As you fall asleep your brain activity may shut off half of the world.
This pattern of brain activity is very similar to a behavioural syndrome displayed after brain injury, called spatial neglect.
Spatial neglect is characterised by a lack of awareness to one side of the body and its surrounding space. This is more likely to occur when one hemisphere is damaged within the brain. For example, if damage occurs within the left hemisphere, patients lose awareness of their right-hand side of space and vice versa. Those with spatial neglect tend to only eat food from one side of the plate, draw incomplete pictures and dress only one side of the body. Stimuli in the affected area of space does not seem to register within the individual’s conscious awareness – they act as if that side of space doesn’t exist at all.
Interestingly, a rightward attentional bias has been identified in previous studies of spatial neglect – patients are more likely to neglect stimuli on the left-hand side of space then the right. It is unclear why this occurs but many have theorised that it is due down to the right hemisphere’s role in spatial cognition.
The right hemisphere has also been implicated in alertness and attention – this provides further evidence for the hemisphere’s influence in spatial neglect. In 2005 the MRC Cognition and Brain Sciences Unit in Cambridge conducted a study which discovered that decreased alertness dramatically altered spatial awareness in healthy people by shifting attention to the right (remember that funny rightward bias?). This effect had been found before where volunteers in a state of reduced alertness (such as sleep deprivation) also displayed a rightward attentional bias.
This is pretty important as it shows spatial neglect can be mirrored and studied within the general population, without the limitations which are usually associated with studies on those with brain injury (for example, generalising the results to the wider population).
To investigate this spatial awareness – alertness link further, researchers recruited 26 healthy participants.
Participants then performed an auditory spatial localisation task whilst transitioning in and out of sleep within a darkened room on a reclining chair. They were presented with a series of randomised tones located either to the left or right-hand side of their body and were asked to report as quickly as possible where the tone was, by pushing one of two buttons. Drowsiness was measured by participant’s reaction times and EEG recordings, providing a direct measure of their alertness levels (something which hadn’t been done in previous studies).
This would not only shed more light on an spatial awareness – alertness link but also on the changes in consciousness before sleep onset.
The error rate recorded for left tones during drowsiness was 24.67% – this decreased to 13.94% during alert performance. Participants were also 17 times more likely to show a rightward attentional shift with drowsiness on a task than leftward shift, or no shift at all. Overall, the degree to which participants displayed an increase in left tone errors was significantly correlated with levels of drowsiness reported by Hori scores (operationalised scale of sleep activity), reaction times and the number of trails without any response, indicating sleep onset.
Researchers concluded that a decreased alertness in healthy participants was associated with a marked asymmetric increase in error rates to auditory stimuli presented in the left-hand space. It mirrored spatial neglect normally shown in patients after a stroke and confirmed the rightward attentional shift previously found in a healthy population.
Why does this happen? Well, spatial awareness has been thought of as a competition between cerebral hemispheres. Therefore, during spatial neglect the ‘intact’ hemisphere may dominate the impaired ‘rival’ hemisphere’s spatial awareness. Thus, if the right hemisphere is damaged, the left hemisphere will have greater influence over spatial cognition and attention, thus explaining the rightward shift in attention. This is further evidenced by the popularity of spatial neglect after unilateral (one-sided) brain injury from stroke.
Therefore, any state of reduced alertness or sleepiness should produce a detectable shift in spatial bias.
Another possible explanation has also been proposed. As you fall asleep the connectivity between frontoparietal and frontotemporal networks fade early on, reducing higher level executive control functions. Therefore, even though participants may respond to stimuli they may not integrate all of its features, including location. If this is the case, it would reveal a rightward attentional bias in normal human spatial attention with an unknown cause, calling for future research.
This study was particularly useful as it extended the literature by showing that rightward attention shifts in healthy individuals can occur in auditory as well as visual modalities. It also points towards another therapeutic target for those with spatial neglect – maintenance of alertness levels.
It also shows, that with the correct methodologies and out-of-the-box thinking, we can study brain injury within the general population, producing deeper insights about the functioning of the brain and consciousness.
Pretty impressive ‘huh?
[ Losing the left side of the world: Rightward shift in human spatial attention with sleep onset. (2014) Corinne A. Bareham, Tom Manly, Olga V. Pustovaya,Sophie K. Scott & Tristan A. Bekinschtein. Scientific Reports 4, Article number: 5092 doi:10.1038/srep05092 ]