Modulation of gaze-evoked blinks depends primarily on extraretinal factors

Gaze-evoked blinks are contractions of the orbicularis oculi (OO)-the lid closing muscle-occurring during rapid movements of the head and eyes and result from a common drive to the gaze and blink motor systems. However, blinks occurring during shifts of gaze are often suppressed when the gaze shift is made to an important visual stimulus, suggesting that the visual system can modulate the occurrence of these blinks. In head-stabilized, human subjects, we tested the hypothesis that the presence of a visual stimulus was sufficient, but not necessary, to modulate OO EMG (OOemg) activity during saccadic eye movements. Rapid, reorienting movements of the eyes (saccades) were made to visual targets that remained illuminated (visually guided trials) or were briefly flashed (memory-guided trials) at different amplitudes along the horizontal meridian. We measured OOemg activity and found that the magnitude and probability of OOemg activity occurrence was reduced when a saccade was made to the memory of the spatial location as well as to the actual visual stimulus. The reduced OOemg activity occurred only when the location of the target was previously cued. OOemg activity occurred reliably with spontaneous saccades that were made to locations with no explicit visual stimulus, generally, back to the fixation location. Thus the modulation of gaze-evoked OOemg activity does not depend on the presence of visual information per se, but rather, results from an extraretinal signal.     

Rhesus monkeys behave as if they perceive the Duncker Illusion

The visual system uses the pattern of motion on the retina to analyze the motion of objects in the world, and the motion of the observer him/herself. Distinguishing between retinal motion evoked by movement of the retina in space and retinal motion evoked by movement of objects in the environment is computationally difficult, and the human visual system frequently misinterprets the meaning of retinal motion. In this study, we demonstrate that the visual system of the Rhesus monkey also misinterprets retinal motion. We show that monkeys erroneously report the trajectories of pursuit targets or their own pursuit eye movements during an epoch of smooth pursuit across an orthogonally moving background. Furthermore, when they make saccades to the spatial location of stimuli that flashed early in an epoch of smooth pursuit or fixation, they make large errors that appear to take into account the erroneous smooth eye movement that they report in the first experiment, and not the eye movement that they actually make.     

Temporal influences on retinal correspondence: ocular motor findings in paradoxical spatial projection.

BACKGROUND: Paradoxical diplopia is a condition in which objective eye position contradicts subjective localization in visual space. The term "paradoxical" is usually reserved for instances when known sensory adaptations cannot explain the contradiction. The development of this condition begins with infantile or childhood strabismus, followed by the development of a common sensory adaptation, anomalous retinal correspondence (ARC). ARC causes a reduction in the subjective angle of strabismus compared with the objective angle, and in its completed form the subjective angle decreases to zero. There is no "adaptive" mechanism that would increase the subjective angle such that it would be greater than the objective. In cases of treatment by corrective surgery, the anatomically based motor correction leads to a contradiction between eye position and binocular perception. In this event, the objective angle is less than the subjective, and the result is a paradoxical perception. We encountered a 25-year-old woman who experiences paradoxical localization on cover testing in the absence of a manifest strabismus and with no previous surgical intervention. METHODS: Using a magnetic search coil technique, we evaluated eye movements during fixation, smooth pursuit, saccades, and during cover test conditions to determine how these eye movements correlated to the subjective perception in space. RESULTS: Although smooth pursuit and saccades were normal, there were two elements during cover test that could explain the paradoxical projection. One was the phenomenon that during the cover test the paradoxical projection appeared only when the eye was covered for >4 s. The second was that there was a regression from the full exophoria position toward the midline while the eye was under cover that correlated with a possible paradoxical projection situation. DISCUSSION: ARC, usually associated with a beneficial sensorimotor adaptation, can express itself as a detrimental sensorimotor manifestation. Paradoxical perception also can exist without previous surgical intervention and without the influence of prisms and instrumentation such as the synoptophore. Further studies are indicated to explore the sensorimotor feedback mechanism between eye position and spatial perception.