Selection in touch: negative priming with tactile stimuli

Abundant experimental evidence has demonstrated attentional selection within the visual modality. Furthermore, the consensus view is currently that two processes contribute to selection: the amplification of the target stimuli and the ignoring (or suppression) of any distractor information. However, at present it is less clear how selection is achieved within the tactile modality. In the present study, we analyze the aftereffects of ignoring tactile distractors, in a novel tactile variant of the negative priming paradigm. In the typical negative priming paradigm, repeating an ignored distractor stimulus as the target on the following trial usually leads to a cost in terms of reaction times (RTs) or error rates, thereby indicating that selection is achieved in part by the ignoring of distractors. In two experiments, we observed significant RT costs when a previously ignored vibrotactile stimulus constituted the target in the next trial. This result shows that tactile selection is in part achieved by active ignoring of distractor representations, as has been shown previously in both the visual and auditory modalities.     

Numerosity discrimination of tactile stimuli.

The ability of 40 college students to discriminate the number of tactile stimuli presented simultaneously was measured in two experiments. Simulation was provided by 12 solenoids fixed to points on the arms and legs. Exp. I showed a mean correct discrimination level of between 1.55 and 2.10 solenoids. Exp II compared discrimination of numerosity by a group who received immediate feedback with a no-feedback control group. Feedback produced a small increase in this ability. However, the most stimulation points correctly discriminated did not exceed three. The data are discussed in terms of the possible role of cutaneous masking.     

Amplitude and spatial-period discrimination in sinusoidal gratings by dynamic touch

We measured discrimination thresholds for sinusoidal gratings using active dynamic touch. In the first experiment we measured the thresholds for amplitude discrimination as a function of amplitude and spatial period. Thresholds ranged between 10.8%, and 15.8% of the standard amplitude. We showed that amplitude differences as small as 2 microm can be detected. We found that Weber fractions for amplitude discrimination are constant over a range of amplitudes, but are influenced by the spatial period of the grating; discrimination improved with increasing spatial period. In the second experiment we determined the thresholds for spatial-period discrimination. We used the same design as in the first experiment. Weber fractions ranged from 6.4% to 11.8%. Amplitude was found to have no effect on the Weber fractions for spatial-period discrimination. However, the spatial period had an effect on the Weber fractions: larger spatial periods yielded lower Weber fractions.     

Investigating the time course of tactile reflexive attention using a non-spatial discrimination task

Peripheral cues are thought to facilitate responses to stimuli presented at the same location because they lead to exogenous attention shifts. Facilitation has been observed in numerous studies of visual and auditory attention, but there have been only four demonstrations of tactile facilitation, all in studies with potential confounds. Three studies used a spatial (finger versus thumb) discrimination task, where the cue could have provided a spatial framework that might have assisted the discrimination of subsequent targets presented on the same side as the cue. The final study circumvented this problem by using a non-spatial discrimination; however, the cues were informative and interspersed with visual cues which may have affected the attentional effects observed. In the current study, therefore, we used a non-spatial tactile frequency discrimination task following a non-informative tactile white noise cue. When the target was presented 150 ms after the cue, we observed faster discrimination responses to targets presented on the same side compared to the opposite side as the cue; by 1000 ms, responses were significantly faster to targets presented on the opposite side to the cue. Thus, we demonstrated that tactile attentional facilitation can be observed in a non-spatial discrimination task, under unimodal conditions and with entirely non-predictive cues. Furthermore, we provide the first demonstration of significant tactile facilitation and tactile inhibition of return within a single experiment.     

Blindness enhances tactile acuity and haptic 3-D shape discrimination

This study compared the sensory and perceptual abilities of the blind and sighted. The 32 participants were required to perform two tasks: tactile grating orientation discrimination (to determine tactile acuity) and haptic three-dimensional (3-D) shape discrimination. The results indicated that the blind outperformed their sighted counterparts (individually matched for both age and sex) on both tactile tasks. The improvements in tactile acuity that accompanied blindness occurred for all blind groups (congenital, early, and late). However, the improvements in haptic 3-D shape discrimination only occurred for the early-onset and late-onset blindness groups; the performance of the congenitally blind was no better than that of the sighted controls. The results of the present study demonstrate that blindness does lead to an enhancement of tactile abilities, but they also suggest that early visual experience may play a role in facilitating haptic 3-D shape discrimination.     

Frequency discrimination between and within line gratings by dynamic touch

Line gratings were used to investigate the tactual discrimination thresholds for line frequency. In Experiment 1, participants were asked to discriminate between two gratings, each with a different line frequency. We used four standard frequencies in the eightfold range from 0.5 to 4 lines/cm. Thresholds were found to be constant at about 10.6%. In this experiment, we also measured hand speed and contact force. Hand speed was roughly in the range between 0.12 and 0.44 m/sec; contact force ranged from 0.62 to 2.76 N. In Experiment 2, we determined discrimination thresholds for line frequency transitions within a single grating. We used two frequencies and three transition lengths. The transition length had no effect on the threshold. In a third experiment, line frequency was modulated periodically. Varying the standard frequency and the size of the modulation period was found to have no effect on the discrimination thresholds. We conclude three things. First, Weber fractions for line frequency discrimination decrease as a function of line frequency within the experimental range. Second, discrimination thresholds are not altered by the length of the transition between two adjacent gratings with different line frequencies. And finally, the size of a modulation period in periodically modulated gratings is of no influence on the modulation detection threshold.     

Taste,odor, and tactile discrimination before and after smoking

Differential sensitivity was measured before and after smoking a cigarette by seven smokers, using six non-smokers as controls. Stimuli consisted of the tastes of sucrose, sodium chloride, citric acid, caffeine, and quinine hydrochloride; the odors of vanillin and 2-butanone; and the viscosity produced by a carrageenan gum. No significant jnds were observed between smokers and non-smokers, nor before vs after smoking. Therefore, the data supported neither longterm nor short-term influences of cigarette smoking. Practice effects were noted among the control group, as more correct responses were obtained on the second than on the first set of solutions in most of the test stimuli.     

A tactile stimulation device for measuring two-point and gap discrimination thresholds in humans

A device to measure two-point-discrimination and gap-discrimination thresholds is described. The apparatus is capable of controlled and reliable delivery of forces up to 42.50 g while determining point of skin contact and subsequent depth of skin indentation during threshold determination. Stimulus delivery is controlled by computer software that deactivates a solenoid, allowing a counterbalanced beam with one of two stimulus wheels mounted on its end to rise and stimulate the skin. Depth of skin indentation is recorded by hardware that monitors the output of a photocell reflecting the position of the beam. The apparatus allows complete computer control of all data acquisition and recording.     

Quokkas (Setonix brachyurus) demonstrate tactile discrimination learning and serial-reversal learning

Two male quokkas (Setonix brachyurus: a herbivorous macropod marsupial) were trained to discriminate pairs of stimuli in the laboratory. Quokkas indicated their choice by pulling on 1 of 2 simultaneously presented cords. The quokkas' discrimination abilities were tested on 6 tactile and 6 visual discrimination tasks. Correct responses were rewarded with food. For both quokkas, all tactile tasks were learned to a criterion of 75% correct in up to 4 20-trial sessions. No visual task maintained criterion performance in 4 sessions. One tactile discrimination was reversed 10 times. After the 1st reversal, the error rate declined sharply and fell to a level well below the initial discrimination.     

Temporal ventriloquism in a purely temporal context

This study examines how audiovisual signals are combined in time for a temporal analogue of the ventriloquist effect in a purely temporal context, that is, no spatial grounding of signals or other spatial facilitation. Observers were presented with two successive intervals, each defined by a 1250-ms tone, and indicated in which interval a brief audiovisual stimulus (visual flash + noise burst) occurred later. In "test" intervals, the audiovisual stimulus was presented with a small asynchrony, while in "probe" intervals it was synchronous and presented at various times guided by an adaptive staircase to find the perceived temporal location of the asynchronous stimulus. As in spatial ventriloquism, and consistent with maximum likelihood estimation (MLE), the asynchronous audiovisual signal was shifted toward the more reliably localized component (audition, for all observers). Moreover, these temporal shifts could be forward or backward in time, depending on the asynchrony order, suggesting perceived timing is not entirely determined by physical timing. However, the critical signature of MLE combination--better bimodal than unimodal precision--was not found. Regardless of the underlying model, these results demonstrate temporal ventriloquism in a paradigm that is defined in a purely temporal context.     

Relative roles of spatial and intensive cues in the discrimination of spatial tactile stimuli

Two psychophysical measures of tactile sensitivity--grating-orientation (GO) and smooth-grooved (SG) discrimination--were used to determine tactile spatial acuity with and without an intermediate surface (latex glove) interposed between the contactor and the skin. Measures were made at three locations that varied in sensitivity and in density of innervation of the primary afferent fibers: the index fingerpad (fingertip), palmar surface of the proximal phalanx (fingerbase), and the thenar eminence (palm). Neurophysiological studies have suggested that the density of innervation of SAI fibers is a limiting factor in spatial acuity. In the present study, without a glove, the GO thresholds varied as a function of location. With the glove, increases in the GO thresholds were relatively uniform and modest. Without a glove, however, the SG thresholds were well below the GO thresholds, and changing the site of stimulation had little effect on the threshold. With a glove, the SG thresholds increased by 100% to more than 300% as in comparison with those in the no-glove condition. The largest increases occurred at the less sensitive locations. The results of the GO task are consistent with the view that GO is a valid measure of spatial acuity. The results of the SG task, however, are inconsistent with previous results and suggest that both spatial and intensive factors are involved in this task.     

Spatial asymmetries in tactile discrimination of line orientation: a comparison of the sighted, visually impaired, and blind

Thresholds for tactile discrimination of stimulus orientation discrepancy from standard or referent vertical, horizontal, and diagonal orientations were determined for sighted, visually impaired, and blind subject groups. The stimuli were presented to the ventral distal portion of the tip of the subject's left index finger via an Optacon. Although the subject groups did not differ in overall discrimination accuracy, for each group the deviations from vertical and horizontal standard orientations were discriminated reliably more accurately than the deviations from standard diagonals, ie the oblique effect was obtained. The bases for this tactual spatial anisotrophic effect appear to reflect both sensory--neurological and experiential factors.     

The influence of stimulus tilt on haptic curvature matching and discrimination by dynamic touch

We investigated whether haptic comparison of the curvature of strips is influenced by the tilt (the average slope relative to the horizontal) of the curved strips. This particular stimulus manipulation was chosen to decide between two broad ways in which dynamic curvature comparison might be done: on the basis of the attitude (slope) differences over the surfaces or on the basis of the attitude (slope) differences between successively presented surfaces. For this purpose we conducted matching and discrimination experiments in which strips of constant curvature 9 cm long were touched dynamically and the tilts of the strips were varied. The results of the matching experiment showed some influence of tilt: the strips were judged to be slightly more curved if the tilt was nonzero than if it was zero. The results of the discrimination experiment did not show an effect of tilt. Because the attitude (slope) differences over the surfaces were independent of the tilt it is concluded that curvature comparison is based primarily on attitude differences over the curved surfaces.     

Measure and integral with purely ordinal scales

We develop a purely ordinal model for aggregation functionals for lattice-valued functions, comprising as special cases quantiles, the Ky Fan metric and the Sugeno integral. For modelling findings of psychological experiments like the reflection effect in decision behaviour under risk or uncertainty, we introduce reflection lattices. These are complete linear lattices endowed with an order reversing bijection like the reflection at 0 on the real interval [−1,1]. Mathematically we investigate the lattice of nonvoid intervals in a complete linear lattice, then the class of monotone interval-valued functions and their inner product.