Seeing the future: Predictive control in neural models of ocular accommodation.

Ocular accommodation is the process of adjusting the eye's crystalline lens so as to bring the retinal image into sharp focus. The major stimulus to accommodation is therefore retinal defocus, and in essence, the job of accommodative control is to send a signal to the ciliary muscle which will minimize the magnitude of defocus. In this article, we first provide a tutorial introduction to control theory to aid vision scientists without this background. We then present a unified model of accommodative control that explains properties of the accommodative response for a wide range of accommodative stimuli. Following previous work, we conclude that most aspects of accommodation are well explained by dual integral control, with a "fast" or "phasic" integrator enabling response to rapid changes in demand, which hands over control to a "slow" or "tonic" integrator which maintains the response to steady demand. Control is complicated by the sensorimotor latencies within the system, which delay both information about defocus and the accommodation changes made in response, and by the sluggish response of the motor plant. These can be overcome by incorporating a Smith predictor, whereby the system predicts the delayed sensory consequences of its own motor actions. For the first time, we show that critically-damped dual integral control with a Smith predictor accounts for adaptation effects as well as for the gain and phase for sinusoidal oscillations in demand. In addition, we propose a novel proportional-control signal to account for the power spectrum of accommodative microfluctuations during steady fixation, which may be important in hunting for optimal focus, and for the nonlinear resonance observed for low-amplitude, high-frequency input. Complete Matlab/Simulink code implementing the model is provided at https://doi.org/10.25405/data.ncl.14945550.

Non-dominant hand use increases completion time on part B of the Trail Making Test but not on part A.

The Trail Making Test (TMT) is used in neuropsychological clinical practice to assess aspects of attention and executive function. The test consists of two parts (A and B) and requires drawing a trail between elements. Many patients are assessed with their non-dominant hand because of motor dysfunction that prevents them from using their dominant hand. Since drawing with the non-dominant hand is not an automatic task for many people, we explored the effect of hand use on TMT performance. The TMT was administered digitally in order to analyze new outcome measures in addition to total completion time. In a sample of 82 healthy participants, we found that non-dominant hand use increased completion times on the TMT B but not on the TMT A. The average completion time increased by almost 5 seconds, which may be clinically relevant. A substantial number of participants who performed the TMT with their non-dominant hand had a B/A ratio score of 2.5 or higher. In clinical practice, an abnormally high B/A ratio score may be falsely attributed to cognitive dysfunction. With our digitized pen data, we further explored the causes of the reduced TMT B performance by using new outcome measures, including individual element completion times and interelement variability. These measures indicated selective interference between non-dominant hand use and executive functions. Both non-dominant hand use and performance of the TMT B seem to draw on the same, limited higher-order cognitive resources.

The influence of APACHE II score on the average noise level in an intensive care unit: an observational study.

BACKGROUND: Noise levels in hospitals, especially in intensive care units (ICUs) are known to be high, potentially affecting not only the patients' well-being but also their clinical outcomes. In an observational study, we made a long-term measurement of noise levels in an ICU, and investigated the influence of various factors on the noise level, including the acute physiology and chronic health evaluation II (APACHE II) score. METHODS: The average noise level was continuously measured for three months in all (eight) patient rooms in an ICU, while the patient data were also registered, including the APACHE II score. The 24-hour trend of the noise level was obtained for the patients of length-of-stay (LOS) ≥1 day, which was compared to the timeline of the ICU routine events. For the patients with LOS ≥4 days, the average noise levels in the first four days were analyzed, and regression models were established using the stepwise search method based on the Akaike information criterion. RESULTS: Features identified in the 24-hour trends (n = 55) agreed well with the daily routine events in the ICU, where regular check-ups raised the 10-minute average noise level by 2~3 dBA from the surrounding values at night, and the staff shift changes consistently increased the noise level by 3~5 dBA. When analyzed in alignment with the patient's admission (n=22), the daytime acoustic condition improved from Day 1 to 2, but worsened from Day 2 to 4, most likely in relation to the various phases of patient's recovery. Regression analysis showed that the APACHE II score, room location, gender, day of week and the ICU admission type could explain more than 50% of the variance in the daily average noise level, LAeq,24h. Where these factors were argued to have causal relations to LAeq,24h, the APACHE II score was found to be most strongly correlated: LAeq,24h increased by 1.3~1.5 dB when the APACHE II score increased by 10 points. CONCLUSIONS: Patient's initial health condition is one important factor that influences the acoustic environment in an ICU, which needs to be considered in observational and interventional studies where the noise in healthcare environments is the subject of investigation.

Human stereopsis is not limited by the optics of the well-focused eye.

Human stereopsis, the perception of depth from differences in the two eyes' images, is very precise: image differences smaller than a single photoreceptor can be converted into a perceived difference in depth. To better understand what determines this precision, we examined how the eyes' optics affects stereo resolution. We did this by comparing performance with normal, well-focused optics and with optics improved by eliminating chromatic aberration and correcting higher-order aberrations. We first measured luminance contrast sensitivity in both eyes and showed that we had indeed improved optical quality significantly. We then measured stereo resolution in two ways: by finding the finest corrugation in depth that one can perceive, and by finding the smallest disparity one can perceive as different from zero. Our optical manipulation had no effect on stereo performance. We checked this by redoing the experiments at low contrast and again found no effect of improving optical quality. Thus, the resolution of human stereopsis is not limited by the optics of the well-focused eye. We discuss the implications of this remarkable finding.

Eye movements during action preparation.

Looking at actions of others activates representations of similar own actions, that is, the action resonates. This may facilitate or interfere with the actions that one intends to make. We asked whether people promote or block those effects by making eye movements to or away from the actions of others. We investigated gaze behavior with a cup-clinking task: An actor shown on a video grabbed a cup and moved it toward the participant who next grabbed his own cup in the 'same' or in a different, 'complementary', way. In the 'same' condition, participants mostly looked at the place where the actor held the cup. In the 'complementary' condition, gaze behavior was similar at the start of the actor's action. To our surprise, as the action reached completion, participants started to look at the cup's site that corresponded to the grabbing instruction for their own action. A second experiment showed that this effect grew with delay of the go-signal. This indicates that a reason for the effect may be to support memorizing the instructed action. The bottom line of the study is that passively viewed scenes (passive in the sense that nothing in the observed scene is manipulated by the viewer) are scanned to support preparation of actions that one intends to make. We discuss how this finding relates to action resonance and how it relates to links between representations of actions and objects.

Modeling inter-human movement coordination: synchronization governs joint task dynamics.

Human interaction partners tend to synchronize their movements during repetitive actions such as walking. Research of inter-human coordination in purely rhythmic action tasks reveals that the observed patterns of interaction are dominated by synchronization effects. Initiated by our finding that human dyads synchronize their arm movements even in a goal-directed action task, we present a step-wise approach to a model of inter-human movement coordination. In an experiment, the hand trajectories of ten human dyads are recorded. Governed by a dynamical process of phase synchronization, the participants establish in-phase as well as anti-phase relations. The emerging relations are successfully reproduced by the attractor dynamics of coupled phase oscillators inspired by the Kuramoto model. Three different methods on transforming the motion trajectories into instantaneous phases are investigated and their influence on the model fit to the experimental data is evaluated. System identification technique allows us to estimate the model parameters, which are the coupling strength and the frequency detuning among the dyad. The stability properties of the identified model match the relations observed in the experimental data. In short, our model predicts the dynamics of inter-human movement coordination. It can directly be implemented to enrich human-robot interaction.

Image-size differences worsen stereopsis independent of eye position.

With the eyes in forward gaze, stereo performance worsens when one eye's image is larger than the other's. Near, eccentric objects naturally create retinal images of different sizes. Does this mean that stereopsis exhibits deficits for such stimuli? Or does the visual system compensate for the predictable image-size differences? To answer this, we measured discrimination of a disparity-defined shape for different relative image sizes. We did so for different gaze directions, some compatible with the image-size difference and some not. Magnifications of 10-15% caused a clear worsening of stereo performance. The worsening was determined only by relative image size and not by eye position. This shows that no neural compensation for image-size differences accompanies eye-position changes, at least prior to disparity estimation. We also found that a local cross-correlation model for disparity estimation performs like humans in the same task, suggesting that the decrease in stereo performance due to image-size differences is a byproduct of the disparity-estimation method. Finally, we looked for compensation in an observer who has constantly different image sizes due to differing eye lengths. She performed best when the presented images were roughly the same size, indicating that she has compensated for the persistent image-size difference.

Acute alerting effects of light: A systematic literature review.

Periodic, well timed exposure to light is important for our health and wellbeing. Light, in particular in the blue part of the spectrum, is thought to affect alertness both indirectly, by modifying circadian rhythms, and directly, giving rise to acute effects. We performed a systematic review of empirical studies on direct, acute effects of light on alertness to evaluate the reliability of these effects. In total, we identified 68 studies in which either light intensity, spectral distribution, or both were manipulated, and evaluated the effects on behavioral measures of alertness, either subjectively or measured in reaction time performance tasks. The results show that increasing the intensity of polychromatic white light has been found to increase subjective ratings of alertness in a majority of studies, though a substantial proportion of studies failed to find significant effects, possibly due to small sample sizes or high baseline light intensities. The effect of the color temperature of white light on subjective alertness is less clear. Some studies found increased alertness with higher color temperatures, but other studies reported no detrimental effects of filtering out the short wavelengths from the spectrum. Similarly, studies that used monochromatic light exposure showed no systematic pattern for the effects of blue light compared to longer wavelengths. Far fewer studies investigated the effects of light intensity or spectrum on alertness as measured with reaction time tasks and of those, very few reported significant effects. In general, the small sample sizes used in studies on acute alerting effects of light make it difficult to draw definitive conclusions and better powered studies are needed, especially studies that allow for the construction of dose-response curves.

Spectral Tuning of White Light Allows for Strong Reduction in Melatonin Suppression without Changing Illumination Level or Color Temperature.

Studies with monochromatic light stimuli have shown that the action spectrum for melatonin suppression exhibits its highest sensitivity at short wavelengths, around 460 to 480 nm. Other studies have demonstrated that filtering out the short wavelengths from white light reduces melatonin suppression. However, this filtering of short wavelengths was generally confounded with reduced light intensity and/or changes in color temperature. Moreover, it changed the appearance from white light to yellow/orange, rendering it unusable for many practical applications. Here, we show that selectively tuning a polychromatic white light spectrum, compensating for the reduction in spectral power between 450 and 500 nm by enhancing power at even shorter wavelengths, can produce greatly different effects on melatonin production, without changes in illuminance or color temperature. On different evenings, 15 participants were exposed to 3 h of white light with either low or high power between 450 and 500 nm, and the effects on salivary melatonin levels and alertness were compared with those during a dim light baseline. Exposure to the spectrum with low power between 450 and 500 nm, but high power at even shorter wavelengths, did not suppress melatonin compared with dim light, despite a large difference in illuminance (175 vs. <5 lux). In contrast, exposure to the spectrum with high power between 450 and 500 nm (also 175 lux) resulted in almost 50% melatonin suppression. For alertness, no significant differences between the 3 conditions were observed. These results open up new opportunities for lighting applications that allow for the use of electrical lighting without disturbance of melatonin production.

Crowding degrades saccadic search performance.

The identity of a target is more difficult to acquire when it is surrounded by distracters. The purpose of the present experiments was to investigate the implications of this crowding phenomenon for performance and eye movements in a real-life task as search with eye movements. The participants searched for a target in a one dimensional search strip. Above and below this search strip additional elements were added. In three conditions, the similarity of these mask elements to the search elements was varied. The spatial extent of crowding is known to increase with target-mask similarity [Nazir, T. A. (1992). Effects of lateral masking and spatial precueing on gap-resolution in central and peripheral vision. Vision Research, 32, 771-777, Kooi, F. L., Toet, A., Tripathy, S. P., & Levi, D. M. (1994). The effect of similarity and duration on spatial interaction in peripheral vision. Spatial Vision, 8(2), 255-279]. One condition did not contain masks. In a visibility experiment, we firstly validated this crowding manipulation. In the search experiment, we subsequently found that with increasing crowding search times were up to 76% longer. Eye movements were also affected. The number of fixations and fixation duration increased and saccade amplitude decreased with increasing crowding. We conclude that in order to understand eye movements in (everyday) tasks that require active exploration of the visual scene, crowding should be taken into account.

Circadian-Time Sickness: Time-of-Day Cue-Conflicts Directly Affect Health.

A daily rhythm that is not in synchrony with the environmental light-dark cycle (as in jetlag and shift work) is known to affect mood and health through an as yet unresolved neural mechanism. Here, we combine Bayesian probabilistic 'cue-conflict' theory with known physiology of the biological clock of the brain, entailing the insight that, for a functional pacemaker, it is sufficient to have two interacting units (reflecting environmental and internal time-of-day cues), without the need for an extra homuncular directing unit. Unnatural light-dark cycles cause a time-of-day cue-conflict that is reflected by a desynchronization between the ventral (environmental) and dorsal (internal) pacemaking signals of the pacemaker. We argue that this desynchronization, in-and-of-itself, produces health issues that we designate as 'circadian-time sickness', analogous to 'motion sickness'.

Dyadic movement synchronization while performing incongruent trajectories requires mutual adaptation.

Unintentional movement synchronization is often emerging between interacting humans. In the present study, we investigate the extent to which the incongruence of movement trajectories has an influence on unintentional dyadic movement synchronization. During a target-directed tapping task, a participant repetitively moved between two targets in front of another participant who performed the same task in parallel but independently. When the movement path of one participant was changed by placing an obstacle between the targets, the degree of their unintentional movement synchronization was measured. Movement synchronization was observed despite of their substantially different movement trajectories. A deeper investigation of the participant's unintentional behavior shows, that although the actor who cleared the obstacle puts unintentional effort in establishing synchrony by increasing movement velocity-the other actor also unintentionally adjusted his/her behavior by increasing dwell times. Results are discussed in the light of joint action, movement interference and obstacle avoidance behavior.

Crowded visual search in children with normal vision and children with visual impairment.

This study investigates the influence of oculomotor control, crowding, and attentional factors on visual search in children with normal vision ([NV], n=11), children with visual impairment without nystagmus ([VI-nys], n=11), and children with VI with accompanying nystagmus ([VI+nys], n=26). Exclusion criteria for children with VI were: multiple impairments and visual acuity poorer than 20/400 or better than 20/50. Three search conditions were presented: a row with homogeneous distractors, a matrix with homogeneous distractors, and a matrix with heterogeneous distractors. Element spacing was manipulated in 5 steps from 2 to 32 minutes of arc. Symbols were sized 2 times the threshold acuity to guarantee visibility for the VI groups. During simple row and matrix search with homogeneous distractors children in the VI+nys group were less accurate than children with NV at smaller spacings. Group differences were even more pronounced during matrix search with heterogeneous distractors. Search times were longer in children with VI compared to children with NV. The more extended impairments during serial search reveal greater dependence on oculomotor control during serial compared to parallel search.

The venetian-blind effect: a preference for zero disparity or zero slant?

When periodic stimuli such as vertical sinewave gratings are presented to the two eyes, the initial stage of disparity estimation yields multiple solutions at multiple depths. The solutions are all frontoparallel when the sinewaves have the same spatial frequency; they are all slanted when the sinewaves have quite different frequencies. Despite multiple solutions, humans perceive only one depth in each visual direction: a single frontoparallel plane when the frequencies are the same and a series of small slanted planes-Venetian blinds-when the frequencies are quite different. These percepts are consistent with a preference for solutions that minimize absolute disparity or overall slant. The preference for minimum disparity and minimum slant are identical for gaze at zero eccentricity; we dissociated the predictions of the two by measuring the occurrence of Venetian blinds when the stimuli were viewed in eccentric gaze. The results were generally quite consistent with a zero-disparity preference (Experiment 1), but we also observed a shift toward a zero-slant preference when the edges of the stimulus had zero slant (Experiment 2). These observations provide useful insights into how the visual system constructs depth percepts from a multitude of possible depths.

Saccadic search performance: the effect of element spacing.

In a saccadic search task, we investigated whether spacing between elements affects search performance. Since it has been suggested in the literature that element spacing can affect the eye movement strategy in several ways, its effects on search time per element are hard to predict. In the first experiment, we varied the element spacing (3.4 degrees -7.1 degrees distance between elements) and target-distracter similarity. As expected, search time per element increased with target-distracter similarity. Decreasing element spacing decreased the search time per element. However, this effect was surprisingly small in comparison to the effect of varying target-distracter similarity. In a second experiment, we elaborated on this finding and decreased element spacing even further (between 0.8 degrees and 3.2 degrees). Here, we did not find an effect on search time per element for element spacings from 3.2 degrees to spacings as small as 1.5 degrees . It was only at distances smaller than 1.5 degrees that search time per element increased with decreasing element spacing. In order to explain the remarkable finding that search time per element was not affected for such a wide range of element spacings, we propose that irrespective of the spacing crowding kept the number of elements processed per fixation more or less constant.