Ian R. Summers

A broadband tactile array on the fingertip

A stimulator array is described which can deliver a wide range of displacement waveforms from each contactor, allowing vibratory stimuli to be targeted towards different populations of mechanoreceptors in the skin. The array has a working bandwidth of 20-400 Hz and 100 moving contactors covering an area of 1 cm2 on the fingertip. The array was validated with two experiments on the perception of moving vibratory targets within a uniform background vibration. In the first experiment, with target and background at the same frequency, equivalent discrimination of target movement was obtained at higher values of target/background amplitude ratio for 40-Hz stimuli than for 320-Hz stimuli. In the second experiment, discrimination of target movement within a different-frequency background (320-Hz target and 40-Hz background, or vice versa) was found to be much easier than within a same-frequency background. These results suggest that tactile spatial acuity is better at 320 Hz than 40 Hz and that it is possible to target different receptor populations in the skin by using these frequencies. However, there are problems with this interpretation: on the basis of characterization of touch receptors in previous studies, spatial acuity is expected to be worse at 320 Hz than at 40 Hz.

A vibrotactile approach to tactile rendering

While moving our fingertip over a fine surface we experience a sensation that gives us an idea of its properties. A satisfactory simulation of this feeling is still an unsolved problem. In this paper, we describe a rendering strategy based on vibrations that play an important role in the tactile exploration of fine surfaces. To produce appropriate excitation patterns we use an array of vibrating contactor pins. Similar to the colour model in computer graphics, we simulate arbitrary vibrations as a superposition of only two sinewaves. Each sinewave is intended for the excitation of a specific population of mechanoreceptors. We carried out first tests of our rendering strategy on Brownian surfaces of different fractal dimensions.

A functional-magnetic-resonance-imaging investigation of cortical activation from moving vibrotactile stimuli on the fingertip.

Using a 100-element tactile stimulator on the fingertip during functional-magnetic-resonance imaging, brain areas were identified that were selectively activated by a moving vibrotactile stimulus (the sensation of a moving line being dragged over the fingertip). Activation patterns elicited by tactile motion, contrasted to an equivalent stationary stimulus, were compared in six human subjects with those generated by a moving visual stimulus, contrasted to an equivalent stationary stimulus. Results provide further evidence for a neuroanatomical convergence of tactile-motion processing and visual-motion processing in humans. The sites of this convergence are found to lie in the middle temporal complex (hMT+V5), an area with known specialization for visual-motion processing, and in the intraparietal area of the posterior parietal cortex. In an advance on previous studies, the present study includes separate delineation of activations for moving tactile stimuli and activations for moving visual stimuli. Results suggest that the two sets of activations are not entirely collocated. Compared to the visual-motion activations, the tactile-motion activations are found to lie nearer the midline of the brain and further superior.

INFORMATION FROM TIME-VARYING VIBROTACTILE STIMULI

Experiments have been carried out to investigate the information transfer available via a single vibrator on the fingertip. In a first experiment, for stimuli with durations 80 to 320 ms, discrimination of a one-octave step change in frequency at the halfway point was investigated. Results were similar for three stimulus types--sinewave, monophasic pulse and tetraphasic pulse--suggesting temporal cues are more important than spectral cues in this task. In a second experiment, subjects were required to perceive changes in a sequence of stimulus elements. A presentation rate of 6.25 elements s-1 was found to give better results than a rate of 12.5 elements s-1. In the former case, the potential information transfer per element was estimated to be approximately 1.0 bits, corresponding to an information transfer rate of around 6 bits s-1. Implications for the design of a tactile aid to lipreading are discussed.

The Role of the Lateral Prefrontal Cortex and Anterior Cingulate in Stimulus-Response Association Reversals

Many complex tasks require us to flexibly switch between behavioral rules, associations, and strategies. The prefrontal cerebral cortex is thought to be critical to the performance of such behaviors, although the relative contribution of different components of this structure and associated subcortical regions are not fully understood. We used functional magnetic resonance imaging to measure brain activity during a simple task which required repeated reversals of a rule linking a colored cue and a left/right motor response. Each trial comprised three discrete events separated by variable delay periods. A colored cue instructed which response was to be executed, followed by a go signal which told the subject to execute the response and a feedback instruction which indicated whether to hold or flip the rule linking the colored cue and response. The design allowed us to determine which brain regions were recruited by the specific demands of preparing a rule contingent motor response, executing such a response, evaluating the significance of the feedback, and reconfiguring stimulus-response (SR) associations. The results indicate that an increase in neural activity occurs within the anterior cingulate gyrus under conditions in which SR associations are labile. In contrast, lateral frontal regions are activated by unlikely/unexpected perceptual events regardless of their significance for behavior. A network of subcortical structures, including the mediodorsal nucleus of the thalamus and striatum were the only regions showing activity that was exclusively correlated with the neurocognitive demands of reversing SR associations. We conclude that lateral frontal regions act to evaluate the behavioral significance of perceptual events, whereas medial frontal-thalamic circuits are involved in monitoring and reconfiguring SR associations when necessary.

Vibrotactile and electrotactile perception of time‐varying pulse trains

To establish the best strategy for transmitting speech-derived information via a single tactile channel, measurements were made on the perception of frequency- and/or amplitude-modulated pulse-train stimuli, with a comparison of the electrotactile and vibrotactile modalities. In one experiment, vibrotactile perception of 2-oct step changes in stimulus frequency was found to be significantly better than electrotactile on a time-scale appropriate for the transmission of speech features (e.g., with practiced subjects, information transfer of 69% with 200-ms vibrotactile stimuli, 32% with 200-ms electrotactile stimuli). Perception of step changes in stimulus amplitude was similar in the two modalities when changes in amplitude were tailored to match the different dynamic ranges available. In a second experiment, vibrotactile-perception of voice fundamental frequency with various codings was investigated. Both experiments showed information transfer for vibrotactile stimuli to be greater when frequency and amplitude modulation were used together rather than with one or the other in isolation (sentence-stress identification scores: 66% for FM stimuli, 69% for AM stimuli, 80% for FM/AM stimuli). It is concluded that frequency- and amplitude-modulated vibratory stimulation is a good choice in a practical device for the profoundly hearing impaired.

Magnetic resonance imaging of rheumatoid arthritis in metacarpophalangeal joints.

Abstract  Objective. To report the development of high-resolution targeted magnetic-resonance imaging (MRI) techniques (not using injections of contrast media) to investigate and monitor rheumatoid arthritis (RA) in the metacarpophalangeal (MCP) joints. Design and patients. A total of 25 RA patients (age range 30–68 years) with varying degrees of disease severity ranging from early onset through active disease to the burnt-out stage, were imaged. (One patient subsequently underwent surgery and histological data was obtained.) A series of 10 control subjects were also studied – two for each 10-year age range. All the RA subjects were assessed for disease activity using standard clinical criteria and radiography as part of normal procedures. MRI was carried out using a targeted system and novel radiofrequency coil. Images of the MCP were performed at very high resolution with 1.5 mm slice thickness and in-plane resolution 130 µm. Standard gradient-echo (GE) sequences were used for anatomical imaging, multiple-echo GE sequences used to produce effective spin-spin relaxation time (T2*) maps and optimised binomial-pulse presaturation used in conjunction with a GE sequence to generate magnetization-transfer (MT) ratio maps. Results. High-quality high-resolution images of the MCP joints were obtained which highlighted normal anatomy and key features characterising the disease state (e.g. pannus, bone erosions, vascularity). Accurate measurements of T2* and MT with variations of ±4% and ±2% respectively were achieved. In active disease, variations in T2* and MT could be determined throughout areas of pannus, clearly demonstrating the heterogeneity of this erosive tissue. Pannus in MCP joints with active destruction was found to have high values of T2* varying from 25 ms to 40 ms with pockets up to 100 ms, whereas pannus present in chronic destruction, or burnt-out disease, had T2* values ranging from 21 to 29 ms. MT-active tissue was uniformly distributed in burnt-out disease, which was confirmed histologically in one case, compared with a more heterogeneous distribution in active disease. Conclusion. The MRI sequences and targeted system developed allow high-resolution studies of RA disease progression and activity. The data confirm the variable pattern of the disease and, in particular, heterogeneity of pannus.

Tactile information transfer: A comparison of two stimulation sites

Two experiments on the discrimination of time-varying tactile stimuli were performed, with comparison of stimulus delivery to the distal pad of the right index finger and to the right wrist (palmar surface). Subjects were required to perceive differences in short sequences of computer-generated stimulus elements (experiment 1) or differences in short tactile stimuli derived from a speech signal (experiment 2). The pulse-train stimuli were distinguished by differences in frequency (i.e., pulse repetition rate) and amplitude, and by the presence/absence of gaps (approximately 100-ms duration). Stimulation levels were 10 dB higher at the wrist than at the fingertip, to compensate for the lower vibration sensitivity at the wrist. Results indicate similar gap detection at wrist and fingertip and similar perception of frequency differences. However, perception of amplitude differences was found to be better at the wrist than at the fingertip. Maximum information transfer rates for the stimuli in experiment 1 were estimated at 7 bits s(-1) at the wrist and 5 bits s(-1) at the fingertip.

Design of array stimulators for synthetic tactile sensations

We have produced a 100-contactor array on the fingertip to provide passive stimulation, and a 25-contactor array on the fingertip which allows active exploration in 2D. This paper discusses various aspects of the design of such devices.

Sound level monitoring apparatus

Sound level monitoring apparatus for a deaf person comprises a microphone, a discriminator connected to the microphone to provide an output signal in response to the attainment of a predetermined sound level threshold in the ambient sound detected by the microphone, and a sensory transducer responsive to the discriminator to provide a representation of the output signal. In use the threshold suitably represents the normal speech level of the user, and may be adjustable, and the transducer accordingly indicates whether the user should raise or lower his speech level in a given environment. The apparatus can operate in association with several thresholds with the lowest denoting normal speech and with one or more transducers providing respectively, mutually distinctive representations. The transducer or transducers can be of visual form, with use of differently colored light outputs or of vibratory tactile form, with different operating frequencies.