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Chapter 4

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York University
PSYC 1010
Jennifer Steeves

Chapter 4: Sensation and Perception  Psychophysics: Basic Concepts and Issues o Psychophysics  The study of how physical stimuli are translated into psychological experience o Thresholds: Looking at Limits  Sensation begins with the stimulus, any detectable input from the environment  Threshold is a dividing point between energy levels that do and do not have a detectable effect  An Absolute threshold for a specific type of sensory input is the minimum amount of stimulation that an organism can detect  Boundaries of sensory capabilities o Weighting the Differences: the JND  A just noticeable difference (JDN) is the smallest difference in the amount of stimulation that a specific sense can detect  JDN’s very by sense, and the smallest detectable difference is a fairly stable proportion of the size of the original stimulus  Weber’s Law  States that the size of just noticeable difference is a constant proportion of the size of the initial stimulus  The constant proportion is called Weber Fraction  Different fractions apply to different sensory input  As stimuli increase in magnitude, the JDN becomes larger o Psychophysical Scaling  Scale the magnitude of sensory experience  Fecher’s Law  Which states that the magnitude of a sensory experience is proportional to the number of JDNs that the stimulus is causing the experience is above the absolute threshold  Constant increments in stimulus intensity produce smaller and smaller increased in the perceived magnitude of sensation  Example: three equal increases in stimulus intensity (the amount of light) produce progressively smaller differences in the magnitude of sensation (perceived brightness) o Signal-Detection Theory  Proposes that the detection of stimuli involves decision processes as well as sensory processes which are both influenced by a variety of factors besides stimulus intensity  Account for the influence of decision-making processes son stimulus detection  Your responses will depend in part on the criterion you set for how sure you must feel before you react  Depends on your expectations and on the consequences of missing a signal or reporting a false alarm 1  Also dependent on the amount of “noise” in the system  The more noise there is in a system, harder to pick up weak signal  Detectability  Measured in terms of probability and depends on decision- making processes as well as sensory processes o Perception without Awareness  Subliminal perception  The registration of sensory input without conscious awareness  Weak effects overall o Sensory Adaptation  Is a gradual decline in sensitivity due to prolonged stimulation  With prolonged exposure to a stable stimulus, the sensitivity to it decreases  Example: when jump into cold pool, with time you adapt to the temperature and don’t feel it as cold anymore  Autonomic  Keeps people tuned in to the changes rather than the constants in their sensory system  Behavioural adaptation  Our Sense of Sight: The Visual System o The Stimulus: Light  Light is a form of electromagnetic radiation that travels as a wave, moving at the speed of light  Light waves vary in amplitude (height) and in wavelength (the distance between the peaks)  Amplitude affects the perception of brightness, while wavelength affects mainly the perception of colour  Purity (how varied the mix is) influences perception of the saturation or richness, of colours o The Eye: A living Optical Instrument 2  Two main purposes of the eye:  1. They channel light to the neural tissue that receives it (retina)  2. House that tissue  Light enters through the window at the front, the cornea  The cornea and lens (located behind it) form an upside down image on the retina  Lens is a transparent eye structure that focuses the light rays falling on the retina  Accommodation occurs when the curvature of the lens adjust to alter visual focus  Close object = lens get fatter (rounder) to give you a clear image  Distant object = lens flattens out to give you better image  Nearsightedness  Close objects are seen clearly but distant objects appear blurry  Short on the retina  Cornea or lens bends the light too much, or when the eyeball is too long  Farsightedness  Distant objects are seen clearly but close objects appear blurry  Focus of light from close objects falls behind the retina  Eyeball is too short  Iris is the coloured ring of muscle surrounding the pupil  Pupil is the opening in the centre of the iris that helps regulate the amount of light passing into the rear chamber of the eye  When pupil constricts, it lets less light into the eye but it sharpens the image falling on the retina  When the pupil dilates, it lets more light in but the image is less sharp  In bright light, the pupils constrict to take advantage of the sharpened image  Dim light the pupils dilate, image sharpness is sacrificed to allow more light to fall on the retina so that more remains visible  Saccades  Eye movements  Tiny movements are essential to good vision  If someone were able to track your eye movements, they would be able to determine what really interests you, what is capturing your attention, even though you denied it and tried to avert your gaze and look elsewhere  Strength of visual distracters 3 o The Retina: The Brain’s Envoy in the Eye  Retina is the neural tissue lining the inside back surface of the eye, it absorbs light, processes images, and sends visual information to the brain  Retina processes images  Axons that run from the retina to the brain converge at the optic disc, a hole in the retina where the optic nerve fibers exist the eye  Because the optic disc is a hole in the eye, you cannot see the part of an image that falls on it  Blind spot  Retina contains two types of receptors  Rods o Are specialized visual receptors that play a key role in night vision and peripheral vision o Handle night vision because they are more sensitive than cones to dim light o Outnumber cones in the periphery of the retina o When you want to see something in faintly illuminated place, its best to look slightly above or below the place where the object should be  Cones o Specialized visual receptors that play a key role in daylight vision and colour vision o Special sensitivities of cones also allow them to play a major role in the perception of colour o Do not respond well to dim light, that’s why don’t see colour very well in dim light o Better visual acuity  Sharpness and precise detail o Heavily concentrated in the center of the retina  Fovea o Is the tiny spot in the centre of the retina that contains only cones, visual acuity is greatest at this point  Dark adaptation is the process in which the eye becomes more sensitive to light in low illumination  Around 30 minutes  Cones adapt more rapidly than rods  Light adaptation is the process whereby the eyes become less sensitive to light in high illumination 4  Receptive field of a visual cell is the retinal area that when stimulated affects the firing rate of the cell  Centre-surround arrangement o Light falling in the centre has the opposite effect of light falling in the surrounding areas  Lateral antagonism (lateral inhibition) is the most basic of interactive effects o Occurs when neural activity in a cell opposes activity in a surrounding cell o Visual system can compute the relative amount of light at a point instead of reacting to absolute levels of light o Patterns of contrasts o Example: Hermann grid o Vision and the Brain  Optic chiasm is the point at which the optic nerves from the inside half of the each cross over and then project to the opposite site of the brain  Ensures both eyes go to both hemispheres  After reaching optic chiasm, the optic nerve fibers diverge along two pathways 5  Main Pathway o Projects to thalamus o 90% of axons from the retina go to LGN (lateral geniculate nucleus) o Distributed to areas in the occipital love that make up the primary visual cortex  Second Pathway o Leaving optic chiasm branches off to an area in the midbrain called the superior coliculus o Thalamus (LGN) o Occipital Lobe (Primary visual cortex) o PURPOSE: coordination of visual input with other sensory input  Individual cells in the primary visual field don’t really respond much to little spots, they are 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 are most responsive if a line sweeps cross their receptive field but only if its moving in the “right” direction  Highly specialized cells in the visual cortex 6  Feature detectors, neurons that respond selectively to very specific features of more complex stimuli  After input processed in primary visual cortex it sends off for additional processing  Ventral Stream, which processes the details of WHAT objects are out there (form and colour)  Dorsal Stream which processes WHERE the objects are (depth and motion)  Visual Disorders  Visual agnosia o Inability to recognize objects with normal eye function  Prosopagnosia o Which is inability to recognize familiar faces o Including one’s face  Methods in Vision Research  fMRI  Observations of the performance of individual’s suffering from brain damage  Example: McCollough Effect o After image appearing after starting at different line and colour orientation o Viewing the World in Colour  In the visible spectrum, lights with the longest wavelengths appear red, whereas those with the shortest appear violet  Two kinds of colour mixing:  Subtractive colour mixing o Works by removing some wavelengths of light, leaving less light than originally there o Yellow + blue = green o Pigments absorb most wavelengths, selectively reflecting specific wavelengths that give rise to particular colours  Additive colour mixing o Works by superimposing lights, putting more light in the mixture than exists in any one light by itself  The trichromatic theory of colour vision holds that the human eye has three types of receptors with differing sensitive’s to different light wavelengths  According to this model, people can see all of the colour of the rainbow because the eye does its own colour mixing by varying the ratio of neural activity among these three types of receptors  Colour blindness encompasses a variety of deficiencies in the ability to d
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