John J. Whalen

Localizing arithmetic processes in the brain: Evidence from a transient deficit during cortical stimulation

Although substantial progress has been made in characterizing the cognitive processes involved in simple arithmetic, the localization of these processes in the brain is not yet well understood. In this article we consider the localization of a specific arithmetic process, the retrieval of arithmetic table facts from memory. We report a single-patient study in which cortical stimulation was used to create transient disruption of brain activity in localized regions of the cortex. We show that stimulation at a left parietal site impaired performance on simple multiplication problems and further that the impairment reflected stimulation-induced disruption of arithmetic fact retrieval. Our findings support the hypothesis (e.g., Warrington, 1982) that the left parietal lobe is implicated in the arithmetic fact retrieval process.

Materials and Fractal Designs for 3D Multifunctional Integumentary Membranes with Capabilities in Cardiac Electrotherapy

Advanced materials and fractal design concepts form the basis of a 3D conformal electronic platform with unique capabilities in cardiac electrotherapies. Fractal geometries, advanced electrode materials, and thin, elastomeric membranes yield a class of device capable of integration with the entire 3D surface of the heart, with unique operational capabilities in low power defibrillation. Co-integrated collections of sensors allow simultaneous monitoring of physiological responses. Animal experiments on Langendorff-perfused rabbit hearts demonstrate the key features of these systems.

Electrochemical Deposition of Platinum from Aqueous Ammonium Hexachloroplatinate Solution

The morphology, microstructure, and electrochemical properties of platinum thin films deposited from aqueous ammonium hexachloroplatinate are dependent on the deposition potential. With increasing overpotential, the grainsize increases and the films exhibit stronger (111) texture. Films deposited at low overpotential exhibit very small grain size and a preferred (311) texture. In addition, films deposited at -0.5 and -0.6 V are porous with very high surface areas. The electrochemical response of the films is dependent on the morphology and microstructure.

Electrodeposition and Characterization of Thin-Film Platinum-Iridium Alloys for Biological Interfaces

An efficient platinum-iridium thin film alloy electrodeposition method has been evaluated to modify the surface of platinum or gold microelectrodes that are being developed for neural recording and stimulation applications. A large number of electrodeposition process variables have been investigated in terms of how they affected the properties of the electrodeposited films. Three sets of Pt-Ir films of a certain composition were electroplated on gold substrates using a potential cycling technique and characterized using microscopy, elemental analysis, nanoindentation, and electrochemical techniques to evaluate the repeatability of the electrodeposition process. Deposition rates were estimated by determining film mass and thickness as a function of deposition time. The surface morphology of the Pt-Ir films was characterized using scanning electron microscopy (SEM) and the chemical composition was determined using wavelength dispersive spectroscopy (WDS). Nanoindentation measurements showed that the hardness of the electroplated Pt-Ir thin films was nearly 100% higher than that of a Pt foil. The electrochemical properties of the films were evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and compared with those of pure Pt, pure Ir and 80-20% Pt-Ir foils. The final electroplating process resulted in 60-40% Pt-Ir alloys. The film thickness increased with electrodeposition time at a rate of 16.5 nm/min and stable films with up to 500 nm thickness were obtained. Characterization by SEM and EIS revealed that the real surface area of the Pt-Ir films was much larger than that of pure Pt and increased significantly with increasing deposition time.

Testosterone replacement treatment options for HIV-infected men

Hypogonadism is well documented in HIV-infected men, particularly as they progress to AIDS and in those with symptoms of wasting. Testosterone deficiency can be diagnosed with simple laboratory tests, and various treatment options exist. The benefits of androgen replacement are well documented from a large body of literature and experience with hypogonadal men without HIV infection. Hypogonadal men who are given testosterone replacement have improved sexual thoughts and functioning, more energy, and improved mood. Generally, quality of life improves with such therapy. Testosterone replacement tends to maintain or improve lean body mass. The benefit, dose, and timing of testosterone replacement treatment for men with HIV infection, however, are less clear and require further study. Appropriate history and a high degree of clinical suspicion, coupled with relatively simple laboratory measurements, can confirm the diagnosis of hypogonadism in men with HIV. Various options for testosterone replacement, including injections of testosterone esters and the use of transcutaneous patches, are discussed, as are the uses of pharmacologic doses of testosterone, primarily for its potential anabolic effect.

The Dependence of Spectral Impedance on Disc Microelectrode Radius

As microelectrodes gain widespread use for electrochemical sensing, biopotential recording, and neural stimulation, it becomes important to understand the dependence of electrochemical impedance on microelectrode size. It has been shown mathematically that a disc electrode, coplanar in an insulating substrate and exposed to a conducting media, exhibits an inhomogeneous current distribution when a potential step is applied. This distribution is known as the primary distribution, and its derivation also yielded an analytic solution for electrical resistance of the conducting media (Rs), between the disc surface and a distant ground, which is inversely proportional to disk radius [Rs = 1/(4Kr), where k is media conductivity and r is disk radius]. The dependence of spectral impedance on microelectrode radius, however, has not been explored. We verify the analytical solution for resistance using high-frequency (100 kHz) electrochemical impedance data from microelectrodes of varying radius (11-325 mum). For all disc radii, as we approach a lower frequency (rarr 10 Hz), we observe a transition from radial to area dependence (e.g., 1/r rarr 1/r2). We hypothesize that this transition is driven by the fact that the derivation of the primary distribution ignores concentration gradients, but that these gradients cannot be ignored at lower frequencies.

Electrochemical Characterization of Charge Injection at Electrodeposited Platinum Electrodes in Phosphate Buffered Saline

Platinum exhibits biocompatibility and chemical stability over a wide potential range and hence is a candidate material for implantable electrodes. Electrodeposition is of interest for the fabrication of implantable electrodes since it is compatible with a wide range of patterning techniques, including conventional microfabrication, soft-lithography, and electrochemical template synthesis. We show that surface roughness of platinum electrodes deposited from ammonium hexachloroplatinate solution is dependent on the deposition potential. We demonstrate that 600 μm diameter electrodeposited platinum microelectrodes can inject 200 nC of charge in a 2 ms biphasic pulse with a potential window as small as 200 mV.

Towards a completely implantable, light-sensitive intraocular retinal prosthesis

A completely implantable, light-sensitive retinal prosthesis must include the capability for both phototransduction and stimulus current generation, two power intensive functions. These devices will be implanted in the eye, creating a significant source of heat that must be dissipated by the ocular tissue without damaging the retina. However, significant technological hurdles remain. The amount of heat that can be safely dissipated by the eye and surrounding head without harming the retina, depends significantly on the position of the heater. Technology for, a high-density electrode array is advancing, but high-quality wires of biocompatible material in dimensions usable for a retinal prosthesis material have yet to be achieved.

Surface modification of neural stimulating/recording electrodes with high surface area platinum-iridium alloy coatings

High-surface area platinum-iridium alloys were electrodeposited by on Pt and Au microelectrodes using a potential sweep technique. Detailed investigations of the structure and morphology and the electrochemical properties of the electrodeposited Pt-Ir alloy coatings were performed. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used for the determination of the surface morphology and the chemical composition of the Pt-Ir coatings, respectively. The elemental analysis by EDS showed a nearly 60–40% Pt-Ir composition of the coatings. The electrochemical properties of the Pt-Ir coatings were evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV and EIS measurements revealed that the Pt-Ir coated electrodes exhibit significantly increased charge storage capacity and real surface area compared to uncoated Pt electrodes. Charge injection experiments of the Pt-Ir coated microelectrodes revealed low potential excursions, indicating high charge injection capabilities within safe potential limits.

Improved electrode material for deep brain stimulation

Deep brain stimulation (DBS) devices have been implanted for treatment of basic tremor, Parkinsons disease and dystonia. These devices use electrodes in contact with tissue to deliver electrical pulses to targeted cells, to elicit specific therapeutic responses. In general, the neuromodulation industry has been evolving towards smaller, less invasive electrodes. However, current electrode materials do not support small sizes without severely restricting the stimulus output. Hence, an improved electrode material will benefit present and future DBS systems. In this study, five DBS leads were modified using a cost-effective and materials-efficient process for applying an ultra-low impedance platinum-iridium alloy coating. One DBS lead was used for insertion test and four DBS leads were chronically pulsed for 12 weeks. The platinum-iridium alloy significantly improved the electrical properties of the DBS electrodes and was robust to insertion into brain and to 12 weeks of chronic pulsing.