Comparison of Tactile Temporal Numerosity Judgments Between Unimanual and Bimanual Presentations

Underestimation in temporal numerosity judgments computationally explained by population coding model

The ability to judge numerosity is essential to an animal’s survival. Nevertheless, the number of signals presented in a sequence is often underestimated. We attempted to elucidate the mechanism for the underestimation by means of computational modeling based on population coding. In the model, the population of neurons which were selective to the logarithmic number of signals responded to sequential signals and the population activity was integrated by a temporal window. The total number of signals was decoded by a weighted average of the integrated activity. The model predicted well the general trends in the human data while the prediction was not fully sufficient for the novel aging effect wherein underestimation was significantly greater for the elderly than for the young in specific stimulus conditions. Barring the aging effect, we can conclude that humans judge the number of signals in sequence by temporally integrating the neural representations of numerosity.

Numerosity Identification Used to Assess Tactile Stimulation Methods for Communication

Finger-Braille is a tactile communication method used by people who are Deafblind. Individuals communicate Finger-Braille messages with combinations of taps on three fingers of each of the hands of the person receiving the communication. Devices have been developed to produce Finger-Braille symbols using different tactile stimulation methods. Before engaging in communication studies based on technologically-mediated Finger-Braille, we evaluated the relative efficacy of these methods by comparing two devices similarly constructed; the first based on widely employed eccentric rotating-mass vibrating motors and the other using specifically designed tapping actuators. We asked volunteers to identify the numerosity of presented items and for each device we measured (1) error-rate, (2) reaction time, (3) confidence ratings, and (4) a comparison of confidence ratings to actual performance. The four measures obtained for each device showed a net advantage of the tapping stimulation method over the method of vibrations. In this article, we conclude that the tapping stimulation method is recommended for use in the design of tactile communication devices based on Finger-Braille and fingerspelling methods reliant on finger tapping actions. The results did not demonstrate clear evidence for tactile subitising with passively experienced stimulation on the fingers.

Body-Mounted Vibrotactile Stimuli: Simultaneous Display of Taps on the Fingertips and Forearm

In this article, we present a body-mounted tactile display to deliver haptic feedback to the forearm and user-initiated haptic feedback to the fingertips. The display mounts two vibrotactile actuators on the forearm, leaving the user's hands free for manipulation tasks when the hands are not interacting with the tactile display, while also exploiting the tactile sensitivity of the fingertips when needed. We test the effectiveness of the display using paired vibrotactile taps sensed through the forearm and the fingertips, either separately or simultaneously. We measure the ability of participants to identify the vibrotactile taps. The results show that mounting the device on the forearm, so that the participant touches the forearm-mounted device with their fingertips receiving feedback to both locations simultaneously, decreases performance relative to mounting on the fingertips unless large amplitudes are used. We also test the accuracy with which participants identified different numbers of vibration taps (4, 8, 16, and 25 signals). The results show that as the number of signals changes, participant accuracy is not different when stimulating the fingertips alone compared to stimulating the fingertips and forearm together. We conclude with an example of a portable and wearable vibration display, and discuss future use cases of such a display.

Raised Dot Enumeration Via Haptic Exploration

In two experiments we investigated blindfolded, sighted participants' capacity to extract the number of raised dots from arrays of braille cells that they scanned once via active touch. The arrays could contain between one and 12 raised dots and estimates were based on scanning with one or more fingers on one or both hands (Experiment 1), or when the dots were as maximally or minimally spaced as the braille code permits (Experiment 2). We sought evidence of discontinuities in performance that reflect more than one mode of enumeration. We found that participants' estimates of numerosity increased in a linear fashion with actual numerosity, but were increasingly underestimated beyond numerosity of six, and confidence in the judgment declined linearly with increasing numerosity. Finger combinations made no difference to accuracy, errors, or confidence. Increasing dot density had the effect of diminishing perceptual accuracy, exaggerating underestimation and reducing confidence. While perceptual accuracy was generally high up to six raised dots, patterns of confusions and scaling analyses suggest that numerosities of four or less are perceptually unique. In this article, we discuss these data in terms of enumeration in touch and other modalities, and consider whether this discontinuity in enumeration signifies a subitize-to-count or a count-to-estimate transition.

Representing Numerosity Through Vibration Patterns

It can be useful to display information about numerosity haptically. For instance, to display the time of day or distances when visual or auditory feedback is not possible or desirable. Here, we investigated the possibility of displaying numerosity information by means of a sequence of vibration pulses. From previous studies on numerosity perception in vision, haptics and audition it is known that numerosity judgment can be facilitated by grouping. Therefore, we investigated whether perception of the number of vibration pulses in a sequence can be facilitated by temporally grouping the pulses. We found that indeed temporal grouping can lead to considerably smaller errors and lower error rates indicating that this facilitated the task, but only when participants knew in advance whether the pulses would be temporally grouped. When grouped and ungrouped series of pulses were presented randomly interleaved, there was no difference in performance. This means that temporally grouping vibration sequences can allow the sequence to be displayed at a faster rate while it remains possible to perceive the number of vibration pulses accurately if the users is aware of the temporal grouping.