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Effects of stimulus onset and image motion on contrast sensitivity
EFFECTS OF STIMULUS ONSET AND IMAGE MOTION ON CONTRAST SENSITIVITY and BECKYJ. BENNIS
MILKER TULUNAY-KEESEY
Department of Ophthalmology,
University of Wisconsin Madison, Wisconsin 53706. U.S.A.
(Received 13 February 1978; in
revised jbrm
27
November
1978)
Abstract-Spatial sinusoidal gratings were viewed under both stabilized and unstabilized viewing conditions. They were presented either with a gradual or a sharp onset. The results indicate that contrast sensitivity to a range of frequencies centered around 2-3 deg is decreased by a maximum of 0.3 log units when high frequency temporal stimulation induced either by the sharp onset of the spatial pattern or by the motion of the retinal image is minimized. Data are presented which suggest that under conditions of minimal temporal stimulation. if fading of the stabilized image is also allowed, sensitivity may be decreased by a larger amount.
INTRODUCTION
and flickering sinusoidal gratings have clarified the interaction between the spatial and temporal variables of the stimulus in determining contrast sensitivity (Robson, 1966; Kelly, 1972; Kelly, 1977). It has been shown, for example, that counterphase flicker of 8 Hz or lower rates serves to increase sensitivity to spatial frequencies lower than the frequency which yields peak sensitivity; the higher spatial frequencies seem to be immune from the effects of such temporal modulation. Counterphase flicker of higher rates renders contrast sensitivity equal across the low spatial frequencies, as well as affecting a reduction of sensitivity to ah spatial frequencies. Because the range of temporal frequencies that is most effective in changing sensitivity to low spatial frequencies is comparable to the range of frequencies exhibited by normal eye movements which occur during fixation, it is reasonable to expect that temporal modulation caused by these movements determines the shape of the contrast sensitivity curve when it is obtained with a stationary pattern. For example, one class of models concerned with spati*temporal interactions assumes that the motion of the retinal image of the target and its spatial frequency interact in such a way as to boost sensitivity to the midddle range of spatial frequencies, and predicts equal sensitivity for the low spatial frequency range in the absence of retinal image motion (e.g. Arend, 1976). This expectation was tested by several investigators by the method of image stabilization which renders the image motionless with respect to the retina (see Tulunay-Keesey and Jones, 1976 for a review). One of these studies showed that threshold contrast for the detection of a grating depended on the exposure duration of the target and not on the presence or absence of image motion during the exposure. Maximum sensitivity was achieved for a 3 c/deg grating, for example, with an exposure duration of IOOmsec or longer whether the image was stabilized or not. Attenuation of sensitivity for lower spatial frequencies under either viewing condition was more prominent for exposures longer than 1OOmsec. Exposures as
Studies with stationary
767
long as 4sec. a reasonable period of tixation during which the whole range of normal image motion can occur, failed to reveal any differences between the stabilized and unstabilized contrast thresholds (TulunayKeesey and Jones, 1976). In the same study, it was demonstrated that if the target was viewed for an indefinite period of time, sensitivity was reduced for both the stabilized and the unstabilized target. In addition, the stabilized thresholds were found to be somewhat higher, by about 0.2 log units, than the unstabilized. The shape of the contrast sensitivity curve remained unchanged. It was judged therefore, that if there was an effect of image motion on contrast sensitivity, it was small (Tulunay-Keesey and Jones, 1976). It may be argued that the lack of difference between the stabilized and unstabilized thresholds for each spatial frequency when the targets were presented for short, finite ‘durations is due to the form of the temporal waveform. There is evidence (Breitmeyer and Julezs, 1975) attesting to the importance of stimulus onset waveform even under unstabilized conditions. In the Tulunay-Keesey and Jones study, gratings appeared suddenly upon the evenly illuminated oscilloscope screen whose luminance was matched to the space average of the target. The sharp onset of such flashed gratings may have produced luminance variations of high temporal frequency and large enough amplitude to mask or equal the effect of the temporal changes resulting from image motion. When gratings were presented for an indefinite duration the method of adjustment was used for setting thresholds instead of the staircase method which was employed when gratings were flashed. The method of adjustment required that the observer search for threshold starting from zero contrast, or an evenly illuminated screen, by manipulating a control knob. This method introduced a temporal modulation of indeterminate waveform and, in addition, it allowed local adaptation through allowing for long periods of inspection. Therefore, elevation of thresholds evidenced by this method, in relation to the thresholds obtained with flashed targets of finite duration, is likely to be due to both the lack of sharp transients and to local
adaptation. %rnilarly, the rclativelv htgh thresholds obtained with stabilized gratings with the method of adjustment may well reflect both the effects of lack of image motion and the well know-n phenomenon of stabtlized viewing, that of gradual image disappearance which cccompanies long periods of inspection. Kelly (1977) reports differences as large as 1 log unit between unstabilized and stabilized contrast thresholds obtained by the method of adjustment. It is clear that if image stabilization is to be used in studying the effects of eye movements on contrast sensitivity tt is desirable to parcel the effects of image fading and onset transients as well as the effect of image motion. In this paper w’e report our efforts in this direction. We presented targets of long but finite duration with a slow onset under both the stabilized and unstabilized viewing conditions. We compared the thresholds obtained with such a gradual onset with the thresholds obtained when the stimulus. stabilized or unstabilized. was presented with a sharp onset. This provided an estimate of the relative effects of image motion and onset transients. The method of staircase was used to secure thresholds. In order to gain an insight into the effect of image fading on contrast sensitivity. we obtained contrast thresholds with the method of adjustment under conditions when fading was allowed, and compared them with thresholds obtained under stabilized and unstabilized conditions where minimum disappearance was permitted. The design of this experiment also allowed comparisons between the two methods of threshold acquisition, staircase and adjustment. Finally, in this paper we report data from an earlier study with the intent of gauging the range of temporal frequencies for obtaining optimal contrast sensitivity. 3lETHODS General Vertically oriented sinusoidal gratings were generated on an oscilloscope screen. Intensity distribution in the horizontal direction was given by Lo = 1 + m cos 2 r$x where Lo is the average luminance, m is modulation, and fx is the spatial frequency. Lo was kept constant at 5 ml. and contrast could be varied up to 70’?/, with low distortion. The contrast could either be held constant, turned on and off. increased and decreased in a ramp fashion or varied sinusoidally as a function of time. Contrast for a given
spatial
frequency
was expressed
as
:iewmg condition when the image was motionless on the retina and under the unstabilized condition. when it was allowed to move normally; each stimulus was presented either with a ramp (trapezoidal waveform) or step onset (rectangular waveform). Hence there were four experimental conditions altogether. In the trapezoidal mode, contrast was increased linearly irom 0 to predetermind level over 2.5 sec. and held at that level for 5 sec. It was subsequently decreased to 0 within 25sec. In the rectangular mode, contrast was increased from zero to a predetermined level instantaneously and held there for 7.5 sec. The time-integrated contrast for the stimulus in the two modes of presentation for any one contrast the spatial frequency was therefore equal. To examine the extent :o which the trapezoidal waveform reduces the high frequency content of the pulse. we looked at the spectrum of both the rectangular and the trapezoidal pulse. Calculations were made according to the formula given in Mason and Zimmerman (1960): C’(w)
=
h +
sin[(b + 4),2]w
sin[(b - u)/2]~
[(b + 0)i2]w
[fb - a,/2]w
4
1
where 4 = 3.75. h = 3.75 in the case oi the rectangular. and a = 2.5. b = 5 in the case of the trapezoidal waveforms. The second term in the equation gives the ratio of the amplitude in the trapezoid to the amplitude in the rectangular pulse. These values are plotted in Fig. I. At 0 Hz. the d.c. level. both waveforms have the same amplitude and the ratio is 1. At 0.2 Hz. the trapezoid has ag proximately 60:; of the amplitude of the rectangular waveform. At I Hz. the trapezoid has less than IS”;, of the amplitude of the rectangle, and the ratio decreases as the frequency increases. We see that the amplitude of the higher temporal frequencies is reduced significantly by using the slower onset of the trapezoidal waveform. .A modified method of staircase was used to obtain thresholds in the main part of the experiment. Each session started with a few minutes of adaptation to the evenly illuminated screen. Thresholds were obtained for a few spatial frequencies which were viewed under either the stabilized or the unstabilized conditions and which were presented either with a ramp or a step. The contrast was increased by a given amount (j-20% of the previous setting)
12
09
I
1
C = I,,, - L,./
+ L,,,. The oscilloscope was viewed at a distance of L lyccm. and the display subtended 3.2’ vertically and 4’ horizontally. Target contrast was varied with a IO-turn linear potentiometer. The gratings were viewed through a previously described apparatus (Jones er al., 1972: Jones and Tulunay-Keesey. 1975) which provided either unstabilized or stabilized conditions. Stabilization accuracy was such that the maximum residual image movement in 1Osec of viewing was well below I’ arc peak-to-peak, for all subjects. An unstabilized fixation dot was provided for both viewing conditions, and the subject was instructed to maintain fixation under the unstabilized conditions. Experimental design: main experimenr Contrast thresholds were obtained for ten spatial frequencies ranging from 0.5 to l2c/deg for one subject and for nine spatial frequencies ranging from 0.37 to I8 c/d% for the other. Each grating was viewed under the stabilized
-0.3[ -20
’
’ - 1.2
h
’ 0.4 -0.4 0 Frequency Hz
1.2
1 20
Fig. 1. Ratio of the amplitude as a function of temporal frequency in the ramp (trapezoidal waveform) to the step (rectangular waveform) onset of the stimulus at a given contrast. For the ramp onset. the contrast was increased linearly from 0 to a predetermined level in 2.5 sec. It stayed at that level for 5 sec. before returning to 0 over a 2.5 set period. For the rectangular waveform, the contrast was introduced instantaneously. It had a duration of 7.5 sec. For any one contrast at OH& the d.c. level, both waveforms have the same amplitude and the ratio is 1. At 1 Hz the trapezoid has less than lSoi, of the amplitude of the rectangle. at 1.8 Hz, less than 7.5’,,. etc.
Stimulus onset and image motion on contrsastsensitivity every time the observer made a negative response and decreased by the same amount when she made two Consefutive correct responses, The mean of at least four such sequences was taken as the threshold. The procedure was repeated untif. all the spatial frequencies were viewed under each of the four experimental conditions. Each stimulus presentation was preceded by approx 2 see by an auditory signal, and between each stimulus presentation. the observer readapted to &, for a minimum of Ssec Two observers. one experienced (UTK) and one new farB1 to both psychophysics and stabilized image viewing participated in the main study. B)B was subjected to lengthy training sessions.The experiment was repeated three times for both subjects, the average of all three thresholds are presented for LTK and the average of the last two repetitions are shown for 818.
Contrast thresholds were obtained by the method of adjustment on a restricted number of spatial frequencies to determine if stabilized and unstabilized thresholds could be brought closer or driven apart by the way the stimulus is introduced or the instructions given to the subject. In one set of instructions. the observer was asked to reach threshold as quickly as possible by turning a knob that controlled the contrast levek fn a second set of in-
structions
the observer
was asked to go slowly. Two
observers participated in this part of the experiment. A third set of instructions was designed specifically to
incorporate stabilized image disappearance into threshold settings. The observer was asked to wait after the initial threshold determination until the threshold grating disappeared. and to search slowly for another “threshold”. He!
she was encouraged to go through as many iterations as necessary to reach a level of contrast that represented a steady “threshold”. Three observers. BIB. RMJ. and UTK. were available for this study. In each case rhe target was introduced at zero contrast. that is. the observer started searching for threshold from &. and after threshold was reached he turned the control knob so that the screen was again evenly iiluminated. In between threshold settings the observer adapted to L,,. In other words. there were RO transients due to the sharp onset of the stimulus: the only temporal modulation was produced by the subject’s adjustment of the contrast, In yet another version of the method of adjustment. the observer was asked to reach threshold slowly. but after rhe final threshold setting. the contrast was not decreased
to zero. The next frequency *as presented with a sharp onset as the contrast of the previous setting. Thus during the period of threshofd setting a mixed-mode of WtIpOrd~ changes occurred.
RESL’LTS AND DISCUSSION
Effects of image rnorion cs onser of the target A comparison of contrast thresholds obtained with the ramp onset under the stabilized and the unstabilized viewing conditions shoutd give an indication of the effect of image motion alone on contrast sensitivity. in Fig. 2. we present contrast sensitivity functions obtained with such stimuli for both subjects. It appears that for both subjects the presence of image motion serves to increase sensitivity to spatial contrast. When the data for both subjects are considered together the sensitivity enhancing effect of image motion seems to be most pronounced for frequencies between I.2 and 4.5 c/deg. There is a difference between the data of the two subjects: UK‘S results would indicate that image motion may continue to enhance sensitivity to frequencies lower than 0.5 Hz. and that it may serve to decrease sensitivity to frequencies higher than 6c/deg. BIB’s data, on the ocher hand, would indicate that image motion has no effect in increasing sensitivity to frequencies lower than 0.5 Hz. and that it continues to serve as a sensitivity increasing mechanism for frequencies higher than 6 cjdeg, finally losing effectiveness at the highest frequency, 18c/deg. used in this study. A comparison of thresholds obtained under the stabilized conditions with the ramp and the step onsets should indicate the effects of transients resulting from the onset of the stimulus. Figure 3 represents contrast sensitivity curves obtained under these conditions for both subjects. It would appear that the sudden onset of the grating pattern, the farge amplitude, high frequency temporal luminance changes resulting from it has a similar effect on contrast sensitivity as do the motions of the image: thresholds to spatial frequencies ranging from O.jc.‘deg (0.375 for WE) to about 9c,‘deg are
30 a3
1
3
IO
1
3
IO
SPATtAt WQUENCV tc/dagcI)
Fig. 2. Effect of image motion: threshold contrast to detect spatial sinusoidal gratings when the target
presented with a ramp. The open circtes and broken lines designate stabilized. the solid circles and continuous lines unstabilized viewing. Staircase method was used for fhreshold acquistion. Vertical bars show 4 1S.E. was
769
770
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TLLLXAY-KEESEY
SPAtM
and
BECKY J. BENNIS
WfQUWCY
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Fig. 3. Etfect of stimulus onset: Threshold contrast under stabilized conditions. The open circles and broken lines designate the ramp onset (2.5 set on. 5 set steady. 2.5 set 0m of stimulus presentation. the solid circles and continuous lines the step onset (7.5sec onl. Staircase method was used. Vertical bars show 5 I S.E.
lower in presence of onset transients. The maximum effect appears to be centered around 2-3 cjdeg. In Fig. 4 we show contrast thresholds obtained with a target presented in the rectangular-wave mode with a step onset. It was viewed under the stabilized and unstabilized conditions. For both subjects the stabilized and unstabilized thresholds appear to be very similar. These results suggest that the transients resulting from the onset of the target determine the level of contrast at which a spatial pattern is detected but whether or not the image moves within the period of inspection is inconsequential. Yet, as suggested in Fig. 2, if onset transients are not available, the motion of the retinal image appears to regulate sensitivity. The question therefore arises of whether or not these two sources of transients are interchangeable. The best indication of equivalence can be found in a comparison of thresholds obtained with a stimulus viewed under the unstabilized conditions where the onset transients are minimized and the main effective factor
SPATIAL
is image motion, with the thresholds obtained with a stabilized stimulus when the effective transients are mainly those caused by the onset of the stimulus. Figure 5 supplies this comparison. While BJB‘S data would indicate that transients instigated by the norma! motion of the retinal image and the onset of the target are equivalent, UTK’S data would suggest that onset transients weigh more heavily in determining contrast thresholds. The following conclusions can be derived from the common points of the data presented. In a normal period of voluntary fixation of a given duration, sensitivity to spatial contrast is determined by the transients resulting from the onset of the target. The additional transients which result from the motion of the retinal image that take place during.the inspection of the target have little effect in setting sensitivity. On the other hand, if the onset transients are minimixed, it is found that image motion increases sensitivity to a large range of spatial frequencies. The
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Fig. 3. Threshold contrast obtained with a target presented with step onset and offset. Duration 7.5 sec. Open circles, broken lines depict stabilized; solid circules. continuous lines. unstabilized viewing conditions. Staircase method was used. Error bars for BIB are not shown because of the proximity of data points.
771
Stimulus onset and image motion on contrast sensitivity lr
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1
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Fig. 5. Contrast thresholds obtained with the ramp onset (2.5 set on. 5 set steady. 2.5 sec. otTI under unstabilized viewing conditions are designated by continuous lines and solid circles; thresholds obtained with a step onset (7.5 set duration) under stabilized conditions are represented by open circles and broken lines. Staircase method was used. + 1S.E.‘is shown for each data point. middle range of frequencies centering around 2.5 c/deg are especially influenced by temporal modulation resulting from either source. While the com-
bined effect of image motion and onset transients is not greater than the effect of onset transients alone, whether these transients are interchangeable in regulating contrast sensitivity is not clear. Effect of image disappearance method
The results are presented in Figs 6 and 7. The bottom line in Fig. 6 represents stabilized contrast thresholds obtained when the observer was instructed to search for the threshold slowly. The observer increased the contrast if at the initial threshold setting the stabilized target disappeared. This procedure took
and the psychophysics!
Figure 2 indicates that elevation of contrast sensitivity due to image motion, at best, is about 0.2-0.3 log units. Kelly (1977) on the other hand, reports that lack of image motion can drive thresholds down more than 1 log unit for l-2 c/deg and about 0.6 log units for very low (0.2 c/deg) and high (12 c/deg) frequencies. Kelly also presents data which suggest that the psychophysical methods of adjustment and forced choice produce similar measures of contrast sensitivity. Our data (Tulunay-Keesey and Jones, 1976) suggest that the method of adjustment yields the higher thresholds. In an attempt to resolve the differences between Kelly’s and our data we attempted a series of experiments to produce conditions which would yield large differences between stabilized and unstabilized viewing, and small differences between psychophysical methods of threshold estimat ion. First we assured that image stabilization achieved by Kelly’s and our apparatus was of comparable accuracy. We obtained the contrast value for a number of stabilized and unstabilized gratings which produced an after-image when the stimulus was replaced by an evenly illuminated field after 5 set of viewing. We found that a stabilized grating of 20 c/deg at Zoo,/, contrast produced an after image. This indicated a stabilization accuracy of better than 1.5’ arc, and compared well with Kelly’s (1977) data which showed that a stabilized grating of 12 at looo/, contrast produced an after-image. As pointed out in the methods section, when we measured stabilization accuracy by tracking position of an after-image in relation to a real-light stabilized image, we found it to be better than 1’ arc (Jones and Tulunay-Keesey, 1975).
f
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g f
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g u
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Spatial frequency(c/de@ Fig. 6. A comparison of methodology and stimulus onset: Contrast thresholds obtained under stabilized viewing conditions. Solid circles and continuous lines depict thresholds obtained by a method of adjustment under instructions to search for the threshold slowly. If the image were to disappear, the observer was to find a new threshold. The con-
trast is brought to 0 aRer final adjustment. The dotted lines and solid triangles represent contrast values reached by the method of adjustment when the observer was asked to set the threshold quickly. The contrast was brought to 0 after each threshold adjustment. Open squares represent thresholds obtained under the stabilized conditions again by a method of adjustment. The difference is in the mode of stimulus onset. the target is presented with a step onset at an initially high contrast (see text). Unstabilized conditions: Open circles and continuous lines represent contrast values obtained by a method of adjustment when the target was presented wtth a step onset at an initially high contrast. The x’s represent thresholds obtained by the staircase method when the target was presented with a ramp onset.
772
ULKER TLLLNAY-KEESEY
dnd
BECKY J. BENWS
Spatial Frequancy tc/degl
Fig. 7. Effect of image fading: Open circles and dotted lines depict contrast thresholds for stabilized gratings. The subject was specifically instructed to wait for image disappearance and subsequently search for a further “threshold” until a steady threshold was reached. Solid circles and continuous lines represent unstabilized thresholds. Method of adjustment was used.
a minimum of 15sec. The middle dotted line represents stabilized thresholds obtained under instructions which asked for a quick decision. They represented the “initial” contrast setting for detecting a stabilized grating. This procedure took 5-15sec. Without exception the thresholds obtained with the “quick” method were lower than thsoe obtained with the “slow” method. It would appear that the method of adjustment, because it aIIows for temporal frequencies that vary according to the observers’ rate of adjustment, and because it allows for image disappearance that varies according to the time spent inspecting the target, also allows for a wide range of criteria.
The upper solid line in Fig. 6 again represents data obtained with the method of adjustment, but under the unstabilized condition. In opposition to the procedure followed with the stabilized gratings, the contrast was not reduced to zero after each threshold setting, but the new target was introduced with a sharp onset at the contrast value of the previous setting. In other words, this method contained both the sharp transients due to the onset of the ‘pattern and image motion, and aIso temporal variations due to the observers’ manipulating of the contrast. It yielded the lowest’thresholds of all. The stabilized thresholds resulting from this mixed-mode of transients fell on the middle dotted line. They are represented by open
Stimulus onset and image motion on contrast sensitivity
773
of the mammalian retina. Our data reported presquares. On this graph we also plotted the thresholds viously (Tulunay-Keesey and Jones, 1976) and the obtained with the ramp onset by the star&se method present data reaflirm that this neural mechanism is under the unstabilized conditions. These points, not dependent on image motion, but that the temrepresented by the symbol x1 are close to the upper poral variations supplied by them or the stimulus line. This provides a comparison between the method onset modifies the effect of these interactions in a of adjustment and staircase, and suggests that when quantitative manner. the method of adjustment allows for sharp onset transients, thresholds can be brought closer to those Effect of Picker obtained by the staircase method. The data we have presented in conjunction with The important point of Fig. 6 is that stabilized and unstabilized thresholds obtained with the method of a set of unpublished data allow us to speculate on adjustment can be driven close or apart chiefly the range of temporal frequencies necessary for maximum contrast sensitivity. according to the instructions given to the observer. A comparison of the spectrum analysis of the trapeThis is not entirely due to the intrinsic inadequecy zoidal and the rectangular waveforms for a given conof the psychophysical method itself, but partly due to the changes that the visual system undergoes when trast value shows that the two waveforms contain the retina is confronted with a motionless image fcr comparable amplitudes at the very low temporal frelong periods of time. This notion is strengthened by quencies below 1 Hz. and that the rectangular waveform has considerably more amplitude at frequencies the data shown in Fig. 7. above 1 Hz (see Fig. 1). As Fig. 3 shows, the rectanguThey were obtained according to a set of instruclar tiaveform, in the absence of image motion. protions which incorporated image disappearance. The vides the lower of the thresholds for spatial frequenobservers was asked to wait, under the stabilized concies lower than 8 c/deg. It would be expected therefore ditions. for the threshold target to fade and subsequently to search for another “threshold”. He/she was that, for the low and the middle range of spatial frequencies, temporal changes of a frequency of 1 Hz to repeat this procedure as many times as necessary. The target at the initial setting did disappear very or higher would be needed to increase sensitivity to quickly, but subsequent “threshold” targets took in- levels above those obtained with a stabilized image. A set of data previously obtained in this laboratory creasingly longer times to fade. According to the subjects, the “final threshold” represented a level of conconfirm this notion. Contrast thresholds were set by trast which “looked as if it would not fade easily”. the method of adjustment for stabilized sine wave This procedure involving multiple settings took a gratings of 0.75, 3 and 7.5 c;deg, which were flickered in a temporal sine wave fashion, as well as for steady minimum of 45 sec. It can be seen in Fig. 7 that at 3 c/deg, image fading stabilized gratings of the same frequency. The results yielded thresholds approx 1 log unit higher than the for the 0.75 and the 7.5 c;deg gratings are shown in thresholds obtained when the image moved normally Fig. 8. The data points for the 3 c/deg target are not on the retina. The differences between the stabilized plotted: they follow closely the curve for the and unstabilized thresholds were larger for the lower 0.75c/deg target, with the exception that the threshspatial frequencies and smaller (about 0.5-0.3 log old for the steady grating designated as 0 Hz is at units at 8 c/deg) for the higher ones. Another feature ‘3% contrast. of these curves is that when image fading is allowed, It is seen that sensitivity for the low frequency gratthe frequency at which highest sensitivity is achieved ing of 0.75 c/deg increases close to.five times (1.7 times is displaced toward a higher spatial frequency. This for the 3 c/deg) when it is flickered in counterphase is perhaps indicative of the “steady state” of the at 1 and 2 Hz. Flicker starts to loose its effectiveness system. at about 4Hz for the 0.75 and the 3c/deg target. If image disappearance is indeed an indication of However, even the higher rates of flicker are effective the “‘steady state” of the system responsible for conin increasing sensitivity above the levels obtained with trast sensitivity and if steady state is under question, the steady grating. Kelly (1977) reports a similar it is perhaps best to use a method which could allow image disappearance and subsequent readjustment of thresholds. If the question under study is the role of Iimage motion due to the movements of the eye which take place during a normal period of fixation, as it was in this study, then it seems to us best to study contraSt sensitivity with a method that can separate the effect of eye movements from the effect of target disappearance in determining sensitivity. It should be noted that even if the mction of the image that takes place during a restricted period of inspection supplies a boost of sensitivity to a middle range of spatial frequencies, the absence of such image I I I I I motion does not render equal sensitivity across the $” 03 I 3 IO low and middle frequency range under any condition. Tun~rol traqrancy(Hz) This invariance of the shape of the contrast sensitivity Fig. 8. Contrast thresholds for stabilized gratings of 0.75 curve suggests that low frequency attenuation is sub- (open circles) and 7.5 (solid circles) c,‘deg as a function served by a mechanism of spatial interactions. such of frequency of counter-phase flicker. 0 Hz depicts when as center surround antagonism of the ganglion cells me gratmg was steady. - _
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GLKERTLLCNAY-KEESEY and BECKYJ. BE>\IS
trend. For the higher spatial frequency of 7.5 c!deg. on the other hand. flicker up to 4 Hz does not increase sensitivity to a level higher than that obtained with the steady stabilized grating: in fact. the higher temporal frequencies of 8 and 10 Hz result in a decrease of sensitivity. This agrees with the data shown in Figs 2 and 3 which indicate that the presence of image motion or the rectangular waveform containing the higher temporal frequencies results in a decrease of sensitivity for the higher spatial frequencies belou the levels obtained with the ramp wageform under the stabilized conditions when presumably the prominent temporal frequency is very low. around 0.2 Hz. The efficiency of temporal changes with frequencies lower than 1 Hz in determining contrast sensitivity remains a question however. As the advantage of the rectangular waveform in increasing sensitivity is lost for spatial frequencies higher than about 8 c/deg, and since the ramp and the step provide comparable amplitudes at the very low temporal frequencies, we may be justified in concluding that the high spatial frequencies benefit as much or more from the very low temporal frequencies alone as they do from a combination of the low and high temporal frequencies. It should be pointed out that these very low temporal frequencies are within the same range that would be supplied by drifting the high spatial frequency targets at velocities comparable to the normal drift motion of the eye. If our reasoning is correct. than the large differences between the stabilized and unstabilized measures of sensitivity (Figs 6 and 7) found with the method of adjustment for the high spatial frequencies can be ascribed to an absence of the very low temporal frequencies as well as the long term effects of image stabilization. CONCLUSIONS
The main conclusions of this set of studies are as follows : (1) The shape of the contrast sensitivity curve is invariant, i.e. low spatial frequency attenuation is still present under conditions wherein the high frequency transients, resulting from image motion or stimulus
onset, are minimized. This suggests a spatial rather than a temporal neural interaction as the mechanism [or contrast sensitivitv. (2) In a normal period of fixation. the effect of high frequency temporal changes resulting from either image motion or stimulus onsef is to increase sensitivity by a small amount. (3) In the absence of image motion. thresholds can be driven upward, sensitivity lowered. primarily by allowing for long periods of fixation and image disappearance. This magnifies the differences between stabilized and unstabilized thresholds. rlc~no~ledyemenrs-This research was supported by an NIH grant EYO0308. We thank Drs L. A. Ripgs and J. 0. Limb for their comments on the first draft of the manuscript and Dr Elmer Johnson who kindly supplied the limbal seating contact lenses necessary for image stabilization. Secretarial help of Miss Jan Livingstone and Miss Josephine Anderson is Qratefully acknowledged.
REFERENCES
Arend L. E. (1976) Temporal determinants of the form of the spatial contrast threshold MTF. C%ion Res. 16. 1035-1042. Breitmeyer B. and Julesz B. 11975) The role of on and ofi transients in determining the physchophysical spatial frequency response. Msion Res. 15, 411-416. Jones R. M. and Tulunay-Keesey U. (1975) Accuracy of image stabilization by an optical-electronic feedback system. Vision Res. 15, 57-61. Jones R. M.. Webster J. G. and Keesey U. T. (1972) An active feedback system for stabilizing visual images. IEEE Pans B,ME-19 pp. 2%33. Kelly D. H. (1977) Visual Contrast Sensitivity. Oprica Micra 24, 107--129. Kelly D. H. (1972) Adaptation effects on spatio-temporal sine wave thresholds. .Vision Res. 12, 89-101. Mason S. J. and Zimmerman H. J. (1960) Electronic Circuirs. Signals and Sysrems. Wiley. New York. Robson J. G. (1966) Spatial and temporal contrast sensitivity functions of the visual system. J. opr. Sac. ,-lm. 56, 1141-l 142. Tulunay-Keesey U. and Jones R. M. (1976) The effect of micromovements of the eye and exposure duration on contrast sensitivity. Vision Res. 16, 481-488.
MILKER TULUNAY-KEESEY
Department of Ophthalmology,
University of Wisconsin Madison, Wisconsin 53706. U.S.A.
(Received 13 February 1978; in
revised jbrm
27
November
1978)
Abstract-Spatial sinusoidal gratings were viewed under both stabilized and unstabilized viewing conditions. They were presented either with a gradual or a sharp onset. The results indicate that contrast sensitivity to a range of frequencies centered around 2-3 deg is decreased by a maximum of 0.3 log units when high frequency temporal stimulation induced either by the sharp onset of the spatial pattern or by the motion of the retinal image is minimized. Data are presented which suggest that under conditions of minimal temporal stimulation. if fading of the stabilized image is also allowed, sensitivity may be decreased by a larger amount.
INTRODUCTION
and flickering sinusoidal gratings have clarified the interaction between the spatial and temporal variables of the stimulus in determining contrast sensitivity (Robson, 1966; Kelly, 1972; Kelly, 1977). It has been shown, for example, that counterphase flicker of 8 Hz or lower rates serves to increase sensitivity to spatial frequencies lower than the frequency which yields peak sensitivity; the higher spatial frequencies seem to be immune from the effects of such temporal modulation. Counterphase flicker of higher rates renders contrast sensitivity equal across the low spatial frequencies, as well as affecting a reduction of sensitivity to ah spatial frequencies. Because the range of temporal frequencies that is most effective in changing sensitivity to low spatial frequencies is comparable to the range of frequencies exhibited by normal eye movements which occur during fixation, it is reasonable to expect that temporal modulation caused by these movements determines the shape of the contrast sensitivity curve when it is obtained with a stationary pattern. For example, one class of models concerned with spati*temporal interactions assumes that the motion of the retinal image of the target and its spatial frequency interact in such a way as to boost sensitivity to the midddle range of spatial frequencies, and predicts equal sensitivity for the low spatial frequency range in the absence of retinal image motion (e.g. Arend, 1976). This expectation was tested by several investigators by the method of image stabilization which renders the image motionless with respect to the retina (see Tulunay-Keesey and Jones, 1976 for a review). One of these studies showed that threshold contrast for the detection of a grating depended on the exposure duration of the target and not on the presence or absence of image motion during the exposure. Maximum sensitivity was achieved for a 3 c/deg grating, for example, with an exposure duration of IOOmsec or longer whether the image was stabilized or not. Attenuation of sensitivity for lower spatial frequencies under either viewing condition was more prominent for exposures longer than 1OOmsec. Exposures as
Studies with stationary
767
long as 4sec. a reasonable period of tixation during which the whole range of normal image motion can occur, failed to reveal any differences between the stabilized and unstabilized contrast thresholds (TulunayKeesey and Jones, 1976). In the same study, it was demonstrated that if the target was viewed for an indefinite period of time, sensitivity was reduced for both the stabilized and the unstabilized target. In addition, the stabilized thresholds were found to be somewhat higher, by about 0.2 log units, than the unstabilized. The shape of the contrast sensitivity curve remained unchanged. It was judged therefore, that if there was an effect of image motion on contrast sensitivity, it was small (Tulunay-Keesey and Jones, 1976). It may be argued that the lack of difference between the stabilized and unstabilized thresholds for each spatial frequency when the targets were presented for short, finite ‘durations is due to the form of the temporal waveform. There is evidence (Breitmeyer and Julezs, 1975) attesting to the importance of stimulus onset waveform even under unstabilized conditions. In the Tulunay-Keesey and Jones study, gratings appeared suddenly upon the evenly illuminated oscilloscope screen whose luminance was matched to the space average of the target. The sharp onset of such flashed gratings may have produced luminance variations of high temporal frequency and large enough amplitude to mask or equal the effect of the temporal changes resulting from image motion. When gratings were presented for an indefinite duration the method of adjustment was used for setting thresholds instead of the staircase method which was employed when gratings were flashed. The method of adjustment required that the observer search for threshold starting from zero contrast, or an evenly illuminated screen, by manipulating a control knob. This method introduced a temporal modulation of indeterminate waveform and, in addition, it allowed local adaptation through allowing for long periods of inspection. Therefore, elevation of thresholds evidenced by this method, in relation to the thresholds obtained with flashed targets of finite duration, is likely to be due to both the lack of sharp transients and to local
adaptation. %rnilarly, the rclativelv htgh thresholds obtained with stabilized gratings with the method of adjustment may well reflect both the effects of lack of image motion and the well know-n phenomenon of stabtlized viewing, that of gradual image disappearance which cccompanies long periods of inspection. Kelly (1977) reports differences as large as 1 log unit between unstabilized and stabilized contrast thresholds obtained by the method of adjustment. It is clear that if image stabilization is to be used in studying the effects of eye movements on contrast sensitivity tt is desirable to parcel the effects of image fading and onset transients as well as the effect of image motion. In this paper w’e report our efforts in this direction. We presented targets of long but finite duration with a slow onset under both the stabilized and unstabilized viewing conditions. We compared the thresholds obtained with such a gradual onset with the thresholds obtained when the stimulus. stabilized or unstabilized. was presented with a sharp onset. This provided an estimate of the relative effects of image motion and onset transients. The method of staircase was used to secure thresholds. In order to gain an insight into the effect of image fading on contrast sensitivity. we obtained contrast thresholds with the method of adjustment under conditions when fading was allowed, and compared them with thresholds obtained under stabilized and unstabilized conditions where minimum disappearance was permitted. The design of this experiment also allowed comparisons between the two methods of threshold acquisition, staircase and adjustment. Finally, in this paper we report data from an earlier study with the intent of gauging the range of temporal frequencies for obtaining optimal contrast sensitivity. 3lETHODS General Vertically oriented sinusoidal gratings were generated on an oscilloscope screen. Intensity distribution in the horizontal direction was given by Lo = 1 + m cos 2 r$x where Lo is the average luminance, m is modulation, and fx is the spatial frequency. Lo was kept constant at 5 ml. and contrast could be varied up to 70’?/, with low distortion. The contrast could either be held constant, turned on and off. increased and decreased in a ramp fashion or varied sinusoidally as a function of time. Contrast for a given
spatial
frequency
was expressed
as
:iewmg condition when the image was motionless on the retina and under the unstabilized condition. when it was allowed to move normally; each stimulus was presented either with a ramp (trapezoidal waveform) or step onset (rectangular waveform). Hence there were four experimental conditions altogether. In the trapezoidal mode, contrast was increased linearly irom 0 to predetermind level over 2.5 sec. and held at that level for 5 sec. It was subsequently decreased to 0 within 25sec. In the rectangular mode, contrast was increased from zero to a predetermined level instantaneously and held there for 7.5 sec. The time-integrated contrast for the stimulus in the two modes of presentation for any one contrast the spatial frequency was therefore equal. To examine the extent :o which the trapezoidal waveform reduces the high frequency content of the pulse. we looked at the spectrum of both the rectangular and the trapezoidal pulse. Calculations were made according to the formula given in Mason and Zimmerman (1960): C’(w)
=
h +
sin[(b + 4),2]w
sin[(b - u)/2]~
[(b + 0)i2]w
[fb - a,/2]w
4
1
where 4 = 3.75. h = 3.75 in the case oi the rectangular. and a = 2.5. b = 5 in the case of the trapezoidal waveforms. The second term in the equation gives the ratio of the amplitude in the trapezoid to the amplitude in the rectangular pulse. These values are plotted in Fig. I. At 0 Hz. the d.c. level. both waveforms have the same amplitude and the ratio is 1. At 0.2 Hz. the trapezoid has ag proximately 60:; of the amplitude of the rectangular waveform. At I Hz. the trapezoid has less than IS”;, of the amplitude of the rectangle, and the ratio decreases as the frequency increases. We see that the amplitude of the higher temporal frequencies is reduced significantly by using the slower onset of the trapezoidal waveform. .A modified method of staircase was used to obtain thresholds in the main part of the experiment. Each session started with a few minutes of adaptation to the evenly illuminated screen. Thresholds were obtained for a few spatial frequencies which were viewed under either the stabilized or the unstabilized conditions and which were presented either with a ramp or a step. The contrast was increased by a given amount (j-20% of the previous setting)
12
09
I
1
C = I,,, - L,./
+ L,,,. The oscilloscope was viewed at a distance of L lyccm. and the display subtended 3.2’ vertically and 4’ horizontally. Target contrast was varied with a IO-turn linear potentiometer. The gratings were viewed through a previously described apparatus (Jones er al., 1972: Jones and Tulunay-Keesey. 1975) which provided either unstabilized or stabilized conditions. Stabilization accuracy was such that the maximum residual image movement in 1Osec of viewing was well below I’ arc peak-to-peak, for all subjects. An unstabilized fixation dot was provided for both viewing conditions, and the subject was instructed to maintain fixation under the unstabilized conditions. Experimental design: main experimenr Contrast thresholds were obtained for ten spatial frequencies ranging from 0.5 to l2c/deg for one subject and for nine spatial frequencies ranging from 0.37 to I8 c/d% for the other. Each grating was viewed under the stabilized
-0.3[ -20
’
’ - 1.2
h
’ 0.4 -0.4 0 Frequency Hz
1.2
1 20
Fig. 1. Ratio of the amplitude as a function of temporal frequency in the ramp (trapezoidal waveform) to the step (rectangular waveform) onset of the stimulus at a given contrast. For the ramp onset. the contrast was increased linearly from 0 to a predetermined level in 2.5 sec. It stayed at that level for 5 sec. before returning to 0 over a 2.5 set period. For the rectangular waveform, the contrast was introduced instantaneously. It had a duration of 7.5 sec. For any one contrast at OH& the d.c. level, both waveforms have the same amplitude and the ratio is 1. At 1 Hz the trapezoid has less than lSoi, of the amplitude of the rectangle. at 1.8 Hz, less than 7.5’,,. etc.
Stimulus onset and image motion on contrsastsensitivity every time the observer made a negative response and decreased by the same amount when she made two Consefutive correct responses, The mean of at least four such sequences was taken as the threshold. The procedure was repeated untif. all the spatial frequencies were viewed under each of the four experimental conditions. Each stimulus presentation was preceded by approx 2 see by an auditory signal, and between each stimulus presentation. the observer readapted to &, for a minimum of Ssec Two observers. one experienced (UTK) and one new farB1 to both psychophysics and stabilized image viewing participated in the main study. B)B was subjected to lengthy training sessions.The experiment was repeated three times for both subjects, the average of all three thresholds are presented for LTK and the average of the last two repetitions are shown for 818.
Contrast thresholds were obtained by the method of adjustment on a restricted number of spatial frequencies to determine if stabilized and unstabilized thresholds could be brought closer or driven apart by the way the stimulus is introduced or the instructions given to the subject. In one set of instructions. the observer was asked to reach threshold as quickly as possible by turning a knob that controlled the contrast levek fn a second set of in-
structions
the observer
was asked to go slowly. Two
observers participated in this part of the experiment. A third set of instructions was designed specifically to
incorporate stabilized image disappearance into threshold settings. The observer was asked to wait after the initial threshold determination until the threshold grating disappeared. and to search slowly for another “threshold”. He!
she was encouraged to go through as many iterations as necessary to reach a level of contrast that represented a steady “threshold”. Three observers. BIB. RMJ. and UTK. were available for this study. In each case rhe target was introduced at zero contrast. that is. the observer started searching for threshold from &. and after threshold was reached he turned the control knob so that the screen was again evenly iiluminated. In between threshold settings the observer adapted to L,,. In other words. there were RO transients due to the sharp onset of the stimulus: the only temporal modulation was produced by the subject’s adjustment of the contrast, In yet another version of the method of adjustment. the observer was asked to reach threshold slowly. but after rhe final threshold setting. the contrast was not decreased
to zero. The next frequency *as presented with a sharp onset as the contrast of the previous setting. Thus during the period of threshofd setting a mixed-mode of WtIpOrd~ changes occurred.
RESL’LTS AND DISCUSSION
Effects of image rnorion cs onser of the target A comparison of contrast thresholds obtained with the ramp onset under the stabilized and the unstabilized viewing conditions shoutd give an indication of the effect of image motion alone on contrast sensitivity. in Fig. 2. we present contrast sensitivity functions obtained with such stimuli for both subjects. It appears that for both subjects the presence of image motion serves to increase sensitivity to spatial contrast. When the data for both subjects are considered together the sensitivity enhancing effect of image motion seems to be most pronounced for frequencies between I.2 and 4.5 c/deg. There is a difference between the data of the two subjects: UK‘S results would indicate that image motion may continue to enhance sensitivity to frequencies lower than 0.5 Hz. and that it may serve to decrease sensitivity to frequencies higher than 6c/deg. BIB’s data, on the ocher hand, would indicate that image motion has no effect in increasing sensitivity to frequencies lower than 0.5 Hz. and that it continues to serve as a sensitivity increasing mechanism for frequencies higher than 6 cjdeg, finally losing effectiveness at the highest frequency, 18c/deg. used in this study. A comparison of thresholds obtained under the stabilized conditions with the ramp and the step onsets should indicate the effects of transients resulting from the onset of the stimulus. Figure 3 represents contrast sensitivity curves obtained under these conditions for both subjects. It would appear that the sudden onset of the grating pattern, the farge amplitude, high frequency temporal luminance changes resulting from it has a similar effect on contrast sensitivity as do the motions of the image: thresholds to spatial frequencies ranging from O.jc.‘deg (0.375 for WE) to about 9c,‘deg are
30 a3
1
3
IO
1
3
IO
SPATtAt WQUENCV tc/dagcI)
Fig. 2. Effect of image motion: threshold contrast to detect spatial sinusoidal gratings when the target
presented with a ramp. The open circtes and broken lines designate stabilized. the solid circles and continuous lines unstabilized viewing. Staircase method was used for fhreshold acquistion. Vertical bars show 4 1S.E. was
769
770
~LKER
TLLLXAY-KEESEY
SPAtM
and
BECKY J. BENNIS
WfQUWCY
Wdeg)
Fig. 3. Etfect of stimulus onset: Threshold contrast under stabilized conditions. The open circles and broken lines designate the ramp onset (2.5 set on. 5 set steady. 2.5 set 0m of stimulus presentation. the solid circles and continuous lines the step onset (7.5sec onl. Staircase method was used. Vertical bars show 5 I S.E.
lower in presence of onset transients. The maximum effect appears to be centered around 2-3 cjdeg. In Fig. 4 we show contrast thresholds obtained with a target presented in the rectangular-wave mode with a step onset. It was viewed under the stabilized and unstabilized conditions. For both subjects the stabilized and unstabilized thresholds appear to be very similar. These results suggest that the transients resulting from the onset of the target determine the level of contrast at which a spatial pattern is detected but whether or not the image moves within the period of inspection is inconsequential. Yet, as suggested in Fig. 2, if onset transients are not available, the motion of the retinal image appears to regulate sensitivity. The question therefore arises of whether or not these two sources of transients are interchangeable. The best indication of equivalence can be found in a comparison of thresholds obtained with a stimulus viewed under the unstabilized conditions where the onset transients are minimized and the main effective factor
SPATIAL
is image motion, with the thresholds obtained with a stabilized stimulus when the effective transients are mainly those caused by the onset of the stimulus. Figure 5 supplies this comparison. While BJB‘S data would indicate that transients instigated by the norma! motion of the retinal image and the onset of the target are equivalent, UTK’S data would suggest that onset transients weigh more heavily in determining contrast thresholds. The following conclusions can be derived from the common points of the data presented. In a normal period of voluntary fixation of a given duration, sensitivity to spatial contrast is determined by the transients resulting from the onset of the target. The additional transients which result from the motion of the retinal image that take place during.the inspection of the target have little effect in setting sensitivity. On the other hand, if the onset transients are minimixed, it is found that image motion increases sensitivity to a large range of spatial frequencies. The
RKXENCY
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Fig. 3. Threshold contrast obtained with a target presented with step onset and offset. Duration 7.5 sec. Open circles, broken lines depict stabilized; solid circules. continuous lines. unstabilized viewing conditions. Staircase method was used. Error bars for BIB are not shown because of the proximity of data points.
771
Stimulus onset and image motion on contrast sensitivity lr
IC
x
--I
0.3
1
3
I SPATIAL fUEQUWCY tcldegj 10
I”
J
Fig. 5. Contrast thresholds obtained with the ramp onset (2.5 set on. 5 set steady. 2.5 sec. otTI under unstabilized viewing conditions are designated by continuous lines and solid circles; thresholds obtained with a step onset (7.5 set duration) under stabilized conditions are represented by open circles and broken lines. Staircase method was used. + 1S.E.‘is shown for each data point. middle range of frequencies centering around 2.5 c/deg are especially influenced by temporal modulation resulting from either source. While the com-
bined effect of image motion and onset transients is not greater than the effect of onset transients alone, whether these transients are interchangeable in regulating contrast sensitivity is not clear. Effect of image disappearance method
The results are presented in Figs 6 and 7. The bottom line in Fig. 6 represents stabilized contrast thresholds obtained when the observer was instructed to search for the threshold slowly. The observer increased the contrast if at the initial threshold setting the stabilized target disappeared. This procedure took
and the psychophysics!
Figure 2 indicates that elevation of contrast sensitivity due to image motion, at best, is about 0.2-0.3 log units. Kelly (1977) on the other hand, reports that lack of image motion can drive thresholds down more than 1 log unit for l-2 c/deg and about 0.6 log units for very low (0.2 c/deg) and high (12 c/deg) frequencies. Kelly also presents data which suggest that the psychophysical methods of adjustment and forced choice produce similar measures of contrast sensitivity. Our data (Tulunay-Keesey and Jones, 1976) suggest that the method of adjustment yields the higher thresholds. In an attempt to resolve the differences between Kelly’s and our data we attempted a series of experiments to produce conditions which would yield large differences between stabilized and unstabilized viewing, and small differences between psychophysical methods of threshold estimat ion. First we assured that image stabilization achieved by Kelly’s and our apparatus was of comparable accuracy. We obtained the contrast value for a number of stabilized and unstabilized gratings which produced an after-image when the stimulus was replaced by an evenly illuminated field after 5 set of viewing. We found that a stabilized grating of 20 c/deg at Zoo,/, contrast produced an after image. This indicated a stabilization accuracy of better than 1.5’ arc, and compared well with Kelly’s (1977) data which showed that a stabilized grating of 12 at looo/, contrast produced an after-image. As pointed out in the methods section, when we measured stabilization accuracy by tracking position of an after-image in relation to a real-light stabilized image, we found it to be better than 1’ arc (Jones and Tulunay-Keesey, 1975).
f
I
I
1
I
g f
3-
g u
I
I
I
I
3
IO
I
Spatial frequency(c/de@ Fig. 6. A comparison of methodology and stimulus onset: Contrast thresholds obtained under stabilized viewing conditions. Solid circles and continuous lines depict thresholds obtained by a method of adjustment under instructions to search for the threshold slowly. If the image were to disappear, the observer was to find a new threshold. The con-
trast is brought to 0 aRer final adjustment. The dotted lines and solid triangles represent contrast values reached by the method of adjustment when the observer was asked to set the threshold quickly. The contrast was brought to 0 after each threshold adjustment. Open squares represent thresholds obtained under the stabilized conditions again by a method of adjustment. The difference is in the mode of stimulus onset. the target is presented with a step onset at an initially high contrast (see text). Unstabilized conditions: Open circles and continuous lines represent contrast values obtained by a method of adjustment when the target was presented wtth a step onset at an initially high contrast. The x’s represent thresholds obtained by the staircase method when the target was presented with a ramp onset.
772
ULKER TLLLNAY-KEESEY
dnd
BECKY J. BENWS
Spatial Frequancy tc/degl
Fig. 7. Effect of image fading: Open circles and dotted lines depict contrast thresholds for stabilized gratings. The subject was specifically instructed to wait for image disappearance and subsequently search for a further “threshold” until a steady threshold was reached. Solid circles and continuous lines represent unstabilized thresholds. Method of adjustment was used.
a minimum of 15sec. The middle dotted line represents stabilized thresholds obtained under instructions which asked for a quick decision. They represented the “initial” contrast setting for detecting a stabilized grating. This procedure took 5-15sec. Without exception the thresholds obtained with the “quick” method were lower than thsoe obtained with the “slow” method. It would appear that the method of adjustment, because it aIIows for temporal frequencies that vary according to the observers’ rate of adjustment, and because it allows for image disappearance that varies according to the time spent inspecting the target, also allows for a wide range of criteria.
The upper solid line in Fig. 6 again represents data obtained with the method of adjustment, but under the unstabilized condition. In opposition to the procedure followed with the stabilized gratings, the contrast was not reduced to zero after each threshold setting, but the new target was introduced with a sharp onset at the contrast value of the previous setting. In other words, this method contained both the sharp transients due to the onset of the ‘pattern and image motion, and aIso temporal variations due to the observers’ manipulating of the contrast. It yielded the lowest’thresholds of all. The stabilized thresholds resulting from this mixed-mode of transients fell on the middle dotted line. They are represented by open
Stimulus onset and image motion on contrast sensitivity
773
of the mammalian retina. Our data reported presquares. On this graph we also plotted the thresholds viously (Tulunay-Keesey and Jones, 1976) and the obtained with the ramp onset by the star&se method present data reaflirm that this neural mechanism is under the unstabilized conditions. These points, not dependent on image motion, but that the temrepresented by the symbol x1 are close to the upper poral variations supplied by them or the stimulus line. This provides a comparison between the method onset modifies the effect of these interactions in a of adjustment and staircase, and suggests that when quantitative manner. the method of adjustment allows for sharp onset transients, thresholds can be brought closer to those Effect of Picker obtained by the staircase method. The data we have presented in conjunction with The important point of Fig. 6 is that stabilized and unstabilized thresholds obtained with the method of a set of unpublished data allow us to speculate on adjustment can be driven close or apart chiefly the range of temporal frequencies necessary for maximum contrast sensitivity. according to the instructions given to the observer. A comparison of the spectrum analysis of the trapeThis is not entirely due to the intrinsic inadequecy zoidal and the rectangular waveforms for a given conof the psychophysical method itself, but partly due to the changes that the visual system undergoes when trast value shows that the two waveforms contain the retina is confronted with a motionless image fcr comparable amplitudes at the very low temporal frelong periods of time. This notion is strengthened by quencies below 1 Hz. and that the rectangular waveform has considerably more amplitude at frequencies the data shown in Fig. 7. above 1 Hz (see Fig. 1). As Fig. 3 shows, the rectanguThey were obtained according to a set of instruclar tiaveform, in the absence of image motion. protions which incorporated image disappearance. The vides the lower of the thresholds for spatial frequenobservers was asked to wait, under the stabilized concies lower than 8 c/deg. It would be expected therefore ditions. for the threshold target to fade and subsequently to search for another “threshold”. He/she was that, for the low and the middle range of spatial frequencies, temporal changes of a frequency of 1 Hz to repeat this procedure as many times as necessary. The target at the initial setting did disappear very or higher would be needed to increase sensitivity to quickly, but subsequent “threshold” targets took in- levels above those obtained with a stabilized image. A set of data previously obtained in this laboratory creasingly longer times to fade. According to the subjects, the “final threshold” represented a level of conconfirm this notion. Contrast thresholds were set by trast which “looked as if it would not fade easily”. the method of adjustment for stabilized sine wave This procedure involving multiple settings took a gratings of 0.75, 3 and 7.5 c;deg, which were flickered in a temporal sine wave fashion, as well as for steady minimum of 45 sec. It can be seen in Fig. 7 that at 3 c/deg, image fading stabilized gratings of the same frequency. The results yielded thresholds approx 1 log unit higher than the for the 0.75 and the 7.5 c;deg gratings are shown in thresholds obtained when the image moved normally Fig. 8. The data points for the 3 c/deg target are not on the retina. The differences between the stabilized plotted: they follow closely the curve for the and unstabilized thresholds were larger for the lower 0.75c/deg target, with the exception that the threshspatial frequencies and smaller (about 0.5-0.3 log old for the steady grating designated as 0 Hz is at units at 8 c/deg) for the higher ones. Another feature ‘3% contrast. of these curves is that when image fading is allowed, It is seen that sensitivity for the low frequency gratthe frequency at which highest sensitivity is achieved ing of 0.75 c/deg increases close to.five times (1.7 times is displaced toward a higher spatial frequency. This for the 3 c/deg) when it is flickered in counterphase is perhaps indicative of the “steady state” of the at 1 and 2 Hz. Flicker starts to loose its effectiveness system. at about 4Hz for the 0.75 and the 3c/deg target. If image disappearance is indeed an indication of However, even the higher rates of flicker are effective the “‘steady state” of the system responsible for conin increasing sensitivity above the levels obtained with trast sensitivity and if steady state is under question, the steady grating. Kelly (1977) reports a similar it is perhaps best to use a method which could allow image disappearance and subsequent readjustment of thresholds. If the question under study is the role of Iimage motion due to the movements of the eye which take place during a normal period of fixation, as it was in this study, then it seems to us best to study contraSt sensitivity with a method that can separate the effect of eye movements from the effect of target disappearance in determining sensitivity. It should be noted that even if the mction of the image that takes place during a restricted period of inspection supplies a boost of sensitivity to a middle range of spatial frequencies, the absence of such image I I I I I motion does not render equal sensitivity across the $” 03 I 3 IO low and middle frequency range under any condition. Tun~rol traqrancy(Hz) This invariance of the shape of the contrast sensitivity Fig. 8. Contrast thresholds for stabilized gratings of 0.75 curve suggests that low frequency attenuation is sub- (open circles) and 7.5 (solid circles) c,‘deg as a function served by a mechanism of spatial interactions. such of frequency of counter-phase flicker. 0 Hz depicts when as center surround antagonism of the ganglion cells me gratmg was steady. - _
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GLKERTLLCNAY-KEESEY and BECKYJ. BE>\IS
trend. For the higher spatial frequency of 7.5 c!deg. on the other hand. flicker up to 4 Hz does not increase sensitivity to a level higher than that obtained with the steady stabilized grating: in fact. the higher temporal frequencies of 8 and 10 Hz result in a decrease of sensitivity. This agrees with the data shown in Figs 2 and 3 which indicate that the presence of image motion or the rectangular waveform containing the higher temporal frequencies results in a decrease of sensitivity for the higher spatial frequencies belou the levels obtained with the ramp wageform under the stabilized conditions when presumably the prominent temporal frequency is very low. around 0.2 Hz. The efficiency of temporal changes with frequencies lower than 1 Hz in determining contrast sensitivity remains a question however. As the advantage of the rectangular waveform in increasing sensitivity is lost for spatial frequencies higher than about 8 c/deg, and since the ramp and the step provide comparable amplitudes at the very low temporal frequencies, we may be justified in concluding that the high spatial frequencies benefit as much or more from the very low temporal frequencies alone as they do from a combination of the low and high temporal frequencies. It should be pointed out that these very low temporal frequencies are within the same range that would be supplied by drifting the high spatial frequency targets at velocities comparable to the normal drift motion of the eye. If our reasoning is correct. than the large differences between the stabilized and unstabilized measures of sensitivity (Figs 6 and 7) found with the method of adjustment for the high spatial frequencies can be ascribed to an absence of the very low temporal frequencies as well as the long term effects of image stabilization. CONCLUSIONS
The main conclusions of this set of studies are as follows : (1) The shape of the contrast sensitivity curve is invariant, i.e. low spatial frequency attenuation is still present under conditions wherein the high frequency transients, resulting from image motion or stimulus
onset, are minimized. This suggests a spatial rather than a temporal neural interaction as the mechanism [or contrast sensitivitv. (2) In a normal period of fixation. the effect of high frequency temporal changes resulting from either image motion or stimulus onsef is to increase sensitivity by a small amount. (3) In the absence of image motion. thresholds can be driven upward, sensitivity lowered. primarily by allowing for long periods of fixation and image disappearance. This magnifies the differences between stabilized and unstabilized thresholds. rlc~no~ledyemenrs-This research was supported by an NIH grant EYO0308. We thank Drs L. A. Ripgs and J. 0. Limb for their comments on the first draft of the manuscript and Dr Elmer Johnson who kindly supplied the limbal seating contact lenses necessary for image stabilization. Secretarial help of Miss Jan Livingstone and Miss Josephine Anderson is Qratefully acknowledged.
REFERENCES
Arend L. E. (1976) Temporal determinants of the form of the spatial contrast threshold MTF. C%ion Res. 16. 1035-1042. Breitmeyer B. and Julesz B. 11975) The role of on and ofi transients in determining the physchophysical spatial frequency response. Msion Res. 15, 411-416. Jones R. M. and Tulunay-Keesey U. (1975) Accuracy of image stabilization by an optical-electronic feedback system. Vision Res. 15, 57-61. Jones R. M.. Webster J. G. and Keesey U. T. (1972) An active feedback system for stabilizing visual images. IEEE Pans B,ME-19 pp. 2%33. Kelly D. H. (1977) Visual Contrast Sensitivity. Oprica Micra 24, 107--129. Kelly D. H. (1972) Adaptation effects on spatio-temporal sine wave thresholds. .Vision Res. 12, 89-101. Mason S. J. and Zimmerman H. J. (1960) Electronic Circuirs. Signals and Sysrems. Wiley. New York. Robson J. G. (1966) Spatial and temporal contrast sensitivity functions of the visual system. J. opr. Sac. ,-lm. 56, 1141-l 142. Tulunay-Keesey U. and Jones R. M. (1976) The effect of micromovements of the eye and exposure duration on contrast sensitivity. Vision Res. 16, 481-488.