Listening to one of two synchronous messages.

The general situation of using two or more communications channels at once or in alternation gives rise to many specific problems. One of these, the difficulty of speaking and listening simultaneously, has already been studied (2). Another is the varying difficulty of identifying a message as important when it arrives while other messages are already in progress: this is being examined elsewhere (8). The present report is concerned with the possibility of answering one of two messages which start at the same point in time, but one of which is irrelevant.