The letter height superiority illusion.

Letters are identified better when they are embedded within words rather than within pseudowords, a phenomenon known as the word superiority effect (Reicher in Journal of Experimental Psychology, 81, 275-280, 1969). This effect is, inter alia, accounted for by the interactive-activation model (McClelland & Rumelhart in Psychological Review, 88, 375-407, 1981) through feedback from word to letter nodes. In this study, we investigated whether overactivation of features could lead to perceptual bias, wherein letters would be perceived as being taller than pseudoletters, or words would be perceived as being taller than pseudowords. In two experiments, we investigated the effects of letter and lexical status on the perception of size. Participants who had to compare the heights of letters and pseudoletters, or of words and pseudowords, indeed perceived the former stimuli as being taller than the latter. Possible alternative interpretations of this height superiority effect for letters and words are discussed.

Hemisphere-dependent ipsilesional deficits in hemianopia: Sightblindness in the 'intact' visual field.

OBJECTIVES: In addition to exhibiting a severe contralesional deficit, hemianopic patients may also show a subtle ipsilesional visual deficit, called sightblindness (the reverse case of 'blindsight). We have tested for the presence, nature and extent of such an ipsilesional visual field (IVF) deficit in hemianopic patients that we assigned to perform two visual tasks. Namely, we aimed to ascertain any links between this ipsilesional deficit, the lesion side, and the tasks performed or the stimuli used. METHODS: We tested left and right homonymous hemianopic (right brain-damaged RBD and left brain-damaged LBD, respectively) patients and healthy controls. Natural-scene images, either non-filtered or filtered in low or high spatial frequency (LSF or HSF, respectively) were presented in the IVF of each subject. For the two tasks, detection ("Is an image present?") and categorization ("Is the image of a forest or a city?"), accuracy and response time were recorded. RESULTS: In the IVF the RBD (left hemianopes) patients made more errors on the categorization task than did their matched controls, regardless of image type. In contrast, the only task in which the LBD (right hemianopes) patients made more errors than did the controls was the HSF-images task. Furthermore, in both tasks (detection and categorization), the RBD patients performed worse than did the LBD patients. DISCUSSION: Homonymous hemianopic patients do indeed exhibit a specific visual deficit in their IVF, which was previously thought to be unaffected. We have demonstrated that the nature and severity of this ipsilesional deficit is determined by the side of the occipital lesion as well as by the tasks and the stimuli. Our findings corroborate the idea of hemispheric specialization at the occipital level, which might determine the nature and severity of ipsilesional deficits in hemianopic patients.

Characteristics of contralesional and ipsilesional saccades in hemianopic patients.

In order to further our understanding of action-blindsight, four hemianopic patients suffering from visual field loss contralateral to a unilateral occipital lesion were compared to six healthy controls during a double task of verbally reported target detection and saccadic responses toward the target. Three oculomotor tasks were used: a fixation task (i.e., without saccade) and two saccade tasks (eliciting reflexive and voluntary saccades, using step and overlap 600 ms paradigms, respectively), in separate sessions. The visual target was briefly presented at two different eccentricities (5° and 8°), in the right or left visual hemifield. Blank trials were interleaved with target trials, and signal detection theory was applied. Despite their hemifield defect, hemianopic patients retained the ability to direct a saccade toward their contralesional hemifield, whereas verbal detection reports were at chance level. However, saccade parameters (latency and amplitude) were altered by the defect. Saccades to the contralesional hemifield exhibited longer latencies and shorter amplitudes compared to those of the healthy group, whereas only the latencies of reflexive saccades to the ipsilesional hemifield were altered. Furthermore, healthy participants showed the expected latency difference between reflexive and voluntary saccades, with the latter longer than the former. This difference was not found in three out of four patients in either hemifield. Our results show action-blindsight for saccades, but also show that unilateral occipital lesions have effects on saccade generation in both visual hemifields.

Optic neuritis: from magnocellular to cognitive residual dysfunction.

Optic Neuritis (ON) has been associated to both parvocellular dysfunction and to an alteration of the magnocellular pathway. After objective visual field and acuity recovery, ON patients may complain about their vision suggesting a residual subclinical deficit. To better characterize visual abnormalities, 8 patients recovering from a first ON episode as well as 16 healthy controls performed a simple detection task and a more complex categorization task of images presented in low spatial frequencies (to target the magnocellular system) or in high spatial frequencies (to target the parvocellular system) or of non-filtered images. When completing the tasks with their (previously) pathologic eye, optic neuritis patients showed lower accuracy compared to controls or to their healthy eye for low spatial frequency images only. Conjointly, the longest reaction times were observed with the previously pathologic eye regardless the type of images and to a greater extent in the categorization task than in the detection task. Such data suggest two distinct, although associated, types of residual dysfunction in ON: a magnocellular pathway alteration and a more general (magno and parvocellular) visual dysfunction that could implicate the cognitive levels of visual processing.

Ophthalmic disorder may affect visuo-attentional performance in childhood.

The aim of this study was to explore the visuo-attentional skills of children with an ophthalmic disorder. Twenty-four patients and 60 healthy controls between the ages 4 and 7 years, all right-handed with normal or corrected-to-normal close visual acuity, were divided into four age groups. Patients' diagnoses included refractive disorders (e.g., myopia, hypermetropia), strabismus, amblyopia, cataract, and nystagmus. All participants performed nine paper-and-pencil visuospatial tasks aiming to assess selective attention (cancellation tasks), spatial working memory (symbol orientation task), fine visual analysis (embedded figures test), and simple perceptual analysis (shape-matching task). In healthy children, the results showed that performance on all visuo-attentional tasks improved with age. While perception, orientation of attention, and visual working memory develop by the time children begin school (age 5), more sophisticated abilities such as attention disengagement and motor control continue to develop during late childhood. Moreover, a spatial bias in attention orienting appeared with reading acquisition (6-7 years). In ophthalmic children, at 4 years of age defects were observed in all assessed functions, but at 7 years an attentional deficit was virtually the only one remaining. Overall, the results demonstrate that children with an ophthalmologic disorder may experience difficulties with visuospatial tasks despite corrected-to-normal visual acuity.

An FMRI investigation of the cortical network underlying detection and categorization abilities in hemianopic patients.

The current study aims to investigate visual scene perception and its neuro-anatomical correlates for stimuli presented in the central visual field of patients with homonymous hemianopia, and thereby to assess the effect of a right or a left occipital lesion on brain reorganization. Fourteen healthy participants, three left brain damaged (LBD) patients with right homonymous hemianopia and five right brain damaged (RBD) patients with left homonymous hemianopia performed a visual detection task (i.e. "Is there an image on the screen?") and a categorization task (i.e. "Is it an image of a highway or a city?") during a block-designed functional magnetic resonance imaging recording session. Cerebral activity analyses of the posterior areas-the occipital lobe in particular-highlighted bi-hemispheric activation during the detection task but more lateralized, left occipital lobe activation during the categorization task in healthy participants. Conversely, in patients, the same network of activity was observed in both tasks. However, LBD patients showed a predominant activation in their right hemisphere (occipital lobe and posterior temporal areas) whereas RBD patients showed a more bilateral activation (in the occipital lobes). Overall, our preliminary findings suggest a specific pattern of cerebral activation depending on the task instruction in healthy participants and cerebral reorganization of the posterior areas following brain injury in hemianopic patients which could depend upon the side of the occipital lesion.

Finding centre: ocular and fMRI investigations of bisection and landmark task performance.

The line bisection task is used as a bedside test of spatial neglect patients who typically bisect lines to the right of true centre. To disambiguate the contribution of perceptual from motor biases in bisection, previous research has used the landmark task in which participants determine whether a transection mark is left or right of centre. One recent study using stimuli that reliably leads to leftward perceptual biases in healthy individuals, found that ocular judgements of centre were biased to the right of centre, whereas manual bisections were biased leftwards. Here we used behavioural measures and functional MRI in healthy individuals to investigate ocular and perceptual judgements of centre. Ocular judgements were made by having participants fixate the centre of a horizontal bar that was dark at one end and light at the other (i.e., a 'greyscale' stimulus), whereas perceptual responses were made by having participants indicate whether a transection mark on the greyscales stimuli was to the left or right of centre. Behavioural data indicated a leftward bias in the first, second and longest fixations for bisection. Moreover, greyscale orientation (i.e., dark extremity to the right or to the left), and stimulus position modulated fixations. In contrast, for the landmark task, initial fixations were attracted towards the transection mark, whereas subsequent fixations were closer to veridical centre. Imaging data showed a large bilateral network, including superior parietal and lingual cortex, that was active for bisection. The landmark task activated a predominantly right hemisphere network including superior and inferior parietal cortices. Taken together these results indicate that very different strategies and underlying neural networks are invoked by the bisection and landmark tasks.

Colour, face, and visuospatial imagery abilities in low-vision individuals with visual field deficits.

This study investigates to what extent visual perception integrity is necessary for visual mental imagery. Sixteen low-vision participants with severe peripheral visual field loss, 16 with severe central field loss, 6 left brain-damaged patients with right homonymous hemianopia, 6 right brain-damaged patients with left homonymous hemianopia, and 16 normally sighted controls performed perceptual and imagery tasks using colours, faces, and spatial relationships. Results showed that (a) the perceptual and mental image>ry disorders vary according to the type of visual field loss, (b) hemianopics had no more difficulties imagining spatial stimuli in their contralesional hemispace than in their ipsilesional one, and (c) the only hemianopic participant to have perceptual and mental imagery impairments suffered from attentional deficits. Results suggest that (a) visual memory is not definitively established, but rather needs perceptual practice to be maintained, and (b) that visual mental imagery may involve some of the attentional-exploratory mechanisms that are employed in visual behaviour.

Visuospatial processing in schizophrenia: does it share common mechanisms with pseudoneglect?

Schizophrenia patients demonstrate behavioural and cerebral lateralised anomalies, prompting some authors to suggest they exhibit a mild form of right unilateral neglect. To better describe and understand lateralised visuospatial anomalies in schizophrenia, three experiments were run using tasks often utilised to study visuospatial processing in healthy individuals and in neglect patients: the Behavioural Inattention Test (BIT), the manual line bisection task with and without a local cueing paradigm, the landmark task (or line bisection judgement), and the number bisection task. Although the schizophrenia patients did not exhibit the full-blown neglect syndrome, they did demonstrate marked spatial biases that differentiated them from controls on all but two tasks. More specifically, schizophrenia patients showed neither a simple perceptual deficit nor an asymmetry, but demonstrated (1) lateralised anomalies on a simple manual line bisection task; (2) unilateral attentional deficits for line bisection within a local cueing paradigm; and (3) a lateralised deficit in the visuospatial representations of numbers. Altogether, these results suggest a right hemineglect-like deficit in schizophrenia in attentional, representational, and motor-intentional processes. Yet it does not appear to be as strong a phenomenon. Indeed, it could be considered as an accentuation of the normal asymmetry in visuospatial processing.

Bisections in two languages: when number processing, spatial representation, and habitual reading direction interact.

Calabria and Rossetti (2005) demonstrated that spatial biases related to the mental number line can be seen even when bisecting strings of number words. Strings of smaller magnitude number words were bisected further to the left than strings of larger magnitude number words. The current study investigated whether the left-to-right mental number line associated with number processing will result in similar spatial biases despite a habitual, right-to-left reading direction. Monolingual left-to-right readers were compared to bidirectional readers of English and Hebrew. Strings of Arabic numerals and of number words (e.g., THREE, EIGHT) were presented in separate conditions of English and Hebrew. Significant rightward biases were seen among native Hebrew readers, regardless of English reading level; whereas native English readers (both bidirectional and monodirectional) did not show significant biases to either the left or the right. The spatial bias in bisecting either Arabic numeral strings or number words was related to the habitual reading direction of the participant. There was no difference in spatial bias or for frequency of spatial bias based on numerical magnitude for either condition. We discuss the influence of cultural factors, such as reading direction and proficiency, on the representation of spatial and numerical material.

Assessment of visuo-attentional abilities in young children with or without visual disorder: toward a systematic screening in the general population.

In young children, visual attention, analysis or memory is only rarely evaluated. Moreover, tools to test for such higher-order visual capacities in children are limited. In an attempt to develop and refine such tools, we selected nine tests to assess visuo-attentional abilities before formal reading education (grade 1). The battery consisted of gaze fixation, visual field, visual extinction, binocular visual pursuit, visual memory, "A" cancellation, Teddy bears cancellation, embedded figures, and matching tasks. This battery was used in the general population (n=110) to calculate cut-off scores identifying the lower 5% of the general population to obtain a screening measure for neurovisual disabilities in children. To evaluate our battery's sensitivity and specificity to neurovisual disorders over ophthalmological diseases, a neurovisual group (n=9) and an ophthalmologic group (n=13) also completed the tests. Overall, all but three tests of the battery could be used to discriminate between neurovisual and ophthalmologic children. The ophthalmologic children failed the visual field extent examination and the cancellation tasks, consistent with deleterious effects of ophthalmologic disease on visual perception as well as higher-order vision. Using the cut-off scores, the battery identified only 2 out of 13 ophthalmologic patients, but 5 out of 9 neurovisual patients. In the general population, these cut-off scores identified seven children. These children were previously undiagnosed with any disability (i.e., no diagnosis of ophthalmological, neurological, or psychiatric disease) and thus did not receive any rehabilitation. This preliminary study highlights the necessity for a neurovisual disorder screening tool for young children.

Specific impairments in visual processing following lesion side in hemianopic patients.

INTRODUCTION: Following unilateral damage of the primary visual cortex one of the most common visual field defects observed is Homonymous Hemianopia (HH), a loss of vision of the contralesional hemifield in each eye. The ipsilesional ("intact") part of the central visual field is often used to compensate for difficulties encountered in the peripheral hemianopic visual field. However, the quality of vision within the central visual field is not well-known. METHODS: To better describe and understand visual processing in hemianopia, two tasks were conducted with 25 healthy controls, six left hemianopes, and five right hemianopes. Filtered (in high, above 6 cycles/degree, or low, below 4 cycles/degree, spatial frequencies - HSF and LSF, respectively) and unfiltered natural scene images (5° of visual angle) were briefly presented (100 msec) centrally on a computer screen. Participants were required either to respond when a natural scene was presented (yes/no detection task) or to indicate if the stimulus was a city or a highway (categorization task). RESULTS: The three groups showed similar accuracy levels but significant differences were observed in response times. More precisely, left hemianopes were impaired both in the detection and in the categorization tasks whereas right hemianopes were only impaired in the categorization task. However, the three groups had similar responses to spatial frequencies: HSF were processed more slowly than LSF. CONCLUSIONS: Overall these results suggest that central vision is not intact in hemianopia. Lesion side selectively affects reaction times (RTs) in the detection and the categorization tasks, but does not seem to determine a specific deficit in spatial frequency processing.

Visual-perceptual abilities in healthy controls, depressed patients, and schizophrenia patients.

Previous studies have suggested a right hemineglect in schizophrenia, however few assessed possible visual-perceptual implication in this lateralized anomaly. A manual line bisection without (i.e., lines presented on their own) or with a local cueing paradigm (i.e., a number placed at one or both ends of the line) and the Motor-free Visual Perceptual Test-Vertical format (MVPT-V) were used to assess the visual-perceptual abilities of healthy controls, schizophrenia and depressed patients. Whereas healthy controls and depressed patients showed a non-significant leftward bias in manual line bisection, schizophrenia patients bisected significantly to the left of the true centre of the line. Interestingly, the pattern of performances in response to the local cueing paradigm was similar in depressed and schizophrenia patients such that both groups demonstrated a significant change in their bisection performance only in response to a cue placed at the right extremity of the line (control performance was modified by cues at either end of the line). Finally, in the MVPT-V, schizophrenia patients were impaired relative to the other two groups, especially in the spatial working memory and visual closure categories. These results suggest that: 1/a deficit towards the right hemifield, consistent with a mild form of right hemineglect, can be observed in schizophrenia; 2/lateralized anomalies could also be observed in depression using an appropriate tool such as manual line bisection; 3/performances in the MVPT-V suggested that a simple visual-perceptual deficit could not explain the lateralized anomaly observed in the manual line bisection, as it is the case in the hemineglect syndrome.

Pseudoneglect in schizophrenia: a line bisection study with cueing.

INTRODUCTION: Numerous authors have reported the existence of lateralised abnormalities towards the right side in patients with schizophrenia. METHODS: In the present study, a manual line bisection task was used to assess the existence of a visuospatial bias in patients with schizophrenia as compared to healthy subjects and left unilateral neglect patients. In addition, we used a local cueing paradigm (consisting of a number placed on the right, on the left, or at both ends of the line). RESULTS: Healthy subjects showed a leftwards trend in the "no cue" condition (known as pseudoneglect) and neglect patients showed a right bias in all cue conditions. In contrast, patients with schizophrenia placed their manual estimation of the centre further to the left than healthy subjects in all cue conditions, reflecting neglect of the right side of the line. Moreover, like healthy subjects and neglect patients, patients with schizophrenia were affected by the local cueing. CONCLUSION: Hence, patients with schizophrenia show a bias in their spatial representation, which does not interfere with local context processing.

Exaggerated leftward bias in the mental number line of patients with schizophrenia.

Several visuo-motor tasks can be used to demonstrate biases towards left hemispace in schizophrenic patients, suggesting a minor right hemineglect. Recent studies in neglect patients used a new number bisection task to highlight a lateralized defect in their visuo-spatial representation of numbers. To test a possible lateralized representational deficit in schizophrenia, we used the number bisection task in 11 schizophrenic patients compared to 11 healthy controls. Participants were required to orally indicate the central number of an interval orally presented. Whereas healthy subjects showed no significant bias, schizophrenic patients presented a significant leftward bias. Therefore, these results suggest an impairment in higher order representations of the number space in patients with schizophrenia, an impairment that is qualitatively similar to the deficit described in neglect patients.