Rightward Errors Research Articles

Evoking visual neglect-like deficits in healthy volunteers - an investigation by repetitive navigated transcranial magnetic stimulation.

In clinical practice, repetitive navigated transcranial magnetic stimulation (rTMS) is of particular interest for non-invasive mapping of cortical language areas. Yet, rTMS studies try to detect further cortical functions. Damage to the underlying network of visuospatial attention function can result in visual neglect-a severe neurological deficit and influencing factor for a significantly reduced functional outcome. This investigation aims to evaluate the use of rTMS for evoking visual neglect in healthy volunteers and the potential of specifically locating cortical areas that can be assigned for the function of visuospatial attention. Ten healthy, right-handed subjects underwent rTMS visual neglect mapping. Repetitive trains of 5Hz and 10 pulses were applied to 52 pre-defined cortical spots on each hemisphere; each cortical spot was stimulated 10 times. Visuospatial attention was tested time-locked to rTMS pulses by a landmark task. Task pictures were displayed tachistoscopically for 50ms. The subjects' performance was analyzed by video, and errors were referenced to cortical spots. We observed visual neglect-like deficits during the stimulation of both hemispheres. Errors were categorized into leftward, rightward, and no response errors. Rightward errors occurred significantly more often during stimulation of the right hemisphere than during stimulation of the left hemisphere (mean rightward error rate (ER) 1.6±1.3% vs. 1.0±1.0%, p=0.0141). Within the left hemisphere, we observed predominantly leftward errors rather than rightward errors (mean leftward ER 2.0±1.3% vs. rightward ER 1.0±1.0%; p=0.0005). Visual neglect can be elicited non-invasively by rTMS, and cortical areas eloquent for visuospatial attention can be detected. Yet, the correlation of this approach with clinical findings has to be shown in upcoming steps.

Learning from the value of your mistakes: evidence for a risk-sensitive process in movement adaptation

Risk frames nearly every decision we make. Yet, remarkably little is known about whether risk influences how we learn new movements. Risk-sensitivity can emerge when there is a distortion between the absolute magnitude (actual value) and how much an individual values (subjective value) a given outcome. In movement, this translates to the difference between a given movement error and its consequences. Surprisingly, how movement learning can be influenced by the consequences associated with an error is not well-understood. It is traditionally assumed that all errors are created equal, i.e., that adaptation is proportional to an error experienced. However, not all movement errors of a given magnitude have the same subjective value. Here we examined whether the subjective value of error influenced how participants adapted their control from movement to movement. Seated human participants grasped the handle of a force-generating robotic arm and made horizontal reaching movements in two novel dynamic environments that penalized errors of the same magnitude differently, changing the subjective value of the errors. We expected that adaptation in response to errors of the same magnitude would differ between these environments. In the first environment, Stable, errors were not penalized. In the second environment, Unstable, rightward errors were penalized with the threat of unstable, cliff-like forces. We found that adaptation indeed differed. Specifically, in the Unstable environment, we observed reduced adaptation to leftward errors, an appropriate strategy that reduced the chance of a penalizing rightward error. These results demonstrate that adaptation is influenced by the subjective value of error, rather than solely the magnitude of error, and therefore is risk-sensitive. In other words, we may not simply learn from our mistakes, we may also learn from the value of our mistakes.

Near and far space neglect: Task sensitivity and anatomical substrates

Most group studies which have investigated neglect for near and far space have found an increased severity of symptoms in far space compared to near space. However, the majority of these studies used relatively small samples and based their findings almost exclusively on line bisection performance.The aim of the present study was, therefore, to explore the occurrence of neglect for near and far space in a larger group of unselected right brain damaged patients and to evaluate whether neglect specific to near and far space is a task-related deficit or generalises across distance irrespective of task. In addition, a lesion overlap analysis was carried out to identify critical lesion sites associated with distance specific neglect deficits.Thirty-eight right hemisphere damaged patients carried out a line bisection and a cancellation task by using a pen in near space (40cm) and a laser pointer in far space (320cm).The results showed that both the number of left-sided omissions and rightward bisection errors were significantly increased in near compared to far space. Distance specific dissociations, albeit less common, were more frequently observed for cancellation than line bisection.These results suggest that space representation in neglect is more severely impaired in near than in far space. In addition, distance related dissociations in neglect may depend on task demands. Although the anatomical findings were broadly consistent with a dorsal and ventral stream dichotomy for near and far space processing, they also suggest the involvement of intermediate structures in distance related neglect phenomena.

No transfer of calibration between action and perception in learning a golf putting task

We assessed calibration of perception and action in the context of a golf putting task. Previous research has shown that right-handed novice golfers make rightward errors both in the perception of the perfect aiming line from the ball to the hole and in the putting action. Right-handed experts, however, produce accurate putting actions but tend to make leftward errors in perception. In two experiments, we examined whether these skill-related differences in directional error reflect transfer of calibration from action to perception. In the main experiment, three groups of right-handed novice participants followed a pretest, practice, posttest, retention test design. During the tests, directional error for the putting action and the perception of the perfect aiming line were determined. During practice, participants were provided only with verbal outcome feedback about directional error; one group trained perception and the second trained action, whereas the third group did not practice. Practice led to a relatively permanent annihilation of directional error, but these improvements in accuracy were specific to the trained task. Hence, no transfer of calibration occurred between perception and action. The findings are discussed within the two-visual-system model for perception and action, and implications for perceptual learning in action are raised.

The role of the cerebellum in ‘real’ and ‘imaginary’ line bisection explored with 1-Hz repetitive transcranial magnetic stimulation

The role of the cerebellum is well characterized for many motor processes and for some cognitive tasks, although its contribution to lateralized spatial judgement has never been probed directly. To address this omission, we investigated the effects of cerebellar disruption on two different line bisection tasks in eight healthy subjects. Based on previous evidence of crossed cerebellar-cortical connections we predicted a shift in the perceived midline that would occur in opposite directions depending on the cerebellar hemisphere targeted. Repetitive transcranial magnetic stimulation (rTMS), given at 1-Hz (600 pulses), was used as a non-invasive way to interfere with processing in the cerebellar cortex. Performance was assessed for both 'physical' line bisection using a newly developed Landmark variant task and for 'mental' line bisection using number pairs. The effects for number line bisection were lateralized--left but not right cerebellar rTMS increased rightward errors, whereas for physical line bisection rTMS to either hemisphere did not affect performance. Effects due to neck muscle contraction and changes in eye position were ruled out with appropriate control stimulation sites, and eye-tracking. The results confirm the role of the cerebellum in spatial judgement, and, for the first time, demonstrate direct cerebellar involvement in the generation of the midline in 'imaginal' (number) space. The difference between number line and physical line bisection effects is discussed with reference to pre-existing models of cerebellar hemispheric specialization and functional topography.

Spatial Bias and Right Hemisphere Function: Sex-Specific Changes with Aging

Patterns of cerebral asymmetry related to visuospatial functions may change with age. The typical leftward bias on a line bisection task may reflect cerebral asymmetry. With age, such leftward bias decreases. This study demonstrated that the age-related decrease of leftward bias may actually be sex-specific. In addition, previous research suggests that young adults' deviation in line bisection may reflect asymmetric hemispheric activation of perceptual-attentional "where" spatial systems, rather than motor-intentional "aiming" spatial systems; thus, we specifically fractionated "where" and "aiming" bias of men and women ranging in age from 22 to 93 years old. We observed that older men produced greater rightward line bisection errors, of primarily "where" spatial character. However, women's errors remained leftward biased, and did not significantly change with age. "Where" spatial systems may be linked to cortico-cortical processing networks involving the posterior part of the dorsal visuospatial processing stream. Thus, the current results are consistent with the conclusion that reduced right dorsal spatial activity in aging may occur in the male, but not female, adult spatial system development.

Repetita iuvant: Object-centered neglect with non-verbal visual stimuli induced by repetition

Neglect can be ego-centered or object-centered depending on the reference frames for “left” and “right”, of either the body or an object. It has been suggested that object-centered neglect is not a general phenomenon but is limited to words because only they have a true canonical representation. Here, we examined whether object-centered neglect could be observed for non-verbal material by creating, after repeated exposure, a canonical representation of a nonsense figure. Fourteen neglect patients repeatedly bisected a series of asymmetrical nonsense drawings containing two different shapes at their right and left end-points (canonical trials). In the critical trials, which were the last three in the series, the position of the two shapes was mirror-reversed. Afterwards, neglect patients were asked to draw the stimulus, which provided a further measure of whether a canonical representation of the object has been built by the patients. All the patients made rightward errors with the canonical stimuli. With mirror-reversed stimuli, the bisection errors were reversed to the contralesional side in one patient, returned to zero in one patient and significantly decreased in three patients. In addition, 10 patients reliably drew the canonical stimulus at the end of the series of trials, providing an indication that they built up a canonical representation of the stimulus. The present data provide evidence that object-centered neglect is a phenomenon that is not limited to words. The nature of a stimulus, verbal or non-verbal, is not critical for observing object-centered neglect. What is critical is the way in which material is represented by the patients.

Visual bisection of freely-viewed asymmetrical stimuli

When performing line bisection tasks, neurologically normal individuals exhibit leftward biases. Similarly, normals also exhibit leftward biases during judgements of brightness, numerosity, and size. However, when these same judgement stimuli are visually bisected, this results in rightward bisection biases. These apparently contradictory results are complicated by the fact that bisection tasks are typically performed using single stimuli, whereas judgements of brightness, numerosity, and size are performed on pairs of stimuli. The present study examined the effects of visual bisection on pairs of stimuli. A sample of 34 undergraduate psychology volunteers exhibited leftward biases when making judgements of brightness, numerosity, and size. However, these same participants did not exhibit leftward bisection biases with the same stimuli. Instead, overall rightward bisection errors were observed. These results indicate that although these two types of tasks elicit similar perceptual biases, they are probably the result of different perceptual mechanisms.

Rightward and leftward bisection biases in spatial neglect: two sides of the same coin?

Neglect patients, when asked to bisect a horizontal line, typically show large rightward errors with long lines and a decreased error with medium length lines. With very short lines the bisection bias reverses from the right to left side of the line physical centre (the so-called crossover effect). It is commonly pointed out that such a leftward bias is difficult to explain by traditional theories of neglect. Several accounts propose two distinct mechanisms, one that works for short lines and one that works for long. In the present study we demonstrated that the crossover effect can be explained by means of a unitary mechanism that derives from the space anisometry hypothesis. This hypothesis postulates that in neglect patients representational space is progressively 'relaxed' contralesionally and progressively 'compressed' ipsilesionally. In a series of five experiments, we investigated the crossover effect in 26 right-brain damaged patients: 17 with neglect without hemianopia, 4 with neglect and hemianopia and 6 without neglect or hemianopia. Patients were to bisect or extend lines of objectively and subjectively different lengths. The modulation of subjective length was created by an Oppel-Kundt illusion that is thought to resemble the distortion of representational space that occurs with neglect. All groups, except for the patients with neglect and hemianopia, were prone to the illusion. The rightward bias was reduced when the illusion induced a perceptual distortion opposite to that thought to underlie neglect. Importantly, the strength of the illusion decreased with reducing the physical line length and reversed with very short lines. These results argue for a simple and unitary explanation of the crossover effect in spatial neglect within the framework of the space anisometry hypothesis.

Visual attention, emotion, and action tendency: Feeling active or passive

Several visual and emotional processes reflect similar underlying patterns of cortical activation. Characteristic individual perceptual style was measured by lateral attentional errors in a standard visual line-bisecting task. The direction of error indicates a predominance of activation in the contralateral prefrontal cortex. Individual differences in mood were measured by the self-endorsement of emotional adjectives. A total of 27 right-handed adults responded to the trait version of the Positive and Negative Affect Schedule (PANAS). As predicted, rightward errors in visual line bisecting correlated positively and significantly with the Positive Affect subscale. The scale adjectives determined and strong had significant correlations with the visual bisecting measure. The negative affect subscale was correlated with neither the line bisecting measure nor the positive subscale. An extension from trait to state measures tested the self-description of current affect using related synonyms and antonyms in a subset of 22 of the participants. Active, dominant, mighty, powerful, and strong were significantly associated with rightward bisecting errors, whereas passive and cooperative were correlated with errors in the opposite direction. The results were incompatible with the valence model (positive-negative), direction model (approach-withdrawal), and movement model (approach-resistance) of the lateral prefrontal substrate of emotion. The action tendency model (active approach-passive yielding) was supported.

Understanding the effects of moving visual stimuli on unilateral neglect following stroke

Moving visual stimuli have been shown to reduce unilateral neglect (ULN), however, the mechanisms underlying these effects remain poorly understood. This study compared lateralised and non-lateralised moving visual stimuli to investigate whether the spatial characteristics or general alerting properties of moving visual stimuli are responsible for reducing neglect. Post-stroke left neglect patients as well as healthy and patient control subjects were tested on a computerised line bisection task under six visual stimulus conditions. The key finding was that, relative to the no stimulus condition, leftward moving and left-sided moving visual stimuli shifted neglect patients’ bisection errors leftward while the non-lateralised random moving visual stimuli did not reduce neglect patients’ rightward bisection errors. The results provide evidence that spatial characteristics rather than general alerting properties of moving visual stimuli reduce rightward bisection errors in ULN. Moreover, the pattern of findings strongly supports the notion that moving visual stimuli reduce neglect by capturing attention and drawing it to a spatial location rather than by activating the attentional system via superior collicular neurons.

Approaches to Subjective Midpoint of Horizontal Lines in Unilateral Spatial Neglect

Patients with unilateral spatial neglect usually bisect longer lines with greater rightward errors, while they sometimes err leftward for very short lines (e.g., 25 mm). We analysed movements of eye fixation from the time before line presentation to elucidate whether patients with neglect approach the subjective midpoint differently for lines of various lengths. Four patients with left neglect bisected 200 mm, 100 mm, and 25 mm lines that appeared across the centre of a liquid crystal display (LCD) monitor. The fixation immediately before line presentation was located on average near the centre of the lines. Three of the patients approached the subjective midpoint point directly from the left side in more than 70% of the 200 mm and 100 mm trials. The subjective midpoint frequently deviated leftward on the “attended” segment between the leftmost point of fixation and the right endpoint, while it was displaced rightward on the total extent. The three patients initially explored the 25 mm lines searching for the left endpoint. They thereafter bisected the same lines with leftward errors approaching the subjective midpoint from the left side. The remaining patient searched beyond the right endpoint and in turn approached the subjective midpoint from the right side in about half of the trials and independently of line length. In the 200 mm and 100 mm trials, the subjective midpoint divided the attended right segment nearer to the right endpoint. On the attended right extent of a line, patients with neglect may place the subjective midpoint toward the side from which they approached that point. In the bisection of very short lines, approaches from the left endpoint may cause leftward errors of the subjective midpoint. For longer lines, however, approaches from the left side may result in rightward error of bisection for the total length, as the leftward extent from the fixation immediately before line presentation is hardly explored.

Rightward biases in free-viewing visual bisection tasks: Implications for leftward responses biases on similar tasks

Neurologically normal individuals exhibit strong leftward response biases during free-viewing perceptual judgments of brightness, quantity, and size. When participants view two mirror-reversed objects and they are forced to choose which object appears darker, more numerous, or larger, the stimulus with the relevant feature on the left side is chosen 60–75% of the time. This effect could be influenced by inaccurate judgments of the true centre-point of the objects being compared. In order to test this possibility, 10 participants completed three visual bisection tasks on stimuli known to elicit strong leftward response biases. Participants were monitored using a remote eye-tracking device and instructed to stare at the subjective midpoint of objects presented on a computer screen. Although it was predicted that bisection errors would deviate to the left of centre (as is the case in the line bisection literature), the opposite effect was found. Significant rightward bisection errors were evident on two of the three tasks, and the leftward biases seen during forced-choice tasks could be the result of misjudgments to the right of centre on these same tasks.

Weights and measures: A new look at bisection behaviour in neglect

Horizontal line bisection is a ubiquitous task in the investigation of visual neglect. Patients with left neglect typically make rightward errors that increase with line length and for lines at more leftward positions. For short lines, or for lines presented in right space, these errors may ‘cross over’ to become leftward. We have taken a new approach to these phenomena by employing a different set of dependent and independent variables for their description. Rather than recording bisection error, we record the lateral position of the response within the workspace. We have studied how this varies when the locations of the left and right endpoints are manipulated independently. Across 30 patients with left neglect, we have observed a characteristic asymmetry between the ‘weightings’ accorded to the two endpoints, such that responses are less affected by changes in the location of the left endpoint than by changes in the location of the right. We show that a simple endpoint weightings analysis accounts readily for the effects of line length and spatial position, including cross-over effects, and leads to an index of neglect that is more sensitive than the standard measure. We argue that this novel approach is more parsimonious than the standard model and yields fresh insights into the nature of neglect impairment.

Visual line bisection in sinistrals and dextrals as a function of hemispace, hand, and scan direction

Visual line bisection was investigated in 26 sinistral and 24 dextral subjects as a function of hemispace, hand and scan direction. An ANOVA revealed significant main effects for hand preference, due to the mean bisection errors of dextral subjects being significantly leftward of those of sinistral subjects; for hand, due to the bisection errors of the left hand being significantly to the left of the right hand; and for scan, due to the bisection errors following a left scan being significantly to the left of a right scan. One significant interaction was found, that between hand and direction of scan, due to a significant difference between left and right hands following a scan from the left but not following a scan from the right. For dextral subjects the leftward bisection errors of the left and right hands following a scan from the left, but not for a scan from the right, differed significantly from the midpoint. For sinistral subjects the leftward bisection errors following a scan from the left and rightward bisection errors following a scan from the right differed significantly from the midpoint for the left hand but not for the right hand. No significant main effect or interactions for hemispace were found. This confirms that both sinistral and dextral subjects display pseudoneglect when using their preferred hand and scanning from the left. However, sinistrals, but not dextrals, will display reversed pseudoneglect when using their preferred hand and adopting a scan direction from the right. These results are discussed in terms of the interaction between three factors, whose influence can jointly and severally produce misbisections, hemispheric specialisation for visuospatial function, hemispheric activation for a manual response, and the allocation of visual attention.

Line-bisecting performance in highly skilled athletes: Does preponderance of rightward error reflect unique cortical organization and functioning?

A line-bisecting test was administered to 250 highly skilled right-handed athletes and a control group of 60 right-handed age matched non-athletes. Results revealed that athletes made overwhelmingly more rightward errors than non-athletes, who predominantly bisected lines to the left of the veridical center. These findings were interpreted in the context of previous EEG research on athletes and brain localization studies of select personality traits. A preliminary theory (Group Cortical Organization and Activation Theory) that highly skilled athletes and other specific homogeneous populations may have developed a unique cortical organization or response system that mediates relative tendencies in cerebral activation was advanced. The potential utility of the line-bisecting test as an assessment and intervention tool in sports was discussed.

Conflict and integration of spatial attention between disconnected hemispheres

OBJECTIVESTo clarify how the disconnected hemispheres perceive a line and bisect it with successful or unsuccessful integration of spatial attention.METHODSEye movements were recorded when a patient with an extensive callosa...

Significance of Paradoxical Leftward Error of Line Bisection in Left Unilateral Spatial Neglect

We intended to determine if leftward error of line bisection means “right” or “left” neglect in a patient with left unilateral spatial neglect. The patient placed the subjective midpoint to the left of the true center when bisecting lines without cueing. By contrast, when cued to the left endpoint, he showed typical rightward errors that became greater as longer lines were presented. Cueing to the right endpoint increased leftward errors compared with the bisections without cueing. The results suggest that paradoxical leftward error of line bisection is a form of “left” unilateral spatial neglect in that the shortness of the left extent is ignored.

Performance consistency of normal observers in forced-choice tachistoscopic visual line bisection

Pseudoneglect (PN) refers to the leftward error exhibited by normal observers on line bisection tasks (Bowers and Heilman, Neuropsychologia, 18, (1980) 491–8). Although a thorough review of the literature has shown PN to be relatively robust (Jewell and McCourt, Neuropsychologia, 38, (2000) 93–110), controversy remains concerning the reliability of the phenomenon, with some studies reporting a relatively high incidence of normal subjects with rightward bisection errors. The present experiment assesses the consistency of bisection performance in normal young observers. Right-handed subjects ( N=22) participated in a tachistoscopic forced-choice line bisection task. Each subject participated in 7–16 experimental sessions separated by at least 24 h (total bisection measurements=317). Individual bisection performance could thus be evaluated with respect to within-subject variability measures. An eyetracker recorded gaze position during the task in one session. A highly significant mean group bisection error of −0.26° ( P<0.001) was obtained (left negative), and individual subject means ranged from −0.55° to +0.03°. Of the 317 total bisection measurements, 9% (28) deviated rightward. Significant ( P<0.05) mean leftward errors occurred in 91% (20/22) of subjects. Mean bisection error in two subjects was not significantly different from zero. No subject possessed a significant rightward error. Mean gaze deviation from screen (and line) center ranged from ±0.9°, and was positively correlated ( P<0.05) with bisection error. It is concluded that forced-choice tachistoscopic line bisection measures are highly reliable; a mean correlation of +0.87 exists between mean error based on 15 trials and means estimated from a random sample of only two trials. The incidence of true rightward bisection error in the population of normal right-handed subjects is thus estimated to be less than 5%.

Line versus representational bisections in unilateral spatial neglect

OBJECTIVESTo clarify the mechanisms of left unilateral spatial neglect found in the bisection of lines after cueing to the left end point and to determine whether neglect occurs for the...