Our basic approach is to consider the time series of color values at
any spatial location (pixel) and amplify variation in a given temporal
frequency band of interest. For example, in Figure 1 we automatically
select, and then amplify, a band of temporal frequencies
that includes plausible human heart rates. The amplification reveals
the variation of redness as blood flows through the face. For this
application, temporal filtering needs to be applied to lower spatial
frequencies (spatial pooling) to allow such a subtle input signal to
rise above the camera sensor and quantization noise.