Perceptual Coherence : Hearing and Seeing

Gain Control and External and Internal Noise 247
is further transformed in two ways. First, an expansive nonlinear power
function accentuates the higher intensities. Second, the input is squared,
multiplied by the internal multiplicative noise fraction, and added to the
transformed signal. Finally, additive equivalent input noise independent of
the signal also is added so that the effective stimulus becomes
transformed input signal + external noise + independent multiplicative
equivalent input noise + independent additive equivalent input noise
and that stimulus is used to make a decision about the presence or absence
of the signal. The basic model assumes that the calculation-decision process
is independent of the stimulus.
Researchers (Gold, Bennett, & Sekular, 1999; R. W. Li, Levi, & Klein,
2004; Z.-L. Lu & Dosher, 2004) used such a model to investigate whether
practice reduced the equivalent input noise or increased the efficiency of
Figure 6.3. The basic threshold versus external noise function (threshold versus
intensity, TVI) consists of two segments (A). The flat segment is due to the equivalent
internal noise, and the sloping segment is due to the effect of the external contrast
noise. Representations of the effects of reduced internal (B), external (C), and
multiplicative noise (D) are depicted by the darkening line segments and by the direction
of the arrow. Adapted from “Perceptual Learning Retunes the Perceptual
Template in Foveal Orientation Identification,” by Z.-L. Lu and B. A. Dosher, 2004,
Journal of Vision, 4, 44–56.