Movement is an important element for our perception of the world. Helmholtz (1986) proposed that we see movement by taking a copy of the move muscle signal (efference copy) and compare it with the retinal image motion, and this brain signal theory has been supported from findings by Stevens et al. (1976) findings. The motion detector detects movement in the visual system. There is no motion detectors in the retina or LGN (Barlow & Hills, 1963), thus motion is not process until it reaches cortex, V1, V2, V3 and extends to the dorsal regions, V5 / MT, MST, V6, and beyond with each successive area processes more complex motion patterns.
A motion detector can identify what direction a stimulus is travelling, a detector needs to be asymmetrical and with a time delay. The reason is that if a signal travels from A to B, it will excite receptor A before receptor B, and with a time delay being introduced to the output signal from A, it will arrive at the detector at the same time as not-delayed signal B, given the delay is sufficient for the signal to travel. In comparison, if a signal travels from B to A, it will excite detector B first; and when the stimulus reached A, detector A will activate. As there is a time delay to A, signaling from A and B will not reach the detector at the same time. With this schema, it can be ensured that only movement from A to B will excite the detector, which enables the identification of the direction in which the stimulus is travelling in (Borst & Egelhaaf, 1989).
The waterfall illusion was first observed and explained by Addams (1834) from a waterfall at Foyers in Scotland. By staring at the same motion for a period of time we can produce direction specific threshold elevation (Levinson & Sekuler, 1980), and a directional of motion after-effect (Levinson & Sekuler, 1976). A waterfall illusion is where the staring of free fall water for too long, makes looking at a stationary object move in the opposite direction, which is also known as the movement after-effect. As explained by Mather, Verstraten, & Anstis (1998) waterfall illusion happen due to the comparison mechanism in our brain, which compares the activities of up and down detectors to determine whether what has been seen is moving upwards, downwards or even stationary.
Under a normal condition, viewing a stationary stimulus will excite both the up and down detectors slightly, at about the same level. So the comparator will identify the up and down detectors activities to be no different, thus we perceive this as stationary. With the waterfall illusion, individuals are presented and are required to stare at the free fall of water, which is a downward motion. Thus it will excite the down detector more compared to the up detector, hence the down detector will produce a lot more action potential energy, while the up detector producing little. Since the massive difference between the productions of action potential...