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PSYC2030 - Sensation and Perception Chapter 3

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PSYC 2220
Jennifer Steeves

Ben Kim PSYC2030 – Sensation & Perception Page 1 of 4 Spatial Vision: From starts to Stripes Chapter 3 Lecture 2 Visual Acuity What is the path of image processing from the eyeball to the brain? • Eye o Photoreceptors o Bipolar cells o Retinal ganglion cells • Lateral geniculate nucleus • Striate cortex Centre-Surround Field Receptive field: The region in space in which stimuli will activate ON-center ganglion cells • Excited by light that falls on their center and inhibited by the light that falls in their surround Off-center ganglion cells • Inhibited when light falls in their center and excited when light falls in the background. Acuity: The smallest spatial detail that can be resolved Shellen E Test • Herman Snellen invented this method for designating visual acuity in 1862 • Notice that the strokes of the E for a grating pattern • There are several ways to measure visual acuity • Eye doctors use distance to characterize visual acuity, as “20/20 vision” o Your distance vs normal vision distance • Vision scientists: smallest visual angle of a cycle of granting o The smallest the visual angle at which you can identify a cycle of a granting, the better your vision Spatial Frequency: the number of cycles of a grating per unit of visual angle (usually specified in degrees) - Another way to think of spatial frequency is as the number of times a pattern repeats per unit area Ben Kim PSYC2030 – Sensation & Perception Page 2 of 4 Why sine 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 Retinal ganglion cells and stripes Not only is the spatial frequency important, but so is the phase Phase: the phase of a grating refers to its position within a receptive field The LGN (Lateral Geniculate Nucleus): Receives all sensory information from vision and auditory systems. • Magnocellular layers (1 &2) only get information from M-cells • Parvocellular layers (3-6) only get information from P-cells • Information from the left side of the body goes to the left LGN and vice-versa for the right side. Striate Cortex Striate 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 Two important features of Striate cortex: • Topographical mapping • Cortical magnification o Dramatic scaling of information from different parts of visual field o The amo
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