Losing the left side of the world: rightward shift in human spatial attention with sleep onset

Abstract

Unilateral brain damage can lead to a striking deficit in awareness of stimuli on one side of space called Spatial Neglect. Patient studies show that neglect of the left is markedly more persistent than of the right and that its severity increases under states of low alertness. There have been suggestions that this alertness-spatial awareness link may be detectable in the general population. Here, healthy human volunteers performed an auditory spatial localisation task whilst transitioning in and out of sleep. We show, using independent electroencephalographic measures, that normal drowsiness is linked with a remarkable unidirectional tendency to mislocate left-sided stimuli to the right. The effect may form a useful healthy model of neglect and help in understanding why leftward inattention is disproportionately persistent after brain injury. The results also cast light on marked changes in conscious experience before full sleep onset.

Figure 1

(A.) Identical tones were presented at angles between 0° and 60° from, and equidistant to, the participants' midline. (B.) Error rates on tones within different bands of laterality. Negative degrees are to the left of midpoint, positive are to the right of midpoint. Drowsiness is associated with a marked rise in errors for left tones, including those lateralised as far as 40°–60° to the left. Of note, the tones within 15–60° of laterality are only biased during the drowsy periods – with more left than right errors. (C.) Participants made significantly more left-tone errors than right-tone errors in the drowsy quartile, whilst a similar of left and right tone errors were made in the alert quartile. Error bars represent the standard errors of the means (Alert left: ±.021; Alert right: ±.021; Drowsy left: ±.030; Drowsy right: ±.020).

Figure 2. Individual bias scores (proportions left – right tone errors/total errors) during alert and drowsy quartiles.

Whether left or right biased initially, almost all the participants (21 of 26) demonstrate a rightward shift in error performance at drowsy periods.

Figure 3. Individual EEG theta: alpha ratio traces, reaction times and error pattern from a single participant across each trial of the session.

Early trials are characterised by alertness, relatively fast reaction times and few errors on both left and right tones. By 20-minutes rising drowsiness is following by increased reaction times and a marked increase in errors, almost exclusively on left-tones, including those 15–60° to the left.

Figure 4. Proportion of left and right errors for trials within each Hori Scale Sleep stage.

From Hori Scale 4 left error proportions begin to increase incrementally whilst right error proportions do not show substantial changes with Hori Stage progression.