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The simple reaction time as an aid in determining the sign of a visual transient response
Acta PsycEjologfca30 Atte&on and Performance IH (F-V’.G. Koster, ed.) 1969, W-95 0 North-HollandPublishingCompany, Amsterdam
THE SIMPLE REACTION T AgD IN DETERMINING T
AS AN SIGN
OF A VISUAL TRANSIENT RESPONS G. ‘H. MOWBRAY and JOSEPH F. BIRD The Johns Hopkins University, Applied Physics Laboratory, Silver Spring, Md. 20910, U.S.A.
ABSTRACT
A visible transient response at frequencies above fusion is described. The polarity of the response, as subjectively determined, is found to depend upon the order in which two alternating but fused light trains of different frequency are presented. The results of a transient thr :shtild test and a simple reaotion time test confirm the subjective determination. The implications of these results for models of flicker-fursion frequency mechanisms are discussed.
In celebrating the: centenary of the contributions to the scientific understanding of human reaction and decision processes by the great Dutch physiologist, I?. C. Donders, i; is as well to recognize at the: same time that, characteristic of his era, his interests were manifold. beginning his pioneering investigaticns in the field of response latencies, he had already established himself i:l the vanguard of the world’s visual scientists. It bus seems appropriate at this time to offer as small tribute a -paper in which both visual processes and reaction processes are involved. Specifically, we hope to show how properties of the simple reaction time to a visual stimulus were instrumental in the: selection of one model from among many possible models to explain in part the way t?meeye handles time-varying light stimulation. 1.
LICKER-FUSION AND DE
ANGE?-1TYPEMODELS
(1957), a fellow-countryman of F. C. Ponders and himself a visual scientist, rejuvenated an old field of psychophysiology a few years ago with his brilliant work on flicker-fusion thresholds. Recent and summaries of these developments are available and will d&us& here (BROWN, 1965; SPERLING~ 1964). It is important to note, however, that De Lange by studying flicker-fusion threshok~sas a function of the percent amplitude modulation of the flickering source E LANGE
%4
THE SIGN OF
VISUAL TRANSIENT RESPONSE
85
as able to uce attenuation characteristic curves for human eyes. ested that the fusion point of an am litude-modulated light source is directly rel to the amplitude of the first Fourier ~:~~rne flickering so rce and that over most of the frequency cker can be perceived, the eye operates as a linear, lowint, a class of models has been proposed to account for the nse to time-dependent photic stimuli EVINSOW, 1968). These all comm make use of the electrical analogue of cascaded IX integrators with or without the addition of some differentiation. The number of series rations requi in the steep high-frequency cut-off of t3e attenuation chara ived from psychophysical da um*ber most frequently encountered is 10. DGKIN (1964) found necessary to assume as the result of physiological experiments with Lirnulus. For the Iowfrequency response portion of the characteristics, i.e., between 2 and 15 Hz particularly for high brightnesses, Kelly has been able to fit ysical data by assuming two stages of differentiation along rators. Because of the uncertainty surrounding the actual for all flicker conditions, number of integrations re these models have been r atio’n models. It y>eerns riate at this t.ime, in view o 11~‘srecent analysis, to designate them as n-integration, 2-differentiation models. 2.
A VISUAL TRANSIENT RESPONSE AT FREQUENCIES ABOVE FUSION
Consider the stimulus wave form shown in fig. 1. When z represents a total duration of two seconds, T, and 1’, are one second eat is 1 msec, then if the period, t,, is in the neighborhood of 5 msec, a
t------
Time
Fig.
1.
The wave-form
of the stimulus light scmrce.
-w
G. M. MOWBRAY
86
AND JOSBPH F. BIRD
visible transient response is produced. This transient response has same tit,cr~:sting properties with critical implications for the n-integration, 2differentiation models just discussed. As the duration of fi ic increased from near threshold by a very Eittle bit it rapidly beccmes visibk on every presentation. An increase in period of from 1.2%to 1.50 mm: above the threshold per.iod will accomplish this in most cases. ‘Yet, although readily visib?e, the transient still retains much of its elusiveness. In alpprance the transient seems to be a very brief eclipse, almost as thou,gh the stimulus winked its eye. However, if the pulse train of fig. 1 is reversed so that the longer periocl is presented first followed by the shorter period, the transient takes on a different appearance. In precisely what way the transient appears to differ is difficult to describe. Spontaneous responses from naive observers always reveal that the first order produces a transient that is’sharper, cleaner and more definite even though the periods are identical in both cases. Long and careful observation by pmcticed observers reveals that for the first case the transient appears as a brightness decrement and for the second case as a brightness increment 140~ these observations impose restrictions on the De Lange-Kelly class of models with n-integrations and up to two differentiations. The most serious restriction resides in the sign ~;f the transient response. All of the models with the single exception of 3 = 2 predict a sign for the transient which is in direct contradiction to the subjective perception. Thus, it becomes important to know by more objective means whether the sign is as perceived. 3. A
THRESHOLDTEST FOR DETERMINING THE SIGN OF THE VISUAL
TRANSIENT RESPONSE mere
have lxen
many thorough studies made of increment visual thresholds but surprisingly few concerned with direct comparison of increment and decrement thresholds under similar conditions. Within the last two years, however, :;tudies by SHORT (1966) and PAT~L and JONES (1968)
using ,different techniques have yielded essentially similar
resulti;. Irmbot4 the
less
decrement
the. increment threshold was consistently
greater than
threshold with a tendency for the difference
to become
at higher background luminances. Pate1 and Jones, in particular, showed that stimulus duration and area and background ~luminance are important parameters. ‘I’hc!stimulus was viewed peripherally in tlese experiments, atid the
THE SIGN OF A VISUAL TRANSIENT
RESPONSE
87
luminance levels were largely in the scotopic range. For fovea1 viewing the picture is not so clear. LACKWELL (1946) with no fixation and relatively long viewing times found lower decrement thresholds for low tation conditions, and NERRICK (1956) found a slight difference ckground. I-Iowever, with fovea1 viewing of a lo stimulus and no 67) found no differences at photopic levels usi stimulus with no surround. Despite this somewhat contradictory state of affairs, frequency-ofseeing functions were generated to determine thresholds for the appearance of the visual transient response for the two conditions that subjectively provide incremental and decremental 3.1. A ppuruhas and ,mhxdure
The time-varying stimulus train of fig. 1 was generated by standard electronic pulse-triggering and delay units operating a Sylvania glowmodulator tube type R 1131-C. The glow-tube was mounted on an optical bench and the light output directed through appropriate lenses and diaphragm stops to furnish the subject with Maxwellian view. Arcular neutral density filters provided 8 log units of attenuation. The stimulus source was circular, subtending I.dc of visual angle and was viewed foveally in the center of a m -white circular bowl mease served to fix the subject uring 74.5 cm in diameter. A dental firmly in position. Two microswitches were attached to all arm rest: extending from the right-hand side of the apparatus. One switch allotted the subject to initiate the two second duration pulse train, the: other controlled a Hewlett-Packard interval timer and was used, as subsequently will be described, to measure subject’s reaction times. Illumination of the background field was obtained from two Macbeth daylight lamps with crossed Polaroid filters on stands behind the observer positioned to yield a uniform level within the field of view. Illumination of the ci’rcular hole in the bowl through which the stimulus was viewed was furnished by magnesium fluoride tubes housed in al diffus’ing box behind the bowl and was reflected forward by a halfsilvered mirror. The level of this illumination coukl be match& to the general background level with a rheostat. For this experiment, the background luminance was kept constant at about 0.6 mL, while the Talbot level of the alternating pulse train was 60 mL. In exploratory observations, a range of conditions was determined
G. W. MOWBRAY AND JOSEPH F. BIRD
88
ticmtwag fie:ly
to
extencl from near 0 y41seen to near 100 “/o seen for
both the case when the short period was presented first md when it was presnt& second in sequence. In all conditions the short period tween was’ held constant at 1.0 msec while the long period var425 and 6.0 msec. To obtain the frequency-of-seeing functions, a csmplel:e set of increment or decrement thres ‘ids was obtained at one sitting. For any OES con&ion of the variabl eriod, 50 observations were taken before another vatiable was run, and the order of presentation of the variables was randwn. Two series of both increment and decrement thresholds were completed and the results combined, t us making a total of 100 observatiions per experimental point. Instead of using ‘blanks’ (i.e., occasions where no sti ) to check the reliability of the subjective criterion as is customary s type of experiment, another device was adopted. Instead of having the transition between the two frequencies of alternation occur at precisely the midpoint of the 2 set period, it was possible to make it occur anytime between 0.6 see and 1.5 set after the train was initiated by the subject. At the transition point, a pulse from the stimulus generator was used to start the Hewlett-Packard time-inteyvcl meter, The subject was instructed to depress a micro-switch as soon as he observed the tradsieat thus stopping the trmer. False positive responses could thus be easily detected and some useful reaction. time data could be obtzined as Iwell. The time of the occurrence of the transition was nged randomly between each trial.
2 depicts the frequency-of-seeing functions for increment and t conditions for one observer. It will be remember senting the standard frequency first results in a visual transient response that subjectively takes on the appearance of an intensity decrement. From the figure it is obvious that the decrement function (standard first) represents a more sensitive response than the increment function. s is in keeping \ti,th some of those eriments reported that actud.Iy added light to or subtracted light from some cxisti.ng backgr agreement is provocative but by no means conclusive since r~! increment-decrement thresholds were defined by directly easurabk quantities whereas the production of a subjective transient es no such opportunities for objective mensuration.
F A VISUAL
STAMDARO ORS
P
-
1000
TRANSIENT
RESPONSE
88
cps
:
80
E R
c
60
E N T
40
s E
20
N
0
_-I
--_I_.-_.I_____I___L_I
Feriod of
Fig.
4.
2.
A
6.0
5.0
4 .o the Variable
(mlllisecondsl
Frequmxy-of-swing functions for the visual transient as a furxtion the period of the variable intermittence.
SIMPLE
REACTION
OF THE VISUAL
TIME
TRANSIENT
TEST
FOR DETERMINING
of
THE SIGN
RESPONSE
shown that the le reaction time to light increments and decrements is not the sam ep dt=crease in luminanc: providing shorter reaction times than a step incre;sse, This result was not confirmed by JROUFS ef al. (1967) using a stepped current input to a glow-modulator tub: for their stimulus. The reztction times collected while we were obtaining the frequency-of-seeing functions tended to support Steinman’s results. Therefore, we decided to run a further series using frequency-separations that produced transie ts that were visible 100 76 of the time but in which all frequencies were still above the fusion point. STEINMAN
4.1.
(1944)
Apprutus
has
und procec’ure
The equipment and luminance conditions were
same as for the previous experiment. Three subjects were set the tusk of responJQ as quickly as possible by pressing the reaction key when they perceived the transient. One of the two frequencies employed (the sta ard) was always MN Hz, while the other the variable) was selected to cover a range pror;he
G. I-I, MOWBRA’Y AND JOSEPH T. BIRD
90 ducing
a
trancient just barely noticeab!.e nearly 100 $% of t one that was quite clearly distinct. Reaction time trials were run in blocks of SO reaciion times each for each frequency condition presented in random order. A session consisM of a.!1 frequency conditions with a single standard-vari order. There was a total of four sessions in whic’h the standard-variable order was alternated yielding a total of I reaction times per experiment91 point. Subjects were given practice trials on each condition before any reaction times were recorded, and they were never informed which condition was being presented. 4.2. Results F,ig. 3 is a plot of the reaction times as a function of the period of the variable frequency for the two standard-variable orders. Only one subject’s data are illustrated since the resul.:, for al! were essentially the same. From t-h%it is evident that the order in which the two fsequencies are prer;ented i-i a determinant of reaction time. That order which produces ai transient whose sign is negative with res t to subjective brightness also p;,-educes the shortest reaction latencies. The two fir~tionc do not appear to merge as the distinctness of the transient increases but tipper to be asymptotic to different levels with an average difference of 20-25 Eltisx.
ST.~NDWRO - 1000 cps OBS : GHM
350 f? E A c T
b N
300 m =
e =
250
200 ; M E
First
150
-L-.1
4.0 Period
Fig.
3.
6.0 of
8.0
the Variable
IO.0
I2 .o
14.0
I tnsec)
Simple reaction timea to the appearanw of the visual transient as a function of the period of the variable intermittence.
OF .4WSUAL TRANSIENT REdPCMSE
5.
91
~S~US§ION
a stimulus of the for of the two frequencies in the ~~ctiveimpression of lower one creates a positive going effect. Further, plications for the 2-differentiation, n-integration
luminance as the steady source. e reports that a visual transient is produced at the onset an the modulation frequency was around 600 Hz. Levinson further reports that the sign of this transient cannot be distinguished subjectively. Now, comparing our visual transient observations with those of evinson, we may t&e as typical of consider the stimulus shown in fig. 4 w lenfal sine wavk5component the Levinson situation (assuming the fu dominates the response as it apparent1 s at high frequencies), and which is a speci = 0) for our stimulus. We perceive in this by the case a brightness increment, and this perception is support results of the foregoing experiments. We have also observed in this the transient is enhanced by extending the last dark phase, ing a negative ‘flash’. Presumably, therefore, adding a positive flash instead would nullify the perceived brightness increment rent paradox. evinson, on the other hand, states it cannot eived whether the transient response is an increment or a crement. To try to distinguish, he inserts ‘flashes’ at the transition time and finds a negative one enhances the percept and a positive one cancels it, in correspondence with our experience. But he implies that the original stimulus must be giving an apparent brightness decrement, which contradicts our results and our direct perceptions. turning to a consideration of the n-integrator models, we
G. Hi. MOWBRAY AND JOSEPH F. BIRD
92
began by fixing the polarity of all models so that a positive step input g&s a positive response, corresponding to a brightness increment. Then, e type Iof stimulus used in these experiments and pictured as a special case in fig. 4, our calculations yield a transient response whi&:
t L ip
i N
c
__J--L_rLrTlhE
--I--
--
E
Fig. 4.
A special case of the stimulus wave-form s,hown in fig. 1, where t2 = 0.
(a) for all n # 2 is negative, i.e., brightness decrement; (b) for only tt = 2 is nositive, i.e., brightness increment. Thus, only the latter%member of this class of model5 is allowed by our sign determination, and all o hers are ruled out. On the other hand, note that if Levinson’s indirectly determined sign were valid, any of the models, except n = 2, would be permitted. There is the difficulty that with ,?-ras small as 2, the filter cannot by itself give the steep flanks of the .De Lange curves for flicker fusion. However, a small n seems a prereqtisfte for obtaining from such models our detailed experimental results. 111any event, by adding to the model a neural detector which imposes thresholds both on the amplitude of excitation and on its duration, it is passibJe to save the phenomenon of De Lange flanks (and other classic,\1 flicker results) with n still small. Further, with this amplitude-dumtion detector, the apparent paradox obsrved by us on inserting a negative flash, or the related contradictory sign determination by Levinson, may be reconciled with the perceived sign of the transient. Thus, if to the stimulus is added a brief nega the added response of the model, n = 2, is a brief negative pip, und to positive response and then a slow relaxation towards zeroI But thz assumed detector ignores the negative pip and sees only ow positive response, i.e., a brightness increment, which then rices the response due to the original stimulus without flash. rly, a brief positive flash would be detected as an added decrement hich could cancel tine original response. For the models, n # 2, similar
ve enhancement, cause the original transient
n =2
Response
n = 4 Response
__;.CL.T& _/
Fig.
5.
A schematic representation the type 21 = 2 and r~ - 4. going stimulus is markecl for deEetion &u-at.ion threshold
of some calculated model responses for The n = 2 response to the brief positiveindicating that it is below the amplitudeof the neural detector.
the former case, the brief pip, ignored by the assumed detector, is marked for d.eletion. Adding (or subtracting) the latter two responses illustrates the effect of’brief flashes. We may e that the model, n = 2, with the assumed detector kile for the responds oppositely to very short and very long flashes. former the initial upward pip can be too brief to be detected, for the latter the pip dominates the response, which is then a brightness increment!, as it should be. The response may well be of ambiguous polarity for intermediate flash lengths around the detector duration threshold -- several milliseconds. This might relate to the difficulty of perceiving the sign of brief flashes as reported by Slperliwgin LEVINSON (1968).
94
G.H. MOWBRAW
AND JOSEPH F. BIRD
ACKNOWLEDGMENTS
This investigation
was supported in part by the
Weapons, Department of the Navy, under and by U.S. Public HeaIth Service Research Nat.ionaZInstitute of Neurological Diseases md Blindness.
REFERENCES R.,1946. Contrast thresholds of the human eye. 3. optic. 233. Amer. X, 624-643. .H. Graham (ed.), Visio visual perception. New y and Sons. p. 251. nuation characteristics and phase shift characterthe human fovea-cortex system in relation to the flickerfusion phenomena. Delft: Technical University. FUC~RTES, M. G. P. and A. L. HODGKIN, 1964. Changes in time scale and set& tivity in the Ommatidia of Limulus. J. Physiol. 172,239463. HEWWK, Ik.K., 1956. Fovea1 1uminance discrimination as a function of the duration of the decrement or increment in luminance. J. camp. physiol. Bsychol. 49,437--443. KELLY, D. H., l%l. Visual responses to time-dependent stimuli. II. Single channel model of the photo.pic visual system. J.optic. Sot, Amer. 51,747754. LEVNON, J. Z., 1968. Flicker fusion phenomena. Science, 1 PM-EL,A. S.and R. W. JONES,1968. Increment and decreme J. optic, Sot. Amer. 58,696+599. R~uF!Y~, J. A J., J. T. H. LAMMER~and J. A. &EMONS, 1%7. Positive and negative flashes. I.P.O. Annual Progress Report, 2, 129-133. a-r’, A. I)., 1%6. Decremental and incremental visual thresholds. 3. Physiol. 185, 646-654. SP . van der Tweel (eds.), Flicker. 9 CL I%4 In: H. E. Henkes and H. Proceedings of the Symposium on the phylsiology of flicker, The EIague: Dr. W. Junk. 3-15. AN, A R.., 1944. Reaction time to change compared with other psychophysical methods. Arch. Psychol. : BLMXWELL,
H.
e steepness of the transmission curve is never more than that of a ftitem in series and never gets at 10 RC times. is true but even five seems to be too much. ot of rmple talk in terms of these low-pass filters on the evisfence ing being me=ured below about 5 Hz. Everything one finds below
F A VISUAL T
corroboration
of
NSUH’iT RESPONSE
95
is point is that if you 1 ogle make a subjective say one half or something to ysical flicker frequen and then it stops somewhere
ut in a more simp.ified form viz. some then would have a sort of as such would ode1 is completely not 4 nfuse rattle with a tonal
and ‘tonal sensation’ may be very useful.
PeHr: Would it be possible to make inferences about the nature of the filtering from looking at the phase shift, i.e. to think of part of reaction time as the result of the phase shift introduced by whatever filtering is taking place and to look at that as a function of frequency? lp you a thing, because once Schosrtcn: This has been done. But it does not given the frequency characteristic and assumi - which is most hat it is a minimum-phase network, then you just calculate wh r&&n should be. So you get no extra evidence from it. Pelv; As long as you assume it is a minimum-phase network. Mlouter#: In ordinary ii e you encounter no filters which are not rninimumphase network. What Roufs finds in linking up the time function on the one d car-responds with the frequency function on the other.
THE SIMPLE REACTION T AgD IN DETERMINING T
AS AN SIGN
OF A VISUAL TRANSIENT RESPONS G. ‘H. MOWBRAY and JOSEPH F. BIRD The Johns Hopkins University, Applied Physics Laboratory, Silver Spring, Md. 20910, U.S.A.
ABSTRACT
A visible transient response at frequencies above fusion is described. The polarity of the response, as subjectively determined, is found to depend upon the order in which two alternating but fused light trains of different frequency are presented. The results of a transient thr :shtild test and a simple reaotion time test confirm the subjective determination. The implications of these results for models of flicker-fursion frequency mechanisms are discussed.
In celebrating the: centenary of the contributions to the scientific understanding of human reaction and decision processes by the great Dutch physiologist, I?. C. Donders, i; is as well to recognize at the: same time that, characteristic of his era, his interests were manifold. beginning his pioneering investigaticns in the field of response latencies, he had already established himself i:l the vanguard of the world’s visual scientists. It bus seems appropriate at this time to offer as small tribute a -paper in which both visual processes and reaction processes are involved. Specifically, we hope to show how properties of the simple reaction time to a visual stimulus were instrumental in the: selection of one model from among many possible models to explain in part the way t?meeye handles time-varying light stimulation. 1.
LICKER-FUSION AND DE
ANGE?-1TYPEMODELS
(1957), a fellow-countryman of F. C. Ponders and himself a visual scientist, rejuvenated an old field of psychophysiology a few years ago with his brilliant work on flicker-fusion thresholds. Recent and summaries of these developments are available and will d&us& here (BROWN, 1965; SPERLING~ 1964). It is important to note, however, that De Lange by studying flicker-fusion threshok~sas a function of the percent amplitude modulation of the flickering source E LANGE
%4
THE SIGN OF
VISUAL TRANSIENT RESPONSE
85
as able to uce attenuation characteristic curves for human eyes. ested that the fusion point of an am litude-modulated light source is directly rel to the amplitude of the first Fourier ~:~~rne flickering so rce and that over most of the frequency cker can be perceived, the eye operates as a linear, lowint, a class of models has been proposed to account for the nse to time-dependent photic stimuli EVINSOW, 1968). These all comm make use of the electrical analogue of cascaded IX integrators with or without the addition of some differentiation. The number of series rations requi in the steep high-frequency cut-off of t3e attenuation chara ived from psychophysical da um*ber most frequently encountered is 10. DGKIN (1964) found necessary to assume as the result of physiological experiments with Lirnulus. For the Iowfrequency response portion of the characteristics, i.e., between 2 and 15 Hz particularly for high brightnesses, Kelly has been able to fit ysical data by assuming two stages of differentiation along rators. Because of the uncertainty surrounding the actual for all flicker conditions, number of integrations re these models have been r atio’n models. It y>eerns riate at this t.ime, in view o 11~‘srecent analysis, to designate them as n-integration, 2-differentiation models. 2.
A VISUAL TRANSIENT RESPONSE AT FREQUENCIES ABOVE FUSION
Consider the stimulus wave form shown in fig. 1. When z represents a total duration of two seconds, T, and 1’, are one second eat is 1 msec, then if the period, t,, is in the neighborhood of 5 msec, a
t------
Time
Fig.
1.
The wave-form
of the stimulus light scmrce.
-w
G. M. MOWBRAY
86
AND JOSBPH F. BIRD
visible transient response is produced. This transient response has same tit,cr~:sting properties with critical implications for the n-integration, 2differentiation models just discussed. As the duration of fi ic increased from near threshold by a very Eittle bit it rapidly beccmes visibk on every presentation. An increase in period of from 1.2%to 1.50 mm: above the threshold per.iod will accomplish this in most cases. ‘Yet, although readily visib?e, the transient still retains much of its elusiveness. In alpprance the transient seems to be a very brief eclipse, almost as thou,gh the stimulus winked its eye. However, if the pulse train of fig. 1 is reversed so that the longer periocl is presented first followed by the shorter period, the transient takes on a different appearance. In precisely what way the transient appears to differ is difficult to describe. Spontaneous responses from naive observers always reveal that the first order produces a transient that is’sharper, cleaner and more definite even though the periods are identical in both cases. Long and careful observation by pmcticed observers reveals that for the first case the transient appears as a brightness decrement and for the second case as a brightness increment 140~ these observations impose restrictions on the De Lange-Kelly class of models with n-integrations and up to two differentiations. The most serious restriction resides in the sign ~;f the transient response. All of the models with the single exception of 3 = 2 predict a sign for the transient which is in direct contradiction to the subjective perception. Thus, it becomes important to know by more objective means whether the sign is as perceived. 3. A
THRESHOLDTEST FOR DETERMINING THE SIGN OF THE VISUAL
TRANSIENT RESPONSE mere
have lxen
many thorough studies made of increment visual thresholds but surprisingly few concerned with direct comparison of increment and decrement thresholds under similar conditions. Within the last two years, however, :;tudies by SHORT (1966) and PAT~L and JONES (1968)
using ,different techniques have yielded essentially similar
resulti;. Irmbot4 the
less
decrement
the. increment threshold was consistently
greater than
threshold with a tendency for the difference
to become
at higher background luminances. Pate1 and Jones, in particular, showed that stimulus duration and area and background ~luminance are important parameters. ‘I’hc!stimulus was viewed peripherally in tlese experiments, atid the
THE SIGN OF A VISUAL TRANSIENT
RESPONSE
87
luminance levels were largely in the scotopic range. For fovea1 viewing the picture is not so clear. LACKWELL (1946) with no fixation and relatively long viewing times found lower decrement thresholds for low tation conditions, and NERRICK (1956) found a slight difference ckground. I-Iowever, with fovea1 viewing of a lo stimulus and no 67) found no differences at photopic levels usi stimulus with no surround. Despite this somewhat contradictory state of affairs, frequency-ofseeing functions were generated to determine thresholds for the appearance of the visual transient response for the two conditions that subjectively provide incremental and decremental 3.1. A ppuruhas and ,mhxdure
The time-varying stimulus train of fig. 1 was generated by standard electronic pulse-triggering and delay units operating a Sylvania glowmodulator tube type R 1131-C. The glow-tube was mounted on an optical bench and the light output directed through appropriate lenses and diaphragm stops to furnish the subject with Maxwellian view. Arcular neutral density filters provided 8 log units of attenuation. The stimulus source was circular, subtending I.dc of visual angle and was viewed foveally in the center of a m -white circular bowl mease served to fix the subject uring 74.5 cm in diameter. A dental firmly in position. Two microswitches were attached to all arm rest: extending from the right-hand side of the apparatus. One switch allotted the subject to initiate the two second duration pulse train, the: other controlled a Hewlett-Packard interval timer and was used, as subsequently will be described, to measure subject’s reaction times. Illumination of the background field was obtained from two Macbeth daylight lamps with crossed Polaroid filters on stands behind the observer positioned to yield a uniform level within the field of view. Illumination of the ci’rcular hole in the bowl through which the stimulus was viewed was furnished by magnesium fluoride tubes housed in al diffus’ing box behind the bowl and was reflected forward by a halfsilvered mirror. The level of this illumination coukl be match& to the general background level with a rheostat. For this experiment, the background luminance was kept constant at about 0.6 mL, while the Talbot level of the alternating pulse train was 60 mL. In exploratory observations, a range of conditions was determined
G. W. MOWBRAY AND JOSEPH F. BIRD
88
ticmtwag fie:ly
to
extencl from near 0 y41seen to near 100 “/o seen for
both the case when the short period was presented first md when it was presnt& second in sequence. In all conditions the short period tween was’ held constant at 1.0 msec while the long period var425 and 6.0 msec. To obtain the frequency-of-seeing functions, a csmplel:e set of increment or decrement thres ‘ids was obtained at one sitting. For any OES con&ion of the variabl eriod, 50 observations were taken before another vatiable was run, and the order of presentation of the variables was randwn. Two series of both increment and decrement thresholds were completed and the results combined, t us making a total of 100 observatiions per experimental point. Instead of using ‘blanks’ (i.e., occasions where no sti ) to check the reliability of the subjective criterion as is customary s type of experiment, another device was adopted. Instead of having the transition between the two frequencies of alternation occur at precisely the midpoint of the 2 set period, it was possible to make it occur anytime between 0.6 see and 1.5 set after the train was initiated by the subject. At the transition point, a pulse from the stimulus generator was used to start the Hewlett-Packard time-inteyvcl meter, The subject was instructed to depress a micro-switch as soon as he observed the tradsieat thus stopping the trmer. False positive responses could thus be easily detected and some useful reaction. time data could be obtzined as Iwell. The time of the occurrence of the transition was nged randomly between each trial.
2 depicts the frequency-of-seeing functions for increment and t conditions for one observer. It will be remember senting the standard frequency first results in a visual transient response that subjectively takes on the appearance of an intensity decrement. From the figure it is obvious that the decrement function (standard first) represents a more sensitive response than the increment function. s is in keeping \ti,th some of those eriments reported that actud.Iy added light to or subtracted light from some cxisti.ng backgr agreement is provocative but by no means conclusive since r~! increment-decrement thresholds were defined by directly easurabk quantities whereas the production of a subjective transient es no such opportunities for objective mensuration.
F A VISUAL
STAMDARO ORS
P
-
1000
TRANSIENT
RESPONSE
88
cps
:
80
E R
c
60
E N T
40
s E
20
N
0
_-I
--_I_.-_.I_____I___L_I
Feriod of
Fig.
4.
2.
A
6.0
5.0
4 .o the Variable
(mlllisecondsl
Frequmxy-of-swing functions for the visual transient as a furxtion the period of the variable intermittence.
SIMPLE
REACTION
OF THE VISUAL
TIME
TRANSIENT
TEST
FOR DETERMINING
of
THE SIGN
RESPONSE
shown that the le reaction time to light increments and decrements is not the sam ep dt=crease in luminanc: providing shorter reaction times than a step incre;sse, This result was not confirmed by JROUFS ef al. (1967) using a stepped current input to a glow-modulator tub: for their stimulus. The reztction times collected while we were obtaining the frequency-of-seeing functions tended to support Steinman’s results. Therefore, we decided to run a further series using frequency-separations that produced transie ts that were visible 100 76 of the time but in which all frequencies were still above the fusion point. STEINMAN
4.1.
(1944)
Apprutus
has
und procec’ure
The equipment and luminance conditions were
same as for the previous experiment. Three subjects were set the tusk of responJQ as quickly as possible by pressing the reaction key when they perceived the transient. One of the two frequencies employed (the sta ard) was always MN Hz, while the other the variable) was selected to cover a range pror;he
G. I-I, MOWBRA’Y AND JOSEPH T. BIRD
90 ducing
a
trancient just barely noticeab!.e nearly 100 $% of t one that was quite clearly distinct. Reaction time trials were run in blocks of SO reaciion times each for each frequency condition presented in random order. A session consisM of a.!1 frequency conditions with a single standard-vari order. There was a total of four sessions in whic’h the standard-variable order was alternated yielding a total of I reaction times per experiment91 point. Subjects were given practice trials on each condition before any reaction times were recorded, and they were never informed which condition was being presented. 4.2. Results F,ig. 3 is a plot of the reaction times as a function of the period of the variable frequency for the two standard-variable orders. Only one subject’s data are illustrated since the resul.:, for al! were essentially the same. From t-h%it is evident that the order in which the two fsequencies are prer;ented i-i a determinant of reaction time. That order which produces ai transient whose sign is negative with res t to subjective brightness also p;,-educes the shortest reaction latencies. The two fir~tionc do not appear to merge as the distinctness of the transient increases but tipper to be asymptotic to different levels with an average difference of 20-25 Eltisx.
ST.~NDWRO - 1000 cps OBS : GHM
350 f? E A c T
b N
300 m =
e =
250
200 ; M E
First
150
-L-.1
4.0 Period
Fig.
3.
6.0 of
8.0
the Variable
IO.0
I2 .o
14.0
I tnsec)
Simple reaction timea to the appearanw of the visual transient as a function of the period of the variable intermittence.
OF .4WSUAL TRANSIENT REdPCMSE
5.
91
~S~US§ION
a stimulus of the for of the two frequencies in the ~~ctiveimpression of lower one creates a positive going effect. Further, plications for the 2-differentiation, n-integration
luminance as the steady source. e reports that a visual transient is produced at the onset an the modulation frequency was around 600 Hz. Levinson further reports that the sign of this transient cannot be distinguished subjectively. Now, comparing our visual transient observations with those of evinson, we may t&e as typical of consider the stimulus shown in fig. 4 w lenfal sine wavk5component the Levinson situation (assuming the fu dominates the response as it apparent1 s at high frequencies), and which is a speci = 0) for our stimulus. We perceive in this by the case a brightness increment, and this perception is support results of the foregoing experiments. We have also observed in this the transient is enhanced by extending the last dark phase, ing a negative ‘flash’. Presumably, therefore, adding a positive flash instead would nullify the perceived brightness increment rent paradox. evinson, on the other hand, states it cannot eived whether the transient response is an increment or a crement. To try to distinguish, he inserts ‘flashes’ at the transition time and finds a negative one enhances the percept and a positive one cancels it, in correspondence with our experience. But he implies that the original stimulus must be giving an apparent brightness decrement, which contradicts our results and our direct perceptions. turning to a consideration of the n-integrator models, we
G. Hi. MOWBRAY AND JOSEPH F. BIRD
92
began by fixing the polarity of all models so that a positive step input g&s a positive response, corresponding to a brightness increment. Then, e type Iof stimulus used in these experiments and pictured as a special case in fig. 4, our calculations yield a transient response whi&:
t L ip
i N
c
__J--L_rLrTlhE
--I--
--
E
Fig. 4.
A special case of the stimulus wave-form s,hown in fig. 1, where t2 = 0.
(a) for all n # 2 is negative, i.e., brightness decrement; (b) for only tt = 2 is nositive, i.e., brightness increment. Thus, only the latter%member of this class of model5 is allowed by our sign determination, and all o hers are ruled out. On the other hand, note that if Levinson’s indirectly determined sign were valid, any of the models, except n = 2, would be permitted. There is the difficulty that with ,?-ras small as 2, the filter cannot by itself give the steep flanks of the .De Lange curves for flicker fusion. However, a small n seems a prereqtisfte for obtaining from such models our detailed experimental results. 111any event, by adding to the model a neural detector which imposes thresholds both on the amplitude of excitation and on its duration, it is passibJe to save the phenomenon of De Lange flanks (and other classic,\1 flicker results) with n still small. Further, with this amplitude-dumtion detector, the apparent paradox obsrved by us on inserting a negative flash, or the related contradictory sign determination by Levinson, may be reconciled with the perceived sign of the transient. Thus, if to the stimulus is added a brief nega the added response of the model, n = 2, is a brief negative pip, und to positive response and then a slow relaxation towards zeroI But thz assumed detector ignores the negative pip and sees only ow positive response, i.e., a brightness increment, which then rices the response due to the original stimulus without flash. rly, a brief positive flash would be detected as an added decrement hich could cancel tine original response. For the models, n # 2, similar
ve enhancement, cause the original transient
n =2
Response
n = 4 Response
__;.CL.T& _/
Fig.
5.
A schematic representation the type 21 = 2 and r~ - 4. going stimulus is markecl for deEetion &u-at.ion threshold
of some calculated model responses for The n = 2 response to the brief positiveindicating that it is below the amplitudeof the neural detector.
the former case, the brief pip, ignored by the assumed detector, is marked for d.eletion. Adding (or subtracting) the latter two responses illustrates the effect of’brief flashes. We may e that the model, n = 2, with the assumed detector kile for the responds oppositely to very short and very long flashes. former the initial upward pip can be too brief to be detected, for the latter the pip dominates the response, which is then a brightness increment!, as it should be. The response may well be of ambiguous polarity for intermediate flash lengths around the detector duration threshold -- several milliseconds. This might relate to the difficulty of perceiving the sign of brief flashes as reported by Slperliwgin LEVINSON (1968).
94
G.H. MOWBRAW
AND JOSEPH F. BIRD
ACKNOWLEDGMENTS
This investigation
was supported in part by the
Weapons, Department of the Navy, under and by U.S. Public HeaIth Service Research Nat.ionaZInstitute of Neurological Diseases md Blindness.
REFERENCES R.,1946. Contrast thresholds of the human eye. 3. optic. 233. Amer. X, 624-643. .H. Graham (ed.), Visio visual perception. New y and Sons. p. 251. nuation characteristics and phase shift characterthe human fovea-cortex system in relation to the flickerfusion phenomena. Delft: Technical University. FUC~RTES, M. G. P. and A. L. HODGKIN, 1964. Changes in time scale and set& tivity in the Ommatidia of Limulus. J. Physiol. 172,239463. HEWWK, Ik.K., 1956. Fovea1 1uminance discrimination as a function of the duration of the decrement or increment in luminance. J. camp. physiol. Bsychol. 49,437--443. KELLY, D. H., l%l. Visual responses to time-dependent stimuli. II. Single channel model of the photo.pic visual system. J.optic. Sot, Amer. 51,747754. LEVNON, J. Z., 1968. Flicker fusion phenomena. Science, 1 PM-EL,A. S.and R. W. JONES,1968. Increment and decreme J. optic, Sot. Amer. 58,696+599. R~uF!Y~, J. A J., J. T. H. LAMMER~and J. A. &EMONS, 1%7. Positive and negative flashes. I.P.O. Annual Progress Report, 2, 129-133. a-r’, A. I)., 1%6. Decremental and incremental visual thresholds. 3. Physiol. 185, 646-654. SP . van der Tweel (eds.), Flicker. 9 CL I%4 In: H. E. Henkes and H. Proceedings of the Symposium on the phylsiology of flicker, The EIague: Dr. W. Junk. 3-15. AN, A R.., 1944. Reaction time to change compared with other psychophysical methods. Arch. Psychol. : BLMXWELL,
H.
e steepness of the transmission curve is never more than that of a ftitem in series and never gets at 10 RC times. is true but even five seems to be too much. ot of rmple talk in terms of these low-pass filters on the evisfence ing being me=ured below about 5 Hz. Everything one finds below
F A VISUAL T
corroboration
of
NSUH’iT RESPONSE
95
is point is that if you 1 ogle make a subjective say one half or something to ysical flicker frequen and then it stops somewhere
ut in a more simp.ified form viz. some then would have a sort of as such would ode1 is completely not 4 nfuse rattle with a tonal
and ‘tonal sensation’ may be very useful.
PeHr: Would it be possible to make inferences about the nature of the filtering from looking at the phase shift, i.e. to think of part of reaction time as the result of the phase shift introduced by whatever filtering is taking place and to look at that as a function of frequency? lp you a thing, because once Schosrtcn: This has been done. But it does not given the frequency characteristic and assumi - which is most hat it is a minimum-phase network, then you just calculate wh r&&n should be. So you get no extra evidence from it. Pelv; As long as you assume it is a minimum-phase network. Mlouter#: In ordinary ii e you encounter no filters which are not rninimumphase network. What Roufs finds in linking up the time function on the one d car-responds with the frequency function on the other.