Somatosensory Temporal Discrimination Threshold Involves Inhibitory Mechanisms in the Primary Somatosensory Area
- 1Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, WC1N 3BG, London, United Kingdom,
- 2Department of Neurology and Psychiatry, Sapienza University of Rome, 00185 Rome, Italy,
- 3Non-Invasive Brain Stimulation Unit, Clinical and Behavioral Neurology Department, Institute for Inpatient Treatment and Scientific Studies Santa Lucia Foundation, 00142 Rome, Italy,
- 4Department of Neurological and Movement Sciences, University of Verona, 37134 Verona, Italy, and
- 5Institute for Inpatient Treatment and Scientific Studies Neuromed Institute, 86077 Pozzilli, Italy
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Author contributions: L.R., A.B., and J.R. designed research; L.R., E.C., and P.T. performed research; L.R. and E.C. analyzed data; L.R., E.C., P.T., A.B., and J.R. wrote the paper.
Abstract
Somatosensory temporal discrimination threshold (STDT) is defined as the shortest time interval necessary for a pair of tactile stimuli to be perceived as separate. Although STDT is altered in several neurological disorders, its neural bases are not entirely clear. We used continuous theta burst stimulation (cTBS) to condition the excitability of the primary somatosensory cortex in healthy humans to examine its possible contribution to STDT. Excitability was assessed using the recovery cycle of the N20 component of somatosensory evoked potentials (SEP) and the area of high-frequency oscillations (HFO). cTBS increased STDT and reduced inhibition in the N20 recovery cycle at an interstimulus interval of 5 ms. It also reduced the amplitude of late HFO. All three effects were correlated. There was no effect of cTBS over the secondary somatosensory cortex on STDT, although it reduced the N120 component of the SEP. STDT is assessed conventionally with a simple ascending method. To increase insight into the effect of cTBS, we measured temporal discrimination with a psychophysical method. cTBS reduced the slope of the discrimination curve, consistent with a reduction of the quality of sensory information caused by an increase in noise. We hypothesize that cTBS reduces the effectiveness of inhibitory interactions normally used to sharpen temporal processing of sensory inputs. This reduction in discriminability of sensory input is equivalent to adding neural noise to the signal.
SIGNIFICANCE STATEMENT Precise timing of sensory information is crucial for nearly every aspect of human perception and behavior. One way to assess the ability to analyze temporal information in the somatosensory domain is to measure the somatosensory temporal discrimination threshold (STDT), defined as the shortest time interval necessary for a pair of tactile stimuli to be perceived as separate. In this study, we found that STDT depends on inhibitory mechanisms within the primary somatosensory area (S1). This finding helps interpret the sensory processing deficits in neurological diseases, such as focal dystonia and Parkinson's disease, and possibly prompts future studies using neurostimulation techniques over S1 for therapeutic purposes in dystonic patients.
- high-frequency oscillations
- somatosensory evoked potentials
- somatosensory temporal discrimination threshold
- transcranial magnetic stimulation
- Received May 21, 2015.
- Revision received November 4, 2015.
- Accepted November 12, 2015.
- Copyright © 2016 the authors 0270-6474/16/360325-11$15.00/0
