Visual Enumeration and Estimation: Brain mechanisms, Attentional demands and Number representations.

Abstract

The work presented in this thesis explored the roles of attention and number awareness in visual enumeration and estimation through a variety of methods. First, a distinction was made between different attentional modes underlying estimation and enumeration in an in-depth single case study of a patient with simultagnosia. Subsequently I demonstrated that, in visual enumeration, subitizing and counting are dissociable processes and they rely on different brain structures. This was done through a neuropsychological single case study as well as through the first large sample neuropsychological group study using a voxel-based correlation method. Following this, behavioural methods were used to examine the relations between subitizing and estimation. I found that, under conditions encouraging estimation, subitizing is an automatic process and may lead to the exact representation of small numbers, which contrasts with approximate representations for larger numerosities. Finally, a functional MRI study was conducted to highlight the brain regions that are activated for subitizable numerosities, but not for larger numerosities under distributed attention conditions. The imaging study provided converging evidence for automatic subitizing leading to an exact number representation. The last chapter discusses the implications of the contrast between subitization and counting for understanding numerical processing.