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Chapter 4

Textbook Chapter 4 Notes

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Ayesha Khan

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Notes From Reading C HAPTER 4: SENSATION AND P ERCEPTION (PGS.136-197) Psychophysics: Basic Concepts and Issues • Sensation and perception is very relevant to our experiences in our everyday life • Sensation – the stimulation of sensory organs. o Involves the absorption of energy (i.e. light or sound waves) by sensory organs (i.e. eyes, ears) • Perception – the selection, organization, and interpretation of the sensory input. o Organizing and translating sensory input into something meaningful. • Psychophysics – the study of how physical stimuli are translated into psychological experience. o Important psychologist – Fechner (Germany). Thresholds – Looking for Limits • Sensation begins with a stimulus • Stimulus – any detectable input from the environment • Fechner’s question is a concept central to psychophysics: the threshold • 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. o As Stimulus intensity increases, subjects’ responding to stimuli gradually increases. o As such, absolute threshold is the stimulus intensity that is detected 50% of the time. Weighing the Differences: The JND • Fechner was interested in people’s sensitivity to difference between stimuli • Just Noticeable Difference (JND) – smallest difference in the amount of stimulation that a specific sense can detect • Webber’s law – states that the size of a JND is a constant proportion (“Webber Fraction”) of the size of the initial stimulus o Different fractions apply to different types of sensory input o As the stimulus increase in magnitude, the JND becomes larger. Psychophysical Scaling • Fechner’s Law – larger and larger increases in stimulus intensity are required to produce JND in the magnitude of sensation o i.e. Light bulbs in a room • Perceptions can’t be measured on absolute scales Signal-Detection Theory • Signal detection theory is used to measure the way we make decisions under conditions of uncertainty • For instance, you’re recording the amount of times a security guard in an airport is able to detect someone who is carrying a gun o Based on sensory processes (sensitivity) and decision processes (bias) o Hit, miss, false alarm, correct rejection Perception without Awareness • Subliminal Perception – is the registration of sensory input without conscious awareness; it is a genuine phenomenon, but the effects tend to be very weak Sensory Adaptation 1/8 Notes From Reading C HAPTER 4: SENSATION AND P ERCEPTION (PGS.136-197) • Sensory adaptation is a gradual decline in sensitivity to prolonged stimulation • Suggests, sensory adaption probably is a behavioral adaption that has been sculpted by natural selection Our Sense of Sight: The Visual System • What people “sense” and what they “perceive” may be quite different The Stimulus: Light • Light is a form of electromagnetic radiation that travels as a wave, moving, naturally enough, at the speed of light • Vary in amplitude (height), wavelength, purity o Amplitude  perception of brightness o Wavelengths  perception of color o Purity  perception of richness/saturation • Vision is a filter that permits people to sense but a fraction of the real world The Eye: A Living Optical Instrument • The eye channels light to the neural tissue that receives it and houses the retina • Light enters the eye through the cornea (transparent window). • Cornea and Lens form an upside down image of the object on the retina. o Brain knows the rule to the corresponding positions in the world • Lens – focuses light rays falling on the retina • Nearsightedness – close objects are seen clearly, but distant objects blurry. o Due to the focus of light from distant objects falling short of the retina. • Farsightedness – farther objects are seen clearly but close seem blurry o Due to the focus of light from near objects falling behind the retina • Iris – colored ring surrounding pupil (black center of the eye) • Pupil – regulate the amount of light passing into the rear chamber of the eye o When the pupil dilates it lets more light in but the image is less sharp The Retina: The Brain’s Envoy in the Eye • Retina – neural tissue lining the back surface of the eye • Optic Disk – a hole in the retina that corresponds to the blind spot • Visual Receptors: Rods and Cones (organized into receptive fields) o Cones – play a key role in day and color vision and provide greater acuity than rods  Do not respond well to dim light, Cones provide sharpness detail o Rods – play a key role in night and peripheral vision and greatly outnumber cones  More sensitive than cones to dim light, Handle peripheral vision  Density greatest outside the fovea, and gradually decreases o Fovea is a tiny sport in the center of the retina that contains only cones; visual acuity is greatest at this spot • Dark Adaptation – the process in which the eyes become more sensitive to light in low illumination. • Light Adaptation – the process where eyes become less sensitive to light in high illumination • Information Processing in the Retina 2/8 Notes From Reading C HAPTER 4: SENSATION AND PERCEPTION (PGS.136-197) • The Receptive field of a visual cell – retinal area that when stimulated effects the firing of that cell. • Lateral antagonism occurs when neutral activity in a cell opposes activity in surrounding cells Visual and the Brain • Visual input is meaningless until it is processed in the brain • Visual Pathways to the Brain o Optic Chiasm - Ensures that signals from both eyes go to both hemispheres of the brain o After reaching the optic chiasm, the optic nerve splits between 2 pathways – main and secondary. o Main pathway - can be subdivided into the parvocellular channel (what) and magnocellular channel (where), which engage in parallel processing of stimulus input o Secondary pathway function – handles coordination of visual input with other sensory input o Parallel Processing involves simultaneously extracting different kinds of information form the same input • Info Processing in the Visual Cortex o In the occipital lobe handles the initial cortical processing of visual input o Feature Detectors – neurons that respond selectively to very specific features of complex stimuli. o Signals travel through two streams: o Ventral stream – processes the details of what objects are out there (ie. The perception of form and colour o Dorsal stream – which processes where the objects are (ie. The perception of motion and depth o Visual Agnosia – an inability to recognize objects. o Prosopagnosia – inability to recognize familiar faces • Multiple Methods in Vision Research o The McCollough Effect is a well-known afterimage phenomenon that differs from other color afterimage effects because it is contingent on both color and pattern/form Viewing the World in Color • The Stimulus for Color o Wavelength is most closely related to hue, amplitude to brightness, and purity to saturation (for the perception of color) • Subtractive Color Mixing – works by removing some wavelengths of light, leaving less light (i.e. Crayons) • Additive Color Mixing – works by putting more light in the mixture than any one light (Shine red, green, and blue spotlights on a white surface) • Trichromatic Theory of Color Vision o Holds that the human eye has three types of sensitive to wavelengths associated with red, green and blue o Color Blindness encompasses a variety of deficiencies in the ability to distinguish among colors (Dichromats & frequently occur in males) • Opponent Process Theory of Color Vision 3/8 Notes From Reading C HAPTER 4: SENSATION AND P ERCEPTION (PGS.136-197) o Complementary Colors – pairs of colors that produce gray tones when mixed together. o Afterimage – visual image that persists after a stimulus is removed. Trichromatic Theory can’t explain. (Color will be complement) o The opponent process theory of color vision – color perception depends on the receptors that make antagonistic responses to three pairs of colors. • Reconciling Theories of Color Vision o It takes both theories to explain color vision • Effects of Color on Behavior o Colors can have automatic, unconscious effects on behavior o Over the course of human evolution, certain colors may have had adaptive significance for survival or reproduction Perceiving Forms, Patterns and Objects • Reversible Figure – drawing that is compatible with 2 interpretations that can shift back and forth. • Perceptual Set – a readiness to perceive a stimulus in a particular way. • Inattentional blindness involves the failure to see fully visible objects or events in a visual display • Feature Analysis: Assembling Forms o Feature Analysis – the process of detecting specific elements in a visual input and assembling them into a more complex form. o Bottom Up Processing- a progression from individual elements to the wh
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