Microstimulation-evoked neural responses in visual cortex are depth dependent

Abstract

Background: Cortical visual prostheses often use penetrating electrode arrays to deliver microstimulation to the visual cortex. To optimize electrode placement within the cortex, the neural responses to microstimulation at different cortical depths must first be understood. Objective: We investigated how the neural responses evoked by microstimulation in cortex varied with cortical depth, of both stimulation and response. Methods: A 32-channel single shank electrode array was inserted into the primary visual cortex of anaesthetized rats, such that it spanned all cortical layers. Microstimulation with currents up to 14 mA (single biphasic pulse, 200 ms per phase) was applied at depths spanning 1600 mm, while simultaneously recording neural activity on all channels within a response window 2.25e11 ms. Results: Stimulation elicited elevated neuronal firing rates at all depths of cortex. Compared to deep sites, superficial stimulation sites responded with higher firing rates at a given current and had lower thresholds. The laminar spread of evoked activity across cortical depth depended on stimulation depth, in line with anatomical models. Conclusion: Stimulation in the superficial layers of visual cortex evokes local neural activity with the lowest thresholds, and stimulation in the deep layers evoked the most activity across the cortical column. In conjunction with perceptual reports, these data suggest that the optimal electrode placement for cortical microstimulation prostheses has electrodes positioned in layers 2/3, and at the top of layer 5.

Representational drift influences learning of a sensory cortical microstimulation task

“…Chronic electrical microstimulation poses numerous challenges: it is invasive, can inadvertently activate neurons up to millimeters away from the stimulation site (Histed et al ., 2009), and can cause tissue degradation at the site of electrode insertion (Polikov et al ., 2005). Furthermore, simultaneous recording is complicated by a prominent stimulation artifact (Weiss et al ., 2019), and even studies with concurrent recording rarely examine the activity evoked at the stimulation site (Allison-Walker et al, 2021; Butovas and Schwarz, 2003; Chen et al, 2020; Hao et al, 2016) (but see (Sombeck et al, 2022)). Finally, electrical microstimulation cannot target genetically separable neural populations.…”

Intracortical current steering shifts the location of evoked neural activity

“…Electrical stimulation artifacts (figure 2(A)) were removed post hoc by interpolating the signal from 1 ms before the stimulation pulse until the post-stimulation signal returned to ±150 µV of the pre-stimulation amplitude and remained there for 1 ms [30]. This removal method minimized the time that the signal was blanked allowing action potentials to be detected as early as possible.…”

“…Manipulation of neural activity through current steering requires that the neural activity summates without saturation between the stimulating electrodes. This relies on the relationship of response spread with the amplitude of microstimulation [30,45]. To predictably manipulate neural activity, the interactions must be consistent; low-level microstimulation elicits variable results due to local changes in neural state [46] but high levels of stimulation will induce response saturation.…”

Stimulation-Evoked Effective Connectivity (SEEC): An in-vivo approach for defining mesoscale corticocortical connectivity

“…Electrical stimulation of the brain is a versatile clinical and scientific tool. Clinically, stimulation can be used to modify the excitability of cortical regions (1, 2), map functional brain regions prior to neurosurgical resections (3), and drive artificial input into sensory cortices (4, 5). In research settings, stimulation can also be driven through small electrodes implanted into the cortex in a technique known as intracortical microstimulation (ICMS).…”