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The spatial representation of numbers: evidence from neglect and pseudoneglect

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Abstract

The aim of the present paper is to provide an overview of the evidence that links spatial representation with representation of number magnitude. This aim is achieved by reviewing the literature concerning the number interval bisection task in patients with left hemispatial neglect and in healthy participants (pseudoneglect). Phenomena like the Spatial Numerical Association of Response Codes (SNARC) effect and the shifts of covert spatial attention caused by number processing are thought to support the notion that number magnitude is represented along a spatially organized mental number line. However, the evidence provided by chronometric studies is not univocal and is open to alternative, non-spatial interpretations. In contrast, neuropsychological studies have offered convincing evidence that humans indeed represent numbers on a mental number line oriented from left to right. Neglect patients systematically misplace the midpoint of a numerical interval they are asked to bisect (e.g., they say that 〈5〉 is halfway between 〈2〉 and 〈6〉) and their mistakes closely resemble the typical pattern found in bisection of true visual lines. The presence of dissociations between impaired explicit knowledge and spared implicit knowledge supports the notion that neglect produces a deficit in accessing an intact mental number line, rather than a distortion in the representation of that line. Other results show that the existence of a strong spatial connotation constitutes a specific property of number representations rather than a general characteristic of all ordered sequences.

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Notes

  1. Zorzi et al.’s (2002) neglect patients were unimpaired in arithmetical tasks, which, at first sight, seems to contradict the notion that quantity is represented spatially. However, Priftis et al. (2006) have subsequently shown that in neglect patients, spatial representations of numbers are intact and only explicit access to those spatial representations is impaired. In addition, it should be kept in mind that Zorzi et al.’s neglect patients were perfectly able to bisect the number interval by applying an algorithm, that is, through the use of Dehaene’s (1992) verbal code (see above). However, they were strictly asked not to do so.

  2. Alternatively, one may think that smaller numbers are cognitively more available, perhaps because of their higher frequency. However, this interpretation is contradicted by the observation that the effect is modulated by head rotation (Loetscher et al. 2008): Participants showed a bias towards producing smaller numbers when their head (along with their spatial attention, presumably) was rotated to the left, whereas they produced larger numbers when their head was rotated to the right.

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Acknowledgments

Preparation of this paper was supported by grants from MIUR (Prin 2005 and Prin 2006) to CU and MZ and from CARIPARO Foundation to MZ and KP.

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Correspondence to Carlo Umiltà.

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Umiltà, C., Priftis, K. & Zorzi, M. The spatial representation of numbers: evidence from neglect and pseudoneglect. Exp Brain Res 192, 561–569 (2009). https://doi.org/10.1007/s00221-008-1623-2

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