Triggered correlation has been shown to be a useful tool to characterize the frequency selectivity of single units in the vertebrate auditory system. Such characterization is simplified mathematically because the means of the noise stimulus, x(t), the equivalent bandpass filter output, y(t), and its derivative, y, are all zero. Once the x+(t) estimate of h(t) is obtained, it can be Fourier-transformed to find the equivalent frequency response of the bandpass filter, H(jra).

If a sensor is low-pass or has a broad bandpass characteristic in its response to stimuli, TC can also be applied. Most physiologically relevant physical input parameters are non-negative, and thus the means of x and y would need to be considered.

Theoretically, TC can be applied to the definition of the linear spatiotemporal transfer function of ganglion cells (GCs) in the retina. In this case, the stimulus can be a two-dimensional checkerboard overlapping the GS RF. The elements are made to shift rapidly and randomly in intensity, using either white light or monochrome illumination. Every time a GC spike occurs, a two-dimensional average of stimulus previous patterns is made, giving an l+(x, y, t) .

There are probably four reasons TC has not enjoyed wider application in sensory neurophysiology:

1. The theory is difficult to understand.

2. The TC algorithm is difficult to implement.

3. TC says nothing about the underlying physiology giving rise to frequency selectivity, etc. It is a "black box" approach.

4. TC is best applied to "tuned" neuro-sensory systems, such as the vertebrate auditory system and the electroreceptor system of mormyrid electric fish (see Section 2.5.2). Results when the linear filter is not sharply tuned are hard to interpret.

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