In this chapter I consider those psychological processes, phenomena, and models that are specifically concemed with the detection of particular stimuli in a surround. Detection is, as we have seen, a curiously misused and abused word in the lexicon of psychology. Some uses often encroach upon the definition of other weH-defined visual processes. In other cases the intended meaning of the word is so inclusive as to make it virtually meaningless as a scientific designator. One appropriate use of the word, but one with which I am not concemed in this chapter, is energy detection; that is, the consteHation of phenomena and proces ses associated with the absolute threshold for luminosity-in other words, the process by which an ob server determines if there is any photic stimulus there at all. Though there are certainly some cognitive overtones to even this kind of absolute threshold (usually associated with the criterion levels used by an observer in accepting or rejecting a near-threshold event) in fact, our knowledge of the mechanics of the absolute luminosity-detection threshold convinces most contemporary visual scientists that it is an ensemble of properties of the photochemical-receptive processes in rods and cones (as weH as the statistics of individual quanta) that regulates this kind of detection behavior. Indeed, the absolute threshold of luminosity detection has been known for many years (Helson & Fehrer, 1932) to be virtually unaffected by the shape or form of a stimulus as long as it is within the integration area defined by Ricco's law. Helson and Fehrer also note (a conclusion for which I have found no subsequent refutation) that it takes "25 times as much light to perceive forms correctly . . . as com-

pared with just noticeable light .... " (p. 101).1 The energy-detection problem is also weH known to be almost a nonproblem

The word detection is also used in another context (other than measurement of the limits of luminosity thresholds) by students of alphabetic-Ietter perception. The task of finding one particular letter among many is sometimes said to be illuminating an aspect of form "detection." However, a close inspection of this kind of paradigm suggests that what is actually being studied in this case is a kind of successive-recognition behavior. This is not what Iaminterested in, either, in this chapter. Letter "detection" experiments of this recognition genre are , therefore, considered in chapter 5.