Allocentric coding in ventral and dorsal routes during real-time reaching: Evidence from imaging-guided multi-site brain stimulation.

Recent evidence has revealed an advantage for movements to last target positions in a structured visual display, suggesting a mediating role of allocentric, in addition to egocentric, information in goal-directed reaching. This notion is accommodated by the recently updated perception-action model (Milner and Goodale, 2008), which postulates functional roles of ventral and dorsal neural areas in allocentric coding. In the present study, we used imaging-guided multi-site continuous theta burst stimulation (cTBS) over regions of the ventral and dorsal processing streams to unravel their functional contribution on visually guided reaching in two display conditions: the "egocentric" condition where the target appeared in an empty display and the "allocentric" condition where the target appeared in a structured display with placeholders marking possible target locations. Cortical sites for cTBS were identified individually for each participant via coregistration with magnetic resonance scans. Results indicated that cTBS in the egocentric condition did not affect movement time, but cTBS in the allocentric condition modulated movement time contingent on stimulation site and target position. In particular, cTBS over the lateral occipital cortex (part of the ventral stream) and over the angular gyrus (part of the dorsal stream) eliminated the last-target advantage by slowing down reaching to the salient last target position. cTBS over the superior parietal occipital cortex did not affect the last-target advantage. These outcomes provide the first causal evidence for allocentric coding in ventral and dorsal routes during real-time reaching, thereby supporting the updated perception-action model.

Controlled visuomotor preparation deficits in attention-deficit/hyperactivity disorder.

To the best of the authors' knowledge, there are no published reports on visuomotor preparation in attention-deficit/hyperactivity disorder (ADHD). This is unfortunate, because research suggests that ADHD is an output-related deficit, and suboptimal execution of tasks may be the result of incomplete visuomotor preparation. The authors compared 19 children with ADHD with 124 healthy and 120 pathological controls in terms of their performance (speed, speed variability, and accuracy) on the finger precuing test, a test measuring (automatic and controlled) visuomotor preparation. The data implied that children with ADHD have an impaired ability to engage in effortful, controlled visuomotor preparation activities. Fast, automatic response preparation was not affected by ADHD. In addition, children with ADHD showed more variability in overall test performance than other children. No group differences were found in response accuracy.

Physiological, biochemical and psychological markers of strenuous training-induced fatigue.

The purpose of the study was to investigate whether severe fatigue, possibly leading to overreaching, could be diagnosed at an early stage by a combination of parameters. Seven well-trained male subjects (age [mean +/- SD]: 25.3 +/- 4.7 yr; body mass: 76 +/- 6.6 kg; VO2max: 61.1 +/- 7 ml.kg(-1).min(-1)) increased their training load by doubling their training volume and increasing the intensity by 15 % over a period of two weeks. Before and after this intensified training period subjects underwent a series of tests including a maximal incremental cycle ergometer test (Wmax) with continuous ventilatory measurements and blood lactate values, time trial, basal blood parameter tests (red and white blood profile), hormones [growth hormone (GH), insulin-like growth factor 1(IGF-1), adreno-corticotropic hormone (ACTH), cortisol], neuro-endocrine stress test [short insulin tolerance test (SITT), combined anterior pituitary test (CAPT) and exercise], a shortened Profile of Mood State (POMS), the estimated rate of perceived exertion (RPE) and a cognitive reaction time test. The intensified training period resulted in a significant increase of the training load (p <0.01), training monotony (p <0.01) and training strain (p <0.01). The RPE during training increased significantly (p <0.01) during the intensified training period. Total mood score obtained from the POMS tended to increase (p=0.06), reflecting an increase in worse mood state. A novel finding was that reaction times increased significantly, indicating that overreaching might adversely affect speed of information processing by the brain, especially for the most difficult conditions. After the intensified training period, neither changes in exercise-induced plasma hormone values, nor SITT values were observed. During the CAPT only cortisol showed a significant decrease after the intensified training period. Hemoglobin showed a significant decrease after the intensified training period whereas hematocrit, red blood cell count (RBC) and MCV tended to decrease. The intensified training had no effect on physical performance (Wmax or time trial), maximal blood lactate, maximal heart rate and white blood cell profile. The most sensitive parameters for detecting overreaching are reaction time performance (indicative for cognitive brain functioning), RPE and to a lesser extend the shortened POMS. This strongly suggests, that central fatigue precedes peripheral fatigue. All other systems,including the neuro-endocrine, are more robust and react most likely at a later stage in exhaustive training periods.

The one-target advantage: a test of the movement integration hypothesis.

Two experiments were conducted to compare the temporal structure of single aiming movements to two-component movements involving either a reversal in direction or an extension. For reversal movements, there was no cost associated with the movement time for the first segment of the movement. However, regardless of movement direction, initiation instructions, handedness or effector, two-component extension movements were always associated with a longer movement time for the first movement segment. This disadvantage for extension movements, but not reversal movements, is consistent with the notion that there is interference between the execution of the first movement and implementation of the second movement. By contrast, because the muscular force used to break the first movement is also used to propel the second movement, reversal movements are organised as an integrated unit.

Distractor effects on pointing: the role of spatial layout.

There is a debate about how task-irrelevant visual distractors influence motor responses. Inconsistent findings in previous studies may reflect the use of different spatial layouts. In this study, participants pointed to randomly lateralized targets under full viewing conditions, either with or without a single distractor present. Distractor location, size, and spacing from the target were systematically manipulated while target size and movement amplitude remained constant. Larger distractors facilitated reaction times and slowed movement times. Distant distractors facilitated only reaction times. Movement endpoints were biased away from distractors. The single distractor also modulated perceived target size. These results are discussed in the context of current theories of the role of visual perception for action control.

Motor preparation in postural control in seated spinal cord injured people.

Spinal cord injured (SCI) people try to compensate for the loss of postural muscle function by increased use of non-postural muscles. Such alternative muscle use, however, may necessitate important modifications in motor control. In this study motor programming processes were investigated in three groups, i.e. in high thoracic SCI, low thoracic SCI and non-SCI subjects. A bimanual forward-reaching task, in which graded sitting balance perturbation was systematically invoked, was presented to the subjects as a visual precue choice reaction time (RT) task. Effects of movement preparation were examined by precuing reaching distance information. RT and movement times were recorded. Centre of pressure (CP) displacement was used as an indicator for sitting balance perturbation. Results indicated that high thoracic SCI subjects programmed balance-perturbing reaching movements as fast as did non-SCI subjects. Low thoracic SCI subjects, on the other hand, showed substantially longer programming times. This latter outcome is consistent with the hypothesis that the low thoracic SCI subjects adopted a more complex postural control strategy involving residual motor functions in an attempt to actively compensate for loss of postural muscle function. High thoracic SCI subjects, on the other hand, appeared to adopt a less complex and thus easier to programme postural control strategy.

An explanation of orthogonal S-R compatibility effects that vary with hand or response position: the end-state comfort hypothesis.

This study presents an explanation of orthogonal stimulus-response compatibility (SRC) effects that vary with hand or response location: the end-state comfort hypothesis. It posits that responses are spatially transformed and cognitively mapped onto the stimulus dimension according to relative hand posture, thereby mediating the pattern of facilitation and interference in response selection. In the first three experiments, we investigated the eccentricity effect, finding that responses by the left hand in left hemispace are faster with up-left/down-right mapping while responses by the right hand in right hemispace are faster with up-right/down-left mapping (Michaels & Schilder, 1991, Experiment 1). The endstate comfort hypothesis correctly predicted that the eccentricity effect occurred irrespective of the relative position of the stimulus and response device in the sagittal plane (Experiments 1 and 2), and that it reversed when the stimulus-response set was reversed, regardless of the relative position of the stimulus and response device in the fronto-parallel plane (Experiments 2 and 3). Experiment 4 shows a new orthogonal SRC effect that was predicted by the end-state comfort hypothesis. Our results are inconsis tent with other explanations, such as the virtual-lines hypothesis and the salient-features hypothesis.

Effects of spatial and symbolic precues on localization performance.

One of the fundamental properties of spatial vision is the ability to localize objects in space. According to a recent proposal, accurate localization performance involves the operation of two systems: the attention system and the eye movement system. Upon stimulus presentation, attention is shifted to the target area: this provides coarse location information. Subsequently, a saccadic eye movement is executed: this provides fine location information. In this study we tested predictions derived from this model concerning the effects of precue information on localization performance. In a series of five experiments we manipulated duration of precue (71, 400, and 1,000 ms) and type of precue (spatial versus symbolic). Results showed that very short duration (i.e., 71 ms) spatial precues improved localization performance whereas very short duration symbolic precues did not. In contrast, the 1,000 ms duration precue condition showed similar amounts of precuing benefit for the spatial and symbolic precues. This pattern of differential precuing effects corroborated the two-process model of localization performance.

The additivity of stimulus-response compatibility with perceptual and motor factors in a visual choice reaction time task.

One hundred and four participants performed a two-choice reaction time task requiring button-press responses to visual stimuli. Two levels of stimulus-response compatibility were factorially combined with two levels of stimulus discriminability and two levels of response repertoire. Results showed that the effects of these three variables were additive both in terms of mean reaction time as in terms of reaction time variances. The implication of this outcome is discussed in terms of the underlying information processing stage structure.

Control of rapid aimed hand movements: the one-target advantage.

A series of 8 experiments examined the phenomenon that a rapid aimed hand movement is executed faster when it is performed as a single, isolated movement than when it is followed by a second movement (the 1-target advantage). Three new accounts of this effect are proposed and tested: the eye movement hypothesis, the target uncertainty hypothesis, and the movement integration hypothesis. Data are reported that corroborate the 3rd hypothesis, but not the first 2 hypotheses. According to the movement integration hypothesis, the first movement in a series is slowed because control of the second movement may overlap with execution of the first. It is shown that manipulations of target size and movement direction mediate this process and determine the presence and absence of the 1-target advantage. Possible neurophysiological mechanisms and implications for motor control theory are discussed.

Gender differences in choice reaction time: evidence for differential strategies.

This study considered the hypothesis that on some tasks men and women might employ different information processing strategies. Twelve male and 12 female participants performed a 2- and 4-choice, compatible and incompatible, choice reaction time task that required a verbal response to a spatial location target stimulus. Results demonstrated a near-significant overall reaction time advantage for male participants. Moreover, males and females showed a differential pattern of reaction time as a function of stimulus location. Specifically, in the 4-choice-compatible condition, females exhibited a linear increase in reaction time as a function of the left-right dimension; males, on the other hand, showed a two-component, step-like increase. It was suggested that this gender difference in reaction time performance may reflect differences in processing strategy. Specifically, it was argued that in the present task females may have employed a serial, left-to-right, processing strategy, and males a binary, split-half (dichotomizing) strategy.

Effects of age on performance in a finger-precuing task.

This study investigated age-related precuing effects in the finger-precuing task (J. Miller, 1982). In this task, a spatial precue provides partial advance information about which fingers to use for responding. Results indicated a substantial age-related deficit in preparing 2 fingers on 2 hands, but not on 1 hand. This disparate set of findings does not provide strong support for A. A. Hartley's (1993) hypothesis that anterior brain attention systems responsible for selection-for-action are compromised with advancing age. Finally, the authors report that advancing age increasingly slows reaction time more to the inner than to the outer stimulus-response positions. A possible mechanism of this age-related bowed stimulus-response position effect is discussed.

Postural motor programming in paraplegic patients during rehabilitation.

One of the basic aims in the rehabilitation of thoracic spinal cord injured (SCI) patients concerns the regaining of sitting posture control. This implies the development of new postural strategies requiring the adjustment of motor programming processes. The aim of this study was to investigate the time course of postural reorganization during active, clinical rehabilitation of thoracic SCI patients with different SCI levels. Thus changes in motor programming in sitting balance control were investigated in two groups of complete low or high thoracic SCI patients. At several stages during the rehabilitation process an experiment was held in which sitting posture was perturbed systematically using submaximal reaching movements over four reaching distances. This bimanual reaching task was presented as a visual precue choice reaction time (RT) task in which reaching distance (i.e. grade of postural perturbation) was precued. Results indicated that in both high and low thoracic SCI patients RTs in movements involving postural perturbation became shorter during the course of the rehabilitation period. However, low thoracic SCI patients were generally slower in the programming of balance perturbing movements than high thoracic SCI patients, a phenomenon that did not change over time. Furthermore, initial differences in RTs as a function of grade of postural perturbation disappeared in both groups in the course of the rehabilitation phase. Precue benefit, equally large for both groups, did not change as a function of rehabilitation time. It is concluded that the observed phenomena signify the gradual development of new central postural control processes in both SCI groups during rehabilitation. Low thoracic SCI patients, having more residual sensorimotor functions, seem to adopt more complex strategies in maintaining and restoring sitting balance that take longer to specify and to programme. High thoracic SCI patients seem to rely on simpler strategies using more passive postural support.

Aerobic capacity and cognitive performance in a cross-sectional aging study.

In a population unselected for aerobic fitness status, aerobic fitness (VO2max) and its interaction with age were used to predict performance on several cognitive measures known to be affected by chronological age. It was hypothesized that, in particular, cognitively demanding tasks would be sensitive to aerobic capacity. Healthy subjects between 24 and 76 yr of age (N = 132) were recruited from a larger study into determinants of cognitive aging (Maastricht Aging Study-MAAS). All participants took part in a submaximal bicycle ergometer protocol and an extensive neurocognitive examination, including tests of intelligence, verbal memory, and simple and complex cognitive speed. Participants engaged more hours a week in aerobic sports and felt healthier than the nonparticipants of the same age did. No group differences were found in the basic anthropometric characteristics height, weight, and BMI. Two of four subtasks that reflect complex cognitive speed (Stroop color/word interference and Concept Shifting Test) showed main and interaction effects with age of aerobic capacity in a hierarchical regression analysis, accounting for up to 5% of variance in parameter score after correction for age, sex, and intelligence main effects. These findings fit well within a moderator model of aerobic fitness in cognitive aging. They add to the notion that aerobic fitness may selectively and age-dependently act on cognitive processes, in particular those that require relatively large attentional resources.

Effects of age and gender on discrete and reciprocal aiming movements.

Using a cross-sectional design, this study determined the time course of aging effects on rapid discrete and reciprocal aiming movements in men and women. A total of 80 men and 61 women in good health were classified into six age groups (25, 35, 45, 55, 65, and 75 years). The discrete task required participants to make one discrete aiming movement, whereas the reciprocal task required a series of back-and-forth movements. Results indicated for both aiming tasks that greater age was strongly associated with slower movement times. The significant interaction between age and task indicated that the discrete task showed much larger aging effects (54%) than the reciprocal task (25%). This finding is tentatively interpreted in terms of a reduced efficiency of "on-line" control processes.

Spatial localization: tests of a two-process model.

The present study examined the recently proposed two-process model of localization performance in which a shift of attention, providing coarse location information, is followed by a saccadic eye movement, providing fine location information. In experiment 1 the nature of the localization response was manipulated. In contrast to the indirect response mode used in the study by Adam et al., i.e., manipulating the "arrow" keys to move the cursor to the target location, experiment 1 required subjects to point to the target location. The high degree of similarity between the pattern of results obtained with the pointing and cursor response indicated that performance in the localization paradigm was not differentially affected by the nature of the required response. In experiment 2 the characteristics of the backward masking stimulus was manipulated by employing three masking conditions: (1) a long-duration mask; (2) a short-duration mask (100 ms); and (3) a no-mask condition. Results showed that the long-duration mask caused interference at short and facilitation at long intervals between onset of target and mask; the short-duration mask caused interference only at short intervals. Overall the findings were consistent with the two-process model of localization performance.

Interference between saccadic eye and goal-directed hand movements.

The aim of the present study was to investigate the nature of the interference effect when the eye is accompanied by a goal-directed hand movement rather than when the eye moves alone. Latencies of eye and hand movements in response to small and large visual target stimuli were measured while employing dual-task methodology. Experiments 1 and 2 were designed to investigate whether the interference effect is related to a specific temporal bottleneck, i.e. the eye and hand motor systems share limited available processes at a specific point in time. The findings of robust interference effects independent of the temporal organization of eye and hand contradicted this notion. The interference effect was not present in experiment 3, where response preparation and target-localization mechanisms were limited by providing subjects with advance information about target position. Experiment 4 employed randomized target positions again and highly salient stimuli, the latter only limiting target-localization processes. The absence of an interference effect adds weight to the argument that visual spatial attentional mechanisms involved in target localization constitute the locus of the interference. Neurophysiological implications of these findings are discussed.

Measurement of cognitive load in instructional research.

The results of two of our recent empirical studies were considered to assess the usefulness of subjective ratings and cardiovascular measures of mental effort in instructional research. Based on its reliability and sensitivity, the subjective rating-scale technique met the requirements to be useful in instructional research whereas the cardiovascular technique did not. It was concluded that the usefulness of both measurement techniques in instructional research needs to be investigated further.

Effects of a 10-month endurance-training program on performance of speeded perceptual-motor tasks.

This study investigated the effects of a 10-mo. endurance-training program (running) on speeded psychometric skills. On a weekly basis the experimental group of 36 subjects participated in one supervised and three unsupervised training sessions. The control group of 22 subjects engaged in no structural training activities during this period. Subjects' performance on a simple reaction-time task, a choice reaction-time task, and a letter-recognition task was assessed before and after the training intervention. Analysis indicated that a 20% improvement in aerobic fitness in the trained group was not accompanied by a significant improvement in psychometric performance. This result is discussed in terms of the relationship between aerobic fitness and cognitive performance. It is concluded that improved physical fitness is not necessarily accompanied by improved perceptual-motor skills.

Manipulating the spatial arrangement of stimuli in a precuing task.

Within the context of the spatial precuing paradigm a consistent finding is that, with hands placed adjacently, precuing of two fingers on the same hand results in faster discrete finger responses than precuing of two fingers on different hands. This phenomenon is known as the 'hand advantage'. The present study examined the possibility that perceptual factors contribute to this hand advantage. Results showed that the perceptual manipulation of moving the two center stimulus positions each one position inwards significantly reduced the hand advantage over the first 250 ms of preparation interval. This result was interpreted as reflecting a perceptual locus of the hand advantage. Perceptual encoding processes were tentatively postulated to operate according to the familiar subgroup hypothesis, which claims that spatial precues may differ in the 'familiarity' or 'strength' of the subgroups they create. The fact that a significant hand advantage remained present was taken as evidence in support of post-perceptual processes mediating the hand advantage. On the basis of these findings we argue that both perceptual and post-perceptual processes play a role in the spatial precuing task and mediate the hand advantage.