There's an interesting case study in The Lancet, about a woman who began hearing voices with speech impairments following a bicycle accident.
The 63-year-old woman was treated at the University Hospital of Psychiatry in Bern, Switzerland, after falling from her bicycle and hitting her head. Following the accident, she suffered a brain hemorrhage and lost consciousness.
Upon her arrival at the hospital, it was found that the woman had an aneurysm (a blood-filled dilation of a blood vessel in the brain). This was treated, and a craniotomy was performed to after tests showed damage in the frontal and temporal lobes. (A craniotomy involves removal of part of the skull, to minimize the risk of further damage due to swelling of the brain.)
7 days after her injury, the woman regained consciousness, and was found to have right-sided hemiplegia (paralysis) and global aphasia (an inability to speak or comprehend language).
After 4 weeks in the hospital's intensive care unit, the woman's paralysis was significantly reduced, but she still had severe speech deficits - she could communicate using single words or incomplete sentences, and could only understand short phrases.
In December 2006, the patient began suffering from auditory hallucinations. She started hearing her thoughts aloud, as well as echoes of earlier conversations, and also hallucinated the voices of hospital staff. These voices appeared several times a day, for a few minutes at a time.
Although the patient understood that the voices were caused by her head injury, she believed them to be real, and heard them coming from a space outside of her body. She reported that they sounded just like other voices, but that they said only very short and simple sentences.
The auditory hallucinations were thought to be caused by epilepsy, and fMRI scans showed that, despite the brain damage, the auditory regions of the temporal cortices would become active when the patient reported hearing the voices.
The patient was discharged just before Christmas 2006; by this time, some residual speech deficit remained, and she was experiencing the auditory hallucinations only once a week. But she was readmitted in March of this year, when the speech deficit worsened.
Electroencephalogram showed that the patient was experiencing epileptic seizures in the left frontal and temporal lobes. She was prescribed sodium valproate to bring the seizures under control. An EEG performed 3 days later showed no seizure activity; the patent's speech deficit had returned to its previous level, and the auditory hallucinations had stopped.
The authors conclude:
[brain injury-induced] epilepsy seems to have caused auditory hallucinations...[which are] widely thought to derive from inner speech that has been misidentified as coming from outside the self, because of defective monitoring. Our patient's experiences are consistent with this hypothesis, since her hallucinations had speech deficits that...were the same as her own.
Reference:
Hubl, D., et al. (2007). Hearing dysphasic voices. Lancet 370: 538.
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This is either a great way to tell people to wear a helmet, or a great way for helmet-denialists to argue they are useless!
We DO have preformed 'maps' in our brains. For example, even before we perform certain action, the brain areas responsible for bringing about this action becomes active. This can not only be visualized by PET Scan, they are forming the basis of thought controlled devices.
Secondly, when we dream, these areas (visual cortex and association fibers) become active, and weave their own magic, using stored memories.
Thus, it is not surprising that the woman's temporal lobe and some portions of the frontal cortex may have been unduly stimulated to 'activate' the already stored sound patterns.
Lastly, borrowing from your own synaesthesia article, I could conjecture that some form of abnormal neural connections may have developed within her own brain. And this perhaps led to interpretation of some other form of sensation into 'sound'. A syndrome (Frey's Syndrome) can lead to a post operative synaesthesia kind of disorder, for example.
When I saw this I thought: "Beautiful... but how come nobody has described this before?". I can't remember any other case of auditory hallucinations in an aphasic.