Temporal integration of monaural and dichotic frequency modulation

Abstract

Frequency modulation (FM) detection at low modulation frequencies is commonly used as an index of temporal fine-structure processing. The present study evaluated the rate of improvement in monaural and dichotic FM across a range of test parameters. In experiment I, dichotic and monaural FM detection was measured as a function of duration and modulator starting phase. Dichotic FM thresholds were lower than monaural FM thresholds and the modulator starting phase had no effect on detection. Experiment II measured monaural FM detection for signals that differed in modulation rate and duration such that the improvement with duration in seconds (carrier) or cycles (modulator) was compared. Monaural FM detection improved monotonically with the number of modulation cycles, suggesting that the modulator is extracted prior to detection. Experiment III measured dichotic FM detection for shorter signal durations to test the hypothesis that dichotic FM relies primarily on the signal onset. The rate of improvement decreased as duration increased, which is consistent with the use of primarily onset cues for the detection of dichotic FM. These results establish that improvement with duration occurs as a function of the modulation cycles at a rate consistent with the independent-samples model for monaural FM, but later cycles contribute less to detection in dichotic FM.

FIG. 1.

(Color online) Analysis of dichotic FM via a comparison of the signals at the two ears. (A) (sin) and (B) (-cos) illustrate phase offset as a function of time throughout the stimulus duration. (C) illustrates the instantaneous ITD throughout the duration of the stimulus for the sin (S) and -cos (C) stimuli.

FIG. 2.

Experiment I: The effect of stimulus duration on monaural and dichotic FM detection. Mean FM detection thresholds are shown with standard error bars for monaural (closed symbols) and dichotic (open symbols) conditions with the modulation starting phase indicated by the symbol as noted in the key.

FIG. 3.

(Color online) Experiment II: The effect of the number of modulation cycles on monaural FM detection. Mean monaural FM detection thresholds are shown with standard error bars for stimulus durations of 500 ms (open black symbols) and 1250 ms (closed black symbols) as a function of modulation cycles. The black line represents predictions made by the independent-samples cycles model of Eq. (4) with the corresponding adjusted R² value. Data from other studies as indicated in the legend include Buss et al., 2004; Grose and Mamo, 2012; He et al., 2007; Hoover et al., 2019; Strelcyk and Dau, 2009; Wallaert et al., 2016; and Witton et al., 2000. See Table II for details of the modulation and carrier frequencies used in those studies.

FIG. 4.

(Color online) Experiment III: Effect of short stimulus duration on dichotic FM detection. Mean dichotic FM detection thresholds are shown with standard error bars as a function of modulation cycles. Modulation starting phase is indicated by the symbol as shown in the key. The functions represent predictions of different model fits to the data with corresponding adjusted R² values shown in the inset. Data from other studies include Grose and Mamo, 2012; Whiteford and Oxenham, 2015; and Hoover et al., 2019. See Table IV for details about the modulation rate and duration.

FIG. 5.

(Color online) Experiment III: Effect of short stimulus duration on dichotic FM detection, excluding data at 1/8 of a cycle. Mean dichotic FM detection thresholds are shown with standard error bars as a function of modulation cycles.