Restoring Lost Cognitive Function Hippocampal-Cortical Neural Prostheses

O ne of the frontiers in the biomedical sciences is the development of prostheses for the central nervous system (CNS) to replace higher thought processes that have been lost due to damage or disease. Prosthetic systems that interact with the CNS are currently being developed by several groups [1], though virtually all other CNS prostheses focus on sensory or motor system dysfunction and not on restoring cog-nitive loss resulting from damage to central brain regions. Systems designed to compensate for the loss of sensory input attempt to replace the transduction of physical energy from the environment into electrical stimulation of sensory nerve fibers (e.g., a cochlear implant or artificial retina) or the sensory cortex [2]–[4]. Systems designed to compensate for the loss of motor control do so through functional electrical stimulation (FES), in which preprogrammed stimulation protocols are used to activate muscular movement [5], [6], or by decoding premotor/motor cortical commands for the control of robotic systems [7]–[9]. The type of neural prosthesis that performs or assists a cognitive function is qualitatively different from the cochlear implant, artificial retina, or FES. We consider here a prosthetic device that functions in a biomimetic manner to replace information transmission between cortical brain regions [10], [11]. In such a prosthesis, damaged CNS neu-rons would be replaced with a biomimetic system comprised of silicon neurons. The replacement silicon neurons would have functional properties specific to those of the damaged neurons and would both receive as inputs and send as outputs electrical activity to regions of the brain with which the damaged region previously communicated (Figure 1). Thus, the class of prosthesis being proposed is one that would replace the computational function of the damaged brain and restore the transmission of that computational result to other regions of the nervous system. Such a new generation of neural prostheses would have a profound impact on the quality of life throughout society; it would offer a biomed-ical remedy for the cognitive and memory loss accompanying Alzheimer's disease, the speech and language deficits resulting from stroke, and the impaired ability to execute skilled movements following trauma to brain regions responsible for motor control. We are in the process of developing such a cognitive prosthe-sis for the hippocampus, a region of the brain involved in the formation of new long-term memories. The hippocampus is responsible for what have been termed long-term declarative or recognition memories [12]–[18]: the formation of …