 The experimenters examined whether direct microstimulation in the absence of a stimulus could tap into the same neural representations that guided the animal’s report. They isolated neurons in primary somatosensory cortex responsive to vibration frequency on the fingers (S1, the somatosensory homunculus discovered by Penfield [Penfield &amp; Boldrey 1937]; see  figure 1).  The investigators then stimulated the same neurons in S1 in the absence of the test flutter, so used stimulation as a substitute for an actual vibration. Thus, the animal had to make a comparison between the frequency of a mechanical sample to either a subsequent (1) real mechanical test vibration (i.e., the good case with an actual stimulus) or (2) to a microstimulation test stimulus (i.e., the “hallucinatory” case where direct activation of the S1 neurons occurred in the absence of a stimulus). Romo et al. demonstrated that discrimination performance based either on mechanical stimulation or microstimulation was equivalent. In other words, the animals could match either mechanical or microstimulation to a remembered mechanical  sample.[14]