For some time now we’ve known that in the brains of humans, monkeys and cats, visual information is processed by two separate streams – one for working out where things are and the other for processing what they are. Now Stephen Lomber and Shveta Malhotra have conducted an experiment on cats and provided perhaps the strongest evidence to date that, in the mammalian brain, sounds too are processed via two separate “what” and “where” streams.
Lomber and Malhotra used a new cooling method to reversibly knock-out specific areas of the cats’ auditory cortex – the part of the brain used for processing sound. The new technique involves surgically implanting small tubes into the cat’s brain, through which chilled menthol is passed. In mammals, communication between brain cells stops when temperatures drop below 20 degrees Celsius, so cooling of the implanted tubes can be used to inhibit activity in a chosen localised brain region.
Tests on three cats showed that cooling of the more frontal part of their auditory cortex impaired their ability to localise sounds (the “where” function), but didn’t affect their ability to discriminate between sounds (the “what function”). By contrast, cooling of a rear part of the auditory cortex had the opposite effect: it impaired the cats’ ability to discriminate sounds, but didn’t affect their sound localisation skills.
This pattern of results is known as a double dissociation and is the gold standard test in classic cognitive neuropsychology for demonstrating that two separate brain regions are responsible for independent functions. Before now, the evidence for “what” and “where” pathways in the auditory cortex was far weaker, having been based largely on recordings of single cell activity in monkeys or brain imaging in humans.
In a commentary on this new research, Christian Sumner and colleagues agree that this is strong evidence, but they caution that the complete picture may turn out to be more complicated. “‘What’ and ‘where’ are appealing concepts,” they wrote, “but it seems probable that cortical processing is more refined and more plastic.”
Lomber, S.G., Malhotra, S. (2008). Double dissociation of ‘what’ and ‘where’ processing in auditory cortex. Nature Neuroscience, 11(5), 609-616. DOI: 10.1038/nn.2108