IHI 1 | MI 145 110 TH _ A“? i‘fi‘éESY'EfiééW 33“ {3F ‘ N... J‘fi- L‘J‘mfi 85:731‘. E538? £33330! 3‘3 (355 3%?» "“3‘ n3:- MW :E‘i‘i ff? GE‘EWEQ‘ ESE :‘A‘CP‘E‘ T1333 33 f3 3 ‘3‘ {3:33; M33 "3 3:5. .3. 3:33.:Z3‘332333‘ S‘ii‘fiE 3..."? ETY 33:13.53? 33:5 $33323 39:..- “£ 3}“ "‘2! a L 1 u h A K Y Michigan State University ’ q in 2-515 ABSTRACT AN INVESTIGATION OF THE INFLUENCE OF THREE MATCHING INSTRUCTIONS ON SUBFUSIONAL INTERMITTENT BRIGHTNESS by Jonathan David Kazsuk In the past little attention has been paid to the question of instructions relating to the subjective brightness matching of sub- fusional intermittent photic targets to a steady photic target in a psychophysical situation. There are several qualitatively differ- ent sensory end-results over a range of photic intermittency con- ditions ranging from OFF to isolated flashes of light. This in- vestigation was carried out in an attempt to gain some knowledge concerning the influence of three instructions fbr making brightness matches. Two trained observers were instructed to attend to three aspects of subfusional photic intermittency. They were the on phase, the off phase, and a combination of the on and off phases. Pulse lengths were held constant while subfusional intermittency rates were man- ipulated from conditions very close to critical flicker frequency (high frequencies) to conditions of single flashes of light. The influence of matching instructions on subjective brightness matches was shown to increase as the intermittency rates were de- creased. Also, a sensory end-result shift was quantitatively deter- 2 Jonathan David Kazsuk mined within the range of intermittency rates used. Approved: %w,~tfiq \ Date: M‘ql‘iblL AN INVESTIGATION OF THE INFLUENCE OF THREE MATCHING INSTRUCTIONS ON SUBFUSIONAL INTERMITTENT BRIGHTNESS BY Jonathan David Kazsuk A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF ARTS Department of Psychology 1967 /<;£?fi. _~’ ’ 0 .r . I . I“ I.’ .~ , ' I ., I k/ t , ‘ v A CKIIOW LEDGELEK TS I am thankful to Dr. 8. Howard Bartley for his personal in- terest in the conception and propagation of this thesis. For the use of his laboratory facilities and for suggestion and guidance during my program I am deeply indebted. Drs. Bakan and Henley, who served on my committee, were help- ful With suggestions and critical inquiries. Dr. R. J. Ball has through personal interest and technical assistance been an inspiration and a teacher. Dr. C. W. Schneider contributed much to this student's exposure to psychology and gave many hours of his time and concern to make the collection of data possible. My wife Kay deserves the deepest appreciation in return fer her encouragement and support throughout the creation of this thesis. A National Institute of Mental Health fellowship has provided funds and made it possible to concentrate on this thesis and a graduate program. ii I INTRODUCTION . . . I I IVETHOD o o o o o 0 Apparatus Observers Procedure III RESULTS . . . IV DISCUSSION . . . . V SUM'IARYOOOOOO REFERENCES . . . . TABLE OF CONTENTS iii \O\O\l \J 12 10. 11. 12. LIST OF FIGURES Schematic diagram of‘the apparatus . . . . . . . Schematic diagram of the steady and intermittent targets . . . . . . . . . . . . . . . . . . . . Subjective er JK With Subjective er CS with Subjective er JK with Subjective er CS with Subjective er JK with Subjective er CS with Brightness brightness the 20 m3. brightness the 20 ms. brightness the #0 ms. brightness the 40 ms. brightness the 60 ms. brightness matches pulse . matches pulse . matches pulse . matches pulse . matches pulse . matches obtained by observ- obtained by observ- obtained by observ- obtained by observ- obtained by observ- obtained by observ- the 60 ms. pulse. . . . . . . . . . differences between on-matches and off-matches for observer JK . Brightness differences between on-matches and ofTLmatches fer observer CS . Comparison of the ofTBmatch zero brightness point for observers JK.and CS . . . . . . . . . Averaged brightness differences between instruc- tions fbr observer JK . . . . Averaged brightness differences between instruc- tions fer observer CS . . . . iv Page 8 8 1h 15 16 17 18 24 INTRODUCTION During the 1950's Bartley and BishOp (1955a, 1955b) became in- terested in investigating some of the temporal dimensions of visual activity. Using the rabbit as their preparation fer gross neuro- physiological recordings in the optic cortex, they removed the animal's eye and stimulated the stump of the optic nerve With direct intermittent electrical shocks. By manipulating the temporal aspects of the electrical shocks, they found that when the bursts of electri- cal stimulation were spaced 1/5 of'a second apart that all small, all large, or all medium sized responses were elicitable in the cortex. Bartley (1956) continued the neurophysiological work with the rabbit's eye intact and found that photic stimulation to the eye showed the same five per second cortical rhythmicity. From these findings Bartley formulated the alternation of re- sponse interpretation of neurophysiological coding in the visual pathway (Bartley, 1961). The essential core meaning of this inter- pretation is based on the above findings of intrinsic periodicity within the visual modality of the rabbit which seems to provide the neural basis for handling photic and electrical stimulation in a prescribed way. If the photic stimulation is timed to the intrinsic periodicity of the sensory processes, then the maximum cortical re- sponse will result. It is further assumed that when the cortical re- sponse is at its maximum that the sensory end-result of brightness perception in human observers would be at its maximum. Bartley (1958) then shifted his experimental endeavors, using temporal stimulation in the visual modality, to human observers in a psychophysical task and discovered what he called brightness en- hancement. Brightness enhancement is a subjective phenomenon which occurs in certain psychophysical situations when the subjective brightness of an intermittent photic target exceeds the subjective brightness of a steady photic target of the same intensity. In his original findings Bartley (1958) reported that brightness enhancement reached a peak at about 8-10 cycles per second (cps) when decreasing stimulus intermittency rates (SIR's) from critical flicker frequency (OFF). Bartley did not use low SIR‘s (below 3.6 cps) and used a target configuration with the steady and intermit- tent portions somewhat separated. However, since then brightness enhancement has been reported for SIR'e from 2 to 20 cps, and it is not empirically known what aspects of the stimulus complex were most influential in producing these variations. Some of the stim- ulus dimensions which seem to influence the intermittency rates at which brightness enhancement has been found are listed below. 1. Pulse to cycle fraction (abbreviated PCB and also called light-dark ratio). 2. Photic intensity. 5. The spacial dimentions of the targets. 4. Binocular or monocular viewing. u, 9 Systematic studies are needed to investigate the influence played by each of these stimulus dimensions. The conclusion that peak brightness enhancement does not seem to be locked onto any particular rate range should be noted at this point. While in rabbits the peak brightness enhancement may be locked onto 5 cps, experiments showing behavioral data to confirm it have not been reported. It has been shown that brightness en- hancement in the human observer is a complex phenomenological pheno- menon. This brings us to some questions on the nature of the sen- sory end-result itself. Ranney (1964) has reported six levels of qualitative sensory end-results for human observers viewing a range of intermittency rates from CFF to discrete periods of light and dark flashes. The six levels of phenomenology were fusion, flutter, ripple, fine flicker, coarse flicker, and flash. Thus the same sensory end- results are not obtained at all frequencies below CFF. As the rate of intermittency is decreased from high to low, the appearance of light and dark phases becomes more pronounced. In accordance with Banney's qualitative categories there is a certain rate range at which a sensory end-result is perceived as being a series of dis- crete light and dark periods. While making brightness matches Valsi, Bartley, and Bourassa (1959) realized a sensory shift from fluctuations of lighter and darker phases of the intermittency with some brightness always present, to discrete light and dark phases. They considered this sensory shift to involve two very different brightness phenomena and recommended that the rates of intermittency corresponding to these two categories of sensory end-results should not be put on graph without designating their differences. When the SIR is above OFF, the sensory end-result is one of uniform steady brightness. Slightly lower rates introduce what is called flicker. This flicker can be described as a slight brightness fluctuation, but since it is only slight the main characteristic of the field is a steady component of brightness which can be matched to any steady field adjacent or near it. As rates are lowered still further the appearance of brightness fluctuation increases until there is no steady component to be seen. This shift proceeds as rates are decreased until each light pulse is quite discretely sep- arated from the pulse just preceeding and just fellowing it. The separation between pulses may be made great enough so that there is no detectable influence of one pulse on another. when this is the case it can be said that a series of isolated pulses is involved. The supposition which follows is that making brightness matches of a series of light and dark periods to a steady target is a difh ferent phenomenon than making brightness matches between a steady target and a fluctuating target containing a brightness component at all times. With the exception of Colgan (1965) no researcher has reported 5 giving specific instructions to the observers making intermittent brightness matches. Colgan varied the rate of photic intermittency using Sylvania glow modulator tubes and red filters (612-668mu). The intensity was about 2.2 q/ft.2, and the results were reported as showing an increase in brightness enhancement from 20 ops until a rate of 2 Cps was reached. A fairly large sample of naive subjects were given instructions for making brightness matches. One group was instructed to match to the on phase of the intermittency, an- other to the average of the on and the off phase, and a third group was told nothing. Colgan reports that the brightness enhancement was greatest for the group instructed to match to the on phase of the intermittency. Schneider and Bartley (1966) manipulated the rate of intermit- tency from 20 to 1 cps and reported a gradual increase in bright- ness from 20 cps to 1 cps with the greatest rise in brightness being from 3 cps to 1 cps. The observer's instructions fer making a match were implicit and were such that an observer (this author was one of two observers) was to match the steady photic source to an "average" of the intermittent target brightness. The study by Schneider and Bartley was done because it was sus- pected that observers were shifting their brightness match criterian over the range of intermittency rates producing the various sensory end-results. To measure this, standard deviations were calculated for each of two observers for the 40 brightness matches during four sessions. Schneider and Bartley report an increase in standard deviations for the rates from 6 cps to 5 cps. when the observer perceives the intermittent stimulus as a pronounced fluctuation of brightness and is required to match this to steady brightness, he has several alternatives to consider. He can match the light phase of the intermittency to the steady, the dark phase to the steady, or he can try to achieve a match to the "mean" of the fluctuating brightness. This is not a mean in the mathematical sense of the word for the observer is not making a direct observation but is using a criterian which seems to be appro- priate. Hence, whatever the response is, it must not be treated strictly as an observation, 1. e. as a sensation in and of itself but rather as a derivative. When the flicker is not pronounced, the alternatives are essentially reduced to making a match on the basis of‘a predominant steady component. In order to learn more about the quantitative brightness effects related to the three alternatives listed above, two questions are asked. One, what effect does matching according to the three alter- natives have on brightness matches over SIR's from CFF to discrete flashes? Two, at what intermittency condition does the sensory end- result shift from an experience with a brightness component present at all times to an eXperience of discrete flashes? METHOD Apparatus. The apparatus is shown diagrammatically in Figure 1. The photic sources 51 and 82 are Sylvania glow modulator tubes (R 1151 C). S1 is the steady photic source and is driven electron- ically by a Grasen-Stadler flicker-stion apparatus. 52 is the in- termittent source and is driven electronically by a modified Grass 8-4. W1 is a calibrated neutral density wedge which controls the percentage of photic transmission passing to the observer's eye. H1 could be moved by a rack and pinion device by turning the dial D. SZ is a stationary balance wedge. The scale numbers on H1 were used for recording a brightness match. These numbers were then converted to percentage of trans- mission and were accurate to within .1 per cent. RS is a reduction screen preventing any extraneous light from reaching the observer. LH is a light housing keeping the photic energy of S1 and 82 sep- arate. FF is a piece of filter paper used to make the two photic sources 81 and 82 appear uniferm. F is a permanent .4 neutral density filter over the 52 intermittent source. I and S represent the position of the intermittent and steady target images to the observer 0. The observer was located 80 cm. from the target which is drawn in Figure 2. The diameter of I and S is 1 cm. which subtends 45' of are for the observer. The 5 cm. between the I and 8 targets subtends an arc of 20 9'. The overall target configuration is 5 cm. high and subtends an arc of 50 55'. 7 <::) F LH Grass 8-4 H! F gisz <::) I §g’ [:::}_S1 Grason O (observer) (::i- Stadler (position) D I W1 Fig. 1. Schematic diagram of the apparatus showing the arrange- ment of light sources S1 and 82, neutral density wedges W1 and W2, the reduction screen, the dial for the wedge, and the appearance of the intermittent and steady targets. Fig. 2. Schematic diagram of the steady and intermittent targets as they specially appear in actual dimensions. ‘lll') 1 cm. 3cm. {‘IEI’ 1 cm. Observers. Two male graduate students including this author served as the observers. Each had many hours of eXperience serving in subfusional intermittent brightness matching experiments. Procedure. Under all conditions, the observer using monocular vision matched the brightness of the steady target to the perceived brightness of the intermittent target (point of subjective equality). The intensity of the intermittent target was held constant at 1.8 c/ft.2 The current on both the steady and intermittent glow tubes was monitored and held constant at 25.ma. The intensity of the steady target ranged from 5.8 c/ft.2 to .1 c/ft.2 depending on the position of the neutral density wedge. The neutral density wedge controlled percentage of transmission of the steady photic energy to the observer so that the current to the glow tube could be held constant. The luminance of the glow tubes was nearly achromatic. Befere collecting data each observer practiced the instructions fer several weeks in order to become familiar with the task. The on—match instructional set meant that the observer attended to and attempted to match to the on phase of the intermittency. The mean-match instructional set meant that the observer attended to and attempted to match to a derivative of all brightness aspects of the intermittency. The off match instructional set meant that the observer attended to and attempted to match to the off phase of the intermittency. A total of 5060 observations were recorded for each observer. (1) Three instructional sets were used. (2) Three pulse lengths 10 were used. Using a pulse length of 20 ms the frequencies used were 45, 25, 20, 15, 12, 10, 8, 6, 4, 2, 1, .75, and .50 cps. Using a pulse length of 40 ms the frequencies used were 20, 15, 12, 10, 8, 6, 4, 2, 1, .75, and .50 cps. And, using a pulse length of 60 ms the frequencies used were 15, 12, 10, 8, 6, 4, 2, 1, .75, and .50 cps. (5) Thirty observations at each condition were collected over 5 sessions. In addition to the 5060 intermittent observations each observer made a total of 540 brightness matches with both targets at steady luminance. Ten steady observations were recorded at the beginning of each session and an additional 10 were recorded at the end of each session. One pulse length with corresponding SIR's presented in random order was used over three sessions fer the three instructional sets befbre the next pulse length was used. For each condition in each session, each observer made 5 ascending and 5 descending matches in alternate fashion. Ten minutes for adapting to the target were given before each session. All values obtained with the neutral density wedge were con- verted into the percentage of transmission (P of T) required for a match. The mean of 50 observations made under a particular con- dition was used as the percentage of transmission required to make the intermittent and steady targets look equally bright. The mean of the 540 steady matches was recorded as the P of T of the steady 11 source necessary to make the two photic targets look equally bright when the intermittent photic target was made steady without changing its intensity. RESULTS The mean percentage of transmission over 5 sessions for each condition (5 pulse lengths With corresponding SIR's and 5 instruc- tional sets) and fer each subject to indicate a measurement of the point of subjective brightness equality are shown in Figures 5-8. The general observations from these figures pertinent to this in— vestigation are listed below. 1. The instruction variable results in great brightness dif- ferences between the on—matches and the offematches for the low SIR's. That brightness differences tend to decrease as SIR's are increased is shown in figures 9 and 10. 2. The off-match curve drops to zero brightness around 1 or 2 cps. As the pulse length increases the steepness of drop to the zero point increases fer both observers. 5. The mean-match curves tend to fall closer to the on-match curves than to the off-match curves. 4. For the 40 and 60 ms pulses, the lower the SIR the higher the on and mean-match curves. These curves rise above the steady curve at about 10 cps and tend to increase in height as the SIR's decrease. This area above the steady line is the brightness en- hancement area. For the 20 ms pulse the shape of the on and mean- match curves tend to resemble a shallow U. The highest and lowest SIR's show similar brightness levels. No substantial brightness enhancement was shown fer this pulse length. 12 15 Fig. 5. Subjective brightness matches obtained by observer JK With the 20 ms. pulse. OBSERVER: JK ON-NATCR INSTRUCTIONS A MEAN-MATCH INSTRUCTIONS c) OFF-MATCH INSTRUCTIONS Q 20 Is PULSE LENGTH STEADY MATCH .. — — .— 48 .. t I. .\ ,2 56 n. a Ii 0 A A 24 .. '. i—g‘, . - ’9 9 u 6: '5 12 v ‘ ‘3 O (L A v v .50 .75 1 2 4 6 8 1O 12 15 2O 25 45 CYCLES PER SECOND .VII 11‘») ."YI‘. I, H. . ‘55)} 56 241 12 « Fig. 4. 14 Subjective brightness matches obtained by observer CS with the 20 ms. pulse. OBSERVER: CS ON-MATCH INSTRUCTIONS £3 LEAN-MATCH INSTRUCTIONS [3 CYCLES PER SECOND OFF-MATCH INSTRUCTIONS 0 20 NS PULSE LENGTH STEADY MATCH ......_ A A g A c ‘3 5 a u a 4‘ ., a A , C u =\ V.’ ‘ h- ‘7: u ‘ " = ’0‘ 55—53 I) e - e e N e o o .50 .7 1 2 4 6 8 10 12 15 20 25 £15 15 Fig. 5. Subjective brightness matches obtained by observer JK With the 40 ms. pulse. OBSERVER: JK ON-MATCH INSTRUCTIONS A MEAN-MATCH INSTNJCTIONS C] 72... OFF—MATCH INSTRUCTIONS o . to Is PULSE LENGTH STEADY MATCH — _ ._ .— 60...... 48-... 56-41- ~ 24 12mT O A m ' V V )5 .75 1 2 4 6 a 10 12 15 20 CYCLES PER SECOND 16 Fig. 6. Subjective brightness matches obtained by observer CS with the 40 ms. pulse. OBSERVER: CS ON-MATCH INSTRUCTIONS A: MEAN-MATCH INSTFUCTIONS a OFF-MATCH INSTRUCTIONS o 40 MS PULSE LENGTH smwrmmR--—— & -I1> 48 -1 56 '0 24.0 12 + O ~‘(D V 2 4 6 8 10 12 15 2O CYCLES PER SECOND '80 51,0 Fig. 7. 17 Subjective brightness matches obtained by observer JK with the 60 ms. pulse. 78- OBSERVER: JK A ON—HATCH INSTRUCTIONS A 72.. LEAN-HATCH INSTRUCTIONS C] “ OFF-MATCH INSTRUCTIONS O 60 NS PULSE LENGTH STEADY MATCH - - — — 60. A A A 482 T o. , ————————————— -. ’a - a. — -- — — - A... —. .- .— u \ 5 a I 56__1 _;__ _ I: - ‘s- 24“ 12_ O I} A g .56 .775 Y 2’ 4 6 8 1o 12 15 CYCLES PER SECOND 18 Fig. 8. Subjective brightness matches obtained by observer CS with the 60 ms. pulse. OBSERVER: CS ON-HATCH INSTRUCTIONS A MEAN-MATCH INSTRUCTIONS [:1 OPP-HATCH INSTRUCTIONS o 60 MS PULSE LENGTH .. STEADY HATCH ._ _. _. ._ A A A V .5’ 35 1" 2 1+ 6 8 10 12 15 CYCLES PER SECOND 19 Fig. 9. Brightness differences between on—matches and off- matches for observer JK With each pulse length. 78... OBSERVER: JK 72 A ON—MA‘I‘CH - OFF-MATCH 60 MS A " .\ ON—MATCH - OFF-MATCH 40 MS a ON—MATCH - OFF—MATCH 20 MS 0 I'.‘ :1 L‘- 60+» 48.» c a 56 1f. 0 “’7 24.. 12 t. 9 :1 L: ‘A \a Q}————£D O A A‘. .50 .75 1 2 1+ 6 8 1O 12 15 20 2 CYCLES PER SECOND 72 12 20 Fig. 10. Brightness differences between on-matches and offb matches fbr observer CS with each pulse length. OBSERVER: CS .. ON—MATCH - OFF—MATCH 60 NS A ON-MATCH - OFF-MATCH 40 MS [3 A ON-MATCH - OFF-MATCH 20 MS 0 lb A A C . i‘ n c -7 (v 0 5 q. . U I a: B .50 .75 1 2 4 6 8 1O 12 15 2O 25 E5 CYCLES PER SECOHD 21 5. The off- match curves are highest for the high SIR'S. The off-match curves for the 60 ms pulse are flatter at those SIR's above the drop-off than for the 20 and 40 ms pulses. Figure 11 compares the two observers with regards to the in- fluence of pulse length and SIR on the zero point of the offhmatch curves. The two observers agree at the 20 and 60 ms pulse condi- tions, but the zero point for JK‘was 1 cps lower than the zero point for CS at the 40 ms pulse length condition. It can be noticed from Figure 5 that JK took only 11 P of T to make an offbmatch at 2 cps which is the zero point for observer CS. Figures 12 and 15 compare the mean differences between the on- matches and the mean-matches with the differences between the mean- matches and the offhmatches by averaging over SIR's for each pulse length used. In all cases except observer JK at the 60 ms pulse condition, the mean-match is on the average closer to the on-match results. Observer CS had the least difference at this 60 ms pulse length condition. However, mean-matches were not made for the SIR's. below the drop-off because of task difficulty. Table 1 shows that the on-match instructions always resulted in higher brightness values for both observers than the mean-match or offhmatch instructions when averages are taken over SIR'S. This result is in agreement with Colgan's result (1965). It should be noted, however, that in the investigation being reported here brightness values for mean-matches were in some cases higher than Fig. 11. Comparison of the off-match zero brightness point for observers JK and CS. OBSERVER: JK I OBSERVER: CS 1:] JD 20 40 60 PULSE LENGTH “'TCHES [1.11. C‘.‘ 1') h BRTGHTK 2A-. 18~~ 12» Fig. 15. 24 Brightness differences between on-matches and mean-matches compared with brightness differences between mean-matches and off- matches. Each graphic point represents averaging over all SIR's for a given pulse length and instruction before computing the difference. This figure is for obser— ver CS. ON-MATCH - WAN-MATCH I MEAN-MATCH - OFF-MATCH a 20 40 60 PULSE LENGTH Table 1. Averaged brightness comparisons between instruc- tions over all SIR's at a given pulse length. Subject Pulse Length On-match Mean Mean-match Mean Difference in ms. Over Rates PofT Over Rates PofT PofT CS 20 28.27 25.49 4.78 CS 40 44.78 59.69 5.09 CS 60* 44.29 40.92 5.57 Subject Pulse Length Mean-match Mean Offhmatch Mean Difference in ms. Over Rates PofT Over Rates PofT PofT cs 20 25.49 12.50 11.19 CS 40 59.69 14.94 24.75 08 60* 40.92 52.20 8.72 Subject Pulse Length On-match Mean Mean—match Mean Difference in ms. Over Rates PofT Over Rates PofT PofT JK 20 29.48 27.69 1.79 JK 60* 44.02 59.41 4.61 Subject - Phlse Length Mean-match Mean Offhmatch Mean Difference in ms. Over Rates PofT Over Rates PofT PofT JK 20 27.69 14.07 15.62 JK 40 46.82 14.86 51.96 JK 60* 59041 59076 "oij * Mean-matches for these stimulus intermittency rates below the drop-off zero point were not made fer this pulse length. See text for explanation. 25 the on—matches. At the SIR'S below the drop-off, mean-matches were considered by both observers to be extremely difficult if not impossible. For the 20 and 40 ms pulse lengths an attempt was made to make mean-matches in order to determine somewhat of an idea where these matches would fall With regards to the on and offbmatches. Figures 5-6 show the mean-matches made at these SIR's to be closely aligned with the on-match curves. . This task was considered too difficult at the 60 ms pulse length conditions, and each Observer felt he could match only to the ex- treme on or off phases of the intermittency. DISCUSSION The data Show in Figures 9 and 10 that brightness differences between on-matches and offematches tend to increase as SIR is de- creased. As the SIR is decreased while holding pulse length constant, the null period between each pulse is increased by some known amount. Thus, as the null period is increased from OFF for any of the three pulse lengths used in this study the need to specify the criterian for making brightness matches becomes more necessary. In Figures 9 and 10 we can see that brightness differences between the two instructions is not the same fbr each pulse length. Figures 5-8 show that the 40 and 60 ms pulse lengths produce greater on-match brightness than the 20 ms pulse at low SIR's. This explains the position of the curves in Figures 9 and 10 at the low SIR's. The position effects at higher SIR's in these figures cannot be empirically explained at this time. The conclusion from these data is that pulse length as well as null period effects brightness match differences. This study produced data clearly showing two categories of sensory end-results. Too little attention has been paid to this sensory shift. Studies dealing with the brightness of a series of discrete light and dark periods have a different task for the Obser- vers to perform than studies dealing with subfhsional intermittency where there is a brightness component present at all times. The series of discrete light and dark periods produces a sensory end- 26 27 result characterized by abrupt changes of light and dark. The next experimental step is to create studies which will systematically assay temporal, intensity, and spacial variables within each of these sensory categories in order to ferret out similarities and differences of brightness within the two categories. This would mean finding the zero brightness point of the offhmatch curve for a given set of conditions. The findings here, that there is a general increase in bright- ness as SIR is decreased from 20 cps does not agree with Hartley's Q L-L' ‘3‘- ' (1957) original finding that brightness enhancement reached a peak at 8-10 cycles per second and then started to decline as rates were lowered to 5.6 cps. Bartley, however, was working with intensity levels around 100 c/ft.2- much different than the 1.8 c/ft.2 inten- sity level used in this study. Bartley (1951) showed that with a very low intensity level of‘.OO7 c/ft.2 that brightness increased as rate decreased down to 5.6 cps. The indication is that intensity is a crucial variable in determining the shape of a brightness curve over a wide range of'rates. These findings tally with the neurophysiological studies and the alternation of response theory that brightness enhancement may be absent with low levels of stimulation, because not all channels in the optic pathway are involved in such a way as to produce sufh ficient channel synchrony. 28 The practice in the past of not specifying the brightness match- ing criterian seems to leave much room fer improvement. At the low SIR's a naive observer could match anywhere within this range if only told to "match two targets in brightness." While this author would not eXpect him to make an on-match, he also would not eXpect an untrained observer to make an off-match. Instead, what the naive observer seems to do as evidenced through casual observation is to vacillate around somewhere between the on phase and off phase of the intermittency. This type of matching has been called making an average or mean-match. Attempts to make mean-matches in this study showed that in almost all cases the mean-matches were closer to the on-matches than to the offematches even though both observers felt that they were dividing the brightness of the on phase and off phase down the middle. As the SIR's get lower the difficulty of making a mean-match increases according to the verbal reports of the two observers. ‘\ The recommendation according to these findings is that observers be instructed to make brightness matches to some criterian specified by the experimenter rather than being told nothing or simply to match the brightness of two targets. It is the opinion of this author that if brightnesses are to be compared over a wide range of inter- mittency rates, the on phase of the intermittency should be the basis for making a brightness match, but at the same time the nature of the sensory end -result being dealt with should be specified. 817M351 BY In the past little attention has been paid to the question of instructions relating to the subjective brightness matching of sub- fusional intermittent photic targets to a steady photic target in a psychophysioal situation. There are several qualitatively dif- ferent sensory end-results over a range of photic intermittency conditions ranging from CFF to isolated flashes of light. This in— vestigation was carried out in an attempt to gain some knowledge concerning the influence of three instructions for making brightness matches. Two trained observers were instructed to attend to three as- pects of subfusional photic intermittency. They were the on phase, the off phase, and a combination of the on and off phases. Pulse lengths were held constant while subfusional intermittency rates were manipulated from conditions very close to critical flicker frequency (high frequencies) to conditions of single flashes of light. The influence of matching instructions on subjective bright- ness matches was shown to increase as the intermittency rates were decreased. Also, a sensory end-result shift was quantitatively determined within the range of intermittency rates used. 29 REFERENCES Bartley, S. H., and Bishop, G. H. Factors determining the ferm of the electrical response from the optic cortex of the rabbit. AEEE‘.§° Physiol., 1955a, 19;, 175-184. Bartley, S. H. and Bishop, G. .. The cortical response to stim- ulation of the optic nerve in the rabbit. Ageg. i. Physiol., 1955b, 105, 175-184. Bartley, S. H. Temporal and spatial summation of extrinsic impulses with the intrinsic activity of the cortex. 3. cell. comp. 'Puflgle 1956, §, 41-620 Bartley, S. H. Subjective brightness in relation to flash rate and light-dark ratio. g. EXper. Psychol., 1958, 22, 515—519. Bartley, S. H. Intermittent photic stimulation at marginal inten- sity levels. '9. Psychol., 1951, 52, 217-225. Bartley, S. H. A clarification of some of the procedures and con- cepts involved in dealing with the optic pathway. In R. Jung and H. Kornhuber (Eds.), The Visual System: Neurophysiology and Psychgphysics. Heidelburg: Springer-Verlag, 1961. Colgan, C. M. The effect of observational techniques on brightness enhancement. §m3£. g. Psychol. 1965, 28, No. 5, 471-475. Banney, J. E. Perceptions of subfusional photic intermittencies various intensities and pulse-to-cycle fractions. Unpublished ioctoral dissertation, Michigan State University, East Lansing, l-‘Ii chi gan , 1964 . Schneider, C.W., and Bartley, S. H. Changes in sensory phenomena and observer criteria at low rates of intermittent photic stimulation. if 3: Psvchol., 1966, 65, 55-66. Valsi, E., Bartley, S. H., and Bourassa, C., Farther manipulation of brightness enhancement. .2' Psychol., 1959, 48, 47-55.