Bisection Bias Research Articles

Representational neglect for words as revealed by bisection tasks

In the present study, we showed that a representational disorder for words can dissociate from both representational neglect for objects and neglect dyslexia. This study involved 14 brain-damaged patients with left unilateral spatial neglect and a group of normal subjects. Patients were divided into four groups based on presence of left neglect dyslexia and representational neglect for non-verbal material, as evaluated by the Clock Drawing test. The patients were presented with bisection tasks for words and lines. The word bisection tasks (with words of five and seven letters) comprised the following: (1) representational bisection: the experimenter pronounced a word and then asked the patient to name the letter in the middle position; (2) visual bisection: same as (1) with stimuli presented visually; and (3) motor bisection: the patient was asked to cross out the letter in the middle position. The standard line bisection task was presented using lines of different length. Consistent with the literature, long lines were bisected to the right and short lines, rendered comparable in length to the words of the word bisection test, deviated to the left (crossover effect). Both patients and controls showed the same leftward bias on words in the visual and motor bisection conditions. A significant difference emerged between the groups only in the case of the representational bisection task, whereas the group exhibiting neglect dyslexia associated with representational neglect for objects showed a significant rightward bias, while the other three patient groups and the controls showed a leftward bisection bias. Neither the presence of neglect alone nor the presence of visual neglect dyslexia was sufficient to produce a specific disorder in mental imagery. These results demonstrate a specific representational neglect for words independent of both representational neglect and neglect dyslexia.

Numerosities and space; indeed a cognitive illusion! A reply to de Hevia and Spelke (2009)

de Hevia and Spelke (de Hevia and Spelke (2009). Spontaneous mapping of number and space in adults and young children, Cognition, 110, 198–207) investigated the mapping of number onto space. To this end, they introduced a non-symbolic flanker task. Here subjects have to bisect a line that is flanked by a 2-dot and a 9-dot array. Similar to the symbolic line bisection task, a bias towards the larger numerosity was observed. We re-investigated these results both by creating new flanker stimuli that controlled for different (non-numerical) stimulus properties and by developing a new measurement tool. We demonstrate that the bisection bias was caused by the larger area subtended by the 9-dot array compared to the 2-dot array and not numerosity. Our study puts constraints on the results of the study by de Hevia and Spelke. The role of visual cues in numerosity processing in general is discussed.

Age-related changes in visual pseudoneglect

Pseudoneglect is a slight but consistent leftward attentional bias commonly observed in healthy young populations, purportedly explained by right hemispheric dominance. It has been suggested that normal aging might be associated with a decline of the right hemisphere. According to this hypothesis, a few studies have shown that elderly tend to exhibit a rightward attentional bias in line bisection. In the present study, we tested this hypothesis in young and older participants using a perceptual landmark task. Results yield evidence for an age-related shift, from a strong attentional leftward bias in young adults toward a suppressed or even a reversed bias in the elderly. Right hemisphere impairment coupled to a left hemispheric compensation might explain the perceptual shift observed in older adults. However, a decline in corpus callosum function cannot be excluded. Alternatively, these results may be in agreement with the hypothesis of an age-related specific inhibition of return dysfunction, an overt attentional orienting mechanism, and/or a decrease of dopamine.

Blind individuals show pseudoneglect in bisecting numerical intervals

Neurologically normal individuals typically show a leftward bias--known as pseudoneglect--in bisecting physical lines as well as numerical intervals. The latter bias may reflect the spatial nature in which numbers are represented (i.e., the mental number line). In previous studies, we found that congenitally blind individuals show such leftward bias in haptic bisection. Here, we demonstrate that blind individuals also show a consistent leftward bias in numerical bisection. The leftward bias was greater when numbers were presented in descending rather than ascending order, and the magnitude of the leftward bias was comparable to that shown by a control group of blindfolded sighted participants. Our findings thus support the view that pseudoneglect operates at a mental representational level rather than being perceptually based. Moreover, the consistent leftward bias shown by blind individuals in both line and numerical bisection suggests that the right hemisphere dominance in spatial processing, resulting in an overestimation of the left side of space, develops even in the absence of any visual input.

Do supine position and deprivation of visual environment influence spatial neglect?

It has been suggested that in spatial neglect, placing the patient in a supine position and performing tasks in the dark would reduce the rightward bias in line bisection and cancellation tasks. However, these findings remain debated and have not been extended to other tasks such as reading or visual exploration. Here, in the same study, we examined the effect of body position (BP) and visual environment (VE) on relatively ecological tests of spatial neglect. Among 17 patients with right-hemisphere stroke, 12 were neglect and five were non-neglect in clinical tests. They were compared with 12 healthy control participants in four tasks: line bisection, text reading, number reading, and visual exploration. Tasks were performed on a computer screen in two BP (sitting and supine) and two VE (light and dark) conditions. We found that placing patients in darkness reduced contralesional omissions in the visual exploratory task and, to a smaller extent, in number reading. Conversely, the supine position did not influence performance, and even resulted in cognitive slowing, especially in reading. In conclusion, we confirmed that reducing visual information can improve performance, but only to a limited extent. This justifies strict control of peripheral visual information when exploring neglect patients. Conversely, positioning neglect patients in the supine position can have a discrete negative effect on cognitive functioning, and this effect must be taken into account during therapy.

Static versus dynamic judgments of spatial extent

Research exploring how scanning affects judgments of spatial extent has produced conflicting results. We conducted four experiments on line bisection judgments measuring ocular and pointing behavior, with line length, position, speed, acceleration, and direction of scanning manipulated. Ocular and pointing judgments produced distinct patterns. For static judgments (i.e., no scanning), the eyes were sensitive to position and line length with pointing much less sensitive to these factors. For dynamic judgments (i.e., scanning the line), bisection biases were influenced by the speed of scanning but not acceleration, while both ocular and pointing results varied with scan direction. We suggest that static and dynamic probes of spatial judgments are different. Furthermore, the substantial differences seen between static and dynamic bisection suggest the two invoke different neural processes for computing spatial extent for ocular and pointing judgments.

Radial line bisection biases in the periphery

Radial line bisection biases in the periphery

Asymmetrical Effects of Adaptation to Left- and Right-Shifting Prisms Depends on Pre-existing Attentional Biases

Proposals that adaptation with left-shifting prisms induces neglect-like symptoms in normal individuals rely on a dissociation between the postadaptation performance of individuals trained with left- versus right-shifting prisms (e.g., Colent, Pisella, & Rossetti, 2000). A potential problem with this evidence is that normal young adults have an a priori leftward bias (e.g., Jewell & McCourt, 2000). In Experiment 1, we compared the line bisection performance of young adults to that of aged adults, who as a group may lack a leftward bias in line bisection. Participants trained with both left- and right-shifting prisms. Consistent with our hypothesis, while young adults demonstrated aftereffects for left, but not right prisms, aged adults demonstrated reliable aftereffects for both prisms. In Experiment 2, we recruited a larger sample of young adults, some of whom were right-biased at baseline. We observed an interaction between baseline bias and prism-shift, consistent with the results of Experiment 1: Left-biased individuals showed a reduced aftereffect when training with right-shifting prisms and right-biased individuals showed a reduced aftereffect when training with left-shifting prisms. These results suggest that previous failures to find generalizable aftereffects with right-shifting prisms may be driven by participants' baseline biases rather than specific effects of the prism itself.

Oblique spatial shifts of subjective visual straight ahead orientation in quadrantic visual field defects

Patients with postchiasmatic visual field defects often show a contralesional bias towards the scotoma in line bisection or when indicating their visual subjective straight ahead (VSSA). Recent evidence suggests a retinotopic misrepresentation of visual space in patients with homonymous quadrantanopia (HQ). We therefore assessed in the present study whether patients with HQ show an oblique shift of their VSSA towards their scotoma, in addition to the known bias in horizontal line bisection. Moreover, we examined whether eccentric fixation contributes to this shift. To this purpose, 15 non-neglecting stroke patients with HQ and 15 matched healthy control subjects were assessed in horizontal line bisection and in the horizontal and vertical dimension of their VSSA. Additionally, perimetric blind spot mapping was performed. Eight out of nine patients with left quadranopia showed the typical leftsided, horizontal line bisection error, while only one out of seven patients with rightsided quadranopia showed a rightsided shift. Normal subjects showed a non-significant leftward shift in line bisection (pseudoneglect). All 15 patients with HQ showed a large oblique shift of their VSSA towards the blind quadrants, while normal subjects showed no systematic left-rightward shift, but a small downward shift of the VSSA. The position of the blind spot was normal in all testable eyes of patients and control subjects, thus excluding eccentric fixation or cyclorotation of the eyes. In conclusion, our study reveals a hitherto unreported oblique spatial shift of subjective visual body orientation towards the blind quadrants in non-neglecting patients with quadranopia.

The centre is not in the middle: Evidence from line and word bisection

English and German readers have been shown to mark a position to the left of the true centre as the subjective midpoint in word bisection. This effect resembles a well-known phenomenon observed with the bisection of solid lines (pseudoneglect), although this behavioural similarity does not imply a common origin. The purpose of the present study was twofold: on the one hand, to investigate the perceptual and lexical features that influence the bisection of Italian orthographic strings and, on the other hand, to investigate whether identical or partially independent processing mediate bisection of line and orthographic stimuli. Five experiments were carried out to explore to what extent stimulus type (lines, words, pseudowords, consonant strings, symbols), stimulus length (from 3 to 13 characters), list context (pure and mixed), and written word frequency (high and low) affected the bisection performance. The results showed that list context modulated the processing similarities across different materials and that word frequency failed to influence the magnitude of the bisection bias. More critically, across all five experiments, the results showed different effects for solid lines versus orthographic material. Lines were always bisected to the left, independent of length and list context. By contrast, a crossover effect emerged with orthographic material; for long stimuli (above five letters) the bias was consistently to the left, while short stimuli showed a consistent rightward bias. The results indicate that manual bisection involved partly different cognitive mechanisms during word and line perception and that this may depend on the characteristics of the stimuli (words/discrete vs. lines/continuous).

A simple model of the vertical–horizontal illusion

In spite of being one of the most elementary visual effects, the vertical–horizontal illusion is still poorly understood. We provide here a simple quantitative model to describe the overestimation of the vertical segment relative to the horizontal one. The model also includes a necessary factor related to the effect of bisecting a line on the under-estimation of its length. These two factors, orientation anisotropy and length bisection, provide a very good account of various configurations of the illusion when the stimulus looks like a ‘T’, an ‘L’, or a ‘+’-sign, and for different stimulus orientations. The orientation anisotropy bias is on average 6% while the length bisection bias amounts to about 16%. In addition, when uncertainty about line estimation is included, new predictions on the sensitivity of different configurations are obtained and confirmed. In particular, we find that the ‘+’-sign figure suffers from a loss of sensitivity in comparing their vertical and horizontal segments when compared to the ‘L’-figure. This difference can only be accounted for by a late-noise model where uncertainty is at the decision stage rather than on the image measurements.

Line bisection as a neural marker of approach motivation

Approach motivation has been reliably associated with relative left prefrontal brain activity as measured with electroencephalography (EEG). Motivation researchers have increasingly used the line bisection task, a behavioral measure of relative cerebral asymmetry, as a neural index of approach motivation-related processes. Despite its wide adoption, however, the line bisection task has not been confirmed as a valid measure of the precise pattern of activity linked to approach motivation. In two studies, we demonstrate that line bisection bias is specifically related to baseline, approach-related, prefrontal EEG alpha asymmetry (Study 1) and is heightened by the same situational factors that heighten the same approach-related prefrontal EEG alpha asymmetry (Study 2). Results support the line bisection task as an efficient and unobtrusive behavioral neuroscience measure of approach motivation.

Auditory space perception in left- and right-handers

Several studies have shown that handedness has an impact on visual spatial abilities. Here we investigated the effect of laterality on auditory space perception. Participants (33 right-handers, 20 left-handers) completed two tasks of sound localization. In a dark, anechoic, and sound-proof room, sound stimuli (broadband noise) were presented via 21 loudspeakers mounted horizontally (from 80° on the left to 80° on the right). Participants had to localize the target either by using a swivel hand-pointer or by head-pointing. Individual lateral preferences of eye, ear, hand, and foot were obtained using a questionnaire. With both pointing methods, participants showed a bias in sound localization that was to the side contralateral to the preferred hand, an effect that was unrelated to their overall precision. This partially parallels findings in the visual modality as left-handers typically have a more rightward bias in visual line bisection compared with right-handers. Despite the differences in neural processing of auditory and visual spatial information these findings show similar effects of lateral preference on auditory and visual spatial perception. This suggests that supramodal neural processes are involved in the mechanisms generating laterality in space perception.

Emotion and space: lateralized emotional word detection depends on line bisection bias

There is converging evidence, from various independent areas of neuroscience, for a functional specialization of the left and right cerebral hemispheres for positive and negative emotions, respectively (“valence theory” of emotional processing). One subfield, however, has produced mixed results, i.e. work on the detection of parafoveally presented positively or negatively emotional words by healthy subjects. Right or left visual field advantages were described and interpreted as reflecting the superiority of either the left hemisphere (LH) for linguistic material, or of the right hemisphere (RH) for highly emotional stimuli. Here we show that 48 healthy, right-handed participants' performance on a lateralized lexical decision task depends on their individual inclination to bisect a line to the left or right of the objective center. Only those with a bisection bias to the right showed the LH advantage for word detection known from the neuropsychological literature. Negative emotional words were processed with comparable accuracy in the two visual fields. However, a recognition advantage for negative over positive emotional words was found exclusively for those participants with a leftward line bisection bias. These results suggest that in work on functional hemispheric differences state variables like stimulus lateralization and word emotionality may be less decisive than the trait variable of lateral hemispatial attention. We propose a cautious reconsideration of the concept of “hemisphericity,” which once emphasized individual differences in baseline hemispheric arousal, but was later dismissed in a reaction to oversimplifications in popular science accounts.

Corpus Callosum Maturation and Line Bisection Performance in Healthy Children

Line bisection performance in children has been hypothesized to be a measure of corpus callosum maturation. Several previous studies have shown that normal prepubescent children bisect lines to the right of true center with their right hand and to the left with their left hand (symmetrical neglect). In contrast, children entering puberty reportedly bisect lines to the left with both the right and left hands (pseudoneglect). The shift from symmetrical to pseudoneglect has been hypothesized to reflect corpus callosum maturation and its involvement in the transfer of attention-based visuospatial processes. In the current study, line bisection performance and MR quantitative corpus callosum volumes were examined in 46 healthy children ages 8-18 years. A linear relationship between corpus callosum volume and age was found. However, the expected age-contingent line bisection performance pattern was not observed. In addition to the expected two patterns of line bisection bias, pseudoneglect and symmetric neglect, two additional distinct patterns of line bisection were identified. These findings, and other findings in the literature, raise important questions about the reliability and validity of the line bisection test. No relationship was found between corpus callosum volume and amount or direction of line bisection deviation. Our findings do not support previous hypotheses regarding line bisection-corpus callosum relationship.

Length perception of horizontal and vertical bisected lines

In an inverted T figure, the vertical line is largely overestimated (Avery and Day in J Exp Psychol 81:376-380, 1969). This vertical overestimation results from the vertical and bisection biases. Line orientation biases length perception in the sense that the vertical line of a L shape is perceived as longer than the horizontal line of the same physical length. In the inverted T figure, the vertical line is overestimated because of its orientation but also because the horizontal line is bisected. In the current study, we used various two-line configurations to investigate the role of bisection a/symmetry in line length perception and its interaction with the vertical bias. Experiment 1 showed that symmetry and asymmetry of bisection have different consequences on line length perception, as previously shown by Wolfe et al. (Percept Psychophys 67:967-979, 2005). Experiments 2 focused on the relation between the vertical and bisection biases by manipulating orthogonally line orientation and bisection a/symmetry. The results provided evidence that bisection can prevent the manifestation of the vertical bias, so that when the two lines are bisected, vertical lines are not anymore overestimated. These results are discussed in the light of recent findings claiming that saccades could play an essential role in length perception.

The effect of acute ethanol challenge on global visuospatial attention: Exaggeration of leftward bias in line bisection

Acute alcohol challenge has been associated with a selective impairment of right hemisphere function. A hallmark of visuospatial neglect syndrome is that patients with right hemisphere lesions misbisect horizontal lines far rightward of veridical centre. Neurologically intact participants misbisect lines with a systematic leftward bias (pseudoneglect). Neuroimaging studies in neurologically intact participants reveal predominant right hemisphere activation during performance of line bisection tasks. The current study assessed whether acute alcohol challenge alters global visuospatial attention. Participants (N=18; 10 male; strongly right-handed; mean age 23 years) engaged in a forced-choice tachistoscopic line bisection task in both ethanol challenge (mean BAC=.077) and no ethanol control conditions. Mean leftward bisection error in the control condition was −0.238 degrees visual angle (1.05% line length), and leftward bisection error significantly increased (p=.001) under ethanol challenge (−0.333 degrees visual angle, 1.47% line length). Mean bisection precision in the control condition was 0.358 degrees visual angle (1.58% line length); bisection precision significantly deteriorated (p=.008) under ethanol challenge (0.489 degrees, 2.17% line length). Decreased bisection precision indicates that ethanol disrupts the fidelity of visuospatial performance. The exaggerated leftward bisection error implies that ethanol may exert a differential effect on left versus right hemispheric function with respect to the control of global visuospatial attention.

Pseudoneglect for mental alphabet lines is affected by prismatic adaptation

While patients with right parietal damage and spatial neglect bisect lines to the right, the general population bisects lines to the left; a phenomenon known as pseudoneglect. The leftward bias also occurs for mental representations, such as number and alphabet lines. Prismatic adaptation can have a dramatic effect on attentional bias and corrects neglect and pseudoneglect for physical and mental number lines. This study examined whether prismatic adaptation can correct leftward bisection biases for alphabet lines, which may have a different spatial arrangement compared to number lines. In pre-adaptation testing, students (n = 42) were shown letter trigrams (e.g. C H P) and judged whether the alphabetical distance before or after the inner-letter was larger. Participants were then split into three groups and were adapted to left-shifting, control or right-shifting prims. After adaptation, the mental alphabet bisection task was re-administered. The length of left side of the alphabet lines was overestimated by all three groups in the pre-adaptation phase. Right-shifting prisms and control spectacles had no effect on the leftward bias whereas exposure to left-shifting prisms corrected the bias. The results replicate an effect observed for mental number lines and demonstrate that low-level sensory-motor shifts can correct attentional biases associated with high-level representations, such as letters.

Schizotypy and pseudoneglect: A critical update on theories of hemispheric asymmetries

Introduction. Positive schizotypy has been associated with a leftward spatial bias (pseudoneglect) in different tasks and populations. We tested whether this relationship (1) can be observed for two different hemispatial tasks in the same participants, and (2) is specific to positive schizotypy. Methods. Forty right-handed men performed a conventional line bisection task and a whole-body movement task. In the latter task, participants were presented with illuminated target locations on the floor, and had to visit the remembered locations (equal number in right and left hemispace) after a short time interval. We assessed side preferences in both tasks. Positive schizotypy, negative schizotypy, and cognitive disorganisation were assessed with a validated self-report questionnaire. Results. Irrespective of schizotypy, pseudoneglect was observed in both tasks. We found a rightward bisection and walking bias as a function of positive schizotypy, and also as a function of cognitive disorganisation for walking initiation. Discussion. This unexpected finding resulted in a review of hemispheric asymmetry for function in schizotypy, and in the discussion of potential variables that might account for the present discrepancy. We suggest that different schizotypy questionnaires and their presentation mode might be a potential contributor to the opposite findings in the literature.

Influence of Moving Background on Line Bisection Performance in the Normal Elderly Versus Patients with Hemispatial Neglect

Previous studies have shown that when healthy young participants bisect stationary lines on a moving background (MB) or optokinetic stimulation, they perceive the stationary line moving in the opposite direction of the MB (illusory motion [IM]), and they displace their bisection mark in the direction of the IM. This study attempted to learn whether IM also influences attentional biases of the healthy elderly and patients with hemispatial neglect. In experiment 1, healthy elderly participants and patients with neglect bisected lines in conditions where IM was absent or present. To better understand the MB dichotomy between the healthy elderly and neglect patients, in experiment 2, participants' eye movements were recorded using an infrared eye tracker. In experiment 1, healthy elderly participants' biases occurred in the opposite direction of MB when IM was present but in the same direction of MB when IM was absent. In contrast, neglect patients' biases occurred in the same direction of MB regardless of conditions. Eye movements reflect the spatial direction of attention. In experiment 2, the healthy elderly participants were able to selectively attend to the line, whereas neglect patients were impaired in that they fixated on the line. These results suggest that the healthy elderly can selectively fixate on a line, and with MB, they perceive the stationary line moving, resulting in a bisection bias in the direction of the IM. In contrast, when there is an MB, the patients with neglect are impaired in that they fixate on the line. Thus, they do not perceive IM; instead, they are primarily influenced by the MB.