Directional Hypokinesia Research Articles

Bedside video-oculographic evaluation of eye movements in acute supratentorial stroke patients: A potential biomarker for hemispatial neglect

Background and purposeThe presence of hemispatial neglect adversely affects functional outcomes in stroke patients; consequently, it warrants early targeted rehabilitative intervention. Nevertheless, hemispatial neglect in the acute phase of stroke has often been underdiagnosed. In this study, we aimed to detect hemispatial neglect at the bedside in acute stroke patients by measuring eye movements using video-oculography (VOG). MethodsForty-seven patients with acute unilateral supratentorial stroke were enrolled. We quantitatively measured horizontal saccade (latency, velocity, and amplitude) and smooth pursuit (gain) at the bedside using VOG and compared these variables with scores on the Behavioral Inattention Test (BIT), a screening battery to assess hemispatial neglect. ResultsContralesional saccade latency, velocity, and amplitude, and ipsilesional smooth pursuit gain were suppressed compared with those in the opposite directions (p = 0.08, 0.02, 0.04, and 0.02, respectively). These directional ocular hypokinesia values correlated with the total BIT score (correlation coefficients −0.53, 0.48, 0.51, and 0.39, respectively). The association was significant even after adjusting for age and stroke severity. ConclusionsEye movement measurements performed using VOG significantly correlated with the tendency for hemispatial neglect in acute supratentorial stroke patients. Bedside VOG measurement may be a simple biomarker for detecting hemispatial neglect even in patients in the supine position during the acute phase of stroke.

Unilateral neglect post stroke: Eye movement frequencies indicate directional hypokinesia while fixation distributions suggest compensational mechanism.

IntroductionEye movements and spatial attention are closely related, and eye‐tracking can provide valuable information in research on visual attention. We investigated the pathology of overt attention in right hemisphere (RH) stroke patients differing in their severity of neglect symptoms by using eye‐tracking during a dynamic attention task.MethodsEye movements were recorded in 26 RH stroke patients (13 with and 13 without unilateral spatial neglect, and a matched group of 26 healthy controls during a Multiple Object Tracking task. We assessed the frequency and spatial distributions of fixations, as well as frequencies of eye movements to the left and to the right side of visual space so as to investigate individuals’ efficiency of visual processing, distribution of attentional processing resources, and oculomotoric orienting mechanisms.ResultsBoth patient groups showed increased fixation frequencies compared to controls. A spatial bias was found in neglect patients’ fixation distribution, depending on neglect severity (indexed by scores on the Behavioral Inattention Test). Patients with more severe neglect had more fixations within the right field, while patients with less severe neglect had more fixations within their left field. Eye movement frequencies were dependent on direction in the neglect patient group, as they made more eye movements toward the right than toward the left.ConclusionThe patient groups’ higher fixation rates suggest that patients are generally less efficient in visual processing. The spatial bias in fixation distribution, dependent on neglect severity, suggested that patients with less severe neglect were able to use compensational mechanisms in their contralesional space. The observed relation between eye movement rates and directions observed in neglect patients provides a measure of the degree of difficulty these patients may encounter during dynamic situations in daily life and supports the idea that directional oculomotor hypokinesia may be a relevant component in this syndrome.

Prism adaptation speeds reach initiation in the direction of the prism after-effect.

Damage to the temporal-parietal cortex in the right hemisphere often leads to spatial neglect-a disorder in which patients are unable to attend to sensory input from their contralesional (left) side. Neglect has been associated with both attentional and premotor deficits. That is, in addition to having difficulty with attending to the left side, patients are often slower to initiate leftward vs. rightward movements (i.e., directional hypokinesia). Previous research has indicated that a brief period of adaptation to rightward shifting prisms can reduce symptoms of neglect by adjusting the patient's movements leftward, toward the neglected field. Although prism adaptation has been shown to reduce spatial attention deficits in patients with neglect, very little work has examined the effects of prisms on premotor symptoms. In the current study, we examined this in healthy individuals using leftward shifting prisms to induce a rightward shift in the egocentric reference frame, similar to neglect patients prior to prism adaptation. Specifically, we examined the speed with which healthy participants initiated leftward and rightward reaches (without visual feedback) prior to and following adaptation to either 17° leftward (n=16) or 17° rightward (n=15) shifting prisms. Our results indicated that, following adaptation, participants were significantly faster to initiate reaches towards targets located in the direction opposite the prism shift. That is, participants were faster to initiate reaches to right targets following leftward prism adaptation and were faster to initiate reaches to left targets following rightward prism adaptation. Overall, these results are consistent with the idea that prism adaptation can influence the speed with which a reach can be initiated toward a target in the direction opposite the prism shift, possibly through altering activity in neural circuits involved in reach planning.

Neglected premotor neglect.

Unilateral neglect, or neglect for short, is commonly described as the failure to respond and attend to stimuli presented on the contralesional side. It cannot be explained by primary motor and sensory impairment (Heilman et al., 1987), and is usually caused by a stroke. Although neglect patients often recover spontaneously within several weeks, they demonstrate poorer amelioration and require longer hospitalizations following a stroke compared to stroke patients without the affliction (e.g., Buxbaum et al., 2004; Gillen et al., 2005). Many different subforms of neglect have been specified to date (e.g., Saevarsson et al., 2011). One of these, premotor neglect (PMN; also known as intentional motor neglect, directional action neglect, etc.; see Saevarsson, 2013a) denotes an intentional, voluntary, and directional (e.g. eye, hand, and head) motor bias from the ipsilesional side to an object in the contralesional side of space (Watson et al., 1978; Halligan and Marshall, 1989; Bisiach et al., 1990; Goodale et al., 1990; Heilman et al., 2008; Saevarsson, 2013a). For instance, patients may fail to reach an apple on their left side with their right hand (i.e., directional akinesia; Heilman et al., 1987) although they may be visually aware of the object. The foundation of PMN diagnosis is based on various studies that indicate performance improvement or decline when patients perform tasks that require directional movements under different visual conditions (see Saevarsson, 2013a for discussion). PMN is often seen alongside other neglect forms (in approximately 45% of cases), although exact incidence has not been specified (Saevarsson, 2013a). Unfortunately, many neglect reviews and empirical studies ignore PMN altogether (e.g., Saevarsson et al., 2008; Karnath, 2014), or report it merely as an unimportant accompaniment and not specific to neglect (e.g., Himmelbach and Karnath, 2003; Rossit et al., 2009a; Striemer and Danckert, 2013). For example, Himmelbach et al. (2007, p. 1980) claim that PMN is not a “consequence of spatial neglect but rather indicate[s] a phenomenon occurring in some of these patients as well as in other stroke patients (without neglect), i.e., a phenomenon occurring with (so far not further identified) brain damage.” In line with this view, the number of studies on PMN have decreased considerably since the 1990s (Saevarsson, 2013a). Conversely, many authors argue for the importance of PMN (e.g., Mattingley and Driver, 1997; Konczak and Karnath, 1998; Vossel et al., 2010; Saevarsson, 2013b) although non-neglect-based terms such as directional hypokinesia are often used. For instance, the most commonly applied neglect definition of Heilman et al. (1987) refers to PMN when describing the affliction. Controversially, current mainstream literature does not reject this description despite the fact that some authors seem to prefer “spatial” or “hemispatial” neglect as a synonym, although representational neglect is non-spatial in nature. The nature of PMN is poorly understood and may hold the key to advanced neglect assessment and rehabilitation (Punt and Riddoch, 2006; Saevarsson, 2013a), thus we argue for the existence and importance of PMN with regard to various clinical, neuroanatomical, and methodological issues.

How many forms of perseveration? Evidence from cancellation tasks in right hemisphere patients

Neglect patients' performance during cancellation tasks is characterized by left sided omissions and, in many cases, by the production of inappropriate material of various kinds in the ipsilesional space, e.g. additional marks over already cancelled targets, marks drawn away from targets, scribbles, irrelevant drawings. It is unclear whether these behaviours, which have collectively been called perseverative, are functionally and anatomically connected and whether they correlate with the severity of neglect. Here we report a retrospective study on 33 right brain damaged patients with neglect after right hemisphere lesions in whom we measured the intensity of perseveration of the three following kinds: (1) ‘additional marks' (AM) perseveration where patients cancelled a target with two or more well separated marks; (2) ‘scribble’ perseveration, where patients, instead of cancelling the target with a single pen stroke as required by the task, performed multiple pen strokes without breaking the pen-to-paper contact, with the final product being a scribble; (3) ‘flying marks’ (FM) perseveration where patients produced cancellation marks well away from the targets. We found that AM and FM perseveration correlated with neglect severity, while ‘scribble’ perseveration did not. The lesion-symptom mapping showed three separate anatomical areas in the right hemisphere: ‘scribble’ perseveration was associated with lesions of the orbitofrontal cortex and caudate nucleus; AM perseveration was associated with damage to the rolandic operculum, superior temporal gyrus and inferior frontal gyrus; FM perseveration was associated with damage to the dorsal premotor cortex and the temporal pole. Neglect severity followed damage to a region which grossly corresponds to the sum of the regions associated with AM and FM perseveration respectively. This complex behavioural and anatomical pattern is interpreted in terms of a three-factor model, in which AM perseveration is caused by a deficit of disengagement of attention from the right side (also causing omissions), FM perseveration is caused by directional hypokinesia (also causing left-side omissions), and ‘scribble’ perseveration is the consequence of a failure to inhibit an initiated motor act, which is completely separate (both anatomically and functionally) from the disorder inducing omissions.

Directional and spatial motor intentional disorders in patients with right versus left hemisphere strokes.

Motor intentional disorders (MIDs) are characterized by dysfunction in the preparation, initiation, maintenance, and termination of goal-oriented actions. In this study, we investigated (1) whether patients with right hemisphere strokes (RHS) and left hemisphere strokes (LHS) differ in the frequency of delayed action initiation (hypokinesia) and motor impersistence; (2) whether there is a directional or hemispatial component of hypokinesia or motor impersistence; (3) whether there is an association between the presence of hemispatial neglect and tests for MID; and (4) the location of injury associated with MID. Thirty-two patients with acute unilateral stroke (21 with RHS and 11 with LHS) and 12 age-matched healthy controls participated in the study. To determine the presence and severity of directional and spatial hypokinesia and impersistence we used a new apparatus, the Directional Movement Tester (DMT). While being tested with the DMT, the participants held a static bar located either in the right or left hemispace using either their right or left hand and upon stimulus onset pushed the bar either leftward or rightward and maintained a given force for 10 sec. The frequency of hypokinesia and impersistence was higher in the RHS group than in the LHS group, but there were no hypokinetic directional or spatial asymmetries. The RHS group did demonstrate left contralesional directional impersistence, but there were no spatial asymmetries of impersistence. Signs of hemispatial neglect were not associated with these measures of MID. Participants with frontal or subcortical lesions were significantly more likely to demonstrate hypokinesia and impersistence than those with posterior lesions.

Prism adaptation theory in unilateral neglect: motor and perceptual components

Prism adaptation theory in unilateral neglect: motor and perceptual components

Motor Neglect and Future Directions for Research

Motor Neglect and Future Directions for Research

A note on Striemer and Danckert's theory of prism adaptation in unilateral neglect

The nature of the therapeutic effects of prism adaptation (PA; see Striemer and Danckert, 2010a for a description) is a major point of controversy in clinical and neurocognitive psychology (e.g., Saevarsson et al., 2009, 2010). A detailed understanding of these effects could greatly advance the treatment and assessment of unilateral neglect. Many authors have concluded that perceptual aspects of neglect, such as visual perception and higher-order visuospatial cognition, improve following PA (e.g., Pisella et al., 2006; Redding and Wallace, 2006; Serino et al., 2006; Saevarsson et al., 2009, 2011; Nijboer et al., 2010, 2011). In contrast to this mainstream view, Striemer and Danckert (2010a) recently proposed that PA produces beneficial effects on spatial and premotor neglect (PMN; defined as the bias of movement from the ipsilesional to the contralesional side, most commonly reported for hand movements; Watson et al., 1978; Bisiach et al., 1990). For example, patients may have difficulties with initiating contralesional directional movements (directional hypokinesia; see Saevarsson, in press for an overview). Striemer and Danckert (2010b) argued that PA has little or no effect on perceptual biases and that only a few PA studies do, in fact, address perceptual neglect directly. Those that do, they claim, show only limited improvement following PA, especially in tasks requiring more explicit perceptual judgments than standard clinical neglect tests. Striemer and Danckert (2010a) also support their view by referring to plasticity changes in areas in the dorsal visual stream that are sometimes spared in neglect, such as the superior parietal lobule and the intraparietal sulcus. These areas are responsible for visuomotor behavior and attention, but are not involved in “more explicit perceptual judgments” (Striemer and Danckert, 2010a, p. 308), and are believed to play a major role in PA. In line with this, they also refer to studies where PA has been found to improve pointing and eye movements (Dijkerman et al., 2003; Ferber et al., 2003; Angeli et al., 2004; Serino et al., 2006).

Contralesional cross-over in chronic neglect: Visual search patterns reveal neglect of the ipsilesional hemispace

With neuropsychological rehabilitation approaches neglect patients can learn to compensate for the reduced awareness of the contralesional hemispace that is often observed after right brain damage. Here, we report contralesional "cross-over" deviations in line bisection that are hypothesized to be a result of focusing on the contralesional hemispace while the intact hemispace is "neglected". We investigate whether this unexpected pattern of deviation is related to defects in the visual field, motor intention/hypokinesia deficits or deficits in working memory. Neglect patients with and without homonymous field defects were screened for contralesional cross-over deviations in line bisection of long lines. During line bisection eye movements were recorded in two conditions with and without requiring hand movements in order to search for directional hypokinesia. Visual fields were tested with near-threshold perimetry and with supra-threshold campimetry. Of 53 chronic neglect patients only 8 showed cross-over in line bisection. Evidence for directional hypokinesia was found in only one patient. Patients with cross-over focused more often to the left than to the right of the objective line midpoint. Patients with and without visual field defects did not differ in the extent of cross-over deviations. Cross-over deviation and inconsistent stimulus detection in left hemispace were correlated irrespective of the presence of a visual field deficit. Larger cross-over deviations were associated with poorer verbal working memory span, and disorganized patterns of eye movement were related to reduced visuo-spatial working memory capacity. Increasing awareness of the disorder and the use of compensatory strategies may have led to a cross-over shift of visual search dominance towards the neglected side resulting in an exploration deficit of the ipsilesional side.

Understanding the parietal lobe syndrome from a neurophysiological and evolutionary perspective.

In human and nonhuman primates parietal cortex is formed by a multiplicity of areas. For those of the superior parietal lobule (SPL) there exists a certain homology between man and macaques. As a consequence, optic ataxia, a disturbed visual control of hand reaching, has similar features in man and monkeys. Establishing such correspondence has proven difficult for the areas of the inferior parietal lobule (IPL). This difficulty depends on many factors. First, no physiological information is available in man on the dynamic properties of cells in the IPL. Second, the number of IPL areas identified in the monkey is paradoxically higher than that so far described in man, although this issue will probably be reconsidered in future years, thanks to comparative imaging studies. Third, the consequences of parietal lesions in monkeys do not always match those observed in humans. This is another paradox if one considers that, in certain cases, the functional properties of neurons in the monkey's IPL would predict the presence of behavioral skills, such as construction capacity, that however do not seem to emerge in the wild. Therefore, constructional apraxia, which is well characterized in man, has never been described in monkeys and apes. Finally, only certain aspects, i.e. hand directional hypokinesia and gaze apraxia (Balint's psychic paralysis of gaze), of the multifaceted syndrome hemispatial neglect have been described in monkeys. These similarities, differences and paradoxes, among many others, make the study of the evolution and function of parietal cortex a challenging case.

Are drawing perseverations part of the neglect syndrome?

Unilateral neglect patients typically omit to cancel contralesional targets. Moreover, they can repeatedly cancel ipsilesional stimuli exhibiting what is termed ‘perseverative behavior’. Two alternative accounts of this behavior have been proposed. According to one of them, it is considered as integral to neglect and due either to a perceptual (allochiria), or a premotor (directional hypokinesia) pathological mechanism leading to the ipsilesional displacement of contralesional responses. According to the other one, perseverations are interpreted as the consequence of motor–control–disinhibition co-occurring with, although independent of, spatial neglect. We compared some crucial predictions of these two hypotheses on a group of 10 right-brain-damaged patients, eight with neglect and two without neglect, showing a perseverative behavior in both conventional and experimental cancellation tasks. In our experiment, the spatial location and the numerosity of targets were manipulated to obtain different degrees of horizontal alignment between targets on the left and on the right of the central vertical axis of the sheet. We found that ipsilesional perseverations were not influenced by left neglected targets and were not correlated to neglect severity. Additionally, perseverative errors were associated with right basal ganglia lesions rather than with presence of neglect. These findings support the view that two different pathological mechanisms might be involved in left spatial neglect and ipsilesional perseverative behavior.

Productive and Optic Prism Exposureproductive and Defective Impairments in the Neglect Syndrome: Graphic Perseveration, Drawing Productions and Optic Prism Exposure

The effects of adaptation to prisms displacing rightwards the field of vision on omission errors, and on perseveration and other graphic productions in a line cancellation task, were assessed in nine right-brain-damaged patients with left unilateral spatial neglect. Prism adaptation improved both neglect, as indexed by omission errors, and perseveration behaviour, up to a delay of 60 min. No correlation was found between omission and perseveration errors in all assessments. The suggestion is made that perseveration and other complex graphic productions made by right brain-damaged-patients with left spatial unilateral neglect is due to a defective monitoring of complex motor behaviour, frequently associated to cerebral damage involving the right frontal lobe. Interpretations of perseveration behaviour in terms of allochiria and directional hypokinesia are considered, and their limits discussed.

The cortical network for eye–hand coordination and its relevance to understanding motor disorders of parietal patients

Cortical neurons in both superior (SPL) and inferior (IPL) parietal lobules are modulated by a variety of signals concerning planning and execution of eye and hand movement. Thanks to these properties, parietal neurons are ideally suited for eye–hand coordination during reaching. In SPL, a fundamental feature of neurons is the invariance of their directional tuning properties across tasks that require different forms of spatial relationships between the eye and the hand. In such conditions, the orientation of the preferred directions (PDs) of individual SPL cells cluster within a limited sector of space, the global tuning field ( GTF), to be regarded as an ideal frame to dynamically match eye and hand signals on the basis of the orientation of their PDs. At the population level, the mean vectors of the GTF cover the direction continuum in a uniform fashion. These neurons are part of a parietal network richly interconnected with the premotor and motor areas of the frontal lobe. Thus, the reaching disorders of patients with optic ataxia might be interpreted as a consequence of the breakdown of the combinatorial mechanisms of the GTF of parietal neurons, and of their interplay with premotor cortex. In IPL, the main feature of eye and/or hand related neurons is the uneven distribution of their PDs, that mostly point toward the contralateral space. This anisotropy of the representation of directional motor space might explain the movement disorders that characterize directional hypokinesia in neglect patients. In conclusion, the study of the dynamic properties of parietal neurons and of their relationships with the premotor cortex via cortico-cortical connections provides a basis for an interpretation of movement disorders of parietal patients from a neurophysiological perspective.

Neglect of memory after dopaminergic lesions in monkeys

Crossed unilateral dopaminergic lesions of the nigrostriatal bundle and unilateral inferotemporal cortex ablations (DA × IT lesions) in marmoset monkeys produced impaired retention of object discriminations first learnt before, or after, the DA lesion but no impairment on new learning of the same type of task. Retention testing of a pre-operatively learned task was given after new learning of a different task so impairment cannot be attributed to improvement with practice or spontaneous recovery. We argue that the DA lesion produces a form of intentional neglect, a defect of volition, which is the mnemonic counterpart of the volitional neglect of directional hypokinesia, which animals with this lesion also exhibit. The DA lesion was unilateral (for welfare reasons) so the information to be retrieved had to be confined to that hemisphere by the use of an IT ablation in the other hemisphere. Unilateral DA lesion compromises the competence of ipsilateral fronto-striatal interactions and our results parallel those found in monkeys with crossed IT × frontal lesions that are impaired on complex tasks requiring effortful implementation of a cognitive strategy but are not impaired on discrimination learning. Parkinsonian patients with sub-total but bilateral DA loss may lack ‘top-down’ conative mechanisms as well as ‘top-down’ movement initiation mechanisms. They may fail to initiate retrieval strategies, although they may not exhibit retrograde amnesia under test conditions that provoke retrieval. Failure to self-initiate retrieval of relevant knowledge may contribute to the paucity of cognitive style and loss of executive skills exhibited by some patients with Parkinson's disease.

Canceling out both the real and the spectral lines

Neglect patients typically show motor perseveration while canceling targets on the ipsilesional side. This behavior can be influenced by the presence vs. absence of targets on the (neglected) contralesional side ( Bottini & Toraldo, 2003). As alternative explanations, the authors proposed (i) directional hypokinesia—the patient cannot perform reaching movements towards detected left-sided targets, and thus carries on canceling on the right side, and (ii) allochiria—the patient misperceives left-sided targets as located on the right side, and cancels them there. We report here data from a patient (EZ) that might confirm the second hypothesis. EZ was presented with 19 displays in which the number and position of cancellation targets on both sides were varied systematically. EZ showed motor perseveration while canceling, but this tendency did not vary across conditions. Interestingly though, EZ also drew cancellation marks in the empty space between the ipsilesional targets, and this phenomenon was significantly more intense when there were more targets on the neglected side. As EZ’s comments suggested, such a behavior might reflect the attempt to cancel out delusional targets. Our speculation is that those objects were generated by allochiria.

The over-representation of contralateral space in parietal cortex: a positive image of directional motor components of neglect?

The activity of single cells was recorded in behaving monkeys while they performed several eye-hand directional motor tasks. The results revealed that in parietal area 7a there exists a directional representation of eye and hand motor space that, contrary to that of superior parietal, premotor and motor cortex, is highly skewed toward the contralateral workspace. In man, the loss of this representation after parietal lesions might explain the emergence of the directional movement disorders of neglect. In fact, although unilateral neglect is consequence of damage to different brain structures, it is more common and enduring after right inferior parietal cortex lesions. Neglect patients ignore and avoid interacting with events occurring in the contralesional part of their physical and mental space. Current theories distinguish perceptual from motor components of neglect. One key feature of the latter is directional hypokinesia, an impaired representation of space for action, evident as difficulty to plan hand movements toward the contralesional part of egocentric space. An impairment of a similar nature is also observed for eye movements. In this study, we offer an interpretation of directional movement disorders of neglect from a physiological perspective, i.e. by focusing on the mechanisms underlying the representation of visuomotor space in parietal cortex.

Brain Responses to Detection of Right or Left Somatic Targets are Symmetrical in Unilateral Parkinson's Disease: A Case Against the Concept of ‘Parkinsonian Neglect’

Signs of attentional dysfunction mimicking spatial neglect have been described both in humans with lateralised Parkinson's Disease (PD) and in animals with MPTP-related hemiparkinsonism. Such deficits have been attributed to dopamine loss in basal ganglia and cortical targets. However, in previous studies the existence of neglect was assumed from behavioural tests which needed a motor output, thus entailing interpretation ambiguities due to effects of directional hypokinesia. We recorded brain event-related potentials (ERPs) evoked by the presentation of target somatic stimuli to the affected and non-affected sides in 44 patients with unilateral or asymmetrical PD. The N2 and P3 ERP components were specifically analysed, since (a) they are triggered selectively by task-relevant, attended sensory stimuli; (b) their latency reflects stimulus evaluation time, independently from the execution of a motor response, and (c) they have proved to be abnormal in hemineglect syndromes due to focal brain lesions. Irrespective of the side (left or right) of motor symptom predominance there were no significant ERP differences to stimulation of the affected and non-affected limbs, nor was there any correlation between ERP latencies and the degree of dopaminerelated motor impairment. The P3 latency was abnormally delayed in 23% of the patients, but there was no trend for abnormalities to concentrate on the affected side. This study does not confirm the existence of a significant attentional impairment toward the affected limb in lateralised PD, and suggests that previous clinical evidence of ‘neglect’ behaviour in PD might be linked to directional hypokinesia, thus reflecting intentional, rather than attentional lateralised deficits.

Detecting directional hypokinesia: The epidiascope technique

A new technique to detect directional hypokinesia is proposed. Subjects with unilateral neglect were administered cancellation tests with an epidiascope used to dissociate visual input from motor output. Two different cancellation tasks were performed in order to compare this technique with the method devised by Tegnér and Levander [ Brain 114, 1943–1951, 1991]. No difference in detecting hypokinetic deficits was observed; however, our method was easier to perform and more suitable for clinical testing.

Clock-drawing in a case of left visuo-spatial neglect: A deficit of disengagement?

A patient (C.B.) with severe left visuo-spatial neglect and hemianopia after right hemisphere stroke was tested on three conditions of a clock-drawing task. Either on spontaneous drawing or when the number sequence was provided by the experimenter, C.B. drew a clockface with left-sided numbers transposed to the right side of the dial. By contrast, when each number was drawn on a separate dial, its location was correct and there was no transposition. Directional hypokinesia or a representational deficit cannot explain these effects. It is proposed that a deficit in disengaging attention from right-sided visual stimuli plays a critical role in some forms of number transpositions.