Tactile perception and the information processing basis of tactile speech prostheses for the deaf

The research reported in this thesis took as its starting point the question of whether touch has the information processing characteristics required to deal with speech transforms. A review of the current status of tactile speech prostheses and of the range of potential limitations to touch's ability to deal with speech transforms, lead to the identification of two specific research foci. First, it was argued that an appropriate strategy for tactile aid development is to establish where tactile and auditory processes overlap and where they differ, then to take advantage of any similarities, or compensate for any differences, between these modalities in the design of the prosthesis. In line with this argument, a series of experiments was undertaken to test previous suggestions that there is an underlying similarity between auditory and tactile representations of stimuli. In support of these claims, it was found that auditory and tactile versions of patterns are easier to compare than are auditory and visual versions of those patterns. Subsequent research revealed that one aspect of this processing affinity between touch and hearing is that both modalities, unlike vision, process temporally distributed information more efficiently than spatially distributed information. This finding has broader theoretical significance in view of the current controversy regarding the division of senses according to a spatial vs temporal criterion. The second research focus addressed was whether touch has the spatial and temporal acuity required to deal with speech transforms. It was argued that the limiting factors in tactile spatial and temporal acuity were more likely to occur at the higher level of touch's ability to deal with the strong interactive effects between pattern elements, rather than at the lower level of two-point thresholds in time and space. As masking is a primary interactive force between tactile pattern elements, an attempt was made to resolve the ongoing debate regarding the extent to which tactile masking effects either limit the perception of complex tactile patterns by obscuring the identity of pattern elements or facilitate this task via a process of perceptual integration. This question was investigated by measuring the discriminability of three-element tactile patterns as the spatial and temporal separation, and hence the level of masking, between pattern elements was varied. It was expected that performance would De best at closer element spacings, due to the greater opportunity for perceptual integration to occur. Contrary to this prediction, it was found that the increasing levels of masking induced by decreasing the spatial and temporal separation between pattern elements caused a decrease in the discriminability of the patterns. One caveat to the acceptance of this result was the possibility that training may be required before touch can take advantage of any beneficial interactions between pattern elements, a possibility supported by the anecdotal reports of the subjects. Tentative support for this suggestion was provided by a pattern learning experiment involving three subjects from the previous experiment. After brief experience with closely spaced tactile patterns, the subjects were able to discriminate these stimuli at least as well as widely spaced tactile patterns. While failing to demonstrate the proposed beneficial effects of integration, these results did indicate that close spatial and temporal proximity between tactile pattern elements may not adversely affect the discriminability of those patterns. If subsequent research confirms this tentative finding, then the implication for tactile speech prostheses is that the display employed need not avoid the strong masking effects induced by close spatial and temporal proximity between speech pattern elements. In summary, this thesis showed that there is a general similarity between auditory and tactile perceptual representations of patterns which may both assist in the implementation of tactile speech prostheses, and advantage touch over vision for this purpose. Second, it appears that although tactile pattern perception is initially impeded by the occurrence of masking effects between pattern elements, this performance deficit may be removed once the observers have sufficient experience with the stimuli. There is, however, a clear need for further research before this tentative conclusion can be confirmed.