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 act
ivates neurons in the primary somatosensory cortex (S1) that somatotop
ically map to the site of stimulation(4,5). A subset of these neurons-
those with quickly adapting properties, associated with Meissner's cor
puscles-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 flut
ter stimuli. However, are these neurons directly involved in the perce
ption of flutter? Here we investigate this in monkeys trained to discr
iminate the difference in frequency between two flutter stimuli delive
red sequentially on the fingertips(1,7). Microelectrodes were inserted
into area 3b of S1 and the second stimulus was substituted with a tra
in of injected current pulses. Animals reliably indicated whether the
frequency of the second (electrical) signal was higher or lower than t
hat of the first (mechanical) signal, even though both frequencies cha
nged 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 t
he perception of flutter. Furthermore, as far as can be psychophysical
ly quantified during discrimination, the neural code underlying the se
nsation of flutter can be finely manipulated, to the extent that the b
ehavioural responses produced by natural and artificial stimuli are in
distinguishable.