You board a train and immediately, with the glint of urgency in your eyes, look for a seat. You are fighting to get into Central London during the morning rush-hour, so getting a seat would elevate your social status from peasant-to-privileged. Your vigilance has paid dividends; there’s an empty seat by the window. In a frantic tussle, you claim the seat as yours from the present moment and for the remainder of the 34-minute journey. Endorphins and dopamine flood your body, notifying your brain of victory. You sit down whilst also turning your head to view the outside of where you will be departing from; then something weird happens: You become conscious of the sensation of motion – the same feeling you would get if the train were moving — yet when your eyes dart back to the window you see that you are in fact stationary.
In this instance, your brain has constructed an expectation of what, in this situation, you should be perceiving. Basically, your brain is attempting to get ahead of the perceptual game. It figures that from previous experience when you have been on a train, it’s been moving, so why should this journey be any different?
The human brain is an anticipatory system; it constructs predictions of your internal self and the outside environment, which allows the system to plan actions ahead of time and quickly comprehend any incoming sensory information. Your brain, then, attempts to match incoming sensory data with previous experience. Take the example above, when you are on a train, it’s usually in motion, so therefore, your brain predicts movement until new information contradicts that prediction.
In general, our surrounding world is flat-line stable i.e. objects and consequences remain stable, with any change occurring slowly. This allows our brain to construct robust predictions of how the world behaves. Given the unchanging nature of our outside world, the brain feels pretty comfortable leading any form of perception with its internal expectation of how things should play out, which serves to enhance what is perceived. And, usually, these expectations are completely on the money. . . until there not.
When the incoming sensory information is ambiguous, the brain’s gathered prior expectations become ever more important – as sensory information is unreliable — and can often bias perception into fault: meaning, the presented sensory information does not adhere to the expectation given. Such phenomena have been shown in the lab.
Phillip Sterzer and colleagues, for example, found that participants would perceive a set of dots in motion, floating around in an ambiguous pattern (neither discernibly travelling to the right or left), as going in one direction or another, depending on a previously learnt expectation. Sterzer et al. asked participants to wear red-green lensed glasses. One group’s glasses altered the motion of the dots, so they moved in a leftward direction, whilst the other group’s glasses changed the dots’ motion to be perceived as moving in a rightward direction (see figure 1). When each group was again presented with the ambiguous set of dots, participants reported that the motion of the dots was aligned to their prior expectations – even when there was no such sensory information to match the expectations.
Therefore, when you’re on that train and you perceive the sensation of motion when there really isn’t any, your expectations have taken the saddle – presumably filling-in the gaps until the sensory information gathered becomes disambiguated.
The brain can predict future sensory scenes by learning and exploiting statistical regularities gathered from previous experience. Our brains appear to achieve this by gathering regularities. For instance, some sensory features appear more often than others, like that of vertical or horizontal lines in comparison to oblique lines; shadows are more likely to be perceived from below an object than from above, as light from the sun always comes from above; and objects perceived on the periphery of our visual field often recede further from the centre, due to locomotion – no wonder we had that moment of perceiving the train and ourselves were in motion: it’s what the regularities gathered from general “train journey experience” had taught our predictive brains.
This bias of tending to perceive objects caught in the periphery of our vision receding away, as is the case with our train example, has seeded itself deep in the brain. The brain is plastic and can thus be moulded by experience. Sensory regularities, then, become overrepresented and tuned in the sensory areas of the brain. For example, the medial temporal (MT) motion dedicated area of the brain responds with vigour when presented with objects in the periphery moving away, rather than closer to the centre of our visual field. Our MT area of the brain is, therefore, responsible for us perceiving the train in motion when it really isn’t; the brain has been tuned to expect this experience.
More often than not, when we perceive the train and ourselves are in motion, we actually are. But sometimes, our expectations — built from regular experience — are ahead of our senses, which gives us an awareness of events that aren’t true to reality.