Looking while imagining

Background: Subjects with hemispatial neglect often exhibit representational neglect: a failure to report details from the left side of mentally visualized images. This failure could reflect impaired ability to generate the left side of the mental image, or it could reflect failure to explore the left side of a normally generated mental image. When subjects with hemispatial neglect look at pictures or drawings, their attention tends to be drawn to objects on the right side, thereby aggravating their failure to explore the left side. If representational neglect represents a failure to explore the left side of a normally generated mental visual image, then it should be improved by blindfolding, which removes the attention-catching right-sided stimuli. However, if representational neglect represents a failure to generate the left side of the mental visual image, then blindfolding should have little impact on reporting of details of the image. Methods: To determine which of these explanations is correct, we asked eight normal participants and eight brain-damaged patients with left representational neglect to imagine the map of France and to name as many towns as possible in 2 minutes. In different sessions, participants performed the task with eyes open or while blindfolded. Results: Normal participants mentioned more towns while blindfolded than with vision, thus suggesting a distracting effect of visual details on mental imagery. Patients with neglect, however, showed no appreciable effect of blindfolding on reporting of details from either side of mental images. Conclusion: Representational neglect may represent a failure to generate the left side of mental images. NEUROLOGY 2007;68:432–437 Representational neglect has been ascribed to a failure to generate or maintain a normal representation of the contralesional side of mental images.1-3 Representational neglect is commonly assessed by requiring subjects to draw objects from memory4 or to name the towns or the countries on an imagined map.5,6 For example, when subjects with hemispatial neglect are asked to evoke mentally the map of France, they may omit to mention the towns located on the left part of the map,6,7 thus suggesting an amputation of the left part of their mental representation of space.1,8 An alternative explanation is that the mental image of contralesional space was not lacking, but rather that it was not adequately explored. This explanation is consistent with a hypothesis postulating that visual mental imagery involves some of the attentional-exploratory mechanisms that are employed in visual behavior,9,10 in particular, an inability to direct attention to areas of imagined space.1,11 The positive influence of head position,11 sensory manipulations6,12,13 and prismatic visuomotor adaptation14,15 (all of which might be expected to affect exploratory behavior but not the generation of a mental image) on representational neglect in a pure imaging task fits well with this explanation. When patients with neglect were asked to perform a drawing from memory task,4,16-18 with or without blindfolding, left neglect was decreased and even eliminated by blindfolding. These results suggest that visual feedback may exacerbate representational neglect and support the hypothesis that engaging attention through visual input can influence the processing of visual imagery.10 However, even in the blindfolded state, such tasks incorporate a major intentional component that underlies the act of drawing itself as well as the ongoing dynamic process involved in repeatedly comparing what is imagined to have been drawn with the original mental image template. This intentional component could serve to normalize an originally defective visual mental image. Can the presence or absence of visual input influence representational neglect in a similar way in the absence of such an intentional component? The aim of the present study was to answer this question. Editorial, see page 400 From the Université de Lyon, Université Lyon 1, Inserm UMR-S 534, Bron, and Hospices Civils de Lyon, Service de Rééducation Neurologique, Hôpital Henry Gabrielle, Lyon, France (G.R., P.R., Y.R., D.B.); Institut Fédératif des Neurosciences Lyon (G.R., P.R., Y.R., D.B.), Lyon, France; Inserm, U610 and Fédération de Neurologie (P.B.), Hôpital de la Salpétrière, Paris, France. Disclosure: The authors report no conflicts of interest. A preliminary version of this work was presented at the 23rd European Workshop on Cognitive Neuropsychology, Bressanone, Italy, January 23–28, 2005. Received January 17, 2006. Accepted in final form October 27, 2006. Address correspondence and reprint requests to Dr. Gilles Rode, Service de Médecine Physique et Réadaptation Neurologique, Hôpital Henry Gabrielle, Hospices Civils de Lyon, Route de Vourles, BP 57, F-69565 Saint-Genis Laval, France; e-mail: [email protected] 432 Copyright © 2007 by AAN Enterprises, Inc. Methods. We studied eight right brain–damaged patients (six men, two women, mean age 55.6 10.1 years) and eight agematched healthy subjects (five men, three women; mean age 55.1 7.5 years). All subjects were right-handed and gave informed consent. All the patients had been admitted to a neurologic rehabilitation unit for treatment of left hemiplegia. Clinical features and CT scan data are described in table 1. Rightward head and eye deviation were rated on a 4-point scale: score 0 no deviation; score 1 intermittent deviation; score 2 mild deviation that the subject was able to overcome with verbal instruction; score 4 severe deviation that the subject was unable to overcome even with verbal instruction. Anosognosia for motor impairment was assessed using 4-point scale.19 All the patients showed a extensive unilateral lesion. Etiology was always vascular, ischemic in six cases and hemorrhagic in the two other cases. None of subjects had impaired arousal, confusion, dementia, or psychiatric disorders. At the time of examination, 1 month post-onset, all patients showed a marked left-sided visuospatial neglect defined by several tests: a line bisection task,20 a line and star cancellation task,21,22 and reading a text and writing under dictation. All the patients also demonstrated left neglect on drawing from memory (a daisy and a clock) and on copying a daisy and a Gainotti drawing.23 At the time of testing, only three of eight subjects (N2, N5, and N8) showed mild anosognosia. Each subject was asked to mentally visualize the map of France as if he or she could see the map in front of him or her in his or her mind in two conditions: with eyes closed or eyes open. To help participants, they were asked to remember the map of France that they had learned during their first school period or to remember the weather forecast map featured each on television or in the newspapers. Participants had to list all the towns that they could “see” in 2 minutes.24 No instruction was given concerning the direction of mental scanning or the orientation of the mental map.24 Half of the subjects began with the eyes-closed condition, whereas the remaining half proceeded in the reverse order. Responses were recorded in two ways: i) mean total scores, indicating the number of towns named, and total scores were analyzed with a two-way analysis of variance (ANOVA) (subject x condition); ii) mean leftand right-sided scores defined by the position of reported towns on the two halves of the map. Towns located inside a 75-km stripe centered on a vertical meridian line (linking Lille to Perpignan) were not taken into account (middle score). Left-right scores were analyzed with a three-way ANOVA (subject x condition x side). To have a better estimate of the location of named towns on the map in the two experimental conditions, we also measured the distance between each named town and the vertical meridian line. The distances were measured on a map of France (scale: 1/5,000,000; 1 cm 50 km) on which all the towns that were named by the subjects were plotted. A positive value indicates a town located to the right side of the vertical meridian line and a negative value indicates a town to the left of the meridian line. Comparisons of distances were performed with two nonparametric tests: the Kruskal-Wallis ANOVA with one factor (subject or condition) and the median test, which simply counts the number of cases in neglect and healthy controls that fall above or below the common median, and computes the 2 value for the resulting 2 2 samples contingency table. If healthy subjects and neglect patients have identical medians, we expect approximately 50% of all cases in each sample to fall above (or below) the common median. Results. Individual data are summarized in table 2. Healthy subjects had symmetrical scores. For all patients, the left-sided score was less than the right-sided score in both conditions, thus suggesting a deficit in image generation. To estimate more accurately the location of named towns, they were placed on a tracing of a map of France (figure 1). In healthy subjects, the reported towns are distributed over the entire map and in aggregate they create a complete map of France (figure 1A). This is consistent with the idea that performance relied on the exploration of an inner image. In patients with neglect, the named towns were placed mainly on the right half of the map, which, however, looks like the right side of the map produced by healthy subjects. This suggests a fully spared representation on the right side. However, the defective left half of the maps imagined by patients with hemispatial neglect suggests a left representational deficit (figure 1B). Notably, patients with neglect never named a town more than once, whatever its location. In healthy subjects, mean total scores were 225 in the eyes-open condition and 259 in the eyes-closed condition, whereas in patients with neglect, the mean total scores were similar in both conditions (145 and 150). ANOVA revealed that the subject factor as well as the condition factor were significant (F1,7 9.31; and F1,7 12.36) because more towns were mentioned in eyes-closed condition (25.56 vs 23.13), and patients with neglect listed less towns than controls (18.44 vs 30.25). Table 1 Demographic features, clinical and CT assessed lesion site of neglect patients Patient Age/sex Hemiplegia Hemi-anesthesia Hemianopia Ocular and