These complex electrode array geometries were designed to be implanted into cerebral cortex to a depth of 1.5 to 2mm; however, the Utah team discovered that arrays containing very large numbers of individual electrodes cannot simply be pushed into the cortex of experimental animals. Rather, just as a staple can be inserted into a block of wood if it has sufficient momentum and velocity, these arrays require high velocity insertion to fully insert them. To achieve this end, the Utah team has built a pneumatically actuated insertion tool.62 This insertion tool is capable of full insertion of a 100-electrode array in fewer than 200 |isec (an insertion velocity on the order of 7 m/sec).
Both the Utah and the Michigan electrode arrays have been used in scores of electrophysiological experiments in a variety of sites in the nervous system in rats, turtles, cats, monkeys, guinea pigs, and ferrets. They have been used in both stimulation and recording applications in both acute and chronic preparations. Histological studies indicate that there is usually a little localized bleeding associated with their implantation, but this usually resolves itself quickly, and single- and/or multi-unit recordings can be made within an hour or two of implantation. The fact that single-unit recordings can be made with these complex devices provides the best evidence that the neurons near the electrode tips have not been significantly damaged by the insertion process. The fact that recordings can be made on a chronic basis indicates that the materials used in their manufacture are biocompatible.
The efficacy of the Utah Electrode Array as a means to stimulate neural tissues has been studied in a series of chronic behavioral experiments conducted in cats.63 Rather than training cats to respond to a visual task, Rousche decided to train cats to respond to auditory stimuli (cats seem to always attend to auditory stimuli while this is not the case for visual stimuli). Cats were trained to press a lever whenever they detected an auditory tone delivered through a loudspeaker that was initiated by a different lever press. When proficient at this task, the cats were implanted in the auditory cortex with a Utah array. After the implant, the cats were periodically electrically stimulated via the implanted array rather than aurally stimulated. If the electrical stimulation evoked a percept, the cats pressed the lever. These experiments demonstrated that the cats were able to detect presumed electrically evoked auditory percepts for current injections in the 1- to 10-| A range, a range well within the safe limits of chronic stimulation.
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