The Dichotic Difference Score (DDS)

By: Frank Musiek

The dichotic difference score (DDS) has been utilized periodically over the history of dichotic listening. It represents an interesting manipulation of dichotics and relates to its various applications. The DDS simply requires the computation of the differences on performance of dichotic listening between the right and left ears. Generally, this means that if the right ear score is higher, a plus (+) sign is attached to the score. If the left ear score is better, then a minus (-) sign is attached to it. There are other ways to render the difference score, however this is one of the most common and straight forward.

Computing a difference score for dichotic tests has been around for some time as mentioned. One of the earliest reports was published by Kuhn in 1973. This work was a complex and statistically driven way of computing the DDS. Though difficult to follow, it did provide the concept underlying the DDS. It also provided another way to look at dichotic scores rather than simply right and left ear scores. Over the years, the DDS (not always termed that) was used by various researchers and clinicians, but did not seem to be the focus of the many reports on dichotic listening. While at the University of Connecticut, my then Ph.D. student Jeff Weihing, did some work on DDS computations and was interested in determining if it had value over simple left and right ear scores. I know that on several occasions when reviewing cases with various neurological disorders across various dichotic tests, individual ear scores fell within the normal range and the difference score did not. The difference score, in some way, was more insightful to the problem.

The DDS became more of a consideration when one reviews data on two specific types of dichotic test: the dichotic rhyme test and dichotic CVs. The dichotic rhyme test was a test of dichotic fusion ability. This essentially yielded (free recall) responses that were one word, although two words were presented. The word that was reported was either lateralized to the left or right ear, with a slight advantage to the right side. For example, normal scores for the dichotic rhyme test were usually 52% and 48% for the right and left ears respectively (see Musiek et al. 1989). Interestingly in neuroauditory disorders of the central auditory nervous system (CANS), the ear opposite of the lesioned hemisphere would reveal a decreased score, but the opposite ear would often increase in its performance – beyond established normal limits. This was also seen in split brain patients comparing pre-operative to post-operative dichotic rhyme test results (Musiek et al. 1989). This would of course, create a huge DDS. The same has been shown for dichotic CVs, though in this procedure, the increase in scores were not considered abnormal. Thus, the DDS can be very useful, revealing the possible problem at hand. This phenomenon was reported by Berlin in the 1970s on patients with temporal lobectomy/hemispherectomy (Berlin et al. 1972, Berlin 1975). Despite these findings, the DDS in these articles were not in the spotlight because the focus was elsewhere. Therefore, over the years, the concept of DDS did not create great interest in the dichotic listening community.

The DDS reflects an asymmetry in dichotic listening and a number of researchers and clinicians over the years have postulated these asymmetries – especially the marked ones were that were mostly driven by auditory and not supra-modal actions such as: attention, memory, and other cognitive processes. (see Musiek et al, 2005). This thinking is consistent, to some degree, to the recent work by Cameron et al. (2016). Though I cannot review all details of this study, it argues that the dichotic difference test (DDdT) results mostly escape the consequences of memory, attention, etc. These researchers also take advantage of their computations of the monaural digit scores and compare them to the dichotic scores. However, the difference score as stated, reflects asymmetry which many believe is mostly auditory driven.

Of note, the notion that supra-modal processes are Gestaltly applied to sensory processes without specific laterality. That is, unless forced in some manner, there is no major right side or left side differences created by supra-modal actions. Of course, this aspect of brain function has it’s pros and cons and is readily debated across many disciplines and that discussion is far beyond the purpose of this article.

Although DDS has been around for many years, it’s enough to say that it has not been used up to its true potential. It is worthwhile to pay attention to it now as it may provide some help to current challenges we have in studying dichotic listening and applying it.

References

1. Kuhn, G. (1973) The Phi coefficient as an index of ear differences in dichotic listening,  Cortex, 9, 447-457
2. Berlin, C., Cullen, J. Hughes, L., Berlin, H. et al. (1975) Proceedings of a Conference on Central Auditory Disorders, Univ. Nebraska Medical Center, Omaha
3. Berlin, C., Lowe-Bell, S., Jannetta, P. Kline, D. (1972) Central auditory deficits after temporal lobectomy, Archives of Otolaryngol., 96, 4-10
4. Cameron S1, Glyde H1, Dillon H1, Whitfield J1, Seymour J1. (2016) The Dichotic Digits Difference Test (DDdT): Development, Normative Data, and Test-Retest Reliability Studies Part 1. JAAA, 27(6):458-69.
5. Musiek, F.E., Bellis, T.J., & Chermak, G.D.  (2005). Nonmodularity of the Central Auditory Nervous System: Implications for (Central) Auditory Processing Disorder, American Journal of Audiology, Vol 14: 128-138, December.
6. Musiek, F., Kurdziel-Schwan, S., Kibbe, K., Gollegly, K., Baran, J. & Rintelmann, W.  (1989). The Dichotic Rhyme Task: Results in Split Brain Patients. Ear and Hearing, 10, 33-39.