Trying to identify a short Sci-Fi story - virtual cyber-prison

I wonder if the StackExchange hive-mind might be able to identify the following short story I read as part of a brilliant anthology in (I think) the late eighties.

The basic premise is that there’s a virtual cyber-prison that sedated prisoners experience. The cyber-prison is surrounded by a ‘fog’ that knocks the prisoners out. No-one has managed to escape.

One prisoner learns to manipulate this virtual reality (so far, so proto-Matrix). At first he just enjoys doing stuff like making a dandelion seed float into the mouth of a yawning guard; but eventually he ‘escapes’ into the fog, which he also learns to manipulate.