The Eye. Cognitive Neuroscience of Language. Today s goals. 5 From eye to brain. Today s reading

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1 Cognitive Neuroscience of Language 5 From eye to brain Today s goals Look at the pathways that conduct the visual information from the eye to the visual cortex Marielle Lange Today s reading Leff, A.P., Crewes, G.T., Scott, S.K., Kennard, C., Wise, R.J.S. (2001). The functional anatomy of single-word reading in patients with hemianopic and pure alexia. Brain, 124, The Eye Available at the ITO. Visual Perception of Print. Source to identify. 1

2 Structures of the Human Eye Light: The physical stimulus Light is a form of radiant energy. This energy is radiated in waves that have a characteristic wavelength. Psychology: Second Canadian Edition by Santrock and Mitterer. Retina, as seen through pupil The retina is a tissue that is an extension of the brain. Photoreceptors On-line book, Anatomy and Physiology, Martini. 2

3 Structure of the retina Characteristics of Rods and Cones Three types of cones (reacting to blue, red, green ranges of wavelength) million per eye 8 million per eye 5.ppt Psychology: Second Canadian Edition by Santrock and Mitterer. Their stimulation in various combinations provides the perception of different colours. Distribution of rods and cones Ganglion cells on/off surround 3

4 Ganglion Cell, receptive fields In the visual pathway, the message must cross two synapses before it heads toward the brain. In other sensory pathways at most one synapse lies between a receptor and a sensory neuron. The extra synapse adds to the synaptic delay, but it provides an opportunity for the processing and integration of visual information before it leaves the retina. Centre-surround antagonism On-center ganglion cells: excited when light falls in the center of their receptive field. Inhibited when light falls on the surround. (Only a weak response is evoked by a uniform field of light. ) On-line book, Anatomy and Physiology, Martini. ni_fundanaphy_5/bb/obj/14/ch14/html/ch14_4_1.html ng/psyc261/vision/vision.ppt Hubel & Wiesel Retinal Output (Ganglion) Cells 10% are magnocells (large) : fast responses - for timing visual events, visual motion, controlling eye movements, coarse features (low spatial frequencies ), high contrast sensitivity 80% are parvocells (small) : for colour, high acuity, fine detail (high spatial frequencies), low contrast sensitivity Visual magnocellular pathways control eye movements, and are particularly important for maintaining steady fixation 4

5 Optic disc & start of optic nerve Optic disk, optic nerve, optic chiasm, optic tract Optic nerves, Optic chiasm, optic tract Lateral Geniculate Nucleus (LGN) (pupillary reflex, orient eyes towards objects) Lateral Geniculate Nucleus (LGN) courses/np2000/disordersattention-awarenesskok/disorders-attentionawareness-kok.ppt 5

6 Lateral geniculate nucleus (LGN) Axonal pathway to the LGN The Thalamus is mostly a relay center. The Lateral Geniculate Nucleus (LGN) deals with visual information, sending some to reflex centers in the brain stem, other to the the visual cortex. Functions: Enhance information about contrast, organizes information, receives feedback from other areas. ption-and-attention/perception-and-attention.ppt Left eye Right eye The LGN has six layers each of which gets independent input from either the left or the right eye but not both. Magnocellular and parvocellular projections The magno cells (large) are part of the m-pathway -- primarily responsible for processing information about motion and flicker. The parvo cells (small) are part of the p-pathway -- primarily responsible for processing information about form, colour, and texture. In fovea, monitor cones, with 1:1 connections Can receive info from as many as 1000 rods - coarse coding Small ganglion cells Colour sensitive Small Receptive Fields High resolution` Slow, sustained resp. More sensitive at high contrast Large ganglion cells Colour insensitive Large Receptive Fields Low resolution Fast, transient response. More sensitive at low contrast 6

7 Striate cortex (V1, Area 17) Visual Cortex (or striate cortex) Primary and association areas Retina, geniculate-striate system 1,000,000 axons! Retinotopic maps Axons carrying signals from neighbouring parts of the retina are next to one another within the optic nerve. 7

8 V1: Topographic representation What Do Images Look Like in Cortex? Bottom image: Slice through area V1. The cells that have stained dark are those that were responding while the animal viewed the stimulus shown above. Preservation of spatial structure topographic representation. Important because of the vast numbers of cells (~100,000,000 in each hemisphere's V1). Note also how the cortex expands the representation of the fovea relative to the periphery (cortical magnification). Original image Retinal image Cortical image LGN to V1 connections Cortical organization in V1: Layered and Columnar Organization Information travels from the LGN primarily to layer 4 of V1 but not all of the information goes to the same part of layer 4. Magnocellular layers of the LGN project to an upper subdivision of layer 4 in V1 and the he parvocellular layers of the LGN to a lower subdivision of this layer. Separation of information (e.g., motion vs. colour) so that it can be processed separately. 8

9 1: few cells ( molecular layer) 2: -> V2 5: motor cortext, 6: feedback cx Columnar organization The surface of the cortex is divided into functionally distinct regions or microcolumns, each about 30 µm in diameter. Neurons within a column will tend to increase or decrease their firing rates together. Each column can be seen as a computational unit that codes a specific information (orientation, direction, colour). Here: orientation columns Columnar organization (Pinwheel demo on course s website) Cells in V1 do not stick to the same sort of circular, center/surround organization as the ganglion cells Selectivity They have more complex organizations that allow for new sorts of selectivity that does not occur before we reach the visual cortex. Hubel and Wiesel s hierarchical model of visual cortical processing Simple cells respond to lines in a particular orientation Complex cells respond to lines in a particular orientation, which move in a certain direction vision/vision.ppt 9

10 Orientation selectivity in V1 V1: Position selectivity Orientation selectivity The figure shows receptive field maps for several different sorts of simple cells. The + symbols show positions where the cell responds well to a light spot, and the - symbols show positions where the cells responds well to a dark spot. Position selectivity in simple cells in V1. This cell responds best when the stimulus of the appropriate orientation and size is in one particular position. V1: Direction selectivity V1: Colour selectivity This cell responds best when the stimulus moves up and to the left. This response is not based on solely the orientation of the moving bar, because when the same bar moves in the opposite direction, the cell responds at a significantly reduced level. This cell responds best when a blue bar of the appropriate orientation, size, and position is presented on a yellow background. Note that this cell is not responding to just the combination of blue and yellow, because when we reverse the colours of the bar and the background, the cell's response is reduced significantly. 10

11 Beyond V1, Functional specialization: Different areas specialized in different types of processing of the visual information Beyond V1: what and where patwhays 25 ant. 6 post PET study in normals (Zeki et al., 1991) The What (ventral) The Where (dorsal) pathway: deficits in pathway: deficits in object discrimination landmark task 11

12 Two visual streams hypothesis Connectivity in monkey cortex (Felleman and Van Essen 1991) Milner and Goodale (1993, 1998) Number of cortical structures arranged in a quasihierarchical structure Reentrant signaling Influence of context on letter perception 12

13 Occulomotor nerves Eye movements when reading On-line book, Anatomy and Physiology, Martini. Patterns of eye movements Viewer perspective 13

14 Foveal view A visual account of reading impairment A visual account of reading impairment Impairment of the magnocellular pathways Poor readers have been shown to have 30% smaller magnocellular neurons. Stein, Talcott, & Witton (2001) This can lead to unsteady eye fixation and visual confusion of letter order. This leads to poor memory of the visual form of words impeding orthographic skills. Conclusions Retinotopic maps: what is close to each other in the stimulus is coded close to each other in the brain. This remains true all along the way from photoreceptors to cortex. Selectivity in information processing at various levels of organization in the visual system (Photoreceptors, M-, P- ganglion cells, M- P- layers in the LGN, Columns in V1; Dorsal/where & ventral/what pathways beyond V1) Parallel pathways: information from each eye initially processed separately. Canonical microcircuit hypothesis: each cortical area conducts computations of the same form using similar circuitry but different inputs to each area convey different functions. In spite of differences in cytoarchitecture, the types, arrangements, and connections of cortical neurons is similar throughout cortex. 14