The Broca-Sulzer Phenomenon and the “Growth” of Sensation

If the intensity of a brief test stimulus of variable duration is adjusted by the S until the TS appears equal in brightness to a surrounding or adjacent comparison stimulus (CS) of constant intensity and much longer duration, the TS intensity settings for equal apparent brightness gradually decrease up to TS durations of 50 msec, and then increase gradually. The same results may be produced by holding TS constant and having the S adjust CS intensity, in which case the CS intensity settings will gradually increase up to TS durations of 50 msec after which they will gradually decrease (see discussion in Stainton, 1928).

From such experiments it has been generally concluded that the perceived brightness of brief flashes of light (under 100 msec) gradually increases, reaching a peak at abou1 50 msec, and then gradually diminishes to some equilibrium level. This interpretation is often thought to receive strong support from microelectrode studies measuring rates of spike discharge from retinal elements (Adrian, 1928). Such studies have demonstrated that the impulse frequency measured as a function of the duration of a brief luminous stimulus shows an initial rapid increase, reaches a maximum, and then gradually decreases to a new equilibrium level. By superimposing both sets of data on the same graph, Adrian (1928) argued for a direct comparison between studies of pooled discharge frequencies of elements in the optic nerve of the eel and the {23} Broca-Sulzer data. This comparison is still regarded as plausible by many psychologists (see Grossman, 1967, p. 200) who believe that changes in receptor firing rate, which under normal conditions reflect changes in stimulus intensity, produce corresponding changes in sensational strength. As Adrian expressed it, a discontinuous neural effect is converted into a corresponding continuous conscious effect.

Adrian maintained that his comparison between the Broca-Sulzer data on man and the neurophysiological data on the eel was “justified” in spite of the fact that the Broca-Su1zer data were obtained by employing double stimuli (TS and CS) whereas Adrian’s data were obtained by employing only single stimuli. Adrian’s awareness of the interpretive import of this methodological difference is reflected in his own statements in his 1928 monograph:

Comparing the impulse discharge in an eel’s optic nerve and the brightness of a visual image in man may be like the comparison of chalk with cheese. . . . (p. 117)


The effects of two stimuli presented simultaneously or in rapid succession may be quite different from the sum of effects of each stimulus presented singly. (p. 117) (Adrian, 1928)

Metacontrast phenomena and the Crawford effect indicate that this is indeed the case. Yet, in spite of this, most students of the Broca-Sulzer phenomenon using TS-CS stimulus pairs have assumed that the apparent brightness of the TS was primarily a function of its duration. If, however, the TS {24} and CS are arranged so as to always terminate simultaneously, as they were in the studies by Katz (1959, discussed in Boynton, 1961), then any change in the TS duration must also pro- duce a corresponding change in the CS-TS onset asynchrony. It then becomes impossible to determine whether one’s results should be attributed to changes in TS duration or to changes in onset asynchrony!*

*NOTE: The same criticism may be leveled against the experiments of Kolers (1962) who, in most cases, used as an index of masking the TS threshold duration measured as a function of the ISI duration. Clearly, changes in either of these variables must produce concomitant changes in SOA. This fact is particularly relevant in view of Kahneman’s (1967) observation (consistent with the thesis of this paper) that for brief stimuli it is the SOA and not the ISI that is the important variable affecting metacontrast.

Fortunately, the experiments of Raab and Osman (1962) test this problem directly. Their findings throw considerable doubt on the traditional interpretation of the Broca-Sulzer effect. Raab and Osman found that the position of the CS-TS temporal overlap cannot be ignored, for conditions of coincident onset and conditions of cotermination produce opposite effects!

If a 200 msec flash and one of shorter duration are terminated together and then matched for equal brightness, this equality is destroyed when the stimuli are presented with coincident onsets. The brief flash will now appear fainter and will require a doubling or more of its luminance to appear as bright as the standard. (Raab and Osman, 1962, p. 1177)

These findings are not surprising since they were observed under conditions similar to those known to produce type A metacontrast and phi movement (Fehrer and Smith, 1962). In {25} fact, Raab and Osman report that the perception produced under conditions of coincident TS-CS onset “is overlaid with the illusion of motion.”

The conclusion is inescapable that the temporal relationships between a TS and a spatially contiguous CS strongly affect the apparent brightness of the TS, and that the Broca-Sulzer effect, as it is generally produced using asynchronous stimulation of neighboring retinal regions, does not accurately represent the relation between stimulus duration and perceived brightness. This conclusion was also reached by Raab, Fehrer and Hershenson (1961) who carried out a more direct measure of the apparent brightness of brief stimuli as a function of their duration. These experimenters were primarily concerned with measuring visual reaction time to single stimuli within an intensity range generally assumed to produce the Broca-Sulzer effect. But they were unable to produce the Broca-Sulzer effect by using single stimuli:

Early in our experiments it became apparent that our method of stimulus presentation yielded a brightness-duration relation very different from that of Broca and Sulzer. According to the latter, the 50 msec flash should have appeared brighter than the longer flashes, especially when the stimulus luminance was high. Instead we found that apparent brightness increased with stimulus duration up to the 500 msec flash for all three luminance levels.

As the intensity of the stimulus was at a 1 vel that should yield a pronounced Broca-Sulzer effect, it is obvious that this application of the method of single stimuli yielded results al- together at variance with the matching procedures that have been employed by previous investigators of this effect. (Raab, et al., 1961, p. 209) {26}

The findings of Raab et al. (1961) and of Raab and Osman (1962) permit us to conclude that any direct comparison of perceptual effects produced by individual stimuli with the Broca-Sulzer effects produced by stimulus pairs cannot be justified. Furthermore, it is apparent that the neural effects produced by individual pulses of light do not correlate with apparent brightness effects produced under similar conditions of stimulation. For instance, a comparison between the data obtained by Adrian (1928) and that of Raab et al. (1961) would tend to support the conclusion that perceived intensity may increase while receptor firing rate actually decreases!

The belief that sensation “grows” as a function of the stimulus duration, i.e., that the perceived strength of a stimulus passes sequentially through each of the perceivable strength values produced by stimuli of shorter duration, and the belief that the shape of this sensational growth function is similar to that of the Broca-Sulzer function for brief stimuli, have been widely accepted as valid inferences from the physiological and behavioral data. These beliefs continue to serve as general premises in physiological psychology, as may be judged from the following statement in one of the leading textbooks in this field:

The initial or initiatory phase of the visual sensation is characterized by a perceptible increase in the subjective intensity of the sensation to some maximum value followed by a fall to asymptotic levels. This overshoot phenomenon is analogous to the Broca-Sulzer effect . . . for brief stimulus durations. (Grossman, 1967, p. 201) {27}

However, contrary to the general confidence in the soundness of these inferences, neither the experiments of Raab et al. (1961) nor any studies of the Broca-Sulzer phenomenon allow us to conclude that sensation “grows” to a maximum as the duration of the stimulus increases. This criticism was first advanced by Stainton (1928) and most recently by Wilson (1965, cited in Efron, 1967) and by Efron (1967) who discusses this issue at length. The latter author holds that the only possible evidence admissible in answer to the question of sensational growth is that relying on introspective reports of subjects describing exactly what they do in fact experience. But all such evidence directly contradicts the notion of sensational growth.

In fact, following a brief exposure to a stimulus, we have no conscious experience of any stages of lesser specificity, but experience only the most specific identification that the stimulus duration allows.

The awareness of only the most specific identification may be introspectively confirmed by focusing one’s attention on an object in the room. We do not when first observing an object with central vision fleetingly experience the object as it would appear with the most peripheral vision, then as it would appear with less peripheral vision. Nor do we actually experience the “increase” of brightness which is assumed to occur . . . . Nor do we experience, when exposed to a flash of red light, a light that is initially pink but increases in saturation. Nor do we experience, when first touching an object with our finger tips, a fleeting perception of the object as it would feel on our forearm. Similarly, when we shift our attention from one object of awareness to another, there is no experience of “growing” specificity of the new object of awareness–we just perceive the new object. (Efron, 1967, p. 721) {28}

Furthermore, as Efron points out, the conclusion that a sensation is not experienced by an observer as passing through lesser “stages” is identical to the conclusion that,

. . . he has no awareness of the object at all until some moment in time when he experiences it with the maximum specificity* which that exposure time has permitted. (Efron, 1967, p. 722)

It is this premise, first advanced by Efron, which we appealed to earlier in our explanation of the results of experiments measuring reaction time to stimuli masked by metacontrast.

If the Broca-Su1zer phenomenon does not reflect the growth of subjective brightness, what are we to make of the data of Broca-Sulzer?

The data from other studies using multiple brief visual stimuli suggest that (1) there exists a neural processing period requiring about 60-70 msec, (2) a stimulus that is maintained unchanged will have its energy integrated over its processing period, and (3) the apparent brightness of a stimulus is a function of its duration within the limits of its processing time.

Other studies, furthermore, suggest that processing time decreases as stimulus intensity increases, and that when two adjacent brief stimuli are presented differing in intensity, the more intense stimulus will be processed earlier and will produce an illusion of asynchrony and apparent movement.

*NOTE: Efron’s concept of “specificity” may be defined as that attribute of a S’s experience which changes when the same objective event is viewed under different experimental conditions, and which accounts for a S’s altered ability to discriminate that event from other similar events. {29}

In the Broca-Sulzer phenomenon, all of the above factors are operating together. For approximately the first 100 msec of TS durations, with every increasing intensity setting there is the confounding effect of increasing perceptual asynchrony.

The Broca-Sulzer phenomenon, we are now justified in saying, does not represent the relation between stimulus duration and perceived brightness. Nor does it represent the “growth” of sensation as a function of stimulus duration, first, because the concept of sensational growth is directly contradicted by introspective evidence, and second, because the Broca-Sulzer phenomenon is an artifact of stimulus asynchrony and stimulus energy integration. It represents one of the many perceptual effects that can be produced by presenting two or more stimuli to neighboring retinal areas under conditions of interstimulus delay that permit the processing periods of these stimuli to overlap, resulting in a spatiotemporal integration of sensory information.

 Latest revision: April 8, 2014 5:49 pm

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