Somatosensory discrimination based on cortical microstimulation

The sensation of flutter is produced when mechanical vibrations in the range of 5-50 Hz are applied to the skin(1-3). A flutter stimulus activates neurons in the primary somatosensory cortex (S1) that somatotopically map to the site of stimulation(4,5). A subset of these neurons-those with quickly adapting properties, associated with Meissner's corpuscles-are strongly entrained by periodic flutter vibrations, firing with a probability that oscillates at the input frequency(1,6) Hence, quickly adapting neurons provide a dynamic representation of such flutter stimuli. However, are these neurons directly involved in the perception of flutter? Here we investigate this in monkeys trained to discriminate the difference in frequency between two flutter stimuli delivered sequentially on the fingertips(1,7). Microelectrodes were inserted into area 3b of S1 and the second stimulus was substituted with a train of injected current pulses. Animals reliably indicated whether the frequency of the second (electrical) signal was higher or lower than that of the first (mechanical) signal, even though both frequencies changed from trial to trial. Almost identical results were obtained with periodic and aperiodic stimuli of equal average frequencies. Thus, the quickly adapting neurons in area 3b activate the circuit leading to the perception of flutter. Furthermore, as far as can be psychophysically quantified during discrimination, the neural code underlying the sensation of flutter can be finely manipulated, to the extent that the behavioural responses produced by natural and artificial stimuli are indistinguishable.