Neural recordings from the primary motor cortex of monkeys performing movements at an angle to a cue stimulus have yielded two main results: (A) the population vector rotates from the direction of the cue to the direction of movement, and (B) cells with intermediate preferred directions are recruited during the middle of this rotation. These results have been interpreted as the neural correlates of a process of 'mental rotation'. Here we propose that results A and B are also consistent with an alternate hypothesis of 'response substitution', given four well known features of cortical neurophysiology.