Mentioned just once earlier on in the novel (by Ken Erdedy), anhedonia comes up in a more focused and defined way in the 690s, and largely as it pertains to Hal.
Wallace characterizes anhedonia as melancholy, low-grade depression, spiritual torpor, and the loss of ability to enjoy formerly enjoyable things or activities. For anhedonics, “Objects become schemata. The world becomes a map of the world, An anhedonic can navigate but has no location.” 
Things gets specific about Hal on the following page, but the quotation above already has me thinking about Hal’s certain emotional numbness. It carries me back to that phone conversation in which he is describing to Orin his encounters with the grief therapist to whom he finds he is unable to “deliver the goods.” Hal, the consummate student, who has recently found his father’s map splattered via microwave all about the kitchen walls, is only able to approach his grief as another assignment. Is Hal emotionally bereft?
Hal himself hasn’t had a bona fide intensity-of-interior-life-type-emotion since he was tiny; he finds terms like joie and value to be like so many variables in rarified equations, and he can manipulate them will enough to satisfy everyone but himself that he’s in there, inside his own hull, as a human being – but in fact he’s far more robotic than John Wayne. 
Hal’s lack of affect has been apparent for some time, but a statement like the one above about lacking interior-life-type-emotions since he was tiny is something of a revelation, at least for me. I was under the impression that Hal’s disconnectedness came after Himself’s suicide. So the question is why? What separated Hal from his ability to feel at a very early age? Was it the DMZ, the substance, “even just the accidental synthesis of which sent the Sandoz chemist into early retirement and serious unblinking wall-watching…” 
And just how empty is Hal? We know that Schtitt calls Hal his revenant, indicating Hal’s comeback in tennis, but revenant also means “ghost.” Is Hal like a ghost? It’s a question that sends me page-flipping even further:
‘The Incster has the last word once again,’ says Struck. Which invites a chorus:
‘Halation,’ Rader says. ‘A halo-shaped exposure-pattern around light sources seen on chemical film at low speed.’
Halation. Also seems a bit ghostly. Spectral.
On a separate but related topic, Hal first started getting high at the age of 16 to help him sleep through a recurring nightmare in which he, night after night, found himself in a gargantuan tennis court with intricately convoluted white boundary lines “going every which way, and they run oblique or meet and form relationships and boxes and rivers and tributaries and systems inside systems…” In the dream, which used to wake him nightly, he never could make out who the distant opponent was. Bob Hope relieved him of this nightmare.
If I’m not mistaken, Hal’s off the Bob Hope since the Eschaton debacle and the unexpected urologist’s visit. How has the sudden withdrawal affected him? In the middle of the Randy Lenz / Bruce Green nighttime debacle, we shift to Hal lying in his bunk counting the breaths between the sequential appearances at his door of Jim Troeltsch, Michael Pemulis, and John Wayne. He doesn’t move, doesn’t go to lunch, counts breaths, tells Troeltsch he’s photosynthesizing.
Over 200 breaths later, John (‘N.R.’) Wayne opened up the ajar door a little more and put his whole head in and stayed like that, with just his head in. He didn’t say anything and Hal didn’t say anything, and they stayed like that for a while, and then Wayne’s head smoothly withdrew.
I’d love to hear your take on Hal, his anhedonia/emotional numbness, or any of this.