Speech is one important faculty that distinguishes humans from the rest of the animal world. Insights into how speech is hardwired into the brain began with Paul Broca’s discovery in the 19th century that the left cerebral hemisphere is primarily involved in the speech process. Over the years, various researchers have found several pathways that explain aspects of speech, but neuroscience now seems to be heading towards a unified theory of speech function in the brain.
Josef Rauschecker, PhD, with co-author Sophie Scott, has reviewed fMRI studies on humans and physiological and anatomical studies on non-human primates, and published his conclusions in the June issue of Nature Neuroscience. fMRI enables scientists to watch activity inside the brain. Animal studies mainly involved a technique called single-cell recording, wherein changes within a single neuron can be observed. The paper suggests that there are two parallel pathways that control speech function in the brain. These speech loops run from lower to higher-function neural regions.
The two pathways—“what” and “where”, as the authors call them—start in the auditory cortex located in the temporal lobe. The “what" pathway processes complex auditory signals and then sends them forward to the “where” pathway. The “where” pathway, located in the parietal lobe just below the temporal lobe, processes the spatial aspect of sound. It also provides feedback in speech production.
Determining the speech and language loops in the brain would help in devising treatments for disorders like autism and schizophrenia.
In 1861, Broca autopsied a patient who had speech problems while alive. He found irregularities in the frontal cortex (Broca’s Area) of the patient’s l
eft hemisphere. Problems associated with Broca’s Area lead to inability in speaking, known as Broca’s aphasia. Patients with Broca’s aphasia can understand what they hear but are unable to speak.
Then another scientist, Wernicke, in 1876, found that damage to the posterior portion of the temporal lobe (Wernicke’s Area) causes problems in comprehending speech.
These two areas of the brain are connected by arcuate fasciculus, a band of nerve fibres. Damage to this area causes conduction aphasia, wherein the patient can understand language but his speech doesn’t make sense.
Various models have been proposed to describe the speech pathway in the brain. American neurologist Norman Geschwind came up with the “connectionist” model in the 1960s. According to this model, the speech loop was centralised—between Broca’s and Wernicke’s Areas. Wernicke’s Area processed auditory signals and relayed them to Broca’s Area, which finally processed speech. Recent studies however seem to contradict this and other older models where the speech function is centralised. They are of view that speech function is spread across various anatomical structures in the brain, rather than being localised at two primary centres.
There’s place for Broca’s and Wernicke’s Areas in the new theory of speech but the authors don’t buy into the theory that these two are the only governing areas. “Essentially we are saying that ‘Wernicke’s Area’ encompasses much more than the posterior superior temporal lobe, which is where people generally put Wernicke’s Area in the brain,” says Scott, Wellcome Senior Fellow, Institute of Cognitive Neuroscience, London. “In terms of Broca’s Area, we show that it is connected to speech perception areas in two ways—receiving bottom-up information from auditory areas and sending projections back to auditory areas.”