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Somatosensory discrimination based on cortical microstimulation

Abstract

The sensation of flutter is produced when mechanical vibrations in the range of 5–50 Hz are applied to the skin1,2,3. A flutter stimulus activates neurons in the primary somatosensory cortex (S1) that somatotopically map to the site of stimulation4,5. 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 frequency1,6. 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 fingertips1,7. 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 fluttercan be finely manipulated, to the extent that the behavioural responses produced by natural and artificial stimuli are indistinguishable.

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Acknowledgements

We thank W. T. Newsome for comments and discussions. R.R.'s research was partially supported by an International Research Scholars Award from the HHMI and by grants from DGAPA-UNAM, CONACyT and Fundación Miguel Alemán AC.

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Correspondence to Ranulfo Romo.

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Further reading

Figure 1: Sequence of events during standard (up arrow) and microstimulation (down arrow) discrimination trials.
Figure 2: Psychophysical performance using periodic stimuli.
Figure 3: Psychophysical performance using aperiodic comparison stimuli.
Figure 4: Psychophysical performance as a function of comparison amplitude.

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