Collicular microstimulation during passive rotation does not generate fixed gaze shifts

Pathways for Naturalistic Looking Behavior in Primate I: Behavioral Characteristics and Brainstem Circuits

Organisms control their visual worlds by moving their eyes, heads, and bodies. This control of "gaze" or "looking" is key to survival and intelligence, but our investigation of the underlying neural mechanisms in natural conditions is hindered by technical limitations. Recent advances have enabled measurement of both brain and behavior in freely moving animals in complex environments, expanding on historical head-fixed laboratory investigations. We juxtapose looking behavior as traditionally measured in the laboratory against looking behavior in naturalistic conditions, finding that behavior changes when animals are free to move or when stimuli have depth or sound. We specifically focus on the brainstem circuits driving gaze shifts and gaze stabilization. The overarching goal of this review is to reconcile historical understanding of the differential neural circuits for different "classes" of gaze shift with two inconvenient truths. (1) "classes" of gaze behavior are artificial. (2) The neural circuits historically identified to control each "class" of behavior do not operate in isolation during natural behavior. Instead, multiple pathways combine adaptively and non-linearly depending on individual experience. While the neural circuits for reflexive and voluntary gaze behaviors traverse somewhat independent brainstem and spinal cord circuits, both can be modulated by feedback, meaning that most gaze behaviors are learned rather than hardcoded. Despite this flexibility, there are broadly enumerable neural pathways commonly adopted among primate gaze systems. Parallel pathways which carry simultaneous evolutionary and homeostatic drives converge in superior colliculus, a layered midbrain structure which integrates and relays these volitional signals to brainstem gaze-control circuits.

Effects of pupillary light and darkness reflex on the generation of pro- And anti-saccades

Saccades are often directed toward a stimulus that provides useful information for observers to navigate the visual world. The quality of visual signals of a stimulus is influenced by global luminance, and the pupil constricts or dilates after a luminance increase or decrease, respectively, to optimize visual signals for further information processing. Although luminance level changes regularly in the real environment, saccades are mostly studied in the luminance-unchanged setup. Whether pupillary responses triggered by global luminance changes modulate saccadic behavior are yet to be explored. Through varying background luminance level in an interleaved pro- and anti-saccade paradigm, we investigated the modulation of pupillary luminance responses on the generation of reflexive and voluntary saccades. Subjects were instructed to either automatically look at the peripheral stimulus (pro-saccade) or to suppress the automatic response and voluntarily look in the opposite direction from the stimulus (anti-saccade). Level of background luminance was increased (light), decreased (dark), or unchanged (control) during the instructed fixation period. Saccade reaction time distributions of correct pro- and anti-saccades in the light and dark conditions were differed significantly from those in the control condition. Moreover, the luminance condition modulated saccade kinematics, showing reduced performances in the light condition than in the control condition, particularly in pro-saccades. Modeling results further suggested that both pupil diameter and pupil size derivative significantly modulated saccade behavior, though effect sizes were small and mainly mediated by intersubject differences. Together, our results demonstrated the influence of pupillary luminance responses on the generation of pro- and anti-saccades.

Decisions in motion: vestibular contributions to saccadic target selection

The natural world continuously presents us with many opportunities for action, and thus a process of target selection must precede action execution. While there has been considerable progress in understanding target selection in stationary environments, little is known about target selection when we are in motion. Here we investigated the effect of self-motion signals on saccadic target selection in a dynamic environment. Human subjects were sinusoidally translated (f = 0.6 Hz, 30-cm peak-to-peak displacement) along an interaural axis with a vestibular sled. During the motion two visual targets were presented asynchronously but equidistantly on either side of fixation. Subjects had to look at one of these targets as quickly as possible. With an adaptive approach, the time delay between these targets was adjusted until the subject selected both targets equally often. We determined this balanced time delay for different phases of the motion in order to distinguish the effects of body acceleration and velocity on saccadic target selection. Results show that acceleration (or position, as these are indistinguishable during sinusoidal motion), but not velocity, affects target selection for saccades. Subjects preferred to look at targets in the direction of the acceleration-the leftward target was preferred when the sled accelerated to the left, and vice versa. Saccadic reaction times mimicked this selection bias by being reliably shorter to targets in the direction of acceleration. Our results provide evidence that saccade target selection mechanisms are modulated by self-motion signals, which could be derived directly from the otolith system.

Interaction between visual and vestibular signals for the control of rapid eye movements

To investigate interactions between voluntary and reflexive eye movements, five subjects were asked to make pro- or anti-saccades to various oblique locations cued by a head-fixed flash while being rotated sinusoidally in yaw (0.17 Hz; 73 degrees /s peak velocity) in complete darkness. Eye movements were recorded with the coil technique. In the pro-saccade task, targeting responses showed clear compensation for the intervening nystagmus, but there was a marked increase in horizontal scatter. Most quick phases directed into the hemifield opposite to the flash (away trials) were suppressed from ~100 ms onward. By contrast, quick phases directed into the hemifield of the flash (toward trials) continued virtually unabated until visually triggered saccades began to appear. From 80 ms onward, these vestibularly triggered movements showed signs of metrical modification by the visual signal. In the anti-saccade experiments, suppression of quick phases away from the flash was just as strong as in the pro-saccade experiments, and error rates in these trials were almost as low as in stationary control conditions. Suppression of quick phases directed toward the flash was a new phenomenon that emerged only in anti-saccade experiments. Since this inhibition had a late onset and was only partial, error rates in anti-saccade toward trials were very high. At short latencies, both components of most rapid eye movements were wrongly directed toward the flash. This was followed by a stage with frequent incongruent responses in which unsuppressed quick phases provoked an incorrect horizontal movement, whereas the vertical component showed a correct anti-saccade response. At still longer latencies, most responses were correct in both components. The visual modification of short-latency responses in both tasks showed that rapid eye movements could not simply be classified as either voluntary or reflexive, but suggested that signals underlying each class could merge into a compromise response. That vestibular rotation during the anti-saccade task may cause a wrongly directed horizontal component resembling a quick phase, combined with a vertical component expressing a correct anti-saccade signal, reveals a remarkable independence at the component level. These observations suggest that voluntary and involuntary movements can be programmed in parallel. This behavior is explained most parsimoniously by assuming that the two signals converge at a component-coding stage of the system, rather than at a vectorial coding stage.