Accelerated critical neural pathway formation is a medical procedure, a type of genetic engineering, designed to enhance the formation of cognitive pathways in the Human brain. Specifically, the procedure manipulates a person's genetic structure to accelerate the growth of neuronal networks in their cerebral cortex. This can result in tremendously increased intelligence in areas such as analysis, deduction, induction, and theoretical exposition.
There is also, however, a high likelihood of dramatic and often unpredictable side effects on the personality of the patient. These can include, but may not be limited too: anti-social personality, violent tendencies, a reversion to child-like states of interaction and the appearance of a catatonic state.
Because DNA resequencing of sentient beings for any reason other than repairing serious birth defects is illegal in the Federation, those who have undergone the procedure are unable to join Starfleet - in part as a result of the Augments, most notably Khan Noonien Singh. However Julian Bashir was able to enter Starfleet Medical because he and his parents covered up his medical history. (DS9: "Doctor Bashir, I Presume")
In 2374 Doctor Karen Loews took several of her patients, all of whom had undergone accelerated critical neural pathway formation, to Deep Space 9 for interaction with Dr. Bashir. Dr. Lowes believed that because Bashir had also undergone genetic engineering, but without the associated side-effects, that he might be able to help some of her patients relate to the outside world in a normal fashion. (DS9: "Statistical Probabilities")