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Hearing + Perception, part 2 April 10, 2013 Hearing Aids et al. • Generally speaking, a hearing aid is simply an amplifier. • Old style: amplifies all frequencies • New style: amplifies specific frequencies, based on a listener’s particular hearing capabilities. • More recently, profoundly deaf listeners may regain some hearing through the use of a cochlear implant (CI). • For listeners with nerve deafness. • However, CIs can only transmit a degraded signal to the inner ear. Cochlear Implants A Cochlear Implant artificially stimulates the nerves which are connected to the cochlea. Nuts and Bolts • The cochlear implant chain of events: 1. Microphone 2. Speech processor 3. Electrical stimulation • What the CI user hears is entirely determined by the code in the speech processor • Number of electrodes stimulating the cochlea ranges between 8 to 22. • • poor frequency resolution Also: cochlear implants cannot stimulate the low frequency regions of the auditory nerve Noise Vocoding • The speech processor operates like a series of critical bands. • It divides up the frequency scale into 8 (or 22) bands and stimulates each electrode according to the average intensity in each band. This results in what sounds (to us) like a highly degraded version of natural speech. What CIs Sound Like • Check out some nursery rhymes which have been processed through a CI simulator: CI Perception • One thing that is missing from vocoded speech is F0. • …It only encodes spectral change. • A former honors student, Aaron Byrnes, put together an experiment testing intonation perception in CI-simulated speech for his honors thesis. • Tested: discrimination of questions vs. statements • And identification of most prominent word in a sentence. • 8 channels: • 22 channels: The Findings • CI User: • Excellent identification of the most prominent word. • At chance (50%) when distinguishing between statements and questions. • Normal-hearing listeners (hearing simulated speech): • Good (90-95%) identification of the prominent word. • Not too shabby (75%) at distinguishing statements and questions. • Conclusion 1: F0 information doesn’t get through the CI. • Conclusion 2: Noise-vocoded speech might not be a completely accurate CI simulation. Mitigating Factors • The amount of success with Cochlear Implants is highly variable. • Works best for those who had hearing before they became deaf. • The earlier a person receives an implant, the better they can function with it later in life. • Works best for (in order): • Environmental Sounds • Speech • Speaking on the telephone (bad) • Music (really bad) Critical Period? • For congentially deaf users, the Cochlear Implant provides an unusual test of the “forbidden experiment”. • The “critical period” is extremely early-• They perform best, the earlier they receive the implant (12 months old is the lower limit) • Steady drop-off in performance thereafter • Difficult to achieve natural levels of fluency in speech. • Depends on how much they use the implant. • Partially due to early sensory deprivation. • Also due to degraded auditory signal. Practical Considerations • It is largely unknown how well anyone will perform with a cochlear implant before they receive it. • Possible predictors: • lipreading ability • rapid cues for place are largely obscured by the noise vocoding process. • fMRI scans of brain activity during presentation of auditory stimuli. Infrared Implants? • Some recent research has shown that cells in the inner ear can be activated through stimulation by infrared light. • This may enable the eventual development of cochlear implants with very precise frequency and intensity tuning. • Another research strategy is that of trying to regrow hair cells in the inner ear. One Last Auditory Thought • Frequency coding of sound is found all the way up in the auditory cortex. • Also: some neurons only fire when sounds change. And now for something completely different… • Q: What’s a category? • A classical answer: • • All members of the category exhibit the same properties. • • A category is defined by properties. No non-members of the category exhibit all of those properties. The properties of any member of the category may be split into: • Definitive properties • Incidental properties Classical Example • A rectangle (in Euclidean geometry) may be defined as having the following properties: 1. Four-sided, two-dimensional figure (quadrilateral) 2. Four right angles This is a rectangle. Classical Example • Adding a third property gives the figure a different category classification: 1. Four-sided, two-dimensional figure (quadrilateral) 2. Four right angles 3. Four equally long sides This is a square. Classical Example • Altering other properties does not change the category classification: 1. Four-sided, two-dimensional figure (quadrilateral) 2. Four right angles definitive properties 3. Four equally long sides This is still a square. A. Is red. incidental property Classical Linguistic Categories • Formal phonology traditionally defined all possible speech sounds in terms of a limited number of properties, known as “distinctive features”. (Chomsky + Halle, 1968) [d] = [CORONAL, +voice, -continuant, -nasal, etc.] [n] = [CORONAL, +voice, -continuant, +nasal, etc.] … • Similar approaches have been applied in syntactic analysis. (Chomsky, 1974) Adjectives = [+N, +V] Prepositions = [-N, -V] Prototypes • The psychological reality of classical categories was called into question by a series of studies conducted by Eleanor Rosch in the 1970s. • Rosch claimed that categories were organized around privileged category members, known as prototypes. • • (instead of being defined by properties) Evidence for this theory initially came from linguistic tasks: 1. Semantic verification (Rosch, 1975) • Is a robin a bird? • Is a penguin a bird? 2. Category member naming. Prototype Category Example: “Bird” Exemplar Categories • Cognitive psychologists in the late ‘70s (e.g., Medin & Schaffer, 1978) questioned the need for prototypes. • Phenomena explained by prototype theory could be explained without recourse to a category prototype. • The basic idea: • Categories are defined by extension. • Neither prototypes nor properties are necessary. • Categorization works by comparing new tokens to all exemplars in memory. • Generalization happens on the fly. A Category, Exemplar-style “square”