Holographic mesoscope enables simultaneous read-write of neural activity across brain regions

Neuroscientists have long struggled with a fundamental constraint: they could stimulate neurons in one brain region or record from another, but doing both simultaneously across distant areas remained out of reach. A new instrument, described this August in Nature Neuroscience, breaks that barrier. The two-photon holographic mesoscope allows researchers to activate specific neurons with precision in one part of the mouse cortex while simultaneously recording the activity of thousands of neurons across multiple downstream regions—all using light alone.

The device works by combining two existing technologies in a way that had never been attempted before. Optogenetics, which uses genetically modified neurons that respond to light, provides the ability to stimulate. Computer-generated holography, which employs spatial light modulators to shape light into precise three-dimensional patterns, delivers that stimulation with near single-cell accuracy. On the recording side, the system relies on fluorescent proteins that light up when neurons fire, allowing researchers to watch the downstream consequences of their stimulation across a wide area. The challenge was that holography has always been limited to a small field of view—less than two square millimeters—confining previous studies to local circuits or, at best, interactions between two adjacent brain regions.

The team solved this by integrating an optimized holographic system into a commercial two-photon mesoscope, which can record across up to five square millimeters. They added an arrangement of mirrors that repositions the stimulation beam rapidly across this larger area, expanding the accessible stimulation zone by roughly tenfold. This architectural innovation opened a new experimental possibility: activate neurons in one cortical region while mapping the functional consequences across multiple distant areas simultaneously.

In their proof-of-concept experiments, the researchers stimulated groups of neurons in four different visual cortex areas, one after another, while recording from thousands of neurons in six surrounding regions. In one striking demonstration, they identified neurons in visual area V1 that responded to different orientations of black and white stripes. They then stimulated groups of these orientation-tuned cells and trained a machine-learning classifier to decode the downstream activity patterns. The classifier could correctly identify which orientation had been stimulated significantly better than chance, suggesting that the activation patterns the team wrote into one region transmitted visually relevant information to distant areas.

The connectivity maps that emerged from these experiments revealed something unexpected. Local interactions between nearby cortical areas were primarily inhibitory, confirming earlier findings. But the effects on activity in more distant regions tilted toward excitation—a new observation that challenges assumptions about how information flows through the cortex. The researchers emphasized that this work was designed to demonstrate the platform's capabilities rather than provide exhaustive maps or final biological interpretations. What matters is what becomes possible next.

Hillel Adesnik, the study's lead investigator at UC Berkeley, noted that this was the first time researchers could activate specific ensembles of neurons in one cortical area and map the functional impacts across three, five, or even six other areas. The ability to perturb functionally defined groups of neurons across multiple regions simultaneously—something no other technique, optical or otherwise, could do—opens what his team calls a new class of causal experiments. Instead of passively observing how the brain processes information, researchers can now write specific patterns into one region and watch how that information propagates downstream.

The practical implications extend beyond basic neuroscience. Larger field-of-view capabilities could eventually scale the technology to animals with bigger brains, such as nonhuman primates. Brain-machine interfaces, which currently struggle to decode the distributed patterns of activity that carry meaningful information, could benefit from understanding which neural signals matter most for communication. And as neuromodulation becomes increasingly relevant to clinical treatment—helping patients with neurological and psychiatric conditions—the insights gained from mapping how information actually flows through the brain could inform better therapeutic approaches. For now, the tool has scratched the surface of fundamental questions about how the brain encodes and transmits information. The next phase belongs to the broader neuroscience community, armed with a new instrument and a clearer view of what questions become answerable when you can read and write neural activity across the entire landscape of interconnected brain regions.