The mental representation of ordered sequences in visual neglect

Abstract

Recently, left spatial neglect phenomena have been reported for the mental representation of numbers, i.e. the mental number line (Zorzi et al., 2002; Vuilleumier et al., 2004). The present study aimed at replicating the findings of Zorzi et al. (2002) in a number bisection task with small numbers and at extending the investigation to larger numbers up to 100, letters in the alphabet, days of the week and months of the year. Five patients with left spatial neglect and 20 healthy control subjects performed four mental bisection tasks (numbers, letters, days, and months tasks) where participants were asked to estimate the midpoint of a verbally presented interval. Results showed that left spatial neglect may disrupt the mental bisection of numerical intervals when small numbers have to be processed. Size of the numerical interval affected patients' performance in number range 1-9, i.e. longer intervals led to right-sided errors whereas the smallest interval did not (as in Zorzi et al., 2002). Spatial mental representations may thus exist for small numbers, while for larger numbers no consistent pattern was found. In non-numerical bisection tasks a spatial bias was found in letters and days sequences, but not in the months sequence. Patients made more shifts to letters "later" in the alphabet in the letter bisection task and more shifts to days earlier in the week in the day bisection task. It could be argued that shifts to days earlier may correspond to right-sided shifts, when the representation of days in the week is cyclic as hours on a clock.