Why aren鈥檛 we equally capable with our left and right sides? Wouldn鈥檛 this have given us a survival advantage?
Tony Seikel
Inkom, Idaho, US
Advertisement
Non-humans also demonstrate laterality of function, in which one side of the body has different 鈥渟pecialisations鈥� than the other.
This lateralisation occurs because brain areas responsible for those activities are specific to one hemisphere or another. We talk about 鈥渉emispheric dominance鈥� in the sense that, for instance, the left brain hemisphere in 85 per cent of humans governs language and fine-motor function. While one hemisphere may be dominant in a specific task, that hemisphere has come to be seen as generally dominant, such that we often think of the left hemisphere as dominating (and therefore being 鈥渂etter鈥� than) the right. This translates into an unspoken sense of hemispheric superiority.
In humans, as well as fine-motor and language function, the dominant hemisphere controls many important cognitive functions, such as categorisation and linear analysis. In contrast, the non-dominant hemisphere has responsibility for integrating and synthesising the activities and processes of the dominant hemisphere. For instance, the dominant (typically left-hemisphere) fusiform gyrus, a structure of the brain, can identify and label all the parts of faces, but the fusiform gyrus in the non-dominant (typically right) hemisphere is apparently responsible for putting them together to make a recognisable face. Without the non-dominant hemisphere, the dominant side knows and can list off the parts, but can鈥檛 make sense of the whole.
And it is more than 鈥渃an鈥檛 make sense of the whole鈥�: those who work with people who have had a non-dominant-hemisphere stroke know that those who have 鈥渘ormal鈥� left-hemisphere function and a disordered right hemisphere can have wide-ranging conditions, including not realising they can鈥檛 make sense of the whole, and not even realising something is wrong.
These are collectively referred to as misidentification syndromes and include deficits in spatial awareness (hemispatial neglect), and significant language and reading conditions related to the lack of the availability of context provided by the right hemisphere.
In hemispatial neglect, the individual is simply unaware of the presence of the left half of their body (left neglect) or the left visual field until it is brought to their attention. Once their attention flags, that portion of their body or visual field is no longer available to consciousness. It is possible that contextualisation itself is an evolutionary benefit derived from differentiation of function, in that it complements and supports the more active processes.
To come back to the question, we have a cultural assumption that doing (dominant side) is important and being (non-dominant side) is less so. The availability of a synthesising system to complement the dominant doing side may have given us a competitive advantage in itself. We are, after all, human beings, not 鈥渉uman doings鈥�!
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
快猫短视频 retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.