Fine frequency-modulation trigger features of midbrain auditory neurons extracted by the progressive thresholding method - a preliminary study

Spectro-temporal receptive fields (STRFs) are commonly used to characterize response properties of central auditory neurons and for visualizing 'trigger features'. However, trigger features in STRF maps typically have a blurry appearance. Therefore it is unclear what details could be embedded in them. To investigate this, we developed a new method called 'progressive thresholding' to resolve fine structures in the STRFs, and applied the method to FM responses recorded from single units at the auditory midbrain of anesthetized rats. Random FM tones of a narrow frequency range (~0.5 octave) were first presented to evoked spike responses at the cell's best frequency. Perispike modulating time waveforms collected (50 msec long, n = 1,500 to 4,000 tracings) were used to generate STRF based on spike-triggered-averaging. After supra-threshold areas of pixel counts had been determined through a step of progressive thresholding in the map, those peri-spike modulating waveforms passing through each area were dejittered systematically. At what seemed to be an optimal threshold, multiple trigger features (up to a maximum of 4 fine bands) were extracted from the initially simple-looking STRF. Results show that fine FM trigger features are present in STRFs and that they can be resolved with the present method of analysis.