1382 J. Opt. Soc. Am. A/Vol. 17, No. 8 /August 2000 Holger Knau

Thresholds for detecting slowly changingGanzfeld luminances

Holger Knau

Institute of Biophysics and Radiation Biology and Department of Biology, University of Freiburg, Germany

Received October 14, 1999; accepted April 4, 2000; revised manuscript received April 18, 2000

Detection thresholds for luminance increments or decrements are normally related to rapid light changes.The goal of this study was to determine detection thresholds for slowly changing achromatic Ganzfeld lumi-nances before and after adaptation to a constant Ganzfeld illumination, subsequently called Ganzfeld adapta-tion. During Ganzfeld adaptation, perceived brightness decreased slowly and leveled off (on average after 5–7min), despite constant illumination of the retina. The state of adaptation was characterized by using magni-tude estimation. Comparing detection thresholds for changing light intensities before and after Ganzfeld ad-aptation showed that the sensitivity for luminance changes is independent of the perceived brightness. Afurther issue addressed was the time dependence of the luminance change. Is there a limit below which achange of luminance is no longer perceivable? Even for the slowest gradient tested (0.01 log/min), subjectswere able to detect the change of luminance, although they were not able to perceive a continuous brightnesschange. Similar thresholds (ca. 0.24 log unit) for shallow and steep luminance gradients suggest an absoluteluminance detection mechanism. Possible underlying mechanisms and neurophysiological substrates are dis-cussed. © 2000 Optical Society of America [S0740-3232(00)00608-6]

OCIS codes: 330.5510, 330.7320, 330.1880.

1. INTRODUCTIONIn 1804, Troxler1 reported that during strict fixation, aconstant peripherally viewed test spot of low contrast be-came embedded into the background of the visual fieldand was thus invisible. This phenomenon is called theTroxler effect2,3 and was attributed to local adaptation.4–6

Local adaptation is observable when the same retinal re-gion is exposed to a prolonged constant stimulation. Thefaded image of Troxler’s effect is revived by eye blinks orsmall involuntary saccades, because of the temporal in-terruption or the shift of the stimulus to an unadaptedregion.4,7 During Ganzfeld adaptation, where only a ho-mogeneous and uniform achromatic brightness is seen,the perceived brightness soon declines rapidly and thenlevels off at a brightness perception near, but alwaysabove, the Eigengrau.8 (The term Eigengrau is definedas the residual brightness perceived in an absolutely darkroom after dark adaptation.) This kind of adaptationshould not be confused with the better known photo-chemical dark and light adaptation, wherein the sensitiv-ity of the eye changes as a function of the prevailing lu-minance.

This study addresses the question of whether thethreshold for detecting a slowly changing Ganzfeld illumi-nation is influenced by the state of Ganzfeld adaptation(i.e., whether the threshold is related to the actual bright-ness perception or to the initially steady adapting illumi-nation of the Ganzfeld). Burkhardt9 and Sparrock10 in-vestigated the effect of retinal stabilized images onincrement thresholds. During stabilization, the bright-ness of the stabilized stimulus decreased, but in bothstudies the flash increment thresholds were the samewith or without stabilization. This result suggests that

0740-3232/2000/081382-06$15.00 ©

the sensitivity for increment thresholds depends only onthe number of quanta falling on the retina and not on theperceived brightness. Yarbus11 and Waygood12 furtherclaimed from their studies that luminance changes shouldbe undetectable for the visual system if the rate of changeis small enough. This assertion was tested in the presentstudy by measuring luminance increment and decrementthresholds with use of very slow rates of luminancechange, starting at two different adaptation states. Oneadaptation state was associated with a reduced bright-ness perception due to Ganzfeld adaptation (procedure 1),and the other was associated with no brightness fading(i.e., with a brightness perception equivalent to the pre-vailing Ganzfeld illumination) (procedure 2). The resultsprovide tests of three working hypotheses: (a) Detectionthresholds after Ganzfeld adaptation (procedure 1) wouldbe increased compared with the condition without bright-ness fading (procedure 2) because of a global attenuationof the visual system during brightness fading; (b) thethresholds for procedure 1 would be decreased comparedwith the procedure 2 condition because of a reduced noiselevel in the system; or (c) the threshold would be un-changed for both conditions, as found by stabilized-imageexperiments.9,10

2. MATERIALS AND METHODSThe experimental Ganzfeld consisted of halved ping-pongballs, which were fitted over both of the subject’s eyes.The subject (wearing the ping-pong balls) looked into astyrofoam sphere 0.8 m in diameter that was illuminatedwith a daylightlike spectrum from a 450-W xenon arclamp. The technique used approximated that of an Ul-bricht sphere, ensuring that the light was diffusely re-

2000 Optical Society of America

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flected from the inner surface of the styrofoam sphere toevery part of the ping-pong balls and from there into theeyes. Before the experiment, pupils were dilated with amydriatic (Mydriaticum Roche, Grenzach-Whylen, Ger-many) to eliminate changes of retinal illumination due tochanges in pupillary diameter. Luminance of the innersurface of the ping-pong balls was measured with a cali-brated photometer (Graseby Optronics, Orlando, Fla.).The homogeneity of the Ganzfeld was confirmed by usinga calibrated spectroradiometer (Photo Research, Chat-sworth, Calif.). To control the level of illumination, thelight from the lamp was collimated and passed through awater bath plus IR and UV filters and then focused onto apair of counterrotating, computer-controlled neutral-density wedges (Reynards Corp., San Clemente, Calif.)mounted on stepping motors. Thereafter the light wasrecollimated and projected onto a milky diffusing glass(Opalglasscheibe, 0.4-mm thick, Spindler and Hoyer, Got-tingen, Germany), situated above the subject’s head,which admitted the light beam into the styrofoam sphere.The wedges were calibrated and checked for smooth lumi-nance changes with the calibrated photometer.

The observers steadied themselves on a chin/headrestto keep their head positions constant. To ensure thatthey relied exclusively on their brightness perception,they were kept unaware of the nature, as well as the be-ginning and end, of the experiment. In addition, ear-phones were used with white noise to mask any auditorycues that might reveal manipulations of the stimulus bythe experimenter. This procedure also helped to keep at-tention at a steady, passive level. To control for innerstate, we monitored the pulse frequency of the subjectswith an arm collar. The verbal reports by the subjectswere continuously recorded on tape.

A. SubjectsFour subjects participated in this study: two females,NB and GG (26 and 31 years), and two males, OK andHW (31 and 64 years). All had normal vision except forHW, who was presbyopic. Before the actual experiment,subjects participated in several training sessions to famil-iarize themselves with the stimulus situation and the pro-cedure. Following these practice sessions, their perfor-mance was found to be stable and reliable. In separatetests, they were also dark adapted for 30 min to experi-ence their individual Eigengraus.

B. Procedure 1Subjects were adapted to a constant level of Ganzfeld il-lumination. The time course of relative brightnesschange during adaptation was quantified by means ofmagnitude estimation.13 The subject’s task was to pro-vide a running report of any change of brightness, includ-ing qualitative observations. Since we were interested inthe relative brightness loss and not absolute brightnessjudgments, subjects quantified perceived brightness at ar-bitrary intervals by assigning it a value between 100 (ini-tial brightness) and 5 (Eigengrau). Values below 5 werereserved for a still greater darkness, such as the blacksensation seen when the eyes were closed for several sec-onds or when the light was suddenly switched off.14,15

Before each run, subjects were preadapted for 20 min to aroom luminance comparable with that to which they weresubsequently exposed in the Ganzfeld. Preadaptationwas conducted without the halved ping-pong balls andwith free viewing. This served to neutralize the effects ofindividual light history and to provide an initial bright-ness sensation (i.e., before fading) appropriate to the con-stant adapting luminance used in the experiment.

Test luminances of the Ganzfeld ranged from 0.01 to100 cd/m2. Zero to 4 min after reaching the final state ofadaptation [i.e., constant brightness perception for atleast 5 min (for details, see Ref. 8)], the experimenter in-creased or decreased the Ganzfeld luminance using thecomputer-controlled neutral-density wedges. This wasperformed with either 0.1 or 1.0 log/min. For adaptingluminances of 0.1 and 10 cd/m2, an additional gradient of0.01 log/min was used, a rate that according to Yarbus11

and Waygood12 would be too slow to be detectable.The subjects were not informed about the aim of the

study (i.e., they did not know whether the brightness al-teration was due to physiological or physical changes).The threshold criterion was the first signal of a brighteror darker sensation followed by two other observations inthe same direction. The threshold was the difference be-tween the logarithm of the initial adapting luminance (Ii)and the logarithm luminance where the change was de-tected (If).

C. Procedure 2During these sessions, thresholds were measured for thesame luminance slopes, but before brightness fading oc-curred. After a short, randomized period of 0 to 40 s, di-rectly after Ganzfeld stimulation, the luminance was al-tered in the same fashion as described under procedure 1.Figure 1 presents a schematic of the two experimentalprocedures. To exclude an intrusion of brightness fadingduring the second procedure, the subjects were told toclose and open their eyes repeatedly.

3. RESULTSFigure 2 shows the differences in luminance (log Ii2 log If) needed to detect the intensity change as a func-tion of the initial luminance (procedure 2) or the initialadapting luminance (procedure 1) for subject GG. Theplotted thresholds represent the mean values of at leastsix repetitions of each condition. The error bars corre-spond to the standard errors of the mean. For clarity,the error bars are shown only for two luminance gradi-ents. The thresholds are rather similar but show a smalldecrease in absolute value for higher adapting lumi-nances. Although the resulting detection thresholds arevery similar, the time needed to detect the change wasquite different owing to different luminance gradients.For example, the duration needed to detect luminancechanges during the shallowest gradient (0.01 log/min)ranged between 15 and 32 min instead of 5 to 25 s for the1.0-log/min gradient. The results from subject GG arerepresentative of the other subjects as well.

A statistical analysis of the thresholds revealed no sig-nificant differences (at the 1% level) between thresholdsmeasured with procedure 1 or procedure 2 (i.e., before or

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after Ganzfeld adaptation) for up or down slopes. Table1 shows the mean values for the detection thresholds foreach subject, as well as the results of a two-tailed t test,for independent samples.

The same statistical tests were also used to check theindependence of the thresholds from the direction of the

luminance gradient. Again, there was no significant dif-ference at the 1% level, as can be seen in Table 2.

Because there were no significant differences amongconditions, the results were averaged and subsequentlywill be referred to only as luminance change. The meandetection threshold was 0.24 log unit, which corresponds

Fig. 1. Schematic of procedures used for measuring detection thresholds of luminance changes after (left) and before (right) Ganzfeldadaptation.

Fig. 2. Thresholds for slowly changing Ganzfeld luminances as a function of the initial luminance for subject GG. Different symbolscorrespond to different luminance gradients. Filled symbols refer to thresholds without brightness fading (procedure 2), and open sym-bols refer to thresholds after Ganzfeld adaptation (procedure 1). Positive thresholds correspond to luminance decrements, negativethresholds to luminance increments.

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Table 1. Threshold Means and Standard Deviations for Luminance Changes Before and After GanzfeldAdaptation (i.e., Brightness Fading)

Thresholds Thresholdsp Value (t Test)

Thresholds Thresholdsp Value (t Test)for Increments for Increments

for Incrementfor Decrements for Decrements

for DecrementAfter Ganzfeld Before Ganzfeld Thresholds Before After Ganzfeld Before Ganzfeld Thresholds Before

Adaptation Adaptation and After Ganzfeld Adaptation Adaptation and After GanzfeldSubject (Procedure 1) (Procedure 2) Adaptationa (Procedure 1) (Procedure 2) Adaptationa

NB 0.2 6 0.18 0.26 6 0.1 0.93 0.22 6 0.11 0.2 6 0.15 0.77OK 0.25 6 0.14 0.17 6 0.11 0.16 0.20 6 0.12 0.19 6 0.08 0.85HW 0.25 6 0.14 0.18 6 0.11 0.23 0.43 6 0.25 0.3 6 0.14 0.18GG 0.26 6 0.14 0.2 6 0.08 0.24 0.25 6 0.12 0.19 6 0.08 0.30

a Based on a two-tailed, unpaired t test that compares the thresholds before and after Ganzfeld adaptation for luminance increments and decrements.

Table 2. Threshold Means and Standard Deviations for Up and Down Luminance Gradients

Subject

Mean Thresholds forIncrements Before and After

Ganzfeld Adaptation(Procedures 1 and 2)

Mean Thresholds forDecrements Before and After

Ganzfeld Adaptation(Procedures 1 and 2)

p Value (t Test) for MeanThresholds of Up and Down

Luminance Changesa

NB 0.26 6 0.14 0.21 6 0.13 0.27OK 0.21 6 0.13 0.19 6 0.10 0.69HW 0.21 6 0.13 0.36 6 0.21 0.02GG 0.23 6 0.12 0.22 6 0.10 0.79

a Based on a two-tailed unpaired t test between the mean thresholds of luminance increments measured before and after Ganzfeld adaptation and themean thresholds of luminance decrements measured before and after Ganzfeld adaptation.

to the amount of luminance change necessary to be de-tected under Ganzfeld conditions independently of per-ceived brightness (i.e., before and after Ganzfeld adapta-tion).

Surprisingly, the rate of change showed only minor ef-fects on the detection thresholds. Although subjects re-ported a different subjective perception of the differentlight gradients used, the resulting detection thresholdswere very similar. Once luminance change was detected,during the steepest gradient (1.0 log/min) subjects re-ported a dynamic impression, as if they could follow a fur-ther increase or decrease in brightness. This was not thecase for the shallower gradient (0.1 log/min). Here nochange was observed until the subject suddenly noticedthat it had become darker or brighter. But even then,the subjects were not able to keep track of the continuinggradient. Moreover, after awhile they reported that ithad become darker or brighter again. Even for the slow-est luminance gradient the detection threshold was aboutthe same as for the other gradients, although no subjectwas able to perceive a continuous change.

4. DISCUSSIONThe major finding of this study is that there was no sig-nificant difference between detection thresholds beforeand after Ganzfeld adaptation (i.e., independent of theperceived brightness). This result agrees with earlier re-sults of Burkhardt,9 Sparrock,10 and Cornsweet andTeller.16 Burkhardt9 described an experiment in which astabilized field served as the background for incrementthreshold measurements. The test spot was 20 min of

arc in diameter and was illuminated by 0.3-s flashes atdifferent times after the start of stabilization. Althoughthe background increased its darkness, the incrementthreshold remained constant. Sparrock10 also investi-gated the influence of stabilized retinal images on incre-ment thresholds. He did not find a measurable influenceon Weber’s law between the stabilized and the unstabi-lized conditions. Cornsweet and Teller16 presented a cir-cular field of 8.5 (deg) diameter, which was surrounded byan annulus. If the annulus had a higher intensity thanthe test spot, the test spot looked darker owing to simul-taneous contrast, although its luminance remained un-changed. Cornsweet and Teller measured the incrementthreshold for flashes that were superimposed onto thetest spot as a function of the perceived brightness of thetest spot. The authors found that the threshold is inde-pendent of the perceived brightness and depended only onthe retinal illuminance levels.

The present study extends these former results to slowgradients of luminance change (i.e., luminance increaseor decrease) without any spatial contrast information.This finding sheds some light on the Ganzfeld adaptationmechanism or, more generally, on the processing ofsteady and transient light signals. Sparrock10 had pre-viously proposed a model in which the visual system at-tenuates steady signals while allowing transient signalsto pass unattenuated. He concluded that the two mecha-nisms must be independent and are perhaps situated atdifferent locations. The results of this study are not en-tirely consistent with his conclusion.

The data presented here show that the steepness of theluminance gradient did not influence the detection

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thresholds, although the subjects were not able to followthe continuous luminance change. Even the slowest gra-dient of 0.01 log/min, which was claimed to be undetect-able by Yarbus11 and Waygood,12 had the same thresh-olds. This condition is analogous to the hour hand of aclock. One does not see the movement, but after awhile,one can note the change of position.

This observation suggests that under certain circum-stances the visual system is capable of detecting absoluteluminance differences. No spatial contrast or perceiv-able modulation in time is necessary for this ability. In-stead, a sort of internal reference for differentiating lightlevels might be utilized. Alternatively, detection thresh-old might be reached when a certain amount of luminancechange has accumulated. Thus under appropriate condi-tions the visual system seems to act like a steady stateand not like a transient detector.

A similar conclusion can be drawn from a study byGerrits.17 He described an experiment in which a sub-ject, after dark adaptation, was presented a stabilizedsubthreshold retinal image. Then the luminance wasslowly increased. The subject was not able to determinethe shape, size, color, or position of the stimulus becauseof the very slow luminance change. However, the subjectwas able to infer the attained luminance level from the fi-nally perceived brightness.

Schubert and Gilchrist18 tried to answer the question ofwhether, under Ganzfeld conditions, subjects would beable to perceive absolute luminance changes in additionto relative luminance changes. Thus they investigatedsubjects’ ability to perceive a very slow luminance change(0.045 log/min) that, according to Yarbus11 andWaygood,12 should not be detectable. Schubert and Gil-christ found that the luminance change was noticed afterapproximately 10 min. This duration corresponds to aluminance difference of 0.45 log unit. The authors con-cluded that the visual system is capable of discriminatingat least roughly different luminance levels. Since the de-tection threshold in their study was measured only oncein only one subject, a deviation of a factor 2 from themean detection threshold of this study seems not to becontradictory.

Minimal research can be found in the neurophysiologi-cal literature concerning slowly changing stimuli.Enroth-Cugell and Jones19 published extracellular re-cordings of cat ganglion cells stimulated by large expo-nentially changing lights. The spike rate showed a de-pendence on the speed of luminance change. Theshallower the luminance gradient, the lower the activityof the neurons. The gradients used by Enroth-Cugelland Jones were still fast (2 log/s) compared with the gra-dients used in this study. The findings that revealedlowered activity for shallower gradients supports the con-clusion that even slower changes would not further affectthe neuron activity. Perhaps in this case another mecha-nism, which is able to discriminate absolute luminancelevels, must be considered.

A similar conclusion can be drawn from the study ofTaeumer et al.20 They investigated the electrical mo-mentum of the human eye, which is influenced by lightstimuli. The momentum follows a changed light inten-sity with a damped oscillation. Taeumer et al. found

that the oscillation frequency of the dipole is dependenton the change of luminance but not on its luminancelevel. They reported that below 0.1 log unit/min, the os-cillation could no longer be observed. This finding sup-ports the results presented here, which demonstrate thatthe subjects were not able to perceive the continuouschange of brightness if a very shallow luminance gradientwas used.

In this study, however, the luminance change was no-ticed after a decrease or increase of 0.24 log unit was at-tained. Thus the model of Sparrock10 has to be extendedin the sense that the mechanism for steady signals, whileattenuating the brightness perception, is still capable ofdiscriminating between different amounts of light fallingon the retina. This raises the question, Which mecha-nism is responsible for detecting luminance changes witha slope steeper than those tested in this study and shal-lower than the often used 0.5-s flashes of much psycho-physical research? A systematic investigation of detec-tion thresholds as a function of the rate of luminancechange covering this region is still lacking. Such re-search could reveal a crossing point of the response of dif-ferent mechanisms. It would also be interesting to com-pare such a crossing point with measurements ofmodulation transfer functions, which show a clear cross-ing over between magnocellular and parvocellular signaltransmission when the modulation rate changes from fastto very slow flicker.21

ACKNOWLEDGMENTSThis study was conducted at the Brain Research Unit, De-partment of Biophysics and Radiation Biology and theDepartment of Biology, University of Freiburg, Germany.The work was supported by the German Research Councilunder grant SFB 325, B4. I thank L. Spillman for hissupport during the study and A. Gilchrist and J. S.Werner for scientific discussions.

Present address, Department of Ophthalmology, Uni-versity of California, Davis Medical Center, 4860 YStreet, Sacramento, CA 95817. E-mail, hkknau@ucdavis.edu.

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