From: Tovee 2008 center-surround fields  Complex Cell Found in all layers, but most abundant in layers 2, 3, and 5 – Do not receive direct input from LGN, but from other layers of V1 Respond best to moving edges + - or slits of a fixed width and - precise orientation + + – Respond anywhere in receptive field – May be direction selective Combination of simple fields – Interneurons may be present in direction selective fields From: Tovee 2008  Waterfall Aftereffect Motion selective complex cells have spontaneous rates of activity in absence of any stimulus (true of all neurons) We don’t detect this activity as motion because cells of opposite orientations have the same spontaneous rate of firing, so they cancel each other out By watching continuous motion in one direction (e.g., a waterfall), cells responding to that direction become less active (habituate) When the motion is stopped, their spontaneous activity no longer cancels that of cell with responding to motion in the opposite direction We now experience the spontaneous activity of the cells with oppositely oriented motion selectivity as real motion https://www.youtube.com/watch?v=oNhcpOIQCNs  End Limited Cell May have a Simple or Complex type of field Not only sensitive to width and orientation (and movement for Complex types of fields), but also to length of stimulus Stimulus that exceeds optimal length causes inhibition Can rationalize as end-to-end circuits of simple and complex cells, but connectivity is probably more complex  Color Opponent Cells Red/green or blue/yellow color opponent neurons are located within blobs (layers 2-3) – Double color opponent – Only one type (i.e., red/green or blue/yellow) within each blob – More red/green blobs than blue/yellow There are bridges of color-selective neurons between blobs Selectivity of bridging neurons can be the same as the blobs, if they connect like blobs, or spectrally mixed if they connect dissimilar blobs  Disparity Sensitive Cell Receive input from both eyes Have high rates of activity when a stimulus has a positive disparity (A) between the two eyes Have low rates of activity when a stimulus has zero or negative (B) disparity Involved in depth perception for near objects Kalloniatus and Luu, Webvision 2011  4. Retinotopic Mapping S F From Macaque, CO 1 2 staining 3 Same type of mapping as From: Tootell et at. 1988 LGN Preservation of relative relationships between points in space – Reversed L to R and S to I Greater representation of macular area From: Frisby 1979 5. Ocular Dominance Columns Segregation of Input from Each Eye In layer 4C, cells responding to each eye occur in bands alternating with bands of cells driven by the other eye In other layers, cells respond human more strongly to one eye, particularly in the center of the band, but may also respond to the fellow eye No alternation takes place at “blindspot” or in far temporal field From: Rodieck 1998  Ocular Dominance Columns The bands preferentially responding to each eye extend through the cortex forming Ocular Dominance Columns Each column is composed of repeating, functionally equivalent modules Each module contains one blob From: Tovee 2008  Development of Ocular Dominance Columns In experiment at left, one eye of a primate was deprived of visual input (right panel) Ocular dominance columns for the deprived eye fail to develop properly (dark stripes) Visual deprivation has this effect only during a critical period in early life – The first ~7 years of life in humans Reduced vision in deprived eye From: Hubel, Levay and Weisel, 1977 that can’t be restored by simply by restoring proper input  Critical Period Demonstrates that the visual cortex is not “hard wired” Connections form without input, but are refined by visual experience – E.g., “competition” between overlapping inputs Appropriate visual input during critical period is necessary for developing proper connections – Amblyopia is one result of the visual cortex not receiving appropriate input during the critical period Hard to modify system once critical period has passed Critical period can be reactivated in animal models  Amblyopia Reduced acuity due to problem in eye or visual pathway that hinders normal development – Not present at birth – Critical period for development – Not correctable by refractive means Usually unilateral - caused by unequal input between eyes – Can be due to refractive problem, tropia, or visual deprivation (e.g., cataract) – Can be caused by toxins or malnutrition – Can be hysterical or idiopathic Causes changes in the numbers, types, and connections of neurons throughout the visual pathway beyond the retina  Symptoms of Amblyopia Atypical reduction in VA – May read a few letters on even small lines Shallow curve of letters read vs. line size – “Crowding” effect More than reduced VA – Decreased and variable accommodation – Abnormal eye movements – Poor spatial judgments – Decreased depth perception – Decreased contrast sensitivity – Possible pupil problems  Treatment of Amblyopia Most effective during the critical period Correction of refractive or ocular problems Penalization of non-amblyopic eye – Patching to force use of amblyopic eye – Cycloplegia to blur fellow eye at near Success of penalization strategies supports idea that normal development involves competition between overlapping inputs – Penalization of stronger input levels the playing field Penalizing the stronger eye too much can cause it to become amblyopic 6. Orientation Columns  Original Model Orientation selectivity of simple and complex cells varies in an orderly fashion across the cortex Cell responding to the same orientation form columns perpendicular to the ocular dominance columns A complete cycle of orientations for both eyes is From: Gazzaniga et al., 1998 called a “hypercolumn”  Current Model Colors are orientation, light and dark are ocular dominance, white circles are blobs (CO staining) Orientation changes in pinwheel fashion around center of hypercolumn – Appear to change in linear fashion along edge of column, which gave rise to classical model Spatial frequency preference varies from low to high moving away from center of column From: Tovee 2008 and P.C. Foster, Wikipedia  7. Blobs Contain neurons with color opponent or double color opponent receptive fields – Not orientation selective Surrounded by areas (inter-blob) that are not color selective – These areas are orientation selective – e.g., simple and complex receptive fields Implies that the features from color contrast, such as borders are extracted in inter-blob areas, and “fill” color is supplied by blobs 8. Output to Other Visual Cortex Areas V2 – Input from M and P “pathways” – Thin stripe receives color information and inter-stripe receives form information – Thick stripe receives input from layer 4B of V1 (M cells) – Overlapping retinotopic maps for orientation, color, and disparity V4 – Input from V2 – Involved in color constancy V3 and MT (V5) – Input from M pathway (through thick stripe and directly from 4Cα) – Dynamic form (V3), motion and stereoscopic depth (V5) From: Tovee 2008  Effects of Damage to V1 Cortical blindness – Due to extensive damage to V1 – Leads to loss of visual input to other areas of visual cortex and disrupts recurrent visual circuits – Patient has no conscious perception of light, may have “Blind sight” – Scotoma – Partial lesions can lead to a scotoma whose location depends on the area damaged Causes – In adults, usually stroke or TBI – In children, perinatal hypoxia and ischemia – largest cause of childhood visual impairment in developed countries Management – Vision may recover in children – Some visual recovery may be possible in adults by providing stimuli in the “blind” area QUIZ 1. Which of the following stains would best show all of the neurons in an area of cortex? A. Cytochrome oxidase B. Golgi C. Nissl D. Wiegert 1. Which of the following stains would best show all of the neurons in an area of cortex? A. Cytochrome oxidase B. Golgi C. Nissl D. Wiegert 2. Information from which TWO of the following types of LGN cells are LEAST processed in the primary visual cortex? A. Interneuronal B. Koniocellular C. Magnocellular D. Parvocellular 2. Information from which TWO of the following types of LGN cells are LEAST processed in the primary visual cortex? A. Interneuronal B. Koniocellular C. Magnocellular D. Parvocellular 3. Which of the following is NOT an important stimulus characteristic for a complex cell in V1? A. Length B. Movement C. Orientation D. Width 3. Which of the following is NOT an important stimulus characteristic for a complex cell in V1? A. Length B. Movement C. Orientation D. Width 4. Which of the following best describes retinotopic mapping in V1? Everything that appears on the retina appears in the same way in V1 Everything that appears on the retina appears upside down and backwards in V1 Important areas of the retina occupy a larger area in V1 The relative relationships between points in space in each structure are preserved 4. Which of the following best describes retinotopic mapping in V1? Everything that appears on the retina appears in the same way in V1 Everything that appears on the retina appears upside down and backwards in V1 Important areas of the retina occupy a larger area in V1 The relative relationships between points in space in each structure are preserved 5. Which of the following is NOT an important stimulus characteristic for an ocular dominance column in V1? A. Orientation B. Color C. Spatial frequency D. Ambient illumination 5. Which of the following is NOT an important stimulus characteristic for an ocular dominance column in V1? A. Orientation B. Color C. Spatial frequency D. Ambient Illumination