(e) Logging pilot-in-command flight time. (1) A sport, recreational, private, commercial, or airline transport pilot may log pilot in command flight time for flights-
(i) When the pilot is the sole manipulator of the controls of an aircraft for which the pilot is rated, or has sport pilot privileges for that category and class of aircraft, if the aircraft class rating is appropriate;
...
(iv) When the pilot performs the duties of pilot in command while under the supervision of a qualified pilot in command provided—
(A) The pilot performing the duties of pilot in command holds a commercial or airline transport pilot certificate and aircraft rating that is appropriate to the category and class of aircraft being flown, if a class rating is appropriate;
... (and some more conditions)
and from another source
You can ALWAYS log PIC time in an aircraft for which you are rated when you are the sole manipulator of the controls, regardless of the status of your medical and the flight conditions the flight is conducted in. This is true when you are receiving dual instruction and is true even if you are not qualified to act as PIC (although of course somebody needs to be qualified to act as PIC).