One way of describing it is that the brain is experiencing a case of non-shared deception. I say, "non-shared deception", for the following reasons....
People see cars stop, move, move faster, slow down, and stop again. However, a simple analysis of "motion", via the use of nothing but the mind, soon leads you to seeing the fact that all cars are constantly in motion, and that they never stop moving. This constant motion applies to all objects. All that can be done is change the direction of this constant motion as it is constantly on the go within a 4 dimensional structure known as Space-Time.
This simple ABC analysis of motion soon has you independently understanding Special Relativity, and it also has you independently, and successfully, creating all of the Special Relativity equations with absolute ease.
(To watch this easy breezy analysis of motion in action, goto youtube videos 1-9 at http://goo.gl/fz4R0I )
Thus, those who see objects, such as cars, speed up and slow down etc., are those whom are obviously "Seeing something that isn't really there.". They are experiencing an hallucination.
Now if the outward extension of the mind is reduced to an even smaller scale, via drugs or whatever, then the magnitude of the ongoing hallucinations are magnified even further, magnified to an even greater scale than the hallucinations that are being shared on a global scale.