The visual cortex was a part of the cerebral cortex, which was a part of the brain.

When Romulans were brainwashing Geordi La Forge they connected him to a device that could send images directly to his visual cortex, thereby making it impossible for La Forge to look away. (TNG: "The Mind's Eye")

Tampering by solanogen-based lifeforms caused elevated levels of serotonin to concentrate in the visual cortex and hippocampus of several crewmembers of the USS Enterprise-D. Doctor Beverly Crusher realized that this was caused by a neuro-sedative used by the aliens to sedate the victims they experimented on. (TNG: "Schisms")

In 2369 Doctor Julian Bashir performed a neural imaging scan to stimulate Chief O'Brien's visual cortex. (DS9: "Babel")

The same year, Ty Kajada asked Julian Bashir to run a retinal imaging scan on Rao Vantika's corpse to confirm there was no residual activity in the Kobliad's visual cortex. (DS9: "The Passenger")