I have been reading up on paranoia and psychosis during (and after) meth. There's several scientific studies that point to a haywire effect of various unpronounceable chemicals and receptors and interactions in the brain after taking meth, mainly due to a flood of dopamine interfering with flows and regulations of other things, and this leads to a confusion in the brain regarding how to understand external stimuli. It's all inside your head after all - literally. Here's a pull quote and then the link. There's quite a few of these studies if you're interested to have a look around for them.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4105632/
Methamphetamine also increases dopamine release from the VTA to the prefrontal cortex in the mesocortical pathway. At low doses, methamphetamine can increase cognitive performance by activation of D1 receptors in the PFC however high doses impair cognitive function, possibly by over activation of D1 receptors on interneurons in the PFC and excessive inhibition of cortical signals.
Methamphetamine can thus increase glutamatergic signals to the cortex from both the nigrostriatal, as well as the mesolimbic reward circuits, and increase dopaminergic signals from the mesocortical pathway. Excessive glutamate and dopamine in the cortex may overwhelm GABAergic interneuons, causing dysregulation of the signals, which may relate to the psychotic symptoms that can occur during methamphetamine intoxication in some individuals.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4105632/