Chapter 3 Notes PSY 252 PDF

SPATIAL VISION From Spots to Stripes WHICH WAY DO EYE GO? Signals travel from photoreceptors through vertical pathways to the optic nerve via retinal ganglion cells Next they travel through the lateral geniculate nucleus of the thalamus (sensory waystation) Then into striate vis...

SPATIAL VISION From Spots to Stripes WHICH WAY DO EYE GO? Signals travel from photoreceptors through vertical pathways to the optic nerve via retinal ganglion cells Next they travel through the lateral geniculate nucleus of the thalamus (sensory waystation) Then into striate visual cortex (primary visual cortex) in the occipital lobe FROM RETINA TO BRAIN Signals from P cells and M cells follow different pathways through the brain Parvocellular – ventral stream The WHAT pathway Magnocellular – dorsal stream The WHERE/HOW pathway  BUILDING PERCEPTION FROM SENSORY PIECES Ganglion cells respond to points of light in their receptive fields Cells in visual cortex preferentially respond to lines, edges, bars, and stripes, as inferred from multiple ganglion cell inputs Visual cortex is further subdivided into smaller regions responsible for processing orientation, width, colour, etc. The information from each of these areas is then combined in secondary visual cortical areas giving rise to a coherent view of the world  A FEW TERMS Contrast: difference in luminance between an object and its background Acuity: smallest spatial detail that can be resolved at 100% contrast Visual angle: the angle subtended by and object at the retina Grating cycle: a pair consisting of one light bar and one dark bar in a complete grating image CONTRAST Each row shows a specific frequency of visual grating For each circle in the row, report whether it shows left, right, up or blank ACUITY  ACUITY Eye doctors use distance to characterize visual acuity, as in “20/20 vision” Your distance/Normal vision distance 20/40 – you can see at 20 feet what normal vision sees at 40 feet 20/15 – you can see at 20 feet what normal vision sees at 15 feet Smallest visual angle of a cycle of grating that can be perceived The smaller the visual angle at which you can identify a cycle of a grating, the better your vision VISON GRATING TEST CYCLE EXAMPLE  ACUITY Herman Snellen invented this method for designating visual acuity in 1862. Notice that the strokes on the E form a small grating pattern Each stroke is 1/5 the size of the complete letter This ratio stays consistent as the size decreases  SO WHY STRIPES? Our visual system is made of cells that identify spots of light Center-surround cells are organized in columns and rows to effectively detect edges Thus one of the foundational levels or processing is to see the boundaries between objects - stripes are just repeating boundaries  CENTER-SURROUND CELLS REVISITED Center on cells positioned at a shade boundary The receptive fields of the cells in the uniform shaded areas receive the same stimulation in the excitatory centers and inhibitory surrounds The receptive field of the middle cell results in a stronger response to the center because part of the surround is over a lighter area resulting in less inhibition from the surround This results in the boundary appearing darker than it really is ACUITY AND STRIPES Resolutional acuity is the finest high-contrast detail that can be resolved Determined by the spacing of photoreceptors on the retina Sine-wave gratings show how this works:  ACUITY AND STRIPES The visual system “sees” spots across the grating – with each circle representing a cell’s receptive field When the receptive fields are smaller the blackest and whitest areas of the grating, we perceive the striped pattern When the receptive fields measure all or most of a cycle we perceive a uniform grey field  RECEPTIVE FIELD SIZES Cones in the fovea are most densely packed photoreceptors in the retina, meaning receptive fields for single cells are smaller Rods and cones in the periphery are less densely packed resulting in larger receptive fields – but distribution is not uniform across the periphery Horizontal and vertical distributions are not the same More density across the horizontal axis Less density across the vertical axis Vertical meridian asymmetry - more density at any given point below the midline than the homotopic region above the midline  BACK TO STRIPES AGAIN Spatial frequency – the number of grating cycles per degree of visual angle Schade studied the relationship between spatial frequency and contrast SPATIAL FREQUENCY CONTRAST SENSITIVITY FUNCTION CONTRAST SENSITIVITY FUNCTION CONTRAST SENSITIVITY FUNCTION  CONTRAST SENSITIVITY Individual CSF curves vary Myopia reduces contrast sensitivity in severe cases Contrast sensitivity decreases with age The level of ambient light, the adaptation level of your eyes, and other factors change the curve SO, WHY SINE WAVE GRATINGS? We know stripes are just boundaries and the visual system looks for boundaries Sine wave gratings rarely appear in nature So why is our visual system tuned to process these gratings? SO, WHY SINE WAVE GRATINGS? Patterns of stripes with fuzzy boundaries are quite common. The edge of any object produces a single stripe, often blurred by a shadow, in the retinal image The visual system breaks down images into a vast number of components; each is a sine wave grating with a particular spatial frequency This is called Fourier analysis, which is also how our perceptual systems deal with sound waves  Horizontal lines Vertical lines Light bands Dark bands Edges Each one of these instances creates its own sine wave grating Combined, they create a waveform pattern equally as complex as speech or music for the brain to break down and decipher  On-center ganglion cell – different cells have different “tuning” based on the Tuning a Ganglion Cell… size of their receptive field When the grating frequency is too low, the center and surround will both receive light signals, and the surround will inhibit the center signal When the grating frequency is too high, the both light and dark bands fall in the center and wash out the response The grating frequency is just right, the center is activated by the bright band, and the inhibitory surround is not activated, resulting in vigorous firing IT’S JUST A PHASE…  IF YOU HAVE PHOTOSENSITIVE EPILEPSY OR SUFFER FROM LIGHT- TRIGGERED MIGRAINES, YOU MAY WANT TO LOOK AWAY….  FEELING A LITTLE FUNKY? Stripes can cause photosensitive discomfort Worse in people with photosensitive epilepsy and migraineurs – already experience higher levels of cell firing What causes it? Gamma oscillations! Stripes cause ganglion cells to synchronize firing at 30-90 Hz, or 30-90 times per second Can cause nausea, dizziness, headaches, trigger seizures, and cause visual illusions FEELING A LITTLE FUNKY? Gamma oscillations create the visual discomfort by coordinating the firing of cells that connect to visual processing areas The 30-90Hz firing rate is one associated with connectivity – when cells synchronize their firing at this frequency in large groups it creates greater spreading of activation of cells than at any other frequency More cells firing = more sensations THE LATERAL GENICULATE NUCLEUS OF THE THALAMUS  THE LGN IS LIKE AN ONION… IT HAS LAYERS Geniculate – means to bend Divided into two main structures: Magnocellular layers – input from M ganglion cells Specialized for motion Parvocellular layers – input from P ganglion cells Specialized for fine detail Koniocellular layers: the spaces between the parvo- and magno- cellular layers  The visual field is divided in half Each half of the visual field is sensed by one half of the retina in each eye The Left LGN receives information from the left visual field from each retina The right LGN received information from the right visual field of each retina THE LGN IS LIKE AND ONION… Each layer receives information from one eye Layers 1,3, and 6 receive information from the contralateral retina Layers 2,4 and 5 receive information from the ipsilateral retina The layers of the LGN create a complete map of the visual field The map is created using circular receptive fields similar to the ganglion cells receptive fields STRIATE VISUAL CORTEX Also known as primary visual cortex, area 17, or V1 A major transformation of visual information takes place in striate cortex Circular receptive fields found in retina and LGN are replaced with elongated “stripe” receptive fields in cortex It has about 200 million cells – 100x more cells than the LGN STRIATE VISUAL CORTEX consists of six layers (similar to LGN) Fibers from the LGN project mainly (but not exclusively) to layer 4C Magnocellular axons to upper 4C Parvocellular axon to lower 4C  CORTICAL MAPPING AND MAGNIFICATION Shows topography – the regions in the visual field correspond to similar regions in the striate cortex Shows magnification – each of the topographical regions is not equally represented in terms of space – the fovea is given much more processing space in relation to the periphery  TOPOGRAPHY OF VISUAL CORTEX Much of our early information about cortical layout came from animals and humans with cortical lesions Now we map human cortex using MRI and fMRI MRI reveals the structure of the brain. fMRI reveals brain activity through blood oxygen level–dependent (BOLD) signals REMEMBER RECEPTIVE FIELDS? Ganglion cells respond to points or spots of light ON-center cells most strongly respond if light hits the positive center and does not hit the inhibitory surround OFF-center cells show the opposite pattern  HUBEL AND WIESEL Mapping receptive fields in the cat visual cortex Surprisingly, found that cortical cells did not respond at all to spots of light As they were changing the stimulus slide in the visual projector, the neuron they were recording from started to fire! HUBEL AND WIESEL They realized that the slide edge was what triggered the response After trying different shaped stimuli, they discovered that the cat cortex responded to stripes or edges, not spots of light  ORIENTATION SELECTIVITY Cells in striate cortex respond best to bars of light rather than to spots of light Some cells prefer bars of light, some prefer bars of dark (simple cells) Some cells respond to both bars of light and dark (complex cells) Neurons in striate cortex respond most to bars of certain orientations Response rate falls off with angular difference of bar from preferred orientation  How are the circular receptive fields in the LGN transformed into the elongated receptive fields in striate cortex? A cortical neuron that responds to oriented bars of light might receive input from several retinal ganglion cells If you string several retinal ganglion cells together, they can form an oriented bar. A cell that is tuned to any orientation you want could be created in cortex by connecting it up with the appropriate retinal ganglion cells.  RECEPTIVE FIELD TUNING Cortical cells respond to gratings They have narrower spatial frequency tuning than retinal ganglion cells Specialized cells for: Moving lines Bars Edges Specific motion directions Striate cortical cells respond to specific stimulus characteristics filter for the portion of the image that excites the cell  OCCULAR DOMINANCE IN V1 Each LGN cell responds to one eye or the other, never to both In V1 inputs from both eyes are combined, so each striate cortex cell can respond to input from both eyes Cortical neurons tend to have a preferred eye - respond more vigorously to input from one eye or the other  SIMPLE AND COMPLEX CELLS Simple Cells Complex Cells Receptive fields with clearly defined Receptive fields do not have clearly excitatory and inhibitory regions defined excitatory and inhibitory regions + - + ± ± ± ± ± ± + - + ± ± ± ± ± ± + - + ± ± ± ± ± ± + - + ± ± ± ± ± ± + - + ± ± ± ± ± ± SIMPLE AND COMPLEX CELLS Simple cell may only respond to a stripe if it is positioned within the center of its receptive field Complex cells can respond to a stripe no matter where it is positioned in the receptive field as long as the orientation, spatial frequency, and ocular dominance match  Receptive fields of both simple and complex cells represent pooling of several subunits of lower level cells tuned to specific frequencies and orientations Simple cells pool in a way that lines up the excitatory and inhibitory areas of receptive fields in discrete segments Complex cells pool in a way that makes them insensitive to the positioning of the stimulus in the receptive field END STOPPING Not only are cells sensitive to frequency and orientation, some cells respond preferentially to length of gratings Firing rates are slower when a stripe length is shorter or longer than the receptive field Firing rates are most vigorous when the length is just right  COLUMNS Column: A vertical arrangement of neurons. all neurons have the same orientation tuning and similar receptive fields running the full depth of striate cortex Angle of orientation changes by approximately 10 degrees every.05mm moved horizontally All orientations were encountered within a distance of about 0.5 mm Columns can reflect orientation or ocular dominance  HYPERCOLUMNS AND BLOBS Hypercolumn: a 1mm block of striate cortex at least two sets of columns containing cells responding to every possible orientation (0–180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye CO Blobs: regular arrays of blob- shaped columns spaced about.5mm apart in striate cortex  Each column has a particular orientation preference, which is indicated on the top of each column (and color-coded). Adjacent groups of columns have a particular ocular dominance—a preference for input from one eye or the other—as indicated at the bottom of the figure. Blobs are indicated as cubes embedded in the hypercolumn  WHAT HAPPENS WHEN YOU GET TOO MUCH STIMULATION? Adaptation: A reduction in response caused by prior or continuing stimulation. An important method for deactivating groups of neurons without surgery If presented with a stimulus for an extended period of time, the brain adapts to it and stops responding This fact can be exploited to selectively “knock out” groups of neurons for a short period + +  TILT AFTEREFFECT The perceptual illusion of tilt, produced by adapting to a pattern of a given orientation Supports the idea that the human visual system contains individual neurons selective for different orientations  ADAPTATION TO SPATIAL FREQUENCY Selective adaptation can also be observed for spatial frequencies Provides evidence of spatial frequency tuning of cells in striate cortex  SPATIAL FREQUENCY ANALYZERS Human vision is coded in spatial-frequency channels Spatial-frequency channel: A pattern analyzer, implemented by an ensemble of cortical neurons, in which each set of neurons is tuned to a limited range of spatial frequencies Why would the visual system use spatial-frequency filters to analyze images? Different spatial frequencies provide different information HIGH-FREQUENCY MASKS High spatial frequencies can mask low spatial frequencies Without low frequency information, the broad outlines of the face and features are hard to read You can overcome the effect of the mask by squinting or blurring your vision  TUNE IN NEXT TIME… Perception and recognition of objects (and all the wonderful ways it can go wrong….)