Hallucinations are a positive symptom in schizophrenia and psychosis, the most common type of hallucinations are auditory hallucinations, where patients perceive a sound in the absence of stimulation. These auditory hallucinations are mostly verbal hallucinations which means that people hear voices, often from individuals that are important to them (Leudar, Thomas, McNally & Glinski, 1997). The voices typically comment on ongoing-activities (Leudar et al., 1997). Former convictions stated that these voices are perceived as external from the head and are more real than normal voices. This theory was disproved by Junginger and Frame (1985), who assessed self-reports of voice hearers with different diagnoses, because instead he found a negative relation between the perceived location and the clarity of hallucinations.
60% of people with schizophrenia spectrum diagnosis hear voices (Slade & Bentall, 1988) but also 28% with major affective disorder hear voices. Although the majority of patients diagnosed with schizophrenia hear voices, the majority of voice hearers does not have schizophrenia (Ritsher, Lucksted, Otilingam & Grajales, 2004). Despite psychotic diseases, there are other reasons for voice hearing such as drug side effects or spiritual experiences (Ritsher et al., 2004).
The left temporal regions in the superior gyrus, where the auditory cortex is located, are associated with voice hearing (Ritsher et al., 2004). Especially the planum temporale, a region also placed in the superior temporal cortex, which is related to speech production, is known for the origin of voice hearing.
Therefore the planum temporale (PT) as part of the superior and posterior surface of the temporal lobe has received scientific interest in the context of schizophrenia and auditory hallucinations. Normally the PT is bigger in the left than in the right hemisphere (also visible in the picture above) (Geschwind & Levitsky, 1968). In fact, this is the structure with the biggest known lateral asymmetry (Kwon et al., 1999). Falkai (1995) investigated the PT in 24 schizophrenia patients post-mortem and found that this asymmetry is changed in patients suffering from schizophrenia. In fact, the volume of the left PT was 20% reduced in schizophrenia patients. This result supports the theory of disturbed cerebral laterality in schizophrenia patients. Other studies confirmed that the grey matter volume of the left-sided PT is significantly reduced in schizophrenia patients, resulting in a reduced asymmetry (Kwon et al., 1999). Heschl’s gyrus (primary auditory cortex, see picture above TG) remained the same on both brain sides in either group. Moreover, the severity of symptoms on the positive and negative syndrome scale is associated with reduced left PT volume in schizophrenic patients. (Kwon et al., 1999).
The reduced asymmetry in PT is possibly accompanied by thought disorders. Rossi et al. (1994) investigated the different PT volumes of healthy control patients and schizophrenic patients and found a significantly reduced asymmetry in thought disordered patients. Furthermore, there was a greater leftward asymmetry in the sylvian fissure (which is also located in the left temporal lobe). Within hallucinating patients, the leftward asymmetry of the sylvian fissure correlated with hallucinations and thought disorder (Shapleske et al., 2001).
In conclusion, hearing voices in the form of hallucinations is common, especially in schizophrenia. There are indications in the literature that there are anomalies in the brain regions of those affected, where speech and hearing are also located.