the scream no instrument can register
Thus the loneliness: it's a closed circuit: the current is both applied and received from within.
Context
What makes psychotic depression specifically lonely, the essay argues, is that the agony is generated and suffered entirely inside the person, with no external cause anyone can point to or measure — a screamer wired to her own current. The isolation is definitional: the torment is real and the evidence of it is by nature undetectable, so the sufferer is not only in pain but disbelieved, alone with a fire that leaves no burn marks for a witness to read. [sweep · shared-ground depression-anhedonia]