The representation of space in healthy subjects and brain tumour patients

Neuropsychological basic processes of perception of space and the integration of sensory inputs to provide an internal representation of the external space is of primary interest for cognitive science and modern neuroscience. The main purpose of this research is twofold: (1) to improve and increase current knowledge on spatial representation in human daily-life activities and (2) to study and document the impact of visuo-spatial (S- V) functions impairments in patients who underwent to brain tumour resection and also to offer an innovative way to prevent the occurrence of spatial deficits after resection of brain tumour. In order to deepen these aspects we developed two parallel lines of research. On the one hand we studied mental spatial representation in healthy subjects in relation to current knowledge of Unilateral Spatial Neglect (USN) syndrome or neglect, and on the other hand we designed and used a neuropsychological battery to evaluate the incidence of visuo-spatial disorders in patients with right brain tumours (RBTP) pre- and postoperatively, and also to decide which tests could be applied as intra-operative task in the “awake surgery” practice to prevent V-S disorders after the tumour brain resection. According to the first research line, we carried out two behavioural experiments with relation to the neglect patients altered (anisometric) representation of space hypothesised by Bisiach and co-workers (1996), and to the findings of Ricci et al. (2004) simulating the hypothetical distortion of space in neglect patients in healthy subjects. In a nutshell, neglect syndrome is the inability to attend and respond to stimuli presented on the controlesional extra-, peripersonal and personal space. The space anisometry hypothesis foresee a space pathological progressive compression of the controlesional field in neglect patients which may resemble the Oppel-Kundt (O-K) illusion (Kundt, 1863; Watt, 1999). The classical form of this illusion consists of the perception of a filled space as larger than an empty space of equal size. We used a modified version of that illusion composed of a gradient of vertical lines whose spacing decreased progressively from one side to the other: space is expected to be perceived as larger where the lines are more compressed. In our experiments we tested the hypothesis that a horizontal stimulus presented in a space perceived as larger will produce faster RTs in agreement with the Sperandio end collegues’ studies (2010). In the first experiment, twenty-two healthy subjects were asked to respond as quickly as possible to lateralized stimuli (horizontal line, vertical line and circle) presented on different backgrounds (control condition: evenly spaced vertical lines; illusory conditions: vertical lines progressively compressed to the right or left portion of the monitor). For horizontal stimuli only, subjects’ RTs were reliably faster for stimuli presented on the compressed side of the gradient (275.8 ms) than on the relaxed side (279.9 ms). Moreover to verify the illusion of size, the same subjects were asked to adjust the size of a stimulus (horizontal line, vertical line and circle) presented on the same backgrounds as previous experiment to make it equal to a reference stimulus. For horizontal lines only, subjects produced adjustments in accordance with the predicted effect of the illusion. The results found with vertical stimuli, however, go on the opposite direction only for the left-larger condition. Together, these data show that the O-K illusion produces a mental distortion of space only for the horizontal dimension and this illusory perception of horizontal size could have had an effect of the vertical extent: a thinner stimulus is perceived as longer, thus producing an overestimation of its in vertical extent in the present task (the so-called Garner interference, Garner, 1974). In the second experiment we directly tested this hypothesis. We changed some parameters in respect to the previous experiment and results showed again that RTs for stimuli presented where the spaced was perceived as larger (269.66 ms) were reliably faster than those (274.59 ms) for the stimuli presented where the space was perceived as smaller. Once more, this effect strongly indicated that RTs reflect the perceived size instead of the actual size of objects. Moreover, the second adjustment experiment confirmed the Garner effect: thinner stimuli were perceived as longer producing an overestimation of its vertical extention. These results showed that O-K illusion produce an horizontal distortion of mental space representation affecting both RTs and adjustment tasks in relation with the visual stimuli size perception. According with the second research line, in literature, the data on the incidence and the assessment of visuo- spatial impairments and USN in brain tumour patients is very poor. Some authors (Jagaroo et al., 2000, Russell et al., 2005) reported no spatial deficits after resection of brain tumour, while others (Perrine et al., 2000) showed a different picture, with an incidence of around 37%. Probably that is because of the multicomponential nature of the USN syndrome and the difficulty to detect its different clinical manifestations. Given that USN has a dramatic effect on the outcome of these patients, it is of paramount importance to accurately assess all its manifestations. The purpose of this study was to assess visuo-spatial functions with a newly-build neuropsychological battery to determine the incidence of V-S diseases and neglect in RBTP. We tested 22 RBTP (main age: 57.45, DS: 12,25; histology: 10 high grade and 2 low grade glioma, 3 meningioma, 1 metastasis; tumours locations: 7 temporal, 6 frontal, 5 parietal, 2 occipital), both pre-operatively and post-operatively, with an extensive neuropsychological battery to evaluate of the following cognitive functions: (1) behavioural neglect, (2) anosognosia, (3) personal and body representational neglect, (4) motor neglect, (5) representational neglect, (6) peripersonal neglect, (7) extrapersonal neglect, (8) reading and (9) semantic and implicit elaboration skills. In the preoperative testing, twelve (54,54%) and three (13,63%) patients showed respectively V-S impairments and neglect deficits at least one of the twenty-one tests used. And postoperatively, for V-S impairments, two patients (16,67%) were unchanged, three (25%) improved and seven (58,33%) worsened, and four (18,20%) presented new deficits. Specifically for neglect, one patient improved (33,33), two worsened (66,66%) and six (27,27%) presented new deficits. The present study has important implications for a better knowledge of the spatial disorders in RBTP, thus providing a clearer picture on the everyday life problems faced by these patients, both within the family and at working and social contexts. However, the studies and the data reported so far are incomplete, because a more extensive sample of patient is needed both to document a clear and complete incidence of V-S deficits and neglect in RBTP and to define a clinical relation to tumours features, such as their histology, localization and size. Moreover, it is also difficult to detect which tests are reliable and sensitive for intraoperative use in awake surgery. In conclusion, for both lines of research is desirable to improve the following aspects: A) to enhance knowledge on the anatomical localization of the mental representation of space, with neuroimaging techniques in relation with the results found with the behavioral experiments with healthy subjects; B) to increase the sample for the incidence study of V-S impairments and neglect in RBTP; C) to extend the evaluation of V-S functions and neglect as clinical procedure to prevent and improve brain resection’s outcomes in RBTP; D) to broaden the cortical mapping of V-S functions and neglect as prevention procedure in awake surgery.