My infant is 6 months old and has vertical nystagmus when a light source is flashed on her eyes. No other problems. Is it normal or pathological?

Vertical nystagmus . Usually prompts an evaluation to check for central nervous system causes. Horizontal nystagmus can be benign. You are close enough to the specialists at the yale child study center as well as their pediatric ophthalmologists, neurologists, etc. To seek second opinions/evaluations there for your baby. Best of luck in your search for answers for your little one.
Normal. Probably normal especially if you are flashing lights in her eyes.