The role of attention on numerosity perception and number adaptation in adults, typical and atypical children.

There is considerable controversy as to how the brain extracts numerosity information from a visual scene and as to how much attention is needed for this process. Traditionally, it has been assumed that visual enumeration is subserved by two functionally distinct mechanisms: the fast and accurate apprehension of 1 to about 4 items, a process termed “subitizing”, and the slow and error-prone appraisal of larger numerosities referred to as “estimation”. Further to a functional dichotomy between these two mechanisms, an attentional dichotomy has been proposed. Subitizing has been thought of as a pre-attentive and parallel process, whereas estimation is supposed to require serial attention. In this thesis, the hypothesis of a parallel and pre-attentive subitizing mechanism was tested. In the first part of the thesis to this aim, the amount of attention that could be allocated to an Estimation task was experimentally manipulated. We shown that numerosity estimation is composed by different and separable, sub- systems. Results indicated that subitizing strongly depends on attentional resources, while estimation of larger quantities does not. Exactly the same results were found when the attentional resources dedicated to the visual numerical estimation task were limited on other sensory modalities: indeed visual, auditory and also haptic attentional load strongly and similarly impair visual subitizing but much less high numbers. We also demonstrated that visual adaptation to numerosity, absent in the subitizing range under normal condition, emerges under attentional load with a magnitude of the effect highly comparable to that measured for high numbers. Moreover we first demonstrate that the ability to accurately map numbers onto space also depends on attentional resources, showing that the assumption that performance on the ‘numberline task’ is the direct reflection of the internal numeric representation form could be misleading. In last part of the thesis we study how number adaptation affects number perception in two different population; high-functioning autistic and typically developing children. We demostrated that ASD children discriminated numerosity with the same precision as the typical children, but showed much less (about half) the levels of adaptation to number than the control group. These new results show that adaptation, processes, fundamental for efficient processing of variable sensory inputs, is diminished in autism.