Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Neuroscience

Supervisor

Dr. Brian D. Corneil

Abstract

Vision is one of the principal methods used by primates to acquire information about the surrounding environment. As a result, both humans and monkeys have a highly evolved oculomotor system that functions to rapidly relocate the line of sight to areas of interest. These orienting movements are called gaze shifts. Gaze shifts commonly include the coordinated movement of the eyes-in-head and the head-in-space. This thesis examines the muscular and neural control of orienting head movements.

The contextual control of behavior is important as it allows one to act appropriately in response to different situations. A common task used to examine the contextual control of behavior is the pro- and anti-saccade task. Pro-saccades simply require a subject to look towards a stimulus. Anti-saccades require a subject to inhibit a movement towards a stimulus in favor of a volitional movement to the diametrically opposite position. This task is can reveal capabilities of the oculomotor system and its response to varying behavioral states. To understand the neuromuscular control of orienting head movements during various tasks, we recorded the electromyographic (EMG) activity in ten turner and extensor neck muscles. Recording neck EMGs provides an objective and precise measurement of the neural signals received at the neck muscles, circumventing some of the structural and biomechanical complexities of head motion.

Chapter two examines neck muscle activity in a pro- and anti-saccade task. Many neural areas and certain neck muscles become active in response to the presentation of a visual stimulus. This visual response on the neck muscles can result in a head turning synergy that orients the head towards the stimulus. By dissociating the typical stimulus-response paradigm, we can analyze if and how the bottom-up visual activity changes in relation to different contexts. A number of cortical and subcortical areas are involved in the generation of correct anti-saccades. By combining EMG recordings while subjects perform this task, we can examine whether top-down task-related activity is present in the neck muscles. This experiment could reveal flexibility in the eye-head gaze shift system that has previously gone unreported.

Chapter three will elucidate the supplementary eye fields (SEF) role in the control of orienting eye-head gaze shifts. Neck EMG activity was recorded while providing electrical microstimulation to the SEF in a pro-saccade task The combination of EMGs and SEF stimulation is the first to systematically study the cephalomotor command during head-restrained and head-unrestrained orienting eye-head gaze shifts. The evoked activity of EMGs could reveal functional properties of the neural circuitry between the SEF and the motor related neurons responsible for eye and head movements. The timing and metrics of evoked EMG activity and eye-head gaze shifts are consistent with other frontal areas suggesting a functional role of the frontal cortex in influencing eye-head gaze shifts.

Chapter four will combine EMG recordings with SEF stimulation during a pro- and anti-saccade task. The SEF is thought to serve as an interface between high-level cognitive control of gaze shifts and low-level activity associated with the production of saccades. As will be described later in the thesis, neck muscles demonstrate top-down task related activity during anti-saccades. The SEF is a likely candidate for the generation of task-dependent signals observed during anti-saccades. By combining SEF stimulation and neck EMGs in an anti-saccade task, we can reveal if neck muscle activity is consistent with a role for the SEF in the contextual control of eye-head gaze shifts.

In summary, this thesis identifies three central point’s concerning orienting eye-head gaze shifts. First, chapter two emphasizes the complex interaction of sensori-motor processes in orienting head movements. Second, chapter three attests to the consistent nature of certain areas in frontal cortex and their impact on eye-head gaze shifts. Finally, chapter four demonstrates a potential candidate for influencing the contextual control of cephalomotor commands. Combined, these results highlight the complex interactions of sensori-motor transformations in the motor periphery and emphasize the parallel nature of information processing during the contextual control of eye-head gaze shifts.

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