Human Foot Outperforms the Hand in Mechanical Pain Discrimination

Tactile discrimination has been extensively studied, but mechanical pain discrimination remains poorly characterized. Here, we measured the capacity for mechanical pain discrimination using a two-alternative forced choice paradigm, with force-calibrated indentation stimuli (Semmes-Weinstein monofilaments) applied to the hand and foot dorsa of healthy human volunteers. In order to characterize the relationship between peripheral nociceptor activity and pain perception, we recorded single-unit activity from myelinated (A) and unmyelinated (C) mechanosensitive nociceptors in the skin using microneurography. At the perceptual level, we found that the foot was better at discriminating noxious forces than the hand, which stands in contrast to that for innocuous force discrimination, where the hand performed better than the foot. This observation of superior mechanical pain discrimination on the foot compared to the hand could not be explained by the responsiveness of individual nociceptors. We found no significant difference in the discrimination performance of either the myelinated or unmyelinated class of nociceptors between skin regions. This suggests the possibility that other factors such as skin biophysics, receptor density or central mechanisms may underlie these regional differences.

The role of Weber's law in human time perception

Weber's law predicts that stimulus sensitivity will increase proportionally with increases in stimulus intensity. Does this hold for the stimulus of time - specifically, duration in the milliseconds to seconds range? There is conflicting evidence on the relationship between temporal sensitivity and duration. Weber's law predicts a linear relationship between sensitivity and duration on interval timing tasks, while two alternative models predict a reverse J-shaped and a U-shaped relationship. Based on previous research, we hypothesised that temporal sensitivity in humans would follow a U-shaped function, increasing and then decreasing with increases in duration, and that this model would provide a better statistical fit to the data than the reverse-J or the simple Weber's Law model. In a two-alternative forced-choice interval comparison task, 24 participants made duration judgements about six groups of auditory intervals between 100 and 3,200 ms. Weber fractions were generated for each group of intervals and plotted against time to generate a function describing sensitivity to the stimulus of duration. Although the sensitivity function was slightly concave, and the model describing a U-shaped function gave the best fit to the data, the increase in the model fit was not sufficient to warrant the extra free parameter in the chosen model. Further analysis demonstrated that Weber's law itself provided a better description of sensitivity to changes in duration than either of the two models tested.

Weight-Discrimination Sensitivity in Congenitally Blind and Sighted Adults

This study compared the weight-discrimination sensitivity of 41 congenitally blind and 41 normally sighted adults. The superior weight-discrimination sensitivity of the congenitally blind subjects suggests that blindness from birth can cause compensatory adaptations within the cutaneous modality.

Studies in Olfactory Acuity: III. Relative Detectability of n-Aliphatic Acetates by the Rat

The ability of black and albino rats to detect the first seven n-aliphatic acetates in the vapour phase has been investigated by use of an odour choice box. Both the median threshold and the slope of the probit regression line (which describes the relation between concentration and response) tend to decrease logarithmically in value as the series is ascended. The position of methyl acetate in these relations appears anomalous. When expressed as thermodynamic activities median thresholds for acetates of intermediate chain length are about equally stimulating, whilst short chain alcohols excite at decreasing activities with increasing chain length. The close similarity of these findings to those reported in the previous study on n-aliphatic alcohols supports the view that such relations may be of more general occurrence in olfaction in the rat.

The stimulus-response curve for each acetate studied shows one or more reversals in slope. It is suggested that certain of these are due to the presence of two or more olfactory receptor types one or more of which attain maximum response at lower concentrations of a given acetate than do the remainder. The asymptotes of the curves are reached in about 2–3 log units of concentration.

Sensory Factors in the Voluntary Application of Pressure

The experiments described attempt to assess the extent of sensory control of voluntary movements of the forearm and finger when the contractions of the muscles concerned are approximately isometric.

In the first two experiments, records were obtained of the accuracy with which subjects were able to reproduce a particular pressure with the ulnar border of the hand at different pressure values and rates of application. Results show a statistically significant increase in error of reproduction of pressure as the pressure applied decreases and the rate of application is increased.

In the third experiment, using an applied force of 25 gms. weight, a significant difference in accuracy of reproduction of finger pressure between the maximum and a slower rate of application is also shown, but not when the digital nerves are blocked by a local anaesthetic. Furthermore, there is no significant difference in accuracy of reproduction of finger pressure between the anaesthetized and unanaesthetized condition at the fastest rate of application. On the other hand, at the slow rate of application, there is an increasingly significant difference between the anæsthetized and unanaethetized condition as the applied force is reduced below 400 gms. weight, in favour of the control condition.

In the fourth experiment, subjects were trained to apply a force of 25 gms. weight using a small area of skin on the palmar surface of the index finger, then tested for their ability to reproduce the same force before and after changing the size of the area of application. Results show a marked change in applied force corresponding to the change in size of skin area used.

It is concluded that below an applied force of about 400 gms. weight, the maximum accuracy in reproduction of finger pressure is dependent upon cutaneous receptors.

The results of these experiments are discussed in relation to evidence from other experimental, clinical and animal studies and it is suggested that the major function of afferent impulses from a limb in the initiation of voluntary movements is probably facilitatory rather than sensory.

Olfactory receptor neurons use gain control and complementary kinetics to encode intermittent odorant stimuli

Insects find food and mates by navigating odorant plumes that can be highly intermittent, with intensities and durations that vary rapidly over orders of magnitude. Much is known about olfactory responses to pulses and steps, but it remains unclear how olfactory receptor neurons (ORNs) detect the intensity and timing of natural stimuli, where the absence of scale in the signal makes detection a formidable olfactory task. By stimulating Drosophila ORNs in vivo with naturalistic and Gaussian stimuli, we show that ORNs adapt to stimulus mean and variance, and that adaptation and saturation contribute to naturalistic sensing. Mean-dependent gain control followed the Weber-Fechner relation and occurred primarily at odor transduction, while variance-dependent gain control occurred at both transduction and spiking. Transduction and spike generation possessed complementary kinetic properties, that together preserved the timing of odorant encounters in ORN spiking, regardless of intensity. Such scale-invariance could be critical during odor plume navigation.

Reconciling Fechner and Stevens: Toward a unified psychophysical law

How does subjective magnitude, S. increase as physical magnitude or intensity, I, increases? Direct ratings (magnitude scales; partition or category scales) can be fitted by the power function, S = aIb, in which S equals I raised to a power or exponent, b, and multiplied by a measure constant, a. The exponent is typically about twice as large for the magnitude scale (Stevens) as for the corresponding partition or category scale, but the higher exponent may be explained by the overly expansive way people use numbers in making magnitude estimations. The partition or category scale and the adjusted (for the use of number) magnitude scale for a given modality or condition generally agree with the neurelectric scale and the summated just noticeable difference (jnd) scale. A unified psychophysical law is proposed in which each jnd has the same subjective magnitude for a given modality or condition, subjective magnitude increases as approximately a power function of physical magnitude with the exponent ranging from near 0 to 1 (compressive function), and subjective magnitude depends primarily on peripheral sensory processes, that is, no nonlinear central transformations occur. An undue reliance on Weber's law blinded Fechner to the fact that the true psychophysical scale is approximately a power function. Rejecting Weber's law, which is not valid, means that we no longer have to choose between letting the summated jnd scale be a logarithmic function (Fechner's law) and introducing a nonlinear central transformation to make it into a power function (Brentano–Ekman-Teghtsoonian's law). Fechner and Stevens erred equally about the true psychophysical power function, whose exponent lies halfway between that of Fechner (an exponent approaching zero) and that of Stevens. To be reconciled, Fechnerians must give up the assumptions that Webers law is valid and that the jnd has the same subjective magnitude across modalities and conditions; Stevensians must give up the assumption that the unadjusted (for the use of number) magnitude scale is a direct measure of subjective magnitude.

Evaluating tactile sensitivity adaptation by measuring the differential threshold of archers

This study investigated the relationship between the force applied to a finger and the differential threshold of the force. Further, it presented an improvement function for tactile perception in archers by adapting to circumstances in which enhanced tactile perception and finger dexterity are required to practice archery on a daily basis. For this purpose, a tactile display using an air jet was developed. The air was aimed at the center of the fingertip of the index finger. The inner diameter of the nozzle was set to 3 mm. In this study, a psychophysical experiment was conducted to obtain the differential threshold from two subject groups-an archery athlete group and a control group. A total of six levels of standard stimuli ranging from 2.0 gf to 7.0 gf was obtained. As a result, the differential threshold of the archery group was significantly higher than that of the control group. The Weber ratio of the archery group remained around 0.13 and that of control group was 0.10. The experiment also revealed that the differential threshold for archers exhibited less fluctuation between the trials and between the days, which implied that the tactile perception of archery athletes may be more stable than that of non-experienced subjects. This may be a plasticity property of tactile perception.

Comparison is not just subtraction: Effects of time- and space-order on subjective stimulus difference

In five experiments, participants made comparative judgments of paired successive or simultaneous stimuli. Time- or space-order errors were obtained, which varied with the interstimulus interval (ISI) or stimulus duration, as well as with the stimulus level. The results, in terms of scaled subjective differences, are well described by Hellström's (1979) sensation-weighting model. With successive presentation, in comparisons of line length and tone loudness, the first stimulus had the greater weight in determining the subjective difference for short ISIs, the second for longer ISIs. In comparisons of duration (auditory and visual), the second stimulus had the greater weight. For simultaneously presented line lengths, the left stimulus had the greater weight.

Arm weight, adaptation,and weight discrimination

The addition of a large weight to the forearm impairs weight discrimination by an amount which is small by Weber's Law but which could affect skilled tasks. When time is allowed for adaptation to the forearm weight, discrimination with the weight is improved and discrimination without the weight is impaired. Implications for pilots and astronauts under varying g are discussed.