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The term temporal integration (TI) refers to summation of stimulus intensity during the duration of the stimulus. As duration increases, a sensation like loudness increases, or the sound level at which the stimulus can be detected decreases. The stimuli may be various types of signals, such as tones or bands of noise. Similarly, short succeeding stimuli can combine their energies and provide a lower detection level than individual stimuli. The TI has a time limit. For a stimulus longer than this limit, the loudness, or the detection (threshold) level, remains relatively constant.
Interest in studying TI is fueled by the need to understand auditory processing of speech—a signal that, by its nature, changes rapidly in time. Better understanding of the temporal characteristics of hearing should help us improve means for enhancement of speech communication in unfavorable listening environments, and of listeners with impaired hearing.
Graphs of the relationship between the stimulus duration (plotted on the horizontal coordinate, usually in milliseconds with a logarithmic scale) and the intensity level at the threshold of hearing (plotted on the vertical coordinate in decibels, dB) are called temporal integration curves (TICs). Examples of TICs are shown in Figure 1. The detection intensity level first declines as the stimulus duration increases and then, beyond a time limit called the critical duration, remains constant. The magnitude of TI can be expressed by the difference between the detection levels of long and short signals. The rate of decline of TICs is represented by slopes of the curves, which too are often used as indicators of TI magnitude. These slopes (they are negative) are usually expressed as the ratio of the change of level (in dB) per tenfold increase in signal duration [(L2–L1)/dec], or per doubling of signal duration. The slopes of the TICs and the values of the critical duration represent summary characteristics of TI. (The critical duration depends on a time constant τ, a parameter in formulas describing TICs.)
Figure 1..
Temporal integration curves according to the functions shown in the legend. In curves A1 and A2, the time constant τ = 300 ms; in A1, exponent m = 1; in A2, m = 0.8. In B1, τ = 300 ms; in B2, τ = 100 ms. The value of τ = 300 ms is indicated by a mark on the abscissa.
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