The Short Increment Sensitivity Index (SISI): An Auditory Discrimination Application?

Pathways: A 5 minute informative read……

by Frank Musiek


It has been long recognized by many in audiology and hearing science that our field lacks clinically useable measures of auditory discrimination. This shortcoming, though commonly recognized, continues to be an omission in terms of thorough assessments of a patients’ hearing status in audiology clinics throughout the USA. With this in mind, and no new procedures of auditory discrimination on the horizon, I could not help but reflect on a test procedure that most audiologists have available to them on their audiometers – the SISI test.

For the readership that may not remember or have not read about it, the SISI was introduced by Jerger, Shedd and Harford in 1959. Chris Herget and I wrote an historical commentary about it in 2012 and briefly at the end of that article we alluded to the idea that the SISI might be used as a measure of intensity discrimination. It is this idea that we would like to discuss in more detail now — within the framework of some history of auditory discrimination research and a link to the SISI procedure. So, in this commentary, let’s look at the methodology behind the SISI procedure and see if perhaps it does (or doesn’t) qualify as a measure of auditory discrimination.

Auditory discrimination studies have been primarily the domain of psychoacoustics. The early research by J. D. Harris (1963) provided an insightful account of the different methodologies employed for determining just noticeable differences (JNDs) or difference limens (DLs) for auditory discrimination. Briefly discussed will be three main methodologies forwarded by Harris in his 1963 publication. These have been well accepted and are commonly used in psychoacoustics. Each of these methodologies have been slightly modified by researchers over the years but remain the cornerstone of auditory discrimination studies.

One of the discrimination methods to be discussed is sequential memory. This procedure requires that 2 successive tones be presented and the participant makes a judgement if the 2 tones are the same or different. This procedure can be performed with the participant or examiner adjusting the intensity of the tones (method of adjustment or limits) until a jnd is obtained. The overall intensity, duration and rise fall time of the tones and the interstimulus interval need to be controlled as they can influence the results. However, interestingly, Harris comments that these factors (within reason) only have minimal influence on the results. For moderate overall intensities average jnds for normal hearers hovers slightly over 1 dB based on Harris’s (1963) data..

Method number two to be discussed is the modulation technique (Riesz, 1928) which was one of the first systematic approaches to measuring intensity discrimination. This method was one that modulated the intensity of the tone similar to a warble tone often employed in pediatric audiology. The degree of modulation is adjusted by increasing the intensity (the percentage of modulation) until the listener can detect that the tone is no longer steady but rather modulates or “warbles”. Overall intensity and modulation rate are factors to control in this procedure. At moderate overall intensities, the jnds for the modulation technique is slightly under 1 dB but still highly similar to the sequential memory procedure (Harris, 1963).

The third method used in intensity discrimination is one termed the masking technique. The general conditions of this method requires a continuous acoustic stimulus, often a pure tone, be presented. Super imposed on this continuous stimulus is a short increment of intensity. The listener is directed to signify when they detect this increment in intensity. Factors which need to be controlled is, of course, overall intensity of the stimulus, rise – fall time and duration of the increment in intensity. In Harris’s original study normal hearers’ average jnds were almost identical to the modulation technique (slightly under 1 dB) for a moderate overall intensity level.

It should be mentioned that all three methodologies revealed larger jnds for low overall intensities and smaller and similar jnds for moderate to moderately high intensities.

Now let’s look at the SISI a diagnostic test, by comparison to the three auditory discrimination measures just mentioned. The first two are indeed quite different however the third —- the masking procedure is quite similar. As was discussed in a previous Pathways (Musiek and Herget, 2012) the SISI is composed of a continuous tone with a 300 msec (total duration) incremental tone superimposed on it. The incremental tone has rise- fall time that is 50 msec. The continuous tone is presented at 20 dB SL at the frequency selected. The incremental tone can be 5, 2, or 1 dB but only the 1 dB tones are test items. The 5 dB increments are presented to keep the listener engaged & alert. There are 20, 1 dB presentations and the number of those that are correctly identified are multiplied by 5 to derive a percentage score. Scores that are 0 – 70 per cent are considered negative for cochlear pathology while a 75 percent score and higher is a sign of cochlear involvement.

If one compares masking procedures for intensity discrimination to those of the SISI test one could conclude the methods are highly similar. Hence, from a psychoacoustic stand point, the masking method and the SISI could possibly both be used as a measure of intensity discrimination. Certainly, the psychoacoustic procedure is longer and more detailed in it’s approach but basically it is highly similar to the SISI. Granted, it would be ideal to have auditory discrimination tests designed specifically for clinical use but realistically this is not available — however the SISI procedure is.

The SISI can be used as originally intended to define cochlear pathology however it possibly could also be used as a measure of intensity discrimination. Though as I understand it, Jerger did not portray this as a measure of intensity discrimination, however, in light of the lack of this kind of measure clinically, perhaps the SISI could be used in this capacity.

For sure, there is a paucity of research on the SISI as an auditory discrimination index and more must be done. However, one of the possible uses of the SISI as an auditory discrimination measure is for children having difficulty learning/discriminating vowels. This common problem may be related to poor auditory discrimination ability (see Educating Alpcas, 2018) in children with pure tone hearing loss, auditory processing disorder or related hearing problems (see Kumar et al., 2021; Groth et al. 2011). The SISI may be able to shed light on auditory discrimination problem, though obviously it cannot be used to rule out frequency or duration discrimination difficulties which could be the basis of the problem. Therefore, certainly not a complete answer to vowel learning/discrimination problems, however, in some cases, it could point the audiologist in the right direction.



  1. Educating Alpacas, Auditory discrimination of vowel sounds, 8/10/2018
  2. Groth, K., Lachmann, T., Riecker, A. et al. Developmental dyslexics show deficits in the processing of temporal auditory information in German vowel length discrimination. Read Writ 24, 285–303 (2011).
  3. Harris, J.D. (1963) Loudness Discrimination, J. of Speech and Hearing Disorders, Monograph, Number 11.
  4. Jerger, J., Shedd, J. L., & Harford, E. (1959). On the detection of extremely small changes in sound intensity. AMA archives of Otolaryngology, 69(2), 200-211.
  5. Kumar, P., Singh, N.K., Ganapathy, M.K. et al. Coding of consonant–vowel transition in children with central auditory processing disorder: an electrophysiological study. Eur Arch Otorhinolaryngol 278, 3673–3681 (2021)
  6. Musiek, F. and Herget, C., (2012) The Short Increment Sensitivity Index (SISI) – Historical Vignette in Audiology, Hearing Health and Technology Matters: Pathways, February 26, 2012
  7. Riesz, R. (1928) Differential intensity sensitivity of the ear for pure tones, Physics Rev. 31, 867-875



About Pathways

Pathways is both a column that covers topics related to CAPD and Neuroaudiology and a society for people interested in central auditory disorders that regularly meets to discuss these issues.

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