Intracortical current steering shifts the location of evoked neural activity

Abstract

Objective: Intracortical visual prostheses are being developed to restore sight in people who are blind. The resolution of artificial vision is dictated by the location, proximity and number of electrodes implanted in the brain. However, increasing electrode count and proximity is traded off against tissue damage. Hence, new stimulation methods are needed that can improve the resolution of artificial vision without increasing the number of electrodes. We investigated whether a technique known as current steering can improve the resolution of artificial vision provided by intracortical prostheses without increasing the number of physical electrodes in the brain. Approach: We explored how the locus of neuronal activation could be steered when low amplitude microstimulation was applied simultaneously to two intracortical electrodes. A 64-channel, 4-shank electrode array was implanted into the visual cortex of rats (n=7). The distribution of charge ranged from single-electrode stimulation (100%:0%) to an equal distribution between the two electrodes (50%:50%), thereby steering the current between the physical electrodes. The stimulating electrode separation varied between 300-500 μm. The peak of the evoked activity was defined as the ‘virtual electrode’ location. Main Results: Current steering systematically shifted the virtual electrode on average between the stimulating electrodes as the distribution of charge was moved from one stimulating electrode to another. This effect was unclear in single trials due to the limited sampling of neurons. A model that scales the cortical response to each physical electrode when stimulated in isolation predicts the evoked virtual electrode response. Virtual electrodes were found to elicit a neural response as effectively and predictably as physical electrodes within cortical tissue on average. Significance: Current steering could be used to increase the resolution of intracortical electrode arrays without altering the number of physical electrodes which will reduce neural tissue damage, power consumption and potential heat dispersion issues.

Cortical layering disrupts multi-electrode current steering

“…As all of these trends were expected, we wanted to ensure that the lack of peak shift was not due to experimental hardware or data processing. We chose to compare our multilayer current steering results to our previously obtained results when current steering within a single layer [35].…”

“…We have previously implemented current steering [35] in a single layer by changing the array orientation to that shown in figure 4(A). The evoked activity was much closer to the expected output of Gaussianshaped curves with a peak location that was dependent on the current ratio between electrodes.…”

“…Current steering has been shown to shift the phosphene location within the visual field [33,34] and we have previously shown that current steering can shift the peak of activity when all electrodes were at similar cortical depths (single-layer current steering) [35]. However, the cortical layers have differences in cell densities and types meaning that current steering across the cortical layers may not be as straightforward.…”

Activation and depression of neural and hemodynamic responses induced by the intracortical microstimulation and visual stimulation in the mouse visual cortex

“…They have also shown promising results in neuroengineering applications, particularly in the field of neuroprosthetics [9][10][11]. For example, in visual cortical prosthetics, intracortical microstimulation (ICMS) delivered through implanted probes in the visual cortex has been extensively used to investigate evoked neural responses [12][13][14], animal behaviors [15][16][17][18], and even restore some aspects of visual perception and perceived visual experiences in human subjects [19][20][21][22][23]. In terms of the downside of penetrating microelectrodes, the longevity of recording and stimulation performance [24] and the potentially linked tissue reactions like gliosis around electrodes [25] have been explored.…”

A novel simulation paradigm utilising MRI-derived phosphene maps for cortical prosthetic vision

“…Promising progress has been made towards restoring the vision of blind people by artificially activating surviving cells in the visual pathway [1][2][3][4][5][6][7]. In this study, we focus on cortical visual prostheses which deliver electrical stimulation directly to the visual cortex via implanted electrodes bypassing any damage that might occur at the retina or optic nerve [3,5,[8][9][10]. Typically, each electrode elicits a small spot of light in the visual field, called a phosphene [3,5,8].…”