A NEW BRAIN NEURON DISCOVERY! Human Attention Is All You Need. It may help solve ADHD and Autism. The Brain’s Ancient Focus Filter: How a Handful of Brainstem Neurons Decide What You Notice For decades, attention was treated as a late, sophisticated product of the prefrontal cortex. Find a friend in a crowded room, follow one voice in a noisy café, ignore the flashing notification while you finish a sentence—those feats were assumed to require the most recently evolved parts of the human brain. A new paper from Johns Hopkins University upends that story. An evolutionarily ancient cluster of inhibitory neurons in the brainstem, called PLTi (parabigemino-lateral tegmental inhibitory neurons), is required for selective spatial attention. When these cells are temporarily silenced in mice, the animals become hyper-distractible. Weak, irrelevant stimuli steal their focus. The next day, when the same neurons are allowed to fire again, the identical animals ignore even strong distractors. Vision is intact. Movement is intact. Only the ability to pick a winner among competing signals is gone. The finding, published in Nature Communications recasts attention as something far older than the primate cortex. Birds, fish, frogs, and turtles all possess homologous circuits. The implication is stark: the brain solved the problem of “what matters right now” hundreds of millions of years before humans grew a large prefrontal cortex. What PLTi Actually Is PLTi neurons sit in the midbrain tegmentum, a region shared by all vertebrates. They are parvalbumin-positive GABAergic cells. Almost all of them co-express GABA and PV. They receive input from the superior colliculus (SC) and send long-range inhibitory projections back into the intermediate and deep layers of the SC—the SCid, an established hub that builds a topographic map of surrounding space and helps select where to look or turn. The circuit is reciprocal. The SC talks to PLTi; PLTi talks back and quiets the SC. When researchers activated PLTi chemogenetically, firing rates in SCid dropped. When they silenced PLTi, SCid became overactive. That overactivity is not random noise. It erodes the sharp competitive boundary that normally lets one stimulus suppress its rivals. In earlier work, Mysore’s lab had already mapped an analogous circuit in birds (the nucleus isthmi pars magnocellularis, or Imc). The mouse work was the first causal demonstration that the same logic operates in a mammal performing a human-style attention task. The Experiment That Made Distractibility Visible The team trained freely moving mice on a touchscreen flanker task modeled on human psychology experiments. A target grating appeared straight ahead. The mouse had to report its orientation (vertical or horizontal) by poking the correct location. Meanwhile, a flanking grating of varying contrast appeared off to the side. Sometimes the flanker matched the target (congruent). Sometimes it conflicted (incongruent). Contrast of the flanker was titrated so the team could measure how strong a distractor had to be before it won. Normal mice handled the task well. Accuracy stayed high even with moderately strong incongruent flankers. After bilateral chemogenetic silencing of PLTi (inhibitory DREADDs + CNO), performance on incongruent trials collapsed. Accuracy could fall from about 90 percent to chance. The animals were now captured by faint distractors. Congruent trials and single-target trials were largely spared. Motor trajectories, head-orienting, and basic visual discrimination remained intact. 1 of 2