Stimulation Management for a Multichannel Vestibular ... - CLoNS
Stimulation Management for a Multichannel Vestibular Neural Prosthesis Dai Jiang1, Andreas Demosthenous1, Timothy Perkins2 and Nick Donaldson2 1
Department of Electronic and Electrical Engineering Department of Medical Physics and Bioengineering University College London London, United Kingdom [email protected], [email protected], [email protected] 2
Abstract—This paper describes the stimulation management unit for a multichannel vestibular neural prosthesis. The unit is designed as part of a stimulator ASIC in the implantable subsystem of the prosthesis. This digital unit provides the stimulator ASIC the ability to generate biphasic current pulses at specified amplitude, duration and pulse rate to drive electrodes in the semicircular canals. The circuit was implemented in 0.6-µm CMOS technology and post-layout simulations are presented to show its operation.
I.
INTRODUCTION
The vestibular system, located in the inner ear, is a sensory system which contributes to the body balance and sense of spatial orientation. The system senses the head motion and orientation, providing information to the nerve system in order to control the muscles to keep the balance of body, and to control the eyes to sustain a stable vision during movement. Loss of vestibular sensation as a result of an insult to the inner ear (e.g., infection, ototoxic drug exposure, etc), yields postural instability, a blurred vision during head motion and chronic disequilibrium. Neural prostheses using artificial stimulation could significantly improve the quality of life of people affected by vestibular disorders. A closer look of the natural mechanism of the vestibular system indicates the approach to implementing such prostheses. The vestibular system contains two sub-systems: the semicircular canal system which senses rotation movement, and the otolithic organs which indicate linear accelerations. The structure of the vestibular system is shown in Fig 1. The semicircular canal system consists of three orthogonally placed semicircular canals, which sense angular velocity along three axes, while the two otolithic organs, the Saccule and the Utricle, measure the gravitational force and inertial force due to linear acceleration along the three axes [2]. The canals are filled with fluid which gives inertial forces during head motion. These forces are collected by hair cells in the vestibular organs which work as motion sensors, and the head motion is therefore interpreted by the nerve system.
Fig 1 Illustration of a vestibular system.
By applying artificial neural stimulation to the vestibular nerves to represent the head motion, dysfunctional organs could be bypassed and vestibular function could be restored. Similar neural stimulation approaches have proved successful in cochlear prostheses to restore hearing [3]. Research on developing vestibular prostheses only emerged about a decade ago. The work of developing a prototype vestibular prosthesis was published by Gong and Merfeld in 2000 [1]. This prosthesis was built with off-theshelf discrete components aiming at one semicircular canal stimulation. A multichannel semicircular canal prosthesis was report later by Santina et al which aimed at restoring threedimensional (3-D) vestibular sensation [2]. This prosthesis was also developed at board-level. Shkel et al [4] have developed an electronic prosthesis equipped with customdesigned MEMS sensors which are suitable for implantable devices. More recently, work on developing a neural implant ASIC was published, aiming towards a fully implantable solution for multichannel vestibular prostheses [5]-[6]. The work presented in this paper is part of an EU funded project aiming to develop an artificial device for human implantation to restore 3-D vestibular sensation. The device will mimic the vestibular function by stimulating the nerves in the semicircular canals in the inner ear using the
This work was funded by the European Commission under grant number 225929 – CLONS.
978-1-4244-5309-2/10/$26.00 ©2010 IEEE
3481
information provided by sensors attached to the head [7]. As part of the implantable subsystem, a stimulator ASIC is being developed for multichannel stimulation on the three semicircular canals. This paper presents the stimulation management unit of this ASIC to perform stimulations with fine resolution on both the amplitude and pulse rate. The remaining sections of the paper are organised as follows. Section II describes the design of the stimulation management unit including the communication protocol. Section III presents post-layout simulations, followed by conclusions in Section IV. II.
STIMULATION MANAGEMENT UNIT DESIGN
The top-level architecture of the stimulator ASIC is shown in Fig 2. The stimulator consists of three pulse generators functioning in parallel with each generator as a stimulator for one semicircular canal. These three generators are identical, each consisting of a stimulation management unit, a current source and an output stage. The current source generates the stimulation current with required amplitude. The output stage uses this current to form biphasic chargebalanced current pulses through the electrodes in the semicircular canal. The profile of the current pulses, including the pulse amplitude, width, interphase delay and pulse rate, is controlled by a stimulation management unit. This management unit receives and decodes pulse-setting command frames from the external device, which are transmitted via the serial data input ‘Data’ clocked at ‘DCLK’. Each command contains setting information for one canal and commands are distinguished by the ‘Canal Select’ signal. The stimulation on each semicircular canal is multichannel. Channel control is also part of the function of the stimulation management unit. The function of the stimulation management unit comprises two stages: data loading and pulse generation control. The data loading stage receives and stores the settings information of pulse profiles, while the pulse generation control stage controls the current source and output stage to generate pulses at specified amplitude, width and rate. The two stages operate with non-synchronized clocks. The clock for data loading is synchronized to the serial data input at 100 kHz, and the clock for pulse generation control is an internal clock with a faster speed at 1 MHz. A. Communication Protocol The command data to the data loading stage are organised in frames. Fig 3 depicts the construction of a command frame. Each frame consists of three sections: the synchronization word, the data section and the correction part. The frame is transmitted in serial via the serial date input at a rate of 100 kbit/s. While command frames are received by the control unit for all three canals, a command frame contains settings for one canal only. Stimulation for each specific semi-canal is configured with a 2-bit ‘Canal ID’. Once a synchronization word is detected, the data loading stage of the control of each canal verifies the ‘canal
Fig 2 Stimulator ASIC architecture with electrodes.
ID’ against the configuration to decide whether or not to load the frame to the pulse control stage. If the ‘canal ID’ matches, the entire frame is loaded in parallel to the pulse control stage with each setting word stored accordingly. The ‘Channel’ section specifies the multichannel stimulation control in this canal. The pulse profile settings specify the amplitude and width of both the cathodic and anodic phase, as well as the interphase delay and pulse rate. A new command frame is only sent when a change of the stimulation is needed. One frame contains 101 bits. At 100 kHz data clock it takes 1.01 ms to transmit. As one frame controls only one canal, the maximum frame-rate for a 3-D stimulation (i.e., stimulation on all three canals) is around 333.3 frame/s. B. Operation of Stimulation Management The control of pulse generation is performed on four counters as four state machines: pulse rate counter, cathodic counter, interphase counter and anodic counter. Clocked at the internal clock at 1 MHz, the counters load initial values from the stored pulse profile settings at specific time instants then counts down towards 0 with a step size of 1. The values to the cathodic counter, anodic counter and interphase counter are the values from the setting word “Cathodic Pulse Width”, “Anodic Pulse Width” and “Interphase Delay”, respectively. The value to the pulse rate counter is calculated from the “Pulse Rate” setting. The cathodic counter, interphase counter and anodic counter form a chain following the pulse rate counter, which serves as the base state machine for pulse generation control. Fig 4 illustrates the process of generating a biphasic current pulse. The pulse rate counter is initially loaded with the pulse rate setting when a synchronization word is detected by the
3482
Fig 3 Construction of the command frame.
Fig 4 Control sequence of stimulation management.
data loading stage then starts counting down. Once it reaches “0” it loads the updated pulse rate setting and starts counting down again. The pulse generation process starts at four clock cycles before it reaches “0”. At time instant , a stimulation enable signal “stim_EN” is given to switch on the output stage. At time instant , the “Pulse amplitude” setting applies to the current source and “Channel select” to the multichannel control. In the meantime all other three counters load stored pulse profile settings respectively. Once the pulse rate counter is next to “0”, at time instant , the cathodic counter starts counting down and a cathodic pulse enable signal “Cath_stim” is turned to HIGH. Similarly, the interphase counter starts counting down at time instant when the cathodic counter is next to “0” to decide the interphase delay, and the anodic counter starts at time instant for the anodic pulse. Once the anodic phase finishes, the stimulation period completes and “stim_EN” turns LOW. Pulse amplitude setting also turns LOW at the end of the biphasic pulse to switch off the current source. The output stage has four switches to form an ‘H-bridge’ to drive the electrodes. When “stim_EN” is off, all the four switches are off and the electrodes are isolated. During the stimulation period, when “Cath_stim” is on and “Ano_stim” is off, both “Cath_on” switches are on, therefore, the current passes through the top right switch in the bridge to the bottom electrode then the top electrode then the bottom left switch, as highlighted by the red arrow; when “Ano_stim” is on and “Cath_stim” is off, the current passes towards the opposite direction, as highlighted by the blue arrow. A biphasic pulse is therefore generated. The width of both the cathodic and anodic pulse is decided by the duration of “Cath_stim” and
Fig 5 Layout of the stimulation management unit.
“Ano_stim”, respectively. “stim_EN” signal turns on every time by the pulse rate counter, therefore sets the pulse rate. III.
SIMULATION RESULTS
The stimulation management logic was described using Verilog and implemented in Cadence SoC Encounter using a 0.6-µm CMOS technology. The layout is shown as Fig 5. The circuit occupies an area of 1.65 mm2. Post-layout simulations were performed in Cadence NCSim. The stimulation management unit was supplied with serial data input organized in command frames, accompanied by a date input clock at 100 kHz, as well as an operating clock at 1 MHz. These two clocks were non-synchronized. The simulation results are shown in Fig 6. In this figure the stimulation is indicated by the stimulation enable signal “stim_EN” and a biphasic pulse is indicated by “cath_stim” and “ano_stim”, similar to those in Fig 4. After the stimulator
3483
(a)
(b)
(c)
(d) Fig 6 Post-layout simulation on changing the current pulse profile for 3-D stimulation. (a) Baseline pulse rate setting; (b) Pulse rate change on the horizontal canal; (c) Pulse rate change on the posterior and anterior canal; (d) Details of pulse profile change on the posterior and anterior canal.
is turned on, there is no stimulation pulse on all the three canals. The external device firstly sets the pulse rate on all three canals at a baseline frequency of 150 pps to represent the head being without motion [1]. The width for both cathodic and anodic pulse on all the three canals is 200 µs while the amplitude is 200 µA, as shown in Fig 6(a). After 1 second, a command frame is sent to change the pulse rate on the horizontal canal to 250 pps, meaning the head turns along the horizontal panel, as shown in Fig 6(b). After another 2 seconds, two commands are sent to change the pulse rate on both the anterior and posterior canal to 50 pps, shown in Fig 6 (c), indicating a head movement along a vertical panel. The pulse amplitude and width on both canals are also changed. The cathodic pulse and anodic pulse are not symmetrical, with the cathodic pulse having a width of 100 µs and amplitude of 1 mA, while the anodic pulse a width of 500 µs and amplitude of 200 µA. The amplitude setting changes during the interphase delay, as shown in Fig 6 (d). IV.
of the current pulse profile is controlled by the external device. The digital management unit receives command frames from the external device and controls the current source and output stage to generate current pulses as required at the specified rate. The circuit was realised in 0.6-µm CMOS technology occupying an area of 1.65 mm2. Postlayout simulations have been presented to show its operation. REFERENCES [1]
[2]
[3] [4]
[5]
CONCLUSIONS
This paper describes the design of a stimulation management unit as part of a stimulator ASIC for an implantable vestibular neural prosthesis. The management unit provides the stimulator ASIC the ability to generate highly flexible biphasic current pulses with fine resolutions for each canal to restore 3-D vestibular sensation. The setting
[6]
[7]
3484
W. Gong and D. Merfeld, “A prototype neural semicircular canal prosthesis using patterned electrical stimulation,” Annals of Biomedical Engineering, vol. 28, pp. 572–581, 2000. C. C. Della Santina, A. A. Migliaccio, and A. H. Patel, “A Multichannel Semicircular Canal Neural Prosthesis Using Electrical Stimulation to Restore 3-D Vestibular Sensation,” IEEE Transactions on Biomedical Engineering, vol. 54, no. 6, pp. 1016–1030, 2007. http://www.medel.com/ A. M. Shkel, and F.-G. Zeng, “An Electronic Prosthesis Mimicking the Dynamic Vestibular Function,” Audiology and Neurotology, vol. 11, pp. 113–122, 2006. T. G. Constandinou, J. Georgiou and C. Toumazou, “A Partial-CurrentSteering Biphasic Stimulation Driver for Vestibular Prostheses,” IEEE TBCAS, vol. 2, no. 2, pp. 1–8, 2008. T. G. Constandinou, J. Georgiou and C. Toumazou, “A Neural Implant ASIC for the Restoration of Balance in Individuals With Vestibular Dysfunction,” Proc. ISCAS 2009, pp.641-644, May 2009. S. Micera et al, “A Closed-Loop Neural Prostheses for Vestibular Disorders,” Proc. 2009 European Future Technologies Conference (FET 09), April, 2009.
Department of Electronic and Electrical Engineering Department of Medical Physics and Bioengineering University College London London, United Kingdom [email protected], [email protected], [email protected] 2
Abstract—This paper describes the stimulation management unit for a multichannel vestibular neural prosthesis. The unit is designed as part of a stimulator ASIC in the implantable subsystem of the prosthesis. This digital unit provides the stimulator ASIC the ability to generate biphasic current pulses at specified amplitude, duration and pulse rate to drive electrodes in the semicircular canals. The circuit was implemented in 0.6-µm CMOS technology and post-layout simulations are presented to show its operation.
I.
INTRODUCTION
The vestibular system, located in the inner ear, is a sensory system which contributes to the body balance and sense of spatial orientation. The system senses the head motion and orientation, providing information to the nerve system in order to control the muscles to keep the balance of body, and to control the eyes to sustain a stable vision during movement. Loss of vestibular sensation as a result of an insult to the inner ear (e.g., infection, ototoxic drug exposure, etc), yields postural instability, a blurred vision during head motion and chronic disequilibrium. Neural prostheses using artificial stimulation could significantly improve the quality of life of people affected by vestibular disorders. A closer look of the natural mechanism of the vestibular system indicates the approach to implementing such prostheses. The vestibular system contains two sub-systems: the semicircular canal system which senses rotation movement, and the otolithic organs which indicate linear accelerations. The structure of the vestibular system is shown in Fig 1. The semicircular canal system consists of three orthogonally placed semicircular canals, which sense angular velocity along three axes, while the two otolithic organs, the Saccule and the Utricle, measure the gravitational force and inertial force due to linear acceleration along the three axes [2]. The canals are filled with fluid which gives inertial forces during head motion. These forces are collected by hair cells in the vestibular organs which work as motion sensors, and the head motion is therefore interpreted by the nerve system.
Fig 1 Illustration of a vestibular system.
By applying artificial neural stimulation to the vestibular nerves to represent the head motion, dysfunctional organs could be bypassed and vestibular function could be restored. Similar neural stimulation approaches have proved successful in cochlear prostheses to restore hearing [3]. Research on developing vestibular prostheses only emerged about a decade ago. The work of developing a prototype vestibular prosthesis was published by Gong and Merfeld in 2000 [1]. This prosthesis was built with off-theshelf discrete components aiming at one semicircular canal stimulation. A multichannel semicircular canal prosthesis was report later by Santina et al which aimed at restoring threedimensional (3-D) vestibular sensation [2]. This prosthesis was also developed at board-level. Shkel et al [4] have developed an electronic prosthesis equipped with customdesigned MEMS sensors which are suitable for implantable devices. More recently, work on developing a neural implant ASIC was published, aiming towards a fully implantable solution for multichannel vestibular prostheses [5]-[6]. The work presented in this paper is part of an EU funded project aiming to develop an artificial device for human implantation to restore 3-D vestibular sensation. The device will mimic the vestibular function by stimulating the nerves in the semicircular canals in the inner ear using the
This work was funded by the European Commission under grant number 225929 – CLONS.
978-1-4244-5309-2/10/$26.00 ©2010 IEEE
3481
information provided by sensors attached to the head [7]. As part of the implantable subsystem, a stimulator ASIC is being developed for multichannel stimulation on the three semicircular canals. This paper presents the stimulation management unit of this ASIC to perform stimulations with fine resolution on both the amplitude and pulse rate. The remaining sections of the paper are organised as follows. Section II describes the design of the stimulation management unit including the communication protocol. Section III presents post-layout simulations, followed by conclusions in Section IV. II.
STIMULATION MANAGEMENT UNIT DESIGN
The top-level architecture of the stimulator ASIC is shown in Fig 2. The stimulator consists of three pulse generators functioning in parallel with each generator as a stimulator for one semicircular canal. These three generators are identical, each consisting of a stimulation management unit, a current source and an output stage. The current source generates the stimulation current with required amplitude. The output stage uses this current to form biphasic chargebalanced current pulses through the electrodes in the semicircular canal. The profile of the current pulses, including the pulse amplitude, width, interphase delay and pulse rate, is controlled by a stimulation management unit. This management unit receives and decodes pulse-setting command frames from the external device, which are transmitted via the serial data input ‘Data’ clocked at ‘DCLK’. Each command contains setting information for one canal and commands are distinguished by the ‘Canal Select’ signal. The stimulation on each semicircular canal is multichannel. Channel control is also part of the function of the stimulation management unit. The function of the stimulation management unit comprises two stages: data loading and pulse generation control. The data loading stage receives and stores the settings information of pulse profiles, while the pulse generation control stage controls the current source and output stage to generate pulses at specified amplitude, width and rate. The two stages operate with non-synchronized clocks. The clock for data loading is synchronized to the serial data input at 100 kHz, and the clock for pulse generation control is an internal clock with a faster speed at 1 MHz. A. Communication Protocol The command data to the data loading stage are organised in frames. Fig 3 depicts the construction of a command frame. Each frame consists of three sections: the synchronization word, the data section and the correction part. The frame is transmitted in serial via the serial date input at a rate of 100 kbit/s. While command frames are received by the control unit for all three canals, a command frame contains settings for one canal only. Stimulation for each specific semi-canal is configured with a 2-bit ‘Canal ID’. Once a synchronization word is detected, the data loading stage of the control of each canal verifies the ‘canal
Fig 2 Stimulator ASIC architecture with electrodes.
ID’ against the configuration to decide whether or not to load the frame to the pulse control stage. If the ‘canal ID’ matches, the entire frame is loaded in parallel to the pulse control stage with each setting word stored accordingly. The ‘Channel’ section specifies the multichannel stimulation control in this canal. The pulse profile settings specify the amplitude and width of both the cathodic and anodic phase, as well as the interphase delay and pulse rate. A new command frame is only sent when a change of the stimulation is needed. One frame contains 101 bits. At 100 kHz data clock it takes 1.01 ms to transmit. As one frame controls only one canal, the maximum frame-rate for a 3-D stimulation (i.e., stimulation on all three canals) is around 333.3 frame/s. B. Operation of Stimulation Management The control of pulse generation is performed on four counters as four state machines: pulse rate counter, cathodic counter, interphase counter and anodic counter. Clocked at the internal clock at 1 MHz, the counters load initial values from the stored pulse profile settings at specific time instants then counts down towards 0 with a step size of 1. The values to the cathodic counter, anodic counter and interphase counter are the values from the setting word “Cathodic Pulse Width”, “Anodic Pulse Width” and “Interphase Delay”, respectively. The value to the pulse rate counter is calculated from the “Pulse Rate” setting. The cathodic counter, interphase counter and anodic counter form a chain following the pulse rate counter, which serves as the base state machine for pulse generation control. Fig 4 illustrates the process of generating a biphasic current pulse. The pulse rate counter is initially loaded with the pulse rate setting when a synchronization word is detected by the
3482
Fig 3 Construction of the command frame.
Fig 4 Control sequence of stimulation management.
data loading stage then starts counting down. Once it reaches “0” it loads the updated pulse rate setting and starts counting down again. The pulse generation process starts at four clock cycles before it reaches “0”. At time instant , a stimulation enable signal “stim_EN” is given to switch on the output stage. At time instant , the “Pulse amplitude” setting applies to the current source and “Channel select” to the multichannel control. In the meantime all other three counters load stored pulse profile settings respectively. Once the pulse rate counter is next to “0”, at time instant , the cathodic counter starts counting down and a cathodic pulse enable signal “Cath_stim” is turned to HIGH. Similarly, the interphase counter starts counting down at time instant when the cathodic counter is next to “0” to decide the interphase delay, and the anodic counter starts at time instant for the anodic pulse. Once the anodic phase finishes, the stimulation period completes and “stim_EN” turns LOW. Pulse amplitude setting also turns LOW at the end of the biphasic pulse to switch off the current source. The output stage has four switches to form an ‘H-bridge’ to drive the electrodes. When “stim_EN” is off, all the four switches are off and the electrodes are isolated. During the stimulation period, when “Cath_stim” is on and “Ano_stim” is off, both “Cath_on” switches are on, therefore, the current passes through the top right switch in the bridge to the bottom electrode then the top electrode then the bottom left switch, as highlighted by the red arrow; when “Ano_stim” is on and “Cath_stim” is off, the current passes towards the opposite direction, as highlighted by the blue arrow. A biphasic pulse is therefore generated. The width of both the cathodic and anodic pulse is decided by the duration of “Cath_stim” and
Fig 5 Layout of the stimulation management unit.
“Ano_stim”, respectively. “stim_EN” signal turns on every time by the pulse rate counter, therefore sets the pulse rate. III.
SIMULATION RESULTS
The stimulation management logic was described using Verilog and implemented in Cadence SoC Encounter using a 0.6-µm CMOS technology. The layout is shown as Fig 5. The circuit occupies an area of 1.65 mm2. Post-layout simulations were performed in Cadence NCSim. The stimulation management unit was supplied with serial data input organized in command frames, accompanied by a date input clock at 100 kHz, as well as an operating clock at 1 MHz. These two clocks were non-synchronized. The simulation results are shown in Fig 6. In this figure the stimulation is indicated by the stimulation enable signal “stim_EN” and a biphasic pulse is indicated by “cath_stim” and “ano_stim”, similar to those in Fig 4. After the stimulator
3483
(a)
(b)
(c)
(d) Fig 6 Post-layout simulation on changing the current pulse profile for 3-D stimulation. (a) Baseline pulse rate setting; (b) Pulse rate change on the horizontal canal; (c) Pulse rate change on the posterior and anterior canal; (d) Details of pulse profile change on the posterior and anterior canal.
is turned on, there is no stimulation pulse on all the three canals. The external device firstly sets the pulse rate on all three canals at a baseline frequency of 150 pps to represent the head being without motion [1]. The width for both cathodic and anodic pulse on all the three canals is 200 µs while the amplitude is 200 µA, as shown in Fig 6(a). After 1 second, a command frame is sent to change the pulse rate on the horizontal canal to 250 pps, meaning the head turns along the horizontal panel, as shown in Fig 6(b). After another 2 seconds, two commands are sent to change the pulse rate on both the anterior and posterior canal to 50 pps, shown in Fig 6 (c), indicating a head movement along a vertical panel. The pulse amplitude and width on both canals are also changed. The cathodic pulse and anodic pulse are not symmetrical, with the cathodic pulse having a width of 100 µs and amplitude of 1 mA, while the anodic pulse a width of 500 µs and amplitude of 200 µA. The amplitude setting changes during the interphase delay, as shown in Fig 6 (d). IV.
of the current pulse profile is controlled by the external device. The digital management unit receives command frames from the external device and controls the current source and output stage to generate current pulses as required at the specified rate. The circuit was realised in 0.6-µm CMOS technology occupying an area of 1.65 mm2. Postlayout simulations have been presented to show its operation. REFERENCES [1]
[2]
[3] [4]
[5]
CONCLUSIONS
This paper describes the design of a stimulation management unit as part of a stimulator ASIC for an implantable vestibular neural prosthesis. The management unit provides the stimulator ASIC the ability to generate highly flexible biphasic current pulses with fine resolutions for each canal to restore 3-D vestibular sensation. The setting
[6]
[7]
3484
W. Gong and D. Merfeld, “A prototype neural semicircular canal prosthesis using patterned electrical stimulation,” Annals of Biomedical Engineering, vol. 28, pp. 572–581, 2000. C. C. Della Santina, A. A. Migliaccio, and A. H. Patel, “A Multichannel Semicircular Canal Neural Prosthesis Using Electrical Stimulation to Restore 3-D Vestibular Sensation,” IEEE Transactions on Biomedical Engineering, vol. 54, no. 6, pp. 1016–1030, 2007. http://www.medel.com/ A. M. Shkel, and F.-G. Zeng, “An Electronic Prosthesis Mimicking the Dynamic Vestibular Function,” Audiology and Neurotology, vol. 11, pp. 113–122, 2006. T. G. Constandinou, J. Georgiou and C. Toumazou, “A Partial-CurrentSteering Biphasic Stimulation Driver for Vestibular Prostheses,” IEEE TBCAS, vol. 2, no. 2, pp. 1–8, 2008. T. G. Constandinou, J. Georgiou and C. Toumazou, “A Neural Implant ASIC for the Restoration of Balance in Individuals With Vestibular Dysfunction,” Proc. ISCAS 2009, pp.641-644, May 2009. S. Micera et al, “A Closed-Loop Neural Prostheses for Vestibular Disorders,” Proc. 2009 European Future Technologies Conference (FET 09), April, 2009.
