Temporal modulation transfer functions based upon modulation thresholds

The detectability of amplitude modulation in the absence of spectral cues provides a quantitative description of temporal resolution for steady‐state signals with relatively small amplitude changes. Modulation thresholds for sinusoidally amplitude‐modulated wideband noise were measured as a function of modulation frequency. The resulting ’’Temporal Modulation Transfer Function’’ (TMTF) shows a lowpass characteristic for modulation frequencies below about 800 Hz. The lowpass characteristic is extended up to approximately 2 kHz when the increment in average power produced by modulation is eliminated. The important parametric effects are summarized as follows: (1) TMTFs are independent of overall level, except at very low intensities; (2) the time constant indicated by the TMTF decreases as the center frequency of the band‐limited, modulated noise is increased; (3) modulation thresholds generally decrease with increasing duration of modulation, particularly at low modulation frequencies; (4) when the carrier is gated for the duration of modulation, the TMTF shows a highpass segment at low modulation frequencies. Although the TMTFs are not directly consistent with the attenuation characteristic of a simple lowpass filter, a model which incorporates such a filter, with a time constant of 2.5 ms, describes the entire TMTF and also describes the modulation functions obtained with square‐wave and pulse modulation. The wide bandwidth of initial filtering indicated by the model raises the important question of the role of peripheral filtering in determining the detectability of high‐frequency modulation.