Can someone tell me a mechanism that would fundamentally halt the production of new neurons in adult brains? If you can't define that edge well, then it would s

Can someone tell me a mechanism that would fundamentally halt the production of new neurons in adult brains? If you can't define that edge well, then it would seem very likely that new neurons do form in adult brains, given the scale of neuron to brain, even if many fewer of them do in comparison.