by Frank Musiek, PhD
One of the first studies to address the issue of auditory temporal processing was published by Ira Hirsh in 1959. In many ways this study also popularized the term “temporal” to the field of audiology. The study was fundamental as it looked at the most basic aspect of placing in the correct order two short acoustic stimuli that varied in their onset time. Since all acoustic stimuli must exist in and are influenced by time Hirsh, I am sure, realized the importance of his study and its proper interpretation. Of course, related to this concept of acoustics and time is how the auditory system could perceive the order of two quickly occurring stimuli. Hirsh performed several variations of the experiment of temporal ordering but in our brief account here only selected aspects will be discussed.
The set up for the experiment was one that presented high (4000 Hz) and low (440 Hz) pitched 500 msec tones for which the onsets were varied. The participants were asked which of the stimuli came first — the high pitched one or low pitched one. The normal hearing participants were required to perform at a 75% accuracy rate to determine the accepted time difference between the two acoustic events to allow correct ordering. This reduced variability of what turned out to be a rather difficult task.
After much analysis, Hirsh determined that about 20 msec was required for these participants to reliably order the two acoustic stimuli that differed slightly in onset time. (The actual average of the results ended up being 17 msec but Hirsh seemed to “lean” on using a 20 msec value in most of his reports and future discussions. Rounding up to 20 msec may have been easier to remember and utilized?) It is important to acknowledge that these participants were considered “trained” or “practiced” individuals. Without practice, the time or temporal differential was considerably greater.
Introducing Temporal Factors into Hearing Studies
This finding by Hirsh moved the thinking in auditory science from what was a monopoly of frequency and intensity studies to now introducing temporal factors into hearing investigations. The concept of ordering or sequencing acoustic stimuli could easily be appreciated as an important factor in human hearing. Moreover, the critical question arises: What if the system is not working appropriately?
In this regard Hirsh provided an interesting example. He related that perhaps an individual whose ability to order acoustic stimuli was compromised may result in the perception of the word “boots” rather than “boost”. This is a rather striking example. Perhaps rather than the flagrant mis-ordering of phonemes what might be more likely is the distortion or misrepresentation of speech sounds related to poor sequencing resulting in improper recognition of a word.
Perhaps another aspect of Hirsh’s temporal processing study was that it would open the clinician’s mind to the possibility that problems in hearing could have a temporal basis and not always be frequency or intensity driven. This in turn stimulated more research into temporal factors in hearing and hearing disorders as we will mention a bit later in this script.
Another interesting facet of Hirsh’s interpretation of his findings was the site of the underlying mechanism. Hirsh reasoned that the process of ordering brief acoustic stimuli involved the central auditory system. I believe that Hirsh’s consideration for the central mechanisms was based on the elapsed time required to complete the temporal process (20 msec). This was greater than cochlear processing usually would require. True, cochlear travel time down the basilar membrane would weigh in on temporal processing but likely not to extent of the 20 msec time needed for correct ordering in the study. The neural resolving power of the central auditory system needed in the time domain in Hirsh’s task would be critical to the proper ordering of the acoustic stimuli. Cognitive processes would also come into play to complete a response on the part of the participants.
Hirsh’s notion that temporal ordering is dependent on mechanisms in the central auditory system was born out (to some degree) by some of his own research done with Linda Swisher some years later. In this particular study, Swisher and Hirsh (1972) administered the same two element ordering task to patients with aphasia and a control group. Those individuals with aphasia required a markedly longer time difference between the onsets of acoustic elements compared to the control group. Some of the patients in this clinically oriented study required hundreds of msec to correctly order the stimuli! This finding argued for the potential use of this task for the evaluation of individuals with central auditory disorder, however clinical uptake for this temporal procedure was not to be.
There is no question, however, that the Hirsh study would have a lasting and major impact on clinical and basic hearing research in years to come.
Further research on ordering acoustic stimuli (Fitzgibbons and Gordon-Salant, 1998) and the development of temporal patterns (frequency and duration) were likely motivated by Hirsh’s early studies on temporal processes. The addition of a third element to the two-element task to create a pattern made the task even more difficult. However, pattern tests were and are commonly used clinically (Musiek & Pinheiro, 1987; Musiek et al. 1990).
Hirsh’s 1959 publication was an important first step to trigger more thinking and investigation into temporal processing – now accepted as a critical part of clinical and basic research into the auditory system.
Readings……
- Fitzgibbons PJ, Gordon-Salant S. Auditory temporal order perception in younger and older adults. J Speech Lang Hear Res. 1998 Oct;41(5):1052-60
- Hirsh IJ. Auditory perception of temporal order. J Acoust Soc Am 1959;31:759–767
- Musiek, F.E. & Pinheiro, M.L. (1987). Frequency Patterns in Cochlear, Brainstem, and Cerebral Lesions. Audiology, 26, 79-88.
- Musiek, F., Baran, J. & Pinheiro, M. (1990). Duration Pattern Recognition in Normal Subjects and Patients with Cerebral and Cochlear Lesions. Audiology, 29, 304-313.
- Swisher LP, Hirsh IJ. Brain damage and the ordering of two temporally successive stimuli. Neuropsychologia 1972;10:137–152.
