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week 9 psych.odt

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PSYC 100
Ingrid Johnsrude

Psychophysics - study of how sensation relates to perception sensory thresholds - point at which a stimulus triggers start of an afferent nerve impulse difference threshold - JND between two stimuli - min. change required in intensity of something to detect it is stronger or weaker WEBER'S LAW - size of the JND of a stimulus divided by its initial intensity is a constant - but different sensory domains have different constants - does not hold at the extremes of intensity FECHNER'S LAW - each step in JND represents an equal step in the psychological magnitude of a sensation - so changes in stimulus can be compared across sensory domains STEVEN'S POWER LAW - proposed relationship between magnitude of a stimulus and its perceived intensity Detecting Faint Signals absolute threshold - minimum value of a stimulus that can be detected - eg. faintest sound a person can hear, gentlest touch a person can feel, etc. - usually the stimulus level where observers detect stimulus on 50% of trials - detection doesn't just depend on sensitivity - has more to do with cognitive factors (eg. changes in attention, expectation, and alertness) BIAS response bias - person's tendency to say yes or no, when they are not sure of detection of stimulus NOISE - researchers do their best to control for external noise - spontaneous firing is an example of internal noise (if a stimulus is weak, it won't be able to raise the firing above the spontaneous rate -> signal must be strong enough to not be lost in the "noise") = SIGNAL DETECTION THEORY - mathematical theory of detection of stimulation that says every stimulus requires discrimination between signal and noise - proposed that there is no absolute threshold (because threshold changes w noise and bias) - allows independent assessment of sensitivity and bias Psychophysics of Sound Pitch - how high/low sound is (like a musical note) - depends on frequency/how fast alternation b/w compression/rarefaction is fundamental frequency - lowest frequency of a periodic waveform HOW DOES THE BRAIN INTERPRET THE FREQUENCIES? tonotopic organization - anatomical separation of frequencies in the ear - place code to signal which frequences are present in the sound (also used for high-frequency sounds) - temporal code may also be used for lower-frequency sounds line - individual nerve axon can't fire faster than 1000 times/s (1000 Hz) - so if you are hearing a sonds at 4000 Hz, it is possible becuase multiple nerve fibres connect to each place on basilar membrane - each sound wave excites at least a few auditory neurons VOLLEY PRINCIPLE - fibres can take turns generation action potentials - to send temporal codes in synch w frequencies up to 5000 Hz (human limit) - auditory cells must respond in precise sequence (in time with stimulus) harmonics - series of tones whose frequency is a multiple of the fundamental frequency timbre - identifying sound sources by auditory system - kind of pattern recognition - eg. telling a guitar apart from flute loudness - coded by degree to which each auditory nerve fibre fires on each cycle of stimulus waveform - if sound is loud, fibres will be driven as hard as they can and each fibre will look as much like the 'total response' as possible (in volley principle) - if quiet, fibres will still fire in synchrony, but sporadically, so response might not be as faithful to what stimulus waveform really sounds like - humans are most sensitive to middle frequencies (1000- 4000 Hz) Locating Sounds intensity cue - difference in timbre based on location - helped by shape of pinna/outer ear - information is conveyed to the parietal lobe (responsible for sense of space) - eg you identify where your friend is calling your name from, because you probably heard her voice louder in one ear than the other - also, each ear heard her at v. slightly diff. times - humans are more sensitive to noise along midline of head - why we turn our head toward the sound to hear something better - JND in sound position is smaller near midline than at side - spatial info isn't preserved in inner hair cell receptor array - location has to be inferred based on frequency and timing info PINNAE - unique to us (shape determines how sounds are transmitted into ear canal) - also affects timbre = helps us identify elevation of sound (above/below, in front/behind) TIMING CUE - sound coming from one side of head will arrive in ear on that side first - if distance b/w source and each ear is same, sound will arive at exactly same time - brain uses timing cues to locate sound sources - sound travels very slowly (approx. 340 m/s) - why timers at races start their timers when they see smoke, not hear gun INTENSITY CUES - head 'shadows' sounds so they are less intense in the far ear than ear that's close to source Psychophysics of Vision - we do not have access to 3D so we must use knowledge and experience from moving a
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