The brainstem burst generator for saccadic eye movements

Abstract

In the 16 years since we last summarized the behavior of the premotor elements that control saccades, research has revealed shortcomings in previous formulations of the control mechanisms of the brainstem saccadic burst generator. Specifically, complexities in the eye movement plant, a more detailed knowledge of the behaviors of certain bursting neurons, and previously undiscovered anatomical connections have broadened our knowledge but have generated new questions that require rethinking previous concepts. Perhaps the most crucial revelations/insights have come from studies that have implicated the superior colliculus and the midline cerebellum as crucial elements of the burst generator. In summarizing these recent findings here, we have been led to conclude that the superior colliculus issues the saccadic command and receives feedback from the brainstem burst generators, but the feedback does not control saccade size. In addition, the midline cerebellum also contains a feedback path, but only as part of a more generalized circuit that serves multiple functions.

Crossmodal coupling of oculomotor control and spatial attention in vision and audition

“…The shape and the time course of this rate modulation effect is comparable to saccadic inhibition, a knee-jerk effect of decreased frequency of large saccades observed 47 to 70 ms after abrupt onsets of irrelevant stimuli (e.g., Stampe, 2002, 2004). Reingold and Stampe associated saccadic inhibition with inhibitory processes in the superior colliculus (SC), one of the most important brain structures controlling saccade generation (for reviews see Munoz et al 2000;Scudder et al 2002;Sparks 2002). We also propose that the inhibition of microsaccades can be attributed to an enhanced fixation of gaze and a corresponding inhibition of saccade-related neurons in this oculomotor area.…”

“…Findlay and Walker 1999;Rizzolatti et al 1987Rizzolatti et al , 1994) have proposed such a coupling of spatial shifts of attention (independent of modality) and the preparation of saccadic movements for some time. The SC is the usual suspect in being a neurological counterpart to these models since it is involved in the coordination of covert shifts of attention (Albano et al 1982;Desimone et al 1989;Kustov and Robinson 1996) and the initiation of saccades (reviews in Munoz et al 2000;Scudder et al 2002;Sparks 2002). Moreover, cell organization in the SC is well adapted to the task of shifting gaze to peripheral targets.…”

“…Moreover, cell organization in the SC is well adapted to the task of shifting gaze to peripheral targets. Sensory input cells activated by visual information as well as discharges of responsive cells connected to eye movements are topographically organized in a way that information from visual receptive fields may lead to coding saccadic movement vectors (for reviews see Munoz et al 2000;Scudder et al 2002;Sparks 2002). In the deep layers of the SC, visual and auditory inputs converge (reviewed in Stein and Meredith 1993).…”

“…How does our study fit into the current research landscape? Over the last years we have witnessed much progress in relating visual attention and oculomotor processes and especially how both are linked to the dynamics of activation and inhibition in the SC (reviews in Munoz et al 2000;Scudder et al 2002;Sparks 2002).…”

Shortening and prolongation of saccade latencies following microsaccades

“…Microsaccades' binocularity (Ditchburn and Ginsborg 1953;Krauskopf et al 1960;Lord 1951), involuntary occurrence (Ditchburn and Ginsborg 1953;Ratliff and Riggs 1950), and shared kinematic characteristics with large-scale saccades (Zuber et al 1965) hint at common subcortical mechanisms for the generation of both types of movements (see also discussion). A likely neural correlate of microsaccade generation is the rostral pole of the superior colliculus (SC), a brainstem structure critically involved in the control of saccades and fixations (see Munoz et al 2000, Scudder et al 2002, and Sparks 2002, for reviews). The rationale is that saccade amplitudes are coded along the SC rostralcaudal dimension; small amplitudes like those associated with microsaccades being related to activity in the rostral pole (Robinson 1972).…”