Context Dependent Neural Control of Gaze Shifts

We explore our environment by looking at objects of interest. In order to precisely direct our visual axis where we intend, we must generate a series of movements with our eyes and head, which shift our gaze to the desired location. A number of neural structures provide control for such behavior, each acting as a unique contributor to either higher- or lower-level movement properties of the action. Some structures, such as the superior colliculus (SC), influence the metrics and kinematics of gaze shifts, e.g., controlling the direction, amplitude, and velocity of a saccade. Others, like the ventral premotor cortex (PMv), monitor the context of the movement, presumably differentiating whether the gaze shift was made to search for food or to locate a threat. Here, we aim to present an overarching view of these neural systems by exploring both the kinematics and context of gaze shifts. Systematic manipulation of the visual environment was used to explore neural and behavioral features underlying gaze shifts. In the first study, we instructed the subjects to generate saccades to targets on the screen while we recorded single cell spiking activity from the SC. We then utilized statistical techniques to determine instantaneous SC control over saccade velocity. In the second study we recorded from the PMv and instructed the subject to perform gaze shifts that either preceded or followed a head movement. We aimed to establish context-dependent PMv activity modulation. In the last study we explored the ability of the systems to adapt to a predetermined environmental change. We aimed to induce a context (color) dependent saccadic adaptation. In tandem, these studies explore the span of neural control of gaze shifts ranging from low-level kinematics control to high-level context dependency

Context cuedependent saccadic adaptation in rhesus macaques cannot be elicited using color

When the head does not move, rapid movements of the eyes called saccades are used to redirect the line of sight. Saccades are defined by a series of metrical and kinematic (evolution of a movement as a function of time) relationships. For example, the amplitude of a saccade made from one visual target to another is roughly 90% of the distance between the initial fixation point (T0) and the peripheral target (T1). However, this stereotypical relationship between saccade amplitude and initial retinal error (T1-T0) may be altered, either increased or decreased, by surreptitiously displacing a visual target during an ongoing saccade. This form of motor learning (called saccadic adaptation) has been described in both humans and monkeys. Recent experiments in humans and monkeys have suggested that internal (proprioceptive) and external (target shape, color, and/or motion) cues may be used to produce context-dependent adaptation. We tested the hypothesis that an external contextual cue (target color) could be used to evoke differential gain (actual saccade/initial retinal error) states in rhesus monkeys. We did not observe differential gain states correlated with target color regardless of whether targets were displaced along the same vector as the primary saccade or perpendicular to it. Furthermore, this observation held true regardless of whether adaptation trials using various colors and intrasaccade target displacements were randomly intermixed or presented in short or long blocks of trials. These results are consistent with hypotheses that state that color cannot be used as a contextual cue and are interpreted in light of previous studies of saccadic adaptation in both humans and monkeys