Blogger Harold Schellinx figured it out:
" [..] The covers of these two books, each of which in its own way deals with 'music and the brain', are basically the same (notwithstanding the fact that the one is a portrait of Oliver Sacks, and the other not-a-portrait of Henkjan Honing :-) Both prominently show a listener enjoying music played back through a set of headphones. Both listeners have put their right hand upon the very spot where the sounds originate. This is a common gesture, often made in an attempt to shield off possible 'alien' sounds coming in from the 'outside' and get even more 'inside' the music. And both listeners wear a very private smile: their eyes are closed, thus telling us that whatever is going on, it is going on inside their heads. The pictures show their retreat from all that is space, and thus quite forcefully underline that musical hearing (contrary to the hearing of a mere succession of acoustical events, that I like to call factual) is "the manifestation of time eventuating" (as Viktor Zuckerkandl nicely put it in his 1956 Sound and Symbol: Music and the External World.) [..]" HarS Soundblog (2010)
(See also: een kijkje in de keuken [Dutch])