The Bewegungsnacheffekt (also Bewegungsnachbild ; English aftereffect motion , MAE) is an apparent motion , which may be followed by a prolonged, intense observation of a movement when objects are considered after that are actually static. These then seem to move in the opposite direction to the original movement perception. The effect typically occurs when the motion observation is maintained for about 30 to 60 seconds and eye movements are avoided. Since such an after-effect can be achieved in nature when observing a waterfall, the effect is also called a waterfall illusion .
There is a certain analogy to negative retinal afterimages , which occur in particular after viewing a garish color pattern, when a white surface is then viewed and the pattern then appears in its complementary colors . However, unlike afterimages, secondary effects of movement can also occur if the observation is only made with one eye and the eye is changed when the stimulus changes. From this one can conclude that the post-movement effect must have a cortical basis, whereas after-images can be explained purely by sensory means.
It was found that there are after-effects analogous to the waterfall illusion in auditory perception and the sense of touch . Furthermore, cross-modal effects are also possible, for example a constant tone was perceived by test subjects as becoming quieter after the presentation of an expanding square.
Already Aristotle described about 330 BC. Chr. A movement after-effect. He had looked at pebbles at the bottom of a creek through the roaring water and then had the impression that the stones on the bank were moving. Another traditional description of this phenomenon from ancient times comes from Lucretius , a Roman poet. This was about 56 BC. Chr. Got stuck with his horse while crossing a river and had observed how the horse resisted the strong current when, looking around, he noticed that it seemed to be moving.
In contrast to most of the comparable phenomena that were already described in antiquity, the secondary effect of movement was forgotten, and there seems to be no more descriptions until the 19th century, but then it was rediscovered several times independently of one another. The first was Jan Evangelista Purkyně , a Czech scientist, in 1820 , who described various situations after which one can observe a pseudo movement in the opposite direction, for example after observing a parade of cavalry passing by .
A very detailed account was published in 1835 and comes from Robert Addams , a peripatetic lecturer in natural philosophy . He had made his observation during a tour through the Highlands of Scotland at the Falls of Foyers , which opens into Loch Ness :
“Having steadfastly looked for a few seconds at a particular part of the cascade, admiring the confluence and decussation of the currents forming the liquid drapery of waters, and then suddenly directed my eyes to the left, to observe the vertical face of the sombre age -worn rocks immediately contiguous to the water-fall, I saw the rocky surface as if in motion upwards, and with an apparent velocity equal to that of the descending water, which the moment before had prepared my eyes to behold this singular deception. "
“After looking steadily at a certain part of the waterfall and admiring the confluence and crossing of the currents forming a liquid curtain of water, I suddenly turned my eyes to the left to look at the vertical wall of dark, polished rocks immediately next to the waterfall ; I saw the rocky surface as if it were moving upwards, and apparently with the same speed as the tumbling water, which a moment before had prepared my eyes to see this unique illusion. "
This description was in 1880 by Silvanus Thompson picked up, this led it the common name today Waterfall Illusion (dt. Waterfall Illusion ) a.
Although Purkyně's book was well known, the movement after-effect was rediscovered several times independently of one another in the 19th century, several times in connection with the railroad, but often again in connection with running water. One of them was Johann Joseph Oppel , who made this observation at the Rhine Falls in 1856 . In the middle of the 19th century, Joseph Plateau also stumbled upon the post- motion effect during his research in connection with the phenakistiscope he had invented . He investigated the effects of perception in patterns applied to rotating disks. When he used a black disk on which a white Archimedean spiral was applied in the center, he found that after looking at the rotating disk for a long time, the faces observed subsequently became smaller or larger, depending on the direction in which the disk had turned. Such disks, which were later also referred to as plateau disks or variants thereof, were the most widely used aid in the second half of the 19th century for further investigations of the post-movement effect .
Also Siegmund Exner explored the Bewegungsnacheffekt 1888, emphasized the parallels to afterimages . However, he was aware that there was no parallel in the afterimages for the possibility of transmission from one eye to the other, which was already known at the time. The work by Gustav Adolf Wohlgemuth published in 1911 is still the most comprehensive monograph on the post-movement effect. There Wohlgemuth recapitulated the previous research literature and also carried out 34 further experiments. In doing so, he found, for example, that the post- movement effect also occurs after perceiving stroboscopic movement . He also discovered that the post-movement effect can be preserved, so to speak, if the eyes are closed after the movement has been perceived, because then the post-effect occurs even if the eyes remain closed longer than the post-effect would normally work. This phenomenon was published again in a scientific paper almost 50 years later without knowledge of this experiment.
Not least because secondary effects of movement can be transferred from one eye to the other, it is now considered certain that its basis is to be found in the visual cortex . In this area of the brain in humans - as in other mammals - there are neurons that react to movement stimuli and are selective for a specific direction of movement. As has been demonstrated using functional magnetic resonance imaging (fMRI), a region known as the "motion complex" (MT +) seems to be decisive for the post-motion effect.
The currently favored explanation of the effect, supported by fMRI studies, assumes that the movement negative effect leads to a counter-directed population response of the movement-selective neurons in the MT + area. The prerequisite for this is that a certain number of direction-specific neurons are active when there is no movement stimulus at all. In the arousal state there is a balance for all directions. If a long, similar and intense movement stimulus is perceived, this leads to a permanent dominance of the activity of the neurons specific for this direction of movement, but their sensitivity seems to decrease as the stimulus continues. When the stimulus ceases, this reduced sensitivity remains for a while, and the level of activity of the neurons that are selective for the direction previously shown falls below the basic level, which leads to an excess of the activity of the neurons in the opposite direction.
Combination with tactile and acoustic stimuli
At the beginning of the 21st century it was already known that there was also a secondary movement effect in auditory perception and the sense of touch when it was demonstrated for the first time that this also occurs cross-modally . Test subjects were presented with an approaching or retreating object by changing the size of a square and then playing a constant tone. This was perceived as getting quieter when an object was apparently approaching, and as getting louder when it was moving away, which corresponds to the countermovement in the case of a purely optical post-motion effect. If a corresponding acoustic stimulus was presented at the same time as the visual stimulus, i.e. a sound that became louder or softer, the after-effect increased disproportionately. On the other hand, if the visual and acoustic stimuli were contradictory, no after-effects were detectable.
With simple acoustic stimulus patterns such as a change in volume, the opposite effect could not be produced, i.e. a visual movement illusion after an acoustic movement stimulus. This was only possible with more complex, realistic acoustic movement stimuli. For example, an artificial head recording of a sine tone moving past from left to right was used as a movement stimulus . This enabled an effect to be demonstrated when test subjects then had to evaluate the tendency to move using a random point cinematogram .
A post-movement effect in both directions has also been demonstrated with the combination of tactile and visual stimuli. In addition to other more recent findings, for example that the visual cortex appears to be involved in the analysis of tactile stimuli, cross-modal secondary movement effects indicate that the previously assumed stimulus-oriented organization of the cortical structures does not correspond to reality, but that these rather have a process-oriented structure.
- George Mather, Frans Verstraten, Stuart M. Anstis: The Motion Aftereffect: A Modern Perspective. MIT Press, Cambridge (Massachusetts) 1998, ISBN 978-0-262-13343-2 .
- Waterfall Illusion at illusionsindex.org. Animation and very in-depth explanation.
- Scientists clarify "waterfall illusion" at Wissenschaft.de
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- The area MT + consists of the areas MT (“medial temporal”) and MST (“medial superior temporal”). MT is synonymous with V5. MST does not belong to the visual cortex, but to the cerebral cortex (cortex cerebri). The two areas are difficult to differentiate with fMRI in humans and are therefore often considered together, often referred to as hMT +.
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