New voices: to the left - pseudoneglect
If you are a Beyoncé fan, you will know that one of her best-known songs, ‘Irreplaceable’, opens with ‘To the left, to the left, everything you own in a box to the left’. But why don’t these lyrics read ‘To the right, to the right, everything you own in a box to the right?’ Is it just pure coincidence that the writers of the song choose the left instead of the right? Answering this question – in the broader context of cognition and not just American rhythm and blues – has been a main focus of mine for the past few years. In the hope of igniting your curiosity, allow me to outline a little more about how the left side of space captures our attention.
The tendency to pay more attention to the left side of space is best demonstrated when participants are asked to perform the very simple task of bisecting a visually presented horizontal line at its centre (for a comprehensive review on visuospatial pseudoneglect see Jewell & McCourt, 2000). When participants are asked to do this they typically bisect the line slightly leftward of the true centre. This phenomenon has been referred to as ‘pseudoneglect’ (Bowers & Heilman, 1980) by analogy to the performance of right hemisphere impaired patients who neglect the left side of space, bump into objects and people located on the left and fail to groom the left side of their bodies (Halligan et al., 2003). Pseudoneglect, unlike neglect, does not have such severe consequences for everyday life but is a robust and consistent bias and has even been demonstrated in non-human animals such as birds (Regolin, 2006).
One of the most interesting aspects of pseudoneglect is that it can be observed when vision is completely removed. In one notable study healthy adult participants living in Milan, Italy, were asked to imagine the Piazza del Duomo – a typical Italian shopping square with a cathedral, cafés and shops – and recall landmarks from each side of the square. It was found that participants reported more items from the left-hand side of the Piazza del Duomo than from the right-hand side – regardless of imagined viewpoint (McGeorge et al., 2007). For those of you who are wondering, the Piazza del Duomo findings are not distinct to Italy. Similar trends have since been reported for the mental representation of scenes in France (Bourlon et al., 2010) and Canada (Friedman et al., 2012). These findings suggest that the tendency to err left is not simply a function of direct visual processing. Rather, the bias may occur when we are activating highly familiar material from long-term memory. But it gets even more interesting, because a similar effect can be seen for completely novel stimuli as well.
In a study conducted with colleagues at the Department of Psychology, University of Edinburgh, participants were asked to listen to an aural-verbal description of a novel pattern stimulus which was described as having either ‘filled-in’ or ‘empty’ squares (Figure 1). Participants were asked to mentally represent the pattern using a visuospatial template before reporting which side of the pattern had the most filled-in cells, left or right. Participants reported that there were more filled-in cells on the left-hand side compared to the right-hand side of the pattern and they were significantly more certain about decisions made for the left compared to the right (Brooks, Logie et al., 2011). In this case, the bias occurred for the mental representation of a stimulus temporarily held in working memory. We also found similar results when participants were asked to conduct the same task but create, from aural-verbal description, mental representations of real-world street scenes containing landmarks such as ‘café’, ‘bank’, ‘shop’ to be imagined on the left- and right-hand side (Brooks & Brandimonte, 2014). In both these studies, participants were given a visuospatial template of a pattern or street to provide a structure for mentally organising the landmarks, but they were free to represent each individual landmark in whichever format they chose, in terms of colour, size or dimension. Regardless of this ‘scaffolding’ for building the mental representation, pseudoneglect was still observed. But the story doesn’t end there.
Representational pseudoneglect is also demonstrated on tactile rod bisection, a task that involves feeling the length of a wooden rod using one’s index finger. When tactile rod bisection task is conducted in the complete absence of visual processing, a mental representation of the stimulus is built from touch alone. There are many examples of tactile rod bisection in the literature (e.g. Baek et al., 2002; Bowers & Heilman, 1980), and in collaboration with colleagues at the University of Edinburgh I conducted one of the largest tactile rod bisection studies in the absence of vision. Over 600 healthy participants aged between 6 and 96 years of age were asked to explore the length of a series of wooden rods with their right index finger and indicate the middle of the rod in the complete absence of vision (i.e. they were blindfolded). Across the full adult lifespan there was a tendency to err leftward of the veridical centre, and this bias increased with age (Brooks, Della Sala et al., 2011). The fact that blind participants have also been shown to demonstrate leftward biases on tactile rod bisection (Cattaneo et al., 2010; Cattaneo et al., 2011) is further evidence of a purely representational form of pseudoneglect.
So, representational pseudoneglect is an interesting bias, but what are the cognitive mechanisms behind it?
It is well known that different cognitive functions are lateralised to the left and right sides of the human brain. This makes sense from an evolutionary perspective: lateralisation may be a protective factor; so when one part of the brain is impaired, through traumatic injury or stroke, cognitive function within that particular region may be ‘knocked out’ but spared in another region. Spatial attention happens to be lateralised to the right parietal lobe as indicated by neuroimaging and neuroconduction studies with healthy adult participants (for review see Brooks et al., 2014). The fact that right-parietal-lobe-impaired patients cannot freely direct attention to the left side of space (hence the term ‘left neglect’) supports these findings. The system within the right parietal lobe that directs attention leftward may have evolved as a result of the well-known anatomical connections between hemisphere (right vs. left) and hemispace (left vs. right). This system has been called ‘attentional orienting’ (Reuter-Lorenz et al., 1990). It seems that the theory of attentional orienting can readily account for both visual and representational forms of pseudoneglect; the mechanisms that underlie this system may be activated during direct visuospatial processing or the mental representation of spatial layout.
Another fascinating outcome of our recent research is that representational pseudoneglect increases with age (Brooks, Della Sala et al., 2011). This suggests that attentional orienting remains intact in older age – but this finding doesn’t fit neatly into current models of cognitive ageing. A widely accepted theory is that as our brains age there is less lateralised and more bilateral recruitment of the hemispheres for a given task (Cabeza, 2002). For younger adults, a motor task that involves activating the left or right hand may lead to right- or left-lateralised hemispheric activity respectively, but for older adults who perform the same task we may expect to see this activity spread out across the two hemispheres. Current models of cognitive ageing may thus need to be extended in order to account for our findings and others like this that may emerge in the future.At first glance, pseudoneglect may seem to be an idiosyncrasy of spatial attention, an unusual and unique feature. But it is also a robust and consistent behavioural phenomenon that occurs with and without direct visual processing. Consider a jigsaw puzzle: pseudoneglect is an integral part of spatial attention, which in turn is an integral part of the right parietal lobe, which in turn is an integral part of the human brain.
Understanding each part of the human brain and how these parts fit together is the aim of cognitive neuroscience. Ultimately, if we can build a comprehensive model of the human brain then we may learn how to protect and repair it from disease, injury or damage.
As Steve Jobs said in his famous TED talk, ‘you can’t connect the dots looking forward, you can only connect them looking backwards’. One day in the future the dots will connect and reveal an important application of pseudoneglect in both its visual and representational form. Speaking of looking back, I still find myself drawing on previous research with colleagues in other fields (Ellis et al., 2005; Mattys et al., 2009). It’s funny how you can ‘connect the dots’ to allow one area of your research to inform the other.
Recently I have been exploring representational forms of pseudoneglect across the full adult lifespan. Watch this space for some innovative research in this field with children, mid-age and older adults. We are also aiming to understand people’s own perception of their spatial biases and the strategies that they might use to overcome them. Anecdotal evidence from our studies suggests that the tendency to pay more attention to the left side of space seems to be subtle, subliminal and not obvious to the person demonstrating it.
Taken together the studies noted here, and these are just a few, indicate that representational pseudoneglect is an interesting phenomenon and deserving of significantly more attention in the field of cognitive science. Were the writers of ‘Irreplaceable’ displaying representational pseudoneglect when they wrote the song? I’ll let you decide.
Joanna L. Brooks is at
the Centre for Research on Ageing, Health and Wellbeing, Australian National University
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