Effects of lowpass and highpass filtering on the intelligibility of speech based on temporal fine structure or envelope cues

This study aimed to assess whether or not temporal envelope (E) and fine structure (TFS) cues in speech convey distinct phonetic information. Syllables uttered by a male and female speaker were (i) processed to retain either E or TFS within 16 frequency bands, (ii) lowpass or highpass filtered at different cut-off frequencies, and (iii) presented for identification to seven listeners. Psychometric functions were fitted using a sigmoid function, and used to determine crossover frequencies (cut-off frequencies at which lowpass and highpass filtering yielded equivalent performance), and gradients at each point of the psychometric functions (change in performance with respect to cut-off frequency). Crossover frequencies and gradients were not significantly different across speakers. Crossover frequencies were not significantly different between E and TFS speech (∼1.5 kHz). Gradients were significantly different between E and TFS speech in various filtering conditions. When stimuli were highpass filtered above 2.5 kHz, performance was significantly above chance level and gradients were significantly different from 0 for E speech only. These findings suggest that E and TFS convey important but distinct phonetic cues between 1 and 2 kHz. Unlike TFS, E conveys information up to 6 kHz, consistent with the characteristics of neural phase locking to E and TFS.