Lysergic acid diethylamide (LSD) increases primary process thinking—an early, implicit, associative, and automatic mode of thinking typical of dreaming—via activation of serotonin 2A (5-HT2A) receptors. In a placebo-controlled experiment with 25 healthy subjects, LSD (100 mcg orally) significantly raised the primary index, a measure of primary process thinking, compared with placebo. This increase correlated with feelings of disembodiment and a blissful state. Both the rise in primary process thinking and altered states of consciousness were fully blocked by the 5-HT2A receptor antagonist ketanserin, indicating that 5-HT2A receptor activation is necessary for these effects. Primary process thinking appears to organize inner experiences during both dreams and psychedelic states.
Lysergic acid diethylamide (LSD) produces waking mental imagery that resembles dreaming, an effect driven by activation of the 5-HT2A receptor. In a study with 25 healthy subjects, LSD (100 mcg orally) significantly increased cognitive bizarreness in guided mental imagery reports compared with placebo, and this increase correlated with a loss of self-boundaries and cognitive control. Both the imagery changes and altered state of consciousness were fully blocked by the 5-HT2A antagonist ketanserin (40 mg orally). The findings suggest that LSD-induced dreamlike imagery depends specifically on 5-HT2A receptor activation.