Pain-related somatosensory evoked magnetic fields induced by controlled ballistic mechanical impacts.

The purpose of this study was to investigate cortical processing of painful compared with tactile mechanical stimulation by means of magnetoencephalography (MEG) using the novel technique of mechanical impact loading. A light, hard projectile is accelerated pneumatically in a guiding barrel and elicits a brief sensation of pain when hitting the skin in free flight. Controllable noxious and innocuous impact velocities facilitate the generation of different, predetermined stimulus intensities. The authors applied painful as well as tactile mechanical impacts to the dorsum of the second, third, and fourth digit of the nondominant hand. Pain-related somatosensory evoked magnetic fields (SSEFs) were compared with those following tactile stimulation in seven healthy volunteers. Contralateral primary sensory cortical area activation was observed within the first 70 msec after tactile as well as painful stimulus intensities. Only painful impacts elicited SSEF responses assigned to the bilateral secondary sensory cortical regions and to the middle part of the contralateral cingulate gyrus, which were active at latency ranges of 55 to 155 msec and 90 to 220 msec respectively. Additional long-latency responses occurred in these cortical areas as long as 280 msec after painful stimulation in three subjects. In contrast to tactile stimulation, painful mechanical impacts elicited SSEF responses in cortical areas demonstrated to be involved in central pain processing by previous MEG and neuroimaging studies. Because of its similarity to natural noxious stimuli and the possibility of adjustable painful and tactile impact velocities, the technique of mechanical impact loading provides a useful method for the neurophysiologic evaluation of cortical pain perception.

Blink reflex induced by controlled, ballistic mechanical impacts.

The blink reflex was induced by a defined mechanical impact covering the range from light touch to a hard, painful stroke, and by an electrical current. In both modes of stimulation, the R3--but not the R2--thresholds were correlated with subjective pain thresholds, suggesting a connection between R3 and nociceptor activation. However, R3 magnitude did not increase systematically with increasing levels of subjectively felt pain. The R3, induced by painful impacts, habituated quickly and was strongly affected by attention. The functional significance of the R3 component is discussed.

Psychophysical study of stinging pain evoked by brief freezing of superficial skin and ensuing short-lasting changes in sensations of cool and cold pain.

Psychophysical methods were used to investigate pain in human subjects elicited by controlled freezing of the skin using a novel vortex thermode. When cooling stimuli delivered with a small thermode (7 mm diameter) exceeded the normal cold pain threshold into the sub-zero temperature range (-5 to -11 degrees C), all subjects reported an intense, sharp stinging pain sensation which occurred suddenly and was readily differentiated from normal cold pain. The onset of this stinging 'freezing' pain was closely correlated with a sudden increase in skin temperature beneath the thermode of 4.77+/-0.86 degrees C (+/-SD) associated with the phase transition of supercooled water to ice. The mean intensity of freezing pain was rated as 1.7 times as intense as cold pain at threshold. Subjects' mean reaction-time latency to signal stinging pain following the onset of phase transition on the volar forearm was 687+/-220 ms, which was slower than that for mechanically evoked impact pain. Freezing pain is suggested to be mediated by A-delta fibers, based on estimates of conduction velocity and on the observation that the freezing pain took on a burning quality of slower onset during an A-fiber pressure block of nerve fibers. We also investigated changes in skin sensation following the freezing stimulus, and found that freezing led to (a) an immediate, significant decrease in the cold pain threshold (to higher temperatures), which recovered to baseline in < 16 min, (b) a concomitant change in the quality of cold pain from dull to burning, (c) a significant, parallel increase in the threshold for the perception of cooling (to lower temperatures) which frequently manifested as a complete loss of cold sensation, and (d) a mild heat pain hyperalgesia which was still present 24 h later. The changes in thermal sensitivity were not accompanied by consistent changes in mechanical sensitivity. These results indicate that a characteristic sharp, stinging pain is reliably evoked abruptly at the phase transition of supercooled skin water to ice The ensuing brief decrease in cold pain threshold with burning quality, coupled with decreased sensitivity to cold, are speculated to reflect a central disinhibition of C-fiber nociceptor input due to reduced cold fiber activity. These effects may be relevant to frostbite, and distinguish themselves from the more pronounced thermal and mechanical hyperalgesia seen following intense freeze lesion of the skin.

Cochlear mechanics: coiling effects (I, II) and the absorption equation (III).

Mechanical effects of cochlear coiling are examined by comparing wave propagation in straight and coiled two-dimensional models. (1) Compressional waves PC speed up in the coiled system and PC lambda/4 resonance frequencies differ between models by about 1/2 octave. (2) For transpartitional waves PT, there exists a condition of complete model equivalence. (3) Straight models with equal, instead of equivalent depths underestimate coiled PT speed in the long wave limit. (4) Straight model theory alone fails to predict that there is a special radius relation for maximum coiled PT speed at long wave lengths. (5) The case of near-zero modiolar radius is interesting in connection with responses to the stapes near-field. It is suggested that the simple physical assumption implicit to most PT integration procedures has been, that the longitudinal power flow drops at twice the relative rate of the local fields. Two new equations are obtained: the 'absorption equation' and the equation for 'absorption tonotopy'.

Notes on the comparative mechanics of hearing. I.A shock-proof ear.

In woodpeckers, the cochlear windows show functional specialization. The small membrane of the round window is attached to a special bony plate of the columella. This arrangement of coupled windows probably increases power dissipation at the fenestral boundaries and so dampens impulsive mechanical disturbances.

Notes on the comparative mechanics of hearing. II. On cochlear shunts in birds.

Birds differ in cochlear shunts of acoustic volume flow. One can distinguish three types of shunt: peri-, intra- and intercochlear. The functional significance of shunts is discussed.

Notes on the comparative mechanics of hearing. III. On Shrapnell's membrane.

Shrapnell's membrane is very large in many mammals. A large membrane can affect the coupling between sound field and ossicular vibration. Some effects on sound transmission are suggested.

Interaction of light with the organ of Corti. I. Light guide effects in cochlear hair cells.

A cochlear postmortem preparation has been developed which allows for hydromechanical studies of the transilluminated organ of Corti. It was found that cochlear hair cells act as optical waveguides. This poperty of hair cells is important for optical cochlear investigations. It may be used in the study of the motion of single hair cells.

A study of neurone activity in the spiral ganglion of the cat's basal turn.

A small region of the spiral ganglion in the cat was surgically exposed through the round window. Metal microelectrodes were used to record extracellularly the electrical activity of single spiral ganglion cells. The response characteristics of the cells seemed to be, in general, similar to those seen for auditory-nerve fibres recorded with micropipets in the internal auditory meatus. Data are presented on spontaneous activity, tuning curves, responses to clicks, continuous tones, tone bursts and noise bursts. The relation between frequency selectivity of units and location along the basilar membrane is discussed. -Some units differed in behaviour from auditory-nerve fibres with respect to dead times in interspike-interval histograms and shapes of poststimulus-time histograms of responses to tone bursts and noise bursts. The significance of these deviations is unknown.