System Level Assessment of Motor Control through Patterned Microstimulation in the Superior Colliculus

We are immersed in an environment full of sensory information, and without much thought or effort we can produce orienting responses to appropriately react to different stimuli. This seemingly simple and reflexive behavior is accomplished by a very complicated set of neural operations, in which motor systems in the brain must control behavior based on populations of sensory information. The oculomotor or saccadic system is particularly well studied in this regard. Within a visual environment consisting of many potential stimuli, we control our gaze with rapid eye movements, or saccades, in order to foveate visual targets of interest. A key sub-cortical structure involved in this process is the superior colliculus (SC). The SC is a structure in the midbrain which receives visual input and in turn projects to lower-level areas in the brainstem that produce saccades. Interestingly, microstimulation of the SC produces eye movements that match the metrics and kinematics of naturally-evoked saccades. As a result, we explore the role of the SC in saccadic motor control by manually introducing distributions of activity through neural stimulation. Systematic manipulation of microstimulation patterns were used to characterize how ensemble activity in the SC is decoded to generate eye movements. Specifically, we focused on three different facets of saccadic motor control. In the first study, we examine the effective influence of microstimulation parameters on behavior to reveal characteristics of the neural mechanisms underlying saccade generation. In the second study, we experimentally verify the predictions of computational algorithms that are used to describe neural mechanisms for saccade generation. And in the third study, we assess where neural mechanisms for decoding occur within the oculomotor network in order to establish the order of operations necessary for saccade generation. The experiments assess different aspects of saccadic motor control, which collectively, reveal properties and mechanisms that contribute to the comprehensive understanding of signal processing in the oculomotor system

A Novel Brain Stimulation Technology Provides Compatibility with MRI

Clinical electrical stimulation systems — such as pacemakers and deep brain stimulators (DBS) — are an increasingly common therapeutic option to treat a large range of medical conditions. Despite their remarkable success, one of the significant limitations of these medical devices is the limited compatibility with magnetic resonance imaging (MRI), a standard diagnostic tool in medicine. During an MRI exam, the leads used with these devices, implanted in the body of the patient, act as an electric antenna potentially causing a large amount of energy to be absorbed in the tissue, which can lead to serious heat-related injury. This study presents a novel lead design that reduces the antenna effect and allows for decreased tissue heating during MRI. The optimal parameters of the wire design were determined by a combination of computational modeling and experimental measurements. The results of these simulations were used to build a prototype, which was tested in a gel phantom during an MRI scan. Measurement results showed a three-fold decrease in heating when compared to a commercially available DBS lead. Accordingly, the proposed design may allow a significantly increased number of patients with medical implants to have safe access to the diagnostic benefits of MRI

The relative impact of microstimulation parameters on movement generation

Katnani HA, Gandhi NJ. The relative impact of microstimulation parameters on movement generation. J Neurophysiol 108: 528 -538, 2012. First published April 25, 2012 doi:10.1152/jn.00257.2012Microstimulation is widely used in neurophysiology to characterize brain areas with behavior and in clinical therapeutics to treat neurological disorder. Current intensity and frequency, which respectively influence activation patterns in spatial and temporal domains, are typically selected to elicit a desired response, but their effective influence on behavior has not been thoroughly examined. We delivered microstimulation to the primate superior colliculus while systematically varying each parameter to capture effects of a large range of parameter space. We found that frequency was more effective in driving output properties, whereas properties changed gradually with intensity. Interestingly, when different parameter combinations were matched for total charge, effects on behavioral properties became seemingly equivalent. This study provides a first level resource for choosing desired parameter ranges to effectively manipulate behavior. It also provides insights into interchangeability of parameters, which can assist clinical microstimulation that looks to appropriately control behavior within designated constraints, such as power consumption

A test of spatial temporal decoding mechanisms in the superior colliculus

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Kinematics.

<p>The scatter plots compare the duration (left), peak velocity (middle) and average velocity (right) of saccades evoked from stimulation-only trials and stimulation-with-blink trials. Each dot represents the mean value from one stimulation site, and the error bars represent one standard deviation. The four colors correspond to the four animals, as indicated in the key.</p

Temporal waveforms.

<p>An alternate representation of the data illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051843#pone-0051843-g002" target="_blank">Figure 2</a>. Horizontal and vertical eye velocity is plotted as a function of time for stimulation-evoked saccades with and without blink perturbations. All traces are aligned on saccade onset. All other configurations are the same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051843#pone-0051843-g002" target="_blank">Figure 2</a>.</p

Comparison of radial amplitude.

<p>Comparison of mean radial amplitudes for stimulation-evoked saccades without and with a puff-evoked blink. Green dots represent values from monkey 1; cyan, monkey 2; red, monkey 3; gray, monkey 4. Error bars represent one standard deviation from the mean; solid line marks unity slope. The majority of stimulation sites lie above the unity line, indicating an increase of saccade amplitude due to the blink-saccade interaction.</p

Correlation with BREM kinematics.

<p>The peak velocity of BREM movements versus the change in mean radial amplitude of stimulation-evoked saccades colliding with a puff-evoked blink. Green squares correspond to monkey 1; cyan triangles, monkey 2; red circles, monkey 3; gray diamonds, monkey 4.</p