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Chapter three

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University of Toronto Scarborough
Matthias Niemeier

Chapter Three : Spatial Vision From Stars to Stripes the iris regulate the number of light rays entering the eyeball ; and the cornea, lens, and aqueous and vitreous humors focus these rays so that a clear image is formed on the retina retinal translation help us perceive the pattern of light & dark areas in visual field regardless of the overall light level Visual Acuity -contrast is the difference in luminance between objects in the background, or between lighter and darker parts of the same object Acuity the smallest spatial detail that can be resolved ( eye doctors use specify in terms like 2020 ) BUT - scientist like to talk about the smallest visual angle of a cycle of the grating that we can perceive o Cycle for a grating, a pair consisting of one dark bar and one bright bar (simply one repetition of a black & white stripe) visual angle - the angles subtended by an object in the retina o is the angle that would be formed by lines going from the top & bottom (or left & right depending on the orientation of the stripes ) of a cycle on the page through the center of the lens & on to the retina -this resolution acuity represents one of the fundamental elements of spatial vision: it is the finest high contrast detail that can be resolved, the limit is determined by the spacing of photoreceptors in the retina this resolution acuity represents one fundamental limits of spatial vision it is the finest high- contrast detail that can be resolved o the limit is determined primarily by the spacing of photoreceptors in the retina sine wave gratings a grating with a sinusoidal luminance profile o light intensity varies smoothly & continuously across each cycle if the receptors are spaced such that whitest & blackest parts of the grating fall on separate cones, we should be able to make out the grating BUT if entire cycle falls on a single cone , we will see nothing but gray field -- or experience Aliasing (= misperception of the grating due to under sampling misperceive the cycles to be longer than they actually are) cones in the phobia have a center to center separation of about 0.5 minutes of arc [0.008], which fits nicely with the observed acuity limit of 1 minute of arc o Keep in mind we need 2 cones per cycle to be able to perceive it properly rods and cones in the periphery are packed less tightly BUT many receptors converge on each ganglion cell - thus, poorer acuity in periphery than fovea A Visit to the Eye Doctor this method of reading large letter to small letter & assigning ratio - was by Dr. Hermann Snellen (1862) constructed a set of block letters for which the letter as a whole was five times as large as the strokes that formed the letter He defined visual acuity as - the distance at which a person can just identify the lettersthe distance at which a person with normal vision can just identified letters o In later version reviewer was positioned at a constant distance of 20 feet and the size of the letters was altered normal vision would be defined as 2020 In terms of visual angle a 2020 letter is subtends at an angle of 5 arc minutes in the eye & each stroke of 2020 letter subtends an angle of 1 arc minute If you have to be at 20 feet to read a letter that someone w normal vision can read at 40 feet - 2040 vision 2020 is gold standard BUT most healthy young adults have acuity closer to 2015 Acuity for Low-Contrast Stripes Sss Why Sine Wave Gratings? pure sine wave grating may be rare in the real world - on a larger scale the visual system appears to break down real-world images into a vast number of components each of which is essentially a sine wave grating w particular spatial frequency o This method of processing is analogous to way in which auditory system deals w sound Retinal Ganglion Cells & Stripes
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