D. K. Oller

FFT-based digital tactile vocoder system for real-time use

A microprocessor-based real-time digital vibrotactile vocoder system has been developed to train the deaf and for artificial hearing research. The system is composed of a microcomputer module with a digital signal processor interface units and an attenuator/driver circuit. Live or digitised (stored or synthetic) speech is presented to the skin spectrally through a belt housing eight or 16 vibrators. Speech is processed in real time using a fast Fourier transform. The system is also capable of presenting any arbitrary spatiotemporal pattern on the skin for artificial hearing experiments. A preliminary experiment with a deaf subject indicates that the system is potentially an effective device for artificial hearing.

Frequency discrimination for single channel electrocutaneous signals

Frequency discrimination of electrocutaneous signals was investigated for several different base frequencies at signal durations of 50, 100, and 250 ms. Bipolar pulses (height = 10 mA, width = 13 μs) were presented on a single electrode placed 1 cm above and 8 cm to either the left or right Three practiced subjects judged pairs of stimuli (ISI = 300 ms) in a same‐different task. For signal frequencies below 160 Hz, psycho‐metric functions were similar for all subjects and for all signal durations. In this range of frequencies, Δf/f was constant at approximately 0.3. By contrast, for stimuli greater than 200 Hz, the Weber fraction was found to increase both as a function of frequency and signal duration. The results will be discussed in terms of possible intensity coding schemes for electrocutaneous vocoder designs.

Perception of the Stop/Glide Contrast in Infancy

Much of the published research in infant speech perception has emphasized how well infants have done with a number of speech contrasts, and have noted similarities in pattern of discrimination of adults and infants. Often it has been suggested that infants begin life with the ability to perceive any speech contrast, and that the process of acquiring a language involves inhibition of the ability to perceive contrasts not present in the target language. Indeed some studies have shown infants able to discriminate contrasts on which adults fail if the contrasts are not drawn from the native language of the adults. Other studies, however, have suggested that infants may not always be so perceptually capable. The present work focusses on the stop-glide contrast. The results are inconsistent with the prevalent view and with previously reported studies on the perception of the stop-glide contrast by infants. The results indicate that in a vigilance paradigm adapted for both infant and adult testing, infants perform poorly on the contrasts when compared with adults. Furthermore the pattern of relative perception observed in the adults on stimuli with long or short vowels is quite unlike that of the infants. It is concluded that much work remains in order to evaluate the relative performance of infants and adults in speech perception, since it appears that changes in experimental paradigm or particular stimulus parameters may affect outcomes in fundamental ways.

Striking differences in the babbling of deaf and hearing infants

It is widely believed that deaf infants “babble” in much the same way hearing infants do, although the deaf have been said to produce less babbling than the hearing after six or seven months of age. These beliefs have been based upon sketchy and sparse data. Over the past 14 years, we have been collecting tape‐recording samples of vocalizations and now have extensive recordings from 8 (severely and profoundly) deaf infants who are otherwise unimpaired and 21 hearing infants. The hearing babies all began canonical babbling (production of well‐formed syllable sequences such as [bababa], [nanana], etc.) by 10 months of age while not one of the deaf babies started canonical babbling before 11 months. The onset of canonical babbling is considered significant because it represents the point at which infants manifest a capacity to produce syllables that meet all the requirements of linguistic‐phonetic units. The difference in onset of babbling in the two groups of infants is reliably identified by adult listeners.