There are some studies that suggest there are brain activity differences, at least when asked to imagine something the self reported vividness of that imagery w

There are some studies that suggest there are brain activity differences, at least when asked to imagine something the self reported vividness of that imagery was correlated with greater activity in visual cortex while imagining. There also was a recent paper suggesting that increased baseline visual cortex excitability can lead to decreased imagery ability (perhaps due to a "saturation" type effect). But there is not a ton of research out there on these topics yet, so it's all very preliminary.