In reading these post I hear depression, inability to concentrate and focus is a classical symptom of depression. depression is a disease and like PN the symptoms can't be seen but they are just as real. For example: disturbed sleep, anxiety, hopelessness, trouble with word finding, memory problems, not being able to focus on metal tasks (reading) , trouble concentrating, loss of motivation, feeling overwhelmed, etc.
That said many of the medications used to treat PN cause similar symptoms especially brain fog.
Some people are using a low dose ritalin or provigal to improve concentration and social interactions.