Forward masking of the auditory evoked potentials in a dolphin at monaural and dichotic auditory stimulation: implications for the preceding effect and biosonar

Short-latency auditory brainstem evoked potentials (ABR) to paired sound pulses (the conditioning and test stimuli) were recorded non-invasively in a bottlenose dolphin Tursiops truncatus. The stimuli were played through transducers contacting the left and right acoustic windows at the lower jaw. Two manners of stimulation were used: monaural (the both stimuli played through one and the same transducer) and dichotic (the conditioning and test stimuli played through different transducers, contacting the left and right acoustic window). Thе conditioning and test stimuli were equal in level and duration. The inter-stimulus delay varied from 0.15 to 10 ms. At the monaural stimulation, the suppression of the test stimulus was constant at interstimulus intervals from 0.15 to 0.5 ms; at longer intervals, the test response recovered. At the dichotic stimulation, the deepest suppression of the test response appeared at an interval of 0.5 ms; the test response recovered at both shorter and longer intervals. The complete recovery appeared at intervals as short as 0.15 ms and as long as 2 ms. Implications of the found regularities for the preceding effect and biosonar is discussed.