S07 → N400
The Spike of Fascinating & Unexpected
“It’s not what you look at that matters, it’s what you see”. These words by Henry David Thoreau carry the weight of his profound reflections on the human condition and its intricate connection with nature—insights shaped by his years of living beyond the bounds of civilisation. We can consequently believe him when he recommends seeing through the apparent banality of things to see all the fascinating and unexpected aspects of nature. And, indeed, once we start to see carefully, there is an infinity of events in our surroundings that deserve more than just a quick look.
Who would have imagined that certain plants remain hidden underground, patiently waiting for the heat, smoke, and nutrients from a fire to trigger their bloom—drawing in birds and pollinators to help restore their ecosystem? Or that a seemingly simple question like, “Why does this cloud have this shape?” could unravel into a journey through scattering theory, wave propagation, cloud physics, statistical mechanics, and spatial statistics? N400 picks from other systems’ peregrinations some of those intriguing facts that trigger spikes of curiosity and release waves of genuine interest to document them through short notes, called spikes.
Oh, and why N400?
The answer might surprise you—find out in our very first spike!
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SPIKE 00
→ N400.
© 1. Amoruso, L. et al. (2013) — N400 brain activations for words and actions. / 2. Kutas, Lindamood & Hillyard (1984) — Lexical-semantic (meaning) processing.
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“Every morning, I take my coffee with cream and dog.”
Do these sentences strike you as peculiar? Can you sense an anomaly?
Once you finished reading them, something in your brain reflected this anomaly. This something was imperceptible to the naked eye, but if by chance someone was monitoring the electrical activity of your scalp at this moment, they could have observed a negative electrical potential response peaking. Several electrical potentials—called event-related potentials (ERPs)—are generated in the brain when external stimuli activate the neuronal network. The words "cry" and "dog" here, quite unexpected in these contexts, were the stimulus provoking this negative electrical response—a wave, called the N400 effect.
Since its discovery by Marta Kutas and Steven A. Hillyard in the 1980s, the N400 effect—among other ERP responses—has been extensively studied in the context of language processing. Characterised as a broad negative deflection that starts 200-300 milliseconds (ms) after an unexpected word has been presented visually or auditorily, the wave peaks after approximately 400 ms—hence its name, N400. This negativity reflects the brain’s reaction to semantic violations, highlighting how critical context is to understanding language. Kutas and Hillyard found that the amplitude of the N400 response depends on how well a word fits within its sentence context. More predictable words produce smaller N400 amplitudes, while less predictable or surprising words amplify the response.
This predictability is measured using a concept called cloze probability—the likelihood that a specific word will complete a sentence fragment, based on how people typically fill in the blank. Cloze probability is determined through tasks where participants are asked to predict missing words. Using this approach, researchers have observed N400 responses not only to spoken, written, and signed words but also to pseudowords and familiar abbreviations.
Interestingly, N400s were also observed in non-linguistic stimuli, like line drawings, faces, movies, gestures, environmental sounds, and even music. For instance, it is not rare for musicians to use this characteristic in their compositions by starting a melodic pattern and finishing it unexpectedly, playing with the need for meaningful completion.
The N400 offers invaluable insight into the timing and interactions of cognitive processes involved in language comprehension. Beyond research, it has practical applications in evaluating reading impairments and language disorders, particularly in children. From quirky sentences to clinical tools, the N400 reminds us how deeply our brains are wired to seek meaning and context, whether in language, art, or the unexpected twists of everyday life.
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