This is SO COOL. I'd guess (I did analysis for an fMRI lab for a year so I'm not a pro but not totally talking out of my orifice) that detecting images like this is among the easier things you could do (it probably wouldn't be so easy to do things like "guess the words I'm thinking of") and I suspect other sensory stuff might be harder but I have little knowledge there.

One of the biggest issues with any attempt to extract information from an fMRI scan is resolution, both spatial and temporal - this study used 1.8mm voxels which is a TON of neurons (also recall that fMRIs scan blood flow, not neuron activity - we just count on those things being correlated). Temporally, fMRI sample frequency are often <1hz. I didn't see that they mentioned a specific frequency, but they showed images to the subject for 3 seconds at a time so I'd guess that's designed to ensure you get a least a frame or three while the subject is looking at the image. You can sort of trade voxel size for sample frequency - so you can get more voxels, or more samples, but not both. So detecting things that happen quickly (like, say, moving images or speech) would probably be quite hard (even if you could design an ai thingey that could do it, getting the raw data at the resolution you'd need is not currently possible with existing scanners)

Also, not all brain functions are as clearly localized as vision - the visual cortex areas in the back of the brain map pretty directly to certain kinds of visual stimulus, while other kinds of stimulus and activity are much less localize (there isn't a clear "lighting up" of an area). You can get better resolution if you only scan part of the brain (i.e. the visual cortex) (I don't know if that's what they did for this study), but that's obviously only useful for activity happening in a small part of the brain.

ANYWAY SO COOL!!! I wonder if you could use this to draw people's faces with a subject who is imagining looking at a face? fMRI police sketch? How do brains even work!?