During Sleep the Visual Cortex Inverts Its Response to Light Even as the Thalamus Keeps Relaying It, a Boston University EEG-fMRI Study Finds
A Nature Communications study used simultaneous EEG-fMRI to show that early sleep suppresses light responses in the visual cortex while the thalamus stays intact.
Overview
When the brain falls asleep, it does not simply stop receiving signals from the outside world. Instead, light still travels through the closed eyelids and reaches the visual relay deep in the brain, but the visual cortex flips its response and shuts the information out. That is the central finding of a study titled “Cortex-specific inversion of visual responses during sleep,” published in Nature Communications on June 11, 2026 by a team led from Boston University and the Massachusetts Institute of Technology.
Using simultaneous EEG-fMRI while presenting luminance-modulated visual stimuli to sleeping humans, the researchers found that “responses to light remained intact in the visual thalamus during N1 and N2 sleep,” yet “stimulus-evoked responses in early visual cortex were profoundly suppressed,” according to the study’s abstract in Nature Communications.
What We Know
The brain disengages from the external environment during sleep, but the route by which it does so has been unclear. As the authors note, closing the eyes profoundly reduces visual input to the brain, but some light still passes through the eyelid, and people can perceive luminance changes even through closed eyes while awake, according to Nature Communications. The long-standing assumption has been that the thalamus acts as the gate that blocks sensory signals from reaching the cortex during sleep. The study notes, however, that even with that gating, “sensory information can still reach the cortex,” per the Nature Communications abstract.
To trace where visual signals are modulated at each stage, the team combined two imaging methods at once. EEG tracked the electrical signatures of sleep stages, while fMRI localized the brain’s responses to flashes of light. They presented luminance-modulated visual stimuli to participants as they slept inside the scanner, according to Nature Communications.
The key result was a split between two stages of the visual pathway. In the visual thalamus, responses to light “remained intact” during the lighter N1 and N2 stages of sleep. But in the early visual cortex, the responses were “profoundly suppressed,” and more strikingly, they reversed: the data showed “an inverted pattern in which high-intensity visual stimulation evoked visual cortical deactivation,” the authors report in Nature Communications. In other words, the brighter the stimulus, the more the visual cortex turned down its activity rather than ramping it up.
The authors propose that the inversion reflects active gating in the cortex itself, not just upstream filtering. “These findings suggest a cortical mechanism where inhibitory circuits regulate stimulus-driven deactivation in visual cortex, facilitating sensory isolation during early stages of sleep,” according to the Nature Communications abstract.
The study was authored by Nicholas G. Cicero, Michaela Klimova, Louis Vinke, Sam Ling, and Laura D. Lewis, with affiliations spanning Boston University, MIT, the Massachusetts General Hospital Department of Psychiatry, and the Athinoula A. Martinos Center for Biomedical Imaging, as listed in Nature Communications. The work comes from the visual-neuroscience lab of senior author Sam Ling in the Department of Psychological & Brain Sciences at Boston University, which lists the paper among its 2026 publications.
What We Don’t Know
The study characterizes the lighter N1 and N2 stages of sleep; the abstract does not extend the inversion finding to deep slow-wave (N3) sleep or REM, leaving open whether the same cortical reversal holds across the full night. The proposed role for inhibitory circuits is an interpretation the authors advance to explain the inverted response; the EEG-fMRI measurements localize where the suppression occurs rather than directly recording the inhibitory neurons themselves. The broader question of how this cortical gating relates to whether and when a sleeper wakes in response to light is not resolved by these measurements.
Analysis
The result reframes a familiar idea. The thalamus has long been cast as the brain’s sensory gatekeeper during sleep, the checkpoint that decides what reaches conscious processing. This study locates a second, cortex-level layer of control: even when the thalamus passes a visual signal through, the early visual cortex can invert its own response and actively quiet down. That distinction matters for understanding why sleepers can remain oblivious to ambient light while still, in principle, being roused by it — the gate is not a single door but a sequence of them, and the cortical one behaves in a way that earlier models did not predict.