Quantitative electrophysiological assessments as predictive markers of lower limb motor recovery after spinal cord injury: a pilot study with an adaptive trial design.

Study Design: Observational, cohort study.

Objectives: (1) Determine the feasibility and relevance of assessing corticospinal, sensory, and spinal pathways early after traumatic spinal cord injury (SCI) in a rehabilitation setting. (2) Validate whether electrophysiological and magnetic resonance imaging (MRI) measures taken early after SCI could identify preserved neural pathways, which could then guide therapy.

Differential effects of the mode of touch, active and passive, on experience-driven plasticity in the S1 cutaneous digit representation of adult macaque monkeys.

This study compared the receptive field (RF) properties and firing rates of neurons in the cutaneous hand representation of primary somatosensory cortex (areas 3b, 1, and 2) of 9 awake, adult macaques that were intensively trained in a texture discrimination task using active touch (fingertips scanned over the surfaces using a single voluntary movement), passive touch (surfaces displaced under the immobile fingertips), or both active and passive touch. Two control monkeys received passive exposure to the same textures in the context of a visual discrimination task. Training and recording extended over 1-2 yr per animal.

Effects of transcranial direct current stimulation of primary somatosensory cortex on vibrotactile detection and discrimination.

Anodal transcranial direct current stimulation (a-tDCS) of primary somatosensory cortex (S1) has been shown to enhance tactile spatial acuity, but there is little information as to the underlying neuronal mechanisms. We examined vibrotactile perception on the distal phalanx of the middle finger before, during, and after contralateral S1 tDCS [a-, cathodal (c)-, and sham (s)-tDCS]. The experiments tested our shift-gain hypothesis, which predicted that a-tDCS would decrease vibrotactile detection and discrimination thresholds (leftward shift of the stimulus-response function with increased gain/slope) relative to s-tDCS, whereas c-tDCS would have the opposite effects (relative to s-tDCS).

Tactile texture signals in primate primary somatosensory cortex and their relation to subjective roughness intensity.

This study investigated the hypothesis that a simple intensive code, based on mean firing rate, could explain the cortical representation of subjective roughness intensity and its invariance with scanning speed. We examined the sensitivity of neurons in the cutaneous, finger representation of primary somatosensory cortex (S1) to a wide range of textures [1 mm high, raised-dot surfaces; spatial periods (SPs), 1.5-8.

Neuronal correlates of tactile speed in primary somatosensory cortex.

Moving stimuli activate all of the mechanoreceptive afferents involved in discriminative touch, but their signals covary with several parameters, including texture. Despite this, the brain extracts precise information about tactile speed, and humans can scale the tangential speed of moving surfaces as long as they have some surface texture. Speed estimates, however, vary with texture: lower estimates for rougher surfaces (increased spatial period, SP).

Physical determinants of the shape of the psychophysical curve relating tactile roughness to raised-dot spacing: implications for neuronal coding of roughness.

There are conflicting reports as to whether the shape of the psychometric relation between perceived roughness and tactile element spacing [spatial period (SP)] follows an inverted U-shape or a monotonic linear increase. This is a critical issue because the former result has been used to assess neuronal codes for roughness. We tested the hypothesis that the relation's shape is critically dependent on tactile element height (raised dots).

A critical speed for gating of tactile detection during voluntary movement.

This study addressed the paradoxical observation that movement is essential for tactile exploration, and yet is accompanied by movement-related gating or suppression of tactile detection. Knowing that tactile gating covaries with the speed of movement (faster movements, more gating), we hypothesized that there would be no tactile gating at slower speeds of movement, corresponding to speeds commonly used during tactile exploration (<200 mm/s). Subjects (n = 21) detected the presence or absence of a weak electrical stimulus applied to the skin of the right middle finger during two conditions: rest and active elbow extension.

Tactile perception of roughness: raised-dot spacing, density and disposition.

Recently, we showed that tactile speed estimates are modified by the spatial parameters of moving raised-dot surfaces, specifically dot spacing but not dot disposition (regular, irregular) or density. The purpose of this study was to determine the extent to which tactile roughness perception resembles tactile speed with respect to its dependence and/or independence of the spatial properties of raised-dot surfaces. Subjects scaled the roughness of surfaces displaced under the finger.

Instructed delay discharge in primary and secondary somatosensory cortex within the context of a selective attention task.

The neuronal mechanisms that contribute to tactile perception were studied using single-unit recordings from the cutaneous hand representation of primate primary (S1) and secondary (S2) somatosensory cortex. This study followed up on our recent observation that S1 and S2 neurons developed a sustained change in discharge during the instruction period of a directed-attention task. We determined the extent to which the symbolic light cues, which signaled the modality (tactile, visual) to attend and discriminate, elicited changes in discharge rate during the instructed delay (ID) period of the attention task and the functional importance of this discharge.

Tactile speed scaling: contributions of time and space.

A major challenge for the brain is to extract precise information about the attributes of tactile stimuli from signals that co-vary with multiple parameters, e.g., speed and texture in the case of scanning movements.

Independent controls of attentional influences in primary and secondary somatosensory cortex.

The neuronal mechanisms underlying enhanced perception of tactile stimuli with directed attention were investigated using single-unit recordings from primary (S1, n = 53) and secondary (S2, n = 50) somatosensory cortex in macaque monkeys. Neuronal responses to textures scanned under the digit tips (spatial periods, SP, of 2, 3.7 or 4.

Effects of a cross-modal manipulation of attention on somatosensory cortical neuronal responses to tactile stimuli in the monkey.

The role of attention in modulating tactile sensitivity in primary (SI) and secondary somatosensory cortex (SII) was addressed using a cross-modal manipulation of attention, somatosensory versus visual. Two adult monkeys (Macaca mulatta) were trained to perform two tasks: tactile discrimination of a change in the texture of a surface presented to digits 3 and 4 and visual discrimination of a change in the intensity of a light. In each trial, standard texture (2 mm spatial period, SP) and visual stimuli were presented.

Central neural mechanisms contributing to the perception of tactile roughness.

This paper summarizes recent work showing that tactile roughness appreciation increases in a nearly linear fashion as tactile element spacing or spatial period (SP, distance centre-to-centre between raised dots in these experiments) is increased from 1.5 to 8.5 mm.