If you have spent any time reading about the neuroscience of depression, you have probably encountered the same effects in the same brain regions again and again: The hippocampus shrinks. The prefrontal cortex, the part of the brain responsible for planning and emotional regulation, becomes less active. The default mode network, the system that hums along when the mind is at rest, loses its coherence. Depression, on this account, is fundamentally a disorder of thinking, mood, and memory—a problem in the brain’s upper floors.

That story is not wrong, exactly. But a study published this month in Nature Mental Health suggests it is considerably more incomplete than the field has assumed (Hamilton et al., 2026).

What Was Learned About Depression From 23,417 Brain Scans

Researchers at Washington University in St. Louis pooled data from six large population datasets, covering 23,417 participants, making this one of the largest-ever neuroimaging studies of depression. The scale matters because smaller studies are vulnerable to noise; a finding that shows up in 200 participants may disappear in 2,000. With nearly 25,000 participants, the team could be confident that what they found in this study was real—and that what they didn’t find was also real.

Some findings confirmed the standard story: Reductions in gray matter volume and cortical surface area appeared in the frontal cortex, the anterior cingulate, and the insula—the regions associated with mood regulation, emotional processing, and self-awareness. Prior research was right about those.

But two findings surprised even the authors:

No significant changes appeared in subcortical brain regions, including the hippocampus. Reduced hippocampal volume has been described as one of the most consistent findings in depression research for decades, but at this sample size, it simply did not emerge as a reliable correlate.

Perhaps more striking, significant reductions in brain volume and surface area appeared in somatomotor and visual regions—the parts of the brain involved in movement, physical sensation, and visual processing. These areas rarely appear in discussions of depression’s neuroscience.

The Importance of Motor and Visual Regions in Depression

Depression affects motor and visual brain regions because the physical experience of depression—the heaviness, the slowed movement, the dulled sensory world—may have direct neural correlates rather than being simply downstream effects of low mood.

When someone is depressed, they may feel heaviness in their limbs; a sense that movement costs more than it should; or a world looking somehow flatter or duller, as if the contrast has been turned down. Getting out of bed may feel like a physical challenge, not just a motivational one. People experiencing depression describe these experiences routinely, and clinicians tend to treat them as consequences of low mood rather than symptoms in their own right.

If the somatomotor and visual regions are genuinely affected in depression, however, this framing may need to shift. These physical experiences may not be downstream effects of a mood disorder. They may have their own direct neural correlates, running alongside mood and memory changes, shaped by some of the same underlying biology.

Kassandra Hamilton, the study’s first author, is already planning follow-up work to test whether visual and motor regions encode specific symptoms of depression rather than just overall severity. That distinction matters clinically.

At 23,417 participants, this study found no significant functional connectivity differences between depressed and non-depressed brains, a result that directly challenges the widely held “disconnection syndrome” model of depression. That idea, that depression is a “disconnection syndrome” in which the default mode network loses its coherent communication with other brain systems, has generated a substantial body of research and shaped thinking about how antidepressants and brain stimulation work.

For these 23,000 participants, though, that effect did not consistently emerge. We may have been overestimating how central functional connectivity is to depression, at least as a universal feature of the disorder.

Depression Is Both a Mental and Physical Disorder

For anyone who has been told that depression is essentially a problem in the thinking and feeling parts of the brain, this study offers a correction. It is that, but it is also something in the motor system—and the visual system. In other words, it’s in the body’s brain as much as it is in the mind’s.