Interaural asymmetry of dynamic range: Abnormal fusion, bilateral interference, and shifts in attention

Abstract

Speech information in the better ear interferes with the poorer ear in patients with bilateral cochlear implants (BiCIs) who have large asymmetries in speech intelligibility between ears. The goal of the present study was to assess how each ear impacts, and whether one dominates, speech perception using simulated CI processing in older and younger normal-hearing (ONH and YNH) listeners. Dynamic range (DR) was manipulated symmetrically or asymmetrically across spectral bands in a vocoder. We hypothesized that if abnormal integration of speech information occurs with asymmetrical speech understanding, listeners would demonstrate an atypical preference in accuracy when reporting speech presented to the better ear and fusion of speech between the ears (i.e., an increased number of one-word responses when two words were presented). Results from three speech conditions showed that: (1) When the same word was presented to both ears, speech identification accuracy decreased if one or both ears decreased in DR, but listeners usually reported hearing one word. (2) When two words with different vowels were presented to both ears, speech identification accuracy and percentage of two-word responses decreased consistently as DR decreased in one or both ears. (3) When two rhyming words (e.g., bed and led) previously shown to phonologically fuse between ears (e.g., bled) were presented, listeners instead demonstrated interference as DR decreased. The word responded in (2) and (3) came from the right (symmetric) or better (asymmetric) ear, especially in (3) and for ONH listeners in (2). These results suggest that the ear with poorer dynamic range is downweighted by the auditory system, resulting in abnormal fusion and interference, especially for older listeners.

Keywords: aging; binaural hearing; dichotic speech; dynamic range compression; interaural asymmetry; phonological fusion; speech perception.

FIGURE 1

Estimated audiometric thresholds for the left and right ear. The panels on the left and right represent responses from the left and right ear, respectively. Average results are shown in blue or red, offset to the right, and error bars represent one standard deviation. Shapes offset to the left represent individual audiometric thresholds. Results from YNH and ONH listeners are shown on the top and bottom rows, respectively.

FIGURE 2

Illustration of vocoder processing. (A) Processing stages over time, where 16 bandpassed signals are obtained, their envelopes are extracted and compressed, noise-based carrier are modulated in amplitude, and modulated carriers are summed. (B) Envelope compression in dB, where the envelope is compressed to some percentage of its original range in dB. This is completed filling in the dips of the envelope, then normalizing to the original envelope level.

FIGURE 3

Single word accuracy and number of words responded for (A) interaurally symmetric, and (B) interaurally asymmetric vocoder conditions. The x-axis corresponds to the vocoder condition. The y-axis represents the percentage of trials with accurate, one-word responses (△ shown in black) and the percentage of one-word responses (▽ shown in green). Open and closed shapes represent YNH and ONH listeners, respectively.

FIGURE 4

Different vowel accuracy and number of words responded for (A) interaurally symmetric, and (B) interaurally asymmetric vocoding conditions. The x-axis corresponds to the vocoder condition. The y-axis represents the percentage of trials with at least one word accurately identified (△ shown in black) and the percentage of two-word responses (▽ shown in purple). Open and closed shapes represent YNH and ONH listeners, respectively.

FIGURE 5

Vowel responses by ear for different vowel trials for (A) interaurally symmetric or (B,C) interaurally asymmetric vocoder conditions. The x-axis corresponds to the vocoder condition. The y-axis corresponds to the percentage of trials where the vowel of the response was one word and came from the left (× shown in blue) or right (○ shown in orange).

FIGURE 6

(A) Response categories for one example phonological fusion trial. (B–D) Relative frequency of response categories observed by listeners in (B) symmetric and (C,D) asymmetric dynamic ranges. The x-axis corresponds to the response category from (A). The y-axis corresponds to the proportion of responses. The color and pattern represent the vocoder condition given in the figure legend and arranged from highest to lowest mean dynamic range. The height of bars represents the mean across listeners. Error bars represent standard deviation of the mean across listeners.

FIGURE 7

Proportion of responses containing only liquid consonants for fusion and single word trials by vocoder condition. The x- and y-axes correspond to the proportion of trials containing only liquid consonants for fusion and single word trials, respectively. The color and shape represent the vocoder condition given in the figure legend. Panels on the (left) and (right) correspond to YNH and ONH listeners, respectively.