Brain Advance Access published online on November 26, 2008
Brain, doi:10.1093/brain/awn308
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Enhanced discrimination of low-frequency sounds for subjects with high-frequency dead regions
1Department of Experimental Psychology, Cambridge University, Downing Street, Cambridge CB2 3EB, UK and 2Department of Audiology, All India Institute of Speech and Hearing, Manasagangothri, Mysore 570 006, India
Correspondence to:
Brian C. J. Moore, Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, UK
Research using animals suggests that a lesion in the basal portion of the cochlea, causing a high-frequency ‘dead region’, leads to cortical reorganization, such that frequencies just below the edge frequency of the dead region, fe, become over-represented. We set out to determine if this reorganization has functional benefits. Two groups of subjects were tested, with and without acquired high-frequency dead regions, as assessed using the TEN(HL) test. For the ears with dead regions, the value of fe was close to 1000 or 1500 Hz. The two groups were matched in terms of audiometric thresholds for frequencies below fe and in terms of age. Three subjects with unilateral dead regions (with matched low-frequency audiometric thresholds across ears) were also tested. Three tasks were used: (i) frequency discrimination of sinusoidal tones. The level of every stimulus was roved over a 12-dB range to reduce the salience of loudness cues. The center frequencies used ranged from 250 Hz to just below fe; (ii) detection of sinusoidal amplitude modulation of a sinusoidal carrier. Carrier frequencies of 500 and 800 Hz were used with all subjects, and an additional carrier frequency of 1200 Hz was used for ears with fe close to 1500 Hz and their matched counterparts. Modulation frequencies were 4, 50 and 100 Hz; (iii) identification of consonants in nonsense syllables. The syllables were lowpass filtered at 1000 or 1500 Hz (depending on the value of fe) and complementary highpass-filtered noise was presented to prevent use of information from neurons tuned above fe. For the frequency-discrimination task, the ears with dead regions showed a significant local improvement (‘enhanced’ thresholds) for frequencies just below fe, as has been reported previously. For the subjects with unilateral dead regions, the enhancement occurred only for the ears with dead regions. For the amplitude-modulation detection task, thresholds were generally lower for the ears with dead regions than for the ears without, and this effect was statistically significant. For the subjects with unilateral dead regions, thresholds were lower for the ears with dead regions than for the ears without. Consonant identification was significantly better for the ears with than without dead regions, and this was true for the subjects with unilateral dead regions. We conclude that a dead region at high frequencies is associated with a better ability to process information at low frequencies. These effects may reflect cortical plasticity induced by the dead regions.
Key Words: cortical plasticity; cochlear dead region; frequency discrimination; amplitude-modulation detection; consonant identification
Abbreviations:
CD, compact disk; CF, characteristic frequency; 
F, step used in frequency discrimination task; DLF, difference limen for frequency; ERBN, equivalent rectangular bandwidth of the auditory filter for normal-hearing listeners; fcut-off, cut-off frequency of hearing loss; fe, edge frequency of a dead region; IHC, inner hair cell; LE, left ear; m, modulation index; RE, right ear; SD, standard deviation; SL, sensation level; SII, speech intelligibility index; TEN, threshold-equalizing noise
Received August 1, 2008. Revised September 23, 2008. Accepted October 22, 2008.