Some Effects of Multiple Sclerosis on Speech Perception in Noise: Preliminary Findings

Article excerpt

Abstract--The present investigation examined speech perception in noise of adults with and without multiple sclerosis (MS). Institute of Electrical and Electronic Engineers (IEEE) sentences were presented at a constant level of 65 dBA [L.sub.eq] (equivalent continuous noise level [4 dB exchange rate]) from a loudspeaker located at 0-degree horizontal azimuth and 1.2 m from the study participant. Uncorrelated multitalker babble was presented from four loudspeakers positioned at 45-, 135-, 225-, and 315-degree azimuths and 1.7 m from the study participant. The starting presentation level for the babble was 55 dBA Leq. The level of the babble was increased systematically in 1 dB steps until the subject obtained 0% key words correct on the IEEE sentences. Results revealed a significant difference in speech perception between the two groups at nine signal-to-noise ratios. Some clinical implications of these results are discussed.

Key words: adults, auditory function, auditory processing, hearing, IEEE sentences, multiple sclerosis, multitalker babble, noise, signal-to-noise ratio, speech perception.

INTRODUCTION

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) that affects over 400,000 U.S. citizens and almost 2,500,000 individuals worldwide [1]. MS causes damage to myelin, the fatty coating on nerve fibers that aids in transmission of the electrical impulses within the nervous system, and to the nerve fibers themselves. This damage slows, distorts, or halts the transmission of the electrical impulses transmitted throughout the CNS and results in many of the symptoms associated with MS, such as fatigue, slurred speech, and blurred vision.

The incidence of hearing loss associated with MS varies, with estimates ranging from 1 to 86 percent [2]. When a loss in pure tone sensitivity does occur, it generally is considered mild in nature [3]. Despite this, 40 to 60 percent of MS patients with normal pure tone thresholds complain of difficulty hearing [4]. These findings are not surprising, given that MS primarily affects the CNS. In fact, abnormal auditory-evoked potentials have been reported in 32 to 93 percent of MS patients [4-7]. The most common abnormalities noted in these studies include absent or abnormally low wave V amplitude and the increased III-V interwave latency. Additionally, other studies have reported abnormal auditory processing in subjects with MS, such as problems with dichotic listening tasks and auditory temporal processing [8-10].

Because problems understanding speech in background noise are characteristic of individuals with auditory processing problems and disorders of the central auditory nervous system, one might postulate that individuals with MS would also have this type of deficit. In fact, several studies have revealed that a high percentage (33%-69%) of individuals with MS experience difficulty understanding speech when it is presented with a competing stimulus [3,11]. Unfortunately, these prior studies were conducted either under earphones, monaurally, with a small sample size, or without the use of a control group. With these considerations in mind, the National Center for Rehabilitative Auditory Research at the Portland Department of Veterans Affairs (VA) Medical Center (VAMC) is examining speech perception in noise for adults both with and without MS in a diffuse listening situation. This diffuse listening situation more closely represents real-world listening environments and, as such, may more closely capture the subjective auditory complaints reported by individuals with MS. This article presents interim results obtained thus far from this ongoing investigation.

METHODS

Subjects

We evaluated two groups of subjects in this investigation: subjects with MS and control subjects who were matched to the subjects with MS in age, sex, and four frequency pure tone averages (PTAs). These study participants were recruited from the Portland VAMC and the Oregon Health & Science University in Portland, Oregon. …