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(1)

Neural Prosthetic Engineering

Today

• Review

– Intracochlear electrode array

• Questions on homeworks or projects?

• System continued

– Digital filtering – Mapping

– Telemetry – stimulator

1

(2)

Review

(3)

Neural Prosthetic Engineering

In block diagram form,

• A brief conceptual block diagram

3

(4)

Spatial Specificity

• the degree of spread of neural activity across a place in cochlea that responds to a stimuli from an electrode site

• Spatial specificity of stimulation depends on…

– The number and distribution of surviving ganglion cells

– Whether neural processes peripheral to the ganglion cells are present or not

– The proximity of the electrodes to the target neurons

– The electrode coupling configuration

(monopolar, bipolar)

(5)

Neural Prosthetic Engineering

Straight vs. Pre-curved

5

■ Types of cochlear electrode array - Straight vs Pre-curved

Straight cochlear electrode array (by Med-El)

Pre-curved cochlear electrode array (by Cochlear Corporation)

Straight types

– Deep insertion

– Far from target cells – Lateral wall insertion

Pre-curved types

– Close to target cells

– Using stylet or sheath to insert – Perimodiolar insertion

(6)

Peri-modiolar Placement

• Positioning of electrodes in ST

– Place close to inner wall of ST to minimize the distance between electrodes and SG

• Maximize the number of largely non-overlapping populations of neurons

• Improve spatial specificity of stimulation

• Reduce threshold voltage

• Increase battery life

(7)

Neural Prosthetic Engineering

Performance vs. Number of channels

7

- 8 is enough

L.M.Friesen et al., J. Acoust. Soc. Am, 2001

(8)

Auditory system

Auditory system - Wikipedia

(9)

Neural Prosthetic Engineering

Cochleostomy and round window approach

Auditory cortex 9

Cochlear Electrode Array

Oval Window

Round Window

Cochleostomy

(10)

Cochlear Implant

System & Circuit

(11)

Neural Prosthetic Engineering

A Brief Conceptual Block Diagram

Microphone Analog

pre-processor

Speech External audio signal

(phone, MP3 player)

hardwareDSP Power Amplifier

Transmission Coil

Receiver Receiver-stimulator Coil

Integrated circuit Intra-Cochlear

electrode array External

Internal Forward and backward telemetry

11

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Cochlear Implant

Components in cochlear implant

Microphone

DSP processor

Inductive coil

ASIC chip

Electrode arrays

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Neural Prosthetic Engineering

Cochlear Implant – General System Overview

Wearable Speech Processor Microphone

External Coil

Internal Coil

Implantable Current Stimulator

Inserted

Electrode array

① Sound Signal

② RF Modulation

③ Data & Power Transmission

⑤ Stimulation Pulse train

④ Signal Demodulation

⑥ Auditory Cortex

Nurobiosys Corp., Korea13

(14)

Trying to make these waveforms appear at the electrodes

 Waveforms at various points in the system. Top trace in each panel shows a speech input measured at an output node of the analog preprocessor, and the bottom three traces in each

panel show stimuli for the three of the eight channels of

stimulation The lower panel

shows the interlacing of stimulus pulses across channels using an expanded time scale. The

segment shown is indicated by the dotted rectangle in the upper panel.

(15)

Neural Prosthetic Engineering

N

B

Signal Processing performed within the DSP hardware.

A/D conversion

Ex) 10 bit ADC with Sampling frequency of 17 kHz

FFT (Fast Fourier Transform)

Power spectral density extraction

Transform time domain into frequency domain

Average power calculation (the channel-to-frequency allocation rule)

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑝𝑜𝑤𝑒𝑟 = σ 𝑝𝑜𝑤𝑒𝑟 𝑠𝑝𝑒𝑐𝑡𝑟𝑎𝑙 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑜𝑛𝑒 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑏𝑎𝑛𝑑 𝑠𝑖𝑧𝑒 # 𝑜𝑓 𝑠𝑝𝑒𝑐𝑡𝑟𝑎𝑙 𝑝𝑒𝑎𝑘𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑐ℎ𝑎𝑛𝑛𝑒𝑙

Nonlinear mapping

Stimulation level = log (average channel power)

conversionA/D FFT Average power

calculation Nonlinear

mapping Stimulation level for each channel Speech signal

15

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Mapping

 Log mapping

 Log. mapping is to define the stimulation level according to the average channel power.

 Power low function is used.

𝑦 = 𝐴𝑥𝑝 + 𝐵

𝑤ℎ𝑒𝑟𝑒 𝐴 = 𝐶 − 𝑇

𝑥𝑝𝑚𝑎𝑥 − 𝑥𝑝𝑚𝑖𝑛 , 𝐵 = 𝑇 − 𝐴𝑥𝑝𝑚𝑖𝑛,

𝑝 = 0.2

Average Channel Power

Stimulation Level

T level C level

Minpower Maxpower

Volume 6 (Default)

(17)

Neural Prosthetic Engineering

M APPING - EXAMPLE

0

E16

0

E1 E02 E03 E04 E05 E06 E07 E08 E09 E010 E011 E012 E013 E014 E015

255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 255

Cochlear Implant Mapping

C-level T-level 180

80 200 100 160

40 170

50 180

80 180

80 180

80 180

80 180

80 180 200 80

100 200

160 100 220 40

120 190

90 190

90

Frequency Allocation

T-level button C-level button

Electrode Frequency range (Hz)

E1 300- 450

E2 400-550

E3 550-700

E4 700-850

E5 850-1000

E6 1000-1250

E7 1250-1500

E8 1500-1800

E9 1800-2200

E10 2200-2600

E11 2600-3200

E12 3200-3800

E13 3800-4600

E14 4600-5500

E15 5500-6500

E16 6500-7800

(18)

Power Amplifier

 Class E amplifier

 Class E operation: the loss in the switching element of the converter becomes very low.

PWM signal

Lchoke

Cseries External coil

(19)

Neural Prosthetic Engineering

N

B

Telemetry-later-use separate file

 Data can be transferred via inductive link simultaneously with power

 Bi-directional Telemetry

Regulator, Rectifier Envelope

Detector

Load

(20)

Block Diagram of the Internal Device

Transmitter (external)coil

Receiver Coil (internal)

Trans- former

Data Recovery and power

generator

Digital logic circuit

Current Stimulator (Switches and current sources)

Intra-cochlear electrode array Current Stimulator chip (ASIC)

(21)

Neural Prosthetic Engineering

N

B

Current source

 Current mirror scheme

 Binary-weighted transistor

 0 ~ 1.86 mA 255 levels, 7.3 µA step

I I 2I 4I 128I

I 2I 4I 128I

0 ~ 255I

21

(22)

Current Stimulator

 Cathodic stimulation

 Current flow – ref  ch

ref ch

ON

OFF

OFF

ON

(23)

Neural Prosthetic Engineering

N

B

Current Stimulator

 Anodic stimulation

 Current flow – ch  ref

ref ch

ON OFF

OFF ON

23

(24)

Current Stimulator

 Overall schematic

ref ch0 ch1 ch2 ch15

(25)

Neural Prosthetic Engineering

N

B

Result

 Waveforms at various points in the system. Top trace in each panel shows a speech input measured at an output node of the analog preprocessor, and the bottom three traces in each

panel show stimuli for the three of the eight channels of

stimulation The lower panel

shows the interlacing of stimulus pulses across channels using an expanded time scale. The

segment shown is indicated by the dotted rectangle in the upper panel.

25

(26)

Reference

• F.G.Zeng et al., (IEEE Reviews in Biomedical Engineering, 2008)

• P.C.Loizou, (IEEE Engineering in Medicine and biology, 1999)

• B. Wilson et al., (Nature, 1991)

• D.Riss et al., (Otology & Neurotology, 2011)

• N.B.Frederigue, (Brazilian J. Otorhinolaryngology, 2003)

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