The key paper was published in Science in 2017 by Pete Williams, Simon John and colleagues in mice.
https://pubmed.ncbi.nlm.nih.gov/28209901/
Vit B3 at relatively high dose (1 to 4 gm/day) is now being tested in several human clinical trials for glaucoma and other forms of neurodegenerative and neurocognitive age-related diseases.
See ClinicalTrsil.giv
https://clinicaltrials.gov/ct2/results?cond=Glaucoma&term=ni...
Results are promising, but like most therapeutics the efficacy is likely to vary by genotype, disease subtype, age, diet etc.
I’ve been taking 3 g/day for 6 years and have noticed improvements in cognitive function (recall, speech speed). But I freely admit that this could be placebo effect or due to weight loss and more exercise, so take my n=1 uncontrolled trial with big grain of salt.