Biomedical subjects

G K Essick

Publications and source records attributed to G K Essick.

At least 19 recordsLinked to original sources

PubMed2002

Capturing the spatial percepts evoked by moving tactile stimuli: a novel approach.

Forced-choice procedures are conventionally used to study the percepts evoked by stimuli that move across the skin and enable an unbiased estimation of subjects' sensory capacities. These procedures, however, require subjects to assign complicated percepts to one of a small number of experimenter-defined response categories, none of which may satisfactorily describe the perceptual experience. To address this limitation, we developed a psychophysical approach, which graphically captures spatial information about a moving stimulus in a holistic manner. Briefly summarized, the stimulus object controlled for location, velocity, direction and distance is moved across the skin of a blind-folded subject, after which the subject draws its path on a life-size, two-dimensional photograph of the body region stimulated. Using this approach, we demonstrated that the drawings contain perceptually relevant information, estimates of direction discrimination and subjective traverse length derived from the drawings closely parallel data obtained with forced-choice and magnitude estimation methods, respectively, and generate comparable psychophysical functions of stimulus velocity. In addition, information is represented in the complex shapes of the curves and in the locations at which they are drawn. Analyses of these latter features support the hypothesis that non-sensory factors (individual subject biases) also affect the drawings.

Adolescent↗

PubMed2000

Frequency and site-dependent variations in vibration detection thresholds on the face.

An adaptive psychophysical procedure was used to estimate the vibration detection threshold at seven spatially matched sites on the two sides of the face and at one scalp site. Repeated measurements over six testing sessions were made for stimuli vibrating at 1, 10 and 100 Hz for each of 21 neurologically healthy, young adult females. Approximately 14 stimulus trials were required to obtain each estimate of the threshold amplitude. Thresholds varied as a function of frequency (p < 0.0001), side (p < 0.001) and site (p < 0.0001). Compared to stimulation at 100 Hz at which the estimates were lowest, thresholds were 3.1 times greater at 10 Hz and 5.4 times greater at 1 Hz. Thresholds were lowest on the vermilion and highest on the cheek and chin. The preauricular skin and scalp exhibited an intermediate level of sensitivity. Whereas thresholds were comparable on the two sides of the face for stimulation at 1 Hz, they averaged 1.33 times greater on the right side for stimulation at 10 and 100Hz. Moreover, thresholds obtained during the last two sessions were 16% higher than those obtained during the first two sessions (p < 0.02), suggesting that subjects on average became more conservative in reporting the presence of the stimulus. The sensitivity in discriminating differences in tactile function favors use of the rapidly administered testing procedure in a clinical setting.

Adolescent↗

PubMed2000

Three-dimensional nasolabial displacement during movement in repaired cleft lip and palate patients.

The objective of this study was two-fold: (1) to explore the suitability of a novel modified Procrustes fit method to adjust data for head motion during instructed facial movements, and (2) to compare the adjusted data among repaired unilateral (n = 4) and bilateral (n = 5) cleft lip and palate patients and noncleft control subjects (n = 50). Using a video-based tracking system, three-dimensional displacement of 14 well-defined nasolabial landmarks was measured during four set facial animations without controlling for head motion. The modified Procrustes fit method eliminated the contributions of head motion by matching the most stable landmarks of each video-recorded frame of the face during function to frames at rest. Its effectiveness was found to approximate that of a previous method (i.e., use of a maxillary occlusal splint to which stable dentition-based markers were attached). Data from both the unilateral and bilateral cleft lip and palate patients fell outside the normal range of maximum displacements and of asymmetry, and individual patients demonstrated greater right-versus-left asymmetry in maximum displacement than did individual noncleft subjects. It is concluded that the modified Procrustes fit method is fast, is easy to apply, and allows subjects to move the head naturally without the inconvenience of a splint while facial movement data are being collected. Results obtained using this method support the view that facial movements in cleft patients may be severely hampered and that assessment of facial animation should be strongly considered when contemplating surgical lip revisions.

Adolescent↗

PubMed1999

Psychophysical assessment of the affective components of non-painful touch.

A novel psychophysical procedure for the evaluation of the affective components of touch was developed. A fabric material was stroked across the test site at a controlled direction and velocity, after which the subject provided a numerical estimate of pleasantness. Significant differences were detected for the sites tested (FACE vs ARM), the fabric materials used (VELVET, COTTON and PLASTIC MESH), and the velocity of motion (0.5, 5 and 50 cm/s). Attesting to their validity, estimates of pleasantness correlated negatively with estimates of unpleasantness obtained for the same stimuli. Moreover, subjects were reasonably consistent in their ratings upon stimulus replication. These findings demonstrate that the hedonic qualities of touch can be psychophysically evaluated, and that valid and reliable estimates are obtained.

Adolescent↗

PubMed1999

A letter-recognition task to assess lingual tactile acuity.

PURPOSE: This study evaluated a novel neurosensory test based on letter recognition developed for the assessment of spatial acuity of the tongue. PATIENTS AND METHODS: An up-down tracking procedure was used to estimate the threshold height for the recognition of embossed letters of the alphabet examined by the tongue tip. The 48 stimuli consisted of acrylic strips, one side of which bore a letter (A, I, J, L, O, T, U, or W) of 3, 4, 5, 6, 7, or 8 mm in height. Twenty neurologically normal young adults were tested. RESULTS: Stable estimates of the threshold height were obtained after delivery of only 15 trials. Threshold height averaged 5.1 mm (range, 3.7 to 6.6 mm) and was unaffected by gender (P>.88). Although the letters were identified correctly 54% of the time, on average, the tangibility of individual letters varied from 5% correct (for W) to 82% correct (for T). The letter W was never identified by 16 of 20 subjects; T, O, and U were identified by all subjects. Analysis of errors confirmed that subjects relied on spatial cues to make the discriminations; 58% of the incorrect responses were made to 1 or 2 letters with spatial features similar to those of the stimulus letter. CONCLUSIONS: The threshold height for letter recognition can be obtained easily and rapidly, exhibits low among-subject variability, and reflects the capacity to extract and process spatial information. Letters of similar legibility on the tongue should be used to minimize underestimation of subjects' true acuities.

Adolescent↗

PubMed1999

SI neuron response variability is stimulus tuned and NMDA receptor dependent.

Skin brushing stimuli were used to evoke spike discharge activity in single skin mechanoreceptive afferents (sMRAs) and anterior parietal cortical (SI) neurons of anesthetized monkeys (Macaca fascicularis). In the initial experiments 10-50 presentations of each of 8 different stimulus velocities were delivered to the linear skin path from which maximal spike discharge activity could be evoked. Mean rate of spike firing evoked by each velocity (MFR) was computed for the time period during which spike discharge activity exceeded background, and an across-presentations estimate of mean firing rate (MFR) was generated for each velocity. The magnitude of the trial-by-trial variation in the response (estimated as CV; where CV = standard deviation in MFR/MFR) was determined for each unit at each velocity. MFR for both sMRAs and SI neurons (MFRsMRA and MFRSI, respectively) increased monotonically with velocity over the range 1-100 cm/s. At all velocities the average estimate of intertrial response variation for SI neurons (CVSI) was substantially larger than the corresponding average for sMRAs (CVsMRA). Whereas CVsMRA increased monotonically over the range 1-100 cm/s, CVSI decreased progressively with velocity over the range 1-10 cm/s, and then increased with velocity over the range 10-100 cm/s. The position of the skin brushing stimulus in the receptive field (RF) was varied in the second series of experiments. It was found that the magnitude of CVSI varied systematically with stimulus position in the RF: that is, CVSI was lowest for a particular velocity and direction of stimulus motion when the skin brushing stimulus traversed the RF center, and CVSI increased progressively as the distance between the stimulus path and the RF center increased. In the third series of experiments, either phencylidine (PCP; 100-500 microg/kg) or ketamine (KET; 0.5-7.5 mg/kg) was administered intravenously (iv) to assess the effect of block of N-methyl-D-aspartate (NMDA) receptors on SI neuron intertrial response variation. The effects of PCP on both CVSI and MFRSI were transient, typically with full recovery occurring in 1-2 h after drug injection. The effects of KET on CVSI and MFRSI were similar to those of PCP, but were shorter in duration (15-30 min). PCP and KET administration consistently was accompanied by a reduction of CVSI. The magnitude of the reduction of CVSI by PCP or KET was associated with the magnitude of CVSI before drug administration: that is, the larger the predrug CVSI, the larger the reduction in CVSI caused by PCP or KET. PCP and KET exerted variable effects on SI neuron mean firing rate that could differ greatly from one neuron to the next. The results are interpreted to indicate that SI neuron intertrial response variation is 1) stimulus tuned (intertrial response variation is lowest when the skin stimulus moves at 10 cm/s and traverses the neuron's RF center) and 2) NMDA receptor dependent (intertrial response variation is least when NMDA receptor activity contributes minimally to the response, and increases as the contribution of NMDA receptors to the response increases).

Action Potentials↗

PubMed1998

Evocation and characterization of percepts of apparent motion on the face.

The percepts evoked by sequential stimulation of sites in close spatial proximity (< or = 2.5 cm) on the face were studied. Both method-of-limits and magnitude-estimation procedures were used to identify and characterize alterations in the percepts produced by systematic changes in the temporal and spatial parameters of the sequence. Each site was stimulated by a vertically oriented row of miniature vibrating probes. Apparent motion was consistently perceived when the delay between the onsets of sequentially activated rows (interstimulus onset interval, or ISOI) fell within a relatively narrow range of values, the lower limit of which approximated 5 msec. Both the upper limit and the perceived smoothness and continuity of the motion percepts (goodness of motion) increased with the duration for which each row stimulated the skin over the range evaluated, 15-185 msec. For the successive activation of only two rows, goodness of motion was not influenced by changes in their separation from 0.4 to 2.5 cm. The ISOI values at which magnitude estimates of goodness of motion were highest increased with the duration for which each row stimulated the skin. As such, maximum goodness of motion decreased with increases in the apparent velocity of motion. When the number of sequentially activated rows was increased from two to four or more, the quality of the motion percepts improved. For the successive activation of multiple closely spaced rows, values of ISOI at which numerical estimates of goodness of motion were highest approximated integral fractions of the duration for which each row stimulated the skin. In this situation, the probes rose and fell in a regular, step-locked rhythm to simulate an edge-like or rectangular object moving across the skin. The goodness of motion so attained was relatively independent of the apparent velocity of motion.

Adult↗

PubMed1997

Utility of square-wave gratings to assess perioral spatial acuity.

PURPOSE: The usefulness of square-wave gratings to assess perioral spatial resolution acuity was evaluated. MATERIALS AND METHODS: A psychophysical tracking procedure was used to estimate the threshold groove width for discriminating orientation (horizontal or vertical) of square-wave gratings pressed into the skin. Ten positionally matched sites on the two sides of the face of 36 right-handed, healthy young adults were studied. Commercially available gratings provided alternating ridge- and-groove stimuli with element widths from 0.35 to 3.0 mm. RESULTS: Thirty-three of the 36 subjects could discriminate orientation at all six sites on the vermilion (threshold width averaged 1.06 mm for grooves and for ridges). Thresholds were lower on the mid-portion of the lower vermilion than on the mid-portion of the upper vermilion (P < .05). Moreover, thresholds were lower on the right side of the vermilion than on the left side (P < .02). In contrast to the vermilion, only 25 and 30 subjects could discriminate orientation on the left and right hairy upper lip, respectively; and two or fewer subjects, at each of 12 sites on the chin and cheeks. CONCLUSIONS: Clinical use of small square-wave gratings with ridge and groove widths of 3 mm or smaller is limited to the vermilion. Moreover, baseline values are needed for individual patients to minimize false-positive diagnoses of sensory impairment. The size required of coarser gratings to test other perioral sites may preclude their use for evaluation of discrete, suspect skin areas.

Adolescent↗

PubMed1997

The response to sudden change in vocal tract resistance during stop consonant production.

The speech respiratory system is configured in ways that tend to maximize its ability to respond to changes in the airway environment. Intraoral pressures remain at levels sufficient to generate reliably recognized consonant sounds even in the presence of structural deficits such as velopharyngeal inadequacy. Similar respiratory compensations occur when bite blocks and bleed valves are used to vent airway pressures. The purpose of the present study was to determine the sensitivity of the monitoring system psychophysically and to assess its physiological response to sudden, unanticipated perturbations. Twenty adults were asked to produce the utterance/pa/, and a calibrated perturbator valve permitted air to escape from the oral cavity on randomly selected productions. Respiratory responses were recorded using PERCI-SARS instrumentation. The results indicated that sudden openings of 0.14 cm2 (SD = 0.04) were detected by speakers. Compensatory respiratory responses to suprathreshold pressure-venting occurred rapidly (i.e., 27 ms [SD = 8]) after valve opening. Although peak pressure and area under the pressure pulse fell with valve opening, the magnitude of pressure was nevertheless sufficient for sound generation. Measurements of the slope of the rise in intraoral pressure after subthreshold pressure-venting in 10 participants were compared to measurements obtained from an-elastic model of the upper airway. The data demonstrated a significant difference between vented and unvented conditions for the model, but not the participants. This suggests that elastic recoil is actively and unconsciously controlled in humans to compensate for losses in airway pressure during speech.

Adult↗

PubMed1997

Low-threshold mechanoreceptive afferents in the human lingual nerve.

Intrafascicular multiunit activity and impulses in single mechanoreceptive afferents were recorded from the human lingual nerve with permucosally inserted tungsten microelectrodes. Nylon filaments and blunt glass probes were used for mechanical stimulation of the mucosa of the dorsal surface of the tongue. The innervation territories of nine nerve fascicles were mapped during multiunit recordings. All fascicle fields included the tip of the tongue, suggesting a particularly high innervation density for this area. Thirty-three single mechanoreceptive afferents were isolated and studied. Of these afferents, 22 were characterized by very small mucosal receptive fields (range: 1-19.6 mm2; geometric mean: 2.4 mm2) and responded to extremely low mechanical forces (force threshold range: 0.03-2 mN; geometric mean: 0.15 mN). As such, it was concluded that these "superficial" units terminated near the surface of the tongue. The remaining 11 units responded to probing of large areas of the tongue (> 200 mm2) and exhibited high force thresholds (> or = 4 mN). It was concluded that these "deep" units terminated in the muscle mass of the tongue. Fourteen of the superficial units were classified as rapidly adapting and resembled the fast-adapting type I afferents described for the glabrous skin of the human hand. The rapidly adapting units responded both during the application and removal of, but not during maintenance of, the mechanical stimuli on the receptive field. Two types of slowly adapting responses were observed. One type (characteristic of only 2 units) was characterized by a pronounced sensitivity to force change during the application and removal of the mechanical stimuli and an irregular static discharge during maintenance of the stimulus on the receptive field. In contrast, the other six units exhibited a weak sensitivity to force change, a highly regular static discharge, and spontaneous activity. As such, these two types of slowly adapting units resembled the slowly adapting I and II afferents, respectively, described for the hand. All 11 deep units were slowly adapting, and 7 were, in addition, spontaneously active. The units were not equally sensitive to the application and removal of the mechanical stimuli, suggesting at least two different modes of termination in tongue muscle. The deep units reliably encoded information about tongue movements in the absence of direct contact with the receptive field. In contrast, the superficial units responded vigorously when the tongue was moved to bring the receptive field into physical contact with other intraoral structures.

Adult↗

PubMed1995

Gender-, side- and site-dependent variations in human perioral spatial resolution.

Twenty-eight right-handed, young adults participated in a sensory testing experiment to evaluate spatial resolution at 10 positionally matched sites on the right- and left-hand sides of the face. An adaptive psychophysical (i.e. tracking) procedure was used to estimate the threshold spatial separation for perceiving two points of contact at each site. Estimates of the threshold at one site on both sides of the face were also obtained with a method-of-limits procedure similar to that employed for clinical evaluation of patients. In addition, each individual was asked to rate (i) his(her) overall facial sensitivity to touch and (ii) the degree to which he(she) could discern subtle changes in lip, cheek and chin position during speech, chewing and facial expression. Analysis of the estimates of the threshold separation obtained with the tracking procedure revealed a significant effect of gender (p < 0.04) and of site (p < 0.001). Females were more spatially sensitive than males: average threshold separations were 1.55 mm less. Most notably, the threshold increased ninefold with distance posterolaterally from the oral opening. The vermilion of the upper lip was the most spatially sensitive site (population geometric mean = 2.4 mm) and the preauricular skin the least spatially sensitive site (20.9 mm). Significant effects of side and of interactions among gender, side and site were not observed. The estimates obtained with the method-of-limits procedure were very similar to those obtained with the tracking procedure: the latter were 0.67 mm less on the average. Individuals' ratings of overall facial sensitivity to touch were similar for males and females (p > 0.70). Females, however, reported greater ability to discern subtle changes in lip, cheek and chin position than males (p < 0.03). The ratings of this sensory function correlated negatively with the estimates of the threshold separation on the vermilion of the upper lip (p < 0.03).

Adaptation, Physiological↗

PubMed1995

Receptor encoding of moving tactile stimuli in humans. I. Temporal pattern of discharge of individual low-threshold mechanoreceptors.

The response of 70 cutaneous, low-threshold mechanoreceptors in the human median, radial and inferior alveolar nerves to well controlled brush stimuli moving across the receptive field was quantitatively studied. Microneurography was used to obtain the response of each to multiple velocities from 0.5 to 32 cm/sec in at least two opposing directions. A high degree of response consistency was observed from the slowly adapting receptors to replication of the same stimulus and to a lesser, but significant degree from the fast adapting receptors. The evoked discharge reflected up to three partially overlapping phases of the moving stimulus: skin compression, indentation, and stretch. Although the overall discharge rate increased with both stimulus velocity and force, the spatial discharge pattern was preserved to a high degrees. In contrast, the discharge patterns differed for opposing and orthogonal directions. Reducing the area of skin surrounding the receptive field that was contacted by the moving stimuli had little effect on the evoked response. Individual mechanoreceptors display highly reliable differences to brush stimuli moving at different velocities. to brush stimuli moving at different velocities. Moreover, different directions of movement evoke differences in the discharge that are consistently observed upon replication of the same stimuli. Despite the richness and consistency in the spatial discharge pattern displayed by individual receptors, it is argued that the details of the patterns are not likely used by the CNS to infer information about direction and velocity of movement across the skin. Rather, the intensity of discharge is proposed as a plausible information-bearing attribute of the stimulus-evoked response.

Adult↗

PubMed1995

Receptor encoding of moving tactile stimuli in humans. II. The mean response of individual low-threshold mechanoreceptors to motion across the receptive field.

The mean firing rate evoked in 70 cutaneous, low-threshold mechanoreceptors in the human median, radial, and inferior alveolar nerves by stimulus motion across the skin was quantitatively studied. Moving stimuli, controlled for velocity, direction, and length of skin traversed, were provided by a servo-controlled motor that carried a brush across the receptive field. Each unit was studied with stimuli delivered at multiple velocities from 0.5 to 32 cm/sec in at least two opposing directions. A power function provided an excellent description of the MFR-versus-velocity relationship. The exponent n was interpreted to reflect the receptor's sensitivity to changes in stimulus velocity, and the multiplicative constant c, the predicted response to stimuli moving at 1.0 cm/sec. The fast adapting mechanoreceptors exhibited higher sensitivity to stimulus velocity than the slowly adapting mechanoreceptors. The mean velocity at which the fast adapting units were predicted to first respond to movement was also higher. Estimates of n, c, or both differed significantly for stimuli delivered in opposing directions for more than 70% of the mechanoreceptors. No direction of motion consistently led to power function parameters with higher values so as to suggest a "preferred" regional direction of motion for the entire population. Neither the directional difference in n nor c could be attributed to directional differences in the forces applied across the receptive fields. These findings suggest that information about velocity and direction is represented in the mean firing rate responses evoked in the population of mechanoreceptors activated by a moving tactile stimulus.

Adult↗

PubMed1994

Effect of stimulus force on perioral direction discrimination: clinical implications.

Clinicians have used the same instrument (viz, Semmes-Weinstein pressure aesthesiometers [Research Design, Inc, Houston, TX] or "von Frey hairs") for tests of both contact detection and direction discrimination. However, patients' ability to discriminate direction may be underestimated by barely detectable moving stimuli. To determine whether the aesthesiometers underestimate direction discrimination, we evaluated the capacity of 13 normal subjects to distinguish opposing directions provided by 10 different hairs. The hairs were selected to deliver forces below and above the contact-detection threshold. Each was stroked over 1.0 cm of perioral skin at the velocity at which the subject was predicted to best discriminate direction of motion. It was found that valid estimates of perioral direction discrimination can be obtained with appropriately selected aesthesiometers. Specifically, the least stiff hair whose handle displays a manufacturer's marking two units greater than that of the contact-detection "threshold hair" should be used to deliver the moving stimuli. The resultant force applied by this hair will exceed 10 times the subject's contact-detection threshold force. If a less-stiff hair is used, the capacity to distinguish direction may be underestimated.

Adolescent↗

PubMed1993

Spatial and gender-dependent variations in perioral pinprick sensitivity.

Twenty-eight right-handed young adults participated in a sensory testing experiment to evaluate pinprick sensitivity at ten spatially matched sites on the right and left sides of the face. Stimuli were provided by a sharp-pointed dental explorer on which a rubber eraser had been positioned to minimize variations in the extent to which the skin was indented. Sharpness was defined as the magnitude at which abrupt, localized pricking/stinging sensations were evoked. A magnitude matching procedure was used to reduce among-subject variability in the data. Specifically, each estimate of sharpness was adjusted (i.e., divided) by the subject's mean estimate of the brightness of a visual stimulus. Prior to data collection, subjects were carefully instructed on the use of a common numerical scale for assignment of values of sharpness and brightness. Repeated-measures analysis of variance of the adjusted estimates of sharpness revealed a non-significant effect of gender (p > 0.4), a highly significant effect of side (p < 0.0001), and a highly significant effect of test site (p < 0.0001). Pinprick percepts were sharper on the left side of the face than on the right. Moreover, the vermilion of the upper lip exhibited the greatest sensitivity to pinprick; the vermilion of the lower lip exhibited the least sensitivity. These results suggest that use of a patient's sensitivity to pinprick during clinical neurosensory examination must be undertaken in an informed manner. A conclusion of pathological alteration in sensation can be made only after consideration of the normal spatial variations in the percept of sharpness.

Adolescent↗

PubMed1993

The response of SI directionally selective neurons to stimulus motion occurring at two sites within the receptive field.

Data from two classes of primary somatosensory (SI) neurons (termed "direction-invariant" and "direction-variant") were analyzed to evaluate their capacity to process the directional information provided by two moving (i.e., brushing) stimuli delivered to nonoverlapping skin sites within the receptive field (RF). The stimulus sites were arranged either end to end or side by side on the skin. The two stimuli were delivered at the same time (i.e., simultaneously) or asynchronously in precisely defined orders. For both classes of neurons, and with both the end-to-end and side-by-side dual-stimulus arrangements, the response elicited by dual-site stimulation was usually much less than a linear summation of the responses elicited by independent stimulation of each site. For the direction-invariant neurons, when the two sites were arranged end to end and direction of motion at both sites was the same, directional sensitivity with dual-site stimulation most often matched or exceeded a vectorial sum of the sensitivities observed at each site when stimulated alone. In contrast, with the side-by-side arrangement, the level of directional sensitivity achieved with dual-site stimulation often failed to attain that predicted by vectorial summation of the sensitivities observed at each site. Instead, directional sensitivity under this dual-stimulus condition only approximated that attained with single-site stimulation at the more sensitive site. When noncorresponding directions of motion were presented at two sites within the RF (using either the end-to-end or side-by-side arrangement), direction-invariant neurons failed to respond differentially to opposing patterns of dual-site stimulation. For the direction-variant SI neurons, a particular end-to-end arrangement of the two sites within the RF was studied: Sites were identified on opposite sides of the within-RF boundary that in these neurons separates regions with opposite directional preferences. With this arrangement, the differential response was greater when opposite directions of motion were applied to the two sites than it was when the same direction of motion was delivered at both sites. The observations suggest that for both groups of SI neurons, the magnitude of directional sensitivity is dependent on the same attributes of dual-site stimulation that influence cutaneous directional sensitivity--that is, on the spatial arrangement of and temporal delay between the two stimuli, and on the correspondence of their directions. The effects of dual-site stimulation on the behavior of these two neuron populations appear to be in good agreement with the hypothesis that they subserve a function in tactile motion perception.

Afferent Pathways↗

PubMed1992

Characterization of the percepts evoked by discontinuous motion over the perioral skin.

The capacity of human subjects to process information about discontinuous and continuous movement was evaluated. Constant-velocity brushing stimuli were delivered through aperture plates that rested lightly upon the mandibular skin. Each plate consisted of either two spatially separated, slit-like openings or a single continuous, longer opening. It was discovered that percepts of smooth apparent motion were achieved with the split apertures (i.e., from discontinuous movement) for only limited ranges of stimulus velocity. Moreover, the optimal velocity supporting smooth apparent motion increased with the separation between the slit-like openings. In a second series of experiments, subjects' ability to discriminate opposing directions of discontinuous and continuous movement was evaluated. It was found that subjects could derive directional information from percepts elicited by discontinuous movement. However, the capacity to discriminate opposing directions of continuous movement cannot be explained solely in terms of the ability to process information about the change in position of a stimulus from its onset to its offset.

Discrimination Learning↗

PubMed1991

Directional sensitivity along the upper limb in humans.

The capacity of four neurologically healthy young adults to distinguish opposing directions of cutaneous motion was determined at five different sites along the proximal-distal axis of the upper limb. Constant-velocity brushing stimuli (ranging from 0.5 to 32.0 cm/sec) were delivered through an aperture in a Teflon plate that was securely positioned in light contact with the skin. In one series of experiments, directional sensitivity in d' units was assessed at each site, using an aperture length of 0.75 cm. In a second series of experiments, the aperture length required to obtain the same criterion level of directional sensitivity at each site was determined. To attain the sensitivity reached at distal sites, a proximal stimulus had to traverse a longer chord of skin. Specifically, chords 5.9 times longer on average (range = 5.4-6.2) were required on the proximal forearm than on the index finger pad. This finding suggests that relative directional sensitivity increases sixfold from the proximal forearm to the finger pad. Moreover, relative directional sensitivity on the shoulder was comparable to that observed on the proximal forearm for two of the subjects, and approximately one-half that observed on the proximal forearm for the other two subjects. In addition to such a prominent spatial gradient in relative directional sensitivity, the velocity of stimulus motion at which directional sensitivity was highest increased systematically as the test site was shifted from the finger pad to the proximal forearm. Specifically, the optimal velocity on the finger pad varied among subjects from 1.5 to 9.4 cm/sec (mean = 5.4 cm/sec), and on the proximal forearm from 11.5 to 31.2 cm/sec (mean = 18.6 cm/sec). The optimal velocity on the shoulder was not significantly different from that observed on the proximal forearm. The results suggest that effective and informed clinical testing of patients' capacity to distinguish opposing directions of motion on cutaneous regions that differ in peripheral innervation density requires appreciation of the sensitivities of different skin regions, as well as the unique velocity dependency of direction discrimination at each skin site.

Adolescent↗