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Chapter 4

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PSYC 1200
Jason Leboe- Mcgowan

 Ian Howard is known internationally as a pioneer in sensation/perception research  People can rely on 3 types of cues to determine which way is up - visual, gravity, and body direction  Sensation: the stimulation of sense organs  Perception: the selection, organization, and interpretation of sensory input  Psychophysics: the study of how physical stimuli and translated into psychological experience Thresholds: Looking for Limits  Stimulus: any detectable input from the environment o Sensation begins with a stimulus  Threshold: a dividing point between energy levels that do and do not have a detectable effect  Absolute threshold: for a specific type of sensory input is the minimum amount of stimulation that an organism can detect Weighing the Differences  Just noticeable difference (JND): the smallest difference in the amount of stimulation that a specific sense can detect  Weber's law: states that the size of a JND is a constant proportion of the size of the initial stimulus o As the stimulus increases in magnitude, the JND becomes larger Psychophysical Scaling  Fechner's law: states that the magnitude of a sensory experience is proportional to the number of JNDs that the stimulus causing the experience is above the threshold Signal Detection Theory  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 other than stimulus intensity o The more 'noise' in a system, the harder it will be for you to pick up a weak signal Subliminal Perception  Subliminal perception: the registration of sensory input without conscious awareness ('limen' is another term for threshold, so 'subliminal' means below threshold) o Subliminal messaging controversy began in 1957 - with James Vicary hiding 'eat popcorn' on the screen at a film showing o Cooper and Cooper demonstrated subliminal presentations of Coca-Cola cans and the word 'thirsty', in 2002 Sensory Adaptation  Sensory adaptation: a gradual decline in sensitivity due to prolonged stimulation The Stimulus: Light  Light is a form of electromagnetic radiation that travels as a wave, moving, naturally enough, at the speed of light  Light waves vary in amplitude (height) and in wavelengths (the distance between peaks) o Amplitude affects the perception of brightness o Wavelength affects the perception of color  Light can also vary in its purity (how varied the mix is)  Purity influences perception of the saturation, or richness, of colors The Eye: A Living Optical Instrument  The eye serves two main purposes o Channels light into neural tissue that receives it - retina o Houses that tissue  Light enters the eye through the cornea o The cornea and the crystalline lens located behind it form an upside down image of objects on the retina  Lens: the transparent eye structure that focuses the light rays falling on the retina  Nearsightedness: close objects are seen clearly but distant objects appear blurry o Occurs when cornea bends light too much - light falls short of retina - or eyeball is too long  Farsightedness: distant objects are clear, but close objects are blurry o Focus of light from close objects falls behind the retina o Typically occurs when eyeball is too short  Pupil: the opening in the center of the iris that helps regulate the amount of light passing into the rear chamber of the eye  Saccades: when looking at something, eyes make brief fixations at various parts of the stimuli The Retina: the Brain's Envoy in the Eye  Retina: neural tissue lining the inside back surface of the eye o Absorbs light, processes images, and sends visual information to the brain o Contains two types of receptors - rods and cones  Cones: specialized visual receptors that play a key role in daylight vision and color vision o Provide better visual acuity - sharpness and precise detail - then rods  Fovea: a tiny spot in the center of the retina that contains only cones o Visual acuity is greatest at this spot  Rods: specialized visual receptors that play a key role in night vision and peripheral vision  Optic disk: a hole in the retina where the optic nerve fibers exit the eye  Dark adaptation: the process in which eyes become more sensitive to light in high illumination  The receptive field of a visual cell: the retinal area that, when stimulated, affects the firing of that cell  Lateral antagonism: occurs when neural activity in a cell opposes activity in surrounding cells Vision and the Brain  Optic chiasm: the point at which the optic nerves from the inside half of each eye cross over and then project to the opposite half of the brain  Optic nerve divides into 2 paths o The main path projects into the thalamus, the brains major rely station  Here, about 90% of the axons from the retinas synapse in the lateral geniculate nucleus (LGN)  Subdivided into 2 more pathways - magnocellular and parvocellular  These channels engage in parallel processing: involves simultaneously extracting different kinds of info from the same input o The second path braches off into an area in the midbrain called the superior colliculus  Principle function appears to be the coordination of visual input with other sensory input  Most visual input eventually arrives in the primary visual cortex, located in the occipital lobe o Explaining how the cortical cells in this are respond to light posed a perplexing problem  David Hubel and Torsten Wiesel discovered solution by accident  Projector slide with crack, cat's receptive cells fired like crazy  Individual cells in the primary visual cortex don't really respond much to little spots  Much more sensitive to lines, edges, and more complicated stimuli  Simple cells respond best to a line of the correct width, oriented at the correct angle, and located in the correct position in its receptive field  Complex cells also care about width and orientation, but they 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 o Neurons here involved in perceiving faces can learn from experience  The brain can be rewired by experience  Dorsal stream: processes w
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