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Julie Castronovo, Xavier Seron; Numerical estimation in blind subjects: Evidence of the impact of blindness. Journal of Vision 2007;7(9):528. doi: 10.1167/7.9.528.
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Vision was for a long time considered as essential in the elaboration of the semantic numerical representation (e.g., Simon, 1999). However, early visual deprivation does not prevent the elaboration of a numerical representation with similar spatial properties as the one elaborated in sighted people: a mental continuum oriented from left to right (Castronovo & Seron, in press; Szïcs & Csépe, 2005). Here we investigated the impact of blindness and its following experience on the third property of the semantic numerical representation: its obedience to Weber's law. According to the “sensory limitation hypothesis”, vision presents an advantage over the other senses in the apprehension of numerosities. Early visual deprivation should therefore involve less proficient experience with numbers and more sensitivity to Weber's law. According to the “cognitive compensatory mechanisms hypothesis”, blind people develop compensatory mechanisms to access and represent numerosities in daily life situations, in which sighted people are not used to rely on numerical information (e.g., locomotion). Blindness should therefore involve more experience with numbers and less sensitivity to Weber's law. A group of blind and sighted subjects undertook two numerical estimation tasks: 1) key-press estimation task; 2) auditory events estimation task. Blind and sighted participants' performance obeys Weber's law. However, blind participants also show better numerical estimation abilities than sighted subjects, especially in a numerical context involving proprioception. The theories postulating an important role of vision in the elaboration of numerical cognition and the “sensory limitation hypothesis” cannot account for these results. Blind participants' performance support the “cognitive compensatory mechanisms hypothesis” suggesting that because of their particular experience with numbers, blind people might have developed the ability to compensate for Weber's law, resulting better numerical estimation skills, especially in numerical context involving proprioception. Therefore blindness and its following experience with numbers might have a positive impact on numerical processing.
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