544 CHAPTER 12
Dopamine hypothesis
The view that schizophrenia arises from an
overproduction of dopamine or an increase
in the number or sensitivity of dopamine
receptors.Another biological marker for schizophrenia is sensory gating (Freedman et al.,
1996), which is assessed as follows: Participants hear two clicks, one immediately
after the other. Normally, the brain responds less strongly to the second click than
to the fi rst. However, people with schizophrenia and their fi rst-degree relatives don’t
show the normal large drop in the brain’s response to the second click. This has
been interpreted as a manifestation of the diffi culties that people with schizophrenia
can have in fi ltering out unimportant stimuli.
A third type of biological marker for schizophrenia has been reported by re-
searchers who performed careful analyses of home movies of children who were later
diagnosed with schizophrenia (Grimes & Walker, 1994; Walker et al., 1993). They
found that the children who went on to develop schizophrenia were different from
their siblings: They made more involuntary movements, such as writhing or excessive
movements of the tongue, lips, or arms. This tendency for involuntary movement was
particularly evident from birth to age 2, but could be seen even through adolescence
in individuals who later developed the disorder (Walker, Savoie, & Davis, 1994).
Moreover, those who displayed more severe movements of this type later developed
more severe symptoms of schizophrenia (Neumann & Walker, 1996).
These researchers also found that the children who displayed more involuntary
movements also cried more when coming into contact with adults. Similar results were
reported by researchers who studied Finnish people with schizophrenia. Looking back
at the patients’ school performance, these researchers found that the children who went
on to develop schizophrenia did not do signifi cantly worse academically than other
children—but they were signifi cantly worse at sports and handicrafts, both of which
require motor coordination (Cannon et al., 1999). Further support for the relation-
ship between movement abnormalities and schizophrenia comes from a study in which
researchers compared the extent of abnormal movements in two groups of people:
One group consisted of people with schizotypal personality disorder—a disorder on
the schizophrenia spectrum—and the other group was composed of healthy control
participants or people who had another type of personality disorder. Only people with
schizotypal personality disorder had abnormal movements (Mittal et al., 2008).
This said, we note that not everyone who is clumsy or uncoordinated goes
on to develop schizophrenia. Biological markers are correlated with underlying
problems—but sometimes can also arise for other reasons.Neural Communication
Schizophrenia is likely to involve a complex interplay of many brain systems and neu-
rotransmitters, including dopamine, serotonin, and glutamate, as well as the stress hor-
mone, cortisol. Let’s look more closely at these neurotransmitter substances and their
interactions, as well as the possibility that sex hormones may play a role in this disorder.Dopamine
One neurotransmitter that is clearly involved in schizophrenia is dopamine: Neu-
roimaging studies of people with schizophrenia have found abnormally low numbers
of dopamine receptors in their frontal lobes (Okubo et al., 1997), as well as increased
production of dopamine (possibly to compensate for the reduced numbers of recep-
tors in the frontal lobes) in the striatum (parts of the basal ganglia that produce
dopamine; Heinz, 2000). The dopamine hypothesis proposes that an overproduc-
tion of dopamine or an increase in the number or sensitivity of dopamine receptors
is responsible for schizophrenia. According to this hypothesis, the excess dopamine
or extra sensitivity to this neurotransmitter triggers a fl ood of unrelated thoughts,
feelings, and perceptions. Delusions are then attempts to organize these disconnected
events into a coherent, understandable experience (Gottesman, 1991; Kapur, 2003).
This hypothesis was supported by the discovery that medications that decrease do-
pamine levels also reduce the positive symptoms of schizophrenia (but not the negative
symptoms; Rosenbaum et al., 2005). Further support for the dopamine hypothesis
comes from studies of people who do not have schizophrenia: When they are given
LSD, an hallucinogenic that increases dopamine activity (as well as blocking theThe child in this photo went on to develop
schizophrenia. Notice the unusual hand postures
(indicating involuntary movement) of the
child; studies of home movies of the child who
later developed schizophrenia revealed many
such involuntary movements, particularly of the
tongue, lips, or arms.Walker © 1994. Used by permission of Oxford University Press.