Auditory Remnants

April 5, 2012

Equal Loudness Curves• Perceived loudness also depends on frequency.

Audiograms• When an audiologist tests your hearing, they determine your hearing threshold at several different frequencies.

• They then chart how much your hearing threshold differs from that of a “normal” listener at those frequencies in an audiogram.

• Noise-induced hearing loss tends to affect higher frequencies first.

• (especially around 4000 Hz)

Age• Sensitivity to higher frequencies also diminishes with age. (“Presbycusis”)

Note: the “teen buzz”

Otitis Media• Kids often get ear infections, which are technically known as otitis media.

• = fluid fills the middle ear

• This leads to a form of conduction deafness, in which sound is not transmitted as well to the cochlea.

• Auditorily, frequencies from 500 to 1000 Hz tend to drop out.

Check out a Praat demo.

Loudness• The perceived loudness of a sound is measured in units called sones.

• The sone scale also exhibits a non-linear relationship with respect to absolute pressure values.

Masking• Another scale for measuring auditory frequency emerged in the 1960s.

• This scale was inspired from the phenomenon of auditory masking.

• One sound can “mask”, or obscure, the perception of another.

• Unmasked:

• Masked:

• Q: How narrow can we make the bandwidth of the noise, before the sinewave becomes perceptible?

• A: Masking bandwidth is narrower at lower frequencies.

Critical Bands• Using this methodology, researchers eventually determined that there were 24 critical bands of hearing.

• The auditory system integrates all acoustic energy within each band.

• Two tones within the same critical band of frequencies sound like one tone

• Ex: critical band #9 ranges from 920-1080 Hz

• F1 and F2 for might merge together

• Each critical band 0.9 mm on the basilar membrane.

• The auditory system consists of 24 band-pass filters.

• Each filter corresponds to one unit on the Bark scale.

Bark Scale of Frequency

• The Bark scale converts acoustic frequencies into numbers for each critical band

Bark TableBand Center Bandwidth Band Center

Bandwidth

1 50 20-100 13 1850 1720-2000

2 150 100-200 14 2150 2000-2320

3 250 200-300 15 2500 2320-2700

4 350 300-400 16 2900 2700-3150

5 450 400-510 17 3400 3150-3700

6 570 510-630 18 4000 3700-4400

7 700 630-770 19 4800 4400-5300

8 840 770-920 20 5800 5300-6400

9 1000 920-1080 21 7000 6400-7700

10 1170 1080-1270 22 8500 7700-9500

11 1370 1270-1480 23 10500 9500-12000

12 1600 1480-1720 24 13500 12000-15500

Spectral Differences

• Acoustic vs. auditory spectra of F1 and F2

Cochleagrams• Cochleagrams are spectrogram-like representations which incorporate auditory transformations for both pitch and loudness perception

• Acoustic spectrogram vs. auditory cochleagram representation of Cantonese word

• Check out Peter’s vowels in Praat.

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.

• Depends a lot on the person

• Possibly because of reorganization of the brain

• 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.

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.

Vocal Tract Physiology

April 5, 2012

The Toolkit• There are four primary active articulators in speech.

• (articulators we can move around )

1. The lips

2. The lower jaw (mandible)

3. The tongue

4. The velum

• The pharynx can also be constricted, to some extent.

• Separate sets of muscles control each articulator...

Articulatory Speed• The gold medal goes to the tongue tip...

• which is capable of 7.2 - 9.6 movements per second.

• The rest:

• Mandible 5.9 - 8.4 movements per second

• Back of tongue 5.4 - 8.9

• Velum 5.2 - 7.8

• Lips 5.7 - 7.7

• Note: lips can be raised and lowered faster than they can be protruded and rounded.

1. The Lips• The orbicularis oris muscle surrounds the lips.

• Contraction compresses and rounds the lips.

• A muscle called the mentalis also protrudes the lips.

• Contraction of the risorius muscle retracts the corners of the lips...

• and spreads them.

By the way...• The vowel [i] is typically produced with active lip spreading.

• “Say cheese!”

• What acoustic effect would this have?

• Lips Normal:

• Lips Spread:

• Check ‘em out in Praat.

2. The Jaw• Several different muscles are used to both lower and raise the mandible.

• Primary raisers:

• Masseter

• Temporalis

• Internal pterygoid

2. The Jaw• Several different muscles are used to both lower and raise the mandible.

• Lowerers:

• Anterior belly digastricus

• Geniohyoid

• Mylohyoid

• Note: in lowering, the mandible also retracts.

Articulatory Control• People can produce vowels perfectly fine even when a bite block holds their jaws open. (Lindblom, 1979)

• Adults get the formants right, right from the start...

• But kids need a little time to adjust.

• Abbs et al. (1984) experimented with pulling down people’s jaws...

• when they had to say sequences like [aba] and [afa]!

• Lip muscles adjust immediately for the sudden jaw lowering...

• Adjustment happens faster than electrical signals can travel to the motor cortex and back!

Abbs et al. EMG data

3. The Tongue• The muscles controlling the tongue consist of:

1. Intrinsic muscles

• (completely within the tongue)

2. Extrinsic muscles

• (connect the tongue to outside structures)

• The intrinsic muscles include:

1. The superior longitudinal muscle

2. The inferior longitudinal muscle

3. Transverse muscles

4. Vertical muscles

Tongue: Sagittal View• The superior longitudinal muscle pulls the tongue tip up and back.

• Instrumental in producing alveolars and retroflexes.

• The inferior longitudinal muscle pulls the tongue tip down and back.

• Helps with tongue blade articulations.

Tongue: Coronal View• The transverse muscles pulls in the edges of the tongue, and also lengthens the tongue to some extent.

• Helpful in producing laterals.

• Contraction of the vertical muscles flattens the tongue.

• Interdentals?

Extrinsic #1: Genioglossus• The genioglossus connects the tongue to both the mandible and the hyoid.

• Contraction of the posterior genioglossus moves the whole tongue up and forwards.

• Crucial in palatals.

• Contraction of the anterior genioglossus curls the tongue tip down and back.

Gene-ioglossus

Gene Simmons, of the rock band KISS, is famous for his use of the genioglossus muscle.

Extrinsic #2: Styloglossus• The styloglossus connects the tongue to the “styloid process” in front of the ear.

• Pulls the tongue up and back.

• ...for velar articulations.

• May also help groove (sulcalize) the tongue.

Extrinsic #3: Hyoglossus

• The hyoglossus connects the tongue to the hyoid bone.

• Pulls the tongue down and back.

• = pharyngeals

• Can also pull the sides of the tongue down.

Extrinsic #4: Palatoglossus• The palatoglossus connects the tongue to the soft palate.

• Can be used to raise the back of the tongue.

• And also to lower the velum!

• Lowering the back of the tongue may inadvertently pull the velum down...

• leading to passive nasalization of low vowels.

• Note: Great Lakes vowel shift

Chain Shifting• The Great Lakes Shift is called a chain shift, because first one vowel moves...

• And then a series of others follow.

• In this case, the first shift was:

• Theory: vowels have to stay distinct from one another.

• So listeners can understand what’s being said.

Back to the Shift• The Great Lakes Shift was first noticed in the 1960s.

The Shift, Diagrammed

4. Velar Muscles• The levator palatini raises the velum.

• (connects the velum to the temporal bone)

• The velum is lowered by both the palatoglossus and the palatopharyngeus...

• which connects the palate to the pharynx.