Motor-cortical activity in tetraplegics.
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Adult↗
Biomedical subjects
E M Maynard
Publications and source records attributed to E M Maynard.
Visual prostheses.
The development of man-made systems to restore functional vision in the profoundly blind has recently undergone a renaissance that has been fueled by a combination of celebrity and government interest, advances in the field of bioengineering, and successes with existing neuroprosthetic systems. This chapter presents the underlying physiologic principles of artificial vision, discusses three contemporary approaches to restoring functional vision in the blind, and concludes by presenting several relevant questions to vision prostheses. While there has been significant progress in the individual components constituting an artificial vision system, the remaining challenge of integrating these components with each other and the nervous system does not lie strictly in the realm of neuroscience, medicine, or engineering but at the interface of all three. In spite of the apparent complexity of an artificial vision system, it is not unreasonable to be optimistic about its eventual success.
Biomedical Engineering↗
A technique to prevent dural adhesions to chronically implanted microelectrode arrays.
Minimizing relative movements between neural tissues and arrays of microelectrodes chronically implanted into them is expected to greatly enhance the capacity of the microelectrodes to record from single cortical neurons on a long-term basis. We describe a new surgical technique to minimize the formation of adhesions between the dura and an implanted electrode array using a 12 microm (0.5 mil) thick sheet of Teflon film positioned between the array and the dura. A total of 15 cats were implanted using this technique. Gross examination of 12 implant sites at the time of sacrifice failed to find evidence of adhesions between the arrays and the dura when the Teflon(R) film remained in its initial position. In six implants from which recordings were made, an average of nine of the 11 (81%) connected electrodes in each array recorded evoked neural activity after 180 days post implantation. Further, on average, two separable units were identified on each of the implanted electrodes in these arrays. No significant change was found in the density of cell bodies around implanted electrodes of four of the implanted electrode arrays. However, histological evaluation of the implant sites revealed evidence of meningeal proliferation beneath the arrays. The technique described is shown to be effective at preventing adhesions between implanted electrode arrays and improve the characteristics of chronic recordings obtained with these structures.
Animals↗
Neuronal interactions improve cortical population coding of movement direction.
Interactions among groups of neurons in primary motor cortex (MI) may convey information about motor behavior. We investigated the information carried by interactions in MI of macaque monkeys using a novel multielectrode array to record simultaneously from 12-16 neurons during an arm-reaching task. Pairs of simultaneously recorded cells revealed significant correlations in their trial-to-trial firing rate variation when estimated over broad (600 msec) time intervals. This covariation was only weakly related to the preferred directions of the individual MI neurons estimated from the firing rate and did not vary significantly with interelectrode distance. Most significantly, in a portion of cell pairs, correlation strength varied with the direction of the arm movement. We evaluated to what extent correlated activity provided additional information about movement direction beyond that available in single neuron firing rate. A multivariate statistical model successfully classified direction from single trials of neural data. However, classification was consistently better when correlations were incorporated into the model as compared to one in which neurons were treated as independent encoders. Information-theoretic analysis demonstrated that interactions caused by correlated activity carry additional information about movement direction beyond that based on the firing rates of independently acting neurons. These results also show that cortical representations incorporating higher order features of population activity would be richer than codes based solely on firing rate, if such information can exploited by the nervous system.
Action Potentials↗
A neural interface for a cortical vision prosthesis.
The development of a cortically based vision prosthesis has been hampered by a lack of basic experiments on phosphene psychophysics. This basic research has been hampered by the lack of a means to safely stimulate large numbers of cortical neurons. Recently, a number of laboratories have developed arrays of silicon microelectrodes that could enable such basic studies on phosphene psychophysics. This paper describes one such array, the Utah electrode array, and summarizes neurosurgical, physiological and histological experiments that suggest that such an array could be implanted safely in visual cortex. We also summarize a series of chronic behavioral experiments that show that modest levels of electrical currents passed into cortex via this array can evoke sensory percepts. Pending the successful outcome of biocompatibility studies using such arrays, high count arrays of penetrating microelectrodes similar to this design could provide a useful tool for studies of the psychophysics of phosphene perception in human volunteers. Such studies could provide a proof-of-concept for cortically based artificial vision.
Animals↗
The Utah intracortical Electrode Array: a recording structure for potential brain-computer interfaces.
We investigated the potential of the Utah Intracortical Electrode Array (UIEA) to provide signals for a brain-computer interface (BCI). The UIEA records from small populations of neurons which have an average signal-to-noise ratio (SNR) of 6:1. We provide specific examples that show the activities of these populations of neurons contain sufficient information to perform control tasks. Results from a simple stimulus detection task using these signals as inputs confirm that the number of neurons present in a recording is significant in determining task performance. Increasing the number of units in a recording decreases the sensitivity of the response to the stimulus; decreasing the number of units in the recording, however, increases the variability of the response to the stimulus. We conclude that recordings from small populations of neurons, not single units, provide a reliable source of sufficiently stimulus selective signals which should be suitable for a BCI. In addition, the potential for simultaneous and proportional control of a large number of external devices may be realized through the ability of an array of microelectrodes such as the UIEA to record both spatial and temporal patterns of neuronal activation.
Action Potentials↗
Single unit recording capabilities of a 100 microelectrode array.
We have developed a three-dimensional silicon electrode array which provides 100 separate channels for neural recording in cortex. The device is manufactured using silicon micromachining techniques, and we have conducted acute recording experiments in cat striate cortex to evaluate the recording capabilities of the array. In a series of five acute experiments, 58.6% of the electrodes in the array were found to be capable of recording visually evoked responses. In the most recent acute study, the average signal-to-noise ratio for recordings obtained from 56 of the electrodes in the array was calculated to be 5.5:1. Using standard window discrimination techniques, an average of 3.4 separable spikes were identified for each of these electrodes. In order to compare the two-dimensional mapping capabilities of the array with those derived from other technologies, orientation preference and ocular dominance maps were generated for each of the evoked responses. Histological evaluation of the implant site indicates some localized tissue insult, but this is likely due to the perfusion procedure since high signal-to-noise ratio neural responses were recorded. The recording capabilities of the Utah Intracortical Electrode Array in combination with the large number of electrodes available for recording make the array a tool well suited for investigations into the parallel processing mechanisms in cortex.
Animals↗