One dose of antidepressant is all it takes to change the brain, finds a small new study published in the journal Current Biology.

The study authors took brain scans of 22 healthy people who weren’t depressed and who had never before taken antidepressants. Some were randomized to take a dose of the most common kind of antidepressant, an SSRI (selective serotonin reuptake inhibitors).

After another brain scan three hours later, researchers saw a dramatic change: a widespread drop in connectivity throughout the brain, except where it was enhanced in two brain regions, the cerebellum and thalamus.

The results suggest that antidepressants may alter brain connections much faster than previously thought. “We were surprised,” says study author Julia Sacher, of the Max Planck Institute for Human Cognitive and Brain Sciences, in an email to TIME. “We were not expecting the SSRI to have such a prominent effect on such a short time-scale and the resulting signal to encompass the entire brain.”

Antidepressants are generally thought to take several weeks to kick in. “It is possible that these connectivity changes are the first step in remodeling the brain, as there is evidence from other experiments that such functional connectivity changes can reflect neuroplastic change,” Sacher says.

“However, much work remains before we understand how different antidepressants affect the brains of people with and without depression, not only after the first dose, but also over the longer term. The hope that we have for future studies is to uncover distinct differences in brain connectivity between depression patients who ultimately respond to an antidepressant and those who do not.”

Write to Mandy Oaklander at mandy.oaklander@time.com.