The most likely problem is reduced vertebral artery blood flow to the brain stem and part of the brain from inflammation. The sleep problems associated with whiplash injuries can be a big cause of those symptoms, too. Sleep apnea can be a cause. Chronic sleep and blood flow problems can cause atrophy of the brain cells over time.

Then there is the direct physiological damage to axons (diffuse axonal injury) that reduces the function of the communication structure in the brain. Think of wires that have poor insulation or are corroded or a leaking hose.