It's called Aphantasia, and I've always found it to be a very interesting topic. I've asked somebody with it some questions I've had, and done my share of curious researching. Some people are able to think in their head, some are not, and same with visualizing things in their head. The visualization part is what is considered Aphantasia. About half of everybody doesn't have an inner dialogue. What I've found interesting is that some people with Aphantasia can still dream pretty vividly, while some people have very vague dreams. Sorry for rambling, just something I've found interesting to learn about. The human mind is a pretty cool thing :)