Visual resolution: operational definitions with an eye towards historical precedence

The perceptual task of ‘‘resolving’’ one from two objects can be operationally defined and measured psychophysically. However, experimental methods aside, terms used to describe the measurement can have historical significance and should be chosen carefully as Westheimer (in press) points out. The term in question in this case is ‘‘resolution’’ which historically involves describing the optical resolving power of a telescope for distinguishing a single from a double star. Westheimer (in press) suggests that a resolution task should involve the detection of a trough (a dark thin dimple) between the two stars, so that one is sure that the image did not come from an extended single object. In modern astronomy the task of resolving a double star from a single star makes use of prior knowledge (e.g. stars are point-like objects) and does not rely on detecting a trough in the luminance distribution. Deconvolution methods that take into account the telescope point spread function (see Heydari-Malayeri, Remy, & Magain, 1989; Karovska, 2002) are used to reveal fine detail in modern astronomical images of low contrast objects in noise. The article, ‘‘Using a priori information in image restoration: Natural resolution limit’’ by Terebizh (1999) uses numerical simulation to study the limiting separation between two point-like components of a double star. Westheimer (in press) raises the question about whether prior knowledge should be used in the psychophysical definition of resolution. Consider the task of discriminating a single line from a pair of lines (for psychophysics we will discuss lines rather than dots). If one had prior knowledge that the lines were thin (a signal detection methodology like 2AFC provides the needed prior knowledge), then the task becomes one of blur discrimination in that the two-line stimulus would produce a broader retinal image than the single line. This is the operational definition of resolution used by Carney and Klein (1997) in their one thin line vs. two thin lines two alternative forced choice task (2AFC). If however, there was no