Type A Metacontrast

In the type A metacontrast situation there exists a large difference in intensity or figure-ground contrast favoring the MS. Under these conditions maximum masking is obtained at or near zero At. TS detection increases as the S is supplied with asynchrony information, and reaches a maxi- mum when TS and MS are separated by approximately 100 msec. Under certain stimulus conditions, proactive masking is effective over a wider asynchrony range than is retroactive masking (Fehrer and Smith, 1962). The type A metacontrast situation necessarily confounds the effects of neural asynchrony and figure-ground contrast differences, both of which are produced by intensity differences between the TS and MS. (Neural asynchrony is, of course, additionally influenced by At). Neural asynchrony becomes increasingly influential in determining stimulus detectability as stimulus duration decreases, and becomes maximally influential at stimulus durations below 100 msec. A fact which is particularly significant in as much as the stimulus durations generally employed for the production of metacontrast fall considerably below 100 msec. Since no students of type A metacontrast have, as yet, attempted to experimentally separate out the effects due to neural asynchrony and those due to contrast, our analysis must necessarily remain some- what speculative.

Most students of type A metacontrast have assumed that the masking of the weaker stimulus by the stronger surrounding {17} stimulus is primarily attributable either to simultaneous contrast (a perceptual cause) (Fehrer and Smith, 1962), or to lateral inhibition (a neural cause) (Weisstein, 1968). Explanations based on these assumptions have tended to de-emphasize the possible role played by asynchrony-produced perceptual effects in the production of type A metacontrast. It should be stated at the outset that the assumption that perceptual effects produced by stimulus asynchrony contribute to type A metacontrast is not inconsistent with the fact that type A metacontrast decreases as Δt increases. For, as in the case of type B metacontrast, on may produce numerous perceptual effects which, depending upon the S’s training and response criterion, will either facilitate or hinder TS detection or recognition.

In our earlier discussion of the relationship between RT and perceptual delay we mentioned that the perception of apparent movement could be produced by presenting simultaneously and to adjacent retinal areas two stimuli which differ only in intensity. Under these conditions, apparent movement appears to be directed toward the weaker stimulus. This is because the weaker of the two stimuli is processed more slowly than the more intense stimulus and, therefore, is regarded by the nervous system as having occurred later. In view of this fact, it should not be surprising that apparent movement in the TS region has been reported in studies of both the Broca-Sulzer phenomenon (Raab and Osman, 1962) and type A metacontrast (Fehrer and Smith, 1962). {18}

As has been clearly established in our earlier discussion of type B metacontrast, the perception of apparent movement prevents the S from distinguishing between the TS and MS as two separate events, with the result being that the S re- ports the presence of TS less frequently. It is quite reasonable to assume, therefore, that phi movement also interferes with the perceptual resolution of the TS and MS and is, therefore, at least partially responsible for type A metacontrast “masking”.

This assumption seems to be the only possible explanation for the fact that type A metacontrast “masking” is, like type B, highly form-specific. Pollack (1965) found, for example, that a medium gray hexagonal disk, which was readily “masked” by a concentric white hexagonal ring, was virtually never masked by a surrounding white circular ring. On the other hand, a medium gray circular disk was readily masked by a concentric white hexagonal ring. These facts are incompatible with standard explanations of type A metacontrast entirely in terms of simultaneous contrast masking.

Since the weaker TS is processed more slowly than the MS, apparent movement of the MS flanks toward the center TS would be more exaggerated as the MS is allowed to precede the TS. To the extent that phi movement prevents Ss from reporting the presence of the TS as a separate perceptual component, type A masking would be expected to be greater when the MS precedes the TS. This assumption might account for the data reported by Fehrer and Smith (1962) who found that with large {19} TS-MS intensity differences, “The greatest masking occurs under simultaneous presentation and when the masking stimulus precedes the test flash”, and that the effective Δt range is far greater for proactive than for retroactive masking.

On the other hand, the magnitude of “masking” may depend upon whether or not the perception of phi movement is used as a criterion for the presence of the TS (Fehrer and Smith, 1962), in which case one would expect maximum masking with a highly trained observer to occur at that Δt value where all information produced by neural asynchrony is at a minimum. Our earlier analysis of RT facilitation suggests that minimal neural asynchrony is produced when Δt is adjusted so that the S’s RT to TS is equal to his RT to MS + At. Since these are also the conditions under which facilitation is maximized, it follows that when RT facilitation is at its maximum, TS detection by a trained observer in a metacontrast situation should be at a minimum.

This expectation is confirmed, in part, by the experiments of Fehrer and Biederman (1962) measuring RT to stimuli masked by metacontrast (type A). One of the two Ss tested in the main experiment showed a significant decrease in TS detection under Δt conditions where RT to TS was virtually identical to RT to MS + ∆t (viz. 176.7 msec respectively). At this Δt value of 10 msec correct detection occurred 55% of the time, compared with 63% at Δt or zero, and 68% at Δt of 20 msec. (The second S, whose detection-scores were generally poorer than those of the first, showed no significant change {20} over this Δt range.)

Although this experiment is far from definitive, the results are at least consistent with our assumption that the relative overlap of the neural processing periods of TS and MS plays a crucial role in the production of RT facilitation and TS detection under type A metacontrast.

Of course, any conclusion about the actual cause of type A metacontrast must remain somewhat speculative until some extensive data has been accumulated describing precisely what Ss experience under these stimulus conditions. Our purpose has been merely to raise some doubts about the adequacy of traditional explanations of this phenomenon and to draw general attention to certain similarities between type A and type B metacontrast which many students of these phenomena have tended to disregard or overlook.

These similarities are (1) that apparent movement, of one kind or another, has been reported under both types of metacontrast, and (2) that both types of metacontrast are highly form-specific; the significance of this fact being tha1 form specificity appears to be characteristic of apparent movement in general. Under all metacontrast conditions, when detection criteria are high, the perceptual effects relating to apparent movement will tend to reduce TS detection scores. When detection criteria are low, on the other hand, perceptual effects relating to apparent movement will tend to increase TS detection scores. Thus, while the range over which at will effect a S’s perception of the stimuli is determined {21} by the relative overlap of the TS and MS processing periods, how Δt will affect a S’s performance depends upon his response criterion, i.e., depends upon the significance he attaches to the perceptual effects produced by Δt and upon the instructions given to him by the experimenter.

It is now possible to understand the paradoxical findings of experiments measuring reaction time to stimuli “masked” by metacontrast. A S reacts to the onset of his awareness of the stimulus, which occurs some 60 to 70 msec after the objective onset of the stimulus (Efron, 1967). Any stimulation presented during that 60-70 msec period is neurally integrated and produces the experience of a single event at the end of the processing period. It is the objective onset of the TS that determines the processing period into which all or part of the information from the subsequent MS will be integrated. It is the onset of the TS that insures that something will be perceived at the end of its processing period. What that something will be, however, is determined by the physical attributes of the TS and MS as well as by their spatial and temporal relationships.

These facts constitute the basis for our earlier statement that while the percept to which a S reacts may not contain the TS as a separate and distinct event, it cannot be concluded that a S reacts to an event which is phenomenally absent; the event to which a S reacts is always an integrated event, the objective onset of which occurred some 60 to 70 msec prior to the S’s awareness of it. {22}


Latest revision:  April 8, 2014  5:34 pm

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