MethodsĪBRs were recorded intracranially from the surface of the midbrain inferior colliculus. The results indicate that bats have an adaptation for enhancing sonar receiver gain during a critical timeframe when faint echoes return from nearby small objects. We document an initial suppression phase that recovered within 4 ms and was replaced by an enhancement phase extending 4–15 ms beyond the first pulse, which corresponds to a critical time period for echolocating bats during which increased sensitivity could be very beneficial. To discriminate the overlapping second response from the continuing first response which may last 10–15 ms, we used a double-pulse stimulation protocol similar to the methods used in whales ( Supin et al., 2007 Supin and Popov, 2015) in which averaged ABR responses to a leading masker pulse were digitally subtracted from responses to masker-probe stimulus pairs varying in inter-stimulus interval, thereby revealing in greater detail the response to the trailing probe pulse in each pair. Here, we used ABRs to measure temporal masking effects in the free-tailed bat ( Tadarida brasiliensis). These apparent differences among cetaceans may be due to experimental methods or potentially reflect species-specific physiological adaptations for sonar use in different contexts. Beluga and false killer whales displayed only suppression effects ( Supin et al., 2007 Supin and Popov, 2015), but in dolphins it was discovered that the suppression could be quickly replaced by a pronounced enhancement window under certain circumstances ( Finneran et al., 2016). More recently, ABRs have been used to measure the time course of masking effects in echolocating cetaceans. ABRs have been measured in several bat species ( Wenstrup, 1984 Burkard and Moss, 1994 Obrist and Wenstrup, 1998 Boku et al., 2015 Simmons et al., 2015), but only one investigated forward masking effects in ABRs, finding that a masker could produce either suppression or enhancement depending on timing and level of the masker relative to the probe ( Grinnell, 1963b). The response consists of a stereotyped waveform generated by synchronous neural activity in successive early stages of auditory processing, and has proven to be a useful tool for comparing hearing sensitivities across animals. Despite the potential significance of forward masking effects on biosonar performance, surprisingly little is known about how forward masking effects influence auditory response properties or sonar performance in echolocating bats.Īuditory brainstem responses (ABRs) are acoustically evoked electrical responses that can be recorded with intracranial, subdermal, or superficial electrodes. Echolocating bats have adaptations for attenuating the responses to loud outgoing pulse emissions ( Suga and Schlegel, 1972 Jen and Suga, 1976), however, it has also been hypothesized that the suppressive effects of temporal masking could benefit biosonar by providing an automatic gain control mechanism that stabilizes response amplitudes within an optimal dynamic range ( Supin et al., 2008, 2009). Forward masking effects are problematic for biosonar because outgoing pulse emissions may interfere with the ability to detect and discriminate faint echoes returning soon afterwards. The main effect is a brief reduction in auditory sensitivity lasting ten to hundreds of milliseconds, but under certain conditions forward masking can also produce a response enhancement ( Henry, 1991a, b). Forward masking is an auditory phenomenon in which the response to a sound transiently alters the physiological response to subsequent sounds ( Zwislocki et al., 1959 Abbas and Gorga, 1981 Carlyon, 1988 Relkin and Turner, 1988).
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