Prediction Potentials
The Motion Aftereffect
In the 1980s, a peculiar phenomenon occurred where people started reporting that the pages in books looked red. The text and spaces between them appeared tinted, even though they were purely black and white. Strangely, this phenomenon only happened in the 1980s and did not occur before or after. To understand this, we must step back 2,400 years to Aristotle’s observation of a horse stuck in a river.
Aristotle’s Horse and the Motion Aftereffect
Aristotle observed that after watching the river’s movement for a while, when he looked at the stationary riverbanks, they seemed to move in the opposite direction. This was the first recorded illusion, now known as the motion aftereffect. A modern example of this is staring at a waterfall for a prolonged period and then shifting focus to nearby rocks, which will appear to move upward.
Understanding the Motion Aftereffect
The prevailing hypothesis is that our brain has neurons dedicated to detecting upward and downward motion. These competing populations of neurons inhibit each other to maintain a balanced perception of movement. However, if one set of neurons is overstimulated—such as prolonged exposure to downward motion—then the balance shifts, and once the stimulus is removed, the opposite effect is perceived.
Recalibration, Not Fatigue
The traditional view was that the neurons detecting downward motion simply became fatigued. However, an experiment revealed that even after hours of closing one’s eyes following motion exposure, the aftereffect remained. This suggests that the illusion is not due to neural exhaustion but rather active recalibration—an adjustment of the brain’s baseline expectation to match prolonged stimuli.
Everyday Examples of Recalibration
The Treadmill Illusion
If you run on a treadmill and then step off, the ground seems to move beneath you. This happens because, under normal circumstances, leg movement corresponds with optic flow. On a treadmill, the legs move but the surrounding world does not. The brain adapts by recalibrating this association, causing the illusion of movement when returning to a normal walking environment.
The McCollough Effect
The McCollough effect is a striking example of recalibration where staring at red vertical lines and green horizontal lines for several minutes causes subsequent black-and-white stripes to appear tinted. This illusion can last for months, further proving that the effect is not due to simple fatigue but rather a long-term adjustment in perception.
This also explains why people in the 1980s, accustomed to green monochrome computer screens with horizontal lines, reported that book pages appeared reddish—they were experiencing a prolonged McCollough effect.
The Ganzfeld Effect
When staring at a featureless red field for a long period, the color fades, appearing gray or neutral. This occurs because the brain assumes that the world has not suddenly become uniformly red and actively cancels the constant input to remain sensitive to changes. This effect is commonly observed when wearing tinted sunglasses, where the tint initially distorts color perception but later appears neutral.
The Role of Eye Movements in Perception
Saccades and Microsaccades
The world does not become Troxler-like (i.e., fade into uniformity) because our eyes are in constant motion. The brain prevents objects from disappearing by executing saccades (rapid eye movements occurring three times per second) and microsaccades (tiny, jittery movements). These ensure that even stationary images are refreshed in the brain, preventing them from fading from view.
Ignoring Predictable Features
We are unaware of our own retinal blood vessels because they are fixed in our visual field. The brain expects them to be there and ignores them entirely, just as it ignores the triangle you might draw on a contact lens. This adaptation allows us to focus on changes rather than static features.
Prediction and Surprise
The brain is a prediction machine. It constantly updates an internal model of the world, striving to match its expectations to reality. When the world aligns with expectations, minimal energy is used. When the world deviates, attention is directed toward the unexpected to refine predictions.
Blocking: When Predictions Inhibit Learning
If a person is trained to associate a bell with cheese and later a light is introduced alongside the bell, the brain does not learn that the light also predicts cheese. The brain blocks the second association because it already has a strong predictive model. This is why only violations of expectation lead to meaningful learning.
Addiction and Neuroplasticity
The Brain’s Expectation of Drugs
When a drug is taken repeatedly, the brain begins expecting its presence and modifies receptor levels accordingly. This leads to tolerance, where more of the drug is required to achieve the same effect. When the drug is removed, withdrawal symptoms emerge as the brain struggles to recalibrate to the absence of an expected stimulus.
Heartbreak as Neural Withdrawal
Social connections function similarly. The presence of loved ones forms expectations in our brain. When someone leaves—through breakup, death, or separation—the brain experiences withdrawal, just as it does with a drug. This results in emotional pain akin to physical withdrawal, as the brain must recalibrate to a new reality.
Infotropism: The Brain’s Drive for Information
Maximizing Information
Just as plants exhibit phototropism (moving towards light) and bacteria exhibit chemotropism (moving towards food), the brain exhibits infotropism—the drive to maximize relevant information intake. Neural circuits continuously adapt to enhance sensitivity to important new data while discarding expected information.
Predictive Modeling
The brain builds an internal model of reality and refines it by integrating new information. Predictability conserves energy, and violations of expectation drive learning. This is why unfamiliar environments, such as traveling to a new country, feel stimulating and mentally exhausting—our prediction errors are high, forcing active learning and neural adaptation.
Conclusion
The brain constantly refines its expectations, and surprise is the key driver of neuroplasticity. Whether through motion illusions, sensory recalibration, or learning new skills, the brain actively reshapes itself to become maximally efficient at interpreting the world. Understanding these mechanisms opens pathways for better education, cognitive training, and personal growth.
In the next lecture, we will explore how plasticity changes over time, especially between childhood and adulthood, and whether we can pharmacologically enhance learning potential.