LANGUAGE IN INDIA

Role of Encoding Temporal Fine Structure Cues in
Time Compressed Word Recognition

Vidit Vidyarthi, Ritika Mittal, Imran Anwar Ali Dhamani, and
S.G.R. Prakash

Abstract

With traditional envelope cues and limited spectral and temporal information it's quite challenging for a Cochlear implant listener to understand rapid speech. A deficit in temporal processing may further contribute to difficulty understanding fast speech by a Cochlear implant listener. Hence the present study was aimed at investigating the benefit of encoding temporal fine structure information in understanding rapid speech.

32 normal hearing adults with an age range of 18 to 30 years participated in the study. Two experiments were conducted. In Experiment 1 - Words were compressed to 30% and in Experiment 2 - words were compressed to 50% of their original length. Vocoding of words was done in MATLAB 6.5. In one condition, only envelope cues were given while in the other, fine structure cues were extracted using phase orthogonal demodulation. Low pass filtering was done for getting 400 Hz modulation rate and FM bandwidth of 500 Hz with an envelope cutoff of 500 Hz. Word recognition was measured in both the conditions and subjected to further statistical analysis.

A paired t-test revealed statistically significant improvement in word recognition scores with temporal fine structure cues in both the conditions. (Exp-1: t=6.984, p<0.000 & Exp-2: t=4.399, p<0.000) with a mean difference in the scores in Exp-1 = [1.875] & Exp-2 = [1.0625].

Based on the mean difference in the scores obtained and our clinical observation in both the experiments, we can conclude that in spite of getting statistically significant improvement in the scores (p<0.00) with temporal fine structure information, the improvement is not clinically significant. This may be due to the adequate spectral information (22 channels) provided along with the envelope and fine structure information. Hence, with good spectral resolution there is not much effect of encoding temporal fine structure information on perception of rapid speech.

Introduction

Cochlear implants allow most patients with profound deafness to successfully communicate under optimal listening conditions, with good users being able to communicate on the telephone. Although much progress has been achieved in the design and performance of the cochlear implant systems, much remain to be done. A signal can be discomposed into its amplitude modulation and frequency modulation components. At present most of the cochlear implant signal processing devices are encoding temporal envelope (AM) cues into a restricted number of channels. These devices allows adequate speech perception in quiet environments for most of its users (Stickney et al 2005) but the utility of these temporal envelope cues is seriously limited to only optimal listening conditions (high context speech materials and quite listening environments) (Zeng et al., 2004).

Most of the recent studies showed that encoding fine structure (FM) information could improve CI listener's speech understanding in more realistic listening situations especially when background noise is one of the confounding variable. According to Zeng et. al., 2004, Frequency modulation serves as a salient cue that allows a listener to separate, and then assign appropriately, the amplitude modulation cues to form foreground and background auditory objects. Frequency modulation extraction and analysis can be used to serve as front-end processing to help solve, automatically, the segregating and binding problems in complex listening environments, such as at a cocktail party or noisy cockpit.

Nie et al., (2005) measured sentence recognition in presence of competing voice in 40 normal hearing adult subjects and showed that amplitude modulation from limited number of spectral bands is sufficient to support speech recognition in quiet listening situations but performance deteriorates in presence of competing voice and then it is important to encode frequency modulation along with traditional amplitude modulation (AM) cues to improve speech understanding.

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