Perception Chapter 11: Hearing and Listening Audiogram or Audibility Function (AF) for normal human young adult. Note: peak sensitivity 2-5K frequency range. Audiogram or AF: a graph indicating the threshold intensity of varying frequencies. A normal audiogram show declining threshold intensity from 20hz to about 2,500hz (where it usually bottoms out). Then a steady increase from about 5,000 to 10,000; with a very steep increase from 10,000 on. Afs: ages and species comparisons Top shows different species Bottom shows different ages for humans. Note: as one gets older “contraction” of range occurs, but most pronounced at high frequencies (presbycusis). Presbycusis: increasing age increases threshold intensity for higher frequencies Hearing loss: Conduction deficit Conduction loss: raised threshold levels for entire range of frequencies due to mechanical malfunction in outer ear (can be as simple a ear wax build-up or more serious like severe rupture in tympanic membrane) or middle ear (which usually involves reduced conductive abilities of ossicles) Otitis media: childhood ear infection Otosclerosis: gradual immobilization of stapes (overgrowth). Hearing loss: Sensory-neural Affects response on Basilar membrane. Usually more restricted range of frequencies affected. No differences in bone vs. air conduction. Loud sounds, drugs, presbycusis, trauma to BM all potential causes. Hearing loss: treatments Hearing aids: electronically amplify incoming auditory signal, usually boosts speech range more relative to other frequencies Cochlear implants: electrodes implanted direction in cochlea to directly stimulate ANF. Auditory masking and the Critical Band Auditory masking: The extent to which background noise of varying frequencies (broadband noise) can serve to block or mask the perception of a signal. Ex: trying to listen to the radio (signal) with and without the engine running (masker). The paradigm for studying this works in the following way: 1) a subject attempts to detect a signal of a certain frequency, in the presence of a noise masker. 2) the masker is broadband noise which is varied in two ways (1) location of the center frequency of the noise and (2) the extent of the range of frequencies surrounding the center frequency. Results indicate: 1) masking is asymmetric: lower frequencies mask higher frequencies more effectively than vice-versa 2) masking only occurs within a certain range of frequencies surrounding the center frequency: called the critical band. Two reasons for this: 1) response of basilar membrane to lower and high frequency stimuli 2) response characteristics of frequency tuned neurons in auditory nerve. Auditory masking and the Critical Band Why is masking asymmetrical and why is there a critical band? Two reasons for this: 1) response of basilar membrane to lower and high frequency stimuli 2) response characteristics of frequency tuned neurons in auditory nerve. Loudness Perception Magnitude estimation: this is a procedure where a certain intensity level of sound (say 60dB) is assigned an arbitrary number to describe its loudness. Subject is then presented a series of sounds which vary in their intensity. Subject assigns a number to describe the loudness of the sound relative to the standard sound. Results using ME demonstrate that the relationship between sound intensity and perceived loudness is not a 1 to 1. Loudness tends to increase more slowly than then does intensity (bottom line, open circles on graph) Doubling intensity increases perceived loudness only about 60%. To double loudness, about a 10dB (or tripling) increase in intensity is required Equal loudness contours Loudness Matching: this is where a certain frequency tone is presented at a certain dB level (for example 1000hz tone at 60dB). Then a second tone at a different frequency is presented and subject adjusts intensity level until it is the same loudness. Equal loudness contours: a family of curves representing the required intensity of varying frequencies such that those frequencies match the perceived loudness of a 1000hz tone at a certain dB level. For example, an ELC for 20dB, indicates the dB level required for each freq, such that it sounds as loud as a 1000hz tone at 20dB. All freqs along curve sound as loud as 1000hz tone at 20dB. Neural basis of loudness perception Two mechanisms: Which nerve fibers are active? How widespread is activity on BM? Louder sounds will activate Lower spontaneous activity fibers; and will involve more fibers in total along BM Discriminatory power: usually 1-2dB change in intensity is required to reach JND OHC Hi spon activity Med spon activity IHC Lo spon activity Pitch perception Fundamental frequency: usually lowest frequency produced by vibrating body that accounts for pitch perception. For example: If a musical instrument plays the note B3 at around 220hz, it will create a pattern of overtones consisting of 440,660,880 etc. The perceiver will hear a low 220hz note, with certain timbre qualities. The missing fundamental: what if overtones are played without fundamental? What will the person’s pitch perception be? Missing fundamental Despite absence of fundamental, pitch perception remains the same – person perceives fundamental based on overtones. How is this possible? Place theory: big problem because there is no displacement at the place of the fundamental frequency. Frequency: yes -- the firing rate is determined by the differences between overtones (harmonic spacing) in complex sounds. Notice -- because overtones are multiples, it will be low freqs that require frequency matching by neurons, higher tones will have place, thus this fits in nicely with freq=low, place=hi, freq+place=midrange. Localizing sounds: Where is it coming from? Two cues discussed earlier: Inter-aural intensity differences and inter-aural time differences Intensity differences vary by frequency; greater difference for higher frequencies. Shadowing effect weaker for lower frequencies (left). Localizing sounds: Where is it coming from? • Inter-aural time differences vary with location (left). However, there is a zone on either side of the head where neither ITD nor IID specify a unique location in space: Cone of confusion (right). Duplex theory: ITD used for low frequencies (1,000Hz below); IID used for high frequencies (4,000Hz above). 2-4K Hz most difficult to localize.