Numerical processing overcomes left neglect for the greyscales task

Exploring bias in horizontal and vertical spatial representations using mental number lines and the greyscales task

People have a leftward bias when making visuospatial judgements about horizontally arranged stimuli ("pseudoneglect"), and a superior bias when making visuospatial judgements about vertically arranged stimuli. The leftward visuospatial bias in physical space seems to extend to the mental representation of space. However, whether any bias exists in mental representation of vertical space is unknown. We investigated whether people show a visuospatial bias in the mental representation of vertical space, and if any bias in mental representations of horizontal and vertical space related to the extent of bias in physical space. Participants (n = 171) were presented with three numbers and asked which interval was smaller/larger (counterbalanced): the interval between the first and middle, or middle and last number. Participants were instructed to either think of the numbers as houses on a street or as floors of a building, or were given no imagery instructions. Participants in the houses on a street condition showed a leftward bias, but there was no superior bias in the floors of a building condition. In contrast, we replicated previous findings of leftward and superior bias on greyscales tasks. Our findings reinforce previous evidence that numbers are represented horizontally and ascending left to right by default.

Visuospatial perception is not affected by self-related information

Previous research suggests that attention is drawn by self-related information. Three online experiments were conducted to investigate whether self-related stimuli alter visuospatial perceptual judgments. In a matching task, associations were learned between labels ('Yourself'/friend/stranger's name) paired with cues. Cues were coloured outlines (Experiment 1, N = 135), geometric shapes (Experiment 2, N = 102), or coloured gradients (Experiment 3, N = 110). Visuospatial perception bias was measured with a greyscales task. Cues were presented prior to, and/or alongside greyscales. We hypothesized there would be a bias towards the self-related cue. In all experiments, we found a self-related bias in the matching task. Furthermore, there was an overall leftward visuospatial perceptual bias (pseudoneglect). However, we found anecdotal to moderate evidence for the absence of an effect of self-related cues on visuospatial perception judgments. Although self-related stimuli influence how our attention is oriented to stimuli, attention mechanisms that influence perceptual judgements are seemingly not affected by a self-bias.

Extra-powerful on the visuo-perceptual space, but variable on the number space: Different effects of optokinetic stimulation in neglect patients

We studied the effects of optokinetic stimulation (OKS; leftward, rightward, control) on the visuo-perceptual and number space, in the same sample, during line bisection and mental number interval bisection tasks. To this end, we tested six patients with right-hemisphere damage and neglect, six patients with right-hemisphere damage but without neglect, and six neurologically healthy participants. In patients with neglect, we found a strong effect of leftward OKS on line bisection, but not on mental number interval bisection. We suggest that OKS influences the number space only under specific conditions.

Representational pseudoneglect: a review

Pseudoneglect, the tendency to be biased towards the left-hand side of space, is a robust and consistent behavioural observation best demonstrated on the task of visuospatial line bisection, where participants are asked to centrally bisect visually presented horizontal lines at the perceived centre. A number of studies have revealed that a representational form of pseudoneglect exists, occurring when participants are asked to either mentally represent a stimulus or explore a stimulus using touch in the complete absence of direct visuospatial processing. Despite the growing number of studies that have demonstrated representational pseudoneglect there exists no current and comprehensive review of these findings and no discussion of a theoretical framework into which these findings may fall. An important gap in the current representational pseudoneglect literature is a discussion of the developmental trajectory of the bias. The focus of the current review is to outline studies that have observed representational pseudoneglect in healthy participants, consider a theoretical framework for these observations, and address the impact of lifespan factors such as cognitive ageing on the phenomenon.

Optokinetic stimulation modulates neglect for the number space: evidence from mental number interval bisection

Behavioral, neuropsychological, and neuroimaging data support the idea that numbers are represented along a mental number line (MNL), an analogical, visuospatial representation of number magnitude. The MNL is left-to-right oriented in Western cultures, with small numbers on the left and larger numbers on the right. Left neglect patients are impaired in the mental bisection of numerical intervals, with a bias toward larger numbers that are relatively to the right on the MNL. In the present study we investigated the effects of optokinetic stimulation (OKS) - a technique inducing visuospatial attention shifts by means of activation of the optokinetic nystagmus - on number interval bisection. One patient with left neglect following right-hemisphere stroke (BG) and four control patients with right-hemisphere damage, but without neglect, performed the number interval bisection task in three conditions of OKS: static, leftward, and rightward. In the static condition, BG misbisected to the right of the true midpoint. BG misbisected to the left following leftward OKS, and again to the right of the midpoint following rightward OKS. Moreover, the variability of BG's performance was smaller following both leftward and rightward OKS, suggesting that the attentional bias induced by OKS reduced the "indifference zone" that is thought to underlie the length effect reported in bisection tasks. We argue that shifts of visuospatial attention, induced by OKS, may affect number interval bisection, thereby revealing an interaction between the processing of the perceptual space and the processing of the number space.

The middle range of the number line orients attention to the left side of visual space

Mental representation of numbers is believed to be spatial in nature, with small numbers occupying the left and large numbers the right side of a putative mental number line. Consistent with this, presentation of numbers from the low and high ends of the mental number line induces covert shifts of spatial attention to the left and right side of visual space, respectively. However, the effect of the presentation of the middle range (containing numbers below and above the midpoint) of the number line on visual perception has so far not been studied. Here we show in two experiments, using a line bisection task and a simple target detection task, that processing of middle-range numbers affects allocation of visuospatial attention in a similar way as processing of small numbers, with attention shifted to the left side of space. We suggest that this pattern of results arises due to "anchoring" heuristics that participants use in number processing.