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Motion Aftereffects
In perception, the motion aftereffect (MAE) is a visual phenomenon referring to the illusory motion in a stationary scene as a result of prior exposure to visual motion. This is elicited by consistently gazing at a fixed point surrounded by repetitive movement of a textured pattern for approximately 30 seconds or more. Shifting ones gaze away from the fixed point will elicit an illusory motion in the opposite direction to the movement of the original pattern. Despite seemingly being a simple phenomenon, scientific research indicates that it contains much more complex mechanisms than originally thought, which have in turn enlightened the scientific community to facts about how visual processing occurs in the brain.

Ancient Scholars
The earliest written report on the phenomenon of the MAE originates from Aristotle's book on dreams from approximately 330BC, in which he writes the following… When persons turn away from looking at objects in motion, e.g., rivers, and especially those which flow very rapidly, they find that the visual stimulations still present themselves, for the things really at rest are then seen moving.

There was no report of the phenomenon until almost three centuries afterwards by Lucretius (ca. 56BC), who was the first person known to describe the phenomenon of the aftereffect along with its apparent motion in the opposite direction to the prior stimulus. He states… When our spirited horse has stuck fast in the middle of a river, and we have looked down upon the swift waters of the stream, while the horse stands there a force seems to carry his body sideways and pushing it violently against the stream, and, wherever we turn our eyes, all seems to be rushing and flowing in the same way as we are

19th Century Developments


Despite Aristotle’s and Lucretius's descriptions, the MAE failed to become a predominant topic within subsequent texts on visual processing and optics. The study of the effects of prolonged stimulation on visual perception prior to the 19th century was almost exclusively confined to afterimages involving colours, without the mention of motion. This was the case until the early 19th century when it was brought back into the scientific limelight most notably by Jan Evangelista Purkyně in 1825 and Robert Addams in 1834.

Purkyně, in his report on "Observations and experiments investigating the physiology of senses", reported his personal experience of the phenomenon along with an attempt at an interpretation - suggesting that the illusion was elicited as a result of involuntary eye-movement. 14 years later, Robert Addams reported his account of the phenomenon in his paper "An account of a peculiar optical phenomenon seen after having looked at a moving body." . Within Addams report was the first suggestion that the phenomenon can and should be experimentally studied and manipulated because it has the potential to provide information about how our visual systems work. He further added that this could be achieved by simulating a waterfall in a laboratory via moving stripes.

This suggestion was not put to use until 1849 when Joseph Plateau came to the same conclusion. He introduced a stimulus, a black disk with a white Archimedean spiral, which later became the primary stimulus used in the scientific enquiry of the phenomenon.

From then on until the end of the 19th century, most relevant scientific enquiries used this stimuli, which became known as 'Plateau's Spiral’. However, other types of stimuli were also applied in formative reports. For example, Johann Josef Oppel's (1855) seminal paper on multiple visual illusions from 1855, used the continuous motion of parallel lines consistent with Robert Addams suggestion to measure the effects of the MAE. These two stimuli were what assisted the scientific study into MAEs in the later stages of the 19th century.

Gustav Adolf Wohlgemuth's 'On the Aftereffect of Seen Movement'
The first landmark study for motion after-effects came from Gustav Adolf Wohlgemuth in 1911, with his paper "On the Aftereffect of Seen Movement", which to this day still remains the most comprehensive review of motion after-effects. In his monograph, he extensively reviewed all relevant previous literature on the topic, conducted many of the past experiments himself, as well as 34 of his own. Within these experiments, he reported the following findings…


 * Motion over the retina in the eye is a requirement for the production of a MAE.
 * The intensity of the phenomenon is most dependent upon the still fixation of the gaze on the stimuli.
 * The phenomenon is only generated by the retinal areas which are exposed to prior motion.
 * The phenomenon can be generated within each eye independently, suggesting that what is experienced is a result of the monocular adaptation both eyes individually.
 * There is a wide range at which the speed of the stimulus can generate the phenomenon.
 * The phenomenon can be generated by both real and stroboscopic motion.
 * The phenomenon can be revived from rapid blinking
 * If eyes initially shut after exposure to the stimulus, when opening the eyes again the illusory motion will still be present. This is considered the (storage) of the MAE
 * Adaptation to motion in one direction will slow down the subsequent illusory motion going in the same direction
 * The illusory motion will still occur after exposure to the stimulus whether or not the mind is attending.
 * The motion within the phenomenon can be experienced with eyes closed.

The last point mentioned was referred to as the MAE in the subjective field, and was extended to include homogenous plain surfaces along with eyes being closed. This became an important distinction as it was found that the aftereffect produced within the subjective field lasted for a much shorter period of time and was less visible than in the objective field (when looking at a stationary scene).

Neural Basis
The initial breakthrough for a neural interpretation of the MAE came from H. B. Barlow and R. M. Hill who discovered that a rabbit's retinal ganglion cells are those which signal the motion of objects within the visual field. They exposed the rabbit to a rotating random dot pattern, and reported the firing rates of the retinal ganglion cells, in which they found that the firing rate of these cells gradually slowed from their baseline rate over the first 15-20 seconds. When the dot pattern stopped rotating, the firing rate of the ganglion cells fell below their baseline rate and then recovered gradually over a period of about 30 seconds. There is good reason to think that this finding has relevance to the MAE as the time-periods are very similar to a humans experience of the MAE phenomenon.

Thorough research into human visual processing and its pathways have discovered that motion and direction sensitivity takes place in neurons within the primary visual cortex (V1), with a particular sensitivity in layers 4B & C. Vision is initially registered by the retinal ganglion cells, which are then registered by the lateral geniculate nucleus (LGN) and projected further onto V1. Within the LGN lie different ‘layers’ of cells which register particular aspects involved in visual perception. These include magnocellular, parvocellular and koniocellular layers. The magnocellular layers in particular are important for detecting aspects of motion, such as location, speed and direction. The information from the magnocellular pathways are primarily projected onto layers 4B and 4C within V1, which from there are further projected onto area V5, in which approximately 85% of neurons are sensitive to motion.

This region seems to play a pivotal role in the elicitation of the MAE phenomenon as it has been demonstrated multiple times that applying magnetic stimulation to the region during an experience results in a significant reduction in duration of the MAE.

Explanation
Despite there being a good understanding of the visual system and its pathways in regards to motion, the nature of the MAE is still unknown. The illusion seems to, at least some degree, be a result of reduced responsiveness in the adapted neurons within the primary visual cortex, which would be consistent with Barlow and Hill's (1963) finding.

However, there is good reason to believe that neural fatigue is not the sole explanation for this illusion. This is partly because it is clear that not all neurons within the visual process fatigue. For example, it has been found in cats that only cortical neurons adapt to motion, but the retinal and geniculate neurons do not. Along with this, the time-periods of recovery from the supposed adaptation is not consistent with the time period expected of neural fatigue. An example of this is contingent MAEs (which is when the illusion is dependent on another perceptible feature, such as colour), which can last for weeks. The nature of the MAE remains a mystery, but with the exponentially increasing research in recent years, it is likely that a clear explanation is not far out of sight.