title: Somatosensory Temporal Discrimination Threshold Involves Inhibitory Mechanisms in the Primary Somatosensory Area.
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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, WC1N 3BG, London, United Kingdom, Department of Neurology and Psychiatry, Sapienza University of Rome, 00185 Rome, Italy, rocchi.lor@gmail.com.
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Rocchi
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, WC1N 3BG, London, United Kingdom, Non-Invasive Brain Stimulation Unit, Clinical and Behavioral Neurology Department, Institute for Inpatient Treatment and Scientific Studies Santa Lucia Foundation, 00142 Rome, Italy.
authors.forename: Elias
authors.initials: E
authors.lastname: Casula
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, WC1N 3BG, London, United Kingdom, Department of Neurological and Movement Sciences, University of Verona, 37134 Verona, Italy, and.
authors.forename: Pierluigi
authors.initials: P
authors.lastname: Tocco
authors.affiliation: Department of Neurology and Psychiatry, Sapienza University of Rome, 00185 Rome, Italy, Institute for Inpatient Treatment and Scientific Studies Neuromed Institute, 86077 Pozzilli, Italy.
authors.forename: Alfredo
authors.identifier: http://orcid.org/0000-0003-3598-3142
authors.initials: A
authors.lastname: Berardelli
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, WC1N 3BG, London, United Kingdom.
authors.forename: John
authors.initials: J
authors.lastname: Rothwell
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D004192:Discrimination, Psychological / Q000502:physiology*; D004576:Electromyography; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006225:Hand / Q000294:innervation; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D010812:Physical Stimulation; D011601:Psychophysics; D011930:Reaction Time / Q000502:physiology; D012684:Sensory Thresholds / Q000502:physiology*; D013003:Somatosensory Cortex / Q000502:physiology*; D013223:Statistics as Topic; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
keywords: high-frequency oscillations; somatosensory evoked potentials; somatosensory temporal discrimination threshold; transcranial magnetic stimulation