Striate cortex April 14, 2009 The primary visual cortex (V1) is one of the most studied areas in the mammalian cortex. Pubmed search for striate cortex yielded 35,911 hits Important concepts • General flow of information within the striate cortex • Receptive field characteristics is cortical neurons • Architecture of the striate cortex. • INFORMATION FLOW Information flow Resident cells • There are three basic types of neurons in the primate V1: – Spiny pyramidal cells (excitatory) – Spiny stellate cells (excitatory) – Smooth stellate (almost all are GABAergic). Single-unit recording Receptive field properties of V1 cells • Hubel & Wiesel (1959; 1968) Simple cells • Similar to most retinal ganglion cells and LGN cells – have distinct excitatory and inhibitory subregions within the receptive field. • Orientation selective Receptive field mapping: Find where the neuron is sensitive Find optimal orientation Optimal orientation Receptive field mapping _ + _ + _ + _ • Hubel and Wiesel movie Hubel and Wiesel model of Simple cells • Simple cell receptive fields could be 'built' in the cortex by collecting responses from LGN cells whose receptive fields fall along a line across the retina • In every animal that has been examined there are neurons in V1 that respond to images of bars or edges at a particular orientation. Complex cells – Complex cells are the most numerous in V1 (perhaps making up threequarters of the population). – Orientation selective – No center/surround receptive field organization – Thus, not sensitive to the polarity (bright or dark) of a stimulus. – Many complex cells are also direction-selective, in the sense that they respond only when the stimulus moves in one direction and not when it moves in the opposite direction. H & W model of complex cells • Complex cells receive integrated responses from a collection of simple cells. ANOTHER TYPE OF COMPLEX CELL • Many cortical neurons modulate their responses based on stimulation outside the central receptive field. • Initial experiments of surround modulation measured responses to high contrast bars or gratings of varying lengths. • Some cells called end-stopped cells, decrease their responses when the bars or gratings become larger than the central receptive field. Length-summing cells • Continue to their responses as bars or grating length increases. • All examined mammals, including primates, cats, (Gilbert, 1977), rodents (Girman et al., 1999), and marsupials (Oliveira et al., 2002) have cells that show stopped- and length summing behavior. Columnar organization • All mammals have some degree of columnar organization: neurons below a particular position on the cortical surface have similar functional properties, and these properties tend to change smoothly as one moves across the cortical surface. • Many mammals including have smooth, repeating maps of orientation selectivity in V1. Orientation columns • These orientation maps repeat at 550 micrometer intervals in macaque and tree shrews and 800-900 microns in cat Ocular dominance columns • Another major determinant of cell response is eye-of-origin. Most cortical cells can be driven by stimuli presented in either eye, but they generally prefer (ie. respond more to) one eye or the other - a property called 'ocular dominance'. • Some cells prefer the right eye, and others prefer the left eye. Hubel and Wiesel discovered that ocular dominance is organized in a similar way to orientation preference - dominance is unchanging vertically but alternates as one moves horizontally across the cortex • In a given band, the majority of cells receive monocular visual input that arise from the ipsi or contralateral eye BUT not both. • In the upper and lower layers of cortex, the input becomes mixed. • When V1 is stained for cytochrome oxidase, an enzyme involved in metabolism, distinct blobs show up which are most clearly seen in layers 2 and 3. The blobs are arranged in rows that line up above the centers of the ocular dominance columns in layer 4. • Livingstone and Hubel recorded from cells inside the blob regions, and found that they showed no orientation preference, but instead were concentric, and over half of them responded to color variation The Hubel and Wiesel “ice-cube” model of visual cortex: simplified diagram. (Cortical modules are not REALLY square!). No blobs in layer 4C Pinwheels •The pinwheel fashion of orientation columns was first found using optical recording •Some people say blobs are in the center of the pinwheels/orientat ion columns •Some people disagree • Columns in V1 have spatial frequency preferences. Orientation columns Ocular dominance columns Spatial frequency columns • Spatial frequency tuning may change with depth of column. Cortical maps: what are they good for? • the map of V1 is: – Uniform: making sure the cortex can respond to all possible stimulus properties at every point in visual space • We perceive the environment as uniform – We are not more sensitive to red than to green in one visual area. – Minimizes wiring length • Remember: there are a lot of neurons in the brain and most make local connections in order to conserve space. • Neurons with similar response properties are closer to each other in cortical space. • Neuroscientist once hoped that the columnar organization of V1 would help in understanding the specific rules of one module would generalize to the entire cortex. • However, many studies have failed to identify any role of columnar organization in determining functional properties of single cells (Purves et al., 1992; Horton & Adams, 2005). • NO ocular dominance columns in many mammals (mice, rabbits, squirrels, tree shrews) and vary from animal to animal in squirrel monkeys. • Similar story applies for orientation columns. • May be an aftereffect of development Important stuff! • Flow of information from LGN to V1 and within V1. • Different types of functional properties of V1 cells: simple, complex, and end-stopped cells. • Models of simple and complex cells. END Cortical Magnification Factor! Cortical magnification describes how many neurons in an area of the visual cortex are 'responsible' for processing a stimulus of a given size, as a function of visual field location. In the center of the visual field, corresponding to the fovea of the retina, a very large number of neurons process information from a small region of the visual field. If the same stimulus is seen in the periphery of the visual field (i.e. away from the center), it would be processed by a much smaller number of neurons Chiasm crosses fibers from left and right, no crossing up/down. • The map of the visual world does not exist on the surface of V1 because the brain needs to look at a copy of the image. • Rather the retinotopic map exist – to keep things organized during development – processing visual information usually involves adjacent points in an image. • In primates virtually all visual information that gets to cortex must first go through primary visual cortex. Four main synonyms: • • • • Striate cortex Brodmann’s area 17 Primary visual cortex V1 But why is it called striate cortex? • In 1782 a young medical student named Francesco Gennari described a heavy band of myelinated axons in layer 4B, the stria of Gennari (named after its discoverer), about the only landmark you can see in cortex without special stains. • This striation gave rise to its name: striate cortex Why area 17? • Brodmann (1868-1918) defined areas by cytoarchitecture. • Cytoarchitecture is the organization of the cortex as observed when a tissue is stained for neurons. Why is it called primary? • It’s essential. • First part of cortical vision • If area 17 is ablated – you are cortically blind! Why V1? • There are several cortical areas involved in vision • This is the first one. • V1 is found in all mammals from highly visual primates to animals with subcutaneous eyes such as the blind mole rat (Krubitzer & Kaas, 2005; Cooper et al., 1993) ANATOMY Cortex! • Cortex is a general term, meaning outer covering. • Cerebral cortex consists of two separate flat sheets of cells, one on each side, folded to fit inside the skull. • Gray matter consist of cell bodies and glial cells. • White matter consists of myelinated axons. Laminar organization •Need differential stains to see anything…what you see depends on the staining method •Nissl stains RNA inside of cells •CO stain metabolically active neurons. •Golgi uses potassium chromate and silver nitrate to stain the cell body and dendrites •Weigert stain for myelin (Wertige has many stains) • This operation is analogous to the operation of "edge detectors" in image processing, which process an image by spatial convolution with an edge "kernel" consisting of adjacent positive and negative values. • Most cells of the mammalian visual system from the retina to the cortex respond primarily to local stimulus contrast rather than absolute brightness. • In V1, the tuning width of orientation selective neurons remains constant as stimulus contrast is increased (Sclar & Freeman, 1982). • The influence of spatial frequency on cortical responses is similar to that of stimulus orientation. • The majority of V1 spatial frequency tuning curves have a characteristic band-pass shape. V1 LGN Heimel et al., 2005 Peak preferred temporal frequencies are similar between LGN and V1, but the highest temporal frequency that neurons can follow is on avg slightly lower in the cortex as compared to LGN.