Grab Bag! (Hearing, Fricatives +

Perception, part 2)

April 12, 2011

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 very 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.

Back to Fricatives!• Fricative noise has some inherent spectral shaping

• …like “spectral tilt”

• Note: this is a source characteristic

• This resembles what is known as pink noise:

• Compare with white noise:

Fricative Shaping• The turbulence spectrum may be filtered by the resonating tube in front of the fricative.

• (Due to narrowness of constriction, back cavity resonances don’t really show up.)

• As usual, resonance is determined by length of the tube in front of the constriction.

• The longer the tube, the lower the “cut-off” frequency.

• A basic example:

• [s] vs.

vs.

“sigh” “shy”

[s]

Sampling Rates Revisited• Remember: Digital representations of speech can only capture frequency components up to half the sampling rate

• the Nyquist frequency

• Speech should be sampled at at least 44100 Hz

(although there is little frequency information in speech above 10,000 Hz)

• [s] has higher acoustic energy from about 3500 - 10000 Hz

• Note: telephones sample at 8000 Hz

• 44100 Hz

• 8000 Hz

Further Back

[xoma]

palatal vs. velar

• In more anterior fricatives, turbulence noise is generally shaped like a vowel made at the same place of articulation.

Even Further Back• Examples from Hebrew:

At the Tail End• [h] exhibits a lot of coarticulation

• [h] is not really a “fricative”;

• it’s more like a whispered or breathy voiced vowel.

“heed” “had”

Aspirated Fricatives• Like stops, fricatives can be aspirated.

• [h] follows the supraglottal frication in the vocal tract.

• Examples from Chinese:

[tsa] [tsha]

Back at the Ranch• There is not much of a resonating filter in front of labial fricatives…

• so their spectrum is flat and diffuse

• (like bilabial stop release bursts)

• Note: labio-dentals are more intense than bilabial fricatives

• (channel vs. obstacle turbulence)

Fricative Internal Cues• The articulatory precision required by fricatives means that they are less affected by context than stops.

• It’s easy for listeners to distinguish between the various fricative places on the basis of the frication noise alone.

• Result of both filter and source differences.

• Examples:

• There is, however, one exception to the rule…

Huh?• The two most confusable consonants in the English language are [f] and .

• (Interdentals also lack a resonating filter)

Helping Out• Transition cues may partially distinguish labio-dentals from interdentals.

• Normally, transitions for fricatives are similar to transitions for stops at the same place of articulation.

• Nonetheless, phonological confusions can emerge--

• Some dialects of English substitute [f] for .

• Visual cues may also play a role…

Acoustic Enhancement

• E.g.: is post-alveolar and [s] is alveolar

• more space in vocal tract in front of

• including a “sub-lingual cavity”

• This “filter” of resonates at lower frequencies

• In English, this acoustic distinction is enhanced through lip rounding for

• this extends the vocal tract

• further lowers the resonant frequencies of

• another form of “adaptive dispersion”

• Fricative distinctions can be enhanced through secondary articulations.

The Sub-lingual Cavity

•Let’s check the videotape...

Behind the Constriction

[s]

• Let’s check the ultrasound…

Secondary Articulations• What effect might lowering the center of the tongue have on formant values?

• (think: perturbation theory)

• Check it out in Praat.

And now for something completely different…

• Q: What’s a category?

• A classical answer:

• A category is defined by properties.

• All members of the category exhibit the same 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

3. Four equally long sides

This is still a square.

A. Is red.

definitive properties

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”

Back to Perception• When people used to talk about categorical perception, they meant perception of classical categories.

• A stop is either a [b] or a [g]

• (no in between)

• Remember: in classical categories, there are:

• definitive properties

• incidental properties

• Q: What are the properties that define a stop category?

• The definitive properties must be invariant.

• (shared by all category members)

• So…what are the invariant properties of stop categories?

The Acoustic Hypothesis• People have looked long and hard for invariant acoustic properties of stops, with little success.

• (and some people are still looking)

• Frequency values of compact (synthetic) bursts cueing different places of articulation, in various vowel contexts.

(Liberman et al., 1952)

Theoretical Revision• Since invariant acoustic properties could not be found (especially for velars)…

• It was assumed that listeners perceived (articulatory) gestures, not (acoustic) sounds.

• Q: What invariant articulatory properties define stop categories?

• A: If they exist, they’re hard to find.

• Motor Theory Revision #2: Listeners perceive “intended” gestures.

• Note: “intentions” are kind of impossible to observe.

• But they must be invariant…right?

Another Brick in the Wall• Another problem for motor theory:

• Perception of speech sounds isn’t always categorical.

• In particular: vowels are perceived in a more gradient fashion than stops.

• However, vowel perception becomes more categorical when the vowels are extremely short.

• It’s also hard to identify any invariant acoustic properties for vowels.

• Variation is rampant across:

• tokens

• speakers

• genders

• dialects

• age groups, etc.

• Variability = a huge problem for speech perception.

More Problems• Also: infants exhibit categorical perception, too…

• Even though they don’t know category labels.

• Chinchillas can do it, too!