both the primary motor cortex and the premotor cortex. He used the term M2 for the medial motor cortex now commonly known as the supplementary motor area. (Sometimes in modern reviews M1 is incorrectly equated with the primary motor cortex.) Given this work by Penfield on the human brain and by Woolsey on the monkey brain, by the 1960s the idea of a lateral premotor cortex as separate from the primary motor cortex had mainly disappeared from the literature. Instead M1 was considered to be a single map of the body, perhaps with complex properties, arranged along the central sulcus. Re-emergence The hypothesis of a separate premotor cortex re-emerged and gained ground in the 1980s. Several key lines of research helped to establish the premotor cortex by showing that it had properties distinct from those of the adjacent primary motor cortex. Roland and colleagues studied the dorsal premotor cortex and the supplementary motor area in humans while blood flow in the brain was monitored in a positron emission scanner. When people made complex sensory-guided movements such as following verbal instructions, more blood flow was measured in the dorsal premotor cortex. When people made internally paced sequences of movements, more blood flow was measured in the supplementary motor area. When people made simple movements that required little planning, such as palpating an object with the hand, the blood flow was more limited to the primary motor cortex. By implication, the primary motor cortex was more involved in execution of simple movement, the premotor cortex was more involved in sensory guided movement, and the supplementary motor area was more involved in internally generated movements. Wise and his colleagues studied the dorsal premotor cortex of monkeys. The monkeys were trained to perform a delayed response task, making a movement in response to a sensory instruction cue. During the task, neurons in the dorsal premotor cortex became active in response to the sensory cue and often remained active during the few seconds of delay or preparation time before the monkey performed the instructed movement. Neurons in the primary motor cortex showed much less activity during the preparation period and were more likely to be active only during the movement itself. By implication, the dorsal premotor cortex was more involved in planning or preparing for movement and the primary motor cortex more involved in executing movement. Rizzolatti and colleagues divided the premotor cortex into four