Psych 253 Lecture 2 Spring 14 Wede Lecture 1 History Methods Sensation and Perception Sensation the initial process of detecting and encoding environmental energy Perception the process of organizing and interpreting sensations into meaningful experiences Early Philosophy of Perception Plato and The Allegory of the Cave Perception and your sense of reality are the products of evolution Sensory systems provide a survival advantage Importance of type of energy in the environment for an animal determines which senses have developed Some species sense energies that humans cannot Bees see ultraviolet light Rattlesnakes sense infrared energy e g heat Dogs and cats can hear sounds of higher frequencies Birds turtles and amphibians use magnetic fields to navigate You can never step into the same river twice Heraclitus 540 480 BCE Change is essential for perception Our perceptual systems respond to novelty and change Adaptation A reduction in response caused by prior or continuing stimulation If there is no change in stimulation our senses adapt and we no longer notice the stimulation Perception starts with energy from the environment Sensory transducer A receptor that converts physical energy from the environment into neural energy Johannes Kepler 1571 1630 Astronomer who first demonstrated that the eye used lenses to create an image M ller Lyer Illusion Middle Ages Nativism and Empiricism innate and not learned Nativism Mind produces ideas that are not derived from external sources and that we have abilities that are Dualism The idea that both mind and body exist and are separate entities Both of these ideas were championed by Descartes Empiricism The idea that all knowledge comes through the senses Opposed to nativism which says that some knowledge is innate According to empiricism perception is the source of all knowledge about the world Thomas Hobbes John Locke George Berkeley Doctrine of Specific Nerve Energies 1826 1 Psych 253 Lecture 2 Spring 14 Wede The quality of the sensation elicited by a stimulus depends primarily on the nerve excited and secondarily on the stimulus itself Synesthesia a condition in which stimulation of one sense evokes an involuntary experience in another sense Chromesthesia seeing colors in response to sounds Grapheme or grapheme phoneme synesthesia when certain letters or numbers always appear to be the same colors Current Methods The Dawn of Psychophysics Ernst Weber 1795 1878 Weber s Law As stimulus intensity increases or decreases the magnitude of change must increase proportionately to remain noticeable Difference threshold JND minimum amount of difference between stimuli to reliably notice I the intensity of the original stimulus I the increment in intensity that can just be discriminated from I the just noticeable difference JND k a constant based on the sense being measured Weber s Law I I k Example difference in weights I initial weight 20 lbs k 0 02 I x k I 20 x 0 02 0 4 I 0 4 So Add 0 4 to 20 to find the difference threshold for a 20 lb weight Problem At very high and very low intensities Weber s law breaks down but it is very accurate with moderate levels of stimulation Gustav Fechner 1801 1887 invented psychophysics Psychophysics The science of defining quantitative relationships between physical and psychological subjective events Also believed in panpsychism The idea that all matter has consciousness Developed three methods for finding threshold Proposed the first law of magnitude scaling Fechner s Law S k log R 2 Psych 253 Lecture 2 Spring 14 Wede Absolute threshold the smallest amount of stimulus energy necessary to reliably detect a stimulus Fechner s 3 methods for determining absolute threshold Method of Constant Stimuli A number of stimulus intensities 5 10 typically are selected by the researcher The stimuli are presented numerous times in random order A graph is plotted showing percent of times detected as a function of stimulus intensity The point at which the stimulus was detected 50 of the time is deemed the absolute threshold Problem Very time consuming Advantage Most accurate method most replicable Method of Limits Start with a stimulus clearly above or below threshold Researcher adjusts the intensity of the stimulus down or up in fixed increments This process is repeated several times sometimes ascending and sometimes descending The average of the cross over points is deemed the absolute threshold Example Problems Response bias can affect the results Less accurate than the method of constant stimuli Advantage Can be done more quickly than the method of a constant stimuli Method of Adjustment The intensity of the stimulus is adjusted by the observer until it can just be detected Problem Least accurate of Fechner s 3 methods Advantages easiest to do least boring for the observer Problems with Fechner s 3 methods The observer s criterion for responding greatly affects the results Observer could be lying Magnitude Estimation a k a Magnitude scaling Magnitude estimation Determining the mathematical relationship between stimulus intensity and perceived magnitude Stimulus intensity the physical intensity of a stimulus as measured by physical instruments scale for weight light meter for light decibel reader for sound 3 Psych 253 Lecture 2 Spring 14 Wede Perceived magnitude the subjective impression that one s senses give of intensity e g heaviness Stevens 1961 developed the most accurate law of magnitude scaling based on empirical evidence Magnitude scaling experiments Subjects shown a standard light and told it has a brightness of 10 Lights of various intensities are shown and the subject assigns a value to the brightness of each e g twice as bright 20 1 3 as bright 3 33 Stevens Power Law P KSn P perceived magnitude K and n constants that vary with the stimulus S stimulus intensity n determines the form of the function if n 1 doubling of physical intensity results in less than a doubling of perceived magnitude brightness loudness smell if n 1 doubling of physical intensity results in exactly a doubling of perceived magnitude judging line length if n 1 doubling of physical intensity results in more than a doubling of perceived magnitude electric shock heat Magnitude estimation Signal Detection Theory SDT Green Swets 1966 SDT is a method of determining a subject s sensitivity to a stimulus Takes into account the subject s sensory sensitivity as well as his or her response criterion bias 4 Psych 253 Lecture 2 Spring 14 Wede Detecting a