L-Phenylalanine confers a number of positive effects on cognitive function, including:
Increased focus and concentration
L-Phenylalanine is has been shown to increase dopamine production in the brain.
While the precise mechanisms are not fully understood, it's clear that the amino acid can freely pass the blood-brain barrier and into the central nervous system.
The prevailing explanation is that when cAMP response element-binding protein (CREB) is activated by L-Phenylalanine, the transcription of the enzyme tyrosine hydroxylase is increased, leading to an increase in dopamine metabolism.