A CRJ200 Captain reported confusion in the cockpit as to who was flying the aircraft led to an altitude deviation.

Narrative

During the cruise phase of flight we were given a crossing restriction. The instructions were to cross at 26;000 FT at a DME fix that was south of the VOR. The First Officer was flying. When he began the descent he accidentally depressed the TOGA button; which is located on the thrust lever. This resulted in the autopilot being disconnected. The First Officer heard the autopilot disconnect alarm; but thought that I disconnected the autopilot. He did not realize that he had accidentally disconnected it. I reached up and silenced the alarm and he interpreted the autopilot disconnect and me silencing the alarm as me having taken over the controls. I never announced that I had taken control of the aircraft. He then used the vertical speed scroll wheel to select down; which resulted in pitch mode instead of vertical speed mode due to the TOGA function having accidentally been selected. The aircraft pitched down quickly. I attempted to talk and mentor him out of the flight condition however he thought I had assumed control of the aircraft and he was not making control inputs. I did not realize that he was no longer flying until we reached our assigned altitude and I instructed the First Officer to level off and he didn't. I assumed the controls however; we were already through our assigned altitude. We returned as expeditiously as we safely could. No resolution advisories were received.