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Audition

• Chris Rorden• Deb Hall, MRC Institute of Hearing Research

• Anatomy and function of the auditory system• Brainstem disorders• Word deafness• Amusia• Environmental sound agnosia• Auditory neglect and extinction

www.mricro.com

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Anatomy and function source : Ashmore, 2002

The ‘ear’ is a complex physiological apparatus, not just the visible outer ear

- Reflection of sound in pinna (earlobe) provides spectral cues about elevation of a sound source- Middle ear is a cavity containing an ossicular lever which matches the acoustical impedance of the inner ear so sound energy is effectively transmitted (60%)- Inner ear contains the cochlea where sound is converted into a neural signal

Ear structures

Peripheral– Outer ear– Middle ear– Inner ear– Auditory nerve

Central– Brainstem– Midbrain– Cerebral

Localization and shadowing

– High frequencies: intensity differences: louder if sound is not in head’s sound shadow

– Low frequencies: Inter-aural timing differences– Elevation: Frequencies influenced by location

relative to pinna.

Middle Ear - Ossicles

3 of the smallest bones– Malleus (hammer)– Incus (anvil)– Stapes (stirrup)

Ossicular chain: Transmits acoustic energy from tympanic membrane to inner ear– Delivers sound vibrations to inner ear fluid– Changes impedance: large, weak

movement of ear drum turned to small, forceful movement in cochlear liquid.

– Muscles can dampen response: Prevents the inner ear from being overwhelmed by excessively strong vibrations

Cochlea and neighbors

Tonotopic

Base High Freq

– Apex– Low Freq.

Travelling wave

Always starts at the base of the cochlea and moves toward the apex

Its amplitude changes as it traverses the length of the cochlea

The position along the basilar membrane atwhich its amplitude is highest depends on the frequency of the stimulus

Traveling wave

High frequencies have peak influence near base and stapes

Low frequencies travel further, have peak near apex

A short movie:– www.neurophys.wisc.edu/~ychen/auditory/animation/animationmain.html

– Green line shows 'envelope' of travelling wave: at this frequency most oscillation occurs 28mm from stapes.

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Anatomy and function source : Hackney, 2002

Many sound features are encoded before the signal reaches the cortex

•Cochlear nucleus segregates sound information•Signals from each ear converge on the superior olivary complex - important for sound localization•Inferior colliculus is sensitive to location, absolute intensity, rates of intensity change, frequency - important for pattern categorization•Descending cortical influences modify the input from the medial geniculate nucleus - important as an adaptive ‘filter’

inferior colliculus

medial geniculate body

cortex

superior olivary complex

cochlea

cochlear nucleuscomplex

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Anatomy source : Palmer & Hall, 2002

Primary & non-primary auditory cortex

Sylvian Fissure

Superior Temporal Gyrus

Superior Temporal Sulcus

Medial Temporal Gyrus

Right hemisphere

Heschl’s gyrus (primary AC)

planum temporale (nonprimary AC)

planum polare (nonprimary AC)

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Function source : Palmer & Hall, 2002

• Numerous bilateral regions are frequency-dependent• Overlapping regions are sensitive to intensity and to the temporal changes in sound• One region is sensitive to the spatial properties of sound (R>L) • Speech also activates these regions, but neurons are probably responding to the complex acoustic properties in the sound.•Perceptual attributes may be important

Slow-rate temporal pattern in sound

L

L

LH LH H

Right hemisphere

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Sound intensity and activation Loud sounds (90 dB) activated posterior and medial

temporal gyrus (red) Soft (70 dB) sounds activated area (yellow) is found most

laterally of TTG Medium intensity (82 dB) sounds activated intermediate

area (green). (NeuroImage 2002;17: 710)

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Auditory neuropsychology

Simple modularity of function not clearly apparent- No auditory equivalents of V4 (visual colour area), V5 (visual motion area), fusiform face area etc- Cortical neurons respond to a complex array of stimulus features, and the temporal pattern of those features is important

● Unlike visual or somatomotor systems- A lot of auditory processing is supported by the ascending pathway- Studies in several mammalian species have demonstrated that bilateral ablations of the auditory cortex have little effect on simple sound intensity and frequency-based behaviours

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Brainstem disorders source : Griffiths et al. 1999

Brainstem = cochlear nucleus, superior olivary complex, inferior colliculus

Lesions rarely compatible with lifeMultiple sclerosis can affect brainstem

- Complete deafness is rare- MS patients do not report problems in everyday sound perception - Few systematic studies- Deficit in perceiving frequency changes- Deficit in detecting a gap in noise- Deficit in processing binaural cues for sound localisation

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Auditory agnosia

A deficit in recognition

Acoustical analysis RepresentationsAuditoryinput

Perception Recognition

“Apperceptiveagnosia”

“Associativeagnosia”

Auditory agnosia is of this type

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Auditory agnosia source : Griffiths et al. 1999

Normal brainstem processingMidbrain impairment questionableCortical deficit in perception

- Preserved hearing (pure tones)

- Disordered perception of certain sounds :

Speech - word deafness

Music - amusia

Environmental sounds - environmental sound agnosia

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A case of word deafness source: Ellis & Young, 1988

Hemphill and Stengel (1940) “I can hear you dead plain, but I cannot get what you say. The noises are not quite natural. I can hear but not understand”

- Normal pure tone audiometry

- Fluent speech “no errors of grammar beyond what is common for his particular dialect and standard of education”

- Normal reading

- Normal writing and spelling

- Poor spoken word repetition

- Gross asymmetry between spoken and written word comprehension

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Word deafness source : Ellis & Young, 1988

Associated symptoms- Some hearing loss (> 20 dB HL)

- Production (Broca’s) aphasia

- Perception of melody

- Perception of environmental sounds

Lesion site- Generally large bilateral infarcts

- When unilateral, it’s more often the left hemisphere

- Involves superior temporal lobe (non-primary auditory cortex)

- May or may not involve Heschl’s gyrus (primary auditory cortex)

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Word deafness

- filtered harmonic sounds, broad band noise, silent gaps - transitions in amplitude and frequency on three time scales

(milliseconds, 10s of milliseconds, seconds)

These temporal transitions are rapid and complex

Freq

uency

(kH

z)

Time0

8

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Word deafness source : Ellis & Young, 1988

Inability to make fine temporal discriminations and track rapidly-changing acoustic signals?

“There may be nothing speech specific about the impairment” Ellis & Young, 1988

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A case of amusia source : Peretz, 1993

Patient CN

Symptoms- Unable to recognise even simplest tune- Unable to sing children’s songs that she had known well- No deficit in everyday verbal communication- No deficit in everyday recognition of environmental sounds

Lesion site- Bilateral temporal lobe damage- When unilateral, it’s more often the right hemisphere

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Amusia source : Peretz, 1993

Dissociation within musical perception- Right injury - Deficit in melody perception: the variations in pitch

- Left injury - Deficit in rhythm perception: the temporal organisation of melody over 100s of milliseconds or seconds time scale

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Amusia

As in speech, music contains discrete harmonic sounds that vary over time

-melody: local variation in features from note to note- rhythm: global variations in note duration that relate to a higher order pattern

Frequ

ency

Time

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Environmental sound agnosia

Deficit rarely occurs in isolationsource : Griffiths et al. 1999

Environmental sounds contain fewer changes in acoustic structure over time than an equivalent length segment of speech or music

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A common deficit? No! source : Peretz, 1993

Word deafness, amusia and environmental sound agnosia are distinct- speech and music can dissociate after brain damage

- music and environmental sounds can dissociate after brain damage

- environmental sound perception can be selectively spared

- recovery can follow different patterns (e.g. environmental sounds, then music then speech or in the reverse order)

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A common deficit? Yes! source : Griffiths et al. 1999

Word deafness, amusia and environmental sound agnosia probably co-occur- May not always be report because not all abilities are tested

All 3 types of sound contain a mixture of acoustic features

Deficit in an intermediate level of analysis, which is rarely tested- Analysing the spectro-temporal pattern in sound

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Symptoms(a) Rightward biases in sound localization(b) Poor relative judgements for sounds on the contralesional side(c) Poor elevation judgements for sounds on the contralesional side

Failure to detect contralesional sound, when presented concurrentlyPoor allocation of attention to sounds separated in time

Auditory neglect source : Pavani et al., 2003

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Lesion site – usually right hemisphere- inferior parietal lobe- superior temporal gyrus- temporo-parietal junction

Auditory neglect source : Pavani et al., 2003

30Auditory & visual neglect : A common deficit? Yes! source : Pavani et al. 2003

Many neglect patients exhibit auditory, as well as visual, deficits.

Correlation between severity of clinical visual neglect and experimental auditory neglect measures.

“Neglect can often be caused by damage to brain regions containing multisensory representations of space, with the deficit consequently manifesting across multiple sensory modalities, with correlated severity”.

auditory deficit

vis

ual defici

t

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Symptom- a chronic bias of spatial attention towards the ipsilesional side

Hence, ipsilesional events are perceived earlier than physically synchronous contralesional stimuli. This can be measured using the temporal order judgements test.

Visual extinction source : Rorden et al., 1997

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The same deficit is also found in audition

……and over the same time scale(~ 200 ms)

Visual extinction source : Karnath et al., 2002

33Auditory & visual extinction : A common deficit? Possibly! source : Karnath et al. 2002

Visual and auditory extinction have not been studied in the same patients

...but delay is of the same time scale

“It seems that the costs for information processing of contralesional events in extinction, induced by the bias of spatial attention towards the ipsilesional side, affect awareness of visual as well as auditory events to a similar degree.”

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Seifritz et al. 2002

Heschl’s Gyrus shows sustained response to sounds, surrounding regions respond to onset.

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Key references

(1) Signals and Perception 2002Ch1 The mechanisms of hearing by AshmoreCh3 From cochlea to cortex by HackneyCh4 Imaging central auditory function by Palmer & Hall

(2) Griffiths et al., Disorders of human complex sound processing Neurocase 5: 365-378, 1999

(3) Human Cognitive Neuropsychology by Ellis & Young 1988Ch6 Recognising and understanding spoken words

(4) Thinking in sound: The cognitive psychology of human audition Editors: McAdams & Bigand 1993Ch7 Auditory agnosia: A functional analysis by Peretz

(5) Pavani et al., Auditory and multisensory aspects of visuospatial neglect. Trends in Cognitive Sciences 7:407-414, 2003

(6) Karnath et al., Impaired perception of temporal order in auditory extinction. Neuropsychologia 40: 1977-1982 2002

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Additional references

(7) Review of functional organisation of the auditory cortexHall et al., Relationships between human auditory cortical structure and function. Audiology and Neuro-otology 8: 1-18, 2003

(8) Case studies of auditory agnosia References to many original papers can be found in (2) Griffiths et al., Disorders of human complex sound processing Neurocase 5: 365-378, 1999

(9) A case of non-spatial auditory neglect Cusack et al., Neglect between but not within auditory objectsJournal of Cognitive Neuroscience 12: 1056-1065 2000

(10) Temporal order judgement deficits in visual neglectRorden et al., Visual extinction and prior entry: impaired perception of temporal order with intact motion perception after unilateral parietal damage. Neuropsychologia 35: 421-433 1997