Inter-aural intensity difference (IID) Revisited

September 6, 2023

Pathways: An informative 5 minute read…

by Frank Musiek, PhD

In diagnostic audiology procedures are developed, reported and often show promise. However, for some reason of many these procedures are never adopted for clinical use. In this script we will revisit one such audiological procedure – it was termed the inter-aural intensity difference (IID). Marilyn Pinheiro developed this particular procedure over 50 years ago and I remain surprised that it never achieved clinical use. Please understand the IID measure has been in psychoacoustics for a long time –well before Pinheiro’s procedure. However, Pinheiro’s procedure and related findings were of special note early on in audiology. It is a simple procedure that can be performed with a standard audiometer. It also appeared to have diagnostic significance in that those with central involvement had difficulty with the test.

The methods of the original procedure employed white noise bursts as the stimuli, however tones at various frequencies could be used. These stimuli were about 500 msec in duration with 10 msec rise fall time and were presented to each ear simultaneously. Using current audiometers 200 or 500 msec tone pulses could be utilized. The original research called for initially presenting the signals diotically/dichotically at 20 dB SL in reference to the stimuli employed.

The participants in the study were divided into three groups. One group were normal hearing controls, another was those with cochlear hearing loss and the final group were those with neurological involvement with likely involvement of the central auditory nervous system (CANS). The participants were to track an intracranial sound image as the intensity level at the ears presented through earphones was altered. The final determinant was when the participant relayed that the sound image was localized in the right or left ear.

The tracking of Intracranial imaging is often a stepwise process. In normals simultaneously presented tones at the same level will yield a midline image in the head. This could be perceived in the front, middle or back of the head — but midline. As the intensity level at the ear is varied the stronger signal will bias the image toward that side. As the signal becomes progressively stronger the image moves across the head, closer to the ear receiving the more intense signal.

In the Pinheiro study, the signals were altered in 1 dB steps, hence the intracranial image could be tracked laterally across the head until it was clearly perceived at the ear. Three trials were conducted, and the average of the trials was computed as the intra-aural intensity difference (IID) in dB.

In the control group, the IID was 9 dB for both the left and right sides. Interestingly, other similar studies (though not exactly the same) yielded around a 10 dB IID for normal hearers. That is, when one ear was receiving a signal about 10 dB greater than the other ear it would be lateralized distinctly to that ear. The second group was the cochlear involved and their findings were a bit of a surprise. The IID for the cochlear group was essentially the same as the control group. An interesting approach was then put forth by Pinheiro regarding the cochlear group of participants. Pinheiro tested the cochlear group for those who demonstrated recruitment and those that did not. The results of this approach including the two sub-groups, again showed no difference in IID compared to the control group. The third group – those with CANS involvement did differ from the control and cochlear groups. This finding showed that IID for the ear ipsilateral to the lesion side was similar to the control and cochlear groups, however, IID for the ear contralateral to the lesion side revealed a significantly smaller IID. The group three contralateral ear finding yielded an IID that was slightly over 4 dB while the control and cochlear groups IID was slightly over 9 dB.

The findings of the Pinheiro IID study has several important findings that are of value to neuroaudiology and CAPD – even in todays clinical environment. One is that the procedure is clinically feasible as mentioned earlier. It requires little time and can be performed with commercial audiometers. Another finding with implications is that cochlear participants perform essentially the same as those with normal hearing. Finding central auditory tests that are not influenced by cochlear hearing loss has been a tough challenge for years. This aspect makes IID an attractive procedure, however, it should be further investigated to ensure this finding is valid. If it is valid, it would have a major impact on CAPD testing and evaluation. The third trend of significance for Pinheiro’s IID study is that it is sensitive to CANS involvement. Again, further research should be done to confirm this, but these early findings are indeed impressive.

Why the IID in the contralateral to the lesion side is smaller is somewhat counter intuitive. It seems likely neural timing may be altered in this situation, but further speculation as to how the timing was altered is beyond the scope of this script.

A point worth emphasis is that the Pinheiro study on IID is an example of what has been too common in audiology. That is, solid studies that show great promise are not given their due by the clinical and clinical research communities. Studies such as the one discussed here, deserve more awareness by clinicians and researchers as they have the potential to significantly advance our field and better serve our patients.



  • Pinheiro ML, Tobin H. Interaural intensity difference for intracranial lateralization. J Acoust Soc Am. 1969 Dec;46(6):1482-7.
  • Pinheiro ML, Interaural intensity difference for intracranial lateralization of white noise bursts. A Ph.D.  dissertation, Case Western Reserve University, 1969

Leave a Reply