Stephen E. Folger

fMRI of the Responses to Vibratory Stimulation of Digit Tips

Three studies were carried out to assess the applicability of fMRI at 3.0 T to analysis of vibrotaction in humans. A novel piezoelectric device provided clean sinusoidal stimulation at 80 Hz, which was initially applied in separate runs within a scanning session to digits 2 and 5 of the left hand in eight subjects, using a birdcage RF (volume) coil. Significant clusters of activation were found in the primary somatosensory cortex (SI), the secondary somatosensory cortex (SII), subcentral gyrus, the precentral gyrus, posterior insula, posterior parietal regions (area 5), and the posterior cingulate. Digit separation in SI was possible in all subjects and the activation sites reflected the known lateral position of the representation of digit 2 relative to that of digit 5. A second study carried out in six additional subjects using a surface coil, replicated the main contralateral activation patterns detected in study one and further improved the discrimination of the digits in SI. Significant digit separation was also found in SII and in the posterior insula. A third study to investigate the frequency dependence of the response focused on the effect of an increase in vibrotactile frequency from 30 to 80 Hz, with both frequencies applied to digit 2 during the same scanning session in four new subjects. A significant increase in the number of pixels activated within both SII and the posterior insula was found, while the number of pixels activated in SI declined. No significant change in signal intensity with frequencies was found in any of the activated areas.

Temporomandibular Disorder Modifies Cortical Response to Tactile Stimulation

UNLABELLED Individuals with temporomandibular disorder (TMD) suffer from persistent facial pain and exhibit abnormal sensitivity to tactile stimulation. To better understand the pathophysiological mechanisms underlying TMD, we investigated cortical correlates of this abnormal sensitivity to touch. Using functional magnetic resonance imaging (fMRI), we recorded cortical responses evoked by low-frequency vibration of the index finger in subjects with TMD and in healthy controls (HC). Distinct subregions of contralateral primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and insular cortex responded maximally for each group. Although the stimulus was inaudible, primary auditory cortex was activated in TMDs. TMDs also showed greater activation bilaterally in anterior cingulate cortex and contralaterally in the amygdala. Differences between TMDs and HCs in responses evoked by innocuous vibrotactile stimulation within SI, SII, and the insula paralleled previously reported differences in responses evoked by noxious and innocuous stimulation, respectively, in healthy individuals. This unexpected result may reflect a disruption of the normal balance between central resources dedicated to processing innocuous and noxious input, manifesting itself as increased readiness of the pain matrix for activation by even innocuous input. Activation of the amygdala in our TMD group could reflect the establishment of aversive associations with tactile stimulation due to the persistence of pain. PERSPECTIVE This article presents evidence that central processing of innocuous tactile stimulation is abnormal in TMD. Understanding the complexity of sensory disruption in chronic pain could lead to improved methods for assessing cerebral cortical function in these patients.

Re-wiring the brain: Increased functional connectivity within primary somatosensory cortex following synchronous co-activation

The primary somatosensory cortex shows precise topographical organisation, but can be quickly modified by alterations to sensory inputs. Temporally correlated sensory inputs to the digits can result in the merging of digit representations on the cortical surface. Underlying mechanisms driving these changes are unclear but the strengthening of intra-cortical synaptic connections via Hebbian mechanisms has been suggested. We use fMRI measures of temporal coherence to infer alterations in the relative strength of neuronal connections between digit regions 2 and 4 following 3 hours of synchronous and asynchronous co-activation. Following synchronous co-activation we find a 20% increase in temporal coherence of the fMRI signal (p = 0.0004). No significant change is seen following asynchronous co-activation suggesting that temporal coincidence between the two digit inputs during co-activation is driving this coherence change. In line with previous work we also find a trend towards reduced separation of the digit representations following synchronous co-activation and significantly increased separation for the asynchronous case. Increased coherence is significantly correlated with reduced digit separation for the synchronous case. This study shows that passive synchronous stimulation to the digits strengthens the underlying cortical connections between the digit regions in only a few hours, and that this mechanism may be related to topographical re-organisation.

Frequency-domain measurement of vibrotactile driving responses in first-order afferent populations

Surface recordings made at the wrist during moderate vibrotactile stimulation of a digit display rhythmic activity at the frequency of the driving stimulus. This activity is abolished by local anesthesia of the stimulated digit and by substitution of the corresponding digit of the opposite hand with the recording geometry and the load on the stimulator unchanged. Several additional features of the response are similarly incompatible with an artifactual origin in properties of the stimulus motion or the associated electromagnetic field, but consistent with previous neurophysiological observations. The frequency-domain analysis extends readily to the single-trial level, making the technique potentially useful for a variety of basic research and clinical purposes.

Relationship between Information Processing and Postural Stability in Collegiate Division I NCAA Athletes: Does Concussion History Matter?

Background: Concussions have been associated with deficits in balance and postural stability. Subjects sustaining mild to moderate head injuries showed an increase in inhibition of the primary motor cortex which has been associated with sensorimotor organization and movement execution changes. Purpose: The purpose of this study was to examine the relationship between postural stability and information processing in collegiate athletes with and without a history of concussion. Methods: One-hundred and sixty-five Division I student-athletes completed balance and neurocognitive baseline testing. Thirty-four had a previous history of concussion. Postural sway and spatio-temporal characteristics of center of pressure were measured under four conditions: eyes open firm surface, eyes closed firm surface, eyes open foam surface, eyes closed foam surface. Information processing data came from two composite scores from a neurocognitive assessment tool and from a somatosensory stimulation test. Results: Results showed that student-athletes with a history of concussions, although healthy at the time of testing, had differences in postural control compared to student-athletes without a history of concussion. While sway index scores were not significantly different, spatio-temporal measures showed larger displacements in CoP in previously concussed student-athletes. Reaction times and visual motor speeds were significantly correlated with sway index scores suggesting that processing time does influence balance control in all participants. Conclusion: Sustained balance control differences in previously concussed student-athletes may have implications for compensation strategies and risk of additional injuries.