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Motion after-effect illusions, such as the waterfall illusion, refer to illusions where fixating a screen which shows stimuli moving in a particular direction elicits the perception of motion in the opposed direction when subsequently looking at a static image.

Here is an example: Video Motion After-Effect

The explanation is typically that adaptation effects lead to a saturation of the firing rates of neurons encoding the viewed motion direction.

So far so good - but why do we then perceive afterwards motion in the opposite direction, and not in any other direction where no adaptation has happened?

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This is a more detailed answer about conceptual mechanisms from the same author as AliceD is citing:

The first row represents the sensors (for instance photoreceptors) while the second row represents higher order responses (for instance of MT neurons). The latter's response is their resting level response plus the difference of the corresponding channel minus the opponent channel's response (so yes, the assumption is that there is an opponent mechanism). As you can see, there is no response beyond baseline before adaptation as the difference between the sensor's responses is zero. During adaptation, when subjects are seeing an upward motion, the corresponding response is higher, too.

enter image description here

Now after adaptation, the opposed system shows an above baseline response as now the up sensors display some form of adaptation, hence shifting the response difference of the sensors in the favour of the opponent motion direction:

enter image description here

I am not familiar with the underlying biophysical mechanisms, this model possibly suggests, however, decreased feedforward inhibition after adaptation.

Mather, the motion aftereffect: a modern perspective

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The motion after-effect (MAE) is believed to be primarily due to adaptation of direction sensitive cells in the middle temporal area (MT) (Fig. 1.). The directional cells in this area of the cortex are selectively sensitive to motion in one direction. Hence, looking at a waterfall will selectively cause adaptation in those cells tuned to the direction of the waterfall. When one looks away, the MT cells responsive to opposite directions will become dominant, as their counterparts are momentarily shut down and one will see the waterfall running in opposite direction.

Adaptation thus apparently weakens the opponent input to the adapted MT cells, enabling an enhanced response to motion balanced stimuli. However, the possibility of adaptation occurring in MT neurons themselves cannot be ruled out (Mather et al., 2008).

The MT area is one of the principal visual cortical areas devoted to motion sensing and hence a likely culprit for MAE. However, fMRI studies have indicated that a lot more visual areas in the brain may be involved, either directly, or indirectly, including V1, V2, V3, VP, V3A, V4, and also MT (Mather et al., 2008).

visual streams
Fig. 1. Two visual streams dominate vision: the dorsal 'where" pathway (containing the direction-sensitive MT area), and the ventral 'what' pathway. source: InTech Open

Reference
- Mather et al., Trends Cog Sci (2008); 12(12): 481-7

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    $\begingroup$ Ok thanks, but my question is not so much about the where (I am somewhat aware of MT and the dorsal stream, and that the size of the adapted area allows us to infer if it is a upstream inferred property or happens in MT itself), but why it is exactly the 180 degree opposite direction which then becomes perceptually salient, and not say the direction which is 32 degrees different. I am not sure if your answer addresses my question $\endgroup$ – user234 Jul 30 at 23:10
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    $\begingroup$ @user234 the MT cells are sensitive to specific directions, per the answer, so yeah, I'm covering the Q pretty heads on I guess $\endgroup$ – AliceD Jul 31 at 0:00
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    $\begingroup$ Ok but then I just don't get it from your answer, and would like to understand better: I get that the cells are selective, ie tuned, for a particular motion direction. But when firing rate to this motion direction is attenuated, why is it exactly the opponent direction which becomes perceptually salient? What's the mechanism? Like there must be some opponency mechanism for directions different by 180 degrees I guess? $\endgroup$ – user234 Jul 31 at 0:21

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