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Chapter 3

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PSYB51 Chapter 3: Spatial Vision: From Spots to Stripes (Jan 24 , 2014)h • Iris regulates amount of light entering eyeball • Cornea, lens, humors focus rays so that clear image is formed on retina • Camera take pictures. Visual systems see • Postreceptor layers of retina translate raw light captured by photoreceptors into patterns of spots surrounded by darkness or vice versa, detected by ganglion cells  Retinal translation helps us perceive pattern of light and dark areas in visual field, regardless of overall light level • Neurons in cerebral cortex prefer lines, edges and stripes  This portion of visual cortex organized into ting computers responsible for det orientation, width, colour and other characteristics of stripes in 1 small portion of visual field  Visual system codes images in terms of oriented stripes • Contrast diff in luminance bet object and background, or bet lighter and darker parts of same object • Acuity smallest spatial detail that can be resolved  Fig 3.2 p. 57  eye doctors specify acuity in terms like 20/20  vision scientists talk about the smallest visual angle of a cycle of grating that we can perceive  Fig. 3.3 p. 57  cycle  one’s repetition of a black and white stripe (both gratings in Fig 3.2 have 25 cycles) • a pair consisting of one dark bar and one bright bar • visual angle  angle formed by lines going from top- bottom (or left- right, dep on orientation of stripes) of cycle on the page, through the center of lens onto retina  the angle subtended by an object at the retina • resolution acuity rep one of the fundamental limits of spatial vision  it is the finest high-contrast detail that can be resolved  Fig. 3.4 p 58 o the limit det by spacing of photoreceptors in retina  sine wave gratings  grating w. sinusoidal luminance profile • the light intensity varies smoothly and continuously across each cycle o visual systems “samples” the grating discretely, through the array of receptors at the back of the retina  eye more like digital camera than film camera o if receptors are spaced such that the whitest and blackest parts of the grating fall on separate cones we should be able to make out the grating o if entire cycle falls on single cone we will see nothing but a grey field o grating  collection of lines  aliasing  misperception of grating due to under sampling • high contrast sine wave gatherings can be distinguished from uniform gray field, as long as adj pairs of light/dark stripes are separated by at least 1 arc min of visual angle • Cones in fovea have center – to center separation of 0.5 minute of arc (0.008 degree)  Observed acuity limit of arc = 1 mins ( need 2 cones / cycle to perceive grating accurately) • Rods & cones in periphery packed together less tightly  many receptors converge on each ganglion cell  As a result visual acuity is much poorer in the periphery that in the fovea • Eye doctors do not describe acuity in terms of visual angles and cycles  read letters, decreasing size of letters until you made several errors o visual acuity 20/20 = good o 20/30 = need glasses o possibly 20/10 if you could read the smaller letters on the eye chart • Snellen invented this method for designating visual acuity  constructed a set of block letters for which the letter a whole was 5x as large as the strokes that formed the letter  Fig 3.5 p. 59  He defined visual acuity as follows: • (distance at which a person can just identify the letters)/(distance at which a person with “normal vision can just identify the letters) • In later adaptions of test, the viewer was positioned at constant distance of 20 feet and size of the letters, rather than the position of the viewer was altered • normal vision came to be defined as 20/20 – considered gold standard • Most healthy young adults have acuity level closer to 20/15 • What happens if the contrast of the stripes is reduced – if the light stripes are made darker and the dark stripes are made lighter? o Otto Schade  showed ppl sine wave gratings with diff spatial freq and had them adjust the contrast of the gratings until they could just be detected  Spatial frequency  refer to the # of times pattern, grating cycles, repeats in given unit of space • cycles per degree  # of pairs of dark and bright bars per degree of visual angle • Example Fig 3.6 p 59 • Would think that wider stripes (lower spatial freq) = easier it would be to distinguish light from dark stripes • Schade, Campbell& Green demonstrated that human contrast sensitivity function (CSF) is shaped like an upside-down U  Fig 3.7 p 60 o CSF  function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus.  Window of visibility – object whose freq and contrasts fall in yellow region will be visible  Red line delimits threshold bet seeing and not seeing • contrast threshold  smallest amount of contrast required to detect a pattern  ex. P60 • note that a contrast of 100% corresponds to sensitivity value of 1  60 cycles / degree corresponds to cycle width of 1 min of arc  Fig 3.8 p 60 • “pure” sine wave gratings rare in real world, patterns of stripes with more or less fuzzy boundaries are quite common  trees in a forest ex, books on a bookshelf • Edge of any object prod single stripe, often blurred by shadow, in retinal image • visual systems breaks down real-world images into # of components - each is sine wave grating with a particular spatial frequency  method of processing analogous to the way in which the auditory system deals with sound and is called “fourier analysis” • Each ganglion responds to certain stripes /gratings  Fig 3.9 p. 61 of ON ganglion cell o spatial frequency too low ganglion cell responds weakly because part of the fat, bright bar of the grating lands in the inhibitory surround = damping cell’s response o spatial frequency too high ganglion cell responds weakly because both dark and bright stripes fall within the receptive field centre, washing out the response o spatial frequency just right with bright bar filling the center and darks bars filling the surround, the cell responds vigorously  Retinal ganglion cells are “tuned” to spatial frequency • Each cells responds best to specific spatial frequency that matches its receptive-field size responding less to both higher and lower spatial frequencies st • Enroth& Robson were 1 to record responses of retinal ganglion cells to sinusoidal gratings o discovered that responses depend on the phase of the grating  relative position within the receptive field  Fig 3.1 0 p 62 • When grating has a light bar filling the receptive-field center and dark bars filling the surround this ON –center cell responds vigorously increasing its firing rate o If half the receptive-field center will be filled by a light bar and similarly for the surround  cell’s response rate does not change from its resting state when the grating is turned on • dark bar in the center and the light bars in the surround = -ve response • axons of retinal ganglion cells synapse are in the two lateral geniculate nuclei (LGNs) found in each cerebral hemisphere  lateral geniculate nuclei (LGNs)  structure in thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connection to the visual cortex o act as relay stations on the way from the retina to the cortex  Fig 3.1 p56 o the neurons in the bottom two layers are physically larger than those in the top four layers  bottom two are called magnocelular layers  either of bottom 2 neuron contain layers of lateral geniculate nucleus  receive input from M ganglion cells  response to large, fast moving objects  top 4 parvocellular layers  any of top 4 neuron containing layers of lateral geniculate nucleus  receive input from the P ganglion cells  responsible for processing details of stationary target • visual system splits input from image into diff types of info • koniocellular cells  neuron located between the magnocellular and parvoceullular layers of the lateral geniculate nucleus – known as koniocell layer  each layer involved in different aspects of processing.  one layer is specialized for relaying signals from the S-cones and may be part of a “primordial blue-yellow paths” • First, the left LGN receives projections from the left side of the retina in both eyes, and the right LGN receives projections from right side of retina  Fig 3.12 p 63  Each layer of LGN receives input from 1 or other eye  From bottom to top layers 1,4 and 6 of the right of LGN receive input from the left (contralateral) eye  referring to opp side of body/brain  2,3, and 5 get input from the right (ispislateral) eye referring to same side • Each LGN layer contains a highly organized map of a complete half of the visual field • topographical mapping  orderly mapping of world in LGN and visual cortex  ordered mapping of world onto visual nervous system  provides us with a neural basis for knowing where things are in space • LGN neurons have concentric receptive fields  Respond well to spots and gratings • Why don’t the ganglion cells axons simply travel directly back to the cerebral cortex? o many connection between other parts of the brain and the LGN o More feedback connection from visual cortex to the LGN than from the LGN to the cortex • LGN is a location where various parts of the brain can modulate input from the eyes  LGN part of thalamus  ex. P 63-64 • receiving area for LGN inputs in cerebral cortex lies below the inion  Primary visual cortex (v1), area 17, striate cortex  the area of the cerebral cortex of the brain that receives direct inputs from the lateral geniculate nucleus as well as the feedback from other brain areas and is responsible for processing visual info  Layers important property to neural structures of visual pathway  striate cortex consist of 6 major layers some of which have sublayers Fig 3.13 p 64 • fibres from the LGN project mainly- layer 4 , with magnocelluar axons coming to sublayer 4Calpha and parvocellular axons 4Cbeta • Fig 3.14 p. 65  topography and magnification examples  explanation p. 64 o Information is dramatically scaled from different parts of the visual field  Objects imaged on or near the fovea are processed by neurons in large part of the striate cortex  objects imaged in the far right or left periphery are allocated only a tiny portion of the striate cortex • cortical magnification  amount of cortical area devoted to specific region in visual field o cortical representation of the fovea is greatly magnified compared to the cortical representation of peripheral vision o distortion of visual field map • MRI useful for anatomical imaging of soft tissues. – brain  Lets us see structure of brain  Functional magnectic resonance imaging (FMRI) is a nonavesive technique for measuring and localizing brain activity  does not measure neural activity directly rather it measures changes in blood oxygen level that reflect neural activity • One important consequence of cortical magnification is that visual acuity declines in orderly fashion with eccentricity  Aubert • High resolution req great # of resources:  Dense array of photoreceptors  1-1 lines from photoreceptors =- ganglion cells  Large chunk of striate cortex  To see entire visual field in such high resolution = eyes and brain too large for head  Evolved visual system that provides high resolution in centre and lower resolution in periphery  Need to process details of object from corner of your
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