THE ROUGH GUIDE TO PSYCHOLOGYspheric dominance of 113 participants by asking them to mark the
midpoint of a horizontal line. A left-ward bias in this task is a sign of a
dominant right hemisphere. Subsequent brain scans of the participants
showed that those with a more dominant right hemisphere, whether
male or female or right- or left-handed, tended to have a ten percent
thicker callosum, on average, than participants with a more dominant
left hemisphere.
With the brain sliced in half, other regions buried beneath the cere-
bral cortex become visible. Starting from the brain stem and working
upwards, we find regions like the pons and medulla, which regulate basic
survival functions, including breathing and heart rate, as well as reflexes
like sneezing and being sick. Next is the mid-brain, which includes
basic sensory centres such as the superior colliculi. For many animals,
this is the hub of their visual processing, whereas we have evolved addi-
tional cortical visual pathways. However, one of our visual pathways is
still relayed straight to the superior colliculi for rapid, subconscious
processing, and one day its speed might save your life. If I hurled a ball
in your direction and you amazed yourself and me by catching it before
you’d even consciously registered what was happening, then you can be
pretty sure that feat was thanks to your superior colliculi.
Upwards and forwards, we find the thalamus – the brain’s great
relay station – and the hypothalamus, which is involved in releasingmidbrainmedulla oblongatahypothalamuslimbic systemcorpus
callosum
thalamuspons