"Vection" Illusion (The Psychological Pull)

 

(I'd love to write what I often experience when traveling and sailing, although the transit I was onboard was temporally stationary.)

The brain determines motion by comparing your body to your surroundings.When water moves rapidly in one direction, your eyes register the motion. As the sky and horizon feel fixed, your brain misinterprets the rushing water as your own body moving in the opposite direction. This visual illusion is called vection, and it intensifies the physical feeling of being pulled or pushed by the tide.

That physical sensation is caused by a combination of fluid dynamics, soil mechanics, and a sensory illusion. When a wave recedes, water flows rapidly backward toward the ocean, pulling loose sand grains from around your feet and eroding the support beneath you. At the same time, your body weight presses down on the water-saturated sand, forcing the water out and causing the sand to behave like a dense liquid through localized liquefaction, which makes you sink downward. Conversely, when a new wave crashes into you, it exerts an upward buoyant force equal to the weight of the water your body displaces. This is combined with hydrodynamic lift, which is the upward kinetic force created by the velocity of the incoming water pushing against you. 

This physical push and pull is intensified by a psychological illusion called vection. The eyes track the large, rapid movement of the water against the fixed horizon, and your brain misinterprets the motion and makes you feel as though your own body is moving powerfully in the opposite direction.

The exact same psychological illusion explains your experience on the bus or jeepney.When you sit in a stationary vehicle and a neighboring vehicle starts moving, your eyes track that large, moving object because it fills most of your side view. Your brain assumes that your immediate surroundings should be stable, so it misinterprets the moving vehicle as the fixed background and concludes that your own bus or jeep is rolling backward or forward. This vection happens because your visual system processes the motion faster than your inner ear can signal that your body is actually sitting completely still. The midday noon heat and glare can sometimes worsen this by causing eye fatigue, making your brain rely even more heavily on these quick visual cues rather than your actual physical balance.

Vection is the false sensation of self-motion triggered purely by visual stimuli, occurring when your brain receives visual cues that you are moving even though your body is completely stationary. This phenomenon relies on a sensory conflict between your visual system and your vestibular system, which is the internal accelerometer located in your inner ear. Normally, when you move, your peripheral vision tracks a sweeping pattern of movement called optic flow, while the fluids in your inner ear detect actual physical acceleration. When a large adjacent object like a bus moves smoothly, it fills your peripheral vision and creates an intense pattern of optic flow, causing your visual system to signal that you are moving. However, because you are standing still, your vestibular system signals that you are at a complete standstill.

In the split second this contradiction occurs, your brain resolves the conflict by prioritizing visual information due to the "stable earth assumption," which is an evolutionary trait where your brain assumes the largest objects in your view represent the fixed background. Notice that as the neighboring vehicle usually starts moving smoothly without a sudden jolt, it fails to trigger the acceleration thresholds of your inner ear, making it easier for your brain to trust the visual illusion over your physical reality until you look away at a truly stationary object.

While the fundamental wiring of the human brain is identical, vection is not experienced exactly the same way by everyone because people possess vastly different levels of visual and vestibular susceptibility. Individuals who are naturally "visually dependent" rely heavily on what their eyes see over what their inner ears feel, causing them to experience intense and immediate vection, whereas those with highly sensitive vestibular systems reject the illusion almost entirely. Age also introduces significant variation, as school-aged children experience faster and stronger vection because their balance integration systems are still developing, while healthy older adults over seventy show heightened susceptibility due to the inner ear naturally losing sensitivity with age.

Also, the physical aftermath of this illusion differs, as some people perceive vection as a harmless mental quirk while others suffer from Visually Induced Motion Sickness (VIMS). If a person is already prone to car sickness, sea sickness, or dizziness from first-person video games, this sensory mismatch triggers intense physical distress, cold sweats, or nausea. As such, the habituation and training alter how a person experiences this phenomenon; pilots, sailors, and frequent gamers often train their brains to suppress false visual data through repeated exposure, making them far less likely to be fooled by a moving adjacent bus or shifting ocean waves.