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PSYB51H3 (306)
Chapter

psyb51-perception and cognition

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Department
Psychology
Course
PSYB51H3
Professor
Matthias Niemeier
Semester
Fall

Description
CHAPTER 3: SPATIAL VISION – FROM STARS TO STRIPES Visual Acuity: Oh Say, Can You See? - Contrast: diff. in luminance b/w and object and the background, or b/w lighter and darker parts of the same object - Acuity: smallest spatial detail that can be resolved - Cycle: for a grating, a pair consisting of one dark bar and one bright bar - Visual angle: angle subtended by an object at the retina (formed by lines going from topbottom, through the centre of the lens, and on to the retina o Resolution acuity rep. one of the fundamental limits of spatial vision finest high-contrast detail that can be resolved o Limit is determined primarily by spacing of photoreceptors in the retina - Sine Wave Gratings: a grating with a sinusoidal luminance profile - Aliasing: misperception of a grating due to undersampling Acuity for Low-Contrast Stripes: - Spatial Frequency: number of cycles of a grating per unit of visual angle (specified in degrees) - Cycles per degree: number of dark and bright bars per degree of visual angle - Contrast sensitivity function(CSF): a function describing how the sensitivity to contrast depend on the spatial frequency (size) of the stimulus - Contrast Threshold: smallest amount of contrast required to detect a pattern Why Sine Wave Gratings? - Although “pure” SWG maybe rare in real world, patterns of stripes w/ more or less fuzzy boundaries are quite common (trees in forests, books on bookshelves) - Visual system appears to break down real-world images into vast number of components essentially a SWG with a particular spatial frequency Retinal Ganglion Cells and Stripes: - spatial freq. of grating is too low G.cell responds weakly b/c part of the fat, brighter bar of grating lands in inhibitor surround, damping the cell’s response - spatial freq. of grating is too high-> G.cell responds weakly b/c both dark and bright stripes fall within the receptive-field centre, washing out the response - spatial freq.is just right with a bright bar filling the centre and dark bars in the surround, the cell responds vigorously o Therefore, retinal ganglion cell responds best to a specific spatial freq. that matches its receptive field size - Phase: relative position of a grating Cortical Topography and Cortical Magnification: - Important features of the visual cortex: o Topographical mapping o Dramatic scaling of info. From diff parts of the visual field. - Coritcal Magnification: amount of cortical area (usually in mm) devoted to specific region (eg: 1 degree) in the visual field o imp. Consequence  visual acuity declines in an orderly fashion with eccentricity Receptive Fields in Striate Cortex: - Hubel + Wiesel: cat experiement o Most fundamental discovery: neurons in striate cortex respond to stripes not stars  Receptive fields of striate cortex neurons are not circular, they are elongated, responding much more vigorously to bars, lines, edges, and gratings than to round circles of light Orientation Selectivity: - Other imp. Properties of receptive fields of neurons in S. cortex discovered by Hubel and Wiesel: o Indiv. Neuron will not respond equivalently to just any old stripe in its receptive field. Responds best when line/edge is at just the right orientation and hardly at all when line is tilted more than 30 degrees away from optimal orientation  Orientation Tuning: Tendency of neurons in S. cortex to respond optimally to certain orientations and less to others. Other Receptive-field Properties: - Filter: an acoustic, electrical, electronic, or optical device, instrument, computer, program, or neuron that allows the passage of some frequencies or digital elements and blocks others - Another discovery: many cortical cells respond especially well to moving lines, bars, edges, and gratings o Many respond strongly when a line moves in one direction - Ocular dominance: property of the receptive fields of S. cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other Simple and Complex Cells: - cortical neurons come in variety of types: o Simple cells: clearly defined excitatory and inhibitory regions o Complex cells: receptive characteristics cannot be easily predicted by aping with spots of life Further Complications: - End stopping: cell in the cortex first increases its firing rate as the bar length increases to fill up its receptive field, and then decreases its firing rate as the bar is lengthened further Columns and Hypercolumns - Column: vertical arrangement of neurons o neurons with similar orie
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