Neural and dopamine

In the brain of a person with schizophrenia the chemical messages seem to work differently. The neurotransmitter that is most widely believed to be involved is dopamine. Dopamine is associated with the sensation of pleasure. and has a vital role in the functioing of different brain systems that could be associated with schizophrenia. HIgh levels of dopamine in the brain has been linked with the positive sympotms of schizophrenia. For schizophrenia sufferes messages from neurons that transmit dopamine fire too easily or too often, leading to hallucinations and delusions. Schizophrenics have abnormally high numbers of D2 receptors on receiving neurons resulting in more dopamine binding and therefore more neurons firing.

The original version of this hypothesis focussed on the role of high levels of dopamine in the subcortex (central areas of the brain).

Broca’s area is located in the frontal lobe and it is responsible for speech production and comprehension. 

There is thought to be an excess of dopamine receptors in Broca’s area for sufferers of schizophrenia which could be associated with speech poverty, a negative symptom of schizophrenia.

Recent versions of the dopamine hypothesis have focussed on abnormal dopamine systems in the brain’s cortex. Goldman-Rakic et al. (2004) identified low levels of dopamine in the prefrontal cortex for schizophrenia sufferers, which is responsible for thinking and decision making. This may then lead to the negative symptoms of schizophrenia. 

A strength of the dopamine hypothesis is that drug therapies support the claim that dopamine is associated with schizophrenia. reatments for schizophrenia change levels of dopamine in the brain and these have been successful in helping people suffering from schizophrenia.

Noll argues that anti-psychotic drugs do not alleviate hallucinations and delusions in about a third of the schizophrenia sufferers who experinece them. He also suggests that for some poeple delsuions and hallucinations are present despite having noraml levels of dopmaine so blocking the D2 receptors would have little effort for these people.

Neural correlates are changes in neuronal events and mechanisms that result in the characteristic symptoms of a behaviour or mental disorder, in this case schizophrenia.

The ventral striatum is a region of the brain believed to be involved in the anticipation of reward and motivation.

Avolition may be linked to this area of the brain as it means that suffer of schizophrenia have a lack of motivation to do everyday tasks and activities and this relates to the anticipation of reward and motivation. 

Juckel et al. found lower levels of activity in the ventral striatum of patients with schizophrenia compared to non-sufferers. They observed a negative correlation between activity in the ventral striatum and the severity of negative symptoms.This suggests activity in the ventral striatum is a neural correlate of negative symptoms of schizophrenia.

Allen et al. scanned the brains of patients experiencing auditory hallucinationsand compared them to non-sufferers of schizophrenia. Both groups had to identify pre-recorded speech as theirs or others. Lower activation levels in the superior temporal gyrus and anterior cingulate gyrus were found in the hallucination group. This suggests, reduced activity in these two areas of the brain is a neural correlate of auditory hallucination.

Abnormality in the ventral striatum may cause negative symptoms. However, it may also be that the negative symptoms themselves cause less activity in the striatum. Therefore, the neural correlate may be an effect of schizophrenia, rather than the cause of it. A third explanation on why there is a connection between negative symptoms and activity in the ventral striatum could be that there is another factor which influences both negative symptoms and the activity in the ventral striatum. Therefore, the existence of neural correlates in schizophrenia actually tells us little in itself. 

There is an overwhelming amount of evidence for all the biological explanations. However, there is also evidence that environmental factors (i.e. dysfunctional childhood) have some influence. Twin studies of monozygotic twins, with at least one twin having schizophrenia, do not have a 100% concordance rate. As monozygotic twins share 100% of their DNA if schizophrenia was purely down to biological factors the concordance rates in these studies would be 100%, as it isn’t this tells us that other factors, possibly environmental, have an influence in causing schizophrenia. Therefore, a combination of biological and environmental factors may help us understand schizophrenia.