Head-free Condition Research Articles

Quasi-3D: reducing convergence effort improves visual comfort of head-mounted stereoscopic displays

The diffusion of virtual reality urges to solve the problem of vergence-accommodation conflict arising when viewing stereoscopic displays, which causes visual stress. We addressed this issue with an approach based on reducing ocular convergence effort. In virtual environments, vergence can be controlled by manipulating the binocular separation of the virtual cameras. Using this technique, we implemented two quasi-3D conditions characterized by binocular image separations intermediate between 3D (stereoscopic) and 2D (monoscopic). In a first experiment, focused on perceptual aspects, ten participants performed a visuo-manual pursuit task while wearing a head-mounted display (HMD) in head-constrained (non-immersive) condition for an overall exposure time of ~ 7 min. Passing from 3D to quasi-3D and 2D conditions, progressively resulted in a decrease of vergence eye movements—both mean convergence angle (static vergence) and vergence excursion (dynamic vergence)—and an increase of hand pursuit spatial error, with the target perceived further from the observer and larger. Decreased static and dynamic vergence predicted decreases in asthenopia trial-wise. In a second experiment, focused on tolerance aspects, fourteen participants performed a detection task in near-vision while wearing an HMD in head-free (immersive) condition for an overall exposure time of ~ 20 min. Passing from 3D to quasi-3D and 2D conditions, there was a general decrease of both subjective and objective visual stress indicators (ocular convergence discomfort ratings, cyber-sickness symptoms and skin conductance level). Decreased static and dynamic vergence predicted the decrease in these indicators. Remarkably, skin conductance level predicted all subjective symptoms, both trial-wise and session-wise, suggesting that it could become an objective replacement of visual stress self-reports. We conclude that relieving convergence effort by reducing binocular image separation in virtual environments can be a simple and effective way to decrease visual stress caused by stereoscopic HMDs. The negative side-effect—worsening of spatial vision—arguably would become unnoticed or compensated over time. This initial proof-of-concept study should be extended by future large-scale studies testing additional environments, tasks, displays, users, and exposure times.

Lack of a clear head movement benefit in a spatial release form masking task

The study of spatial benefit in complex listening environments often requires listeners to maintain a fixed head position so that controls are in place for understanding binaural function. In natural listening environments, however, listeners may use head movements (e.g., turns and tilts) to achieve their maximum speech reception. In a previous study, we developed a spatial release from masking task that adapts the location of the maskers to determine the angular threshold for a predetermined masking release (in dB), and in the present study, we modified this task to allow for the listeners to move their heads as they wished to maximize performance on the task. Twenty young normal hearing listeners were tested on the head-fixed and head-free conditions. Head tracking was collected for posthoc analyses. Overall, there was no significant difference in behavioral thresholds between conditions despite wide ranges of head movements in the head-free condition. Interestingly, large individual differences in head movement translated to a narrowing of the variance in behavioral thresholds relative to the head-fixed condition, suggesting that listeners were indeed optimizing their speech reception. Further analyses will dissect the types of head movements and strategic differences among the participants, with implications for listeners with hearing loss.

Saccades and Eye–Head Coordination in Ataxia with Oculomotor Apraxia Type 2

Ataxia with oculomotor apraxia type 2 (AOA2) is one of the most frequent autosomal recessive cerebellar ataxias. Oculomotor apraxia refers to horizontal gaze failure due to deficits in voluntary/reactive eye movements. These deficits can manifest as increased latency and/or hypometria of saccades with a staircase pattern and are frequently associated with compensatory head thrust movements. Oculomotor disturbances associated with AOA2 have been poorly studied mainly because the diagnosis of oculomotor apraxia was based on the presence of compensatory head thrusts. The aim of this study was to characterise the nature of horizontal gaze failure in patients with AOA2 and to demonstrate oculomotor apraxia even in the absence of head thrusts. Five patients with AOA2, without head thrusts, were tested in saccadic tasks with the head restrained or free to move and their performance was compared to a group of six healthy participants. The most salient deficit of the patients was saccadic hypometria with a typical staircase pattern. Saccade latency in the patients was longer than controls only for memory-guided saccades. In the head-free condition, head movements were delayed relative to the eye and their amplitude and velocity were strongly reduced compared to controls. Our study emphasises that in AOA2, hypometric saccades with a staircase pattern are a more reliable sign of oculomotor apraxia than head thrust movements. In addition, the variety of eye and head movements' deficits suggests that, although the main neural degeneration in AOA2 affects the cerebellum, this disease affects other structures.

Measurement of Useful Field of View in Eye and Head-Free Condition while Driving

In this study, we conducted an experiment aimed at developing an effective method for measuring the useful field of view when driving while maintaining free visual (eye and head free) conditions with completely unobstructed head and eye movements. In the experiment, the mental workload was changed in a stepwise manner in order to test the hypothesis that the useful field of view becomes narrower as the mental workload (MWL) increases, as well as to verify the accuracy of the proposed method. The results confirmed the validity and accuracy of the proposed method for measuring the useful field of view under completely unobstructed visual conditions.

Immersive Discovery Method for Exploring Interaction Strategies of an Agent

We propose an alternative approach to find each robotic agent's unique interaction strategies, called Immersing Discovery Method. In this approach, the human manipulator behaves as if she/he becomes the robot and finds efficient interaction strategies based on each robot's shape and modalities. We implement the system including a reconfigurable body robot, an easier manipulation system, and a recording system to evaluate the validity of our method. We evaluate a block-assembling task by the system by turning on and off the modality of the robot's head. Subsequently, the robot's motion during player's motion significantly increases whereas the ratio of confirmatory behavior significantly decreases in the head-fixed design. This result suggests that the robot leads the users and the user follows the robot in head-fixed condition not as in a normal turn-taking interaction style of the head-free condition.

Subthalamic stimulation improves orienting gaze movements in Parkinson’s disease

ObjectiveTo determine the effect of subthalamic stimulation on visually triggered eye and head movements in patients with Parkinson’s disease (PD). MethodsWe compared the gain and latency of visually triggered eye and head movements in 12 patients bilaterally implanted into the subthalamic nucleus (STN) for severe PD and six age-matched control subjects. Visually triggered movements of eye (head restrained), and of eye and head (head unrestrained) were recorded in the absence of dopaminergic medication. Bilateral stimulation was turned OFF and then turned ON with voltage and contact used in chronic setting. The latency was determined from the beginning of initial horizontal eye movements relative to the target onset, and the gain was defined as the ratio of the amplitude of the initial movement to the amplitude of the target movement. ResultsWithout stimulation, the initiation of the head movement was significantly delayed in patients and the gain of head movement was reduced. Our patients also presented significantly prolonged latencies and hypometry of visually triggered saccades in the head-fixed condition and of gaze in head-free condition. Bilateral STN stimulation with therapeutic parameters improved performance of orienting gaze, eye and head movements towards the controls’ level ConclusionsThese results demonstrate that visually triggered saccades and orienting eye–head movements are impaired in the advanced stage of PD. In addition, subthalamic stimulation enhances amplitude and shortens latency of these movements. SignificanceThese results are likely explained by alteration of the information processed by the superior colliculus (SC), a pivotal visuomotor structure involved in both voluntary and reflexive saccades. Improvement of movements with stimulation of the STN may be related to its positive input either on the STN–Substantia Nigra–SC pathway or on the parietal cortex–SC pathway.

Infant eye and head movements toward the side opposite the cue in the anti-saccade paradigm

BackgroundThe anti-saccade task, when people must respond in the direction opposite to a visual stimulus, has been used as a marker of operation of the frontal cortical oculomotor area. However, early development of oculomotor control has been little studied with the infant anti-saccade paradigm, and a few studies did not recognize anti-saccades in infants in light of the results of adult anti-saccade. Since the characteristics of infant eye movements are little known, applying the criteria used in adult study is by no means the best way to study infant anti-saccade. As it is indicated that coordinated eye and head movements often enable infants to control the direction of their gaze, head movements should be examined as an infant orienting response. The aim of this study was to address how infants used eye and head movements during the anti-saccade paradigm. To distinguish infants' responses, we also investigated eye and head movements during a task for an inhibition of return. Inhibition of return, in which delayed responses occur in the direction to which attention had previously been oriented, has been thought to mark activity of the superior colliculus. Since the superior colliculus is thought to develop much earlier in life than the frontal lobes, we thought it useful to compare these task performances during infancy.MethodsInfants were divided into three groups according to age. Anti-saccade and inhibition-of-return tasks were given. Their eye and head movements during tasks were independently recorded by the corneal reflection method in the head-free condition.ResultsYounger infants tended to initiate eye movement less than older ones in both tasks. In the anti-saccade task, responses opposite to the cue tended to show longer latency than responses to the cue. Infants made faster responses toward the side opposite the cue when it was to the right than when it was left of fixation. Regarding the comparison of responses toward the side opposite the cue between two tasks, the leftward eye movement was faster than the leftward head movements in the inhibition-of-return task, while no difference of latency was observed between eye and head movements in the anti-saccade task. A qualitative analysis of the trajectory of these responses revealed that head movement trajectories were steeper in the anti-saccade than in the inhibition-of-return task.ConclusionYounger infants move head and eyes together, with head movements frequently starting first. On the other hand, both the leftward latency difference between eye and head and gentle trajectories of head in inhibition of return indicate that eye movements are more predominant over head movements in the inhibition-of-return task than in the anti-saccade task. This would suggest an earlier developing inhibition-of-return mechanism.

Saccade initiation in ocular motor apraxia

Sirs: Ocular motor apraxia (OMA) is characterized by the loss of voluntary saccade control but the simultaneous preservation of saccades of the optokinetic and the vestibulo-ocular reflex (VOR) [1, 4, 6]. While the suitability of the term is still being debated [6, 10], it is not yet clear which of the conceivable compensation strategies adult patients with OMA use to elicit saccades. Saccades can be initiated in three possible ways: by using the vestibulo-ocular reflex (VOR) [2], signals of voluntary head commands [11], or blinks [2]. We tested for the first time the competing hypotheses in two adult patients with OMA to elucidate the mechanism that helps to restore the saccadic behaviour. The first patient, a 32-year-old woman without a relevant medical history, complained of slowly progressive blurred vision over recent years. She could only produce small vertical but not horizontal saccades on command or to predictive and unpredictive visual targets without concomitant head movements. However, she was able to make small horizontal and vertical saccades in the head-free condition, i. e., during small horizontal head movements or while reading. The vestibular-ocular reflex and optokinetic nystagmus were normal. Neurological examination was otherwise unremarkable, and cranial MRI was normal. The second patient, a 60-yearold man, had had difficulty in reading for several years. A neurological examination revealed that he could hardly make any saccades on command or to predictive and unpredictive targets in the vertical direction without making concomitant head movements. However, saccades could be elicited by optokinetic and vestibular stimuli. In addition, the patient had a right hemiparesis. MRI showed a large left-sided frontoparietal cortical infarction and additional subcortical ischemic lesions on the right side. Eye, head, and lid movements were recorded with the scleral search-coil method, calibrated, and analysed as reported elsewhere [8, 9]. Statistical tests were performed using Student’s t-test and ANOVA with p-values of less than 0.05 for statistical significance. The subject’s head was measured with (head-fixed condition) and without a fixed head support (head-free condition) [8, 9]. Visually guided horizontal and vertical saccades (5 to 20 deg amplitudes) were examined in both conditions. Four healthy aged-matched subjects out of a group of 21 (aged 46 ± 13 years) were used as controls for the head-fixed condition. For the comparison of the two recording conditions (head-fixed vs. head-free) the patients’ latencies were individually compared with two aged-matched control subjects. In the head-fixed condition patient #1 made virtually no visually guided saccades in the horizontal direction (Fig. 1A); patient #2 made virtually none in the vertical direction. Saccades were delayed and hypometric compared with those of the healthy control subjects (Table 1). In the age-matched control subjects there was no difference in the latencies between the head-free (224 + 42 ms) and the head-fixed condition (217 + 38 ms) measured in the same experimental session. In the head-free condition horizontal and vertical visually guided saccades were elicited in both patients (Table 1; Fig. 1B). However, head movements were much more prominent in the patients than in controls (Fig. 1B and E, thin black LETTER TO THE EDITORS

Eye position and target amplitude effects on human visual saccadic latencies.

Gaze shifts vary in the extent of eye and head contribution; a large amplitude and/or an eccentric ocular orbital starting position alter the participation of head movement in the shift. The interval between eye onset and head onset determines compensatory counterrolling before and after the shift and the extent of vestibular ocular reflex reduction during the shift. The latency of eye saccades in the head-fixed condition was measured with respect to target amplitude and orbital position in order to establish base-line operations of these two variables as they apply to the head-free condition. Eye movements were measured during single-step saccades in nine young adult humans. The target step, hereafter called a jump, started from three possible fixation lights; e.g., rightward saccades started from the midline (0 degree) or from -20 or -40 degrees left of the midline, with a maximum amplitude of 80 degrees. The latency of saccades starting from the primary position increased with jump amplitude (amplitude-latency relation). When the eye started eccentrically, the latency was decreased (orbital position-latency relation), with the largest jump amplitudes most affected. These changes can be related to active eye-head coordination. Thus, with a leftward maximal orbital eccentricity, compensatory eye rotation would be impossible with a rightward head movement; however, incorporating the orbital position-latency relation, the forward ocular saccade is expedited by 90 ms. Conversely, with a primary starting position, the ocular component of an 80 degrees gaze saccade could be slowed 125 ms by incorporating the amplitude-latency relation, thus facilitating a head contribution to the gaze shift. The orbital position and amplitude-latency relations were prominent in those subjects with habitually large head contributions to the gaze shift and minimal in individuals with typically small head contributions.

Chapter 7 Firing characteristics of neurones in the superior colliculus and the pontomedullary reticular formation during orienting in unrestrained cats

Cats were trained to fixate a center LED or light spot for 1-10 s and to orient to a target (LED or light spot), which appeared simultaneous with the disappearance of the fixation light. Firing characteristics of neurones in the superior colliculus (SC) and the pontomedullary reticular formation were examined systematically in cats performing the above orienting task. In the head-free condition, a majority of the superficial neurones in the SC was activated when a target appeared in a particular area in the visual field (receptive field) as in head fixed animals. We identified a new group of neurones in superficial layers of the SC, superficial fixation neurones, which fired tonically when fixated on a target light with little activity at fixating a center fixation light. In the intermediate and deep layers of the SC, many neurones fired when directed to a particular area in the visual field (movement field). In addition to these neurones, we found new groups of neurones fired tonically in a particular phase of orienting. These neurones were divided into three principal types, target activated, target suppressed and center fixation activated neurones. Target activated neurones fired tonically during the fixation of a target, but not during fixation of a center spot. Target suppressed neurones were characterized by a pause during fixation of targets irrespective of their location. They had clear phasic and tonic components. The phasic component was characterized by a movement field resembling that seen in a typical intermediate layer neurone. Center fixation activated neurones discharged during fixation of the center spot and exhibited suppression during fixation of the target. Many neurones in the pontomedullary reticular formation fired during orienting. Four major types of neurones, phasic, phasic sustained, pause and tonic neurones, were differentiated on the basis of their firing patterns. Phasic units were characterized by brief phasic firing during orienting and could be divided into four subtypes. Long lead phasic neurones began to fire 50-100 ms prior to the onset of a gaze shift and stopped well before the end of the gaze shift and the total number of firing was best correlated with the total angle of head rotation. Short lead phasic neurones started to fire 10-20 ms prior to a gaze shift and were further subdivided into decrement and plateau types. The decrement type exhibited a brief burst just prior to gaze shift and stopped discharging prior to the end of the gaze shift, while the plateau type fired tonically during the gaze shift and stopped firing after the gaze shift. Total number of discharges of both type was related to maximal angular velocity. The fourth type, gaze neurone, began firing coincident with the onset of the gaze shift and stopped firing at the completion of the gaze shift and the total number of discharges was closely related to total angle of head rotation Phasic sustained (PS) neurones fired 50-100 ms before the onset of a gaze shift and their discharges continued after the end of the gaze shift and were divided into three subtypes: augmenting, pause and plateau. The augmenting type was characterized by a transient increase of firing during the gaze shift and a maintained slow rate of firing following the gaze shift. Total number of discharges related to head movement amplitude and the sustained activity was correlated with head position. The pause type was suppressed during the gaze shift and was tonically active during a head movement. The plateau type had a weak phasic component and discharged at a fairly constant rate. The pause and plateau types appeared to be related to head amplitude and head position, respectively. Tonic units were classified into several types. Target fixation neurones exhibited sustained firing when the cat was fixating a target, irrespective of its location, but did not show sustained firing when the animal fixated the fixation light. Fixation neurones fired tonical

Modeling Slow Correcting Gaze Movements

Recent experimental results show evidence for the corrective role of postsaccadic drifts. This paper addresses the modeling of these slow correcting gaze movements (SCMs). Classical arguments to explain drifts are presented, both in the head fixed condition (pulse-step mismatch) and in the head free condition (vestibulo-ocular reflex (VOR)). The most significant behavioral and electrophysiological experimental data related to SCMs are then briefly reported, with the conclusion that SCMs have a clear corrective role, incompatible with classical explanations of drifts. Based on these experimental data, existing models of the saccadic system are then compared. A theoretical comparison of the classical Robinson model with an alternative model is proposed. Two possible (slow and fast) pathways involved in the control of SCMs are examined, and simulation results are presented. Finally, the discussion addresses the observed species differences in SCMs. The link between natural SCMs and electrical SC stimulations, and the interactions between saccades, VOR, and SCMs are also discussed.

Visual and vestibular reflexes that stabilize gaze in the chameleon.

Spontaneous eye movements as well as visual, vestibular, and proprioceptive cervical reflexes which contribute to gaze stabilization were investigated in the chameleon using the magnetic search-coil technique. The oculomotor range of each eye was very large (180 deg horizontally x 80 deg vertically). Spontaneous ocular saccades were independent in the two eyes and could have very large amplitudes. The fast phases of nystagmus during the stabilization reflexes were also independent in the eyes. In the head-restrained condition, optokinetic nystagmus (OKN) had a low gain in both horizontal and vertical planes (0.35 at 5 deg/s) and showed little binocular interaction. The vestibulo-ocular reflex (VOR) exhibited a low gain (0.2-0.3 from 0.05-1 Hz) and a high-phase lead at low frequency (140 deg at 0.05 Hz). Rotation of the animal in the presence of a visible surround increased the overall gain of gaze stabilization to 0.4-0.5 (P < 0.01) and considerably reduced the phase lead (38 deg at 0.05 Hz). In the head-free condition, head and eye reflexes were active simultaneously during both optokinetic and vestibular stimulation, but nystagmic head movements appeared only occasionally with a rather loose eye-head coordination. During optokinetic stimulation, eye movements contributed more than head movements to gaze stabilization, whereas, during vestibular or visuo-vestibular stimulation, the relative contribution of eye and head responses varied with stimulus frequency. When the head was freed, overall gain for gaze stabilization increased from 0.35 to 0.45 (P < 0.05) for optokinetic stimulation at 5 deg/s and from 0.2-0.3 to 0.4-0.75 (P < 0.001) for vestibular stimulation at 0.05-1 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous Gaze Shifts in Intact Head-free Rats and Following Inferior Olive and Cerebellar Lesions.

Recent experiments have shown that after lesions of the inferior olive or of the flocculus and paraflocculus of the cerebellum, in the pigmented rat, spontaneous saccades made when the head is completely restrained are followed by a large postsaccadic drift. The aims of the present paper were to study (i) the strategies and the characteristics of spontaneous eye - head coordinated gaze shifts in intact pigmented rats and to compare them with those described in other mammals, (ii) how they are affected by inferior olive and flocculus - paraflocculus lesions, and (iii) whether in these groups of animals the stability of the gaze is more deficient when the head is free to rotate in the horizontal plane (head-free condition) relative to the head-fixed condition. Three types of gaze shift strategy of intact rats are described and characterized. Following inferior olive or flocculus - paraflocculus lesion the dynamic parameters of such gaze shifts (the main sequences of head, gaze and eye and the timing of eye and head movement onset) are not significantly affected. The main deficits of lesioned animals affect the stability of gaze at the end of gaze shifts. After inferior olive lesion the amplitude of the postsaccadic drift of the gaze is 43.2% of the gaze saccade in the head-fixed condition, which is reduced to 22.9% in the head-free condition. Following flocculus - paraflocculus lesion the postsaccadic drift of gaze is even more reduced than after inferior olive lesion, changing from 39.2% in the head-fixed condition to only 9.7% in the head-free condition.

Dynamic feedback to the superior colliculus in a neural network model of the gaze control system

The role of the superior colliculus (SC) has been explored in the context of the gaze control system, with the conclusion that recent data place it inside the control loop calculating gaze error. A neural network proposed for the SC achieves both spatial and temporal integration of gaze signals. When placing the SC inside a gaze control loop, linear time-space transformations can be achieved even with restricted feedback on the edges of the map. The proposed gaze control model can reproduce all of the properties of previous head-fixed models. However, for the first time, a collicular mechanism is shown to provide plausible explanations for conflicting results upon electrical stimulation of rostral versus caudal sites on the colliculus, and can generate reasonable trajectories for both eye and head platforms in the head-free condition. The key assumption is the distribution of a common motor (gaze) error to the platforms contributing to the movement.

Visual control of steering in locust flight: the effects of head movement on responses to roll stimuli

The contribution of head movement to the control of roll responses in flying locusts (Locusta migratoria) has been examined (i) on a flight balance, recording the angles through which the locust turns when following an artificial horizon; (ii) by recording activity in a pair of flight muscles in restrained conditions; and (iii) by observations on free flying locusts. Responses were compared when the head was free to turn about the thorax, as normal, and when the head was waxed to the thorax, blocking any relative motion between the two (‘head-fixed’). These experiments suggest that the major signal generating corrective roll manoeuvres is the visual error between the angle of the head and the horizon, rather than a signal that includes a measure of the head-thorax angle. 1. On the flight balance in the head-free condition the roll angle of the thorax was consistently less than in the head-fixed state, and followed the stimulus with longer response lags. Furthermore, the difference between the angle of the thorax assumed during head-free and head-fixed rolls was close to the angle of the head relative to the thorax during head-free responses. 2. Records of activity of the forewing first basalar muscles (M97) were made during rotation of the horizon about immobilized animals. When the head could follow the horizon, the relative latency between activity in the left and right basalar muscles decreased as the head position turned to approach the displaced horizon. When head-fixed, the relative latency was directly proportional to horizon angle. 3. The relative latency between left and right M97 flight muscles correlates better with the visual error signal than with the horizon position signal, lagging by approximately 40 ms. 4. In the open air, head-fixed locusts appear able to fly as well as head-free locusts.

The contribution of coordinated eye and head movements in hand pointing accuracy.

The accuracy of pointing movements of the hand, directed at visual targets 10 degrees to 40 degrees from the midline, was measured in normal human subjects. No visual feedback from the moving hand was available to the subjects. The head could be either maintained stationary (head-fixed condition) or free to move (head-free condition) during the pointing movements. It was found that the error in pointing was reduced for all targets in the head-free condition. This reduction was more important for the more eccentric target (40 degrees). Improvement in accuracy was observed without any significant change in either the latency or the duration of eye, head or hand movements. In the head-free condition, it was found that the head was displaced in the direction of the target by an amount representing no more than 2/3 of the target amplitude. The improvement in accuracy was not influenced by the amplitude of the head movement. A model is proposed which shows how coordinated eye and head movements could improve the encoding of target position.