Catálogo de publicaciones - libros

The auditory brainstem implant (ABI) provides auditory sensations, recognition of environmental sounds and aid in spoken communication in more than 300 patients worldwide. It is no more a device under investigation but it is widely accepted for the treatment of patients who have lost hearing due to bilateral tumors of the vestibulocochlear nerve. Most of these patients are completely deaf when the implant is switched off. In contrast to the cochlear implants (CI), only few of the implanted patients achieve open-set speech recognition without the help of visual cues. In the last few years, patients with lesions other than tumors have also been implanted. Auditory perceptual performance in patients who are deaf due to trauma, cochlea aplasia or other non-tumor lesions of the cochlea or the vestibulocochlear nerve turned out to be much better than in NF2 tumor patients. Until recently, the target region for ABI implantation has been the ventral cochlear nucleus (CN). The electrodes are implanted via the translabyrinthine or retrosigmoid approach. Currently, new targets along the central auditory pathways and new, minimally invasive techniques for implantation are under investigation. These techniques may further improve auditory perceptual performance in ABI patients and provide hearing to a variety of types of central deafness.

Functional imaging techniques have demonstrated a relationship between the intensity of tinnitus and the degree of reorganization of the primary auditory cortex. Studies in experimental animals and humans have revealed that tinnitus is associated with a synchronized hyperactivity in the auditory cortex and proposed that the underlying pathophysiological mechanism is thalamocortical dysrhythmia; hence, decreased auditory stimulation results in decreased firing rate, and decreased lateral inhibition. Consequently, the surrounding brain area becomes hyperactive, firing at gamma band rates; this is considered a necessary precondition of auditory consciousness, and also tinnitus. Synchronization of the gamma band activity could possibly induce a topographical reorganization based on Hebbian mechanisms. Therefore, it seems logical to try to suppress tinnitus by modifying the tinnitus-related auditory cortex reorganization and hyperactivity. This can be achieved using neuronavigation- guided transcranial magnetic stimulation (TMS), which is capable of modulating cortical activity. If TMS is capable of suppressing tinnitus, the effect should be maintained by implanting electrodes over the area of electrophysiological signal abnormality on the auditory cortex. The results in the first patients treated by auditory cortex stimulation demonstrate a statistically significant tinnitus suppression in cases of unilateral pure tone tinnitus without suppression of white or narrow band noise. Hence, auditory cortex stimulation could become a physiologically guided treatment for a selected category of patients with severe tinnitus.

Visual impairment and blindness is primarily caused by optic neuropathies like injuries and glaucomas, as well as retinopathies like agerelated macular degeneration (MD), systemic diseases like diabetes, hypertonia and hereditary (RP). These pathological conditions may affect retinal photoreceptors, or retinal pigment epithelium, or particular subsets of retinal neurons, and in particular retinal ganglion cells (RGCs). The RGCs which connect the retina with the brain are unique cells with extremely long axons bridging the distance from the retina to visual relays within the thalamus and midbrain, being therefore vulnerable to heterogeneous pathological conditions along this pathway. When becoming mature, RGCs loose the ability to divide and to regenerate their accidentally or experimentally injured axons. Consequently, any loss of RGCs is irreversible and results to loss of visual function. The advent of micro- and nanotechnology, and the construction of artificial implants prompted to create visual prostheses which aimed at compensating for the loss of visual function in particular cases. The purpose of the present contribution is to review the considerable engineering expertise that is essential to fabricate current visual prostheses in connection with their functional features and applicability to the animal and human eye. In this chapter, 1) Retinal and cortical implants are introduced, with particular emphasis given to the requirements they have to fulfil in order to replace very complex functions like vision. 2) Advanced work on material research is presented both from the technological and from the biocompatibility aspect as prerequisites of any perspectives for implantation. 3) Ultimately, experimental studies are presented showing the shaping of implants, the procedures of testing their biocompatibility and essential modifications to improve the interfaces between technical devices and the biological environment. The review ends by pointing to future perspectives in the rapidly accelerating process of visual prosthetics and in the increasing hope that restoration of the visual system becomes reality.

Microsystem technologies offer significant advantages in the development of neural prostheses. In the last two decades, it has become feasible to develop intelligent prostheses that are fully implantable into the human body with respect to functionality, complexity, size, weight, and compactness. Design and development enforce collaboration of various disciplines including physicians, engineers, and scientists. The retina implant system can be taken as one sophisticated example of a prosthesis which bypasses neural defects and enables direct electrical stimulation of nerve cells. This micro implantable visual prosthesis assists blind patients to return to the normal course of life. The retina implant is intended for patients suffering from retinitis pigmentosa or macular degeneration.

In this contribution, we focus on the epiretinal prosthesis and discuss topics like system design, data and power transfer, fabrication, packaging and testing. In detail, the system is based upon an implantable micro electro stimulator which is powered and controlled via a wireless inductive link. Microelectronic circuits for data encoding and stimulation are assembled on flexible substrates with an integrated electrode array. The implant system is encapsulated using parylene C and silicone rubber. Results extracted from experiments in vivo demonstrate the retinotopic activation of the visual cortex.

Macular degeneration (MD) and retinitis pigmentosa (RP), two diseases that cause degeneration of retinal photoreceptor cells, are the leading causes of blindness in the United States. Anatomical studies have shown that other retinal neuronal cells (bipolar cells, ganglion cells) are preserved in these diseases and they are capable of eliciting visual percepts when electrically stimulated. We describe the design of a prototype 16-electrode retinal prosthesis, and the physiological and clinical results on six blind patients with RP who had the device implanted. The US Department of Energy is described. The goal of the program is construction of a 1000-electrode retinal neuroprosthesis with the potential of enabling blind patients to read large print and ambulate with ease.

Degenerations of the outer retina such as retinitis pigmentosa (RP) lead to blindness due to photoreceptor loss. There is a secondary loss of inner retinal cells but significant numbers of bipolar and ganglion cells remain intact for many years. Currently, no therapeutic option to restore vision in these blind subjects is available. Short-term pattern electrical stimulation of the retina using implanted electrode arrays in subjects blind from RP showed that ambulatory vision and limited character recognition are possible. To produce artificial vision by electrical retinal stimulation, a wireless intraocular visual prosthesis was developed. Images of the environment, taken by a camera are pre-processed by an external visual encoder. The stimulus patterns are transmitted to the implanted device wirelessly and electrical impulses are released by microcontact electrodes onto the retinal surface. Towards a human application, the biocompatibility of the utilised materials and the feasibility of the surgical implantation procedure were stated. In acute stimulation tests, thresholds were determined and proved to be within a safe range. The local and retinotopic activation of the visual cortex measured by optical imaging of intrinsic signals was demonstrated upon electrical retinal stimulation with a completely wireless and remotely controlled retinal implant. Potential obstacles are reviewed and further steps towards a successful prosthesis development are discussed.

Motor cortex stimulation (MCS) is a promising clinical technique used to treat chronic, otherwise intractable pain. However, the mechanisms by which the neural elements that are stimulated during MCS induce pain relief are not understood. Neither is it known which of the main neural elements, i.e. cell bodies, dendrites or fibers are immediately excited by the electrical pulses in MCS. Moreover, it is not known what are the effects of MCS on fibers which are parallel or perpendicular to the cortical layers, below or away from the electrode. The therapy and its efficacy are less likely to be improved until it is better understood it may work.

In this chapter, we present our efforts to resolve this issue. Our computer model of MCS is introduced and some of its predictions are discussed. In particular, the influence of stimulus polarity and electrode position on the electrical field and excitation thresholds of different neural elements is addressed. Such predictions, supported with clinical evidence, should help to elucidate the immediate effects of an electrical stimulus applied over the motor cortex and may ultimately lead to optimizations of the therapy.

Modeling of the basal ganglia has played a substantial role in gaining insight into the mechanisms involved in the computational processes performed by this elusive group of nuclei. Models of the basal ganglia have undergone revolutionary changes over the last twenty years due to the rapid accumulation of neuroscientific data. In this chapter, we present distinct modeling approaches that can be used to enhance our understanding of the functional dynamics of information processing within the basal ganglia, and their interactions with the rest of the brain. Specific examples of recently developed models dealing with the analysis of computational processing issues at different structural levels of the basal ganglia are discussed.

Cognitive functions, such as memory, attention, and perception, decline with age. Besides other neuroanatomical changes, the level of dopamine also attenuates during aging. We review how computational modeling can provide insights in how these lifetime changes in dopamine levels are expressed at the behavioral level yielding a bridge across different levels. Results indicate that attenuation of dopamine lowers the signal to noise ratio providing a less distinctive neural representation, and detrimental cognitive performance.

In this chapter, we report that blood pressure can be increased or decreased depending on whether an electrode is in ventral or dorsal PAG. We also describe that it is theoretically possible to treat orthostatic hypotension. These are exciting developments not only because they provide an example of direct research from animal research to humans but also because they highlight a potential for future clinical therapies. The control of essential hypertension without drugs is attractive because of the side effects of medication such as precipitation of heart failure []. Similarly, drug treatment of orthostatic hypotension cannot differentiate between the supine and standing positions and can therefore lead to nocturnal hypertension [, ]. A stimulator could be turned off at night or contain a mercury switch that reacts to posture.