Final answer:
Hallucinogens primarily affect the serotonin neurotransmitter system, with substances like mescaline and LSD activating serotonin receptors, resulting in altered mood, perception, and cognition.
Step-by-step explanation:
Hallucinogens primarily impact the serotonin neurotransmitter system. Substances such as mescaline and LSD are known to be serotonin agonists, meaning that they activate serotonin receptors in the brain. These receptors are involved in a wide variety of psychological and physiological functions, including mood, perception, and cognition. In contrast, substances like PCP and ketamine act as antagonists at the NMDA glutamate receptor, yet they are also considered hallucinogens but with different mechanisms of action. Hallucinogens can cause pronounced changes in sensory and perceptual experiences, leading to their classification within the group of psychedelic drugs.