D44 Measurement of sensory thresholds

Topic

The measurement of an insect sensory system's threshold determines its minimum sensitivity—specifically, the lowest stimulus intensity the system can reliably detect. Conversely, the system's psychometric function maps the probability of response against stimulus intensity; this function is fitted using an appropriate statistical distribution to derive the threshold—a value that, within this set of models, defines the system's minimum sensitivity. If the response probability reaches its midpoint at a lower stimulus intensity, the resulting threshold derived from the psychometric function is correspondingly lower. In contrast, the thermal noise threshold establishes the system's fundamental physical limit—a constraint dictated by the laws of physics (as defined in another model in this set) rather than merely by the system's specific technology. Consequently, no physical system can detect a signal smaller than this background thermodynamic fluctuation; the best-known biological receptors approach this fundamental physical limit—much like the extreme olfactory capability of a moth (described in another model in this set), which operates near the stochastic limit imposed by a single molecule. The closer a specific biological receptor comes to this fundamental thermal noise limit, the more extreme its sensitivity becomes. In contrast, the Weber fraction for this sensory system—reflecting its ultimate differential resolution—measures the very Weber law established by another model in this set regarding a differential threshold proportional to a background level; that is, it determines how much a stimulus must change for the system to detect it as different from the preceding one. If the background level of a particular stimulus is higher, the minimum detectable difference—according to this Weber fraction—becomes absolutely larger as well.

Signal detection theory—another model in this set—decouples the sensory receptor's actual sensitivity from the animal's response criterion; this distinguishes sensitivity from discriminative capacity (defined by a different model in the set), thereby allowing for the comparison of sensitivities across experiments. If the animal's response criterion varies significantly between experiments, signal detection theory becomes all the more essential for isolating the sensory receptor's true sensitivity.

Consequently, this model—characterized by the minimum sensitivity and weakest detectable intensity with which it began—enables the resulting audiogram (plotting the system's threshold against stimulus frequency) to reveal both the system's optimal hearing range and the characteristic communication frequencies of specific species, facilitating the interspecies comparisons established by another model in the set. When the audiograms of two different species reveal distinct optimal hearing ranges, the resulting comparison of their sensitivities offers even deeper insights into their respective evolutionary histories.

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