A mouse with a half-human cortex remembers a maze better than one without

The readout is a camera rig. Multiple cameras tracked the mouse as it wandered a small arena, and a computer charted its position and speed, leaving Pong-like traces on a monitor ^[9].

Pasca's group modified the mice so their brains do not fully develop, which leaves them missing most cells of the cortex and the hippocampus ^[4]. Human cells injected into that space take it over. "Human cells that are placed in these animals will divide, will grow, and within a few weeks to a few months they will take most of that space," Pasca said ^[5]. Nearly half the resulting brain volume is human cells ^[2], and more than half of it is still mouse ^[18].

The engineered mice without human cells walked around and squeaked and seemed fairly normal, but in a maze test they could not remember which parts they had explored ^[6]. The mice that received human cells did better on that test, which the team takes as evidence that the human tissue plays some role in cognition ^[7]. MIT Technology Review's account describes the comparison as grafted against ungrafted among the modified animals, and does not report how either group did against an ordinary mouse ^[19]. So the measured effect is partial repair of a deficit the researchers built.

Pasca calls them "xenocortical mice" and expects them to be useful in studying brain injuries ^[8]. That matches what the maze measures. A team hoping for a substrate that behaves like a human cortical network is relying on something weaker: the volume figure, plus the fact of connection. "What's most remarkable to me is the extent to which human neural tissue introduced after birth grew and connected with the mouse nervous system across a species barrier," said Carsten Charlesworth, a scientist in a different Stanford lab who was not involved in the research ^[10][11].

Organoids are already being wired to computers in labs to see whether they can play video games, and other scientists have proposed using them as replacement parts to treat stroke victims ^[13]. Last year Pasca convened a group of ethics experts to study the implications of neural organoid technology, including the odds that an animal could develop human consciousness and the risk that "organoid therapy clinics" might sell scam treatments to desperate patients ^[14]. He said he is not concerned that these rodents have human cognitive capacities, because their brains are relatively tiny and the evolutionary distance between people and mice is so great ^[15]. On the species above, he was blunt: "One of the things that I see as a very clear red line is doing this experiment in a primate," he said. "I don't think that is justified at this point in any way." ^[16] He convened the panel, and the red line is his own sentence.

For a lab weighing this model, the useful question is which single measurement would show the human cells behaving the way that lab needs. For brain injury work, behavioural rescue in a lesioned animal is the right shape of evidence. For work that depends on human-specific circuit behaviour, the volume share describes the graft rather than the result. The same question sits under the ethics: Pasca's stated grounds for comfort are brain size and evolutionary distance, and the experiment he ruled out is engineering a monkey to lack a cortex and filling it with human brain organoids ^[17].