Human visual search does not maximize the post-saccadic probability of identifying targets.

Researchers have conjectured that eye movements during visual search are selected to minimize the number of saccades. The optimal Bayesian eye movement strategy minimizing saccades does not simply direct the eye to whichever location is judged most likely to contain the target but makes use of the entire retina as an information gathering device during each fixation. Here we show that human observers do not minimize the expected number of saccades in planning saccades in a simple visual search task composed of three tokens. In this task, the optimal eye movement strategy varied, depending on the spacing between tokens (in the first experiment) or the size of tokens (in the second experiment), and changed abruptly once the separation or size surpassed a critical value. None of our observers changed strategy as a function of separation or size. Human performance fell far short of ideal, both qualitatively and quantitatively.

Very slow search and reach: failure to maximize expected gain in an eye-hand coordination task.

We examined an eye-hand coordination task where optimal visual search and hand movement strategies were inter-related. Observers were asked to find and touch a target among five distractors on a touch screen. Their reward for touching the target was reduced by an amount proportional to how long they took to locate and reach to it. Coordinating the eye and the hand appropriately would markedly reduce the search-reach time. Using statistical decision theory we derived the sequence of interrelated eye and hand movements that would maximize expected gain and we predicted how hand movements should change as the eye gathered further information about target location. We recorded human observers' eye movements and hand movements and compared them with the optimal strategy that would have maximized expected gain. We found that most observers failed to adopt the optimal search-reach strategy. We analyze and describe the strategies they did adopt.

Cutting-edge molecular modelling to unveil new microscopic insights into the guest-controlled flexibility of metal-organic frameworks.

Metal-organic frameworks are a class of porous solids that exhibit intriguing flexibility under stimuli, leading often to reversible giant structural changes upon guest adsorption. DUT-49(Cu) and MIL-53(Cr) are fascinating flexible MOFs owing to their guest-induced breathing and negative gas adsorption behaviors respectively. Molecular simulation is one of the most relevant tools to examine these phenomena at the atomistic scale and gain a unique understanding of the physics behind them. Although molecular dynamics and Monte Carlo simulations are widely used in the field of porous materials, these methods hardly consider the structural deformation of a soft material upon guest adsorption. In this work, a cutting-edge osmotic molecular dynamics approach is developed to consider simultaneously the fluid adsorption process and material flexibility. We demonstrate that this newly developed computational strategy offers a unique opportunity to gain unprecedented molecular insights into the flexibility of this class of materials.

Gambling in the visual periphery: a conjoint-measurement analysis of human ability to judge visual uncertainty.

Recent work in motor control demonstrates that humans take their own motor uncertainty into account, adjusting the timing and goals of movement so as to maximize expected gain. Visual sensitivity varies dramatically with retinal location and target, and models of optimal visual search typically assume that the visual system takes retinal inhomogeneity into account in planning eye movements. Such models can then use the entire retina rather than just the fovea to speed search. Using a simple decision task, we evaluated human ability to compensate for retinal inhomogeneity. We first measured observers' sensitivity for targets, varying contrast and eccentricity. Observers then repeatedly chose between targets differing in eccentricity and contrast, selecting the one they would prefer to attempt: e.g., a low contrast target at 2° versus a high contrast target at 10°. Observers knew they would later attempt some of their chosen targets and receive rewards for correct classifications. We evaluated performance in three ways. Equivalence: Do observers' judgments agree with their actual performance? Do they correctly trade off eccentricity and contrast and select the more discriminable target in each pair? Transitivity: Are observers' choices self-consistent? Dominance: Do observers understand that increased contrast improves performance? Decreased eccentricity? All observers exhibited patterned failures of equivalence, and seven out of eight observers failed transitivity. There were significant but small failures of dominance. All these failures together reduced their winnings by 10%-18%.

The nonlinear structure of motion perception during smooth eye movements.

To perceive object motion when the eyes themselves undergo smooth movement, we can either perceive motion directly-by extracting motion relative to a background presumed to be fixed-or through compensation, by correcting retinal motion by information about eye movement. To isolate compensation, we created stimuli in which, while the eye undergoes smooth movement due to inertia, only one object is visible-and the motion of this stimulus is decoupled from that of the eye. Using a wide variety of stimulus speeds and directions, we rule out a linear model of compensation, in which stimulus velocity is estimated as a linear combination of retinal and eye velocities multiplied by a constant gain. In fact, we find that when the stimulus moves in the same direction as the eyes, there is little compensation, but when movement is in the opposite direction, compensation grows in a nonlinear way with speed. We conclude that eye movement is estimated from a combination of extraretinal and retinal signals, the latter based on an assumption of stimulus stationarity. Two simple models, in which the direction of eye movement is computed from the extraretinal signal and the speed from the retinal signal, account well for our results.

Reference frames in early motion detection.

To perceive the real motion of objects in the world while moving the eyes, retinal motion signals must be compensated by information about eye movements. Here we study when this compensation takes place in the course of visual processing, and whether uncompensated motion signals are ever available. We used a paradigm based on asymmetry in motion detection: Fast-moving objects are found easier among slow-moving distractors than are slow objects among fast distractors. By coupling object motion to eye motion, we created stimuli that moved fast on the retina but slowly in an eye-independent reference frame, or vice versa. In the 100 ms after stimulus onset, motion detection is dominated by retinal motion, uncompensated for eye movements. As early as 130 ms, compensated signals become available: objects that move slowly on the retina but fast in an eye-independent frame are detected as easily as those that move fast on the retina.