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It is very seldom that psychological theory is applied to human - computer interface design — because very few theories have yet been formulated which are applicable. For the most part designers have to be content to use guidelines and models, which have less applicability. So, the work described in this chapter is unusual, because it describes an interface to a program which teaches about musical harmony, based on psychological theories. The success of that approach is borne out by the fact that the theories suggest the use of a specific style of interface, based on a two-dimensional spatial representation of harmony relationships. This in turn has been shown to be very successful in teaching novice users about harmony.

Recent developments in the design of human-machine interfaces have resulted in interfaces which make access to computer-based equipment more difficult for visually disabled people. The aim of this project was to explore whether it is possible to adapt such interfaces so as to make them usable by people who cannot see a screen. The approach adopted was based upon two principles: the replacement of visual interlace entities by auditory analogues and appropriately constraining the resultant interlace. Two forms of sound were used to embody the auditory interface: musical tones and synthetic speech. The 'auditory screen' so produced consists of a grid of 'auditory objects', each of which is associated with a spatial location, a tone, a name and an action. In order to test the principles a word processing program was designed and implemented to demonstrate how a visual program might be adapted to be accessed through such an interface. This program was evaluated with the assistance of a number of visually disabled testers. They were trained to use the word processor through completing a graded set of exercises. Data were obtained in a number of ways during and after the completion of the exercises. Since the auditory interface had some novel components, special evaluation methods were applied. The nature of the interaction was analyzed, using an approach based on work on keystroke models of visual interfaces. This yielded a simple model of the 'hand-ear' coordination employed, which forms a basis for prediction of user behaviour. It was also necessary to evaluate aspects of the program, such as ease-of-learning and these were investigated by interviewing the subjects. The results demonstrate that the approach is viable. The thesis also discusses a number of problems in using such an interface, most of which are related to the memory load on the user.