How can a seizure that starts in the visual cortex spread to both hemispheres? · Grey Matter

A focal seizure in the occipital lobe first produces what that cortex makes, typically coloured lights or shapes, and it can spread along the cortex to neighbouring areas.

How can a seizure that starts in the visual cortex spread to both hemispheres?

A focal seizure in the occipital lobe first produces what that cortex makes, typically coloured lights or shapes, and it can spread along the cortex to neighbouring areas. It becomes bilateral when the discharge recruits networks that reach both hemispheres, above all the loops between cortex and thalamus: imaging during focal to bilateral tonic-clonic seizures shows the thalamus becoming involved at the moment of spread, and the thalamus relays the rhythm to widespread cortex on both sides. How far a given seizure spreads depends on how well inhibition along the way holds.