Effects of attentional dispersion on sensory-motor processing of anticipatory postural control during unilateral arm abduction

ObjectiveWe investigated effects of attentional dispersion on sensory-motor processing of anticipatory postural control during unilateral arm abduction.MethodsThirteen adults performed arm abduction under two types of attentional dispersion conditions. A target stimulus was presented with 30% probability in two- or three-positions. By cue signal presentation, subjects either focused their attention on one position or divided attention for two or three positions and abducted right arm for target stimulus. Event-related potentials and onset time of postural muscles were measured.ResultsP1-N1 and N2 amplitudes decreased with attentional dispersion in both conditions, but P3 did not change. With attentional dispersion to three-positions, N2 latency increased and start of late CNV was delayed, and also the onset time of gluteus medius was late in correlation to the late CNV changings, with no changings in two-positions.ConclusionsWith attentional dispersion, brain activation decreased in the area related to the sensory processing and especially in the stimulus discrimination area. With increasing attentional dispersion, the delay in motor preparation or anticipatory attention to target stimuli was related to the delay in stimulus discrimination and onset time of postural muscle activation.SignificanceEffects of attentional dispersion on sensory-motor processing of anticipatory postural control were experimentally demonstrated.     

Effects of expected perturbations on the velocity control of fast arm abduction movements

A triphasic electromyographic pattern of sequential activation of agonist, antagonist, and again agonist muscles underlies rapid or ballistic limb movements in humans. The first agonist burst reflects muscular force accelerating the limb, the antagonist burst is mainly related to the braking process of movement, while the second agonist burst is considered a reactive adjustment to the deceleration. The duration of the first agonist burst has been reported to be constant for movements of different amplitudes, thus suggesting that only changes in its amplitude contribute to the velocity control of movement. The present research has been undertaken to investigate the strategy whereby the nervous system increases agonist impulsive force for ballistic performance in response to experimental conditions requiring perceptual and/or provisional processes related to expected changes in load or accuracy constraints. The effects of expected perturbations of different strengths on some kinematic and electromyographic variables of fast arm abduction movements performed in a step-tracking task and in an outer-stop terminated task were analyzed in normal subjects. All motor performances were characterized by triphasic electromyographic patterns. In the absence of expected perturbations, the mean velocity of the movements was markedly higher in the outer-stop terminated task. Correspondingly, greater amplitudes and durations of the first agonist burst were observed. In both types of motor tasks, when expected perturbations were inserted, the velocity of the movement increased as well as the amplitude and the duration of the first agonist burst. These results, in agreement with previous observations, indicate that the normal mechanism, whereby the nervous system increases agonist impulsive force in rapid movements, comprises changes not only in the amplitude but also in the duration of the first agonist burst.     

Flexibility of anticipatory postural adjustments revealed by self-paced and reaction-time arm movements

In standing humans, it is not certain whether anticipatory postural adjustments associated with rapid, voluntary elbow flexion movements (focal movements) originate as a selection from preset synergies or as the result of specific planning of motor commands. We studied these muscle recruitment patterns when the same focal movement was made under behavioral conditions of a self-paced task (SPT) and a reaction-time task (RTT). While standing still, eight normal subjects performed focal movements under the SPT and RTT behavioral conditions and under three different biomechanical conditions: (1) unloaded-upright, (2) loaded-upright (holding a 3800-g metal bar), and (3) unloaded-forward leaning. Anticipatory postural adjustments were quantified using the latency and duration of electromyographic (EMG) data and the center of pressure (COP) displacement data. Postural kinematic [joint and body center of gravity (COG) motion] data were used to quantify the anticipatory postural program's effectiveness at preventing postural movement. Focal movement kinematics and associated EMG activity differed due to biomechanical but not behavioral (SPT vs. RTT) conditions. The maximum and net displacement of the body COG measurements did not differ between the behavioral conditions. The amplitude, timing, and net movements of lower extremity joints were influenced by the behavioral conditions. However, the behavioral conditions significantly affected the phasing (including order of activation) and duration of anticipatory postural EMG activity and the phasing of COP displacements under certain biomechanical conditions. These findings support the theory that anticipatory postural adjustments are planned in detail.     

Effect of visual attention on postural control in children with attention-deficit/hyperactivity disorder

We compared the effect of oculomotor tasks on postural sway in two groups of ADHD children with and without methylphenidate (MPH) treatment against a group of control age-matched children. Fourteen MPH-untreated ADHD children, fourteen MPH-treated ADHD children and a group of control children participated to the study. Eye movements were recorded using a video-oculography system and postural sway measured with a force platform simultaneously. Children performed fixation, pursuits, pro- and anti-saccades. We analyzed the number of saccades during fixation, the number of catch-up saccades during pursuits, the latency of pro- and anti-saccades; the occurrence of errors in the anti-saccade task and the surface and mean velocity of the center of pressure (CoP). During the postural task, the quality of fixation was significantly worse in both groups of ADHD children with respect to control children; in contrast, the number of catch-up saccades during pursuits, the latency of pro-/anti-saccades and the rate of errors in the anti-saccade task did not differ in the three groups of children. The surface of the CoP in MPH-treated children was similar to that of control children, while MPH-untreated children showed larger postural sway. When performing any saccades, the surface of the CoP improved with respect to fixation or pursuits tasks. This study provides evidence of poor postural control in ADHD children, probably due to cerebellar deficiencies. Our study is also the first to show an improvement on postural sway in ADHD children performing saccadic eye movements.     

Feature-specific attention allocation overrules the orienting response to emotional stimuli

Emotional stimuli are generally thought to be processed in an unconditional fashion. Recent behavioral studies suggest, however, that emotional stimulus processing is critically dependent on attention toward emotional stimulus features. We set out to test this hypothesis using EEG measurements and a modified oddball paradigm. Unexpected emotional stimuli evoked amplitude variations of the P3a (an ERP marker of attention orienting) when attention was directed to emotional stimulus properties but not when non-emotional stimulus properties were attended to. We conclude that emotional stimulus processing is not unconditional, but dependent on top-down attentional control.     

Exogenous attention to facial vs non-facial emotional visual stimuli

The capacity of the two types of non-symbolic emotional stimuli most widely used in research on affective processes, faces and (non-facial) emotional scenes, to capture exogenous attention, was compared. Negative, positive and neutral faces and affective scenes were presented as distracters to 34 participants while they carried out a demanding digit categorization task. Behavioral (reaction times and number of errors) and electrophysiological (event-related potentials—ERPs) indices of exogenous attention were analyzed. Globally, facial expressions and emotional scenes showed similar capabilities to attract exogenous attention. Electrophysiologically, attentional capture was reflected in the P2a component of ERPs at the scalp level, and in left precentral areas at the source level. Negatively charged faces and scenes elicited maximal P2a/precentral gyrus activity. In the case of scenes, this negativity bias was also evident at the behavioral level. Additionally, a specific effect of facial distracters was observed in N170 at the scalp level, and in the fusiform gyrus and inferior parietal lobule at the source level. This effect revealed maximal attention to positive expressions. This facial positivity offset was also observed at the behavioral level. Taken together, the present results indicate that faces and non-facial scenes elicit partially different and, to some extent, complementary exogenous attention mechanisms.     

Intermodal selective attention. I. Effects on event-related potentials to lateralized auditory and visual stimuli.

The effects of intermodal selective attention on event-related brain potentials (ERPs) were examined in 2 experiments. In experiment 1, auditory ERPs were compared (1) when subjects responded to easy and difficult-to-detect target tones in sequences of tone bursts; and (2) when they ignored the same auditory sequences and played a demanding video game. In experiment 2, auditory ERPs to tone bursts and visual ERPs to vertical line gratings were compared as subjects responded to difficult-to-detect targets in one modality or the other. Attention to auditory stimuli resulted in biphasic enhancements in auditory ERPs, the Nda (negative auditory difference wave, latency 120-160 msec) and the Pda (positive auditory difference wave, latency 200-240 msec) waves. These had longer latencies and somewhat different scalp distributions than N1 and P2 components evoked by non-attended tones. The Nda and Pda could be contrasted with the monophasic processing negativities typically found in dichotic selective attention tasks. Nda amplitudes were larger for difficult-to-detect targets (closely resembling standards) than for standards themselves, but no Ndas were recorded to highly deviant targets. Deviant auditory stimuli evoked mismatch negativities (MMNs) that persisted during visual attention. MMN amplitudes to difficult-to-detect deviants were enlarged with attention, but no change was found in MMN amplitudes to easy-to-detect deviants. In experiment 2 intermodal attention was associated with biphasic changes in visual ERPs over the posterior scalp: the occipital Pdv (100-130 msec), and contralateral-temporal Ndv (120-320 msec) deflections. Deviant visual stimuli also elicited mismatch negativity/N2b components, largest over the inferotemporal cortex contralateral to the stimulated visual field. Like the auditory MMN, the MMN increased in amplitude with attention, but it was also evident during attend auditory conditions. The results suggest that sustained, intermodal attention depends primarily in processing modulations in modality-specific cortex. We found no evidence of the participation of modality non-specific cortex. This excludes the possibility that intermodal attention depends on a single, supramodal attention system. The relatively long latency of intermodal effects suggests that they may depend on the reafferent (top down) modulation, and do not index "template matching" operations.     

Temporal allocation of visual attention in adult attention deficit hyperactivity disorder

In two experiments, we examined the ability of adults with attention deficit hyperactivity disorder (ADHD) to process multiple targets appearing in a rapid serial visual presentation (RSVP) stream. Using a standard attentional blink (AB) task, subjects were required to both identify a target in the RSVP stream and detect a probe appearing in one of several posttarget serial positions. In Experiment 1, ADHD adults exhibited a protracted AB compared to controls, in that their probe detection did not improve as a function of increasing probe-to-target intervals (450-720 msec). In Experiment 2, the ADHD group performed as well as controls in detecting probes appearing immediately (i.e., 90 msec) after the target. Taken together, the results demonstrate that adults with ADHD exhibit a selective deficit in rapidly shifting attention between the target and the probe, when the two appear several hundred milliseconds apart. These results suggest that adults with ADHD can use automatic (reflexive) attention to detect items in close temporal proximity in the RSVP stream, but have difficulty allocating controlled attention to multiple stimuli separated by several hundred milliseconds.     

Optokinetic nystagmus as an assessment of visual attention to divided stimuli.

Attentional resources are finite and decline with age. We measured subjects' abilities to generate optokinetic nystagmus (OKN), to suppress it with fixation and to continue to suppress it when fixating while simultaneously paying covert attention to a feature of the optokinetic (OK) stimulus. During fixation with a red laser spot, OKN was almost fully suppressed. When subjects suppressed the OKN while simultaneously paying covert attention to a feature of the OK stimulus, suppression of the OKN was less well suppressed. The active OKN was vigorous. Age affected only the divided attention task, perhaps reflecting a diminution in resources of attention with age. The neural pathways serving attention and those serving eye movements appear to be closely related. We suggest the test presented here represents an objective measurement of the ability to divide attention, and that it has the potential to be developed for much more widespread, possibly clinical, use.     

Effects of foreperiod duration on anticipatory postural adjustments: determination of an optimal preparation in standing and sitting for a raising arm movement

The purpose of the present experiment was to examine the motor and postural preparation processes evolving during the foreperiod (known to be optimal at 500 ms in sitting) in a voluntary upper limb movement executed while standing. The foreperiod duration (300, 500, 700 and 900 ms) and the postural conditions (sitting versus standing) were manipulated using a priming procedure. Two types of prime were provided: (1) a prime giving valid information on the side of the raising arm movement to execute, and (2) a prime giving non-valid information (prime opposite to the required side). In a sitting and standing condition, eight normal subjects performed a raising arm movement with a 1 kg load at the level of the wrist. Premotor time and acceleration of arm movement were recorded in the two postural conditions. In the standing condition, anticipatory postural adjustments (APAs) were also quantified using the relative latency of electromyographic (EMG) data and the centre of foot pressure (CoP) displacements. Results (1) showed that the optimal foreperiod duration (i.e., leading to the shortest premotor time) increased as a function of postural conditions (500 versus 700 ms in the sitting and standing conditions, respectively), and (2) emphasized the existence of a temporal modulation in the central organization of the postural and focal commands according to the foreperiod duration.     

Cross-modal selective attention to visual and auditory stimuli modulates endogenous ERP components

In two experiments event-related potentials (ERPs) to visual and auditory stimuli were measured in 12 healthy subjects. A cross-modal and delayed response paradigm was used that allows ERPs to be obtained separately to attended and unattended stimuli under conditions in which unattended stimuli are less likely to be covertly or randomly attended. The results showed: (1) N1 enhancement with attention for standard stimuli in auditory and visual modalities and for deviant stimuli in the visual modality; (2) The onset time and scalp distribution of both the N1 for attend condition and Nd1 were similar regardless of standard or deviant stimuli in the auditory and visual modality; the onset time of Nd1 elicited by auditory and visual deviant stimuli was earlier than that of the unattended N1, and their scalp distributions were different; and (3) The Nd1 components elicited by auditory and visual deviant stimuli were distributed over the respective primary sensory areas, but Nd1 components evoked by auditory and visual standard stimuli were distributed over the frontal scalp. These results suggest that the attended N1 enhancement is primarily caused by a component with endogenous origins and that the early attention effect occurs before the exogenous components. The results support the view that the cross-modal attention to deviant stimuli modulates modality-specific processing in the brain, whereas attention to standard stimuli affects modality-nonspecific or supramodal brain systems.     

The effect of low arousal on visuo-spatial attention

Clinical and experimental evidence suggests a special role of the right hemisphere for alerting and orienting. However, it is not clear whether these attentional aspects should be taken to be independent or interacting. At least on the functional neuroanatomical level there seems to be an overlap in right parietal and frontal cortical structures. In this sleep deprivation study we examined the effect of different levels of arousal on covert orienting of attention repeatedly over a period of 28 h in 11 healthy subjects. The main finding is a disproportionate and significant slowing of responses to stimuli presented to the left visual hemifield that could only be observed in a state of maximally reduced arousal at 5:00 a.m. Furthermore, a facilitation of attentional shifts to the right could be observed in invalid trials when attention had to be reoriented. These results suggest an interaction of arousal and orienting, at least in situations of maximally reduced activation when attentional asymmetries could be provoked even in healthy subjects. The findings are compatible with a right hemisphere dominance for alerting and orienting, and they are discussed in the light of theoretical accounts of visual orienting.     

The effects of muscle vibration on anticipatory postural adjustments

The current study investigated the influence of changes in sensory information related to postural stability on anticipatory postural adjustments (APAs) in standing subjects. Subjects performed fast arm movements and a load release task while standing on a stable force platform or on an unstable board. We manipulated sensory information through vibration of the Achilles tendons and additional finger touch (contact forces under 1 N). Changes in the background activity of leg, trunk, and arm muscles and displacements of the center of pressure (COP) were quantified within time intervals typical for APAs. In the arm movement task, leg and trunk muscles showed a significant drop in the APAs with finger touch, while the vibration and standing on the unstable board each led to an increase in the APA magnitude. In the load release task, ventral muscles decreased their APA activity with touch, while dorsal muscles showed increased inhibition during APAs. During vibration, dorsal and ventral muscles showed increased excitation and inhibition during APAs, respectively. An additional analysis of APAs at a joint level, has shown that in both tasks, an index related to the co-activation of agonist-antagonist muscle pairs (C-index) was modulated with touch, vibration, and stability particularly in leg muscles. Small changes in the other index related to reciprocal activation (R-index) were found only in trunk muscles. Light touch and vibration induced opposing changes in the C-index, suggesting their opposite effects on the stabilization of a reference point or vertical. We conclude that the central nervous system deploys patterns of adjustments in which increased co-contraction of distal muscles and reciprocal adjustments in trunk muscles are modified to ensure equilibrium under postural instability.     

Eye tracking and visual attention to threating stimuli in veterans of the Iraq war

Theoretical and clinical characterizations of attention in PTSD acknowledge the possibility for both hypervigilance and avoidance of trauma-relevant stimuli. This study used eye tracking technology to investigate visual orientation and attention to traumatic and neutral stimuli in nineteen veterans of the Iraq war. Veterans saw slides in which half the screen had a negatively valenced image and half had a neutral image. Negatively valenced stimuli were further divided into stimuli that varied in trauma relevance (either Iraq war or civilian motor vehicle accidents). Veterans reporting relatively higher levels of PSTD symptoms had larger pupils to all negatively valenced pictures and spent more time looking at them than did veterans lower in PTSD symptoms. Veterans higher in PTSD symptoms also showed a trend towards looking first at Iraq images. The findings suggest that post-traumatic pathology is associated with vigilance rather than avoidance when visually processing negatively valenced and trauma-relevant stimuli.     

Defensive responses to looming visual stimuli in monkeys with unilateral striate cortex ablation.

A number of residual visual functions including detection, localization and discrimination of visual stimuli, have been demonstrated in the "blind" fields of monkeys and human patients following damage to striate cortex. We report here that avoidance movements of the head can also be elicited from monkeys with unilateral striate cortex ablations when a "looming" stimulus is presented within the hemianopic and presumably "blind" field. The possible role of the retinofugal projection to superior colliculus in the mediation of these defensive head movements is discussed.     

Effects of sleep deprivation on lateral visual attention

Sleep loss temporarily impairs vigilance and sustained attention. Because these cognitive abilities are believed to be mediated predominantly by the right cerebral hemisphere, this article hypothesized that continuous sleep deprivation results in a greater frequency of inattention errors within the left versus right visual fields. Twenty-one participants were assessed several times each day during a 40-h period of sustained wakefulness and following a night of recovery sleep. At each assessment, participants engaged in a continuous serial addition task while simultaneously monitoring a 150 degrees visual field for brief intermittent flashes of light. Overall, omission errors were most common in the leftmost peripheral field for all sessions, and did not show any evidence of a shift in laterality as a function of sleep deprivation. Relative to rested baseline and postrecovery conditions, sleep deprivation resulted in a global increase in omission errors across all visual locations and a general decline in serial addition performance. These findings argue against the hypothesis that sleep deprivation produces lateralized deficits in attention and suggest instead that deficits in visual attention produced by sleep deprivation are global and bilateral in nature.     

Effects of musical tempi on visual attention ERPs

The purpose of this work was to examine whether fast and slow musical tempi have different effects on selective attention evaluated through ERPs and task performance. A high demanding visual selective attention task was performed by the subjects: without music (BL) and with Bach's music in slow (ST) and fast tempi (FT). Difference waves were obtained substracting non-target from target. FT caused a reduction in reaction time and N2d and P3d latencies and in P3d amplitude. N2d latency was longer in ST than BL and FT. Music played in FT induced a faster stimuli evaluation and response than ST.     

The influence of visual control on postural stability in Parkinson disease

Background and purposeThe aim of the study was to evaluate the influence of visual control on parameters of postural stability among patients with Parkinson disease (PD) in comparison with control subjects.Material and methodsFifty patients diagnosed with idiopathic PD and 50 control subjects without features of central nervous system injury were selected for the study. The clinical diagnosis of idiopathic PD was established according to the clinical criteria of the United Kingdom Parkinson's Disease Society Brain Bank. Only patients in stages I-III according to the Hoehn-Yahr scale were included. The range of sway of the centre of foot pressure (COP) in the frontal plane (COPx) and in the sagittal plane (COPy), as well as the total path length in both axes (COPxy), was tested during quiet standing with and without visual control.ResultsCOPxy with and without visual control was the smallest in the group of patients in stage II in comparison with patients in stage I and III according to Hoehn-Yahr and in comparison with the control group.ConclusionsVisual control significantly affects the parameters of postural stability in PD patients.