Attentional demands of perception of passive self-motion in darkness.

The purpose of this study was to determine whether significant attentional resources are required to accurately monitor changes in bodily orientation, using vestibular information. This question was addressed firstly using a dual-task paradigm in which orientation perception tasks and a speeded auditory tone discrimination task were carried out either singly or in combination. For the active orientation perception task, subjects were seated in darkness on a motorised chair which could be rotated about an earth-vertical axis. Following passive angular displacements, subjects were required to return the chair to their perceived starting position, using a joy-stick which controlled chair motion. For the speeded auditory task, subjects pushed a hand-held button as fast as possible when a tone was presented over headphones. When the two tasks were combined, reaction times on the auditory task increased. Reaction time also increased when subjects were simply asked to fixate during rotation. A second experiment demonstrated that if attention was occupied by performance of a demanding mental arithmetic task during the passive rotation, accuracy of subsequently repositioning the chair to the origin declined, implying that change in orientation had been less accurately registered when performing the concurrent mental task. In combination, these findings indicate that a small but significant degree of attention or cognitive effort is necessary to monitor accurately the direction and amplitude of a brief angular rotation, and to suppress vestibulo-ocular reflex eye movement.     

Visual mental imagery interferes with allocentric orientation judgements

The subjective visual vertical is determined when a subject judges the orientation of an indicator (e.g. a short line segment) as apparently vertical. The mechanisms that underlie this perceptual performance are usually assumed to be based predominantly on bottom-up processing of primarily vestibular and visual information. However, it is also possible that top-down processes play a role in such abilities. We used an interference paradigm in order to investigate the effects of mental images on the perception of the visual vertical. The results demonstrate for the first time that visual mental imagery can exert the same directional influence on the subjective visual vertical as a perception of the corresponding stimulus.     

Hemispheric lateralisation in a manual-verbal task combination: the role of modality and gender

Differences in hemispheric lateralisation between males and females were tested using a manual-verbal task combination. The manual task was finger tapping and the verbal task required reciting words. Words were presented either visually or aurally in order to examine a possible role of modality of presentation on hemispheric lateralisation. The influence of the verbal task on motor task performance was evaluated by changes in the number of taps from single to dual-task condition. The influence of the motor task performance on the verbal task was examined by changes in the number of words recalled. Cognitive performance differences between males and females were also examined in a mental rotation task. The results showed a greater right finger (RH) tapping than left finger (LH) tapping interference, but only when the verbal task was presented in the visual mode. There was no difference in this pattern between males and females, both showing a greater RH tapping than LH tapping interference. The interference in finger tapping for both RH and LH was greater when the verbal task was presented aurally than when presented visually. Furthermore, females compared to males showed a greater interference in finger tapping when the verbal task was presented aurally than when presented visually. Later recall of verbal information was impaired equally by concurrent RH or LH tapping; however, later recall was better when the verbal task was presented visually than when presented aurally. No gender differences were found in delayed recall. Performance in the mental rotation task was better in males than in females. The data are discussed on the basis of theories of dual task interference and/or of brain asymmetry.     

Dissociation between subjective vertical and subjective body orientation elicited by galvanic vestibular stimulation

Previous studies demonstrated that sensory stimulation could differentially affect the subjective vertical (SV) and the subjective body orientation (SBO). This suggests that the central nervous system elaborates various references of verticality in function of the task demands and of the available sensory information. In this study, we tested whether the dissociation between SV and SBO appears for a selective stimulation of the vestibular system, by using galvanic vestibular stimulation (GVS). Seated subjects performed vertical settings by controlling the orientation of a visual rod during GVS. Subjects were also instructed to evaluate the orientation of the head and trunk relative to gravity. The results revealed a large variability in the way SV and SBO were affected. In all cases, the effect of GVS on SV was not a mirror image of a distorted SBO. We propose that this dissociation is mainly determined by central processes involved in the estimation of sensory cues reliability. GVS also yielded a tilt of the head when the head was unrestrained. The results suggest that changes in actual head orientation yielded by GVS may be related to the perceived direction of gravity but cannot be explained by a compensation of an illusory orientation of the head.     

Interference between postural control and mental task performance in patients with vestibular disorder and healthy controls

OBJECTIVES: To determine whether interference between postural control and mental task performance in patients with balance system impairment and healthy subjects is due to general capacity limitations, motor control interference, competition for spatial processing resources, or a combination of these. METHOD: Postural stability was assessed in 48 patients with vestibular disorder and 24 healthy controls while they were standing with eyes closed on (a) a stable and (b) a moving platform. Mental task performance was measured by accuracy and reaction time on mental tasks, comprising high and low load, spatial and non-spatial tasks. Interference between balancing and performing mental tasks was assessed by comparing baseline (single task) levels of sway and mental task performance with levels while concurrently balancing and carrying out mental tasks. RESULTS: As the balancing task increased in difficulty, reaction times on both low load mental tasks grew progressively longer and accuracy on both high load tasks declined in patients and controls. Postural sway was essentially unaffected by mental activity in patients and controls. CONCLUSIONS: It is unlikely that dual task interference between balancing and mental activity is due to competition for spatial processing resources, as levels of interference were similar in patients with vestibular disorder and healthy controls, and were also similar for spatial and non-spatial tasks. Moreover, the finding that accuracy declined on the high load tasks when balancing cannot be attributed to motor control interference, as no motor control processing is involved in maintaining accuracy of responses. Therefore, interference between mental activity and postural control can be attributed principally to general capacity limitations, and is hence proportional to the attentional demands of both tasks.     

Posture and mental task performance when viewing a moving visual field

We investigated the characteristics of standing posture and performance of concurrent cognitive tasks in subjects confronted by whole field visual motion. Movements of the head and centre of pressure (COP) were recorded in 12 subjects who performed modified Brooks spatial and verbal tasks when in quiet stance viewing a chequerboard pattern, planar, visual field, moving with uniform velocity (25 degrees /s, 50 degrees /s and 76 degrees /s). Eight subjects were also tested seated to control for the effect of stance. Task load was monitored by heart rate and eye movements were recorded to ensure viewing compliance. Subjects rated their quotidian susceptibility to visual disorientation on a validated scale. In both lateral and antero-posterior directions there were small amplitude but significant increases in COP sway path length and standard deviations of both COP and head sway during exposure to visual motion in proportion to visual flow speed. Performing cognitive tasks during visual motion attenuated sway S.D. The effects on sway of task and visual flow were independent. Visual motion induced a slight tilt and turn of the head and body in the direction of flow together with slight neck flexion. Errors on both verbal and spatial tasks increased >250% during visual motion both when standing and when seated. Ratings of subjects' susceptibility to disorientation were un-related to either verbal or spatial task error rates. A current hypothesis is that the enhancement of sway by visual motion is destabilisation. We propose an alternative explanation that sway enhancement could be exploratory 'testing of the ground' movements to check for self motion. Hence decrease in sway magnitude during a cognitive task could be caused by a reduction in exploratory movement because attention is diverted from postural control to a secondary task. Mere passive viewing of a moving visual field may interfere with cognitive tasks possibly because the threat of disorientation by whole field motion diverts attentional resources.     

Visual mental imagery during caloric vestibular stimulation

We investigated high-resolution mental imagery and mental rotation, while the participants received caloric vestibular stimulation. High-resolution visual mental imagery tasks have been shown to activate early visual cortex, which is deactivated by vestibular input. Thus, we predicted that vestibular stimulation would disrupt high-resolution mental imagery; this prediction was confirmed. In addition, mental rotation tasks have been shown to activate posterior parietal cortex, which is also engaged in the processing of vestibular stimulation. As predicted, we also found that mental rotation is impaired during vestibular stimulation. In contrast, such stimulation did not affect performance of a low-imagery control task. These data document previously unsuspected interactions between the vestibular system and the high-level visual system.     

Vestibular integration in human cerebral cortex contributes to spatial remapping

The process of visuo-spatial updating is crucial in guiding human behaviour. While the parietal cortex has long been considered a principal candidate for performing spatial transformations, the exact underlying mechanisms are still unclear. In this study, we investigated in a patient with a right occipito-parietal lesion the ability to update the visual space during vestibularly guided saccades. To quantify the possible deficits in visual and vestibular memory processes, we studied the subject's performance in two separate memory tasks, visual (VIS) and vestibular (VES). In the VIS task, a saccade was elicited from a central fixation point to the location of a visual memorized target and in the VEST task, the saccade was elicited after whole-body rotation to the starting position thus compensating for the rotation. Finally, in an updating task (UPD), the subject had to memorize the position of a visual target then after a whole-body rotation he had to produce a saccade to the remembered visual target location in space. Our main findings was a significant hypometria in the final eye position of both VEST and UPD saccades induced during rotation to the left (contralesional) hemispace as compared to saccades induced after right (ipsilesional) rotation. Moreover, these deficits in vestibularly guided saccades correlated with deficits in vestibulo-ocular time constant, reflecting disorders in the inertial vestibular integration path. We conclude that the occipito-parietal cortex in man can provide a first stage in visuo-spatial remapping by encoding inertial head position signals during gaze orientation.     

Upper/lower visual field asymmetry on a spatial relocation memory task

The purpose of this study was to investigate whether the lower visual field advantage reported on a number of visual tasks depends on the activity of neural systems which process information from different spaces. To this end, a double dissociation logic was followed by observing the effects of visual and spatial interference on a relocation memory task performed by 80 volunteers. Results showed that participants were better at relocating stimuli presented in the lower than in the upper visual field. Moreover, a concurrent spatial task, but not a concurrent visual task, disrupted the visual field vertical asymmetry. Those findings confirm that the vertical asymmetry of visual field depends on the spatial processing of incoming stimuli.     

Goal-directed behavior under emotional distraction is preserved by enhanced task-specific activation

Despite the distracting effects of emotional stimuli on concurrent task performance, humans are able to uphold goal-directed behavior. Here, we investigated the hypothesis that this effect is due to the enhanced recruitment of task-specific neural resources. In a two-step functional magnetic resonance imaging study, we first localized those areas involved in mental arithmetics by contrasting arithmetic problems with a number detection task. The resulting activation maps were then used as masks in a second experiment that compared the effects of neutral and emotional distracter images on mental arithmetics. We found increased response times in the emotional distracter condition, accompanied by enhanced activation in task-specific areas, including superior parietal cortex, dorsolateral and dorsomedial prefrontal cortex. This activation increase correlated with larger behavioral impairment through emotional distraction. Similar error rates in both conditions indicate that cognitive task performance is preserved through enhanced recruitment of task-specific neural resources when emotional distracter stimuli are present.     

Anxiogenic properties of yohimbine

To study the pharmacological induction of stress along with psychological stress and their possible interaction, 20 mg yohimbine and placebo orally were administered to 8 panic patients on placebo treatment, 7 panic patients on alprazolam treatment and 12 controls in a double-blind crossover design. Two structured situations which can be considered as 'neutral' stressors were included: a mental arithmetic task and a continuous performance task. Mental arithmetic induced robust increases in ratings of panicky, anxiety, nervousness, heart rate and electrodermal activity, while the continuous performance task induced increases exclusively in skin conductance reaction. Patients responded to these tasks less than controls with regard to subjective ratings and electrodermal activity. Yohimbine did not potentiate the response to the tasks in the patients. In controls, heart rate during the mental arithmetic task, but not during rest, was increased after yohimbine. In contrast to other yohimbine challenge studies no panic attacks were observed. It is hypothesized that the experimental design together with an instructional set that reduces expectancy factors and the inclusion of structured and time-limited tasks in a challenge paradigm is able to reduce the anxiogenic effects of yohimbine.     

Shift of subjective reference and visual orientation during slow pitch tilt for the seated human subject

We examined the ability to assess subjective orientation and orientation of an external visual object during pitch titl. Subjects were seated, restrained, and in darkness in a simulator and estimated when they were 0°, 45°, and 90° forwards and backwards from upright during pitching at 1°/s. They temporarily stopped in these positions and set a 5 cm luminous cube, cockpit mounted at 60 cm from the nasium, to earth vertical. Estimates of subjective tilt were consistently greater than actual tilt. Overestimations were increased by preceding tilts in the opposite direction, particularly when tilting from forwards, where subjects sometimes estimated they were tilted backwards when the machine was tilted forwards. Subjects were surprised with their estimates, and reported disorientation. Regardless, settings of the visual vertical made “intuitively” were largely accurate. Subjective estimates could be construed as “accurate” if one assumes that the rostro-caudal axis of the head was referenced for estimates of upright and forwards and a trunk-leg axis for backwards. Because labyrinthine defective patients behaved as normal subjects, task performance must have been based on proprioception. The overestimation of tilt is exploited in fairground illusions and may account for the common experience when driving, that hills seem much steeper than they are.     

Differential effects of ambivalent visual-vestibular-somatosensory stimulation on the perception of self-motion

The direction of perceived self-motion was determined as a function of combined visual-vestibular and vestibulo-somatosensory stimulations about the earth's vertical z-axis by means of a rotary chair and drum system. The predominant influence of concurrent antagonistic vestibular stimulation on circular vection (CV) even at small accelerations has been demonstrated by several studies in the past. The results of the present paper do not confirm the generally assumed influence of the vestibular system on exocentric motion perception, but instead produce evidence of the dominance of the visual channel even at high acceleration levels. Using a joystick to indicate perceived self-motion, we found the following: Constant CV (visual stimulus velocity: 40 degrees/s) could only be cancelled by vestibular stimulations at oppositely directed mean angular accelerations of 26.9 +/- 9.1 degrees/s2. Smaller accelerations led merely to an acceleration level-related decrease in the perceived velocity of CV. Despite a clear decision with respect to the direction of the perceived motion, subjects (Ss) reported dizziness or even strong motion sickness symptoms. Similar results were obtained with vestibulo-somatosensory interactions. The results are interpreted in terms of an intensive visual support in the processing of visual-vestibular signals, particularly at cortical level, assuming a cortical velocity generator (CVG).     

On-line versus off-line vestibular-evoked control of goal-directed arm movements

The present study tested whether vestibular input can be processed on-line to control goal-directed arm movements towards memorized visual targets when the whole body is passively rotated during movement execution. Subjects succeeded in compensating for current body rotation by regulating ongoing arm movements. This performance was compared to the accuracy with which subjects reached for the target when the rotation occurred before the movement. Subjects were less accurate in updating the internal representation of visual space through vestibular signals than in monitoring on-line body orientation to control arm movement. These results demonstrate that vestibular signals contribute to motor control of voluntary arm movements and suggest that the processes underlying on-line regulation of goal-directed movements are different from those underlying navigation-like behaviors.     

Subtraction in addition to addition: dual task performance improves when tasks are presented to separate hemispheres

This research links neuro- and cognitive psychology by asking whether performance of two concurrent cognitive tasks is facilitated by presentation of each task to a different cerebral hemisphere. Subjects were required to perform two arithmetic problems which were presented simultaneously. One problem required addition; the other subtraction. In Experiment I, briefly exposed numbers were exposed for 100 ms and were arranged so that a digit at fixation had to be added to a top number and subtracted from a bottom number. In Experiment II, the numbers were reversed so that the addition task was below the subtraction task. During Bilateral/bihemispheric trials, the addition problem was presented to one visual field and the subtraction problem to the other visual field. During Unilateral/single hemisphere trials, the addition and subtraction problems were projected to only one visual field. Results indicated that a higher proportion of problems were correctly solved during the Bilateral/bihemispheric trials than during the Unilateral/single hemisphere trials. These data suggest that dividing simultaneous inputs so that each hemisphere is confronted with a task requiring only one kind of cognitive operation facilitates performance, perhaps by minimizing intertask interference. This study illustrates one kind of hemispheric cooperation which facilitates dual task performance.     

Processing of visual motion direction in the fronto-parallel plane in the stationary or moving observer

To examine the effect of concurrent self-motion on the perception of the direction of object-motion, random-dot kinematograms were employed in which the strength of the directional signal was manipulated by varying the percentage of coherently moving pixels. The subject's task was to indicate the motion direction of briefly presented displays while undergoing whole body rotations with angular accelerations of 0, 5, 15, or 45°/s². The perception of the direction of visual motion in the horizontal plane was impaired only when visual and vestibular motion directions were incongruous. The impairment increases with both increasing angular acceleration and decreasing percentage of coherently moving pixels. For object-motion in the vertical plane, an impairment was found for both congruous and incongruous combination of visual and vestibular stimulation, although not as pronounced for the latter (i.e., visual upward, vestibular downward stimulation, and vice versa). These results are discussed in terms of postnatal development and neurophysiological optimization processes resulting from intersensory ‘updating’ through every-day experience of object-motion during self-motion.     

Cerebral compensation during motor imagery in Parkinson's disease

In neurodegenerative disorders, neural damage can trigger compensatory mechanisms that minimize behavioural impairments. Here, we aimed at characterizing cerebral compensation during motor imagery in Parkinson's disease (PD), while controlling for altered motor execution and sensory feedback. We used a within-patient design to compare the most and least affected hand in 19 right-handed PD patients with markedly right-lateralized symptoms. We used a motor imagery (MI) task in which the patients were required to judge the laterality of hand images, rotated either in a lateral or in a medial orientation with respect to the body sagittal plane. This design allowed us to compare cerebral activity (using fMRI) evoked by MI of each hand separately, while objectively monitoring task performance. Reaction times and parieto-premotor activity increased in a similar manner as a function of stimulus rotation during motor imagery of left and right hands. However, patients were markedly slower when judging images of the affected hand in lateral orientations, and there was a corresponding increase in activity in the right extrastriate body area (EBA) and occipito-parietal cortex during mental rotation of the affected hand. Furthermore, these regions increased their connectivity towards the left PMd for right (affected) hands in a lateral orientation. We infer that, in strongly lateralized PD patients, motor imagery of the most-affected hand exploits additional resources in extrastriate visual areas. These findings characterize the cerebral bases of the increased dependence on visual information processing during the generation of motor plans in PD, pointing to its compensatory role.     

Visual field dependence-independence before and after unilateral vestibular loss

To investigate whether visual field dependence-independence changed after unilateral vestibular loss, Menière's patients were tested before and after unilateral vestibular neurotomy and compared with controls. Using the rod and frame test, visual vertical perception was tested under four visual contexts (with a frame tilted either clockwise or counterclockwise, with a vertical frame, and without visual reference). Both controls and Menière's patients before unilateral vestibular loss split into visual field dependent and independent subpopulations. Unilateral vestibular loss did not change this partition but, for both subpopulations, it induced asymmetrical visual field dependence with a reduced or abolished contralesional dependence. Finally, when vertical/horizontal references were provided, visual vertical perception was improved in both subpopulations, suggesting that all patients relied on the same allocentric strategy.     

Central vestibular disorders

Dizziness or vertigo is an erroneous perception of selfmotion or object-motion as well as an unpleasant distortion of static gravitational orientation. It is caused by a mismatch between the vestibular, visual, and somatosensory systems. Thanks to their functional overlap, the three systems are able to compensate, in part, for each other's deficiencies. Thus, vertigo is not a well-defined disease entity, but rather a multisensory syndrome that results when there is a pathological dysfunction of any of the stabilizing sensory systems (e.g., central vestibular disorders, peripheral vestibular diseases with asymmetric input into the vestibular nuclei). This article provides an overview of the most important and frequent forms of central vestibular vertigo syndromes, including basilar/vestibular migraine, which are characterized by ocular motor, postural, and perceptual signs. In a simple clinical classification they can be separated according to the three major planes of action of the vestibulo-ocular reflex: yaw, roll, and pitch. A tonic imbalance in yaw is characterized by horizontal nystagmus, lateropulsion of the eyes, past-pointing, rotational and lateral body falls, and lateral deviation of the perceived straight-ahead. A tonic imbalance in roll is defined by torsional nystagmus, skew deviation, ocular torsion, tilts of head, body, and the perceived vertical. Finally, a tonic imbalance in pitch can be characterized by some forms of upbeat or downbeat nystagmus, fore-aft tilts and falls, and vertical deviation of the perceived straight ahead. The thus defined syndromes allow for a precise topographic diagnosis as regards their level and side.     

Lateralized interference effects of concurrent verbal tasks on sequential finger tapping

Twenty-four right-handed subjects concurrently performed both a sequential finger tapping task and a verbal task with or without visual guidance. The concurrent verbal tasks consisted of shadowing with vocalization and of memorizing without vocalization. The result showed that (1) both verbal tasks interfered with right hand tapping but not with left hand tapping, (2) lateralized interference was stronger in the shadowing than in the memorizing condition, and (3) performance did not differ under the visual and the non-visual guidance conditions. These data suggest that non-vocalized verbal tasks interfere with concurrent motor performance by the right hand, and that this lateralized interference effect is weaker than the effect produced by vocalized verbal tasks.