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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”