Increased functional activity, bottom-up and intrinsic effective connectivity in autism

Sensory perceptual alterations such as sensory sensitivities in autism have been proposed to be caused by differences in sensory observation (Likelihood) or in forming models of the environment (Prior), which result in an increase in bottom-up information flow relative to top-down control. To investigate this conjecture, we had autistic individuals (AS) and neurotypicals (NT) perform a decision-under-uncertainty paradigm while undergoing functional magnetic resonance imaging (fMRI). There were no group differences in task performance and in Prior and Likelihood representations in brain activity. However, there were significant group differences in overall task activity, with the AS group showing significantly greater activation in the bilateral precuneus, mid-occipital gyrus, cuneus, superior frontal gyrus (SFG) and left putamen relative to the NT group. Further, when pooling the data across both groups, we found that those with higher AQ scores showed greater activity in the left cuneus and precuneus. Effective connectivity analysis using dynamic causal modelling (DCM) revealed that group differences in BOLD signals were underpinned by increased activity within sensory regions and a net increase in bottom-up connectivity from the occipital region to the precuneus and the left SFG. These findings support the hypothesis of increased bottom-up information flow in autism during sensory learning tasks.

Media-multitasking and cognitive control across the lifespan

The exponential rise in technology use over the past decade, and particularly during the COIVD-19 pandemic, has been accompanied by growing concern regarding the consequences of this technology use for our cognition. Previous studies on the influence of technology-multitasking (the use of two or more technologies simultaneously) on cognitive performance have provided mixed results. However, these past studies have generally ignored the considerable developmental trajectories that cognitive abilities undergo across the lifespan. In a large community-based science project we investigated the relationship between media-multitasking and cognitive flexibility (multitasking ability) in participants aged 7–70 years. Higher levels of every-day technology multitasking were associated with higher levels of multitasking performance across an age range in which multitasking ability undergoes developmental change. These findings suggest that age is an important moderator of the relationship between technology use and cognition.

Reduced Context Updating but Intact Visual Priors in Autism

A general consensus persists that sensory-perceptual differences in autism, such as hypersensitivities to light or sound, result from an overreliance on new (rather than prior) sensory observations. However, conflicting Bayesian accounts of autism remain unresolved as to whether such alterations are caused by more precise sensory observations (precise likelihood model) or by forming a less precise model of the sensory context (hypo-priors model). We used a decision-under-uncertainty paradigm that manipulated uncertainty in both likelihoods and priors. Contrary to model predictions we found no differences in reliance on likelihood in autistic group (AS) compared to neurotypicals (NT) and found no differences in subjective prior variance between groups. However, we found reduced context adjustment in the AS group compared to NT. Further, the AS group showed heightened variability in their relative weighting of sensory information (vs. prior) on a trial-by-trial basis. When participants were aligned on a continuum of autistic traits, we found no associations with likelihood reliance or prior variance but found an increase in likelihood precision with autistic traits. These findings together provide empirical evidence for intact priors, precise likelihood, reduced context updating and heightened variability during sensory learning in autism.

A systematic, large-scale study of synaesthesia: implications for the role of early experience in lexical-colour associations

For individuals with synaesthesia, stimuli in one sensory modality elicit anomalous experiences in another modality. For example, the sound of a particular piano note may be 'seen' as a unique colour, or the taste of a familiar food may be 'felt' as a distinct bodily sensation. We report a study of 192 adult synaesthetes, in which we administered a structured questionnaire to determine the relative frequency and characteristics of different types of synaesthetic experience. Our data suggest the prevalence of synaesthesia in the adult population is approximately 1 in 1150 females and 1 in 7150 males. The incidence of left-handedness in our sample was within the normal range, contrary to previous claims. We did, however, find that synaesthetes are more likely to be involved in artistic pursuits, consistent with anecdotal reports. We also examined responses from a subset of 150 synaesthetes for whom letters, digits and words induce colour experiences ('lexical-colour' synaesthesia). There was a striking consistency in the colours induced by certain letters and digits in these individuals. For example, 'R' elicited red for 36% of the sample, 'Y' elicited yellow for 45%, and 'D' elicited brown for 47%. Similar trends were apparent for a group of non-synaesthetic controls who were asked to associate colours with letters and digits. Based on these findings, we suggest that the development of lexical-colour synaesthesia in many cases incorporates early learning experiences common to all individuals. Moreover, many of our synaesthetes experienced colours only for days of the week, letters or digits, suggesting that inducers that are part of a conventional sequence (e.g. Monday, Tuesday, Wednesday...; A, B, C...; 1, 2, 3...) may be particularly important in the development of synaesthetic inducer-colour pairs. We speculate that the learning of such sequences during an early critical period determines the particular pattern of lexical-colour links, and that this pattern then generalises to other words.

Abnormal line bisection judgements in children with Tourette's syndrome

Tourette's syndrome (TS) has been associated with loss of normal basal ganglia asymmetry, as well as loss of normal functional asymmetry, including the leftward bias on traditional visuospatial tasks such as line bisection and turning bias tests. The aim of the present study was to examine the lateralisation of visuospatial attention in TS. We examined the effect of an irrelevant moving-dot background on line bisection judgements. Nine children with a DSM IV diagnosis of TS participated, in addition to 9 healthy controls, individually matched for age, sex and IQ. Horizontal lines of varying length were presented on a computer screen with either a blank background, or a moving, random-dot field. The dots moved either leftward or rightward across the screen at 40 or 80 mm/s, and participants were instructed to ignore these distracting stimuli when judging the lines. TS children were found to be abnormally right-biased in line bisection in a similar fashion to unmedicated ADHD children who, in a previous study, showed a similar small, yet significant, right-bias in line bisection. Matched controls showed a small, nonsignificant left bias, consistent with past research. Unlike previous findings with hemineglect patients, the irrelevant moving background had no effect on bisection performance for TS children or healthy controls. The present findings suggest a deficit in visuospatial attention consistent with the emerging picture of a lateralised dysfunction of frontostriatal circuitry in TS.

Unilateral hemispheric activation does not affect free-viewing perceptual asymmetries

Strong leftward perceptual biases have been reported for the selection of the darker of two left/right mirror-reversed luminance gradients under free-viewing conditions. This study investigated the effect of unilateral hemispheric activation on this leftward bias in two groups of dextrals (N = 52 and N = 24). In Experiment 1, activation was manipulated by asking participants to tap with their left or right fingers along their midline. In Experiment 2, participants clenched their left or right hands in their respective hemispaces. Participants selected the stimulus that was darker on the left-hand side 73% of the time. Despite manipulations of activation strength and hemispace, activation had no effect on the asymmetry. If activation was important, the leftward bias should have been enhanced when the left hand/right hemisphere was active and reduced (or reversed) when the right hand/left hemisphere was active. The contribution of left-to-right scanning biases to free-viewing perceptual asymmetries is discussed as an alternative.

Unconscious priming eliminates automatic binding of colour and alphanumeric form in synaesthesia

Synaesthesia is an unusual perceptual phenomenon in which events in one sensory modality induce vivid sensations in another. Individuals may 'taste' shapes, 'hear' colours, or 'feel' sounds. Synaesthesia was first described over a century ago, but little is known about its underlying causes or its effects on cognition. Most reports have been anecdotal or have focused on isolated unusual cases. Here we report an investigation of 15 individuals with colour-graphemic synaesthesia, each of whom experiences idiosyncratic but highly consistent colours for letters and digits. Using a colour-form interference paradigm, we show that induced synaesthetic experiences cannot be consciously suppressed even when detrimental to task performance. In contrast, if letters and digits are presented briefly and masked, so that they are processed but unavailable for overt report, the synaesthesia is eliminated. These results show that synaesthetic experiences can be prevented despite substantial processing of the sensory stimuli that otherwise trigger them. We conclude that automatic binding of colour and alphanumeric form in synaesthesia arises after initial processes of letter and digit recognition are complete.

A kinematic analysis of distractor interference effects during visually guided action in spatial neglect

Patients with left spatial neglect following right hemisphere damage may show anomalies in ipsilesional-limb movements directed to targets on their affected side, in addition to their characteristic perceptual deficits. In this study we examined the extent to which visually guided movements made by neglect patients are susceptible to interference from concurrent visual distractors on the contralesional or ipsilesional side of a designated target. Eleven right hemisphere patients with visual neglect, plus 11 matched healthy controls, performed a double-step movement task upon a digitizing tablet, using their ipsilesional hand to respond. On each double-step trial the first component of the movement was cued to a common central target, whereas the second component was cued unpredictably to a target on either the contralesional or ipsilesional side. On separate trials lateral targets either appeared alone or together with a concurrent distractor in an homologous location in the opposite hemispace. In addition to being significantly slower and more error prone than controls, neglect patients also exhibited a number of interference effects from ipsilesional distractors. They often failed to move to left targets in the presence of a right-sided distractor, or else they moved to the distractor itself rather than to a contralesional target. The initial accelerative phase of their movements to contralesional targets tended to be interrupted prematurely, and they spent significantly more time in the terminal guidance phase of movements to contralesional targets in the presence of an ipsilesional distractor. In contrast, contralesional distractors had little effect on patients' movements to ipsilesional targets. We conclude that right hemisphere damage induces a competitive bias that favors actions to ipsilesional targets. This bias affects multiple stages of processing within the visuomotor system, from initial programming through to the final stages of terminal guidance.

Distinguishing sensory and motor biases in parietal and frontal neglect

Left neglect after right-hemisphere damage may involve perceptual and/or motor impairments. Here we discuss the limitations of previous attempts to separate these components, and introduce a new method. Six neglect patients (three with right inferior parietal lesions and three with right inferior frontal lesions) moved their right hand to a target light, which appeared unpredictably on either the left or the right of central fixation. The target appeared alone or with a distractor light in the opposite hemifield. Any directional motoric bias was measured by comparing reaches from a central start position with those for the same visual displays, but starting from the left of both possible targets (thus requiring only rightward reaches) or from the right (requiring only leftward reaches). All patients were slower to initiate reaches to left than right targets from a central start, which could reflect perceptual and/or motor biases. Critically, in the parietal neglect group only, initiation speed for left targets improved when a rightward reach was required to these (from a left start) rather than a leftward reach. This suggests a deficit in programming leftward movements into left hemispace, in addition to any visual impairment, for parietal neglect. A control task confirmed that this effect of start position was due to the associated change in reach direction and not to afferent inputs from the hand as it rested at the start position. Frontal neglect patients were slow to execute reaches to left targets, regardless of movement direction. Right visual distractors slowed visual reaction times to left targets more than vice versa in frontal neglect patients, and likewise for reach execution times in parietal neglect patients, suggesting that visual distractors on the neglected side have less impact. Distractor effects were unaffected by start position in the frontal neglect group (suggesting a perceptual basis), but distractors slowed reach initiation in the parietal neglect group only from left and central starts. Taken together, these findings demonstrate a directional motor component to parietal but not frontal neglect, and suggest that in man the inferior parietal lobe plays a role not only in perception but also in the programming of selective reaches. These conclusions are related to recent single-unit data from the monkey parietal lobe.

Free-viewing perceptual asymmetries for the judgement of brightness, numerosity and size

Perceptual asymmetries under free-viewing conditions were investigated in 24 normal dextral adults. Three tasks were administered that required participants to chose between a pair of left/right reversed stimuli on the basis of their brightness, numerosity or size. These stimulus features were represented asymmetrically within the stimuli, so that each stimulus appeared darker, larger or more numerous on the left or right sides. Participants more often selected the stimulus with the relevant feature on the left-hand side for all three tasks. Response times for leftward responses were faster than rightward responses. Split-half reliabilities revealed a high level of consistency within the tasks. However, the correlation between tasks was low. These results suggest that the different tasks, while showing similar levels of perceptual asymmetry, engage distinct sets of lateralised processes.

Effects of stimulant medication on the lateralisation of line bisection judgements of children with attention deficit hyperactivity disorder

OBJECTIVES

Deficits in the maintenance of attention may underlie problems in attention deficit hyperactivity disorder (ADHD). Children with ADHD also show asymmetric attention deficits in traditional lateralisation and visuospatial orienting tasks, suggesting right hemispheric (and left hemispace) attentional disturbance. This study aimed to examine the lateralisation of selective attention in ADHD; specifically, the effect of a moving, random dot background, and stimulant medication in the line bisection task.

METHODS

The performance of children with ADHD, on and off methylphenidate, was examined using a computerised horizontal line bisection task with moving and blank backgrounds. Twenty children with a DSM-IV diagnosis of ADHD participated with 20 controls, individually matched for age, sex, grade at school, and IQ. Twelve of the 20 children with ADHD were on stimulant medication at the time of testing. Horizontal lines of varying length were presented in the centre of a computer screen, with either a blank background, or a moving, random dot field. The random dots moved either leftward or rightward across the screen at either 40 mm/s or 80 mm/s.

RESULTS

The children with ADHD off medication bisected lines significantly further to the right compared with controls, who showed a small leftward error. Methylphenidate normalised the performance of the children with ADHD for the task with the moving dots.

CONCLUSIONS

These results support previous evidence for a right hemispheric hypoarousal theory of attentional dysfunction, and are consistent with the emerging picture of a lateralised dysfunction of frontostriatal circuitry in ADHD.

Phasic alerting of neglect patients overcomes their spatial deficit in visual awareness

Patients with extensive damage to the right hemisphere of their brain often exhibit unilateral neglect of the left side of space. The spatial attention of these patients is strongly biased towards the right, so their awareness of visual events on the left is impaired. Extensive right-hemisphere lesions also impair tonic alertness (the ability to maintain arousal). This nonspatial deficit in alertness is often considered to be a different problem from spatial neglect, but the two impairments may be linked. If so, then phasically increasing the patients' alertness should temporarily ameliorate their spatial bias in awareness. Here we provide evidence to support this theory. Right-hemisphere-neglect patients judged whether a visual event on the left preceded or followed a comparable event on the right. They became aware of left events half a second later than right events on average. This spatial imbalance in the time course of visual awareness was corrected when a warning sound alerted the patients phasically. Even a warning sound on the right accelerated the perception of left visual events in this way. Nonspatial phasic alerting can thus overcome disabling spatial biases in perceptual awareness after brain injury.

Parietal neglect and visual awareness

The last decade has seen a resurgence of interest in the neural correlates of conscious vision, with most discussion focused on the 'blindsight' that can follow damage to primary visual cortex, in the occipital lobe. We suggest that new insights into the neural basis of visual awareness may be gleaned from a different neuropsychological phenomenon, namely visual 'neglect' after injury to regions in the parietal lobe. Neglect provides several revealing contrasts with occipital blindsight. Here we summarise four key findings. First, unlike the deficits caused by damage to primary visual cortex, the loss of awareness in parietal neglect is characteristically not strictly retinotopic. Second, visual segmentation processes are preserved in neglect, and can influence what will reach the patient's awareness. Third, extensive unconscious processing takes place for those stimuli on the neglected side which escape awareness, including some degree of object identification. Finally, parietal damage affects initial stages of motor planning as well as perception. These findings are consistent with recent data on single-cell activity in the monkey brain. They also suggest why areas in the inferior parietal lobe may play a prominent role in visual awareness.

The effects of competition and motor reprogramming on visuomotor selection in unilateral neglect

Patients with unilateral neglect following right hemisphere damage may have difficulty in moving towards contralesional targets. To test the hypothesis that this impairment arises from competing motor programs triggered by irrelevant ipsilesional stimuli, we examined 16 right hemisphere patients, eight with left visual neglect and eight without, in addition to eight healthy control subjects. In experiment 1 subjects performed sequences of movements using their right hand to targets on the contralesional or ipsilesional side of the responding limb. The locations of successive targets in each sequence were either predictable or unpredictable. In separate blocks of trials, targets appeared either alone or with a simultaneous distractor located at the immediately preceding target location. Neglect patients were significantly slower to execute movements to contralesional targets, but only for unpredictable movements and in the presence of a concurrent ipsilesional distractor. In contrast, healthy controls and right hemisphere patients without neglect showed no directional asymmetries of movement execution. In experiment 2 subjects were required to interrupt a predictable, reciprocating sequence of leftward and rightward movements in order to move to an occasional, unpredictable target that occurred either in the direction opposite to that expected, or in the same direction but twice the extent. Neglect patients were significantly slower in reprogramming the direction and extent of movements towards contralesional versus ipsilesional targets, and they also made significantly more errors when executing such movements. Right hemisphere patients without neglect showed a similar bias in reprogramming direction (but not extent) for contralesional targets, whereas healthy controls showed no directional asymmetry in either condition. On the basis of these findings we propose that neglect involves a competitive bias in favour of motor programs for actions directed towards ipsilesional versus contralesional events. We suggest that programming errors and increased latencies for contralesional movements arise because the damaged right hemisphere can no longer effectively inhibit the release of inappropriate motor programs towards ipsilesional events.

Motor role of human inferior parietal lobe revealed in unilateral neglect patients

The exact role of the parietal lobe in spatial cognition is controversial. One influential hypothesis proposes that it subserves spatial perception, whereas other accounts suggest that its primary role is to direct spatial movement. For humans, it has been suggested that these functions may be divided between inferior and superior parietal lobes, respectively. In apparent support of a purely perceptual function for the inferior parietal lobe (IPL), patients with lesions to this structure, particularly in the right hemisphere, exhibit unilateral spatial neglect (deficient awareness for the side of space opposite to that of their lesion). Here we show that patients with right IPL lesions also have a specific difficulty in initiating leftward movements towards visual targets on the left side of space. This motor impairment was not found in neglect patients with frontal lesions, contrary to previous proposals that motor aspects of neglect are particularly associated with anterior damage. Our results suggest that the human IPL operates as a sensorimotor interface, rather than subserving only perceptual functions.

Attentional competition between modalities: extinction between touch and vision after right hemisphere damage

The phenomenon of extinction, which occurs frequently after unilateral brain damage, involves a failure to detect the more contralesional of two simultaneously presented stimuli, but with preserved detection of single ipsilesional or contralesional stimuli. Current accounts suggest that the disorder reflects a bias of selective attention, in which ipsilesional stimuli have a competitive advantage. Extinction may be manifested within any one of the major sensory modalities (vision, audition, touch), or it may occur within several modalities in a given individual. Given recent evidence in normals for attentional links between separate sensory modalities, we examined whether extinction can also occur cross-modally, i.e. for double-simultaneous stimuli in separate sensory modalities. We tested whether an ipsilesional event sufficient to extinguish a contralesional stimulus within the same modality may also extinguish a contralesional stimulus in a different modality. Our three patients had right hemisphere damage, and reliable within-modality extinction for visual and tactile stimuli. They also showed significant cross-modal extinction, such that an ipsilesional tactile (or visual) event extinguished awareness of a simultaneous visual (or tactile) event on the contralesional side. These results, which provide the first quantitative evidence for cross-modal extinction, were replicated in a second experiment in which visual and tactile stimuli in the cross-modal conditions were presented at non-homologous elevations within each hemispace. We conclude that after unilateral damage, ipsilesional stimuli have a competitive advantage over contralesional stimuli, and that this affects competition between stimuli from different modalities as well as stimuli within the same modality. These findings are consistent with recent evidence for competitive interactions between tactile and visual events in the control of spatial attention in normals.

Visual extinction and prior entry: impaired perception of temporal order with intact motion perception after unilateral parietal damage

Two patients with left-sided visual extinction after right parietal damage were each given two 'prior entry' tasks that have recently been used to study attentional biases in normals. The first task presented two unconnected bars, one in each visual field, with the patients asked to judge which appeared sooner. Both patients reported that the right bar preceded the left unless the latter led by over 200 msec, suggesting a severe bias to the right affecting the time-course of visual awareness. The second task presented one continuous line in a scrolling format across the same spatial extent, with the patients asked to judge which direction the line moved in. The patients now performed normally. Thus, the perception of temporal order for separate events was impaired by the lesions, but without disrupting motion perception within single events. The implications are discussed for theories of normal and pathological attention, visual awareness, and motion perception.

Preattentive filling-in of visual surfaces in parietal extinction

Unilateral brain damage frequently produces "extinction," in which patients can detect brief single visual stimuli on either side but are unaware of a contralesional stimulus if presented concurrently with an ipsilesional stimulus. Explanations for extinction have invoked deficits in initial processes that operate before the focusing of visual attention or in later attentive stages of vision. Preattentive vision was preserved in a parietally damaged patient, whose extinction was less severe when bilateral stimuli formed a common surface, even if this required visual filling-in to yield illusory Kanizsa figures or completion of partially occluded figures. These results show that parietal extinction arises only after substantial processing has generated visual surfaces, supporting recent claims that visual attention is surface-based.

Responses of neurons in the inferior colliculus of the rat to interaural time and intensity differences in transient stimuli: Implications for the latency hypotheses

Although the sensitivity to interaural intensity differences (IIDs) of neurons receiving excitatory - inhibitory binaural input (EI neurons) has been examined in numerous studies, the mechanisms underlying this sensitivity remain unclear. According to the 'latency hypotheses' neuronal sensitivity to IIDs reflects sensitivity to differences in the timing of ipsilateral and contralateral inputs that are produced as a consequence of the effects of intensity upon latency. If the latency hypothesis is correct, a neuron's responses over any given IID range should be predicted by its responses to the interaural time differences (ITDs) that are 'equivalent' to the IIDs tested, in the sense that they produce the same changes in the relative timing of inputs. This prediction from the latency hypotheses were examined by determining the sensitivity of EI neurons in the inferior colliculus of anesthetized rats to IIDs and ITDs in click stimuli, under conditions that allowed 'equivalent' ITDs to be estimated. In approximately 10% of the 41 neurons tested, the IID-sensitivity function was a perfect or near-perfect match to the equivalent-ITD function, indicating that IID sensitivity could be entirely accounted for in terms of sensitivity to intensity-produced neural time differences, as asserted by the latency hypothesis. For the majority of neurons, however, sensitivity to equivalent ITDs accounted only partially for the characteristics of the IID-sensitivity function; other features of the function in these cases appeared to reflect the operation of an additional factor, most probably the relative magnitude of the inputs from the two ears. Although the conclusions are qualified by the fact that one of the assumptions on which the estimation of equivalent ITDs was based was probably not satisfied for some neurons, the results suggest that intensity-produced changes in both the magnitude and the timing of excitatory and inhibitory inputs shape the IID sensitivity of most EI neurons.

Selective attention in humans: normality and pathology

Human attention is now studied with a variety of methods, ranging from neuroimaging to behavioural studies of normals and brain-damaged patients. Recent results obtained using these methods converge on several conclusions. First, attention can affect early stages of perception. Second, in low-load conditions, unattended stimuli can be processed to high levels, albeit in a tacit manner. Third, the distribution of attention depends on an interplay between reflexive and voluntary factors. Finally, there are strong attentional links between the sensory modalities, and between perception and action. These links might be exploited to remediate attentional deficits after brain injury.

The effects of unilateral visuospatial neglect on perception of Müller-Lyer illusory figures

Left visuospatial neglect after right hemisphere damage is a lateralised disorder of spatial perception and cognition. A study is reported of the extent to which patients with left visuospatial neglect are susceptible to the illusory effects normally elicited by Müller-Lyer figures, in which inducing fins are located on one end or on both ends of a horizontal line. Seven patients with left visuospatial neglect and seven normal controls were tested on a task of horizontal-line bisection in which stimuli consisted of plain lines of three lengths (100, 150, and 200 mm), randomly interleaved with lines with unilateral or bilateral fins. As a group, normals made accurate bisection judgments in the baseline (no-fins) condition, and exhibited significant illusory effects in both the unilateral-fin and the bilateral-fin conditions. In contrast, patients made substantial rightward errors in the baseline condition, consistent with their neglect of the left end of the line. However, as a group, patients still exhibited significant illusory effects with left-sided outward-projecting fins on 100 mm lines and with left-sided inward-projecting fins on 150 mm lines. Moreover, at least one patient exhibited consistent illusory effects both for inward-projecting and for outward-projecting left-sided fins at all line lengths. Normal illusory effects in patients were also obtained with stimuli containing unilateral right-sided and bilateral fins. The existence of such effects with inducing elements on the contralesional extremity of horizontal line stimuli suggests preservation of low-level, perhaps preattentive, perceptual mechanisms responsible for coding elementary visual features. The results may be relevant for an understanding of the influence of attentional factors on illusory perception.

Recovery from directional hypokinesia and bradykinesia in unilateral neglect

Patients with acute right hemisphere (RH) damage and left unilateral neglect may exhibit slowness in both the initiation and execution of goal-directed movements toward the contralesional side (respectively, directional hypokinesia and directional bradykinesia). We retested 13 patients with RH damage, who on initial testing 1 year earlier had shown these impairments on a visually cued sequential movement task. Initiation and execution times were measured separately for leftward and rightward movement sequences, performed in either hemispace and across the body midline. Patients with anterior/subcortical lesions exhibited directional hypokinesia and bradykinesia on initial testing, but no longer showed either after 12 months. It is suggested that the recovery exhibited by these patients was determined conjointly by residual functioning of flexible anterior/subcortical structures involved in response production, in addition to the resolution of transient secondary dysfunction (i.e., diaschisis) of posterior (parietal) areas responsible for attentional orienting and target selection. In contrast, patients with posterior (parietal) lesions continued to exhibit directional hypokinesia after 12 months, probably as a result of permanent damage to a region whose dedicated or specialised functions cannot readily be compensated by other brain areas. These results support the notion that spatial attention and goal-directed movement are subserved by a distributed network involving separate but interconnected brain regions. Distinct functional losses may be predicted, in both the acute and chronic stages postinjury, on the basis of temporary or permanent dysfunction of discrete components of this network.

Horizontal visual motion modulates focal attention in left unilateral spatial neglect

Patients with unilateral spatial neglect are impaired in directing focal attention toward the contralesional side of space. Provision of static spatial cues on the neglected side has previously been shown to help overcome this deficit. Common movement of visual stimuli may also guide the allocation of spatial attention, although such effects have not been examined in patients with unilateral spatial neglect. Eleven patients with right hemisphere damage and clinical evidence of left unilateral spatial neglect, and 11 matched, healthy controls were tested on a task of horizontal line bisection. Lines were presented on a computer display, with a neutral, static, or slowly drifting, random dot background. Under conditions of motion, background stimuli drifted either leftward or rightward, across the full width of the display, at speeds that did not elicit optokinetic nystagmus or perceptual aftereffects. Controls were accurate in all conditions, and showed minimal effects of background conditions. By contrast, patients with left unilateral spatial neglect were sensitive to leftward background motion, showing a significant leftward shift in bisection error, relative to neutral, static, and rightward moving backgrounds. There was no significant effect of rightward motion in comparison with the neutral and static conditions. The extent to which patients were susceptible to the effects of background motion was not related to severity of unilateral spatial neglect, as measured by clinical tests. The benefits of leftward motion may reflect activity of preserved motion processing mechanisms, which provide input to an otherwise dysfunctional attentional network. The use of visual motion to assist in contralesionally guiding focal attention may be useful in the rehabilitation of unilateral spatial neglect.

Impairments of movement execution in unilateral neglect: a kinematic analysis of directional bradykinesia

A kinematic analysis was performed on goal-directed movements made by 14 patients with right hemisphere damage and left unilateral neglect (seven mild, seven severe), and 14 healthy controls. Leftward and rightward pen strokes of varying extents were made to targets of varying size on a centrally located digitising tablet. While mild unilateral neglect patients performed like controls, patients with severe unilateral neglect were slower to initiate leftward than rightward strokes and were slow and inefficient in movement execution. Leftward strokes made by severe unilateral neglect patients were characterised by prolonged movement time, lower peak velocity and departed from optimal bell-shaped velocity profiles. Their leftward strokes also showed prolonged accelerative phases, implying difficulties in force production, while the high proportion of their total movement time spent in decelerating with rightward strokes suggested an abnormal emphasis on terminal visual guidance. Leftward strokes made by these patients also contained more submovements than rightward strokes, suggesting poor force control. An impaired internal representation of the location of left-sided targets and desired movement trajectories in severe unilateral neglect causes breakdown in the temporal control of goal-directed movements.

Effects of hand and age upon abductive and adductive movements: a kinematic analysis

Older and younger dextral subjects performed targeting movements to left and right with their preferred and nonpreferred hands upon a computer graphics tablet. Kinematic analysis revealed that older subjects produced larger constant errors than younger, paused more, and differed from younger individuals in a number of ways with respect to adductive/abductive asymmetries. The right hand was associated with shorter stroke durations and higher peak velocities, and both shorter times to peak velocity and from peak velocity to zero, suggesting superior ballistic preprogramming by the preferred right hand which was also more accurate. While both hands showed small abductive superiorities in terms of peak velocity and time from peak to zero, the largest directional asymmetries, stroke duration, showed leftward superiorities by both hands. We cannot therefore conclude either that experience with the rightward patterns of writing or that a reported tendency towards mirror-symmetrical movements by the two hands can account for the present results. Rather a right-hemisphere mediation of visually directed movements into left hemispace, along with a left-hemisphere mediation of fast, precise, temporal sequencing may jointly determine observable asymmetries. These may appear as a vector representing the opposing contributions of the two specialized hemispheres.

Can task specific perceptual bias be distinguished from unilateral neglect?

The present study examined visuoperceptual bias in 12 right hemisphere damaged patients, eight of whom showed left unilateral neglect on standard clinical tests, and in 30 normal controls. In the chimeric faces task, subjects were required to judge which of a pair of faces appeared happier. Stimuli comprising each pair were mirror images, with the smiling half on the left of one face and on the right of the other. In the grey scales task, subjects were required to indicate which of two shaded rectangles appeared to be darker overall. Again, stimuli were mirror images, with the darker end appearing either on the left or on the right. Patients exhibited a significant rightward bias on both experimental tasks, in contrast to the significant leftward bias exhibited by controls. There was no significant correlation between patients' performances on standard clinical tests and the extent of bias on the two experimental tasks, suggesting that such patients exhibit distinct impairments of spatial cognition which are differentially indexed by the two types of task. Moreover, for both patients and controls, scores obtained on the two perceptual bias tasks were unrelated, suggesting that they may engage stimulus-specific processes which have different underlying patterns of asymmetrical processing. These data provide further support for models which propose that the heterogeneity of disorders of spatial cognition arise from disruption of distinct neural mechanisms.

Residual rightward attentional bias after apparent recovery from right hemisphere damage: implications for a multicomponent model of neglect

Unilateral neglect may be a multicomponent attentional disorder consisting of an initial automatic orienting of attention toward the ipsilesional side and a subsequent impairment in contralesionally reorienting attention, both of which are superimposed on a generalised reduction in attention resources. It has been hypothesised that patients' ability to reorient attention contralesionally may recover relatively quickly, but that the ipsilesional attention bias may be relatively persistent. This hypothesis was tested by consecutively examining 13 patients who had had a right hemisphere stroke, and who had left unilateral neglect. They were examined once shortly after the stroke and again 12 months later, using a battery of standard clinical and experimental tasks. Patients initially showed a strong and consistent rightward attentional bias in addition to a failure to reorient their attention leftward. After 12 months patients continued to show an abnormal ipsilesional attentional bias, though most were now able to fully reorient their attention toward the contralesional side. These results suggest that restitution of the capacity to reorient attention contralesionally may underlie the apparent recovery from clinical signs of unilateral neglect. The presence of a residual ipsilesional attentional bias in most patients, however, may, at least in part, account for the poor functional outcomes in some apparently "recovered" patients.

Age-related motor slowness: simply strategic?

BACKGROUND

While older adults typically exhibit slower hesitant movements, this may simply reflect a preference for a cautious movement strategy, rather than any pathological process.

METHOD

To separate strategic preferences from any impairment in the coordination of movement, the present experiment trained older adults to move at the preferred speed of younger adults (and vice versa) in a simple zigzag drawing task on a digitizing tablet which sampled pen position at 200 Hz. Twelve older adults (mean age 69 yrs 8 mo) and 12 young adults (mean age 21 yrs) joined 9 targets 125 mm apart, of either 5, 10 or 20 mm diameter. Once the age groups were matched for movement duration, movement kinematics were examined to determine whether there were differences in the quality or accuracy of their movements.

RESULTS

When strategic differences are controlled for, older adults performed the task with comparable overall accuracy, but exhibited greater hesitancy and more submovements.

CONCLUSION

The results suggest a decline in motor coordination rather than any simple strategic preference for caution in movement. The hesitancy of movement to some extent parallels that seen in Parkinson's disease.

Hand-hemispace spatial compatibility, precueing, and stimulus-onset asynchrony

The role of attention and the resolution of coding conflicts in hand-hemispace spatial-compatibility effects was examined in a precueing experiment in which visual and vibrotactile precues, with various stimulus-onset asynchronies (SOAs), were presented in blocked and random order. It was expected that precues at the shorter SOAs would fail to facilitate the shifting of attention, as they occur too close to the imperative stimulus to be informative. The task would therefore approximate one of choice reaction time (RT), resulting in a hand-hemispace spatial-compatibility effect. Conversely, the longer SOAs would provide the subject with sufficient time in which to shift attention fully, and would therefore result in a task more like that of simple reaction time (SRT). It was expected that the hand-hemispace spatial-compatibility effect would then be absent. As was expected, this effect was present at the shorter SOAs, and absent at the longer SOAs. In Experiments 2 and 3, provision of a visual precue further facilitated attentional deployment, as did blocking the presentation of various SOAs in Experiment 3. Vibrotactile and visual precues did not differ in their ability to direct attention, implying that these modalities orient attention and precue location in essentially similar ways. These findings are discussed within the context of the mechanisms though to underlie the time course of spatial compatibility and the dissipation of a fading trace of interfering spatial codes.

Reduction in external cues and movement sequencing in Parkinson's disease

To identify the focus of impairment in the performance of sequential movements of patients with Parkinson's disease, the extent of their reliance on external cues was examined. Eighteen patients with idiopathic Parkinson's disease and their matched controls performed a series of button presses at sequential choice points along a response board. The illuminated pathway to be followed successively extinguished ahead of each move according to three levels of reduction of external cues. Patients with Parkinson's disease were particularly disadvantaged with high levels of reduction of external cueing in terms both of movement preparation time (button down time) and movement execution time (movement time between buttons). Moreover, with high levels of reduction of external cueing, patients with Parkinson's disease were particularly subject to progressive slowing (movement time, not down time) further down the sequence. The basal ganglia may help generate internal cues for releasing successive stages of a predefined movement sequence.

Allocation of attention to programming of movement sequences in Parkinson's disease

The allocation of attention to the programming and execution of movement sequences was examined in Parkinson's disease (PD). The time taken to initiate and execute sequences of one, three, and five button taps was examined, while also varying the hand used (left or right) and the attentional resources that could be allocated to sequencing (using single- versus dual-task conditions). These results showed that performance anomalies in PD were most apparent with the preferred right hand under single-rather than dual-task conditions. Subjects suffering from PD may tend to divert attention from the right hand under single-task conditions, and perhaps with short sequences, as well as being less likely to prepare sequences of more than three movements in advance with that hand. These effects were unlikely to reflect asymmetric pathology. If the right hand of such subjects has in some respects now come to behave more like a "clumsy" left hand, this may reflect a deliberate strategic choice in an attempt to cope with a movement impairment.

Can tactile neglect occur at an intra-limb level? Vibrotactile reaction times in patients with right hemisphere damage

Visual reaction time (RT) studies on patients with right hemisphere (RH) damage have demonstrated that the attentional imbalance to stimuli occupying left and right positions exists even within the "intact" ipsilesional hemifield. The purpose of the present study was to test whether such patients might also exhibit relative left-sided impairments in the tactile modality, where stimuli and responses involve the index and middle fingers of the non-hemiplegic ipsilesional hand. Eight patients with RH damage, and eight matched normal controls, were tested using a vibrotactile choice RT paradigm, with the responding hand held in prone or supine posture, and located either at the body midline, or in left or right hemispace. Patients showed significantly slower RTs with the left than the right finger in both hand postures, a difference which remained constant as a function of the hemispatial location of the responding hand. In the prone posture, patients' left finger RTs were slower than those of controls, who showed no difference between left and right finger RTs, while their right finger RTs were faster than those of controls. In the supine posture, both patients and controls exhibited slower left than right finger RTs, though in controls the left finger disadvantage was attributed to biomechanical rather than attentional factors. Patients also made more errors with left than right finger stimuli, both as failures of detection and as incorrect responses, while controls made fewer errors overall and showed no differences between fingers. These data demonstrate a bias in the distribution of attention to tactile stimuli at an intra-limb level, and suggest that the attentional imbalance created by RH damage may be supramodal.

An evaluation of the role of internal cues in the pathogenesis of parkinsonian hypokinesia

Our animal studies suggest that the basal ganglia provide an internal non-specific cue to trigger movement and imply that Parkinson's disease involves a deficiency in this cueing mechanism. Indeed parkinsonian patients typically rely upon external visual cues. To assess the effects of such non-specific cueing mechanisms on movement, we examined patients' utilization of a variety of auditory cues. Ten patients suffering from Parkinson's disease, and their matched controls, pressed buttons at a series of two-way choice points sequentially down a pathway, both when the latter remained illuminated throughout its length, and when it had to be followed from memory alone. In other experimental conditions, auditory cues were also provided, either contingent upon the previous response, at its initiation (a medium level of advance information) or at its completion (a low level of advance information), or as a series of regularly paced (non-contingent) auditory cues (from a metronome). In addition to error data, we recorded down time (DT, time to initiate each next response) and movement time (MT, time to execute each next response). However, both DT and MT measurements showed that parkinsonian patients were enormously disadvantaged by the absence of external cues. While contingent auditory cues were of some help, the performance of patients with Parkinson's disease was dramatically improved by the provision of non-contingent auditory information. Moreover, parkinsonian patients, unlike controls, were greatly affected by the length of individual sub-movements, especially in the absence of external cues. When the pathway to be followed remained illuminated, sub-movement length had little effect. We conclude that for well-learnt, predictable sequences the basal ganglia provide a non-specific internal cue that is necessary for switching between one movement and the next in a movement sequence, and also for development of preparatory activity for each sub-movement in the sequence.

To see or not to see: the effects of visible and invisible cues on line bisection judgements in unilateral neglect

Patients with left unilateral neglect and matched controls were tested in two experiments to examine the effects of lateralized cues on line bisection judgements. Unlike previous studies in which letter or number cues were placed beyond the endpoint(s) of each line, we adopted a procedure which maintained the perceptual point of balance in the horizontal axis of each line. We also related the cueing task more closely to the act of bisection by requiring subjects to place a small mark in direct alignment with the endpoint(s) of each line. In the first experiment, it was found that, for controls, the presence or absence of visible lateralized cues did not differentially affect the magnitude of bisection errors. However, the magnitude of bisection errors made by neglect patients was significantly reduced (and reversed) in the presence of a visible left-sided cue, but remained well to the right of the midpoint in the absence of such cues. In a second experiment, subjects engaged in an otherwise identical cueing procedure, except that no visible marks appeared on the stimulus lines. Neither subject group was affected by the presence or absence of right-sided cues. However, the presence of left-sided cues again reduced the magnitude of bisection errors in neglect patients, but not in controls. These results indicate that the extent of the attentional bias exhibited by neglect patients can be ameliorated even in the absence of lateralized visible cues.

Reversed perceptual asymmetry for faces in left unilateral neglect

Twelve patients with left unilateral neglect and 12 matched controls were examined on two tests of face perception. In the chimeric faces task, subjects were required to make a judgement of happiness on pairs of photographic chimeras, while in the face-matching task, they were required to indicate which of two symmetrical face composites more closely resembled the original. Whereas controls showed a significant leftward perceptual bias on both tasks, left neglect patients showed an even stronger rightward (reversed) perceptual bias. Patients with and without left-sided visual field defects exhibited the reversed asymmetry, and the degree of perceptual bias shown by patients was not related to the severity of their symptoms as measured by standard clinical tasks. There was no relationship between the extent of rightward bias exhibited by patients or controls on the two tasks. These results contribute to our understanding of the mechanisms underlying unilateral neglect. Moreover, the tasks themselves may be employed as a simple and sensitive adjunct to the clinical assessment of this disorder.

Re-orientation of attention in Parkinson's disease: an extension to the vibrotactile modality

Parkinson's disease (PD), a disorder of the dopaminergic nigro-striatal pathway, is associated with both motor and cognitive impairments, including perhaps the ability to focus attention. Disturbances of attentional processes were further examined in a series of vibrotactile choice reaction time (cRT) experiments involving biased probabilities of event occurrence, or valid/invalid precueing. Results of three experiments suggest that PD subjects, compared to controls, are less adept at maintaining attention in space. The performance of the PD subjects improved, however, when they were permitted to direct their gaze to one hand or the other, and when valid external precues were provided before each trial. Observed similarities between the cognitive and motor deficits of PD subjects support the notion that there is a close coupling between the mechanisms coordinating attention and movement.

Impairments of movement initiation and execution in unilateral neglect. Directional hypokinesia and bradykinesia

Patients with unilateral neglect may exhibit slowness in the initiation of contralesionally directed movements in peripersonal space (directional hypokinesia). The present study used a sequential movement task to characterize any such impairment in a group of 24 patients with right hemisphere lesions, 18 of whom had left neglect. A further five patients with left hemisphere lesions, one of whom had right neglect, were also tested. We measured movement initiation and execution times for leftward and rightward movements in either hemispace and across the body midline. Most left neglect patients, particularly those with lesions involving posterior cortex, showed directional hypokinesia. Left neglect patients with anterior and/or subcortical lesions also showed directional bradykinesia, i.e. a slowing in the execution phase of contralesionally directed movements. This impairment occurred regardless of the spatial location of the apparatus and was exacerbated as patients moved closer to their neglected side. The patient with right neglect showed directional hypokinesia but not directional bradykinesia. Right hemisphere and left hemisphere lesion patients without neglect performed in a manner comparable to controls, who did not exhibit directional hypokinesia or directional bradykinesia. These results suggest that directional hypokinesia is associated with both left hemisphere and right hemisphere damage, but only in the context of unilateral neglect. Moreover, the site of hemispheric lesion may determine the temporal characteristics of movement impairments in neglect. Damage to posterior cortex produces deficits in detecting contralesional targets and initiating movements toward them, while damage to anterior or subcortical structures may disrupt the internal representation of an intended trajectory.