CamKII positive autofeedbackloop
One proposed mechanism of memory formation and learning is activity dependent strengthening of synapses. Long term potentiation or LTP is one theses mechanisms.
Calcium/Calmodulin -dependent protein Kinase II is a protein present in the brain and activated during LTP. Once activated, it auto-phosphorylates once this positive autofeedback is tought one of the causes of LTP and memory formation.