Edward F. Kelly

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.

Abnormal cortical sensory activation in dystonia: An fMRI study

Despite the obvious motor manifestations of focal dystonia, it is recognised that the sensory system plays an important role in this condition. This functional magnetic resonance imaging study examines the sensory representations of individual digits both within the subregions of the primary sensory cortex (SI) and in other nonprimary sensory areas. Patients with focal dystonia and controls were scanned during vibrotactile stimulation of both the index (digit 2) and little (digit 5) fingers of their dominant hand (which was the affected hand in all the dystonic subjects). The activation maps obtained were analysed for location, size, and magnitude of activation and three‐dimensional (3‐D) orientation of digit representations. Data from both groups were compared. There were significant differences in the average 3‐D separation between the two digit representations in area 1 of SI between subject groups (9.6 ± 1.2 mm for controls and 4.1 ± 0.2 mm for dystonic subjects). There were also strong trends for reversed ordering of the representation of the two digits in both the secondary sensory cortex and posterior parietal area between the two groups. In addition, in dystonic subjects, there was significant under activation in the secondary somatosensory cortex (SII/area 40) for both digits and in the posterior parietal area for digit 5. These results indicate the presence of widespread activation abnormalities in the cortical sensory system in dystonia.

Functional neuroimaging studies of human somatosensory cortex

Two studies were carried out to assess the applicability of echoplanar fMRI at 3.0 T to the analysis of somatosensory mechanisms in humans. Vibrotactile stimulation of the tips of digits two and five reliably generated significant clusters of activation in primary (SI) and secondary (SII) somatosensory cortex, area 43, the pre-central gyrus, posterior insula, posterior parietal cortex and posterior cingulate. Separation of these responses by digit 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 employed microneurographic techniques in which individual median-nerve mechanoreceptive afferents were isolated, physiologically characterized, and microstimulated in conjunction with fMRI. Hemodynamic responses, observed in every case, were robust, focal, and physiologically orderly. These techniques will enable more detailed studies of the representation of the body surface in human somatosensory cortex, the relationship of that organization to short-term plasticity in responses to natural tactile stimuli, and effects of stimulus patterning and unimodal/cross-modal attentional manipulations. They also present unique opportunities to investigate the basic physiology of the BOLD effect, and to optimize the operating characteristics of two important human functional neuroimaging modalities-high-field fMRI and high-resolution EEG-in an unusually specific and well-characterized neurophysiological setting.

Evoked potentials as indices of adaptation in the somatosensory system in humans: A review and prospectus ☆

Population-level behavior of large neural aggregates can be efficiently monitored by corresponding population-level indices such as somatosensory evoked potentials (SEPs). The literature reviewed clearly indicates that SEPs undergo systematic and often marked changes under conditions of repetitive stimulation. Similar results have been reported for several mammalian species and with a diversity of stimulation, recording and analysis protocols. The most characteristic finding is a loss of SEP component amplitude as a function of decreasing time between stimulus presentations. The effects become larger and appear at longer ISIs at higher levels of the somatosensory pathway, are more readily evoked by stimulus trains than by stimulus pairs and are most pronounced for response components generated in the upper cortical layers. These findings are consistent with a recently proposed neurophysiological model of short-term plasticity in somatosensory cortex, which incorporates detailed and current information on cortical microcircuitry, receptor and neurotransmitter characteristics, topographical organization and dynamic response to repetitive sensory drive. Recommendations are provided for further research, emphasizing the potential of frequency-domain analysis methods in conjunction with mechanical vibrotactile stimuli as a vehicle for more detailed testing of the proposed neurophysiological model and for closer integration with psychophysical studies of vibrotactile adaptation.

Cortical responses to single mechanoreceptive afferent microstimulation revealed with fMRI.

The technique of intraneural microneurography/microstimulation has been used extensively to study contributions of single, physiologically characterized mechanoreceptive afferents (MRAs) to properties of somatosensory experience in awake human subjects. Its power as a tool for sensory neurophysiology can be greatly enhanced, however, by combining it with functional neuroimaging techniques that permit simultaneous measurement of the associated CNS responses. Here we report its successful adaptation to the environment of a high-field MR scanner. Eight median-nerve MRAs were isolated and characterized in three subjects and microstimulated in conjunction with fMRI at 3.0 T. Hemodynamic responses were observed in every case, and these responses were robust, focal, and physiologically orderly. The combination of fMRI with microstimulation will enable more detailed studies of the representation of the body surface in human somatosensory cortex and further studies of the relationship of that organization to short-term plasticity in the human SI cortical response to natural tactile stimuli. It can also be used to study many additional topics in sensory neurophysiology, such as CNS responses to additional classes of afferents and the effects of stimulus patterning and unimodal/crossmodal attentional manipulations. Finally, it presents unique opportunities to investigate the basic physiology of the BOLD effect and to compare the operating characteristics of fMRI and EEG as human functional neuroimaging modalities in an unusually specific and well-characterized neurophysiological setting.

Multichannel PC-based data-acquisition system for high-resolution EEG

Describes a complete, high-performance data-acquisition system for high-resolution EEG. The system includes preamplified EEG electrodes, rigid helmets, differential instrumentation and software-controlled gain amplifiers, digitizing and control circuitry, and optical coupling to a 486 PC. Miniaturized preamplifiers mounted on individual electrodes reduce coupling to external electromagnetic fields and minimize signal distortion caused by increased and/or imbalanced electrode impedances. Electrodes are applied quickly and with minimal skin preparation, yet with high packing densities and repositioning consistent to about 1-3 mm in repeated applications. Extensive testing under realistic conditions demonstrates that the system provides superior electrical performance compared to currently available commercial systems.

Frequency-dependent response of SI RA-class neurons to vibrotactile stimulation of the receptive field

Three types of experiment were carried out on anesthetized monkeys and cats. In the first, spike discharge activity of rapidly adapting (RA) SI neurons was recorded extracellularly during the application of different frequencies of vibrotactile stimulation to the receptive field (RF). The second used the same stimulus conditions to study the response of RA-I (RA) cutaneous mechanoreceptive afferents. The third used optical intrinsic signal (OIS) imaging and extracellular neurophysiological recording methods together, in the same sessions, to evaluate the relationship between the SI optical and RA neuron spike train responses to low- vs high-frequency stimulation of the same skin site. RA afferent entrainment was high at all frequencies of stimulation. In contrast, SI RA neuron entrainment was much lower on average, and was strongly frequency-dependent, declining in near-linear fashion from 6 to 200 Hz. Even at 200 Hz, however, unambiguous frequency-following responses were present in the spike train activity of som

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.

Combined MRI-EEG techniques for correlation of anatomy and function in human somatosensory cortex

Recent advances in high-resolution EEG imaging methods have made it advantageous to decrease inter-electrode distance to approximately 1 - 2 cm. To take full advantage of this increased recording density, it has become imperative to consider inter-subject anatomical variability and even intra-subject anatomical asymmetry. The present study used anatomical information from MRI to augment functional data obtained through EEG. Specifically, acrylic helmets made for each subject and normally used during EEG were utilized to orient NMR sample tubes filled with a marker medium (H2O(DOT)Cu2SO4) radially from the scalp at selected EEG recording sites during MRI. Using the software package AVS, the MRI data could then be volumetrically 3-D rendered, 3-D isosurface rendered, or arbitrarily sliced. The tubes appeared in the 3-D renderings as pointers from recording sites to underlying cortical anatomy. Our task was simplified by our focus on a limited area of the cortex. The renderings provide subject-specific anatomical templates for mapping of EEG topographic patterns and clearly reveal individual variations of cortical surface topography that are usually unaccounted for in EEG analysis.

A Potential Difficulty in A/D Conversion Using Microcomputer Systems

Special precautions are urged upon designers and users of A/D conversion systems based on laboratory microcomputers. Even when normal design procedures and manufacturer specifications are conscientiously followed in organizing the hardware, serious degradation of A/D system performance can result from synchronous noise generated by the controlling software, unless that software is properly configured to minimize execution of noisy instructions at critical times. Simple procedures are outlined to test for the presence of such effects.