Auditory repetition enhancement at short interstimulus intervals for frequency-modulated tones.

Frequency-modulated (FM) sweeps are important components of most natural sounds. To examine the processing of these stimuli we applied a two-tone paradigm. Repeated stimulus presentation usually leads to reduced neuronal responses. However, in a former study repetition enhancements which have been observed when FM tones were separated by short interstimulus intervals (ISIs) of ≤200ms. To further investigate this repetition effect in response to FM tones, we recorded magnetoencephalogram (MEG) in humans during the presentation of consecutive FM sweep pairs separated by ISIs between 100 and 600ms. We presented FM sweep pairs in six experimental conditions: a) two upward FM tones, b) two downward FM tones, c) an upward followed by a downward FM tone and d) a downward followed by an upward FM tone. Sequences of single upward and single downward FM tones served as control conditions. N1m amplitude was enhanced for repeated compared with different FM-direction tone pairs. This effect was found for the shortest ISI of 100ms and disappeared at longer ISIs. Furthermore, mean peak latencies in response to the second tone were prolonged in same-direction pairs at the shortest ISI of 100ms. At ISIs ≥300ms slight enhancement effects occurred between 180 and 400ms after the second stimulus. This is in accordance with a previous MEG study from our laboratory which demonstrated an enhancement effect for sustained fields at latencies of 150-350ms after the second stimulus for same compared to different FM tone pairs separated by an ISI of 200ms.