Attentional mechanisms and number representation

The work presented in this thesis has explored attentional mechanisms and numerosity representations in visual estimation through a variety of methods. After a brief literature review, in chapter 2 I present a study of visual attention that demonstrates a clear example of how perception is influenced by past experience (Priming of Pop-out). Besides the classical measures of Reaction Times, I investigated whether attentional mechanisms can be measured with pupillometry measures. The results suggest that pupillometry may be a valid, non-invasive and fast tool to evaluate some cognitive and perceptual processes, particularly in clinical populations (such as ASD) when behavioral measures may fail. The focus of the thesis then shifts to investigate another capacity of the visual system: estimating number of objects in a visual scene. In Chapter 3 I investigated whether reaction times, which have classically been used to assess differences between very low and intermediate number, can be used to prove the existence of a third regime of number perception, that comes in to play when the items on the scene are too packed to be perceived independently. The results show that not only do thresholds change from intermediate to high densities, but reaction times also follow this pattern. In Chapter 4 I present a study showing how linking elements influence number perception at various ranges of numerosity. Contribution of both number and texture-density are examined. This study strengthens the idea that the Approximate Number System operates only when items are sparse enough to permit spatial segregation. After that limit, ANS gives way to another separate perceptual system responding to texture-density: for sparse stimuli, numerosity – but not element density – can be directly perceived, without being calculated indirectly from other perceptual features. In the fifth and sixth Chapter, I move back to attentional mechanisms, with particular interest in number estimation processes. In the fifth chapter, the role of visual and auditory attentional resources on discrimination thresholds over all the three ranges (subitizing, estimation and texture-density) was investigated. Attentional load greatly affected the subitizing range, to the extent that thresholds became similar to those of the estimation range. More important, numerosities higher than 60-80 dots were more affected by attentional load (both visual and auditory) than lower (non-subitizing) numerosities. Following this, the last empirical chapter assessed the contribution of increasing attentional engagement, instead of decreasing it, on enumeration of numerosity. Here, we examined whether presentation of a visual cue that increased attentional engagement in a given task can facilitate the estimation process, leading to less compressive representation of number in space compared to when attention is diverted elsewhere. Results revealed that enumeration of a collection of dots in the location previously cued led to more precise and accurate (and linear) judgements than enumeration in uncued locations. Overall, this thesis assessed the distinction between intermediate and large numerosity using reaction times, precision, linking elements, and attentional techniques. All studies point to the existence of separate regimes of number perception. Moreover, data on location and object-based attention may suggest that researches focusing on a mechanism that can improve rather than impair enumeration processes could prove helpful when considering rehabilitation in conditions such as dyscalculia. Finally, data on priming effects support an increasing body of evidence that pupillometry can be very useful in tracking perceptual processes, providing information that cannot be gathered from standard psychophysics. Likewise, it would seem that these pupillometry measures may be more sensitive to variations in perceptual styles, and their dependency on personality traits.