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The modification of critical flicker fusion frequency by an adapting stimulus of flickering light
Vision Res. Vol.5,
pp. 463470.
Pergamon Press 1965. Printed inGreat Britain.
THE MODIFICATION FUSION
FREQUENCY
OF CRITICAL BY AN ADAPTING
OF FLICKERING PAUL
FLICKER STIMULUS
LIGHT
TURNER~
Department of Pharmacology,St. Bartholomew’sHospital MedicalCollege,London E.C.1 (Received 18 February 1965)
THE CRITICAL flicker fusion frequency (c.f.f.f.) may be modified by exposure to a light flickering at varying frequencies. SIMONSON (1959) noted that exposure to a flickering light below c.f.f.f. produced a fall in c.f.f.f., but that light of intermittency above c.f.f.f. did not change the threshold. ALPERN and SUGIYAMA (1961), however, found that c.f.f.f. was elevated by first exposing the eye to a light source with an intermittency greater than the c.f.f.f. and confirmed that it was lowered by pre-exposure to a light of lower frequency. The present work has confirmed and extended the findings cited above and reports the effect of hyperventilation and of rebreathing on c.f.f.f. METHODS The procedure used was that of TURNER (1964), being essentially as follows. Subjects were seated in the centre of a room 25 x 20 ft with grey walls and ceiling, illuminated by four fluorescent discharge units, each of 80 W operated from a 50 cycle 230 V alternating current supply. Eye level was adjusted so that the eyes and a flickering light source were in the same horizontal plane but separated by 60 cm (24 in.). The source was a neon lamp of luminance 35 lumens/f@ exposing an area of 3 ems, driven by a rectangular pulse generator with a 1: 1 mark space ratio which was constructed on the electronic “brick” or module system of BELL (1961). The lamp was viewed by fovea1 fixation against the background of a brown sheet of hardboard, on which its socket was mounted. The flicker rate of this lamp was adjustable in O-5 c/s steps by the experimenter who was seated opposite the subject but separated from him by the apparatus. During the changes of flicker frequency the subject closed his eyes. On opening them he was invited, without a limit on time, to decide whether the source flickered or not. Judgement time varied from 1 to 5 sec. The c.f.f.f. was taken as the fastest rate at which the subject considered the source to be flickering as opposed to being steady. The threshold was determined either by an approach from a lower to a higher frequency (ascending threshold) or vice versa (descending threshold). To eliminate order effects, ascending and descending thresholds were determined randomly using a Latin Square arrangement. Results were considered statistically significant if their P values were equal to or less than O-05. The subjects were healthy volunteers, male and female, aged between 16 and 45 years. 1 Present address: MedicalProfessorialUnit, St. Bartholomew’s Hospital, London E.C. 1. 463
464
PAUL TURNER RESULTS
(1) Experiment No. 1 tested the effect of exposure to intermittent light of 25 and 50 c/s from the test lamp on c.f.f.f. In each of 8 subjects, the ascending and descending thresholds were determined, with and without immediate pre-exposure to the light source flickering at 25 and 50 c/s for I min. TABLE 1 Mean thresholds before exposure (c/s)
_~
A D
37.0 35.5
37.0 35.0
25
SO
35.5 34.0
37.5 35.75
~~
Adapting frequency (c/s) Mean thresholds after exposure (cls)
A D
Experiment No. 1. The effect of exposure to intermittent light of 2.5 and 50 c/s for 1 min on mean ascending (A) and descending (D) thresholds of c.f.f.f. in eight subjects. n=8 SE. of difference=0433 c/s 5% critical diff.=0.9 c/s.
The results in Table 1 show that there was no significant difference between either the mean ascending or descending thresholds before exposure. While the fall in both thresholds following 25 c/s was significant, the rise in each after 50 c/s did not quite reach the 5 per cent level. When the mean thresholds before and after 50 c/s were averaged, however. the mean rise then achieved a 5 per cent level of significance. (2) In Experiment No. 2, 10 subjects were exposed to adapting frequencies of 10,20, 40 60 and 80 c/s for 1 min and ascending and descending thresholds measured. A rest period of 2 min elapsed between the determination of each threshold. The results are shown graphically in Fig. 1. There was a significant difference between the mean ascending and descending thresholds. Both show the steepest rise between adapting frequencies of 20 and 40 c/s, followed by a lesser increase in mean ascending and little variation in mean descending threshold. 38.0
“‘“t
20
40 60 Adapting frequency, Q’S
80
FIG, 1. Experiment No. 2. Relation between frequency of adapting light source (IO-80 c/s) and mean c.f.f.f. for ascending thresholds (solid line) and descending thresholds (broken line). The 95 per cent confidence limits were all of the same value and have been illustrated only for the descending threshold at 80 c/s (n= 10).
The Modification
of Critical Flicker Fusion Frequency by an Adapting Stimulus of Flickering Light
465
In experiment No. 3, 8 subjects were exposed for 1 min to adapting frequencies of 24,36, 48 and 54 c/s and the results are shown in Fig. 2. Once again there was a significant difference between mean ascending and descending thresholds. There was a significant increase in the ascending threshold between 24 and 36, and 36 and 54 c/s, and in the descending threshold between 36 and 48 c/s. 39.0
$
u
37.0 2 6 5"
=:
35.0 I
I
I
I
24
I
I
I
36
Adapting
I
48
frequency,
c/s
FIG. 2. Experiment No. 3. As for Fig. 1, except that the range of frequencies of the adapting light source is 24-54 c/s. The 95 per cent confidence limits are illustrated only for the ascending threshold at 54 c/s (n=8).
(3) In order to determine whether the above results arose from a central or peripheral phenomenon, one eye was exposed to the adapting stimulus and then the c.f.f.f. of the contralateral eye was measured. Precautions were taken to ensure that both eyes received the same background illumination. Control values without adaptation were not measured as this would have involved a large number of determinations with its attendant possibility of producing fatigue. Ascending and descending thresholds after 25 and 50 c/s were compared in experiment No. 4. Both eyes were tested in eight subjects, and the results are shown in Table 2. TABLE
2
Adapting frequency (c/s) 25 50 Mean Thresholds
A
(c/s)
D
35.75 35.25
Experiment No. 4. The effect of exposure of one eye to intermittent eye. Both eyes were tested in 8 subjects. n= 16 S.E. of difference=0*34 c/s 5 % critical diff. =0.69 c/s.
36.5 35.75 light on the c.f.f.f. of the contralateral
The difference between the mean ascending thresholds after 25 and after 50 c/s reached a 5 per cent level of significance, while that between mean descending thresholds just failed to do so. When the mean ascending and descending thresholds are averaged, the difference between the average reaches significance.
466
PAUL TURNER HYPERVENTILATION
In Experiments Nos. 5, 6 and 7 the effect of hyperventilation on the adaptation phenomenon was investigated. Experiment No. 5. Ascending and descending thresholds were measured in 8 subjects with and without preceding maximal voluntary hyperventilation for 1 min. In 6 subjects such hyperventilation produced a sensation of light-headedness and paraesthesiae in the hands and feet. TABLE 3 Mean thresholds (c/s) A D Without hyperventilation
38.75
38.0
With hyperventilation
40.0
39.25
Experiment No. 5. Effect of voluntary hyperventilation ing (D) thresholds of c.f.f.f. in 8 subjects. n= 8 SE. of difference=0.39
for one minute on mean ascending (A) and descend-
c/s 5 y0 critical diff. =0.82 c/s.
Hyperventilation produced a highly significant elevation of both ascending and descending thresholds (Table 3). In Experiment No. 6, 8 subjects were exposed to adapting frequencies of 25 and 50 c/s for 1 min with and without voluntary hyperventilation, followed by determinations of ascending and descending thresholds. TABLE 4 Adapting frequency (c/s) 25
50
A
D
A
D
Mean
Without hyperventilation
36.25
34.5
37.5
36-5
Threshold
With hyperventilation
36.5
36.0
38.5
38.0
Experiment No. 6. Effect of voluntary hyperventilation for one minute together with intermittent 25 and 50 c/s on mean ascending (A) and descending (D) thresholds of c.f.f.f. in 8 subjects. n = 8 S.E. of difference=O-45 c/s 5 % critical difference=090 c/s.
light of
Hyperventilation (Table 4) produced a significant elevation of all thresholds except the ascending threshold after 25 c/s. It did not appear to modify the response to adaptation to 25 and 50 c/s. Experiment No. 7. Eight subjects were exposed to adapting frequencies of 10,20,40 and 80 c/s, for 1 min with and without voluntary hyperventilation, and measurements of ascending threshold were then made. The results are illustrated in Fig. 3. Hyperventilation produced a significant elevation of c.f.f.f. when the means were summed overall. A significant difference was found between both thresholds after exposure to 20 and 40 c/s and between those after 40 and 80 c/s, but the difference between 10 and 20 c/s was not significant. There was no significant difference between the slopes of the curves.
The Modification
of Critical Flicker Fusion Frequency by an Adapting Stimulus of Flickering Light
467
39.0
c 0
_ 37.0 t: s i 5
35.01
I
20
40
60
Adaptingfrequency,
80
c/5
FIG. 3. Experiment No. 7. Relation between the mean ascending threshold and the frequency of the adapting light source with (broken line) and without (solid line) voluntary hyperventilation for 1 min. The 95 per cent confidence limits were all of the same value and have been illustrated only for the threshold at 40 c/s, with hyperventilation (n= 8). REBREATHING In Experiments Nos. 8, 9 and 10, the effects on c.f.f.f. of rebreathing for 1, 2 and 3 min respectively were studied in 8 subjects. Ascending and descending thresholds were measured immediately after breathing and without previous rebreathing. The subject inhaled deeply and then breathed out into an empty 1 1bag, continuing to rebreath from this mixture. Four subjects in the second experiment and 5 in the third showed considerable increase in the rate and depth of their respiration, and one became distressed. TABLE
a
Without rebreathing With rebreathing 5 % critical diff. (c/s)
A
D
41.25 41.0
41.0 41.0 0.53
5
Mean threshold (c/s) b A D 38.5 39.0
38.25 38.0 0.24
C
A
D
37.5 37.5
37.0 37.5 0.67
Experiments Nos. 8-10. The effect of rebreathing for (a) 1 min, (b) 2 min, and (c) 3 min on mean ascending (A) and descending (D) thresholds of c.f.f.f. in 8 subjects.
The results of these experiments are shown in Table 5. In Experiments Nos. 8 and 10 there was no effect of rebreathing on either the ascending or descending thresholds. Although the difference between ascending and descending thresholds without rebreathing was 0.25 and O-5 c/s respectively, these values did not reach significance at the 5 per cent level and there was no difference between mean ascending and descending thresholds after rebreathing. In view of the consistent demonstration of a significant difference between ascending and descending thresholds in other experiments, and in Experiment 9, the failure to demonstrate such a difference probably represents a sampling effect. In Experiment 9, although there was no significant overall effect from rebreathing for 2 min, a significant rise in the ascending threshold and a fall in the descending threshold was observed. The difference between ascending and descending thresholds was significant, both with and without rebreathing.
468
PAUL TURNER
DISCUSSION These results not only confirm the findings of other workers (SIMONSON, 1959: ALPERN and SUGIYAMA,1961) that c.f.f.f. is modified by intermittent light of varying frequency, but demonstrate the relation between c.f.f.f. and the adapting frequencies between 24 and 54 c/s (Fig. 2). The effect was less marked with frequencies below 20 and above 60 c,is and the relation tends to be S-shaped. The idea that this phenomenon is centrally mediated is supported by the finding that the c.f.f.f. of one eye can be modified by exposure of the other to the adapting stimulus, which accords with the conclusion of ALPERN and SUGIYAMA (1961). Hyperventilation has been shown to elevate c.f.f.f. (RUBINSTEIN and THERMAN. 1935; ALPERN and HENDLEY, 1952). The present results confirm this. but, in addition. demonstrate that no change occurred in the adaptation phenomenon, nor in the difference between ascending and descending thresholds. Other workers (RUBINSTEIN and THERMAN, 1935; IKEDA, 1960) are inclined to the view that the effect of hyperventilation is upon the retina locally rather than centrally. If this be so, then it is not surprising that the centrally mediated adaptation is not altered by hyperventilation. In view of the effect of hyperventilation on c.f.f.f. it is not surprising that rebreathing a mixture of 7?4 CO:! and 93% 02 for 1 min has been claimed by ALPERN and HENDLEY (1952) to produce a fall in c.f.f.f. The present results, on the other hand, fail to show any consistent decrement in c.f.f.f. after rebreathing expired air. The findings in Experiment No. 9 are of doubtful significance as they were not seen in either experiments Nos. 8 or 10. It is possible that the pCO2 value did not attain a level in these experiments which would modify the c.f.f.f., but 9 subjects showed clinical symptoms of marked CO2 retention. It is of interest to view adaptation to flicker as an aspect of general perception. GUILDFORD and PARK (1931) have investigated subjective judgements of weights and found that judgement could be modified by interpolation of weights of varying value. HELSON (1948) treated their results and those of others mathematically in support of a postulate that in subjective measurements the position of a “neutral point” determines how all stimuli are judged, and thus fixes a frame of reference. If stimuli above this point are called positive, and those below negative, then when the adaptation level is high, positive responses are few or absent and negative ones predominate, while with a low level of adaptation, negative responses are few or absent and positive ones abound. When the adaptation has an intermediate value, positive, negative and neutral responses are more or less balanced. From this HELSON (1948) suggested that in weight and colour discrimination, at least, the value of the “neutral point” tends to move towards that of the adapting stimulus. It is tempting to suggest that c.f.f.f. represents a “neutral point” in another modality and that adaptation to a lower frequency induces a fall, while that to a high frequency is followed by a rise in the “neutral point”. There is evidence to suggest that such adaptation may also be achieved by intersensory effects, visual c.f.f.f. being modified by intermittent auditory stimuli (TURNER and SMART, 1964). SUMMARY (1) C.f.f.f. was modified by previous exposure to an adapting light of varying frequency. This relationship tended to be S-shaped between adapting frequencies of 10 and 80 c/s. It was unaltered by voluntary hyperventilation for 1 min, although this procedure produced an increase in c.f.f.f. at all adapting frequencies.
The Modification
of Critical Flicker Fusion Frequency by an Adapting Stimulus of Flickering Light
469
(2) As exposure of one eye to the adapting light altered the c.f.f.f. of the contralateral unexposed eye, the changes in c.f.f.f. appeared to be central and not peripheral in origin. (3) Rebreathing for periods of l-3 min failed to produce a significant change in c.f.f.f. (4) There was a significant difference between ascending and descending thresholds which was not altered by the adaptation phenomenon or by hyperventilation. (5) It is suggested that the postulate of HELSON (1948) that the “neutral point” of a sensory modality tends to move towards the value of the adapting stimulus can be applied to c.f.f.f. Acknowledgements-1 am grateful to Professor J. P. QUILLIAM for his advice and encouragement, and to Mr. P. M. G. BELL, who designed the electronic flickering source. It is a pleasure to thank Mr. J. V. SMART for his assistance in the statistical design and analysis of the experiments.
REFERENCES ALPERN, M. and HENDLEY, C. D. (1952). Visual functions as indices of physiological changes in acid-base balance of the blood. Amer. J. Optom. 29, 301-314. ALPERN, M. and SUGIYAMA, S. (1961). Photic driving of the critical flicker frequency. J. opt. Sot. Amer. 51, 1379-l 385. BELL, P. M. G. (1961). A unit form of construction of electrical apparatus. J. Physiol. 161, 6-7. GUILFORD, J. P. and PARK, D. G. (1931). The effect of interpolated weights upon comparative judgements. Amer. J. Psychol. 43, 589-599. HELSON, H. (1948). Adaptation-level as a basis for a quantitative theory of frames of reference. Psycho/. Rev. 55, 297-313. IKEDA, H. (1960). The eficts of certain abnormal conditions on critical flicker frequency. Ph.D. thesis. University of London. RUBINSTEIN, B. and THERMAN, P. 0. (1935). The influence of hyperventilation on the fusion frequency of intermittent visual stimuli. Skand. Arch. Physiol. 72, 26-34. SIMONSON,E. (1959). The fusion frequency of flicker as a criterion of C.N.S. fatigue. Amer. J. Ophthal. 47, 556-565. TURNER, P. (1964). Critical flicker fusion frequency and its modification by a conditioning stimulus of flickering light. J. Physiol. 171. 6-8. TURNER, P. and SMART, J. V. (1964). Modification of visual critical flicker fusion frequency by intermittent auditory stimuli. Nature Lond. 203, 1387.
Abstract-The critical flicker fusion frequency is elevated by adaptation to intermittent light of high frequency and depressed by intermittent light of lower frequency. This relationship is almost linear between adapting frequencies of 24 and 54 c/s. Monocular experiments demonstrate that this phenomenon is centrally mediated, and that it is unaffected by hyperventilation or rebreathing, although hyperventilation significantly elevates the fusion threshold. It is suggested that this adaptation phenomenon is another example of the tendency of a “neutral point” to move towards the adapting stimulus as has been described in other modalities.
RCumC-La fmquence critique de fusion du papillotement est elevee par adaptation a une lumitre intermittente de haute Wquence et abaissee si elle est de Wquence plus basse. Cette relation est presque lintaire entre les frtquences d’adaptation de 24 et 54 c/s. Des experiences monoculaires prouvent que ce phenomene est d’origine centrale et qu’il n’est pas affectt par les conditions respiratoires, quoique l’hyperventilation Cl&e d’une facon significative le seuil de fusion. On suggkre que ce phtnomene d’adaptation est un nouvel exemple de la tendance d’un “point neutre” a se deplacer dans la direction du stimulus d’adaptation, comme on le dtcrit dans d’autres modalites.
PAUL
470 Zusammenfassung-Die eines interrnittenden
TURNER
kritisch flackemde Strahlenvereinigung Lichtbogen von hoher Welle, und emiedrigt
ist erhoht im Vergleich
bei Anpassung eines intermitDieser Vergleich ist im Bezug fast linear zwischen
tenden Lichtbogen von tiefer Welle. anpassenden Wellen von 24 und 54 c/s. Einlugige Versuche zeigen das diese Erscheinung dagwischen vermittelt, und das sie micht beeindruckt ist bei hyperbolische Ventilation oder Wiederatmung, trotzdem hyperbolische Ventilation die Verbindungsschwelle erhoht. Es ist vorgeschlagen das diese Anpassungsfahigkeit ein weiteres Beispiel ist, nahmlich dieses, der Neigung eines “neutralen Btmkt” zu einem passenden Stimulus, wie zuvor beschrieben in andern Modalitien.
Pe3~Me-KpHTwECKaX YaCTOTa CAM5IHUR CFlCTOBbIX MeAbKaHUfi IIOBbIIIIaeTCR IIpU aAaI-ITaIIIlIl K IT~pbIBIiCTOMy CBeTj’ BbICOKOZi YaCTOTbI Ii IIOHIiIKaeTCII IIpH aAaIITaIlHM 3TEI OTHOIIEHUR HOCIIT IIO’ITII AUH&HbIfi K IIpepbIBIiCTOMJ’ CBtZTy HU3KOfi SPCTOTM. XapaKTep B HHTepBaAe MemAy aAanTIipyIOILIUMI4 =IaCTOTaMII 24-54 repn. 3KCIICpMMeHTbI, OCyI.IlcCTBAeHHbIe C IIOMOIIlbIO MOHOKyJIapHOrO 3lJCHIla, nOKa3bIBaIOT ‘IT0 3TO RBJIeHEie 06yCAOBAeHO UCHTpZLIIbHO; HO Ha Her0 He OKa3bIBaeT BAnRHIiIl rIInepBeHTIIJISIIIEiRIIJUI BAbIXaHEie KHCAOpOAa, XOTII rHIIepBCHTkIJISILIUII II IIOBbIIIlaeT 3HaYHTeJIbHO YTO 3~0~ QKHoMen ananrauwki aBnueTcx OAHUM noporri cmimirig . LIpqtnonarae-rca, II3 IlpHMepOB TCHJI6?HL@iX4 “HeiT&UJIbHOrO I’IYHKTB” K CMlXIleHHIO II0 HaIIpaBAeHnIO aAaIITHpyfOIAer0 CTHMyAa, KaK 310 OIIHCaHO AJ’IR Ap)‘I-UX MOAaJIbHOCTet.
pp. 463470.
Pergamon Press 1965. Printed inGreat Britain.
THE MODIFICATION FUSION
FREQUENCY
OF CRITICAL BY AN ADAPTING
OF FLICKERING PAUL
FLICKER STIMULUS
LIGHT
TURNER~
Department of Pharmacology,St. Bartholomew’sHospital MedicalCollege,London E.C.1 (Received 18 February 1965)
THE CRITICAL flicker fusion frequency (c.f.f.f.) may be modified by exposure to a light flickering at varying frequencies. SIMONSON (1959) noted that exposure to a flickering light below c.f.f.f. produced a fall in c.f.f.f., but that light of intermittency above c.f.f.f. did not change the threshold. ALPERN and SUGIYAMA (1961), however, found that c.f.f.f. was elevated by first exposing the eye to a light source with an intermittency greater than the c.f.f.f. and confirmed that it was lowered by pre-exposure to a light of lower frequency. The present work has confirmed and extended the findings cited above and reports the effect of hyperventilation and of rebreathing on c.f.f.f. METHODS The procedure used was that of TURNER (1964), being essentially as follows. Subjects were seated in the centre of a room 25 x 20 ft with grey walls and ceiling, illuminated by four fluorescent discharge units, each of 80 W operated from a 50 cycle 230 V alternating current supply. Eye level was adjusted so that the eyes and a flickering light source were in the same horizontal plane but separated by 60 cm (24 in.). The source was a neon lamp of luminance 35 lumens/f@ exposing an area of 3 ems, driven by a rectangular pulse generator with a 1: 1 mark space ratio which was constructed on the electronic “brick” or module system of BELL (1961). The lamp was viewed by fovea1 fixation against the background of a brown sheet of hardboard, on which its socket was mounted. The flicker rate of this lamp was adjustable in O-5 c/s steps by the experimenter who was seated opposite the subject but separated from him by the apparatus. During the changes of flicker frequency the subject closed his eyes. On opening them he was invited, without a limit on time, to decide whether the source flickered or not. Judgement time varied from 1 to 5 sec. The c.f.f.f. was taken as the fastest rate at which the subject considered the source to be flickering as opposed to being steady. The threshold was determined either by an approach from a lower to a higher frequency (ascending threshold) or vice versa (descending threshold). To eliminate order effects, ascending and descending thresholds were determined randomly using a Latin Square arrangement. Results were considered statistically significant if their P values were equal to or less than O-05. The subjects were healthy volunteers, male and female, aged between 16 and 45 years. 1 Present address: MedicalProfessorialUnit, St. Bartholomew’s Hospital, London E.C. 1. 463
464
PAUL TURNER RESULTS
(1) Experiment No. 1 tested the effect of exposure to intermittent light of 25 and 50 c/s from the test lamp on c.f.f.f. In each of 8 subjects, the ascending and descending thresholds were determined, with and without immediate pre-exposure to the light source flickering at 25 and 50 c/s for I min. TABLE 1 Mean thresholds before exposure (c/s)
_~
A D
37.0 35.5
37.0 35.0
25
SO
35.5 34.0
37.5 35.75
~~
Adapting frequency (c/s) Mean thresholds after exposure (cls)
A D
Experiment No. 1. The effect of exposure to intermittent light of 2.5 and 50 c/s for 1 min on mean ascending (A) and descending (D) thresholds of c.f.f.f. in eight subjects. n=8 SE. of difference=0433 c/s 5% critical diff.=0.9 c/s.
The results in Table 1 show that there was no significant difference between either the mean ascending or descending thresholds before exposure. While the fall in both thresholds following 25 c/s was significant, the rise in each after 50 c/s did not quite reach the 5 per cent level. When the mean thresholds before and after 50 c/s were averaged, however. the mean rise then achieved a 5 per cent level of significance. (2) In Experiment No. 2, 10 subjects were exposed to adapting frequencies of 10,20, 40 60 and 80 c/s for 1 min and ascending and descending thresholds measured. A rest period of 2 min elapsed between the determination of each threshold. The results are shown graphically in Fig. 1. There was a significant difference between the mean ascending and descending thresholds. Both show the steepest rise between adapting frequencies of 20 and 40 c/s, followed by a lesser increase in mean ascending and little variation in mean descending threshold. 38.0
“‘“t
20
40 60 Adapting frequency, Q’S
80
FIG, 1. Experiment No. 2. Relation between frequency of adapting light source (IO-80 c/s) and mean c.f.f.f. for ascending thresholds (solid line) and descending thresholds (broken line). The 95 per cent confidence limits were all of the same value and have been illustrated only for the descending threshold at 80 c/s (n= 10).
The Modification
of Critical Flicker Fusion Frequency by an Adapting Stimulus of Flickering Light
465
In experiment No. 3, 8 subjects were exposed for 1 min to adapting frequencies of 24,36, 48 and 54 c/s and the results are shown in Fig. 2. Once again there was a significant difference between mean ascending and descending thresholds. There was a significant increase in the ascending threshold between 24 and 36, and 36 and 54 c/s, and in the descending threshold between 36 and 48 c/s. 39.0
$
u
37.0 2 6 5"
=:
35.0 I
I
I
I
24
I
I
I
36
Adapting
I
48
frequency,
c/s
FIG. 2. Experiment No. 3. As for Fig. 1, except that the range of frequencies of the adapting light source is 24-54 c/s. The 95 per cent confidence limits are illustrated only for the ascending threshold at 54 c/s (n=8).
(3) In order to determine whether the above results arose from a central or peripheral phenomenon, one eye was exposed to the adapting stimulus and then the c.f.f.f. of the contralateral eye was measured. Precautions were taken to ensure that both eyes received the same background illumination. Control values without adaptation were not measured as this would have involved a large number of determinations with its attendant possibility of producing fatigue. Ascending and descending thresholds after 25 and 50 c/s were compared in experiment No. 4. Both eyes were tested in eight subjects, and the results are shown in Table 2. TABLE
2
Adapting frequency (c/s) 25 50 Mean Thresholds
A
(c/s)
D
35.75 35.25
Experiment No. 4. The effect of exposure of one eye to intermittent eye. Both eyes were tested in 8 subjects. n= 16 S.E. of difference=0*34 c/s 5 % critical diff. =0.69 c/s.
36.5 35.75 light on the c.f.f.f. of the contralateral
The difference between the mean ascending thresholds after 25 and after 50 c/s reached a 5 per cent level of significance, while that between mean descending thresholds just failed to do so. When the mean ascending and descending thresholds are averaged, the difference between the average reaches significance.
466
PAUL TURNER HYPERVENTILATION
In Experiments Nos. 5, 6 and 7 the effect of hyperventilation on the adaptation phenomenon was investigated. Experiment No. 5. Ascending and descending thresholds were measured in 8 subjects with and without preceding maximal voluntary hyperventilation for 1 min. In 6 subjects such hyperventilation produced a sensation of light-headedness and paraesthesiae in the hands and feet. TABLE 3 Mean thresholds (c/s) A D Without hyperventilation
38.75
38.0
With hyperventilation
40.0
39.25
Experiment No. 5. Effect of voluntary hyperventilation ing (D) thresholds of c.f.f.f. in 8 subjects. n= 8 SE. of difference=0.39
for one minute on mean ascending (A) and descend-
c/s 5 y0 critical diff. =0.82 c/s.
Hyperventilation produced a highly significant elevation of both ascending and descending thresholds (Table 3). In Experiment No. 6, 8 subjects were exposed to adapting frequencies of 25 and 50 c/s for 1 min with and without voluntary hyperventilation, followed by determinations of ascending and descending thresholds. TABLE 4 Adapting frequency (c/s) 25
50
A
D
A
D
Mean
Without hyperventilation
36.25
34.5
37.5
36-5
Threshold
With hyperventilation
36.5
36.0
38.5
38.0
Experiment No. 6. Effect of voluntary hyperventilation for one minute together with intermittent 25 and 50 c/s on mean ascending (A) and descending (D) thresholds of c.f.f.f. in 8 subjects. n = 8 S.E. of difference=O-45 c/s 5 % critical difference=090 c/s.
light of
Hyperventilation (Table 4) produced a significant elevation of all thresholds except the ascending threshold after 25 c/s. It did not appear to modify the response to adaptation to 25 and 50 c/s. Experiment No. 7. Eight subjects were exposed to adapting frequencies of 10,20,40 and 80 c/s, for 1 min with and without voluntary hyperventilation, and measurements of ascending threshold were then made. The results are illustrated in Fig. 3. Hyperventilation produced a significant elevation of c.f.f.f. when the means were summed overall. A significant difference was found between both thresholds after exposure to 20 and 40 c/s and between those after 40 and 80 c/s, but the difference between 10 and 20 c/s was not significant. There was no significant difference between the slopes of the curves.
The Modification
of Critical Flicker Fusion Frequency by an Adapting Stimulus of Flickering Light
467
39.0
c 0
_ 37.0 t: s i 5
35.01
I
20
40
60
Adaptingfrequency,
80
c/5
FIG. 3. Experiment No. 7. Relation between the mean ascending threshold and the frequency of the adapting light source with (broken line) and without (solid line) voluntary hyperventilation for 1 min. The 95 per cent confidence limits were all of the same value and have been illustrated only for the threshold at 40 c/s, with hyperventilation (n= 8). REBREATHING In Experiments Nos. 8, 9 and 10, the effects on c.f.f.f. of rebreathing for 1, 2 and 3 min respectively were studied in 8 subjects. Ascending and descending thresholds were measured immediately after breathing and without previous rebreathing. The subject inhaled deeply and then breathed out into an empty 1 1bag, continuing to rebreath from this mixture. Four subjects in the second experiment and 5 in the third showed considerable increase in the rate and depth of their respiration, and one became distressed. TABLE
a
Without rebreathing With rebreathing 5 % critical diff. (c/s)
A
D
41.25 41.0
41.0 41.0 0.53
5
Mean threshold (c/s) b A D 38.5 39.0
38.25 38.0 0.24
C
A
D
37.5 37.5
37.0 37.5 0.67
Experiments Nos. 8-10. The effect of rebreathing for (a) 1 min, (b) 2 min, and (c) 3 min on mean ascending (A) and descending (D) thresholds of c.f.f.f. in 8 subjects.
The results of these experiments are shown in Table 5. In Experiments Nos. 8 and 10 there was no effect of rebreathing on either the ascending or descending thresholds. Although the difference between ascending and descending thresholds without rebreathing was 0.25 and O-5 c/s respectively, these values did not reach significance at the 5 per cent level and there was no difference between mean ascending and descending thresholds after rebreathing. In view of the consistent demonstration of a significant difference between ascending and descending thresholds in other experiments, and in Experiment 9, the failure to demonstrate such a difference probably represents a sampling effect. In Experiment 9, although there was no significant overall effect from rebreathing for 2 min, a significant rise in the ascending threshold and a fall in the descending threshold was observed. The difference between ascending and descending thresholds was significant, both with and without rebreathing.
468
PAUL TURNER
DISCUSSION These results not only confirm the findings of other workers (SIMONSON, 1959: ALPERN and SUGIYAMA,1961) that c.f.f.f. is modified by intermittent light of varying frequency, but demonstrate the relation between c.f.f.f. and the adapting frequencies between 24 and 54 c/s (Fig. 2). The effect was less marked with frequencies below 20 and above 60 c,is and the relation tends to be S-shaped. The idea that this phenomenon is centrally mediated is supported by the finding that the c.f.f.f. of one eye can be modified by exposure of the other to the adapting stimulus, which accords with the conclusion of ALPERN and SUGIYAMA (1961). Hyperventilation has been shown to elevate c.f.f.f. (RUBINSTEIN and THERMAN. 1935; ALPERN and HENDLEY, 1952). The present results confirm this. but, in addition. demonstrate that no change occurred in the adaptation phenomenon, nor in the difference between ascending and descending thresholds. Other workers (RUBINSTEIN and THERMAN, 1935; IKEDA, 1960) are inclined to the view that the effect of hyperventilation is upon the retina locally rather than centrally. If this be so, then it is not surprising that the centrally mediated adaptation is not altered by hyperventilation. In view of the effect of hyperventilation on c.f.f.f. it is not surprising that rebreathing a mixture of 7?4 CO:! and 93% 02 for 1 min has been claimed by ALPERN and HENDLEY (1952) to produce a fall in c.f.f.f. The present results, on the other hand, fail to show any consistent decrement in c.f.f.f. after rebreathing expired air. The findings in Experiment No. 9 are of doubtful significance as they were not seen in either experiments Nos. 8 or 10. It is possible that the pCO2 value did not attain a level in these experiments which would modify the c.f.f.f., but 9 subjects showed clinical symptoms of marked CO2 retention. It is of interest to view adaptation to flicker as an aspect of general perception. GUILDFORD and PARK (1931) have investigated subjective judgements of weights and found that judgement could be modified by interpolation of weights of varying value. HELSON (1948) treated their results and those of others mathematically in support of a postulate that in subjective measurements the position of a “neutral point” determines how all stimuli are judged, and thus fixes a frame of reference. If stimuli above this point are called positive, and those below negative, then when the adaptation level is high, positive responses are few or absent and negative ones predominate, while with a low level of adaptation, negative responses are few or absent and positive ones abound. When the adaptation has an intermediate value, positive, negative and neutral responses are more or less balanced. From this HELSON (1948) suggested that in weight and colour discrimination, at least, the value of the “neutral point” tends to move towards that of the adapting stimulus. It is tempting to suggest that c.f.f.f. represents a “neutral point” in another modality and that adaptation to a lower frequency induces a fall, while that to a high frequency is followed by a rise in the “neutral point”. There is evidence to suggest that such adaptation may also be achieved by intersensory effects, visual c.f.f.f. being modified by intermittent auditory stimuli (TURNER and SMART, 1964). SUMMARY (1) C.f.f.f. was modified by previous exposure to an adapting light of varying frequency. This relationship tended to be S-shaped between adapting frequencies of 10 and 80 c/s. It was unaltered by voluntary hyperventilation for 1 min, although this procedure produced an increase in c.f.f.f. at all adapting frequencies.
The Modification
of Critical Flicker Fusion Frequency by an Adapting Stimulus of Flickering Light
469
(2) As exposure of one eye to the adapting light altered the c.f.f.f. of the contralateral unexposed eye, the changes in c.f.f.f. appeared to be central and not peripheral in origin. (3) Rebreathing for periods of l-3 min failed to produce a significant change in c.f.f.f. (4) There was a significant difference between ascending and descending thresholds which was not altered by the adaptation phenomenon or by hyperventilation. (5) It is suggested that the postulate of HELSON (1948) that the “neutral point” of a sensory modality tends to move towards the value of the adapting stimulus can be applied to c.f.f.f. Acknowledgements-1 am grateful to Professor J. P. QUILLIAM for his advice and encouragement, and to Mr. P. M. G. BELL, who designed the electronic flickering source. It is a pleasure to thank Mr. J. V. SMART for his assistance in the statistical design and analysis of the experiments.
REFERENCES ALPERN, M. and HENDLEY, C. D. (1952). Visual functions as indices of physiological changes in acid-base balance of the blood. Amer. J. Optom. 29, 301-314. ALPERN, M. and SUGIYAMA, S. (1961). Photic driving of the critical flicker frequency. J. opt. Sot. Amer. 51, 1379-l 385. BELL, P. M. G. (1961). A unit form of construction of electrical apparatus. J. Physiol. 161, 6-7. GUILFORD, J. P. and PARK, D. G. (1931). The effect of interpolated weights upon comparative judgements. Amer. J. Psychol. 43, 589-599. HELSON, H. (1948). Adaptation-level as a basis for a quantitative theory of frames of reference. Psycho/. Rev. 55, 297-313. IKEDA, H. (1960). The eficts of certain abnormal conditions on critical flicker frequency. Ph.D. thesis. University of London. RUBINSTEIN, B. and THERMAN, P. 0. (1935). The influence of hyperventilation on the fusion frequency of intermittent visual stimuli. Skand. Arch. Physiol. 72, 26-34. SIMONSON,E. (1959). The fusion frequency of flicker as a criterion of C.N.S. fatigue. Amer. J. Ophthal. 47, 556-565. TURNER, P. (1964). Critical flicker fusion frequency and its modification by a conditioning stimulus of flickering light. J. Physiol. 171. 6-8. TURNER, P. and SMART, J. V. (1964). Modification of visual critical flicker fusion frequency by intermittent auditory stimuli. Nature Lond. 203, 1387.
Abstract-The critical flicker fusion frequency is elevated by adaptation to intermittent light of high frequency and depressed by intermittent light of lower frequency. This relationship is almost linear between adapting frequencies of 24 and 54 c/s. Monocular experiments demonstrate that this phenomenon is centrally mediated, and that it is unaffected by hyperventilation or rebreathing, although hyperventilation significantly elevates the fusion threshold. It is suggested that this adaptation phenomenon is another example of the tendency of a “neutral point” to move towards the adapting stimulus as has been described in other modalities.
RCumC-La fmquence critique de fusion du papillotement est elevee par adaptation a une lumitre intermittente de haute Wquence et abaissee si elle est de Wquence plus basse. Cette relation est presque lintaire entre les frtquences d’adaptation de 24 et 54 c/s. Des experiences monoculaires prouvent que ce phenomene est d’origine centrale et qu’il n’est pas affectt par les conditions respiratoires, quoique l’hyperventilation Cl&e d’une facon significative le seuil de fusion. On suggkre que ce phtnomene d’adaptation est un nouvel exemple de la tendance d’un “point neutre” a se deplacer dans la direction du stimulus d’adaptation, comme on le dtcrit dans d’autres modalites.
PAUL
470 Zusammenfassung-Die eines interrnittenden
TURNER
kritisch flackemde Strahlenvereinigung Lichtbogen von hoher Welle, und emiedrigt
ist erhoht im Vergleich
bei Anpassung eines intermitDieser Vergleich ist im Bezug fast linear zwischen
tenden Lichtbogen von tiefer Welle. anpassenden Wellen von 24 und 54 c/s. Einlugige Versuche zeigen das diese Erscheinung dagwischen vermittelt, und das sie micht beeindruckt ist bei hyperbolische Ventilation oder Wiederatmung, trotzdem hyperbolische Ventilation die Verbindungsschwelle erhoht. Es ist vorgeschlagen das diese Anpassungsfahigkeit ein weiteres Beispiel ist, nahmlich dieses, der Neigung eines “neutralen Btmkt” zu einem passenden Stimulus, wie zuvor beschrieben in andern Modalitien.
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