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Chapter 4

PSY100 Chapter 4 Summary

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University of Toronto Mississauga
Dax Urbszat

Chapter 4 - Visual agnosia: losing the ability to assemble what he saw into a meaningful picture of the world; an inability to recognize objects - Sensation: stimulation of sense organs, absorption of energy - Perception: selection, organization, and interpretation of sensory imput, subjective - Psychophysics: study of how physical stimuli are translated into psychological experience, Gustav Fechner - Threshold: dividing point between energy levels that do and do not have a detectable effect - Absolute threshold: the minimum amount of stimulation that an organism can detect for a specific type of sensory input, the stimulus intensity detected 50% of the time - Just noticeable difference (JND): smallest difference in the amount of stimulation that a specific sense can detect - Weber’s law: the size of a just noticeable difference is a constant proportion of the size of the initial stimulus called the Weber fraction - Fechner’s law: the magnitude of a sensory experience is proportional to the number of JNDs that the stimulus causing the experience is above absolute threshold, constant increments in stimulus intensity produce smaller and smaller increases in the perceived magnitude of sensations - Magnitude estimation: asking subjects to assign numbers to stimulus on the basis of how intense they appear to be - Signal-detection theory: the detection of stimuli involves decision processes as well as sensory processes, which are both influences by a variety of factors besides stimulus intensity, detectability - Subliminal perception: registration of sensory input without conscious awareness (limen is another term for threshold so subliminal means below threshold), generally weak effects - Sensory adaptation: gradual decline in sensitivity to prolonged stimulation, people are tuned into the changes rather than the constants - Humans are visual animals - Light is a form of electromagnetic radiation that travels as a wave with varying wavelength (in nm) and amplitude (height) and purity (how mixed it is) - Lens: transparent eye structure that focuses the light rays falling on the retina - Accommodation: occurs when the curvature of the lens adjusts to alter visual focus - Nearsightedness: close objects are seen clearly but distant objects appear blurry b/c the focus of light from distant objects falls short of the retina - Farsightedness: distant objects are seen clearly but close objects appear blurry b/c the focus of light from close objects falls behind the retina - Iris: coloured ring of muscle surrounding the pupil - Pupil: opening in the center of the iris that helps regulate the amount of light passing into the rear chamber of the eye - Retina: neural tissue lining the inside back surface of the eye, it absorbs light, processes images, and sends visual information to the brain - Optic disk: a hole in the retina where the optic nerve fibres exit the eye, creates a blind spot as it is a hole in the retina - Retina has receptor rods and cones (more rods) - Cones: specialized visual receptors that play a key role in daylight vision and color vision, better visual acuity, sharpness and precise detail - Fovea: tiny spot in the center of the retina that contains only cones, visual acuity is greatest at this spot - Rods: specialized visual receptors that play a key role in night vision and peripheral vision, periphery of the retina, averted vision, requires less light - Dark adaptation: process in which the eyes become more sensitive to light in low illumination, cones adapt faster than rods - Light adaptation: process whereby the eyes become less sensitive to light in high illumination - Optic nerve: collection of axons that connect the eye with the brain - Receptive field of a visual cell: retinal area that, when stimulated, affects the firing of that cell, collection of rod and cone receptors that funnel signals to a particular visual cell, circular fields with a center-surround arrangement are most common - Lateral antagonism: occurs when neural activity in a cell opposes activity in surrounding cells, responsible for the opposite effects that occur when light falls on the inner versus outer portions of center-surround receptive field; so visual system can compute the relative amount of light at a point - Optic chiasm: point at which the optic nerves from the inside half of each ee cross over and then project to the opposite half of the brain - Lateral geniculate nucleus (LGN): part of the main pathway, first projects into the thalamus, then the LGN, then into the occipital lobe (primary visual cortex) - The second pathway leaves the optic chiasm branches off to an area in the midbrain called the superior colliculus before travelling through the thalamus and on to the occipital lobe; for coordination of visual input with other sensory input - Main visual pathway is split into the magnocellular and parvocellular channels in parallel pathways (involves simultaneously extracting different kinds of information from the same input - David Hubel and Torsten Wiesel found that the primary visual cortex is much more sensitive to lines, edges, and other more complicated stimuli; simple cells respond best to a line of correct width, oriented at the correct angle and located in the correct position in its receptive field; complex cells respond to any position in their receptive fields - Feature detectors: neurons that respond selectively to very specific features of more complex stimuli - Ventral stream: processes the details of what objects are out there (form and colour) - Dorsal stream: processes where the objects are (motion and depth) - Perceived colour is a function of the dominant wavelength in the mixtures - Wavelength is related to hue - Amplitude is related to brightness - Purity is related to saturation - Colour mixture: o Subtractive colour mixing: works by removing some wavelengths of light, leaving less light than was originally there o Additive colour mixing: works by superimposing lights, putting more light in the mixture than exists in any one light by itself - Trichromatic theory of colour vision: the human eye has three types of receptors with differing sensitivities to different light wavelengths (Hermann von Helmholtz) - Colour blindness: encompasses a variety of deficiencies in the ability to distinguish among colours, more common with males - Complementary colours: pairs of colours that produce gray tones when mixed together - Afterimage: visual image that persists after a stimulus is removed - Opponent process theory of colour vision: holds that colour perception depends on receptors that make antagonistic responses to three pairs of colours - Reversible figure: a drawing that is compatible with two interpretations that can shift back and forth - Same visual input can result in radically different perceptions, as people’s experience subjective - Perceptual set: a readiness to perceive a stimulus in a particular way - Inattentional blindness: the failure to see fully visible objects or events in a visual display - Feature analysis: the process of detecting specific elements in visual input and assembling them into a more complex form - Bottom-up processing: a progression from individual elements to the whole, at least some aspects of form perception involve feature analysis - Top-down processing: a progression from the whole to the elements, people can perceive a word before its individual letters - Subjective contours: perception of contours where none actually exist - Phi phenomenon: the illusion of movement created by presenting visual stimuli in rapid succession o Figure and ground: figure is the thing being looked at, gr
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