M.I. Morad1, M.E. Ibrahim2, H.G. Elmongui3, M.A. Talaat1, M.K. Eldeeb1

Transduction of a Complex Signal Through

the Normal Cochlea and Through the Cochlear Implant

1Department of Otorhinolaryngiology, Unit of AudiovestibularMedicine, Faculty of Medicine, 2Department of Radiology, Faculty of

Medicine, 3Department of Computer and Systems Engineering; faculty of Engineering; Alexandria university, Egypt

• Acoustic signals reaching CI derivedelectrical Signal stimulates the auditorysystem bypassing sensory receptors

• The ascending auditory system willreprocess the electrical signals generatedby the CI.

• An assumed final outcome = signaldetection, discrimination, recognition &comprehension.

CI device Brain Audition

Auditory Sensory Stimulation

Auditory electrical Stimulation

Auditory learning Auditory learning

• Is initiated by a normal cochlea

• Is accomplished by an impairedcochlea through amplification

• Initiate auditory neural stimulationbypassing the auditory sensory system.

• Subsequently bypassing all cochlearactive mechanisms: sensory afferent andsensory efferent controls as well asmiddle and external ears delay times.

• Shift from the natural sequence ofacoustic signal processing:

• External ear - middle ear - cochlearexcitation patterns and sensorytransduction neural firings:

A derived electrical signal that directly excites the ganglia of the auditory nerve

• Precision with which the brain processesphonological structure of spokenlanguage.

• The neural signatures corresponding totiming and spectral variants in speechneural encoding or representation

• Ability to pick speech in backgroundnoise (skill learned with time, improveswith age)

• Central auditory processing abilitiesrobust/ vulnerable/ poor to challengedlistening.

Auditory exposure Auditory exposure

• cABR is a tool to assess brainstemrepresentation of a complex response.

• The stimulus is a CV syllable andconsists of a transient consonant and asustained vowel parts.

• The response consists of an onset respone(waves I, III and V) followed by thefrequency following response.

• Normal cABR indicates normal brainstemencoding of a complex signal presented tothe brain which will subsequentlyinfluence speech understanding andcommunication.

(Johnson et al., 2005)

• It Infers discrimination of spectro-temporalfluctuations in speech signal.

• It Infers discrimination of sounds withrapid acoustic transitions that are easilyconfused e.g stop consonants (momentarystop/ rapid release of airflow).

n= 10, 5.6- 10.92 years old, 5 males and 5 females.

Implanted with right Med-EL standard electrodearray ,full insertion depth.

Coding strategy (FS4 temporal weighting).

Subjects with abnormal CT findings due tomalformed cochlea or meningitis were excluded.

• n=2, 11 years old, males.

• Normal hearing age-matched controls.

• Normal click ABR responses.

• Speech syllable 40 msec /da/ was used to elicitspeech ABR

• Stimulus delivered at a repetition rate of2.1/sec with alternating polarity.

• Biologic navigator pro® and contralateralvertical electrode montage were used.

• A loudspeaker for CI monitored throughRadioshack sound level meter at the subject’shead and right TDH headphones for norms.

• Responses were online bandpass filtered by a30-500 Hz. I/O Latency and RMS-intensityfunctions were done.

• Non-contrast multislice CT of the petrousbones was performed to affirm full electrodeinsertion depth.

cABR Wave V latency: range 1.81 - 4.82 msec at 70 dBHL with a mean = 2.77± 1.06

cABR Trace reproducibility: was maximal at moderate and high intensities (up to

99.65% ) at 60 dB HL

• cABR – stimulus /da/ correlation: range4.55% -27.74% with a = 16.62 ± 0.05

• cABR FFR– vowel correlation: range14.22% -29.39% at 60 and 70 dB HL with a= 19.90% ± 7.62%

• cABR FFR– vowel correlation in normsranged from 20-30% at a delay range of 5.6and 8.1 ms. (Cunningham et al., 2001)

• The growth function of the biologicalsignal(cABR), measured by RMSamplitude, that parallels signal intensitymay be an indication of:

A. Well Developed auditory pathwaywith increased neural density andconsequently increased voltagecapacity. This may reflect theimportance of early stimulation andits organizing factors

B. Decreased RMS may indicate decreasedsurviving neural population which willinfluence the performance with thecochlear implant.

• The biological signal in cochlearimplantees follows the acoustics of thesignal and presents a heterogeneouslatency shift which is less than normsdue to absence of acoustic delay ofexternal and middle ear transfer timesand cochlear travelling wave.

• Variation of response reproducibilitymay reflect a low fidelity neural systemaffected by the etiology of hearing loss.

Auditory experience

Auditory experience

To cochlear implant unit, faculty of medicine, Alexandria university for providing the

participants of the study To Dr. Nina Kraus lab for providing the

brainstem toolbox

• Response correlation to speech stimulusreflects on:

A. Fidelity of CI processing of thatsignal.

B. Integrity and fidelity of brainstemprocessing.

Lag between traces in msec: ideally= zero

Study Lag range Study trace correlation

- 18.875- 21.625 msec( threshold included)

Percentage correlation , ideally almost 100%

- 0.375 - 0.375 msec

17.64% - 99.65% ( threshold included)

31.58%- 99.65%