Neuroprosthetics

The term neural prostheses is understood interfaces between the nervous system and the connection to an electronic component to clinical applications and medical research. Traditionally, single micro- electrodes or electrode arrays (sometimes over 100 electrodes) are used ( Santhanam 2006) to correct for limited, pathological or lost functions of the nervous system and restore or improve normal functions ( Rutten 2002, Schwartz 2004 & 2006). These technical interfaces can be fundamentally divided into motor and sensory neural prostheses according to their fields of application. Neural prostheses to restore nerve function failed completely or partially or represented as so-called substitutive method a replacement.

The neurosciences today include besides biological, medical and psychological meanwhile also philosophical and va more and more ( informations) and technological issues. The question to what extent the activity of the brain can be simulated artificially by electrical stimulation, this is even older than the first official evidence that precisely those action potentials the supporting role in the language of the nervous system play ( Galvani 1791, Du Bois 1849). Already in 1755 undertook the physician Charles le Roy attempts by electrical stimulation of the cortex ( cerebral cortex ) in blind patients elicit visual impressions (Le Roy 1755). Electrical stimulation of the cortex was one of the first methods of neuroscience, which made ​​it possible to establish a relationship between the cortical physiology and perception. The research and medical application for electrical stimulation of neural structures has undergone a revolution within the last few years. Contributing V.A. the new biotechnology developments at the interfaces (electrode interfaces) and new mathematical algorithms. The stimulated neural populations can be directly activated or inactivated, allowing a direct examination of functional relevance, while Ableitstudien only correlations between neural activity and perceptual effects afford ( Cohen 2004). Electrical stimulation thus evolved into a widely used method for the investigation of a variety of neuronal functions: from the cellular functioning on perception through to neural plasticity ( Maldonado 1996, Ma 2005).

Motorized neuroprostheses

The simplest motor prostheses are used to encourage application by a carrier frequency of the activity of certain subcortical nuclei in general or to inhibit. The procedure called deep brain stimulation is used so long been the treatment of Parkinson's disease or similar basalganglionären, motor diseases. Here, for example, the Ncl. subthalamic, in Parkinson's disease chronically overactive, inhibited by high-frequency stimulation ( Volkmann 2004, McIntyre 2004, Tass 2003). The results of such a brain pacemaker are sometimes evaluated by clinical reports very positive, so that in the near future as well as other psychomotor disorders, such as Tourette's syndrome, and classic psychiatric disorders, such as Depression can be treated with deep brain stimulation ( Mayberg 2005). An ambitious target for future motor prostheses is to allow paralyzed patients by derivation of neural signals of the (mainly motor, but not necessarily the primary motor ) cortex and an overpass into control signals for engineering components, a partial recovery of their motor capacity. Above all improvements of multi-electrode techniques for deriving signals of whole cell populations have massive progress allows ( Nicolelis 2001, Chapin 2004). These advances now even have led to clinical trials in human subjects. These should be examined to what extent can also use neural signals to compute motion trajectories for robot arms as possible prosthetic arms, ( Nicolelis 2003, Patil 2004, Hochberg 2006, Nicolelis experiment: Monkey controls arm). The actual fine motor skills of the human hand is to reconstruct, however, can not be reached in the foreseeable future. Recent studies in animal experiments could evoke by electrical stimulation of specific regions of complex motor responses, such as hand - to-mouth movements, defensive reactions or grasping movements. Disagreement one is so far as the complexity of movements triggered by the stimulation site and stimulation parameters used depends. Importance Relevant movements, which are also used in behavioral contexts, have so far from the motor cortex via the premotor cortex up to the posterior parietal or the ventral intraparietal cortex evoked by micro stimulation (Graziano 2002 & 2005 Stepniewska 2005).

Non-invasive, humane Brain- Computer Interfaces: It has managed a patient with locked-in syndrome to move through derivation of an EEG by presentation of certain movements a mouse cursor on a computer screen and write e -mails. Such humane, non-invasive Brain- Computer Interfaces ( BCI) are also still at an early stage of development. Among other things, Niels Birbaumer of the University of Tübingen is working on such a non- invasive human BCIs.

Sensory neural prostheses

The role of sensory prostheses, however, is to translate physical stimuli into neural signals usable to restore lost sensory functions or replace. Sensory prostheses to their users meaningful enable structured percepts and can start at different levels of the sensory pathways. Here, too, are different types of neuroprothetischen approaches already in clinical discourse. The so far only actually successfully used therapeutically sensory neural prosthesis is a prosthesis inner ear ( cochlear implant CI), which the eighth cranial nerve ( N. statoacusticus ) directly within the cochlea stimulated ( Rubinstein 1999 & 2001, Middlebrooks 2005). The mechanical sound transmission through the inner ear and the implementation into an electrical pulse is replaced by the hair cells. The quality of the graft does not reach the natural hearing sensation by far. However, the development of these peripheral accreting prosthesis has already progressed so far that a certain group of hard of hearing to deaf patients actually understanding speech, some is made possible even without lip- reading control ( like on the phone ) again ( Vandali 1995, Wilson 2003 & 2005 Vandali 2005). In contrast to the successes in the auditory system similar developments in ophthalmology neuroprothetischen can so far offer no therapies clinically used. However, the development of a retinal prosthesis based in recent years has made tremendous progress. So already significantly more complex percepts could be produced, as a simple flashes of light, known as phosphenes ( Humayun 1999, Zrenner 2002 Old Hero 2004, Weiland 2005). German research groups that deal with retinal implants include in Tübingen, led by Eberhart Zrenner. A distinction subretinal and epiretinal implants. Peripheral neural prostheses, such as the CI and retinal prostheses, can help only to a specific group of patients in which the injury actually occur before the actual interface (eg lack of sound conduction through the ossicles to the inner ear deafness). In order to achieve a further group of patients, the so-called brain stem implant ( Auditory Brainstem Implant, ABI) has been further developed in recent years. The ABI is a modified CI, although not directly stimulates the auditory nerve, but the first switch node of the afferent auditory system, the Ncl. cochlear brainstem ( Lenarz 2001, Kuchta 2004, Lenarz 2006, Samii 2007). Recent studies by Lenarz and colleagues deal with another form of the central auditory prosthesis in the midbrain ( Auditory Midbrain Implant AMI). They stimulate previously with 16 electrodes specific frequency columns in the superior colliculus ( Samii 2007, Lim 2007, 2008 ). However, the technical complexity and clinical risk increases with such a central grafts as the ABI, enormously. For the visual system, there is already attempts by direct stimulation of the optic nerve ( Veraart ) or by direct stimulation of the cortical visual cortex ( Dobelle etc.) to evoke visual impressions. The direct contact with the somatosensory cortex offers advantages over subcortical strategies. The contact with the biological tissue could be, unlike, for example, in the retina, make very stable by the skull bone serves as a fixation for the neuroprosthetic implant. The size and number of cells in the cortex is much larger and provides purely sterically even more space for larger electrode arrays, which could be important for the recovery of sensory function, from an information technology perspective an advantage. Furthermore, so that could be a therapeutically wider range of Krankheitsätiologien treat, since in principle disorders of the afferent sensory system at all levels would be replaced ( Normann 1999, Donoghue 2002 & 2004).

Modern concepts of an interactive cortical neuroprosthesis

That the generation of visual or auditory percepts by stimulation of cortical tissue with electrical current is possible in principle, however, was shown in a series of human experiments ( Brindley 1968, Dobelle 1974 & 1976, Bak 1990, Schmidt 1996). All examined patients reported here were unanimous in modality- based impressions, as phosphenes and Audene. The assumption that it is possible through multi-channel implants simulate the necessary complexity of cortical activity patterns to actually perceptually to create objects that have been expressed already about 50 years ago (Krieg 1953). Following this she worked for simultaneous electrical stimulation of many places of the visual cortex as a visual prosthesis for the blind approach of Brindley, Dobelle and colleagues. After all, it succeeded Dobelle in 1976 a long time patient blind reading braille letters on such a cortical interface to allow - but only much slower than her this was tactile possible. Responsible for this one made the low spatial accuracy of a superficial stimulation, which put not enough independent channels for information. Hence the spatially far -specific administrable Intracorticale Micro stimulation developed. But even hereby were until now only meaningless and unstructured percepts are evoked ( Troyk 2003).

Ultimately, the classical coding approach for cortical sensory neural prostheses must be regarded as having failed. All studies on sensory neural prostheses since Brindley and Lewin did not perform any fundamentally new insights, as with previous stimulation parameters and electrode configurations, complex information can be transmitted in the brain ( Dobelle, 2000). Current efforts to explain this failure to focus here mainly on a conceptual development of the coding approach, which regards the necessary learning performance, in terms of application-oriented practice, as a main criterion to evoke significant relevant cortical excitations ( Sheikh 2002). Modern concepts cortical neural prostheses might try the biophysical and physiological parameters of electrical stimulation ( Butovas & Schwartz 2003, 2006 & 2007 Tehovnik 2006) with the concepts of sensory substitution of Bach -y -Rita (1969 & 2004) to combine. An electrical stimulus would then no longer seen as a restoration of the lost inputs, rather than abstract, stimulus- coupled input, the importance of which should be learned. The generation of significant relevant percepts is related to the philosophy of embodied cognition as a construct of the alternating play between body, brain and environment discussed ( Chiel 1997, Thompson 2001). This view is supported by many empirical findings. Such a strategy for the development of an interactive, sensory cortex prosthesis in animal studies undertaken by the working group " neural prostheses " at the Leibniz Institute for Neurobiology in Magdeburg under the direction of Frank W. Ohl.

Ethical and philosophical aspects

The creation or modification of behavior by electrical stimulation of the cerebrum is an ethically and philosophically complex problem. The question of self- causation is already under physiological point of controversy ( John Searle 2007), but a study by Talwar et al. (2002) navigated the researchers laboratory rats by implantation of three microelectrodes quasi remotely through even the most complex mazes and over the perceived risks of time - called the Roborat. The approach is as ingenious as it is simple: an electrode innervation in the medial forebrain bundle to stimulate the dopaminergic reward system. Two other electrodes simulate each sensory influences of the left and right whiskers in the barrel field of the somatosensory cortex. The Belohungselektrode is paired with each of the virtual tactile percept Left or Right and the rat starts to move ( Talwar 2002, Nicolelis 2002). The American military is assisting such research in the millions. The central research agency of the Pentagon ( Defence Advanced Research Projects, DARPA ), the program has enhanced human performance called into being in order " not to let the man be the weakest link in the U.S. military ," according to Darpa Director Anthony catheter.