Leftward Extent Research Articles

Mental representation of a line when patients with left unilateral spatial neglect bisect it: A study with an endpoint reproduction task.

Patients with left unilateral spatial neglect (USN) typically place the subjective midpoint to the right of the objective centre. Based on the previous findings (e.g., Ishiai etal. 1989, Brain, 112, 1485), we hypothesized that the patients with left USN may see the representational image of a line that extends equally towards either side of the subjective midpoint depending not upon the information about the leftward extent. The present study tested whether patients with left USN would place the subjective midpoint at the centre of their mental representation of the line. The participants were 10 patients with left USN and 10 neurologically healthy controls. We devised a new 'endpoint reproduction task' using a computer display with a touch panel to seek the representational image when patients with left USN bisect lines and asked the participants to reproduce the location of the right or left endpoint after bisecting lines. The results showed that the representational image of the bisected line depends primarily on the location of the objective right endpoint, not on the location of the objective left endpoint in space. The analyses of the estimated right and left representational extents confirmed our hypotheses that patients with left USN would bisect a line seeing the representational line image that centred across their subjective midpoint. We believe that the findings of the present study with the use of the endpoint reproduction task will contribute to a better understanding of the visuospatial process underlying line bisection of patients with left USN.

Beyond the leftward limit of the perceptual span: Parafoveal processing to the left of fixation in Chinese reading.

During reading, Chinese readers have been found to obtain useful visual information from one character to the left to three characters to the right of fixation. The perceptual span is asymmetrical, and its leftward extent seems to be limited compared with the rightward extent. We conducted an experiment to investigate whether Chinese readers could process written information beyond the leftward extent of the perceptual span. We did this by using a variation of the gaze-contingent display change paradigm (Rayner, Cognitive Psychology, 81, 65-81, 1975) in order to manipulate the parafoveal "postview" that was available to the left of where readers were fixating. Each sentence contained an invisible boundary. Once the readers' eyes crossed the boundary, all of the characters to the left of the boundary except for one, two, or three characters directly to the left of the boundary were replaced with visually similar characters. The change lasted for only one single fixation, resulting in four different "postview" conditions including a control condition (n - 1, n - 2, n - 3, control). The results showed that, compared with the control condition, there were more regressions to the display change area immediately after readers' eyes crossed the boundary in the n - 1, n - 2, and n - 3 conditions, demonstrating that readers can acquire information from the three characters to the left of fixation at least.

Investigating the Effectiveness of Spatial Frequencies to the Left and Right of Central Vision during Reading: Evidence from Reading Times and Eye Movements.

Printed words are complex visual stimuli containing a range of different spatial frequencies, and several studies have suggested that various spatial frequencies are effective for skilled adult reading. But while it is well known that the area of text from which information is acquired during reading extends to the left and right of each fixation, the effectiveness of spatial frequencies falling each side of fixation has yet to be determined. To investigate this issue, we used a spatial frequency adaptation of the gaze-contingent moving-window paradigm in which sentences were shown to skilled adult readers either entirely as normal or filtered to contain only low, medium, or high spatial frequencies except for a window of normal text around each point of fixation. Windows replaced filtered text either symmetrically 1 character to the left and right of each fixated character, or asymmetrically, 1 character to the left and 7 or 13 to the right, or 1 character to the right and 7 or 13 to the left. Reading times and eye-movement measures showed that reading performance for sentences presented entirely as normal generally changed very little with filtered displays when windows extended to the right but was often disrupted when windows extended to the left. However, asymmetrical windows affected performance on both sides of fixation. Indeed, increasing the leftward extent of windows from 7 to 13 characters produced decreases in both reading times and fixation durations, suggesting that reading was influenced by the spatial frequency content of leftward areas of text some considerable distance from fixation. Overall, the findings show that while a range of different spatial frequencies can be used by skilled adult readers, the effectiveness of spatial frequencies differs for text on each side of central vision, and may reflect different roles played by these two areas of text during reading.

A further look at postview effects in reading: An eye-movements study of influences from the left of fixation.

When reading from left to right, useful information acquired during each fixational pause is widely assumed to extend 14 to 15 characters to the right of fixation but just 3 to 4 characters to the left, and certainly no further than the beginning of the fixated word. However, this leftward extent is strikingly small and seems inconsistent with other aspects of reading performance and with the general horizontal symmetry of visual input. Accordingly, 2 experiments were conducted to examine the influence of text located to the left of fixation during each fixational pause using an eye-tracking paradigm in which invisible boundaries were created in sentence displays. Each boundary corresponded to the leftmost edge of each word so that, as each sentence was read, the normal letter content of text to the left of each fixated word was corrupted by letter replacements that were either visually similar or visually dissimilar to the originals. The proximity of corrupted text to the left of fixation was maintained at 1, 2, 3, or 4 words from the left boundary of each fixated word. In both experiments, relative to completely normal text, reading performance was impaired when each type of letter replacement was up to 2 words to the left of fixated words but letter replacements further from fixation produced no impairment. These findings suggest that key aspects of reading are influenced by information acquired during each fixational pause from much further leftward than is usually assumed. Some of the implications of these findings for reading are discussed.

Direct pressure on a pseudomeningocele resulting in intraoperative cerebral ischemia

When positioning patients with meningocele and meningomyelocele, it is standard practice to avoid direct pressure on the lesions. That caution is intended to prevent injury to neural elements within the lesion and violation of the cerebrospinal fluid space. We herein report an additional hazard of direct intraoperative pressure on such lesions. An adult patient with a lumbosacral pseudomeningocele sustained a cerebral ischemic injury as a consequence of direct pressure on the lesion during general anesthesia. A 32-yr-old male with spina bifida and a pseudomeningocele related to recent lumbar surgery underwent a urologic procedure in the lithotomy position. Because the lesion was recognized to lie to the left of the midline, cushioning was placed under the patient's left hip and buttock. The patient was slow to awaken and has sustained significant long-term cognitive deficits. Imaging is consistent with a diffuse cerebral ischemic insult. In retrospect, the size and leftward extent of the pseudomeningocele were not appreciated preoperatively, and in spite of the care taken, intraoperative pressure was placed on the lesion. This report cautions that intraoperative pressure related to positioning patients with extra-axial lesions containing cerebrospinal fluid (CSF), e.g., meningoceles and pseudomeningoceles, can result in increases in CSF pressure and thereby a reduction in cerebral perfusion pressure sufficient to result in cerebral ischemia.

What’s left? An eye movement study of the influence of interword spaces to the left of fixation during reading

In English and other alphabetic systems read from left to right, the useful information acquired during each fixational pause is generally reported to extend 14-15 character spaces to the right of each fixation, but only 3-4 character spaces to the left, and certainly no farther than the beginning of the fixated word. However, this leftward extent is remarkably small and seems inconsistent with the general bilateral symmetry of vision. Accordingly, in the present study we investigated the influence of a fundamental component of text to the left of fixation-interword spaces-using a well-established eyetracking paradigm in which invisible boundaries were set up along individual sentence displays that were then read. Each boundary corresponded to the leftmost edge of a word in a sentence, so that as the eyes crossed a boundary, interword spaces in the text to the left of that word were obscured (by inserting a letter x). The proximity of the obscured text during each fixational pause was maintained at one, two, three, or four interword spaces from the left boundary of each fixated word. Normal fixations, regressions, and progressive saccades were disrupted when the obscured text was up to three interword spaces (an average of over 12 character spaces) away from the fixated word, while four interword spaces away produced no disruption. These findings suggest that influential information from text is acquired during each fixational pause from much farther leftward than is generally realized and that this information contributes to normal reading performance. Implications of these findings for reading are discussed.

Time-on-task effect in pseudoneglect

Neurologically normal subjects systematically misbisect space during visual line-bisection or similar tasks, generally erring to the left of the veridical center when bisecting horizontal lines, a phenomenon referred to as pseudoneglect. This phenomenon is usually interpreted as enhanced attention toward the left hemispace resulting in an overestimation of the leftward extent of a line. While most studies have examined the role of attention in spatial bias using spatial cueing methods in bisection tasks, Manly et al. (Neuropsychologia 43(12):1721-1728, 2005) proposed an original paradigm in which the participants' alertness was diminished by sleep deprivation or prolonged execution of a line-bisection task. The authors reported a significant rightward shift in attention related to declining alertness, but they did not control eye movements and, consequently, modifications of scanning and fixation strategies with fatigue cannot be ruled out in their study. Here we examine whether a diminution in alertness induced by a 60-min-long Landmark task would diminish (or even reverse) this attentional bias, when eye movements are absent. Participants performed a forced-choice judgment about the location of a transaction mark in relation to the veridical center of a horizontal line. The results confirmed a significant decrease in the leftward bias over the course of the session but, in contrast to the findings of Manly et al. (2005), we did not observe a reverse bias from the left to the right hemispace. The results are discussed within the context of the hemisphere-activation model.

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.

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.

Leftward Movement in Severe Neglect

Abstract The difficulty that patients with unilateral spatial neglect (USN) have in exploring into the contralesional space may be explained by motor or attentional disorder. We experienced a patient with severe USN following cerebral infarction in the right postrolandic region, who showed a strong resistance to leftward movement. We devised two sets of tasks using a whiteboard. In the first experiment, the patient showed great difficulty in tracing a line from the right endpoint to the left endpoint. The examiner barely made him start further tracing even by pushing the hand leftward. By contrast, he quickly erased a whole line leftward with an eraser pen. In the second experiment, he was required to erase a line with or without an attention-attracting stimulus at the right endpoint. Not only the leftward extent, but also the velocity of erasing, were decreased when there was a stimulus at the right endpoint. The results of the two experiments suggest that the ability of leftward movement itself was preserved and overattention to the right-sided stimuli impaired his leftward movement. We consider that use of the line-tracing and line-erasing tasks may contribute to a better understanding of interaction of attentional andmotor and motor factors in severe USN.

Leftward movement in severe neglect

The difficulty that patients with unilateral spatial neglect (USN) have in exploring into the contralesional space may be explained by motor or attentional disorder. We experienced a patient with severe USN following cerebral infarction in the right postrolandic region, who showed a strong resistance to leftward movement. We devised two sets of tasks using a whiteboard. In the first experiment, the patient showed great difficulty in tracing a line from the right endpoint to the left endpoint. The examiner barely made him start further tracing even by pushing the hand leftward. By contrast, he quickly erased a whole line leftward with an eraser pen. In the second experiment, he was required to erase a line with or without an attention-attracting stimulus at the right endpoint. Not only the leftward extent, but also the velocity of erasing, were decreased when there was a stimulus at the right endpoint. The results of the two experiments suggest that the ability of leftward movement itself was preserved and overattention to the right-sided stimuli impaired his leftward movement. We consider that use of the line-tracing and line-erasing tasks may contribute to a better understanding of interaction of attentional andmotor and motor factors in severe USN.

Extent of the Left Branching Solution to Certain Bifurcation Problems

A special class of nonlinear eigenvalue problems which exhibit branching to the left of. (below) the smallest eigenvalue is considered. This particular class serves to illustrate a procedure for bounding the leftward extent of a nonnegative solution branch. The procedure relies upon a well-known result about upper and lower solutions associated with a monotone operator. In the situation of left-branching bifurcation, the more difficult determination of a suitable lower solution is achieved by using an explicit solution to a simpler nonlinear problem. A physical example relative to the buckling of a nonlinearly elastic rod is worked out in detail.