Positive symptoms of schizophrenia, specifically hallucinations and delusions, are thought to be caused by hyperactivity of which neurotransmitter?

Dopamine.

Explanation:

Schizophrenia is a mental illness characterized by psychotic episodes and the inability to relate thoughts, feelings and actions, accompanied by delusions, hallucinations (auditory, visual, olfactory and tactile), incoherent thoughts, memory disorders and, in occasions, confusion. Dopamine is a hormone synthesized by the brain that acts as a neurotransmitter and allows the connection between neurons. Studies suggest that some of these alterations in mental functions are caused by an excess in the activity of dopaminergic synapses, that is, there is a hyperactivity in those connections where dopamine is produced and expelled, which range from the ventral tegmental area to the nucleus accumbens and the amygdala, which are the main ones responsible for synthesizing and expelling dopamine from brain reward systems.