Under which conditions do the skin and probe decouple during sinusoidal vibrations?

Previous experiments performed on monkey and human fingertips suggested that the skin surface and stimulus probe decouple for sinusoidal displacements applied perpendicularly to the skin surface. From these observations, it was concluded that sinusoidal vibration may not be a suitable stimulus for understanding and modeling the tactile system. We repeated these experiments on human observers using stimulus frequencies ranging from 0.5 to 240 Hz and with displacement amplitudes up to 1 mm peak-to-peak (p-p). The skin and probe movements were measured in the steady-state using stroboscopic illumination and video microscopy. Contrary to previous conclusions, we found that decoupling did not occur for amplitudes less then 0.25 mm p-p, regardless of stimulus frequency. Decoupling was only observed for stimulus amplitudes greater than 0.25 mm over the stimulus-frequency range investigated. To further investigate this effect, a modified stimulus contactor was used, which permitted the measurement of the skin's movement using reflected light. Measurements were made on both the index fingertip and the thenar eminence. Regardless of body site, no decoupling between the skin and stimulus probe was observed for frequencies ranging from 20 to 100 Hz up to displacements of 0.25 mm p-p. These levels are well within the range used in most human psychophysical experiments performed on these parts of the body. We conclude that sinusoidal vibration can be used reliably to stimulate the tactile system and is an appropriate stimulus for developing models of touch.

Effects of a dorsal column lesion on temporal processing within the somatosensory system of primates.

A dorsal column (DC) lesion has lasting effects on behavioral tasks that require temporal processing of tactile information (e.g., frequency and duration discrimination). The present experiments describe physiological correlates of these deficits in temporal discrimination. Compound action potentials evoked by electrocutaneous stimulation were recorded from the major white matter subdivisions of the spinal cord in anesthetized monkeys, and relationships between stimulation frequency and evoked potential (EP) amplitude were determined for the ascending pathways. At 10 pulses per second (Hz) EPs recorded in the lateral spinal columns were attenuated slightly (by 15% or less, relative to 1.5 Hz), whereas potentials recorded from the DCs were not attenuated. The attenuation increased with stimulation frequencies up to 50 Hz, reaching 80% for the anterolateral column and 38% for the dorsolateral column, but only 15% for the DC. Epidural EPs were recorded, before and after interruption of the contralateral DC, from awake animals with electrodes chronically implanted over primary somatosensory cortex (SI). Following the lesion. EP responses to 1.5-Hz stimulation were 46% of preoperative responses to the same stimulus. At 10 Hz, EP amplitudes were attenuated even more, to 27% of the preoperative amplitude at 1.5 Hz. Principal components analysis was employed to quantify alterations in EP conformation and stimulus frequency was varied from 1.5 to 10 Hz, before and after a DC lesion. Interruption of the DC resulted in a significant decrease in the information provided by the EP about changes in stimulus frequency. EPs were also recorded from different locations along the anterior-posterior dimension of the hindlimb region of SI in lightly anesthetized animals. Principal components analysis revealed that there was less information present in the EP about changes in stimulus frequency (1.5-10 Hz) at all recording locations in animals with a DC lesion, compared with the cortex of normal animals. The DC lesion significantly decreased the amplitude of cortical EPs evoked by repetitive stimulation. At 10 Hz the EP was nearly buried in noise, consistent with behavioral deficits in discrimination of the duration of 10 Hz stimulation following interruption of the DC. Also, significantly less information was present in the cortical EPs about changes in stimulus frequency in the absence of intact DCs, which is consistent with deficits in frequency discrimination. This reduction could be explained in part by a lesser capacity of spinal pathways in the lateral column to follow repetitive stimulation above 10 Hz. However, more rostral manifestations of a DC lesion, at either the thalamus or the cortex, are likely to contribute to the reduced capacity of animals with DC lesions to make temporal discriminations.

The effects of static indentation on vibrotactile threshold.

Vibrotactile thresholds were measured on the thenar eminence and the volar forearm at different static depths of skin indentation. Three stimulus frequencies (1, 20, and 200 Hz) were delivered through either a 0.008- or a 2.9-cm2 contractor. The indentation depths ranged from 0 to 1 mm (0.25-mm steps) relative to the point of skin contact with the stimulator. There was a significant effect of indentation in all stimulus combinations of contactor size, location, and frequency. These results resolve an apparent discrepancy in the literature regarding threshold reduction with increasing contactor size observed on the forearm at low frequencies.

Decay in the effect of vibrotactile masking.

Vibrotactile forward masking experiments were conducted on the thenar eminence of the hand to determine the time course of masking in the Pacinian (P) or a non-Pacinian (NP I) channel. Brief masking and test stimuli that contained energy centered either below 27 or at 500 Hz were used to preferentially activate the NP I or P channels, respectively. Test thresholds were measured at different fixed delays ( Delta t) between the masking and test stimuli that ranged from 5 to 995 microseconds. Masking stimulus level was also varied from 5 to 25 dB above threshold. The masking effect followed an exponential decay with different time constants for each of the channels, but a similar asymptote (residual masking) for both channels. At the higher masking stimulus levels, the time constant for the P channel was about 40 microseconds, while the time constant for the NP I channel was about 100 microseconds. Residual masking in both channels increased about 1 dB for every 5-dB increase of masking stimulus level.

Vibrotactile forward masking: effects of the amplitude and duration of the masking stimulus.

Tactile thresholds for detecting a 50-ms signal presented 25 ms after the termination of a masking stimulus increased as a function of the amplitude level and duration of the masking stimulus. The effects were similar in both the P and NP I channels measured at 250 and 20 Hz, respectively. It was concluded that the increased masking caused by increasing the duration of the masking stimulus resulted from processes other than or in addition to temporal integration--the latter being a characteristic of the P, but not the NP I, channel. The slopes of the masking functions, in which threshold shifts were plotted as a function of masking-stimulus sensation level, were consistently greater for 20-Hz than for the 250-Hz stimuli, suggesting that masking efficiency is greater in the NP I than in the P channel.

A critical band filter in touch.

Separate mechanoreceptor systems in humans were isolated by varying the spectra of vibrotactile stimuli. First, the function relating threshold to frequency of a sinusoid was obtained on the fingertip for each of four subjects, and it was found to comprise two limbs: a Pacinian and a non-Pacinian limb. The peak sensitivity within the Pacinian limb (mediated by Pacinian corpuscles) was around 250 Hz and spanned the region from 65 to 400 Hz. The non-Pacinian limb showed no detectable change in sensitivity in the region between 10 and 65 Hz. These two limbs were then treated as psychophysical channels in experiments in which narrow band noise and individual sinusoids were used to excite one or both channels. In the second and third experiments, the noise stimuli varied in bandwidth from 8 to 70 Hz and varied in center frequency from 25 to 218 Hz. Masking functions were obtained for ON-frequency conditions (the sinusoidal test and noise masker occupied the same frequency region) and for OFF-frequency conditions (the test and masker occupied different frequency regions). The ON-frequency experiments were used to estimate the signal-to-noise ratio (S/N) of the Pacinian channel at threshold. The OFF-frequency masking experiments were used to infer the shape of the Pacinian channel at frequencies below 65 Hz, where thresholds for Pacinian activation were above detection threshold. The results of these three experiments predicted the findings of a fourth masking experiment with a parameter free model that treated the Pacinian channel as a filter that integrates stimulus power. The results show that the Pacinian channel is analogous to a critical band in the auditory system.

Physiological changes during recovery from a primate dorsal column lesion.

The evoked potential (EP) over primary somatosensory cortex (SI) was monitored before and after a complete lesion of the primate dorsal column (DC) pathway on one side. The EP was elicited by electrocutaneous or mechanical stimulation of either foot, and was recorded from the contralateral cortical surface for periods of up to 3 months after the lesion. The amplitudes of the three major peaks (P20, N50, and P90) of the cortical somatosensory EP were significantly reduced following interruption of the contralateral DC. Over weeks following the lesion, there was a significant increase in amplitude of the P90 component of the EP that was not evident in the other peaks. The postlesion increases in P90 amplitude were correlated with improved performance on a task that required grasping with either foot, suggesting that behavioral recovery from a DC lesion results in part from neural plasticity, as opposed to a simple relearning of the task.

Two-dimensional sound localization by human listeners.

This study measured the ability of subjects to localize broadband sound sources that varied in both horizontal and vertical location. Brief (150 ms) sounds were presented in a free field, and subjects reported the apparent stimulus location by turning to face the sound source; head orientation was measured electromagnetically. Localization of continuous sounds also was tested to estimate errors in the motor act of orienting with the head. Localization performance was excellent for brief sounds presented in front of the subject. The smallest errors, averaged across subjects, were about 2 degrees and 3.5 degrees in the horizontal and vertical dimensions, respectively. The sizes of errors increased, for more peripheral stimulus locations, to maxima of about 20 degrees. Localization performance was better in the horizontal than in the vertical dimension for stimuli located on or near the frontal midline, but the opposite was true for most stimuli located further peripheral. Front/back confusions occurred in 6% of trials; the characteristics of those responses suggest that subjects derived horizontal localization information principally from interaural difference cues. The generally high level of performance obtained with the head orientation technique argues for its utility in continuing studies of sound localization.

Directional sensitivity of sound-pressure levels in the human ear canal.

Changes in sound pressures measured in the ear canal are reported for broadband sound sources positioned at various locations about the subject. These location-dependent pressures are one source of acoustical cues for sound localization by human listeners. Sound source locations were tested with horizontal and vertical resolution of 10 degrees. Sound levels were measured with miniature microphones placed inside the two ear canals. Although the measured amplitude spectra varied with the position of the microphone in the ear canal, it is shown that the directional sensitivity at any particular frequency of the broadband stimulus is independent of microphone position anywhere within the ear canal. At any given frequency, the distribution of sound pressures as a function of sound source location formed a characteristic spatial pattern comprising one or two discrete areas from which sound sources produced maximum levels in the ear canal. The locations of these discrete areas varied in horizontal and vertical location according to sound frequency. For example, around 8 kHz, two areas of maximum sensitivity typically were found that were located laterally and were separated from each other vertically, whereas, around 12 kHz, two such areas were found located on the horizontal plane and separated horizontally. The spatial patterns of sound levels were remarkably similar among different subjects, although some frequency scaling was required to accommodate for differences in the subjects' physical sizes. Interaural differences in sound-pressure level (ILDs) at frequencies below about 8 kHz tended to increase monotonically with increasing distance of the sound source from the frontal midline and tended to be relatively constant as a function of vertical source location. At higher frequencies, however, ILDs varied both with the horizontal and with the vertical location of the sound source. At some frequencies, asymmetries between the left and right ears in a given subject resulted in substantial ILDs even for midline sound sources. These results indicate the types of horizontal and vertical spatial information that are available from sound level cues over various ranges of frequency and, within a small subject population, indicate the nature of intersubject variability.

Directional sensitivity of the auditory midbrain in the mustached bat to free-field tones.

To ascertain the directional characteristics of the auditory system in the mustached bat, Pteronotus parnellii, we measured the summated neural response at the lateral lemniscus (N4) in response to pure tones at 30, 60 and 90 kHz, frequencies that are typical of the harmonics of this species' biosonar signal. Stimuli were presented at various vertical and horizontal locations in the contralateral hemifield. Intensity-response functions were measured at different horizontal locations for the second harmonic, and showed no variation in shape with variations in azimuth. There was little difference in directionality measured from either threshold or amplitude of N4 potentials. Our results show that areas of maximum sensitivity (best areas) were significantly different for each of the harmonics (P less than 0.05). The centers of the best areas were: first harmonic (30 kHz), 39 degrees azimuth and -19 degrees elevation; second harmonic, 20 degrees azimuth and 0 degrees elevation; and third harmonic, 12 degrees azimuth and -11 degrees elevation. Thus, with increasing frequency best areas shifted toward the vertical midline. Directionality to first harmonic stimuli was broader than to either of the two higher harmonics.