Line bisection by eye and by hand reveal opposite biases

The vision-for-action literature favours the idea that the motor output of an action - whether manual or oculomotor - leads to similar results regarding object handling. Findings on line bisection performance challenge this idea: healthy individuals bisect lines manually to the left of centre, and to the right of centre when using eye fixation. In case that these opposite biases for manual and oculomotor action reflect more universal compensatory mechanisms that cancel each other out to enhance overall accuracy, one would like to observe comparable opposite biases for other material. In the present study, we report on three independent experiments in which we tested line bisection (by hand, by eye fixation) not only for solid lines, but also for letter lines; the latter, when bisected manually, is known to result in a rightward bias. Accordingly, we expected a leftward bias for letter lines when bisected via eye fixation. Analysis of bisection biases provided evidence for this idea: manual bisection was more rightward for letter as compared to solid lines, while bisection by eye fixation was more leftward for letter as compared to solid lines. Support for the eye fixation observation was particularly obvious in two of the three studies, for which comparability between eye and hand action was increasingly adjusted (paper-pencil versus touch screen for manual action). These findings question the assumption that ocular motor and manual output are always inter-changeable, but rather suggest that at least for some situations ocular motor and manual output biases are orthogonal to each other, possibly balancing each other out.

Line bisection in simulated homonymous hemianopia

Hemianopic patients make a systematic error in line bisection, showing a contra-lesional bias towards their blind side, which is the opposite of that in hemineglect patients. This error has been attributed variously to the visual field defect, to long-term strategic adaptation, or to independent effects of damage to extrastriate cortex. To determine if hemianopic bisection error can occur without the latter two factors, we studied line bisection in healthy subjects with simulated homonymous hemianopia using a gaze-contingent display, with different line-lengths, and with or without markers at both ends of the lines. Simulated homonymous hemianopia did induce a contra-lesional bisection error and this was associated with increased fixations towards the blind field. This error was found with end-marked lines and was greater with very long lines. In a second experiment we showed that eccentric fixation alone produces a similar bisection error and eliminates the effect of line-end markers. We conclude that a homonymous hemianopic field defect alone is sufficient to induce both a contra-lesional line bisection error and previously described alterations in fixation distribution, and does not require long-term adaptation or extrastriate damage.

Attenuation of perceptual asymmetries in patients with early-onset schizophrenia: Evidence in favour of reduced hemispheric differentiation in schizophrenia?

Lateral biases in visual perception have been demonstrated in normal individuals and in patients with unilateral brain lesions. It has been suggested that the absence of structural and functional asymmetries in schizophrenia could be due to a failure in lateralisation that may be most pronounced in those patients whose illness onset is at an early age. Here we examined lateral biases in patients with schizophrenia of an early onset (N = 21) and a late onset.(N = 19), and their respective age-matched control groups, using the greyscales task, a sensitive measure of asymmetries in visual processing. The stimuli consisted of two rectangles, one above the other, shaded in opposite directions and matched overall for darkness. Participants judged which of the two rectangles looked darker overall. Previous studies using this task in healthy participants have reported a reliable bias, such that the rectangle with the darker end on the left is selected preferentially. Whereas the late-onset patients in this study exhibited a perceptual bias of similar direction and magnitude to that of controls, this was not the case for the early-onset patients, who exhibited significantly less bias than their control group. The reduced perceptual bias seen in the early-onset group, but not the late-onset group, suggests an attenuation of right hemisphere mechanisms dedicated to processing vistiospatial information. The attenuated perceptual asymmetry in the early-onset group only may be consistent with the view that (i) an earlier illness onset reflects a greater loss of hemispheric differentiation and (ii) reduced functional asymmetries in the early-onset group are a manifestation of a failure to allocate functions to one or the other hemisphere.

Ocular scanning and perceptual size distortion in hemispatial neglect: effects of prism adaptation and sequential stimulus presentation

When asked to compare two lateralized shapes for horizontal size, neglect patients often indicate the left stimulus to be smaller. Gainotti and Tiacci (1971) hypothesized that this phenomenon might be related to a rightward bias in the patients' gaze. This study aimed to assess the relation between this size underestimation and oculomotor asymmetries. Eye movements were recorded while three neglect patients judged the horizontal extent of two rectangles. Two experimental manipulations were performed to increase the likelihood of symmetrical scanning of the stimulus display. The first manipulation entailed a sequential, rather than simultaneous presentation of the two rectangles. The second required adaptation to rightward displacing prisms, which is known to reduce many manifestations of neglect. All patients consistently underestimated the left rectangle, but the pattern of verbal responses and eye movements suggested different underlying causes. These include a distortion of space perception without ocular asymmetry, a failure to view the full leftward extent of the left stimulus, and a high-level response bias. Sequential presentation of the rectangles and prism adaptation reduced ocular asymmetries without affecting size underestimation. Overall, the results suggest that leftward size underestimation in neglect can arise for a number of different reasons. Incomplete leftward scanning may perhaps be sufficient to induce perceptual size distortion, but it is not a necessary prerequisite.

Ultra-fast recovery from right neglect after 'awake surgery' for slow-growing tumor invading the left parietal area

It is now possible to perform resections of slow-growing tumors in awake patients. Using direct electrical stimulation, real-time functional mapping of the brain can be used to prevent the resection of essential areas near the tumor. Simple clinical observations of patients with a resection of slow-growing tumors have demonstrated substantial recovery within a few days of such 'awake surgery'. The aim of this study was to investigate the kinetics of recovery following the resection of slow-growing tumors invading the left parietal area and to focus mainly on its rapidity. Two patients were assessed by standard line bisection tests and compared with eight healthy individuals. Independently of the pure nature of the symptoms, we report that the patients rapidly and substantially recovered from pronounced right neglect. They were tested 48 hours after the surgery and the recovery was significant for both patients after less than 4 hours. Strikingly, for one patient, recovery was ultra fast and substantial in the first practice session within less than 7 minutes: it occurred without verbal feedback and was substantially retained during the following testing session. Its rapidity suggests a process of unmasking redundant networks. With the slow growth of the lesion, the contralesional hemisphere is probably progressively prepared for rapid unmasking of homologue networks. These results have major clinical implications. For patients with an invading left-side tumor, it is now clear that line bisections are required before, during, and after awake surgery to: plan the surgery, control the quality of the resection, and also optimize the rehabilitation of the patient

"Dov'è la sinistra?" Hemispatial neglect in stroke

As long as the incidence of stroke continues to grow, patients with large right hemisphere lesions suffering from hemispatial neglect will require neuropsychological evaluation and rehabilitation. The inability to process information especially that coming from the left side accompanied by the magnetic orientation to the ipsilesional side represents a real challenge for rehabilitation. This dissertation is concerned with crucial aspects in the clinical neuropsychological practice of hemispatial neglect. In studying the convergence of the visual and behavioural test batteries in the assessment of neglect, nine of the seventeen patients, who completed both the conventional subtests of the Behavioural Inattention Test and the Catherine Bergego Scale assessments, showed a similar severity of neglect and thus good convergence in both tests. However, patients with neglect and hemianopia had poorer scores in the line bisection test and they displayed stronger neglect in behaviour than patients with pure neglect. The second study examined, whether arm activation, modified from the Constraint Induced Movement Therapy, could be applied as neglect rehabilitation alone without any visual training. Twelve acute- or subacute patients were randomized into two rehabilitation groups: arm activation training or traditional voluntary visual scanning training. Neglect was ameliorated significantly or almost significantly in both training groups due to rehabilitation with the effect being maintained for at least six months. In studying the reflections of hemispatial neglect on visual memory, the associations of severity of neglect and visual memory performances were explored. The performances of acute and subacute patients with hemispatial neglect were compared with the performances of matched healthy control subjects. As hypothesized, encoding from the left side and immediate recall of visual material were significantly compromised in patients with neglect. Another mechanism of neglect affecting visual memory processes is observed in delayed visual reproduction. Delayed recall demands that the individual must make a match helped by a cue or it requires a search for relevant material from long-term memory storage. In the case of representational neglect, the search may succeed but the left side of the recollected memory still fails to open. Visual and auditory evoked potentials were measured in 21 patients with hemispatial neglect. Stimuli coming from the left or right were processed differently in both sensory modalities in acute and subacute patients as compared with the chronic patients. The differences equalized during the course of recovery. Recovery from hemispatial neglect was strongly associated with early rehabilitation and with the severity of neglect. Extinction was common in patients with neglect and it did not ameliorate with the recovery of neglect. The presence of pusher symptom hampered amelioration of visual neglect in acute and subacute stroke patients, whereas depression did not have any significant effect in the early phases after the stroke. However, depression had an unfavourable effect on recovery in the chronic phase. In conclusion, the combination of neglect and hemianopia may explain part of the residual behavioural neglect that is no longer evident in visual testing. Further research is needed in order to determine which specific rehabilitation procedures would be most beneficial in patients suffering the combination of neglect and hemianopia. Arm activation should be included in the rehabilitation programs of neglect; this is a useful technique for patients who need bedside treatment in the acute phase. With respect to the deficit in visual memory in association with neglect, the possible mechanisms of lateralized deficit in delayed recall need to be further examined and clarified. Intensive treatment induced recovery in both severe and moderate visual neglect long after the first two to first three months after the stroke.

Pseudo-neglect in Huntington's disease correlates with decreased angular gyrus density

Visuospatial attentional bias was examined in Huntington's disease (HID) patients with mild disease, asymptomatic gene-positive patients and controls. No group differences were found on the grey scales task (which is a non-motor task of visuospatial attentional bias), although patients' trinucleotide (CAG) repeat length correlated with increasing leftward bias. On the line bisection task, symptomatic patients made significantly larger leftward bisection errors relative to controls, who showed the normal slight degree of leftward error (pseudo-neglect). The asymptomatic group showed a trend for greater leftward error than controls. A subset of participants went on to have structural MRI, which showed a correlation between increased leftward error on the line bisection task and reduced density in the angular gyrus area (BA39) bilaterally. This finding is consistent with recent literature suggesting a critical role for the angular gyrus in the lateralization of visuospatial attention.

Os efeitos da idade sobre o padrão de assimetrias táteis, visuais e hemiespaciais em destros e canhotos

Em testes de bisecção de linhas horizontais, os indivíduos normais tendem a partilhá-las à esquerda do verdadeiro ponto zero, fenômeno este denominado de pseudonegligência. As investigações clínicas revelaram que os mecanismos de controle sobre a alocação de atenção ao lado esquerdo do corpo foram profundamente afetados quando houve lesão localizada na parte inferior do lóbulo parietal no hemisfério direito do cérebro; e mais, a omissão de atenção à esquerda da linha média do corpo foi igualmente observada em certas modalidades motoras e táteis, não se limitando apenas ao sistema visual. Neste contexto, estudos evidenciam, ainda, que mudanças na preferência e proficiência manual ocorreram durante o processo de envelhecimento. Por exemplo, diversos estudos mostraram que, com o avanço da idade, a prevalência de canhotismo tende a diminuir; além do mais, em alguns estudos, a partir da idade de 55 anos, a direção de pseudonegligência foi distribuída à direita do ponto zero. Surgem, então, três hipóteses que buscam explicar a fonte das variações através das faixas etárias: A hipótese da degradação diferencial dos hemisférios cerebrais; a hipótese de perda bilateral; e a hipótese de assimetria invariante. Neste estudo, uma amostra de 61 indivíduos foi selecionada, sendo composta de destros e canhotos, de ambos os sexos, em três faixas etárias: de 18 a 30 anos, de 35 a 55 anos e 60 anos e acima. Os indivíduos foram submetidos ao Inventário de Preferência Manual, ao Teste Tátil de Biseccionar Linhas (TTBL) e ao Teste Visual de Biseccionar Linhas (TVBL), com o objetivo de averiguar os efeitos da idade sobre a assimetria manual e hemiespacial através do desempenho dos mesmos. Os resultados, no TTBL, mostram que houve poucas diferenças significativas no desvio e as tendências que emergiram foram ou inconsistentes ou orientadas em direções contrárias às predições de Bowers & Heilman. Não foi identificada nenhuma divergência no grau e na direção do desempenho entre destros e canhotos. O modelo interdependente de ativação, não foi sustentado por estes achados, concluindo-se que a presente versão do TTBL não constitui uma medida válida da lateralidade desta função. Em contraste, no TVBL a pseudonegligência evidenciou-se consistentemente, sob condições específicas em todos os grupos. O modelo interdependente foi apoiado pela interação significativa existente entre as mãos e os hemicampos, em particular quando a mão esquerda partilhou as linhas no campo esquerdo e central. Não foi encontrada nenhuma divergência entre os canhotos e os destros (nas três faixas etárias), no nível e direção de desempenho no TVBL. Contudo, não existiu evidência empírica indicando que a degeneração precoce do hemisfério direito afetou a atenção visuoespacial no grupo de idosos. Por conseguinte, o modelo de assimetria invariante parece mais plausível.

Cognitive performance in high-altitude climbers: a comparative study of saccadic eye movements and neuropsychological tests

Impairment of cognitive performance during and after high-altitude climbing has been described in numerous studies and has mostly been attributed to cerebral hypoxia and resulting functional and structural cerebral alterations. To investigate the hypothesis that high-altitude climbing leads to cognitive impairment, we used of neuropsychological tests and measurements of eye movement (EM) performance during different stimulus conditions. The study was conducted in 32 mountaineers participating in an expedition to Muztagh Ata (7,546 m). Neuropsychological tests comprised figural fluency, line bisection, letter and number cancellation, and a modified pegboard task. Saccadic performance was evaluated under three stimulus conditions with varying degrees of cortical involvement: visually guided pro- and anti-saccades, and visuo-visual interaction. Typical saccade parameters (latency, mean sequence, post-saccadic stability, and error rate) were computed off-line. Measurements were taken at a baseline level of 440 m and at altitudes of 4,497, 5,533, 6,265, and again at 440 m. All subjects reached 5,533 m, and 28 reached 6,265 m. The neuropsychological test results did not reveal any cognitive impairment. Complete eye movement recordings for all stimulus conditions were obtained in 24 subjects at baseline and at least two altitudes and in 10 subjects at baseline and all altitudes. Measurements of saccade performances showed no dependence on any altitude-related parameter and were well within normal limits. Our data indicates that acclimatized climbers do not seem to suffer from significant cognitive deficits during or after climbs to altitudes above 7,500 m. We demonstrated that investigation of EMs is feasible during high-altitude expeditions.

"mind the gap": The size-distance dissociation in visual neglect is a cueing effect

There is a growing body of evidence that the processes mediating the allocation of spatial attention within objects may be separable from those governing attentional distribution between objects. In the neglect literature, a related proposal has been made regarding the perception of (within-object) sizes and (between-object) distances. This proposal follows observations that, in size-matching and bisection tasks, neglect is more strongly expressed when patients are required to attend to the sizes of discrete objects than to the (unfilled) distances between objects. These findings are consistent with a partial dissociation between size and distance processing, but a simpler alternative must also be considered. Whilst a neglect patient may fail to explore the full extent of a solid stimulus, the estimation of an unfilled distance requires that both endpoints be inspected before the task can be attempted at all. The attentional cueing implicit in distance estimation tasks might thus account for their superior performance by neglect patients. We report two bisection studies that address this issue. The first confirmed, amongst patients with left visual neglect, a reliable reduction of rightward error for unfilled gap stimuli as compared with solid lines. The second study assessed the cause of this reduction, deconfounding the effects of stimulus type (lines vs. gaps) and attentional cueing, by applying an explicit cueing manipulation to line and gap bisection tasks. Under these matched cueing conditions, all patients performed similarly on line and gap bisection tasks, suggesting that the reduction of neglect typically observed for gap stimuli may be attributable entirely to cueing effects. We found no evidence that a spatial extent, once fully attended, is judged any differently according to whether it is filled or unfilled.

Perception while watching movies:effects of physical screen size and scene type

Over the last decade, television screens and display monitors have increased in size considerably, but has this improved our televisual experience? Our working hypothesis was that the audiences adopt a general strategy that “bigger is better.” However, as our visual perceptions do not tap directly into basic retinal image properties such as retinal image size (C. A. Burbeck, 1987), we wondered whether object size itself might be an important factor. To test this, we needed a task that would tap into the subjective experiences of participants watching a movie on different-sized displays with the same retinal subtense. Our participants used a line bisection task to self-report their level of “presence” (i.e., their involvement with the movie) at several target locations that were probed in a 45-min section of the movie “The Good, The Bad, and The Ugly.” Measures of pupil dilation and reaction time to the probes were also obtained. In Experiment 1, we found that subjective ratings of presence increased with physical screen size, supporting our hypothesis. Face scenes also produced higher presence scores than landscape scenes for both screen sizes. In Experiment 2, reaction time and pupil dilation results showed the same trends as the presence ratings and pupil dilation correlated with presence ratings, providing some validation of the method. Overall, the results suggest that real-time measures of subjective presence might be a valuable tool for measuring audience experience for different types of (i) display and (ii) audiovisual material.

Pseudoneglect and visual attention networks

Pseudoneglect represents the tendency for healthy individuals to show a slight but consistent bias in favour of stimuli appearing in the left visual field. The bias is often measured using variants of the line bisection task. An accurate model of the functional architecture of the visuospatial attention system must account for this widely observed phenomenon, as well as for modulation of the direction and magnitude of the bias within individuals by a variety of factors relating to the state of the participant and/or stimulus characteristics. To date, the neural correlates of pseudoneglect remain relatively unmapped. In the current thesis, I employed a combination of psychophysical measurements, electroencephalography (EEG) recording and transcranial direct current stimulation (tDCS) in an attempt to probe the neural generator(s) of pseudoneglect. In particular, I wished to utilise and investigate some of the factors known to modulate the bias (including age, time-on-task and the length of the to-be-bisected line) in order to identify neural processes and activity that are necessary and sufficient for the lateralized bias to arise. Across four experiments utilising a computerized version of a perceptual line bisection task, pseudoneglect was consistently observed at baseline in healthy young participants. However, decreased line length (experiments 1, 2 and 3), time-on-task (experiment 1) and healthy aging (experiment 3) were all found to modulate the bias. Specifically, all three modulations induced a rightward shift in subjective midpoint estimation. Additionally, the line length and time-on-task effects (experiment 1) and the line length and aging effects (experiment 3) were found to have additive relationships. In experiment 2, EEG measurements revealed the line length effect to be reflected in neural activity 100 – 200ms post-stimulus onset over source estimated posterior regions of the right hemisphere (RH: temporo-parietal junction (TPJ)). Long lines induced a hemispheric asymmetry in processing (in favour of the RH) during this period that was absent in short lines. In experiment 4, bi-parietal tDCS (Left Anodal/Right Cathodal) induced a polarity-specific rightward shift in bias, highlighting the crucial role played by parietal cortex in the genesis of pseudoneglect. The opposite polarity (Left Cathodal/Right Anodal) did not induce a change in bias. The combined results from the four experiments of the current thesis provide converging evidence as to the crucial role played by the RH in the genesis of pseudoneglect and in the processing of visual input more generally. The reduction in pseudoneglect with decreased line length, increased time-on-task and healthy aging may be explained by a reduction in RH function, and hence contribution to task processing, induced by each of these modulations. I discuss how behavioural and neuroimaging studies of pseudoneglect (and its various modulators) can provide empirical data upon which accurate formal models of visuospatial attention networks may be based and further tested.

[the Use Of Fish On Buccal Smear To Investigate Mosaicism With A 45,x Cell Line: Study On Healthy Men And Patients With Disorders Of Sex Development].

To verify whether fluorescence in situ hybridization (FISH) of cells from the buccal epithelium could be employed to detect cryptomosaicism with a 45,X lineage in 46,XY patients. Samples of nineteen 46,XY healthy young men and five patients with disorders of sex development (DSD), four 45,X/46,XY and one 46,XY were used. FISH analysis with X and Y specific probes on interphase nuclei from blood lymphocytes and buccal epithelium were analyzed to investigate the proportion of nuclei containing only the signal of the X chromosome. The frequency of nuclei containing only the X signal in the two tissues of healthy men did not differ (p = 0.69). In all patients with DSD this frequency was significantly higher, and there was no difference between the two tissues (p = 0.38), either. Investigation of mosaicism with a 45,X cell line in patients with 46,XY DSD or sterility can be done by FISH directly using cells from the buccal epithelium.

Graded Meshes In Bio-thermal Problems With Transmission-line Modeling Method.

In this study, the transmission-line modeling (TLM) applied to bio-thermal problems was improved by incorporating several novel computational techniques, which include application of graded meshes which resulted in 9 times faster in computational time and uses only a fraction (16%) of the computational resources used by regular meshes in analyzing heat flow through heterogeneous media. Graded meshes, unlike regular meshes, allow heat sources to be modeled in all segments of the mesh. A new boundary condition that considers thermal properties and thus resulting in a more realistic modeling of complex problems is introduced. Also, a new way of calculating an error parameter is introduced. The calculated temperatures between nodes were compared against the results obtained from the literature and agreed within less than 1% difference. It is reasonable, therefore, to conclude that the improved TLM model described herein has great potential in heat transfer of biological systems.