Illusory Motion Reveals Velocity Matching, Not Foveation, Drives Smooth Pursuit of Large Objects

When small objects move in a scene, we keep them foveated with smooth pursuit eye movements. Although large objects such as people and animals are common, it is nonetheless unknown how we pursue them since they cannot be foveated. It might be that the brain calculates an object’s centroid, and then centers the eyes on it during pursuit as a foveation mechanism might. Alternatively, the brain merely matches the velocity by motion integration. We test these alternatives with an illusory motion stimulus that translates at a speed different from its retinal motion. The stimulus was a Gabor array that translated at a fixed velocity, with component Gabors that drifted with motion consistent or inconsistent with the translation. Velocity matching predicts different pursuit behaviors across drift conditions, while centroid matching predicts no difference.We also tested whether pursuit can segregate and ignore irrelevant local drifts when motion and centroid information are consistent by surrounding the Gabors with solid frames. Finally, observers judged the global translational speed of the Gabors to determine whether smooth pursuit and motion perception share mechanisms. We found that consistent Gabor motion enhanced pursuit gain while inconsistent, opposite motion diminished it, drawing the eyes away from the center of the stimulus and supporting a motion-based pursuit drive. Catch-up saccades tended to counter the position offset, directing the eyes opposite to the deviation caused by the pursuit gain change. Surrounding the Gabors with visible frames canceled both the gain increase and the compensatory saccades. Perceived speed was modulated analogous to pursuit gain. The results suggest that smooth pursuit of large stimuli depends on the magnitude of integrated retinal motion information, not its retinal location, and that the position system might be unnecessary for generating smooth velocity to large pursuit targets

A Subconscious Interaction Between Fixation and Anticipatory Pursuit

Ocular smooth pursuit and fixation are typically viewed as separate systems, yet there is evidence that the brainstem fixation system inhibits pursuit. Here we present behavioral evidence that the fixation system modulates pursuit behavior outside of conscious awareness. Human observers (male and female) either pursued a small spot that translated across a screen, or fixated it as it remained stationary. As shown previously, pursuit trials potentiated the oculomotor system, producing anticipatory eye velocity on the next trial before the target moved that mimicked the stimulus-driven velocity. Randomly interleaving fixation trials reduced anticipatory pursuit, suggesting that a potentiated fixation system interacted with pursuit to suppress eye velocity in upcoming pursuit trials. The reduction was not due to passive decay of the potentiated pursuit signal because interleaving “blank” trials in which no target appeared did not reduce anticipatory pursuit. Interspersed short fixation trials reduced anticipation on long pursuit trials, suggesting that fixation potentiation was stronger than pursuit potentiation. Furthermore, adding more pursuit trials to a block did not restore anticipatory pursuit, suggesting that fixation potentiation was not overridden by certainty of an imminent pursuit trial but rather was immune to conscious intervention. To directly test whether cognition can override fixation suppression, we alternated pursuit and fixation trials to perfectly specify trial identity. Still, anticipatory pursuit did not rise above that observed with an equal number of random fixation trials. The results suggest that potentiated fixation circuitry interacts with pursuit circuitry at a subconscious level to inhibit pursuit. SIGNIFICANCE STATEMENT When an object moves, we view it with smooth pursuit eye movements. When an object is stationary, we view it with fixational eye movements. Pursuit and fixation are historically regarded as controlled by different neural circuitry, and alternating between invoking them is thought to be guided by a conscious decision. However, our results show that pursuit is actively suppressed by prior fixation of a stationary object. This suppression is involuntary, and cannot be avoided even if observers are certain that the object will move. The results suggest that the neural fixation circuitry is potentiated by engaging stationary objects, and interacts with pursuit outside of conscious awareness

A Covered Eye Fails To Follow an Object Moving in Depth

To clearly view approaching objects, the eyes rotate inward (vergence), and the intraocular lenses focus (accommodation). Current ocular control models assume both eyes are driven by unitary vergence and unitary accommodation commands that causally interact. The models typically describe discrete gaze shifts to non-accommodative targets performed under laboratory conditions. We probe these unitary signals using a physical stimulus moving in depth on the midline while recording vergence and accommodation simultaneously from both eyes in normal observers. Using monocular viewing, retinal disparity is removed, leaving only monocular cues for interpreting the object\u27s motion in depth. The viewing eye always followed the target\u27s motion. However, the occluded eye did not follow the target, and surprisingly, rotated out of phase with it. In contrast, accommodation in both eyes was synchronized with the target under monocular viewing. The results challenge existing unitary vergence command theories, and causal accommodation-vergence linkage

Optometric Measurements Predict Performance but Not Comfort on a Virtual Object Placement Task With a Stereoscopic 3D Display

Twelve participants were tested on a simple virtual object precision placement task while viewing a stereoscopic 3D (S3D) display. Inclusion criteria included uncorrected or best corrected vision of 20/20 or better in each eye and stereopsis of at least 40 arc sec using the Titmus stereo test. Additionally, binocular function was assessed, including measurements of distant and near phoria (horizontal and vertical) and distant and near horizontal fusion ranges using standard optometric clinical techniques. Before each of six 30 minute experimental sessions, measurements of phoria and fusion ranges were repeated using a Keystone View Telebinocular and an S3D display, respectively. All participants completed experimental sessions in which the task required the precision placement of a virtual object in depth at the same location as a target object. Subjective discomfort was assessed using the Simulator Sickness Questionnaire (SSQ). Individual placement accuracy in S3D trials was significantly correlated with several of the binocular screening outcomes: viewers with larger convergent fusion ranges (measured at near distance), larger total fusion ranges (convergent plus divergent ranges, measured at near distance), and/or lower (better) stereoscopic acuity thresholds were more accurate on the placement task. No screening measures were predictive of subjective discomfort, perhaps due to the low levels of discomfort induced

An Ideal Observer for Discrimination of the Global Direction of Dynamic Random Dot Stimuli

Extended the ideal-observer analysis to discrimination of global direction produced by dynamic random-dot cinematograms. Ss were 3 experienced observers, including the author. Discrimination of global direction was measured for various exposure durations, stimulus areas, and dot densities and bandwidths of the distribution of directions. Increasing the duration produced a greater improvement in performance than did increasing either the area or the density. Performance decreased as the distribution bandwidth increased. An ideal-observer model was developed, and the absolute efficiency for human direction discrimination was evaluated. Efficiencies were highest at large distribution bandwidths, with average efficiencies reaching 35%. A local–global noise model of direction discrimination, based on the ideal-observer model, containing a spatial and temporal integration limit as well as internal noise, was found to fit the human data well

Visible Persistence Is Reduced by Fixed-Trajectory Motion but Not Random Motion

Conducted 4 experiments with 2 experienced observers (including the author) to examine whether there is a motion-specific influence on visible persistence. Specifically, points moving in constant directions, or fixed trajectories, show less persistence than points moving with the same spatial and temporal displacements but taking random walks, randomly changing direction each frame. Ss estimated the number of points present in the display for these 2 types of motion conditions. Under conditions chosen to produce good apparent motion, the apparent number of points for the fixed-trajectory condition was significantly lower than the apparent number in the random-walk condition. Thus, the enhanced suppression of persistence observed for a target moving in a consistent direction depends on the activation of a directionally tuned motion mechanism extended over space and time

The Predictive Power of Trajectory Motion

When the central region of an obliquely oriented line is bisected by a wide, vertical opaque occluder, observers misperceive the two line segments as being misaligned (the Poggendorff illusion). If the oblique line segment is replaced with a spot moving on an oblique trajectory, little if any misalignment is perceived. This accurate alignment of oblique segments depends upon the consistent motion of the dot along the oblique trajectory and not other temporal or spatial characteristics of the motion-defined segments since random plotting of the dot along each oblique segment resulted in robust misalignment. The nullification of the Poggendorff illusion was also obtained if only one of the segments was defined by a moving spot so long as the spot moved in a direction that ‘pointed’ to the static segment. Moreover, if the occluder boundary was defined by rows of vertically moving dots, was filled with vertically moving dots or was a real (cardboard) occluder, the motion-defined oblique segments were still perceived to be aligned with little error, consistent with the unimpaired detection of a trajectory dot in noise interrupted by similar occluders [Watamaniuk, S. N. J. & McKee, S. P. (1995). ‘Seeing’ motion behind occluders. Nature, 377, 729–730]. The results are interpreted as evidence that trajectory motion produces a cascade of activity in appropriately aligned motion detectors, in the direction of motion, that continues after the moving object has been occluded to produce a prediction of where the moving object should reappear

Temporal and Spatial Integration in Dynamic Random Dot Stimuli

Random-dot cinematograms comprising many different, spatially intermingled local motion vectors can produce a percept of global coherent motion in a single direction. Three observers\u27 thresholds for discriminating the direction of global motion were measured under various conditions. Discrimination thresholds increased with the width of the distribution of directions in the cinematogram. Thresholds decreased as duration of area of the cinematogram increased. Temporal integration for global direction discrimination extends over about 465 msec (9.3 frames) while the spatial integration limit is at least as large as 63 deg₂ (circular aperture diameter = 9 deg). The large spatial integration area is consistent with the physiology of higher visual areas such as the middle temporal area and the medial superior temporal area