After staring at a waterfall for several minutes, you look at a stationary rock face nearby, and it appears to be moving upward. This "waterfall illusion" occurs because:

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Examining the waterfall illusion involves understanding motion aftereffects in the visual system. 
Option 1: 'Motion-sensitive neurons in your visual cortex have adapted to downward motion, reducing their response and causing an imbalance when viewing stationary objects' – This aligns with the classic explanation of the motion aftereffect: neurons tuned to the downward motion from the waterfall adapt and fatigue, so when you shift gaze to a stationary scene, the unadapted upward-tuned neurons dominate, making the stationary scene appear to move upward. The reasoning is coherent with known MAE mechanisms. 
Option 2: 'The rock face is actually moving slightly due to vibrations caused by the waterfall, which are imperceptible without prior motion adaptation' – While physical vibrations can occur, the waterfall illusion is a perceptual aftereffect that persists after the stimulus is removed; this option incorrectly attributes the illusion to real motion rather than neural adaptation, and it doesn’t explain why the effect emerges after adaptation to downward motion. 
Option 3: 'Your visual system creates compensatory eye movements to track the waterfall that continue when looking at the stationary surface' – Although the eyes do engage in micro-saccades and tracking during dynamic scenes, a sustained illusory drift of stationary objects upward after viewing motion is better explained by neural adaptation rather than ongoing consensual tracking movements, which would not produce a consistent illusory motion in the stationary scene after the motion stops. 
Option 4: 'Your visual system automatically enhances edge contrast after extended exposure to flowing water, creating an illusion of movement' – Enhanced edge contrast can influence perceived boundaries, but the typical motion aftereffect is not about edge contrast amplification; it is about differential adaptation across motion-sensitive pathways, leading to a directional illusion of movement rather than a general contrast-based edge effect. 
Overall, the reasoning points to the motion aftereffect arising from adaptation of motion-sensitive neurons to the waterfall’s downward motion, causing stationary scenery to appear to move upward due to imbalance in directional signals.

PSC 135 001 SQ 2025 Lecture 11 Quiz: Finishing Visual Perception

After staring at a waterfall for several minutes, you look at a stationary rock face nearby, and it appears to be moving upward. This "waterfall illusion" occurs because:

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