The passage of visual sensation from âinstantaneousâ to âdurableâ occurs at Bash durations of .... When S would be unable to perceive differences within.
A PSYCHOPHYSICAL STUDY OF THE VISUAL PERCEPTION OF “INSTANTANEOUS” AND -*DURABLE” JACQLES SERVILE. DOM MICELIand YES GALIFRET Sensorielle I, 4. Place Jussieu, 75005 Paris. France
Laboratoire de Psychophysiologie
(Receiced 9 Janunr!: 1976; in reckedform 19 .\fa.~ 1976) Abstract-The perception of light stimulus duration as related to “instantaneous-durable” (I-D) threshold was analysed in experienced (class A) and experimentally naive (class B) subjects using a category judgement and a four response forced-choice method. As subjective luminance is directly dependent on duration (Bloch’s law) for the brief stimulus presentations used in the experiments. it was necessary to eliminate the brightness cue in the I-D threshold measurements (luminance compensation curves). The passage of visual sensation from “instantaneous” to “durable” occurs at Bash durations of 60-65 msec (class A) and 100-125 msec (class B). Certainty of perception of -durable” occurs at durations longer than 140 msec (class A) and 190 msec (class B).
METHOD
A very brief light stimulus such as a photographic flash produces a sensation of “instantaneous” whereas anv light stimulus whose duration exceeds a certain c&Cal value elicits a sensation of “durable” or “temporal thickness”. For PiPron (1913) the sensation of duration is a process comprising a beginning and an end, separated by a perceived interval. All stimuli, shorter than a certain critical value are indistinguishable in regards to duration, whatever their real physical length. Previous experimental studies concerned with the perception of duration have not directly investigated the transition from instantaneous to durable. Durup and Fessard (1929), studying the effect of duration on the perception of intensity and visual acuity, determined a duration value of 140 msec above which the durable nature of a light stimulus was perceived with certitude. More recently, Haber and Standing (1970), Allan, Kristofferson and Wiens (1971) and in particular Efron (1970a,b,c, 1973a,b) have attempted to relate perceived duration to physical stimulus duration in different sensory modalities (visual, auditory and tactile). The purpose of the present investigatiori was to analysk by means of different psychophysical methods, the variations in sensation of “instantaneous” or “durable” as a function of real physical duration. The work was divided into two parts: the first involved subjective equalization of intensity; the second concerned the determination of the various stages in the passage from “instantaneous” to “durable”. The former study was necessary as the percep tion of duration is dependent on stimulus intensity, and thus, the intensity factor had to be eliminated. Consequently all light stimuli presented in “instantaneous-durable” threshold determinations were of subjectively constant intensity, whatever their duration. Correlations between psychophysical results and recorded visual evoked potentials were later attempted.
(i) Srimulution Binocular presentation in a Maxwellian view optical system employing two identical stimulus channels (illustrated in Fig. II was used for subjmive luminance equalizations, and monocular presentation for *‘instantaneous-durable” (I-D) discriminations. As the diameter of the light source image in the plane of the pupil was less than O.jmm, an artificial pupil was not used. In order to ensure that the image always fell on the same retinal locus, two combined methods were used: (a) the head was immobilized by means of an adjustable dental bite, forehead and chin rest, and (b) stabilization of the optic axis of the eye was achieved by having the subject fixate binocularly a red luminous point, or monocularly a cross-hair reticle. The light sources were glow modulator tubes (Sylvania, Type R 1131 C) which accurately follow the rectangular electric pulses delivered to these lamps. Possible colour changes resulting from variations in duration of the pulses were restricted by insertion of a yellow filter (Kodak, Wratten 15) into the light beam. Flash durations were varied from 2 to 8OOmsec in steps of 0.2log units: 2. 3.2, 5 8 12.5 20 32. 50, 80. 125. 200, 320, 500 and 8OOmsec, cbr;espoAdiig to a logarithmic scale of 0.3, 0.5,. .2.9.The angular diameter of the stimulus field was either 1’30’ or S’, and the luminance was adjusted by interposing graded neutral density filters in various combinations alIowing step variations of 0.1 log units. The maximum luminance of the stimulus field was 16OOcd/m’ (log 1: 3.2). and general room lighting was in the order of 501x. (ii) Suh&ts Students were used as subjects in the psychophysical experiments and were divided into two classes. The five Ss of class A had previous esperience with perceptual discrimination tests, but they had no knowledge of the exact purpose of our experiments. The two Ss of class B were completely naive in regards to the matter under study and had never previously participated in such tests. (iii) Psychophysical measurements (a) Subjectiue hminunce equalixtion viewed binocularly. either to the right or the left of the fixation point, a hemiMd of constant stimulus duration and luminance (standard field), and on the opposite S
’ Equipe de Rccherche Associ& au CNRS (No. 333). 57
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Fig. 1. Schematic representation of the two channel Maxwellian view optical system used in determining luminance equalization curves. S: light source (glow modulator): L,. L?: optic lens: f,. f2. I,: neutral density or colour filters: D: diaphragm: P: pupil of the eye; Rg. Rd: left and right retina.
side a hemifield whose duration and luminance was varied (test field). Together the two hemifields. as seen individually by each eye, formed a circular disc [apparent dia 1’30’ or 5’) centered on the fixation point. The hemifields were displayed either simultaneously. or successively separated by an interval of 400 msec. Imm~iate~y f5llowing each presentation, S was required to judge the intensity of the right field in relation to that of the left held, and reply either “more”. “less” or “equal”. The different durations tested (2-800 msec) were presented in randomized order. and for each duration of the test field, the luminance which was subjectively equal to that of the durable standard field of reference was measured. The equalizations were performed for a luminance of the latter of 160 cd,‘m’. (b) Instanraneous-durnhle rhresl~olil judgement method. This method (Cox, 1967; Torgenson, 1960; Olson and Ogiivie, 1972) is an application of the conventional method of const~t-stimu~, in that the I-D threshold is set at SOS&of both possible responses (~nst~t~eous or durable), and the lower and upper limits of the range of subjective uncertainty is defined as 2jP; and 7ja0 responses durable. Certainty of perception of durable is taken as 10+X, durable responses, in spite of the difficulty of determining a significantly accurate value in the region of the asymptote. S viewed monocularly the appearance of a circular field of variable duration and constant subjective luminance, centered on the fixation point. At the start of each experimental session S was given the following instructions: “light flashes of different durations will be presented. Some of these will appear to possess a certain temporal thickness or certain duration. others will Iack temporal thickness or duration. When you feel you are ready you yourseif will trigger the stimulus. Only one presentation is allowed, and following each pr~entation you must say whether the flash appears: instantaneous, durable, uncertain, somewhat instantaneous, or somewhat durable. In order to facilitate classification of the flashes. we are now going to present you two reference flashes easily distinguishable one from the other” (presentation of 2 and 800 msec gash). Preliminary tests indicated that Ss were relatively more at ease when additional response categories “somewhat instantaneous” and “somewhat durable” were also made available. The different durations tested were each presented 10 times in randomized order. To search for possCaregory
ible interaction effects of cues such as geometric (surface and contour) and chromatic characteristics of the field. this procedure w-as performed employing field diameters of 1’30’ and 5’ and .*white” and “yellow” light. Four response forced-choice merhod. In this experiment S was presented monocularly with a sequence of four-flash stimuli (field dia: 5’) spaced 4OOmsec apart and centered on the same fixation point. Each sequence comprised three stimuli of the same duration-comp~ison stimuli (IO, 20, 30, 50 or 60msec) and one test stimulus whose duration was varied (lO-180msec) (Fin. 2). The oosition of the test Bash was altered randomly Within each’ sequence, end for each duration of the test flash a total of 20 or 100 sequences were presented to Ss of class A and B respectively-. Regardless of test stimulus duration, its luminance was equal to that of the comparison stimuli (Section iii-a). S’s task was to indicate the position of the test flash within the sequence, his instructions being: “for each sequence of four flashes, you must indicate the position (1, 2, 3 or 4) of the flash which appears different”. Ss undergoing this test had not as yet been used in the former I-D dis~iminat~ons, and consequently had no knowledge of the variable parameter. This experiment was designed to permit testing of the different durations without mvolving reflective processes on the part of S. By taking into account initial data derived from a number of other Ss, the comparison stimuli were selected such as to fall within the range of instantaneous durations. Consequently. correct discriminations were interpreted as indicating that the duration of the test flash was located within the range of durable stimuli, and the results obtained with this indirect procedure would be expected to overlap with results provided in the preceding test.
r--lr-f!
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~ 2
3
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1.2.4 : comparison stmwh 3: test stimulus.
Fig. 2. Schematic representation of a sample sequence used in the four response forced-choice method. 1. 2, 4: comparison stimuli; 3: test stimulus equal in subjective luminance to comparison stimuli.
Visual perception of “instantaneous“
59
and “durable” RESULTS
(i) Subjecrire luminance equakarion Equalization curves were in general constructed in the same manner and a representative family of curves for S D (class A) are illustrated in Fig. 3. The points plotted represent the required luminance variation of the test field as a function of duration in order to obtain a subjective luminance which is constant and equal to that of the standard field. Over a range of brief durations whose upper limit lies in the vicinity of 70msec, it is possible to compensate an increase in duration by a respective decrease in intensity (1.t” = constant, where n = 1). Beyond this duration range lies a transition zone where compensation is no longer proportional (0 < n < l), and for
stimulus durations longer than 50 msec. subjective luminance is independant of duration (n = 0). Essentially similar results were obtained regardless of mode of presentation-successive or simultaneous hemifield display-and regardless of apparent field dia of 5’
Fig. 3. Luminance equalization curves showing evolution of test field luminance (log ,5.)as a function of its duration (r), in order to produce a subjective brightness equal to that of the standard field (160 cd.m’ corresponding to log L = 2.2). Experimental data for 3 Ss (D: +: J: 0: M: 0) of class A using simultaneous hemifield display are shown.
or 1’30’. (ii) Instantaneous-durable rhreshold (a) Category judgment method. The five response categories were designated with the following coefiicients: instantaneous: 0, somewhat instantaneous: 0.25, uncertain: 0.5, somewhat durable: 0.75, durable: 1. Each value listed in Table 1 represents the mean of 10 judgements for a given flash duration. The response distribution for a S of class A (field dia 1’30 and 5’) and for class B Ss (l”30’) are shown in Figs. 4(a) and (b) respectively. The graphs may be considered as representing the evolution of perception of temporal thickness as a function of stimulus duration. The passage of sensation from “instantaneous” to “durable” is not abrupt, and between these extremes lies a range of subjective uncertainty,
When S would be unable to perceive differences within the range of instantaneous durations, he would be expected to make a random response choice. With four available responses, the probability P of a random correct response is 0.25 and depending on the number of sequence presentations (N = 20 or iL‘= lOO), the probability of obtaining x random correct responses could be calculated (0.25 + 0.75)‘. Setting sampling error at 0.01; with i\’ = 20, a minimum of 10 correct responses would indicate (validity coefficient 0.99) that S was able to distinguish the test flash from comparison flashes; similarly with N = 100, a minimum of 35 correct responses would be required.
Table 1. Category judgement method: subjective estimate of stimulus duration. Ss categorize each flash displayed into one of five available categories: “durable”, “somewhat durable”, “uncertain”, “somewhat instantaneous”, “instantaneous” attributed respectively with coefficients 1, 0.75. 0.5, 0.25, 0. The values listed are the mean of coefficients for 10 responses. I-D threshold is set at 50% responses durable, the range of uncertainty between 25 and 75% responses durable
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certainty of sensation was attained onl! for stimulus durations of appros 190 mssc for class B. (b) Four respotw ,Cwceii-rizoice me~rimti. Table 2 shows the percentage of correct discriminations as a function of test flash duration for different comparison flashes. Estimatss of I-D threshold values were obtained by interpolation and appear in the righthand column. The same general pattern of response was seen for all subjects within each class. A sample graphic representation of the data is shown in Figs. 5(a) and (b). It can be noted that independently of the durations of comparison stimuli (10, 20 or 30 msec). the percentage of correct responses randomly oscillates around a chance level of performance (2%;) for test stimulus durations shorter than 60 msec for class A. 1~l~Omsec and IlO-1iOmsec for the two Ss of class B. With progressivei]; longer test stimulus durations. the percentage of correct responses evolves more uniformly and exceeds discrimination threshold levels (50“” for class A. 35’,~ for class B-see Methods). The data thus indicate the threshold of appearence of the sensation of temporal thickness to be in the vicinity of 60 mscc for Ss experienced in perceptuai discrimination testing. For esperimentally naive Ss. this sensation appears with stimulus durations of about 1IO msec.
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(b) Fig. 4. Category judgement method. Evolution of the perception of “durable” as a function of stimulus presentation time. Data points are the mean of 10 judgements, each attributed with coefficients ranging from 0: “instantaneous” to 1: “durable” (ordinate). Stimulus durations (abscissa) are plotted on a logarithmic scale. (a) results obtained with subject M (class A) tested with tieid dia of 1’30’ (0) and 5’ (B). (b) results obtained with subjects A and C [class B) tested with field dia 1’30’ (0).
the lower and upper limit of which is measured respectively at 25 and 75% responses “durable”. I-D threshold is calculated by interpolation and is set at SOY;responses “durable”. The data shown in Table 1 were derived from a relatively small S sampling (7 Ss). Although marked inter-subject differences in I-D threshold values were observed within each class, the data nevertheless provided a certain amount of information regarding the perception of duration. Field diameter does not appear to affect I-D threshold. Comparing threshold values for class A Ss tested with field dia 5” and 1’30’, in the case of subject D a higher t~eshoid value was obtained with the smaller field, whereas for subject M the reverse effect was observed. It is however worth noting that for these same Ss, there is wider spread of the range of uncertainty in those series where the smaller field was tested. Striking inter-class differences in I-D threshold values were obtained, with relatively higher values being found for class B Ss (113-l 18 msec). Certainty of temporal thickness perception (loon/;, responses durable) was generally attained at 140msec for class A, whereas this same
Figure 6 shows resuits obtained in the present study b. and for comparison purposes, also includes experimental curves derived from data provided by Blonde1 and Rey (19 11)--d. Durup and Fessard (1929-c. Aiba and Stevens (196-t)---a,-al. Regrouping of the various curves was accomplished by also bearing in mind the different luminances of durable fields reported by these authors. Blonde1 and Rey performed measurements near absolute threshold, i.e. in the vicinity of 10-6cd/m’, and the luminance used by Durup and Fessard was approx 24 cd/m’. The two curves extracted from data reported by Aiba’ and Stevens relate to luminance levels of 64 and 640 cd,‘ma. The trends of the various data plotted are simitar. For the longer duration vatues. where luminance is independent of duration. the slopes are approx 0. With decreasing duration, the slopes of these curves increase tending towards 1 and approach 1 in the region of briefest durations (Bloch’s law). AS previously observed by Aiba and Stevens (1964). the curves exhibit a progressive shift towards shorter duration values as the luminance level increases. Our experimental points (for 3 Ss, luminance level 160 cd mZ) are seen to fall approximately parallel and between the experimental curves obtamed by Aiba and Stevens (at 64 and 64Ocd m’). (ii) fnsrontaneaus-riurrbi~
rirrrsfiolcf meastlremenrs
The overall results indicate that the I-D threshold (6@-6j msec: class A; 100-125 msec: class B) does not vary si_mificantly depending on the testing procedure used. In the category judgement method. S is expected to provide his own criteria for instantaneous or durable, a task which entails the performance of an abso-
Visual perception of “instantaneous”
61
and “durable”
Table 2. Four response forced-choice method. Percentage of correct positioning of test flash within a sequence of four tlashes as a function of its duration, for variouscomparison flash=. Percentages are the mean of correct positioning responses provided by Ss. Depending on the number of sequence presentations, 20 (class A) or 100 (class BL discrimination threshold is set at 50 and 357; respectively. In certain series, intermediate duration values (55 and or 65 msecl were tested in order to obtain a more accurate threshold evaluation Duration
DURATIO?IOF
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20
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22
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70
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75
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37
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20
40
60
80
100
120
140
160
180
95
100
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33
27
27
76
80
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Fig. 5. Graphic representation of the percentage of correct positioning responses obtained with the four response forced-choice method. (Table 2). (a) Subject Ja (class A) using comparison flashes of 20msec (0) and 30 msec (W); (b) subject A (class B) using 30-msec comparison flashes.
Fig. 6. Comparison of curves representing the evolution of physical luminance (log L) of a stimulus as a function of its log duration, in order to produce a constant subjective brightness. a, and a a-Aiba and Stevens (1964). b-experimental curves previously illustrated in Fig. 3, c-Durup and Fessard (I 929). d-Blonde1 and Rey (19 11). Reference luminances (cd:‘m’) are respectively. from top to bottom, 640, 160. 64, 24 and 10e6.
61
JKQCES SER~I~RE. 00~
MICEU and YVES GXLIFRET
lute judgement. The intellectual operation involved in the forced-choice method where S has no conscious knowledge of the variable being tested is more “immediate” than that which comes into play in the former method. As a result. this test would be expected to furnish more valid estimates of I-D threshold values. Although similarly providing information related to I-D threshold. the category judgement method further permits measurements of the certainty of perception of durable stimuli (14Omsec: class A; 190msec: class Bf. In the detection experiment, s’s inability to distinguish between two stimuli whose durations are shorter than 60msec is interpreted as resulting from the fact that each stimulus by itself evokes a sensation of instantaneous. Using a method of comparison of stimulus-pairs of different durations and equal subjective luminance, we were further able to verify that SS could distinguish between 100 and 130 msec stimuli. but were unable to differentiate a 10.msec from a 40-msec gash. Interclass differences in the results co&m the important role of prior training in performing at a maximum of efliciency, even in a relatively simple perceptual task. For the purposes of attempting to relate the psychophysical results to basic physiological mecanisms of vision, the data provided by experienced Ss should be considered preferentially. The 140msec duration value obtained with class A for certainty of perception of durable stimuli is in agreement with data derived from experiments conducted by Durup and Fessard (1929) using themselves as Ss. According to these authors, who as Ss are “comparable“ to our trained class A, “clear” perception of duration intervened “as of 140 msec”. No comparable data in respect to the range of instantaneous duration was provided by these authors who did not attempt to explore the perception of instantaneous. Studies by Efron (1970a,b,c, 1973ab), Haber and Standing (1970), Alian er al. (1971), have been carried out in order to determine the lower limit of real physical durations below which all durations are perceived as identical. In our study, this constitutes the range of durations where S’s responses are random and unrelated to actual physical duration Furthermore, in view of previous findings and the data presented here, it is particularly evident that the short duration stimuli, between which discrimination is impossible. are precisely those which evoke a sensation of instantaneous. Correspondingly. accurare discrimination implies a drift into the range of “temporal thickness” (Pieron, 1923). The term “instantaneous” as it is used in the present paper may therefore be considered as the equivalent of what in recent reports has been referred to as the “critical duration”. In an experiment conducted by Efron (197Oc), SS were asked to confine a flash within two clicks, one click coinciding with the flash stimulus onset, and the other with the flash stimulus offset. By this procedure it was possible to determine a critical duration below which the inter-click interval ceased to decrease. Employing a red field ~ratten 25) of 1’40’ dia, and a luminance of approx 12cdim’ (dark background), Efron found this critical duration to be 130msec. It was further noted that lowering the stimulus luminance resulted in a widening of the minimum inter-
click interval. ConsequentIs with a luminance of 1.2 sd,m”, the minimum inter-click interval was 170 msec and for a luminance minimum of 0.22 cd, m’ the minimum inter-click intertal attained 200 msec. By extrapolating from these results. it is possible to arrive at an estimate of the critical duration. based on the 160cd;m’ luminance level used in our experiments. This predicted critical duration is in the order of 95-IOOmsec. However a basic difference must be taken into consideration before attempting any further comparison of these findings. Our measurements, as well as those of Durup and Fessard, were conducted under conditions of constant prrc&xd luminance, whereas Efron operated at constant ph_vsicai luminance irrespective of duration. According to data provided by Durup and Fessard. for a luminance ieve of 14cd:m’, perceived luminance ceases to be independant of duration for values of the latter below ljOmsec. Therefore in order to maintain a constant subjective luminance level. Efron working at llcd:m’, would have had to compensate for a decrease in duration by a certain increase in intensity. Such a procedure would have resulted in a still further reduction of the minimum inter-click interval For a constant perceived luminance of a durable field of 160cdim’, this interval would have been less than 95-100 msec, an estimate which corresponds well with salues obtained with Efron’s moderately trained Ss (1970~). These values are also compatible with the threshold values obtained in the present study i6C-65 msec for trained Ss and 100-125 msec for untrained Ss).
REFERENCES
Aiba T. S. and Stevens S. S. (1964) Relation of brightness
to duration and luminance under light and dark adaptation. Vision Res. 4, 391-401. Allan L. G., Kristofferson A. 3. and Wiens E. W. (1971) Duration discrimination Ps.whophys.
of brief light flashes. Percept.
9, 327-334.
Bloch A. M. (1885) Experiences sur la vision. C.r. Sot. Biol. 37, 493-495.
Blonde1 A. and Rey J. (1911) Sur la perception des Iumieres breves a la limite de leur portee. i. Phrs. 1, 530. Cos D. R. (1957) Note on grouping. J. .+I. srutisr. ;Iss. 52* 543-547. Durup G. and Fessard A. (1929) Sur la variation de I’enerpie lumineuse et de I’acuite visuelle en fonction de Ia duree a intensiti apparenre constants. .-lnnPe psychol. 30, 72-85. Efron R. (1970a) The relationship between the duration of a stimulus and the duration of a perception. Neurops+zologia
8, 37-55.
Efron R. (1970b) The minimum duration of a perception. Seuropsychoiogia 8, S7-63. Efron R. (197Oc) Effect of stimulus duration on perceptual onset and offset latencies. Percepr. Psychophys. 8, 25 l-254.
Efron R. (197&i) The measurement of perceptual durations. Srudium Generale 23. 550-561. Efron R. (1973) Conservation of temporal information by perceptual systems. Percept. Ps~chop~~s. 14, S18-S30. Efron R. (1973) An invariant characteristic of perceptual systems in the time domain. In Atrenrion and Performance IV (Edited by Kornbium S.) Academic Press, New York.
Visual perception of “instantaneous” Haber R. N. and Standing L. G. (1969) Direct estimates of the apparent duration of a flash. Can. J. Pr,rchol. 24, 216129. Olson C. L. and Ogilvie J. C. (1972) J. marh. Ps~chol. 9, 3X-333.
and “durable”
63
Pieron H. (1923) Les probltmes psychologiques de la perception du temps. .4&e psyhol. 24, l-25. Torgerson W. S. (1960) Quantitative judgement scales. In Psychological Seding: Theory and rlpplicntion (Edited by Gulliksen and Messick). Wiley, New York.
R&urn&-On anaiyse la perception de la duree de stimulations lumineuses chez des sujets entrain& aux tlches perceptives (classe A) et chez des sujets totalement niiifs (classe B) en determinant leur seuil -‘instantan&durable” (I-D). Comme la luminance percue est directement dipendante de la durte pour les presentations breves utilisees dam l’experience (loi de Bloch) il Ctait necessaire d’tliminer le facteur intensiti en tant qu’indice de jugement (courbes de compensation). Le passage de la sensation visuelle d’instantane a celle de durable intervient pour des durees de 60-65 msec (classe A) et 100-125 msec (classe B). La certitude de la perception de durable s’etablit pour des durees egales ou sup&ieures a 14Omsec (classe A) et 190 msec (classe B). Zusammenfassun~Die Aufnahme der Dauer van Lichtstimulanzen im Zusammenhang zu der “Unmittelbar-bleibenden” (U-B) Schwelle wurde bei erfahrenen (Klasse A) und forschungsmassig ungeschulten (Klasse B) Personen, welche ein Kategorie Urteil und eine Methode wobei zwischen vier Antworten gewahlt werden ~011,beniitzen, analysiert. Wlhrend der subjektive Lichteindruch unmittelbar von der Dauer (Gesetz von Bloch) fur die kurtzen Stimulanzversuche, welche beniitzt wurden abhlngig ist, war es notwendig, die Klarheitindizen bei der U-B Schwelle Vermessungen (Lichtkompensierungskurven) auszuschliessen. Der Uebergang des Sichtbaren Gefuhls von Momentan zu Bleibend stellt sich heraus bei einer Lichtdauer von 60-65 msek (Klasse A) und 100-125msek (Klasse B). Sicherheit der Daueraufnahme kommt bei einer Dauer welche ILnge ist als 140 msek (Klasse A) und 190 msek (Klasse B) vor.
