Accelerated critical neural pathway formation was a medical procedure, a type of genetic engineering, designed to enhance the formation of cognitive pathways in the Human brain. Specifically, the procedure manipulated a person's genetic structure to accelerate the growth of neuronal networks in their cerebral cortex. This could result in tremendously increased intelligence in areas such as analysis, deduction, induction, and theoretical exposition.
There was also, however, a high likelihood of dramatic and often unpredictable side effects on the personality of the patient. These included, but may not have been limited to, 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 was illegal in the Federation, those who have undergone the procedure were 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 childhood medical history; he had had the procedure performed at the age of seven in order to correct a condition which caused him several disadvantages.
According to Bashir, his procedure had several beneficial effects: "My IQ jumped five points a day for over two weeks. Followed by improvements in my hand-eye coordination, stamina, vision, reflexes, weight, height. In the end, everything but my name was altered in some way."(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, he might be able to help some of her patients relate to the outside world in a normal fashion. (DS9: "Statistical Probabilities")