Theoretical and Experimental Background

The perceptual experience produced by the presentation of a brief visual stimulus may be radically different from that produced by the same stimulus when immediately followed by one or more additional stimuli to adjacent or overlapping retinal regions. Under these latter conditions Ss may report changes in the perceived brightness, duration, size, color, contour, location, or persistence of the initial stimulus. In addition to one or more of these changes, Ss may also show apparent reduction in reaction time to the initial stimulus, or may report various kinds of apparent movement, viz. beta or phi movement, associated with either of the stimuli. De- pending upon the conditions of stimulation, these perceptual changes have been called variously the Crawford effect, the Broca-Sulzer effect, type A or type B metacontrast, and facilitation. (For recent reviews see Raab, 1963; Kahneman, 1968; and Lefton, 1973.) Because these perceptual phenomena have generally been measured and defined in terms of changes in absolute or difference thresholds, they have come to be regarded as masking effects–“masking” being broadly defined as the process by which one stimulus raises the threshold for the perception of another stimulus.

The purpose of this thesis is three-fold: (1) to show that the measurement of backward masking phenomena exclusively {2} in terms of detection threshold or response magnitude variations has led to an inadequate and distorted view of their nature and cause, and that the description of these phenomena as “masking” effects is often both misleading and unwarranted; (2) to show how far the principle of informational integration during neural processing can carry us toward an understanding of the seemingly “retroactive” and “masking” nature of those visual phenomena traditionally referred to as the Crawford effect, the Broca-Sulzer effect, and metacontrast; and (3) to show that an adequate understanding of the nature and cause of these visual phenomena requires that the changes in the Ss perceptual experiences be carefully taken into consideration when analyzing the experimental data.

The Neural Processing Period

A considerable amount of evidence has accumulated in recent years strongly suggesting the existence of a neural processing period during which sensory information is integrated to produce a perception of the stimulus. Moreover, there is good reason to believe that this processing period (for brief stimuli, at least) has a duration of approximately 60-70 msec. (For a discussion of the evidence supporting this conclusion, see Efron, 1967, pp. 722-726.)

Hogben and Lollo (1974) presented their Ss with a 5 x 5 array of 50 microsecond dots displayed sequentially and in random order. They found that for cycle times of 80 msec or less, the dots were perceived as occurring simultaneously, and a missing dot could be easily detected. At cycle times {3} greater than 80 msec, successiveness was perceived, and the location of the missing dot was increasingly more difficult to identify.

Efron (1967) has observed that a pulse of light having a duration of less than 60 to 70 msec is perceived as durationless, (i.e., undifferentiable from instantaneous). Furthermore, Lindsley (1958, discussed in Dember, 1960, p. 134) demonstrated that under the same stimulating conditions, two identical pulses of light must be separated by a dark interval of about 73 msec if they are to be perceived as successive. At ISIs less than 73 msec the two pulses are perceived as a single continuous pulse.

Efron (1967) has also demonstrated that if a 20 msec red light is followed immediately upon its offset by a 20 msec green light (delivered to the same retinal area), the S re- ports the perception of a single pulse of yellow light. But if the S is presented a 30 msec red flash followed by a 30 msec green flash followed again by a 30 msec red flash (again delivered to the same retinal area), he reports seeing a yellow pulse followed by a red pulse. If a red-green-red-green sequence of four 30 msec flashes is presented, the S again reports seeing a single yellow pulse.

Efron explains these observations in terms of a neural processing period functioning in such a manner that any sensory information from the same or immediately adjacent receptor areas reaching the central nervous system during the same processing period will be integrated into a single perceptual {4} experience (composed of one or more discriminable components). “The existence of such a processing period,” Efron suggests, “can also account for examples of ‘retroactive’perceptual effects of a second stimulus.”

 Latest revision: April 30, 2014 at 1:50 pm

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