The modern era of oculomotor research began with the advent of the chronic single-unit recording method in the late 1960s. Research carried out in the intervening years has made it possible to provide a detailed description of the saccadic command signals that are generated by motor neurons and the formation of these signals in premotor brainstem regions. These findings have been assimilated in control-systems models that simulate important behavioural features of saccades. Despite these great advances, key issues, such as the nature of the feedback signal and the location of the comparator, are unresolved and some of the factors that have impeded progress can be identified.

/pdf/the-brainstem-control-of-saccadic-eye-movements-fz5vp6ibpm.pdf

The brainstem control of saccadic eye movements

Three types of looming-selective neurons have been found in the nucleus rotundus of pigeons, each computing a different optical variable related to image expansion of objects approaching on a direct collision course with the bird. None of these neurons respond to simulated approach toward stationary objects. A detailed analysis of these neurons’ firing pattern to approaching objects of different sizes and velocities shows that one group of neurons signals relative rate of expansion τ (tau), a second group signals absolute rate of expansion ρ (rho), and a third group signals yet another optical variable η (eta). The ρ parameter is required for the computation of both τ and η, whose respective ecological functions probably provide precise ‘time-to-collision’ information and ‘early warning’ detection for large approaching objects.

Computation of different optical variables of looming objects in pigeon nucleus rotundus neurons.

1. Orienting movements, consisting of coordinated eye and head displacements, direct the visual axis to the source of a sensory stimulus. A recent hypothesis suggests that the CNS may control gaze position (gaze = eye-relative-to-space = eye-relative-to-head + head-relative-to-space) by the use of a feedback circuit wherein an internally derived representation of gaze motor error drives both eye and head premotor circuits. In this paper we examine the effect of behavioral task on the individual and summed trajectories of horizontal eye- and head-orienting movements to gain more insight into how the eyes and head are coupled and controlled in different behavioral situations. 2. Cats whose heads were either restrained (head-fixed) or unrestrained (head-free) were trained to make orienting movements of any desired amplitude in a simple cat-and-mouse game we call the barrier paradigm. A rectangular opaque barrier was placed in front of the hungry animal who either oriented to a food target that was visible to one side of the barrier or oriented to a location on an edge of the barrier where it predicted the target would reappear from behind the barrier. 3. The dynamics (e.g., maximum velocity) and duration of eye- and head-orienting movements were affected by the task. Saccadic eye movements (head-fixed) elicited by the visible target attained greater velocity and had shorter durations than comparable amplitude saccades directed toward the predicted target. A similar observation has been made in human and monkey. In addition, when the head was unrestrained both the eye and head movements (and therefore gaze movements) were faster and shorter in the visible- compared with the predicted-target conditions. Nevertheless, the relative contributions of the eye and head to the overall gaze displacement remained task independent: i.e., the distance traveled by the eye and head movements was determined by the size of the gaze shift only. This relationship was maintained because the velocities of the eye and head movements covaried in the different behavioral situations. Gaze-velocity profiles also had characteristic shapes that were dependent on task. In the predicted-target condition these profiles tended to have flattened peaks, whereas when the target was visible the peaks were sharper. 4. Presentation of a visual cue (e.g., reappearance of food target) immediately before (less than 50 ms) the onset of a gaze shift to a predicted target triggered a midflight increase in first the eye- and, after approximately 20 ms, the head-movement velocity.(ABSTRACT TRUNCATED AT 400 WORDS)

Gaze control in the cat: studies and modeling of the coupling between orienting eye and head movements in different behavioral tasks.

Vision and the statistics of the visual environment.

The behavior of the vestibulo-ocular reflex at high velocities of head rotation.

Oblique saccades of the cat: A comparison between the durations of horizontal and vertical components

Responses of visual cells in cat superior colliculus to relative pattern movement.

A comparison between saccades and quick phases of vestibular nystagmus in the cat.

Colour inputs to orientation detectors in the human visual system.

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Oblique saccades of the cat: A comparison between the durations of horizontal and vertical components

Responses of visual cells in cat superior colliculus to relative pattern movement.

A comparison between saccades and quick phases of vestibular nystagmus in the cat.

Colour inputs to orientation detectors in the human visual system.