Temporal Masking in Electric Hearing

Abstract

Temporal masking can be defined as the detection threshold of a brief signal as a function of the signal delay in a relatively long masker. The temporal masking pattern in normal acoustic hearing reveals temporal edge enhancement in which the signal detection threshold is greater near the masker onset than in the steady-state portion. Both peripheral and central mechanisms appear to underlie temporal edge enhancement, but their relative contributions remain elusive. Cochlear implants bypass cochlear mechanical processing and stimulate the auditory nerve directly, thereby providing a unique opportunity to separate the peripheral mechanisms from the central mechanisms. Here, we systematically measured temporal masking in electric hearing by examining whether a brief signal was harder to detect at the onset than in the steady-state portion of a long masker (the Bovershoot^effect). The signal and the masker were presented (1) either to the same electrode or to different electrodes, (2) at the same stimulation or different rates, and (3) in a simultaneous or an interleaved fashion. A consistent pattern of results was observed, depending on the stimulus configuration between the signal and the masker. Simultaneous stimulation at the same rate and with the same electrode produced no difference in sensitivity between the onset and the steady-state conditions, but interleaved stimulation at different rates or with different electrodes produced a significant difference. Unlike acoustic hearing, high masker levels produced an overshoot effect, and low masker levels produced an undershoot effect. Although the present results are consistent with the Bon-frequency vs. off-frequency^hypothesis for the overshoot effect, results also suggest a central Bsame vs. different^mechanism underlying temporal masking. These results have practical implications for improving cochlear implant design.

Auditory Cortex Phase Locking to Amplitude-Modulated Cochlear Implant Pulse Trains

“…Cochlear implant users report increases in perceived pitch corresponding to increases in the rate of unmodulated pulse trains to rates as high as 300 pps (Geurts and Wouters 2001;Kong et al 2004;McKay et al 1994;Xu et al 2002;Zeng 2002). Based on the present results, it seems highly unlikely that discriminations of such high pulse rates rely on phaselocked activity in the auditory cortex.…”

Spatial and Temporal Effects of Interleaved Masking in Cochlear Implants

“…The forward masking slope in acoustic hearing decreases as the masker-probe interstimulus delay becomes larger (Jesteadt et al 1982). Indeed, similar to acoustic hearing, the slope of GOM in on-channel simultaneous masking in electric hearing has been found to be close to 1 (Zeng et al 2005) and less than 1 in forward masking (Chatterjee and Shannon 1998;Nelson and Donaldson 2001). Although the presentation of the masker and probe in the present experiment was not truly simultaneous, the effect of the masker was expected to resemble that of simultaneous masking because the interpulse (interleaved) period between masker and probe was fairly small (≤1 ms), as shown in Figure 1, except for the 125-Hz condition (4 ms).…”

“…This appeared to be attributed to by a shallow growth of masking with maskers at low levels, suggesting a possible effect of neural summation. A phenomenon of negative masking, or "facilitation" by a masker, can sometimes be observed in electric hearing, i.e., detection threshold is lower when the masker is present than in quiet, as seen in both behavioral (Eddington et al 1978;Nelson and Donaldson 2001;Tong and Clark 1986;Zeng et al 2005) and physiological measurements (e.g., Middlebrooks 2004). In those studies, the masker was commonly at a low (almost subthreshold) level.…”

“…The probe was placed in the center of the masker duration (225 ms after the onset of the masker), where the neural discharge from the masker was considered to be in a steady state. While it has been observed that a peripheral "over-shoot" or "undershoot" effect in the discharge rate at the vicinity of temporal edges of onset or offset of stimuli may affect the measurement of masking in electric hearing (Smith et al 2003;Zeng et al 2005), the present study primarily sought to examine masking without such temporal edge effect. The stimulation rate of both masker and probe was one of the following: 125, 500, 2,000, 6,410 Hz.…”

“…Therefore, simultaneous masking in electric hearing can be studied in the context of interleaved stimulation rather than in the strict sense of simultaneous stimulation. Despite the prevalent use of interleaved stimulation in clinical applications, masking in interleaved stimulation (throughout this paper it will be referred to as "interleaved masking") has not been studied as widely as it deserves, as only limited data are available in the literature on this type of masking (Tong and Clark 1986;Zeng et al 2005). Interleaved masking is different from forward masking in that there is a temporal overlap of the masker and probe presentation.…”