The experiments of Raab et al. (1961), Raab and Fehrer (1962), and Kaswan and Young (1965) indicate that RT (Reaction Time) to a stimulus of any given intensity is constant regardless of the stimulus duration–provided, of course, that the stimulus duration is above detection threshold (Kaswan and Young, 1965). Furthermore, as might be expected from the above, for any given luminance RT to a stimulus was found to be independent of its apparent brightness (Raab et al., 1961).
Raab and Fehrer (1962) reported that for stimuli of moderate intensity RT increased by approximately 30 msec as stimulus intensity decreased over a range of three log units (3000 to 3.0 ft-L). Raab et al. (1961) reported an average RT increase of about 40 msec as intensity decreased over a range {11} of four log units (3000 to 0.3 ft-L). These findings are complemented by those of Efron (1964), who conjectured from the results of his own experimental work and that of others that the onset of awareness of a stimulus is delayed by approximately 10 msec for each log unit reduction in intensity.
The fact that changes in stimulus intensity produce changes in average perception delay that are of the same order of magnitude and in the same direction as the corresponding average changes in reaction delay is consistent with the assumption that RT is a measure of the delay between the objective onset of a stimulus and the S’s voluntary reaction initiated in response to the onset of his awareness of the stimulus. The fact that RT is independent of apparent brightness lends further support to our assumption that it is the mere occurrence of the onset of the experience and not the magnitude of the experience that determines RT.
This assumption allows us to understand the results of experiments measuring RT to stimuli “masked” by metacontrast. RT to a double-flash stimulus consisting of a central TS followed by a surrounding or adjacent MS is determined by the first component of the stimulus pair to be completely processed. This is true even if the S’s final experience is such that the TS and MS are not perceptually discriminable as separate events.
When the TS and MS are of equal intensity, as in the case of type B metacontrast, the first information to be processed is that emanating from the TS. In this case, it is valid to conclude that it is the TS that triggers the onset of the S’s {12} awareness of the stimulus complex and, therefore, determines RT. This assumption was appealed to in our earlier discussion of experiments measuring RT to stimuli “masked” by type B metacontrast (Fehrer and Raab, 1962; Harrison and Fox, 1966; Schiller and Smith, 1966). Our basic assumption that RT is determined by the first stimulus component to be processed is equally applicable in the analysis of experiments measuring RT to stimuli “masked” by type A metacontrast (Fehrer and Biederman, 1962).
RT measured under type A metacontrast conditions produces certain complications that require further discussion. Type A metacontrast is generally produced by employing a central TS of much lower intensity than that of the surrounding or flanking MS. The experiments mentioned earlier, showing that reaction delay and perception delay are longer for weaker stimuli, imply that the neural processing of weak stimuli takes significantly longer than the processing of intense stimuli. It is precisely this assumption that stimuli of different intensities, even when presented simultaneously, are processed at different rates, that serves as the basis for traditional explanations of phenomena such as (1) the Pulfrich stereoscopic phenomenon (Lit, 1949; Efron, 1963; Prestrude and Baker, 1968; Julesz and White, 1969), (2) the perception of asynchrony produced by the simultaneous presentation of two adjacent flashes of light differing only in intensity (Alpern, 1954; Arden and Weale, 1954; Roufs, 1963), and (3) the perception of apparent movement (toward the weaker stimulus) also {13} obtained under the latter conditions (Roufs, 1963).
Even though the TS in a type A metacontrast situation may actually precede the MS, information emanating from the more intense MS may be processed before that emanating from the weaker TS. So long as this is the case, the onset of the observer’s awareness of the stimuli will be determined by the processing period of the stronger and, therefore, the later stimulus. This does not mean that the TS has no effect on the observer’s perception, nor that the MS and TS are perceptually discriminable at the end of their processing. All that is being asserted is that the onset of awareness is determined by the stimulus component that completes its processing first. So long as the delay between the earlier weak TS and the later intense MS is such that the MS is first to complete its processing, one is always measuring RT to the MS, even if the TS triggers the timer.
As the TS-MS onset delay (∆t) increases, the combined RT (i.e., RT to the TS-MS array) gradually becomes consistently determined by the processing period of the TS, thereby causing the combined RT to correspond to the RT of the TS alone. A curious effect is obtained, however, when RT is measured to a multiple stimulus. Such conditions often yield combined RTs that are significantly faster than the RTs to either of the independent stimulus components. When this effect is produced by applying asynchronous stimuli to different sensory modalities, it is thought to be a manifestation of “intersensory facilitation” (Herschenson, 1962). When a single modality {14} is stimulated, as under conditions of type A metacontrast, the effect is simply referred to as “facilitation”. (Fehrer and Biederman, 1962).
Regardless of whether the same or separate modalities are stimulated, facilitation reaches a maximum under conditions where Δt is exactly equal to the difference between the independent RTs of the two stimuli (Hershenson, 1962; Fehrer and Biederman, 1962). Since facilitation is greater under conditions of type A metacontrast, where the TS and MS energy differences are great (Fehrer and Biederman, 1962), than under conditions producing type B metacontrast, where the TS-MS energy differences are smaller and where little (Fehrer and Raab, 1962) or no (Harrison and Fox, 1966) facilitation are found, facilitation cannot be attributed to TS-MS energy summation, nor to the magnitude of the TS and MS neural interactions.
The phenomenon of RT facilitation can be simply explained in terms of our assumptions (1) that RT is determined by the stimulus component requiring the shortest processing time, (2) that the processing periods of each component of a multiple stimulus are not affected by the remaining stimulus components, and (3) that the processing period for any given stimulus will vary slightly from moment to moment as a function of slight variations in the stimulus energy, and metabolic processes affecting the sensitivity of the sensory surface. From these assumptions it follows that as Δt increases, the probability increases that the TS, having the longer mean {15} processing period, will be processed sooner than the MS on any given trial. When At is set equal to the difference between the independent mean RTs of the TS and MS, the means of the independent processing time distributions of the TS and MS will coincide. Under these conditions, mean RT to the Joint event (TS-MS) is minimized when the TS and RS are arranged so that their processing times coterminate, which according to our hypothesis, would mean when the onset of awareness of the TS and the onset of awareness of the MS coincide.
Assuming that RT varies directly with processing time, it follows from the above analysis that the mean combined RT should decrease as Δt increases from zero to some value equal to the difference between the means of the independent RTs oi the two stimuli, after which mean combined RT should increase until it becomes equal to RT for TS.
Thus the assumption of an intensity dependent neural processing period allows us to account for the results of experiments measuring RT to “masked” stimuli and to explain the phenomenon of RT facilitation.
Of equal significance is the fact that our analysis does not depend upon — and, in our explanation of RT facilitation, actually contradicts–the paradoxical notion, often advanced as an explanation of RT to “masked” stimuli, that RT is “triggered” by a package of stimulus energy integrated over some “critical duration” and of such a small magnitude that it may be perceptually subthreshold (Fehrer and Biederman, 1962; Fehrer and Raab, 1962; Raab and Fehrer, 1962). {16}
Latest revision: June 15, 2015 @ 3:04 pm
bioperipatetic 