Scientists settled a 30-year argument about why surprising events are remembered so vividly, and the answer reveals the brain is running two completely different strategies at the same time

Professional tennis players can begin moving toward where a serve will land before the ball is even struck. Ask them afterward to describe exactly where it bounced inside the service box, and their memory is imprecise. But the rare serve that goes somewhere unexpected, the one down the middle when every previous serve went wide, they remember with the kind of spatial precision they cannot summon for the hundred serves that went exactly where they predicted. Neuroscientists have known for decades that this pattern exists. Why the brain trades memory quality for speed on familiar events, and floods attention into unexpected ones at the cost of reaction time, has been the subject of a 30-year unresolved debate. A new study from the University of Sydney has finally settled it, and what the researchers found reveals that the brain is not choosing between two strategies at all.

The debate that needed settling

The question at the center of this argument is called adaptive efficiency: how does the brain decide where to direct its limited neural energy at any given moment? The brain receives an enormous volume of sensory information continuously and cannot process all of it with equal depth. Something has to be deprioritized, and the mechanism governing those decisions has been contested for three decades.

One camp argued that the brain prioritizes expected information, processing familiar inputs more thoroughly because they are more likely to be relevant. The other camp argued the opposite: that surprises command deeper processing because unexpected events carry more information about a world that has just deviated from prediction.

Both positions had experimental support. Neither could fully account for the data from the other side. The reason, as the Sydney team discovered, is that both camps were right, and the brain runs both strategies simultaneously through two entirely different neural mechanisms.

“The debate had been focused on whether the brain prioritised expected or unexpected information,” said lead author Ziyue Hu, a PhD candidate from the University of Sydney’s School of Psychology. “We’ve found the answer is both. The brain has its cake and eats it too.”

What the experiment revealed

The researchers recruited 40 participants and had them watch visual flashes appearing at positions around a circle while EEG equipment recorded their brainwave activity and separate sensors tracked their pupil responses. The flashes followed a predictable pattern most of the time, but the researchers periodically broke the pattern, inserting flashes in unexpected locations.

The behavioral results confirmed the basic paradox the field had been trying to explain. Participants reacted faster and more accurately to the expected flashes. But when asked to recall the precise location of a flash afterward, their memory for the expected ones was significantly worse than their memory for the unexpected ones.

The EEG data explained why. Both types of events appeared in the brain’s cortical activity within 100 milliseconds, confirming that the brain registered both equally fast at the level of initial detection. But the neural signatures diverged sharply after that point.

For expected events, the researchers identified two distinct processing stages happening in rapid succession. In the first stage, the brain generated a predictive signal before the flash even appeared, priming the motor system to respond. In the second stage, when the expected flash arrived and confirmed the prediction, the brain essentially suppressed further processing. It had already committed to a response and did not need the incoming sensory data to build a detailed memory of what it had just perceived.

For unexpected events, neither stage operated. There was no predictive priming. But the incoming signal, which carried genuine new information about a world that had deviated from expectation, received the full weight of the brain’s encoding machinery. The brainwave representation of the unexpected flash was measurably clearer and more detailed than the representation of the expected one, captured within the same 100-millisecond window.

“When the brain is faced with a predictable situation, it goes ‘I already know what this is, I don’t need to spend energy processing it carefully,'” said senior author Dr Reuben Rideaux. “But during unexpected events it’s like a software update or patch. Our brain wants to update its internal memory of the world to make sure we’re prepared for the future, so the energy is dedicated to collecting as much information as possible.”

Why experts react faster but remember less

The tennis player example is not a metaphor. It is a direct prediction of the model the researchers confirmed, and it resolves a puzzle that has frustrated sports psychologists and performance coaches for years.

Expert athletes do not simply have faster reflexes than novices. They have better predictions. A professional tennis player who has faced thousands of serves has built a rich internal model of where serves go based on subtle cues like the opponent’s stance, ball toss position, and shoulder rotation. That model fires predictively before the ball arrives, priming the motor system to move before there is sensory confirmation to move toward.

The cost of that predictive efficiency is exactly what the Sydney study measured: the brain, having already committed to a motor response based on its prediction, does not bother encoding the confirming sensory input in detail. The serve that went exactly where it was predicted leaves only a fuzzy memory trace. The serve that violated every prediction, landing in an unexpected spot, gets encoded with the precision of a photograph.

This explains something coaches observe regularly but have struggled to articulate: experts often cannot precisely describe what they just did on a routine play, while novices, who have no predictive model and process everything fresh, sometimes show surprisingly detailed recall of their own correct responses. The novice’s brain treated the expected as if it were unexpected, because to a novice, it is.

The two-stage discovery changes the model

The finding that the brain processes familiar events in two distinct stages, a predictive preparation phase followed by a suppression phase when the prediction is confirmed, was not anticipated. Previous models assumed a single mechanism governed how expected inputs were handled. The existence of two separable stages, each serving a different function and leaving a different neural signature, gives researchers a more detailed architecture to work with.

It also suggests new angles for understanding what goes wrong in conditions where prediction and reality are persistently misaligned. Anxiety, for example, is characterized in part by a hyperactive prediction system that generates threat signals for events that are actually neutral. If the suppression phase, the second stage of familiar event processing, fails to function normally when predictions are confirmed, the brain would continue treating neutral confirmed events as if they warranted full sensory processing, essentially flooding the system with encoding signals for events that should have been dismissed efficiently.

The researchers plan to investigate how these mechanisms develop across the lifespan and what environmental factors shape them. They also intend to apply the model to artificial neural networks, testing whether the same two-strategy architecture could improve the efficiency of AI systems that currently process all inputs with roughly equal computational weight.

The findings resolve the adaptive efficiency debate without declaring a winner, because neither side was wrong. The brain does prioritize familiar inputs, through predictive priming that buys milliseconds of reaction time. And the brain does prioritize surprising inputs, through deep encoding that builds the clearest possible memory of what just changed in the world. It does both at once, through two mechanisms that operate in parallel rather than in competition, in the hundred milliseconds between seeing something and knowing what it was.

Source

Ziyue Hu, Reuben Rideaux. “Adaptive neural efficiency: cortical tracking of expected and unexpected events.” Journal of Neuroscience, June 22, 2026.
DOI: 10.1523/JNEUROSCI.0154-26.2026