Logatome Discrimination in Cochlear Implant Users: Subjective Tests Compared to the Mismatch Negativity

This paper describes a logatome discrimination test for the assessment of speech perception in cochlear implant users (CI users), based on a multilingual speech database, the Oldenburg Logatome Corpus, which was originally recorded for the comparison of human and automated speech recognition. The logatome discrimination task is based on the presentation of 100 logatome pairs (i.e., nonsense syllables) with balanced representations of alternating "vowel-replacement" and "consonantreplacement" paradigms in order to assess phoneme confusions. Thirteen adult normal hearing listeners and eight adult CI users, including both good and poor performers, were included in the study and completed the test after their speech intelligibility abilities were evaluated with an established sentence test in noise. Furthermore, the discrimination abilities were measured electrophysiologically by recording the mismatch negativity (MMN) as a component of auditory event-related potentials. The results show a clear MMN response only for normal hearing listeners and CI users with good performance, correlating with their logatome discrimination abilities. Higher discrimination scores for vowel-replacement paradigms than for the consonantreplacement paradigms were found. We conclude that the logatome discrimination test is well suited to monitor the speech perception skills of CI users. Due to the large number of available spoken logatome items, the Oldenburg Logatome Corpus appears to provide a useful and powerful basis for further development of speech perception tests for CI users. KEYWORDS: mismatch negativity (MMN), cochlear implant (CI), logatome discrimination, speech tests INTRODUCTION For several decades, adults and children who suffer from severe-to-profound sensorineural hearing loss have been able to benefit from a cochlear implant (CI). Electrical stimulation of the auditory nerve partly Rahne et al.: Logatome Discrimination in Cochlear Implant Users TheScientificWorldJOURNAL (2010) Postimplant, speech intelligibility measures have predominantly relied on behavioral tests. These tests were developed in isolation of the respective language areas, mostly referring to genuine words and sentences. Representative examples are the Freiburger monosyllabic word test (FMW) for Germanspeaking subjects Such speech intelligibility tests are very limited for the assessment of CI users with prelingual deafness or prolonged periods of deafness, as they generally do not have good perception abilities for naturally spoken language. In addition, children CI users, particularly the very young, in many cases are not able to perform behavioral speech intelligibility tests because of their cognitive developmental status. Thus, objective measures are needed that do not demand the subjects' attention and cooperation, or a certain level of speech and language skills. Electrophysiological methods have been proposed as an alternative and supplementary objective measure to behavioral data. Observations to date show evidence of correlations between speech intelligibility ability and objective measures for auditory-evoked event-related potentials, N1, P2, P3[6], the acoustic change complex The MMN component of event-related brain potentials (ERPs), which provides a noninvasive electrophysiological measure of cortical auditory processing, reflects the outcome of a change detection process that is based on the memory of sound regularities (often called the "standard") in ongoing auditory input On the other hand, there is also a need for behavioral tests designed to measure more basic speech perception skills, such as discrimination of speech pattern contrast In this paper, we describe a new logatome discrimination test. Thereby, we benefit from use of the Oldenburg Logatome (OLLO) Corpus, which was described recently by Wesker et al. Our concept of testing low-level speech perception abilities is based on the usage of subsets of the OLLO Corpus. This includes three improvements over traditional speech intelligibility tests. First, the test material consists of speech material that is natural, i.e., recorded from nonprofessional speakers of different dialect regions; hence, approaching real hearing situations more closely than test materials recorded by only one professional speaker. Second, the stimuli are logatomes that are considered appropriate for speech audiometry measures: based on the established significant correlation between logatome perception and pure-tone audiometric thresholds at 1, 2, 3, and 4 kHz Using this new logatome discrimination test, we measured the discrimination ability of a group of normal hearing adult subjects and a group of adult CI users, and compare their outcomes with their results on the commonly used Oldenburg Sentence Test (OLSA), measuring speech intelligibility in competing background noise. The OLSA was designed and evaluated for the German-speaking language area METHODS A prospective comparative study was performed in two subgroups of subjects in order to assess and compare performance on conventional behavioral speech tests, with a newly developed speech discrimination task and objective measures to assess auditory hearing skills. Evaluation measures included the OLSA, electrophysiological measurement of MMN responses, and logatome discrimination test. Subjects Thirteen adults (seven females, six males; 19-35 years of age) with normal hearing participated in the study. Subjects were enrolled following audiometric hearing threshold screening to confirm hearing thresholds for pure tones <10 dB HL (hearing level) for the octave frequencies 125-8,000 Hz inclusively. Eight adults (six females, two males; 48-71 years of age) using a unilateral Nucleus® CI (models CI22M, CI24M, or CI24RE); fitted with a SPRINT, 3G, SPECTRA, or Freedom sound processor; with at least 1-year experience with their current processor were enrolled in the study. Subject demographics are shown in Rahne et al.: Logatome Discrimination in Cochlear Implant Users TheScientificWorldJOURNAL (2010) 10, 329-339 332 Materials OLSA The subjects were seated in a sound-attenuated room. Stimuli were presented in free field, originating from a PC with a studio-quality sound card (RME-HDSP-9632), a studio-quality power amplifier (MAM-PA200), and a single loudspeaker (Tannoy Reveal), positioned 1.5 m in front of the subject. A constant noise level of 65 dB SPL (sound pressure level) was used with an adaptive level for the speech stimuli commencing with 75 dB SPL and altered depending on the individual's response. One test list, consisting of 30 sentences, was randomly selected via the software for presentation. All sentences consisting of a subject-verb-numeral-adjective-object five-word structure are automatically created from a 50-word inventory. As such, sentences cannot be memorized or anticipated by the subject due to contextual factors. Sentences were superimposed by speech-simulating noise and presented successively as a closed-set test, i.e., the subject's task was to select the correct sentence (every word correct) out of 10 possible responses for every word. The response was given by clicking on the choices displayed on a touch-screen display. Using an adaptive speech level algorithm, the speech reception threshold for a 50% correct word score was determined to identify speech intelligibility for each individual, and reported as the corresponding signal-to-noise ratio (dB SNR). Electrophysiological Experiment Three logatomes (/gag/, /bab/, and /geg/) of the OLLO Corpus were used for electrophysiological measurements. They were spoken by one German speaker with no dialect, normal speaking effort, speaking rate, and statement speaking style. The stimuli were presented in two oddball conditions. In the vowel-replacement condition, the series of "standard" stimuli (/gag/) was randomly replaced by "deviant" stimuli (/geg/). In the consonantreplacement condition, the logatome /bab/ was used as "deviant" to the "standard" /gag/. Totally, in each condition, 220 deviants were presented with a probability of 15%, whereas the "deviant" stimuli were separated by at least four "standard" stimuli. Additionally, two control conditions containing 450 of the /bab/ or /geg/ stimuli, respectively, were run to test if the MMN is absent in response to that same stimulus when presented alone The experiment was performed on a PC using the PRESENTATION software (Neurobehavioral Systems, Albany, NY) and insert earphones (E-A-RTONE 3A) for normal hearing subjects or free-field Rahne et al.: Logatome Discrimination in Cochlear Implant Users TheScientificWorldJOURNAL (2010) 10, 329-339 333 presentation for CI users for the best possible adjustment of the SPL. The stimuli were calibrated using a programmable attenuator (gPAH, g.tech medical Engineering, Graz, Austria) at 75 dB SPL and presented in the free field in a sound-attenuated booth, originating from a PC with a sound card and loudspeaker (Hi-TEX), positioned 1.5 m in front of the subject. EEG was continuously recorded with a Neuroscan Synamps AC-coupled amplifier (0.05-200 Hz bandwidth; sampling rate: 1 kHz) using electrodes placed at Fz according to the International 10-20 System, plus the left and right mastoids, and referred to the nose. Impedances were kept below 5 kOhms. The EEG at Fz and the mastoid channels were digitally filtered (1-15 Hz bandpass, 24 dB/oct. rolloff) and then epoched relative to reference marker positions according to the stimulus type (standard, deviant, and controls). Epochs were 400 msec, starting from 100 msec before and ending 300 msec after the onset of the stimulus. An artifact rejection criterion was set at ±50 µV and applied after the epochs were baseline corrected on a prestimulus range of 100 msec. The "deviant" and "control" ERPs were calculated as average of the relative epochs. Difference waveforms were obtained by subtracting the ERPs elicited by the "deviant" stimuli from the ERPs elicited by the "control" stimuli. For every subject and both deviant types, the MMN response was identified as minimum amplitude in the latency range of 100-250 msec of the difference waveforms. The amplitudes of the "deviant" and "control" averages were also determined at this latency time point. A paired t-test was performed to determine the significance of the difference between "deviant" and "control" responses. Logatome Discrimination Test For auditory stimulation, a subset of the OLLO Corpus was used. Recording conditions, speaker instructions, and postprocessing of the recorded material are described in detail in Wesker et al. For the logatome discrimination test described here, we selected the 70 logatomes (L071-L150) of CVC structure, recorded by one male speaker (S03M) with a normal articulation characteristic (V2). The stimuli were calibrated using a programmable attenuator (gPAH, g.tech medical Engineering, Graz, Austria) at 75 dB SPL and presented in the free field in a sound-attenuated booth, originating from a PC with a sound card and loudspeaker (Hi-TEX), positioned 1.5 m in front of the subject. Through the use of a software program written in DELPHI, 100 logatome pairs (50 same, 50 different) were randomly selected and presented in same-different combinations. After the presentation of every logatome pair, the subject's perception was recorded by selection of a two-alternative forced-choice (2AFC), pressing button #1 for a "same" impression and button #2 for a "different" impression. Logatome pairs were presented without feedback regarding correct or incorrect responses. The stimuli and the subjects' responses were stored in a text file, and evaluated by calculating the "hits", "misses", "false alarms", and "correct rejections" to determine the sensitivity index d' for every subject. RESULTS OLSA higher (t (13) = 10.45, p < 0.01). This group was labeled as "good CI performers". For the remaining four CI users, labeled as "poor CI performers", the tests were interrupted due to a lack of specificity approaching floor effects. There was no correlation observed between the age of the participants or their onset of deafness and the speech intelligibility results. Electrophysiological Experiment Rahne et al.: Logatome Discrimination in Cochlear Implant Users TheScientificWorldJOURNAL (2010) 10, 329-339 335 FIGURE 2. (a) Group average ERP waveforms for the normal hearing subjects, and the poor and good CI performers, recorded at the electrode site Fz. A negative difference between the "deviant" (bold line) and the "control" (thin line) waveform in a latency range between 100 and 250 msec was identified as MMN. For the normal hearing subjects and the good CI performers, a clear MMN was observed for both the voweland the consonant-discrimination task of CVC logatomes. No significant difference was found for the poor CI performers (*p < 0.05). (b) The group average difference waveforms show the different MMN potentials for the vowel-and consonant-replacement logatome paradigms. Logatome Discrimination Test All except one CI user finished the logatome discrimination test. For normal hearing subjects, the overall hit rate was mostly 100%. For CI users, the hit rate varied from chance level (50%) to 100%. A two-way mixed ANOVA (discrimination task: two levels, consonant and vowel replacement; speech intelligibility, three levels: normal hearing, good CI performers, and poor CI performers) was performed on the sensitivity index d' of the logatome discrimination test after confirming the normal distribution and homogeneity of the sensitivity index (Mauchly's test, p > 0.05). The results show that the d' of both the two discrimination tasks (F (1, 16) = 46.50, p < 0.01) and the three speech intelligibility groups differed significantly (F (2, 16) = 95.51, p < 0.01). A significant interaction of the discrimination tasks and speech intelligibility was found (F (2, 16) = 24.64, p < 0.01). Posthoc tests revealed that, over all subjects, the discrimination ability was higher for the normal hearing subjects compared to that for the good and poor CI performers. The poor CI performers revealed the poorest discrimination abilities. The posthoc test of the interaction between the discrimination task and speech intelligibility revealed a significantly higher d' for the vowel replacement compared to the consonant replacement only for the good CI performers (Bonferroni test, all p < 0.05). DISCUSSION The results of the experiments presented in this paper demonstrate that the OLLO Corpus can be used to design an effective logatome discrimination test for CI users. With its studio-quality recordings of Rahne et al.: Logatome Discrimination in Cochlear Implant Users TheScientificWorldJOURNAL (2010) 10, 329-339 logatomes from 40 German speakers, it is the first corpus that is available for the German-speaking region. Recently published work also reports the first speaker discrimination test drawing its stimuli from this corpus 336 Only the good CI performers were able to perform the OLSA. As this is a speech intelligibility test, this demonstrates the clinical need for more basic speech perception tests as demonstrated by the newly developed logatome discrimination test, which in contrast, all subjects could perform. To avoid floor effects as experienced on behavioral tests such as the OLSA for poor performing subjects, the logatome discrimination test provides a valuable clinical evaluation alternative. For both the normal hearing control group and the good CI performers, the speech intelligibility results of the OLSA were clustered. The resulting SNR distribution for the normal hearing control group was rather narrow, while it was not possible to obtain results for the poor CI users. Thus, a categorization of the speech discrimination ability of the subjects was revealed by the OLSA. The electrophysiological results support the speech intelligibility results revealed by the OLSA. No differences between the MMN amplitudes following vowel-and consonant-replacement paradigms were obtained. The similarity of the MMN amplitudes for normal hearing subjects and good CI performers, in contrast to that observed for the poor CI performers, justifies use of MMN component measures for screening for speech intelligibility and discrimination abilities. Beyond this categorization, the MMN so far appears not to be applicable to a direct measure of the discrimination abilities. Measurements of the MMN for CI users on a individual level are time consuming and difficult to confirm reliably 337 As expected, the logatome discrimination test revealed a broad distribution of d' for the CI users group and a narrow distribution for the normal hearing subject group. Thus, for the CI users, the logatome discrimination test allows a measure of speech discrimination abilities with a higher resolution than permitted by the OLSA or the MMN measure. The logatome discrimination scores varied from chance level to almost 100% without evidence of a ceiling effect as observed and described for the HINT On the other hand, we also state that the MMN component measure is also reported as sensitive to logatome and syllable differences, and is therefore applicable to screen speech discrimination skills of uncooperative subjects for clinical purpose