Theodore W. Berger
AINS Conference / 2002
Affiliation: Professor of Biomedical Engineering and Neuroscience, Director of the Center for Neural Engineering, University of Southern California, Los Angeles, CA
Title: Toward Replacement Parts for the Brain: Implantable Biomimetic Electronics
Abstract: Dr. Berger will present results of a collaborative effort by neuroscientists and engineers at the University of Southern California to develop the technology required for hardware implementation of neural network models to be used as neural prostheses for the replacement of damaged or dysfunctional brain tissue. Five components of this effort will be described, which include (i) experimental study of cellular and molecular mechanisms, (ii) formulation of biologically realistic models of network dynamics, (iii) hardware implementation of the network models in analog VLSI, and (iv) hybrid neuron-silicon devices for interfacing brain implants with existing neural tissue. Described as part of the latter effort are recent successes in culturing living neurons directly onto silicon-based computer chips. This five-part approach is applied to developing a prosthetic device for the hippocampus, a region of the brain responsible for the formation of long-term memories, and that frequently is damaged as a result of epilepsy, stroke, and Alzheimer's disease. The hippocampus is typical of most neural systems found in the mammalian brain in that it is composed of several populations of neurons, each with a distinct set of functional properties that include high order nonlinear dynamics, and are interconnected through a variety of feedforward and feedback circuits. Because of its special role in memory formation, the strengths of connections between hippocampal neurons also are subject to activity-dependent modification, i.e., the system properties can be nonstationary. In the intact hippocampus, subpopulations of neurons are involved in distributed representations of stimuli and/or behavior that can be altered with changing experience or context. Collectively, these system characteristics present a unique set of challenges to developing a biologically realistic model at the systems level which also provides a sufficient basis for a partial replacement neural prosthetic. Through multidisciplinary efforts of neuroscientists, engineers, and medical researchers, many of these challenges can be overcome with present-day technologies, to the extent that the first applications of silicon-based computational elements having direct communication with brain tissue will be realized in the near future. In this regard, novel “neuromorphic” silicon-based multi-site electrode arrays have been fabricated and tested as neuron-silicon interfaces. The spatial distribution of electrode sites is specifically designed to be consistent with the cytoarchitecture of the hippocampus, and brings the uniform distribution of microchip contact pads into the register with the non-uniform distribution of hippocampal neurons. Dr. Berger also will demonstrate with examples how research efforts to develop such a new generation of neural prostheses inevitably "spin-off" other technologies that are highly useful for real-world applications, e.g., speech recognition and other temporal pattern identification systems.