Multidimensional scaling reveals two dimensions of thermal pain

The Individual Differences Scaling (INDSCAL) model of multidimensional scaling was used to explore the dimensions of thermal pain. The observers made 66 similarity judgments to all pairs of 12 different thermal stimulus intensities ranging from zero to noxious. Analysis of the data revealed a two-dimensional group stimulus space. The major dimension ordered the stimuli with respect to their intensity. This quantitative, strength-of-sensation dimension may be interpreted as indicating how weak or strong a stimulus feels, apart from any secondary qualities of warmth or pain. The second dimension was related to the qualitative aspects of the stimuli. This bipolar dimension contained two attributes: a pain attribute ranging from just detectable warmth to painful, and a warm-hot attribute running from just detectable warmth to hot. This study demonstrates the utility of the INDSCAL approach to the understanding of pain and offers a new technique for answering the age-old question concerning the number and qualities of the dimensions underlying the pain experience.     

Comparison of perceptual dimensions uncovered from infant-cry signals using the method of pair-comparisons and the semantic differential

Since infant cry sounds may provide caregivers with information concerning an infant's needs, several studies have examined the perceptual features of cry signals. This tradition is followed in the present study, which utilises two techniques; the method of pair-comparisons (coupled to INDSCAL) and the Semantic Differential (SD). In each case 24 infant cries (six each of pain, hunger, pleasure and birth) are used as stimuli. Data were collected from mothers; 39 rated each cry on 50 SD scales, and 26 provided ratings of similarity for every possible pairwise combination of cries. The similarity of the perceptual spaces described by the two techniques is demonstrated by a canonical analysis which compared the SD factor matrix with the INDSCAL dimension-weight matrix. Two of three possible common dimensions were uncovered. This similarity is also indicated by a standard multiple regression analysis by which the INDSCAL dimensions were described in terms of the SD scales. However, it appears the SD emphasised emotional characteristics of the cry signals whereas INDSCAL emphasised physical attributes.     

Neural mechanisms underlying pain’s ability to reorient attention: Evidence for sensitization of somatic threat detectors

Pain typically signals damage to the body, and as such can be perceived as threatening and can elicit a strong emotional response. This ecological significance undoubtedly underlies pain’s well-known ability to demand attention. However, the neural mechanisms underlying this ability are poorly understood. Previous work from the author’s laboratory has reported behavioral evidence suggesting that participants disengage their attention from an incorrectly cued visual target stimulus and reorient it toward a somatic target more rapidly when the somatic target is painful than when it is nonpainful. Furthermore, electrophysiological data suggest that this effect is mediated by a stimulus-driven process, in which somatic threat detectors located in the dorsal posterior insula activate the medial and lateral prefrontal cortex areas involved in reorienting attention toward the painful target. In these previous studies, the painful and nonpainful somatic targets were given in separate experiments involving different participants. Here, the nonpainful and painful somatic targets were presented in random order within the same block of trials. Unlike in the previous studies, both the nonpainful and painful somatic targets activated the somatic threat detectors, and the times taken to disengage and reorient attention were the same for both. These electrophysiological and behavioral data suggest that somatic threat detectors can become sensitized to nonpainful somatic stimuli that are presented in a context that includes painful stimuli.     

Algebraic models for integration of painful and nonpainful electric shocks

Subjects rated the average intensity of two sequentially presented electric shocks, which were either painful (intensity range from 1 to 4 mA, Experiment 1) or nonpainful (intensity range from .6 to .9 mA). In both experiments, stimuli were presented in a 4 by 4 factorial design to allow tests of algebraic models of the integration process under analysis of variance (Anderson, 1970, 1974). Both sets of data were fit by the equation R_{i j}=w₁s₁+w₂+s₂, (w₁+w₂=1), where R_{i j}is the rating scale response, s₁ and s₂ the scale values for the first and second stimuli, respectively, and w₁ and w₂ associated weights. When stimuli were painful, w₁=w₂, and when stimuli were nonpainful, w₁< w₂. Subjective scale values as a function of stimulus intensity appear to increase nonlinearly for painful stimuli and linearly for nonpainful stimuli. Some implications of the results for pain research are discussed.     

Bilingualism and the perception of professional concepts

A selected set of professional concepts was subjected to analysis through two separate multidimensional scaling techniques, the INDSCAL and TORSCA models, to evaluate the intergroup perceptual differences of four experimental groups, made up of unilingual French, unilingual English and bilingual students. The linguistic relativism thesis provided the research hypotheses on the relationship between language access and usage and concept perception. The multidimensional scaling techniques were applied to the matrix of subjects' similarity judgments on pairs of concepts, thus enabling the identification of three dimensions. The dimensions were labelled as conjunctive, relational and disjunctive, and were assumed to be related to the criteria employed by the subjects in their similarity rankings. An analysis of variance of the individual saliences on each dimension provided evidence of linguistic relativism for both the relational and disjunctive dimensions. These findings support the contention that unilingual speakers of separate languages differ from each other and from bilingual speakers in their perception of professional concepts.     

Multidimensional scaling of individual differences for an individual person with a hand-held calculator

An extension to the INDSCAL method of individual differences scaling was developed. The method derives measures of deviation from a “standard” observer, who is defined in terms of the INDSCAL subject weights. These weights are calculated for one individual person at a time. The method is implemented in a small hand-held calculator. The calculator is used for data acquisition, and it interacts with the user in a conversational manner through an alpha-numeric display. The present implementation of the method scales 84 judgments of pair similarity among seven objects and derives subject weights on two dimensions. The method can be extended to sets of up to 16 objects and up to six dimensions. The method is a feasible, cost-effective approach to classification of individuals on complex perceptual attributes within a single 1-h session.     

Dimensions of the perception of art: verbal scales and similarity judgements

Abstract.— An INDSCAL analysis was performed on subjects' similarity judgements of all pairs of 12 landscape paintings. Three principal INDSCAL dimensions were identified by analysing responses to 24 unidimensional rating scales derived from previous experiments. The present findings, together with those of previously published studies, suggest that three of the principal dimensions of perception of art appear to be (1) Hedonic-Representational, (2) Clarity, (3) A Dynamic factor involving activity, balance and symmetry. It is suggested that although Complexity may be an important factor influencing responses to simple stimuli, its rôle in the perception of paintings is small, and has frequently been over-emphasised.     

Dimensional dominance and stimulus discriminability.

The relationship between the discriminability of scaling stimuli and assessed dimensional dominance was investigated in three studies. Sixty kindergarten children were assessed using a psychophysical scaling method to determine JND values for the size, brightness, and orientation dimensions. Thirty of the same subjects were then assessed for dimensional dominance using stimuli of two levels of known discriminability, based on the obtained JND values. In a third experiment, the remaining 30 subjects were assessed for dimensional dominance using stimuli that systematically emphasized the values of one dimension relative to the other two dimensions, again based on obtained JND values.The results indicated that (1) kindergarten children were able to perform consistently during psychophysical scaling, and there was little variability between children in their judgments of stimuli, (2) the overall level of discriminability affects dimensional dominance scaling behavior, with subjects more likely to exhibit dominance for one dimension when all the values of scaling stimuli are high in discriminability, and (3) some support for the hypothesis that dimensional dominance scaling behavior can be manipulated by manipulating the relative discriminability of scaling stimuli was found, though the trend was not clear.The relative discriminability of scaling stimuli appears to have some effects on dimensional dominance scaling behavior of young children. However, some pre-experimental bias to attend to a particular dimension seems to remain even when the values of all dimensions present are of equal and known discriminability. Dimensional dominance is a function of an interaction between discriminability of scaling stimuli and the experiential bias or perceptual set of the subject.     

Effects on indscal of non-orthogonal perceptions of object space dimensions

A general question is raised concerning the possible consequences of employing the very popular INDSCAL multidimensional scaling model in cases where the assumptions of that model may be violated. Simulated data are generated which violate the INDSCAL assumption that all individuals perceive the dimensions of the common object space to be orthogonal. INDSCAL solutions for these various sets of data are found to exhibit extremely high goodness of fit, but systematically distorted object spaces and negative subject weights. The author advises use of Tucker's three-mode model for multidimensional scaling, which can account for non-orthogonal perceptions of the object space dimensions. It is shown that the INDSCAL model is a special case of the three-mode model.     

Qualitative hemispheric differences in semantic category matching.

A unilateral category matching task with words as stimuli was employed to investigate semantic processing in the right and left hemispheres (RH, LH). An overall right visual field (RVF)/LH dominance was observed and performances were better than chance, also in the left visual field (LVF)/RH. A qualitative analysis of reaction times with individual differences multidimensional scaling (INDSCAL) revealed that LVF/RH INDSCAL solutions were significantly more differentiated in structure than RVF/LH solutions in terms of number and size of dimensions. These findings support a depth of activation hypothesis of hemispheric processing, with the LH rapidly and focally and the RH slowly and diffusely activating the semantic network.     

Laterality and modality-specific effects of chronic pain

Previous investigations indicated that thresholds to nonpainful tactile stimuli were elevated in chronic-pain patients when compared with pain-free individuals (Seltzer & Seltzer, 1986; Seltzer et al., 1988). The present study attempted to determine whether thresholds to tactual and visual stimuli also were elevated by chronic pain. Furthermore, lateralization of the pain effect on tactile thresholds was assessed by obtaining thresholds from both left and right arms. A decrease in tactile sensitivity to nonpainful stimuli in chronic-pain patients was confirmed, but laterality of the effect was not demonstrated. Visual thresholds were not significantly affected by chronic pain. The data in the present study, taken together with other data, support the proposition that pain does not affect right hemispheric processes more than left hemispheric processes.     

Laterality and modality-specific effects of chronic pain.

Previous investigations indicated that thresholds to nonpainful tactile stimuli were elevated in chronic-pain patients when compared with pain-free individuals (Seltzer & Seltzer, 1986; Seltzer et al., 1988). The present study attempted to determine whether thresholds to tactual and visual stimuli also were elevated by chronic pain. Furthermore, lateralization of the pain effect on tactile thresholds was assessed by obtaining thresholds from both left and right arms. A decrease in tactile sensitivity to nonpainful stimuli in chronic-pain patients was confirmed, but laterality of the effect was not demonstrated. Visual thresholds were not significantly affected by chronic pain. The data in the present study, taken together with other data, support the proposition that pain does not affect right hemispheric processes more than left hemispheric processes.     

Individual differences in perceptual space for tactile textures: evidence from multidimensional scaling

Ratio scaling was used to obtain from 5 subjects estimates of the subjective dissimilarity between the members of all possible pairs of 17 tactile surfaces. The stimuli were a diverse array of everyday surfaces, such as corduroy, sandpaper, and synthetic fur. The results were analyzed using the multidimensional scaling (MDS) program ALSCAL. There was substantial, but not complete, agreement across subjects in the spatial arrangement of perceived textures. Scree plots and multivariate analysis suggested that, for some subjects, a two-dimensional space was the optimal MDS solution, whereas for other subjects, a three-dimensional space was indicated. Subsequent to their dissimilarity scaling, subjects rated each stimulus on each of five adjective scales. Consistent with earlier research, two of these (rough/smooth and soft/hard) were robustly related to the space for all subjects. A third scale, sticky/slippery, was more variably related to the dissimilarity data: regressed into three-dimensional MDS space, it was angled steeply into the third dimension only for subjects whose scree plots favored a nonplanar solution. We conclude that the sticky/slippery dimension is perceptually weighted less than the rough/smooth and soft/hard dimensions, materially contributing to the structure of perceptual space only in some individuals.     

A multidimensional scaling analysis of 16 complex sounds

The INDSCAL multidimensional scaling model was used to investigate the distinctive features involved in the perception of 16 complex nonspeech sounds. The signals differed along four physical dimensions: fundamental frequency, waveform, formant frequency, and number of formants. Scaling results indicated that subjects’ similarity ratings could be accounted for by three psychological or perceptual dimensions. A statistically reliable correspondence was observed between these perceptual dimensions and the physical characteristics of fundamental frequency, waveform, and a combination of the two formant parameters. These results were further explored with Johnson’s (1967) hierarchical clustering analysis. Large differences in featural saliency occurred in the group data with fundamental accounting for more variability than the remaining dimensions. Further analysis of individual subject data revealed large individual differences in featural saliency. These differences were related to past musical experience of the subject and to earlier findings using similar signals. It was concluded that (1) the INDSCAL model provides a useful method for the analysis of auditory perception in the nonspeech mode, and (2) featural saliency in such sounds is likely to be determined by an unspecified attentional mechanism.     

Threshold tracking for assessment of long-term adaptation and sensitization in pain perception

To assess temporal variations in the perception of "phasic" heat pain stimuli a psychophysical tracking procedure was developed that enables repeated assessment of the pain threshold at short intervals. This "double-tracking" procedure produces two tracking curves simultaneously, one that approaches the pain threshold gradually from above, the other from below. The threshold for phasic heat pain was measured in 80 tracking trials with stimuli at temperatures near the pain threshold. Concurrently, the threshold for "tonic" heat pain was determined after every 20 tracking trials with a stimulus adjustment procedure. Eleven healthy subjects (age: 26.4 yr. +/- 6.0) participated in 2 sessions each. Phasic stimulation near the pain threshold did not produce any trends in either of the two threshold measures. Hence there was no long-term adaptation or sensitization. However, there were random variations (random walks) in the tracking curves, which we interpret as resulting from a stochastic relationship between stimulus and sensation. In agreement with other reports, discrimination seemed to be better at painful than at nonpainful temperatures.     

Perceptual dimensions of tactile surface texture: A multidimensional scaling analysis

The purpose of this study was to examine the subjective dimensionality of tactile surface texture perception. Seventeen tactile stimuli, such as wood, sandpaper, and velvet, were moved across the index finger of the subject, who sorted them into categories on the basis of perceived similarity. Multidimensional scaling (MDS) techniques were then used to position the stimuli in a perceptual space on the basis of combined data of 20 subjects. A three-dimensional space was judged to give a satisfactory representation of the data. Subjects’ ratings of each stimulus on five scales representing putative dimensions of perceived surface texture were then fitted by regression analysis into the MDS space. Roughness-smoothness and hardness-softness were found to be robust and orthogonal dimensions; the third dimension did not correspond closely with any of the rating scales used, but post hoc inspection of the data suggested that it may reflect the compressional elasticity (“springiness”) of the surface.     

Combining experiences over time: the effects of duration,intensity changes and on-line measurements on retrospective pain evaluations

Two experiments were conducted to examine the effects of various factors on retrospective pain evaluation. The factors examined in Experiment 1 were the rate and pattern of change, the intensity (particularly the final intensity), and the duration of the painful experience. Experiment 2 manipulated these factors and, in addition, examined the effect of continuous (on-line) ratings on the overall retrospective evaluation. The two experiments utilized different pain modalities, heat in the first and mechanical pressure in the second. In addition, all subjects in Experiment 1 experienced stimuli with the same physical magnitude, while in Experiment 2 stimuli were individually tailored to make them subjectively equivalent. In both experiments, subjects were presented with a series of painful stimuli and evaluated the intensity of each stimulus immediately upon its termination. The stimuli themselves were composed of multiple intensity levels that differentially changed over time (Intensity-Patterns). Subjects' on-line ratings in Experiment 2 closely mirrored the physical patterns of the intensities. The main conclusion from both experiments is that the retrospective evaluations of painful experiences are influenced primarily by a combination of the final pain intensity and the intensity trend during the latter half of the experience. In addition, results indicated that duration has little impact on retrospective evaluations for stimuli of relatively constant intensity. However, when the stimulus intensity changes over time, duration does play a role. Finally, the task of continuously reporting the stimulus intensity had a moderating impact on the retrospective evaluations. © 1998 John Wiley & Sons, Ltd.     

疼痛表征在自我和他人间的一致性效应

在以往研究中“pain matrix”被认为是加工自我和他人疼痛信息的特异性神经机制, 也可能是个体对他人疼痛共情的原因。但是最近的研究发现非疼痛的感觉刺激也能激活该脑区, 因此疼痛表征在自我和他人间是否存在一致性受到质疑。研究采用启动范式从两个方面入手探索自身疼痛和疼痛共情间是否存在一致性关系。实验一使用疼痛和非疼痛图片为启动刺激, 疼痛或热刺激为靶刺激, 发现相对于非疼痛图片, 疼痛图片启动下被试对自身疼痛刺激的反应时减少, 疼痛水平和情绪反应增强。实验二使用疼痛和热刺激为启动刺激, 疼痛或非疼痛图片为靶刺激, 发现相对于热刺激, 疼痛刺激启动下被试对疼痛图片的反应时减少。同时, 在两个实验中非疼痛的靶刺激都不受启动刺激的影响。这说明疼痛表征在自我和他人间存在一致性效应。