Scientists transplanted human cerebral organoids ("minibrains") into rats, to better study brain disorders. The neurons grown in vivo looked more like mature human brain cells than those grown in vitro, and they made better models of Timothy syndrome. The human minibrains formed deep connections with the rat brains, received sensory information, and drove the rat's behavior.

Points of Clarification (Q&A based on common comments)
- Why didn't the rat reject the transplant, which frequently happens with organ transplants? /// They used immunocompromised (athymic) rats to avoid the problem of the immune system attacking the transplanted tissue.
- Was the licking behavior really due to optogenetics, or just seeing the blue light? /// A separate group of rats (control group), with a transplant but no optogenetics, also completed the red/blue light water training. They showed no significant difference in licking behavior during red and blue light. This suggests that the differences were really due to optogenetic stimulation of the organoid (not, for example, seeing the light). You can see how the control group did, compared to the optogenetic transplant group, in Figure 5j of the study, link below.

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More on how minibrains are grown and used, and the issue of organoid consciousness:    • Growing "Mini-Brains" in a Lab: Human...  

On the topic of organoid sentience and playing pong:    • Lab-Grown "Mini-Brain" Learns Pong - ...  

Organoid transplant study: https://www.nature.com/articles/s4158...

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