What Actually Happens During a Seizure

In the operating theatre of the Montreal Neurological Institute, on an afternoon in the spring of 1937, a young woman lay awake on the table with the side of her skull folded open. She had intractable focal seizures, and the surgeon Wilder Penfield was gloved at her exposed cortex, touching a slender bipolar electrode to its glistening surface. The patient felt no pain, because the brain itself has no pain receptors and the scalp had been numbed with local anesthetic. Penfield pressed the electrode to a small numbered tag, sent a brief pulse of current through it, and asked her what she felt. Across the table, his colleague Herbert Jasper watched a row of penned waves climb and fall on a strip-chart recorder, tracing the living electrical rhythm of her cortex.

Point by point, on a single awake skull, they were building a map. Each stimulation site earned a numbered ticket that the surgical team later transcribed onto a sketch of the brain. The technique would define epilepsy surgery for the rest of the twentieth century, and in the same hands it would produce, by 1950, the motor and sensory homunculi that every student of the brain now learns first. But behind that elegant map lay a blunt question: what is actually happening, electrically and biologically, when a person has a seizure?

The Difference Between a Seizure and Epilepsy

It helps to start with two words that people often blur together. A seizure is a single event, a sudden, transient episode of hypersynchronous neuronal firing. Ordinarily the neurons in your cortex fire in a loose, irregular chorus, exciting and inhibiting one another in a balance that produces coherent thought and movement. During a seizure that balance fails, and a population of neurons begins to discharge together, in a runaway rhythm, far more synchronously than normal brain activity ever allows. Depending on where this happens and how far it spreads, the result might be a violent convulsion, a brief blank stare, a strange smell, or a single twitching hand.

Epilepsy is the diagnosis, not the event. It is the term applied when seizures recur unprovoked and are not explained by some acute, reversible cause such as very low blood sugar, alcohol withdrawal, or a high fever in a small child. A person can have a single seizure in a lifetime and never be called epileptic. A person with epilepsy has a brain that is, in some structural or chemical way, predisposed to generate seizures again and again. Roughly sixty-five million people worldwide live with the condition, with a prevalence of around 0.6 to 1.0 percent in most populations, and incidence that peaks at the two ends of life, in young children and in older adults.

Our window onto all of this is the electroencephalogram, or EEG, introduced clinically by the German psychiatrist Hans Berger in 1929. By placing electrodes on the scalp, Berger could record the summed electrical activity of millions of cortical neurons firing beneath the skull. The EEG remains the gold-standard tool for classifying a seizure, because the abnormal synchrony that defines the event leaves a distinctive signature on the trace.

A March Up the Arm, and What It Revealed

Decades before either the EEG or Penfield's electrode, a quiet English physician inferred the deep logic of seizures simply by watching them closely. John Hughlings Jackson worked at the National Hospital for the Paralysed and Epileptic at Queen Square in London, and in 1873 he described an orderly pattern in certain motor seizures. A jerking would begin in a patient's thumb, then climb to the wrist, then up the arm, then to the face, then down the leg, advancing in a fixed sequence as though something were sweeping across a hidden territory.

Jackson drew two conclusions that turned out to be profoundly correct. First, he reasoned that the motor cortex must contain a somatotopic map, an orderly representation in which adjacent body parts are controlled by adjacent patches of brain, because only such a map could explain why the convulsion crawled in that particular order. Second, he understood seizures themselves as "discharging lesions of grey matter," abnormal local explosions of cortical activity that spread outward across the map. The sequence he described is still called the Jacksonian march, and it remains one of the most vivid demonstrations in medicine that the surface of the brain is organized like a chart of the body.

Focal and Generalized: How Modern Medicine Sorts Seizures

Every contemporary classification of seizures begins with a single question: where does the abnormal firing start? Focal seizures begin in one part of one hemisphere. They might stay focal while the person keeps full awareness, they might impair awareness, or they might spread until they engulf both hemispheres in a bilateral convulsion. Generalized seizures, by contrast, involve both hemispheres from the very outset, and they come in several recognized forms: tonic-clonic, absence, myoclonic, tonic, clonic, and atonic. The International League Against Epilepsy formalized this focal-versus-generalized vocabulary in 2017, retiring the older language of "partial" versus "generalized" seizures that had served the field for decades.

This is not mere relabeling. The distinction governs almost everything that follows, from which drug a neurologist reaches for to whether surgery is even conceivable. A focal seizure has, by definition, a place of origin, a seizure-onset zone that can in principle be located and, in the right circumstances, removed. A generalized seizure has no single origin to excise. The two halves of the classification therefore lead down very different therapeutic roads, and getting the category right is the first real task after a person's first event.

The Two Faces of a Generalized Seizure

Most people, asked to picture a seizure, imagine the tonic-clonic event, and it is indeed the most dramatic. The body first stiffens in the tonic phase, then breaks into the rhythmic jerking of the clonic phase, with a complete loss of consciousness throughout. When it ends, the person typically enters a postictal state of confusion, drowsiness, and disorientation that can last anywhere from minutes to hours, because the brain needs time to recover from the metabolic storm it has just endured. This is the seizure of popular imagination, and it is real, but it is only one face of the disorder.

The other face is almost its opposite. An absence seizure is a brief lapse of awareness lasting roughly five to twenty seconds, with no convulsion at all. The person simply goes blank, stops mid-sentence, perhaps blinks or stares, and then resumes as if nothing happened, often unaware that any time passed. On EEG these events carry an unmistakable signature, a generalized spike-and-wave discharge at three cycles per second, the famous 3 Hz pattern of childhood absence epilepsy. Because the outward sign is so subtle, these seizures are routinely mistaken for daydreaming, and a child having dozens of them a day at school may be scolded for inattention long before anyone thinks to order an EEG.

Mapping the Brain to Cut It Safely

Return now to Penfield and the woman on the table in 1937, because their work answers a practical question that the classification raises: if a focal seizure has a place of origin, can you simply remove it? Penfield founded the Montreal Neurological Institute in 1934 precisely to treat drug-resistant focal epilepsy by resecting the seizure-onset zone. The difficulty was that the bad tissue often sat dangerously close to eloquent cortex, the regions that govern movement, sensation, and language, which a surgeon must spare if the patient is to leave the theatre intact.

Penfield's solution was awake cortical stimulation. With the patient conscious under local anesthesia, he delivered short electrical pulses through a bipolar electrode and listened to what each spot produced: a tingle in the hand, a twitch of the lip, a word that suddenly would not come. By stimulating before he cut, he could chart the functional territory in real time and steer the knife around it. The lasting product of thousands of such tickets was the pair of homunculi, the distorted little figures of the motor and sensory cortex with their enormous hands and lips and tiny trunks, published in The Cerebral Cortex of Man by Penfield and Theodore Rasmussen in 1950. Jackson had deduced the body map from a marching convulsion; Penfield had stimulated it into view, one numbered point at a time.

Calming the Storm, From Phenobarbital to the Diet

For most patients, of course, the question is not surgery but medication, and the story of antiseizure drugs is one of slow, often accidental progress. Phenobarbital, introduced in 1912, was the first effective antiseizure drug, and because it is cheap and durable it remains in use in low-resource settings today, though it sedates. Phenytoin followed in 1938, discovered by Tracy Putnam and Houston Merritt, who screened compounds for antiseizure activity in an electrically induced cat seizure model and found the first drug that controlled seizures without putting patients to sleep. The modern pharmacy is broader still: carbamazepine and oxcarbazepine are workhorses for focal seizures, valproic acid and lamotrigine span the generalized types, and levetiracetam, approved by the FDA in 1999, works through a genuinely novel target, the synaptic vesicle protein 2A, which influences how neurons release their chemical signals.

These drugs succeed more often than people expect. Roughly seventy percent of patients achieve full seizure freedom on one or two medications. That leaves about thirty percent with drug-resistant epilepsy, and for them three established adjuncts exist beyond the drugs. The ketogenic diet, a high-fat, low-carbohydrate regimen introduced by Russell Wilder at the Mayo Clinic in 1921, is now first-line for specific syndromes such as GLUT1 deficiency and Dravet syndrome. Vagus nerve stimulation, FDA-approved in 1997, uses an implanted device to deliver pulses to the vagus nerve and reduces seizure frequency by about fifty percent in roughly half of treated patients. And for one well-defined group, those with mesial temporal lobe epilepsy and hippocampal sclerosis, surgery is the most effective option of all: the anterior temporal lobectomy, shown by Samuel Wiebe's 2001 trial in the New England Journal of Medicine to be clearly superior to continued medication, the modern descendant of the operation Penfield pioneered.

Why the Quiet Seizures Matter Most

The most durable misconception about seizures is the belief that every one involves visible, whole-body convulsions. It does not. Absence seizures are silent blank stares. Focal aware seizures may consist of a single isolated symptom, a phantom smell, a jerk of one hand, a sudden flood of déjà vu, while the person remains entirely conscious and able to describe it afterward. Recognizing these non-convulsive events is not a pedantic exercise, because they are missed all the time, and because they respond to specific medications that differ from those used for tonic-clonic seizures. A child who is punished for inattention, an adult whose recurring sense of the uncanny is dismissed as anxiety, may be having seizures that the right EEG and the right drug could resolve. To understand what happens during a seizure, then, is partly to widen the definition past the dramatic image and to see the whole, surprisingly varied family of events that share one underlying fault: too many neurons, firing too well together, at once.

Key Takeaways

A seizure is a single episode of hypersynchronous neuronal firing, while epilepsy is the diagnosis given when such seizures recur unprovoked, a condition affecting roughly sixty-five million people worldwide and read most reliably through the EEG that Hans Berger introduced in 1929. Modern medicine sorts seizures first by origin, focal events beginning in one hemisphere and generalized events engaging both from the outset, a vocabulary the International League Against Epilepsy formalized in 2017; among generalized types, the violent tonic-clonic seizure with its postictal confusion and the silent twenty-second absence seizure with its telltale 3 Hz spike-and-wave sit at opposite extremes of the same disorder. The somatotopic logic that Hughlings Jackson inferred from his 1873 marching convulsions was later stimulated into view by Wilder Penfield's awake cortical mapping at Montreal, which yielded both the homunculi and the surgical principle of sparing eloquent cortex. Around seventy percent of patients reach seizure freedom on one or two antiseizure drugs running from the 1912 phenobarbital to the 1999 levetiracetam, leaving a drug-resistant third for whom the ketogenic diet, vagus nerve stimulation, and temporal lobectomy remain established options, and the most important practical lesson is that many seizures never look like a convulsion at all, so the quiet ones, the stares and smells and jolts of déjà vu, are the ones most worth learning to recognize.