The specific trial is IIRC one for scientists (or experts more generally?) to take LSD and then think about their field of study expertise while their brainwaves are monitored. Sheldrake signs up enthusiastically, muses on fungal compounds and their effects on the brain and network interconnectedness etc, and as Becca says uses that in the book as a springboard for discussing various known and potential/theoretical aspects of fungi interacting with each other and with animal brains including human ones.