much less painful stimulation。 There are two TMS devices called single pulse TMS and repetitive pulse TMS (rTMS) while the latter has greater effect but potential to cause seizure。 TMS can be used for therapy particularly in psychiatry, as a tool to measure central motor conduction and a research tool to study different aspects of human brain physiology such as motor function, vision, and language。 The rTMS method has been used to treat epilepsy with rates of 8–25 Hz for 10 seconds。 The other therapeutic uses of rTMS include parkinson diseases, dystonia and mood diseases。 Also, TMS can be used to determine the contribution of cortical networks to specific cognitive functions by disrupting activity in the focal brain region。[2] Early, inconclusive, results have been obtained in recovery from coma (persistent vegetative state) by Pape et al。 (2009)。[14]

二、Current technologies 1、Cochlear implants[edit]

  Fig 4 Cochlear implant

Cochlear implants have provided partial hearing to more than 120,000  persons worldwide as of 2008。 The electrical stimulation is used in a cochlear implant to provide functional hearing in totally deafened persons。 Cochlear implants include several subsystem components from the external speech processor and radio frequency (RF) transmission link to the internal receiver, stimulator, and electrode arrays。 Modern cochlear implant research started in the 1960s and 1970s。 In 1961, a crude single electrode device was implanted in  two

deaf patients and useful hearing with electric stimulation was reported。 The first FDA approved complete single channel device was released in 1984。[15] In cochlear implants, the sound is picked up by a microphone and transmitted to the behind-the-ear external   processor

to be converted to the digital data。 The digitized data is then modulated on a radio frequency signal and transmitted to anantenna inside a headpiece。 The data and power carrier are transmitted through a pair of coupled coils to the hermetically sealed internal unit。 By extracting the power and demodulating  the  data,  electric  current  commands  are  sent  to the cochlea to stimulate the auditory nerve through microelectrodes。[16] The key point is that the internal unit does not have a battery and it should be able to extract the required energy。 Also to reduce the infection, data is transmitted wirelessly along with power。 Inductively coupled coils are the best candidate for power and data telemetry。 Parameters needed by the

internal unit include the pulse amplitude, pulse duration, pulse gap, active electrode, and return electrode that are used to define a biphasic pulse and the stimulation mode。 An example of the commercial devices include Nucleus 22 device that utilized a carrier frequency of

2。5 MHz and later in the newer revision called Nucleus 24 device, the carrier frequency was increased     to     5 MHz。[17]  The     internal      unit      in      the      cochlear      implants    is an ASIC (application-specific integrated circuit) chip that is responsible to ensure safe and reliable electric stimulation。 Inside the ASIC chip, there is a forward pathway, a backward pathway, and control units。 The forward pathway recovers digital information from the RF signal which includes stimulation parameters and some handshaking bits to reduce the communication  error。  The  backward  pathway  usually  includes  a  back  telemetry  voltage

sampler that reads the voltage over a period of time on the recording electrode。 The stimulator block is responsible to deliver predetermined current by external unit to the microelectrodes。 This block includes a reference current and a digital to analog converter to transform digital commands to an analog current。[18]