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Disinhibition in metacontrast masking of vernier acuity targets: Sustained channels inhibit transient channels
DISINHIBITION IN METACONTRAST MASKING OF VERNIER ACUITY TARGETS: SUSTAINED CHANNELS INHIBIT TRANSIENT CHANNELS’ BRL’NO
G. BREITX.~EYER’
Neurologische Klinik mit Abteilung fur Neurophysiologie der Universitlt D-7800 Freiburg i. Br.. West Germany (R~rirrd
14 Ocroher 1977:
in rerised/orm
Hansastrasse 9.
28 Fehruory 1978)
Abstract-Fovea1
vernier resolution is impaired when a briefly presented target (T) is followed at varying stimulus onset asynchronies (SOA’s) by a spatially adjacent set of metacontrast (M) bars. Impairment of resolution IS optimal at intermediate SOA’s (50-60 msec). When two Ranking (F) bars are in turn placed adjacent to M and are presented either continuously throughout the T-M display sequence or else synchronously with T. a reduction of metacontrast etTects at the previously optimal SOA’s occurs. Such a reduction does not occur when F is presented synchronously with M. In the framework of current neurophvsiological interpretations of metacontrast. these results indicate that sustained channels inhibit transient channels.
I. INTRODUCTION
Recent neurophysiological interpretations of visual masking (Breitmeyer and Ganz, 1976) indicate that inhibitory interactions between transient and sustained neurons (Singer and Bedworth, 1973) can account for a variety of masking phenomena. In particular, according to several current models of metacontrast. the typical U-shaped function relating masking magnitude to stimulus onset asynchrony (SOA) between target and adjacent mask is taken as an indicator of inhibition of slow, target-activated sustained channels by fast, mask-activated transient channels (Breitmeyer and Ganz. 1976; Weisstein, Ozoc and Szoc 1975) or by fast transient channels activated by the target-mask sequence (Matin. 1975). The assumption of latency differences between transient and sustained channels is based on neurophysiological findings (Dow, 1974) as well as psychophysical findings (Breitmeyer, 1975; Lupp, Hauske and Wolf, 1976; Vassilev and Mitov, 1976). Besides establishing the existence of transient-onsustained neural inhibition, Singer and Bedworth’s (1973) study also tentatively pointed to a reciprocal sustained-on-transient inhibition. Exploiting fovea1 and parafoveal stroboscopic motion phenomena, von Griinau (1978) recently obtained results consistent with this latter form of inhibition. The present study ’ This research was carried out while the author was on leave of absence from the Department of Psychology, University of Houston, from July. 1976to July. 1977. during which time he was a fellow of the Alexander von Humboldt Foundation. Bonn-Bad Godesberg, West Germany. He thanks the Foundation for its generous support. Thanks also go to Lothar Spillmann who provided excellent research facilities and support. and to Anne Ransom who served as a subject. This research was supported by Deutsche Forschung Gemeinschaft SFB 70. ’ Reorint reauests should be sent to the author at the followmg address: Department of Psychology, University of Houston, Houston, TX 77004. U.S.A.
exploits systematic modifications of fovea1 metacontrast effects to demonstrate psychophysically not only transient-on-sustained inhibition but also the reciprocal sustained-on-transient inhibition. Fovea1 metacontrast generally has been found to be weaker and less robust than parafoveal metacontrast (Alpern. 1953: Kolers and Rosner. 1960: Saunders. 1977; Stewart and Purcell, 1970). This is most likely due to at least two factors. The first is that the concentration of sustained neurons relative to transient neurons is highest in the central part of the retina. and decreases towards the periphery (Fukuda and Stone, 1974; Gouras. 1968; Hoffmann. Stone and Sherman. 1972). The second is that the relative magnitude of the responses of sustained neurons increases as one approaches the center of the retina, whereas for transient neurons it mcreases as one moves to the peripheral retina (Cleland and Levick, 1974). These factors should lead not only to a decrease in the effective transient-on-sustained channel inhibition in the fovea, i.e. to weaker fovea1 metacontrast, but also to a maximal fovea1 sustained-ontransient inhibition. Exploiting this fact, the present study employs a fovea1 vernier resolution task which is known to yield U-shaped metacontrast functions when the briefly presented vernier target is followed by spatially adjacent masking bars (Westheimer and Hauske. 1975). To demonstrate sustained-on-transient inhibition. the following rationale is employed. The SOA or range of SOA’s at which optimal metacontrast masking occurs is assumed to be a measure of the latency difference between slow sustained channels activated by the vernier target and fast transient channels activated by the adjacent mask bars. Thus, at these SOA’s the fast transient channels activated by the delayed mask can maximally inhibit the slow sustained channels activated by the target. Such a situation would prevail if one restricted the stimulus display to a small vernier target fT’)and two larger adjacent mask bars (M) as shown in Fig. I. However. suppose now that
BRLXOG BREITXE>ER mg in 20 msec steps from S to I1Y mjzc. were used. -\I1 sttmuli were front-tllumlnated m a 4IodeI N-1000 Sclentllic Prototype tachlstoscope. and kere LIeued bmoculsrl) at a distance of I25 cm At the eqeplece of the tachlsroscope the luminance of the uhtte porttons of the vrsual dtspla) was I> ft-I..
Procedure
T
37.0 1
Ftg. I. Spattal Ia)-out of stimulus display. Dimensions are in mmutes of arc. T: target. M. mask. F: flank. See text
for other detalis. two additional Ranking bars (F) in turn are placed immediately adjacent to the mask bars. If the sustained activity generated by the vernier target. Thus wtth Ranking bars of appropriate spatio-temporal activit) generated by the mask bars, one should in turn obtain less inhibition, i.e. disinhibition of the sustained activity generated by the t’ernier target. Thus with Ranking bars of appropriate spatio-temporal relation to the target-mask stimulus sequence. so that sustained activity generated by the flanking bars can inhibit the transient activity generated by the mask bars, one should obtain decreases in masking of the vernier target at those target-mask SOA’s at which optimal target masking is otherwise obtained in absence of flanking bars. 2. GENERAL METHOD Subjects Two experienced psychophysical observers were used. One uias the author (BB). a 30-year-old male with normal vision: the other (AR), was a Z-year-old female research assoctate at Freiburg University with normal vision. AR
was unaware of the purposes of the experiment. Srllrluli and apparatus The stimuli used in this study are shown rn Fig. 1. The dimensions of the stimuli. where indicated, are in minutes of arc. Two coitnear vertical bars and two colinear horizontal bars formed an imaginary cross, the center of which the subjects were instructed to fixate during the experimental sessions. At this central location was presented one of two vernier targets: either the one as shown with the upper portion slightly shifted right relative to the lower one or else one with the upper part shifted an equai amount to the left relative to the lower portion. The width of the target bar segments was 3’. the vernier offset was 0.3’. All stimuli were drawn with black Indian ink on white index cards. The black-to-white contrast was approximately 0.9. For BB. target and mask bars were each presented for 5 msec; for AR, they were each presented for 8 msec. For both subjects. seven target-mask SOA’s vary-
Two ezperlments were run. In Experiment I. -hen the flanking bars Were used. the) here presented contmuousl> at one of t%o spatial separations relauce to the mask. In Expertment ?. the Ranking bars. when used. were presented either s)nchronouslq with the target or else wrth the mask bars In both expertments a total of three conditions were employed. In one condition only the tsr_ger-mask sequence was presented. in the other two condmons. flanking bars m specified spatial (Experiment I) and temporal (Expertmrnt 1) relation to the mask bars v.ere also presented. Data were collected over several Reeks: first for Expertment I. then for Expertment 1. Wlrhm a dzul> session. three ZY trzal blocks were run per subject. one bloch devoted to each of the three conditions. In each block. four trials were devoted to each of the seven SO.A’s. On two of the trials the vermer target with the upper portion displaced to the right aas used: on the remaining t*o maIs. the vernier target with the upper portion displaced to the l:ft. Wlthin a block of ZS tr!sls target and SO.4 uere cross-randomized. Also within a daily session. order of blocks was randomized. Trials were imtiated by the subject. After a trial the subject \tas required to respond either “right” or --left”, depending on whether he beiieked the upper portion of the vernier target to be displaced to the right or left. For each condition of Euperlment 1. 61 trials per SOA were run m 16 nonconsecutive daqs; for each condltton in Expertment f. 1S trials per SOA were run In I.! nonconsecuttve da>s. The proportion of response errors was taken as an index of the mazking effect with higher lfrror proportions correspondmg :c*greater maskmg effects 3. ELPERI%IEST I
In the current experiment. masking effects are measured in the absence of flanking bars (Condition 1) or in the presence of a stationary and continuousl> present set of flanking bars separated from the mask bars either by 4’ (Condition 2) or else by 25’ (Condition 3). In Condition I, one should expect to obtam the usual U-shaped metacontrast masking function with optimal masking at intermediate SOXs. The effect of the continuously presented and stationar? flanking bars should be primarii? to activate sustained channels (Enroth-Cugell and Robson. 1966: Kulikowski and Tolhurst. 1973; Tolhurst. 1975). Provided that. as in Condition 2, the spatial separation of flank and mask bars is sufficientI> small (4’). the inhibitory effect. if any. that this sustained channel activity has on the transient channel activity of the mask should produce less target inhibition, i.e. target disinhibition. This disinhibition should manifest itself In terms of a noticeable reduction in masking effects at intermediate target-mask SOA’s. However. if, as in Condition 3. the spatial separation between flank and mask bars is sufficiently large (20’). little. if an>. such disinhibit~on should be obtained. The results of Experiment 1 are shown separateI> for each subject and each condition in Fig. 2. It should be noted that Condition I (no flank) yields the typical U-shaped metacontrast function with optimal masking at SOA’s ranging from 40 to 85msec.
1403
Metacontrast masking of vernier acuity targets
1
o--o CONOiTlON1
2 --* CONDITION 3 o---q CONOITION
z .1 z z E
l
0,
0
LO 60 80 100 120 STIMLILUS ONSETASYNCHRONY(MSEC)
2G
Fig. 7. The proportion of incorrect identification of the vernier target as a function of SOA and experimental condition. Higher proportions correspond to greater masking.
However. in Condition 2. where the flanking bars were in relatively close spatial proximity to the mask bars. the masking effect is comparatively small in that range of SOA values. In fact. the effect of the nearby flank bars is not only to decrease masking at intermediate SOA’s but also to increase masking at the smallest SOA. Although the latter result is unexpected and somewhat puzzling the former result was expected on the hypothesis that sustained channels can inhibit transient channels. Under viewing conditions used in this experiment. the sustained-on-transient inhibition must be a relatively local mechanism since. compared to Condition 1, little, if any, disinhibition or reduction of masking at intermediate SOA’s was obtained in Condition 3, where the flanking bars were separated by 20’ from the mask bars. However, more precise and complete determinations of the spatial parameters affecting this disinhibition require further study.
sient channei response which is of shorter latency-by an amount equal to the optimal masking SOA-than the sustained channel response. For instance. when the target and mask are presented synchronously, the short latency transient activity generated by the mask cannot effectively inhibit the longer latency sustained activity generated by the target. The same holds when the mask follows the target at large SOA’s. Here the transient activity generated by the mask occurs after the sustained activity generated by the target. Only at intermediate target-mask delays which synchronize mask transient activity with target sustained activity can one expect to obtain optimal target masking. Given also the reasonable interpretation that (cl the target disinhibition results obtained in the prior experiment were produced by a reciprocal, sustained-ontransient inhibition, one zan make specific predictions about how variations in mask-flank temporal separation affect the strength of target disinhibition. In this experiment. when Rank bars were used. only two types of variations of mask-flank temporal separations are employed, as shown in Fig 3. In Condition 2 of the experiment the flank bars, separated by 2.8’ from the mask bars. were presented synchronously with the target. In Condition 3 the same flank bars were presented synchronously with the mask bars. Condition I was the control condition in which only vernier target and mask bars were presented. Condition 1 was expected to yield the usual U-shaped metacontrast function. This follows from assumptions a and b above. However. analyzing Condition 2. one notes that for any trial the temporal separation between mask and flank bars is the same as the temporal separation between vernier target and mask. From assumptions a and b. above. it follows that one should obtain a U-shaped metaconttast function. i.e. the faster transient activity ol’ the mask inhibits optimally at some positive SOA the slower sustained activity of the target. In this argument we neglect. for the time being the reciprocal sustainedon-transient inhibition that the target has on the mask (however. see Discussion). But. taking assumption c in conjunction with assumption b, this predicts a decrease in the metacontrast effect at otherwise optimal SOA’s. That is to say. by temporally locking the flanks bars to the target. one is assuring that at this otherwise optimal masking SOA the sustained activity generated by the flank bars (and target) is optimally
B
A
ON
-S -Bms-
OFF Ic-
4 EXPERIMENT
2
Whereas the previous experiment investigated the elects of variation of spatial separation between mask and continuously presented flank bars on target disinhibition. the current experiment investigates the effects of variations of temporal separation between mask and flank bars. Recall that the theoretical interpretation of U-shaped metacontrast functions relies on (a) transient-on-sustained inhibition and (b) a tran-
8 -128 msec-4 STIMULUS CONOlTlON 1 2 3
A
8
r T&F T
M
M M&F
Fik 3. Temporal sequence of the two successive stimuli (A and B) presented in Experiment 2. The nature of stimuli
A and B varies for the different conditions as shown.
1404
BRLVO G. BRLITME~ ER
superimposed m time on the transient activity generated by the mask. Thus one should optimize the simultaneous occurrence of two reciprocal and antagonistic inhibitory process. On the one hand. one should optimize the mask transient-on-target sustained inhibition giving rise to the target masking effect. but on the other hand one should optimize also the flank sustained-on-mask transient inhibition giving rise to the target disinhibition effect. Consider now Condition 3. Here mask and flank are temporally locked. By an argument similar to the one above. it can be seen that the sustained activity generated by the flank occurs later than the transient activity generated by the mask. and hence little if any disinhibitory effects should be measured. The results for each condition and for each subject are shown separately in Fig. 4. Condition I, the control condition consisting only of the vernier target and mask bars. yielded the typical U-shaped metacontrast function peaking around 40-60 msec. Under Condition 3. U-shaped functions, similar in form and magnitude. were obtained, indicating that the flanking bars. when synchronized with the mask bars. manifested no disinhibitory effect. However. under Condition 1. when the flanking bars were presented synchronously with the target, a reduction of masking. i.e. target disinhibition. occurred at that SOA range in which masking was optimal under Conditions 1 and 3. Thts pattern of results was expected on the basis of a sustained-on-transient neural inhibition.
o----d
---• o-.-o
l
CONDITI”UN 1 CONOITION 2 CONOITION 3
;
. D
0
80 100 120 20 LO 60 STIMULUS ONSET ASYNCHRONY iMSEC)
=
Fig. 4. The proportion of Incorrect identification of the vernier target as a function of SOA and experimental condition.
However. as m Expenment I. Condition 2. t\hlch >lelds disinhlbltion of target bisibihtk at intermedIate SOA’s. increased mask effectiveness at the smallest SOA value. This result *as unexpected. and cannot be explained as a simple addltl\e combmatton of masking effects of mask and flank bars at small target-mask SOA’s. A control expenment &as run m which only the first stimulus (stimulus A of Fig. 3) of each condition was presented The results are shown for each subJect bk the set of three points m the lower right of Fig. 4. Here. under each condition target error rate is roughly 0 I. That is, the flank alone presented synchronously with the targst in Condition 2 had no striking maskmg effect on the target. OnI> in combination with the mask at short target-mask SOA’s. and therefore at short flankmask SOA’s. did the flank bars enhance the effect of the mask bars. This result is puzzling and ma! indicate a potentiailq interesting spatio-temporal interaction in the cisual system. I.
DISCLSSIO\
Disinhibition in backward masking has been reported previously in metacontrast paradigms using spatially non-overlapping disk and ring stimuli (Dember and Purcell. 1967) as well as uniform flash paradigms where the test flash was spatially superimposed on a larger uniform conditioning (mask) flash (Robinson. 1966. 1968). The results of both experiments reported here mdicate that the presence of sustained-on-transient inhibinon in the human visual system ma) be one mechanism for producing such disinhibition phenomena. Experiment I showed that the continua;. i.e. sustained. presence of a set of flanking bars diminished the ability of mask bars to inhibit visibility of a vernier target. Since the sustained presence of the flank bars under steady fixation presumably activated only sustained channels, this reduction of mask effectiveness is very likely due to the inhibition of mask transient channels by flank sustained channels. l\lioreover. this inhibition. at least under the viewing conditions used in Experiment I, must be a spatial& local one. The results of Experiment 2 corroborate the general fmding of Experiment 1 and its interpretation. In Experiment 2, when a flashed instead of continuously presented set of flank bars was used. a reduction in the effectiveness of the mask occurred only when the temporal asynchrony between flank and mask hars was such that. due to inherent latency differences between transient and sustained channels. the activities of the mutually antagonistic channels overlapped maximally in time. This. or a similar mutual antagonism. has been inferred from a variet) of other studies. Recall that the electrophyslological findings of Singer and Bedworth (1973) point conclusively to a transient-on-sustained neural inhibition; however. from their data these investigators also were able to infer a reciprocal sustained-on-transient neural inhibition. Recent psychophysical findings also seem to corroborate this interpretation of reciprocal inhibition between transient and sustained channels. Von Griinau’s (1978) findings on fovea1 and near-fovea1 stroboscopic motion are consistent with this view. Stroboscopic motion
1405
Metacontrast masking of vernier acurty targets appears weaker in the fovea where sustained channeIs are dominant. and may inhibit the transient motion sienals.’ In a complicated set of experiments using stimuli similar to the ones used in the present study. Storer and Banffy (1977) also found a reduction of masking effectiveness in a metacontrast design when mask stimuli where flanked by neighboring ones. Although Stofer and Banffy (1977) interpret this finding in terms of an interaction between flank offset and mask onset. i.e. transient events. they nevertheless did obtain. as in Experiment 1. a reduction of mask effectiveness in the susraine~f presence of the Rank stimuli. Nelson (1974) has recently offered an intriguing hypothesis for one possible functional interpretation of this mutual antagonism between transient and sustained channels. He assumes that sustained channels subserve a focal percepr system whereas the transient channels subserve a tlisrrihrrrrd percept system: the systems are mutually antagonistic. Although Nelson I19741 does not make the following claim. if one were to identify the distributed percept system with the ambient visual system. the focal and ambient visual systems (Schneider, 1969: Trevarthen. 1968) may be interrelated in a mutually antagonistic manner. The importance of a mutual, reciprocal inhibition between sustained and transient channels also has implications for some current theories of visual masking. Weisstein or al. (1975) have incorporated sustainedart-transient inhibition in their recent model of masking as a possible mechanism for paracontrast effects. Whereas both Breitmeyer and Ganz (1976) and Matin (1975) propose different mechanisms for paracontrast. neither explicitly incorporates sustained-~~.transient inhibition in their models to explain any other masking effects. Future models-perhaps versions of Bridgeman’s (1971) model based on a Hartline-Ratliff neural net (Ratliff. 1965) and modified to include reciprocal inhibitory interactions between transient and sustamed channels-may be more adequate from a qualitative as well as quantitative viewpoint than current theories.
of temporal response properties of transient and sustained channels. Won Res. IS. I41 I-1412. Breitmeyer B. G. and Ganz L. (19761 Implications of sustained and transient channels for theories of visual pattern masking. saccadic suppression. and information processing. Ps_&tol. Rrr. 83. l-36. Bridgeman B. (I9711 Metaconttast and lateral lnh~bition. Psycho/. Ret. 78. 5’8-539. Cleland B. G. and Levick W. R. (19741 Brisk and sluggish
concentrically organized ganglion cells in the cat’s retina. J. Physiol.. Lond. 240. 421-456. Dember W. N. and Purcell D. G. (1967) Recovery of masked visual targets b! inhibition of the masking stimulus. Science 157. 133S-1336. Dow B. M. (1974) Functional classes of ceils and their laminar distribution in monkey visual cortex. J. Krurophysiol. 37. 927-946. Enroth-Cuaell C. and Robson J. G. (19661 The contrast sensitivity of retmal ganglion cells ok the cat. f. Plt~sioL. Lond. 187. 517-552. Fukuda Y. and Stone 1. (1974) Retinal distribution and central projection of Y-. X-. and W-cells of the cat’s retina, J. Nrurophysiof. 37. 749-772. Georgeson M. (1976) Antagonism between channels for pattern and movement in human vision. Nature. Lmd. 259, 413-415. Gouras. P. (1968) Identification of cone mechanisms in monkey ganglion cells. J. PhysioL, Land. 199. S33-547. Griinau. M. von (1978) Interaction between sustained and transient channels: Form inhibits motion in the human visual system. vision Rex 1%. 197-201. Hoffmann K. P.. Stone J. and Sherman S. M. (19721 Relay of receptive-field properties in dorsal lateral geniculate nucleus of the cat. f. .~~urop~~~io~. 35. 518-531. Kolers I? and Rosner B. S. (19601 On visual masking (metacontrast): Dichoptic observations. 4rir. J. Pychol. 73. 2-21. Kulikowski J. J. and Tolhurst D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. Physiot. Lmd. 232. 149-162. LUDD U.. Hauske G. and Wolf W. 119761 Perceotual latenties to sinusoidal gratings. Vision Rut. 16. 969-972. MacKay D. M. and MacKay V. (19761 Antagonism between visual channels for pattern and movement” Mawr. Lond. 263. 3 12-3 I4 Matin E. (1975) The two-transient (masking) paradigm. Ps.wltoi. Rrr. 8f. 451-461. Nelson J. I. (19741 Motion sensitivity in peripheral riston. Frrccption 3. 151-152. Ratliff F. (1965) Mach buncls: Qu~tff~~fa~j~~ wdirs on mwrut rteraork\ in r/w wrirtu. Holden-Day. San Francisco. Robinson D. N. (19661 Disinhibition of visually masked stimuli. Scirncr 154. 1S7- 158. Robinson D. N. (1968) Visual disinhibitron with binocular and interocular presentations. J. opr. Sot ._(m. 58. 254-257. Saunders J. E. (1977) Fovea1 and spatial properties or brightness and metacontrast. Kiort Rr\. 17. 375-378. Schneider G. E. (19691 Two visual systems. Scietrce 163. 895-902. Singer W. and Bedworth N. (19731 Inhibitory interaction between X and Y units in the cat lateral geniculatc nucleus. Brairr Rrs. 49. 291-307. Stewart A. L. and Purcell D. G. (19701 U-shaped masking functions in visual backward masking: Effects of iarget configuration and retinal position. Pcrcrpr. P\.I chophys. 7. 253-256. Storer A. and Banfly S. (1977) Relation of split apparent motion to metacontrast. J. up. Psycho/: Hum. Percept. Perform. 3, 258-277. Toihurst D. J. (19751 Sustained and transient channels in human vision. Visiou Re. 15. IISI-IISS. Trevarthen C. 8. (1968) Two mechanisms of vision in
’ Georgeson (19761 also has interpreted a streaming motion after-effect observed after prolonged viewing of a stationary grating pattern as evidence for mutual antagonism between transient and sustained channels. However. this interpretation of the after-effect. as noted by MacKay and MacKay (1976). is highly questionable.
Vassilev A. and Mitov D. (1976) Perception time and spatial frequency. CTsiort Rrr. 16. 719-730. Weisstein N.. Ozoc G. and Szoc R. (19751 A comparison and elaboration of two models of metacontrast. Psyltol. Rec. 82. 325-343. Westheimer G. and Hauske G. (197s) Temporal and spatial interference with Vernier acuit). k’isiori Rrs. 15. 1137-l 141.
REFERESCES
Alpern M. (19.53) Metacontrast. J. opt. SOC. Attt. 43. 648-657. Breitmeyer B. G. (1975) Simple reaction time as a measure
primates. P.~+r&ol.Forsch.
31. 299-337.
G. BREITX.~EYER’
Neurologische Klinik mit Abteilung fur Neurophysiologie der Universitlt D-7800 Freiburg i. Br.. West Germany (R~rirrd
14 Ocroher 1977:
in rerised/orm
Hansastrasse 9.
28 Fehruory 1978)
Abstract-Fovea1
vernier resolution is impaired when a briefly presented target (T) is followed at varying stimulus onset asynchronies (SOA’s) by a spatially adjacent set of metacontrast (M) bars. Impairment of resolution IS optimal at intermediate SOA’s (50-60 msec). When two Ranking (F) bars are in turn placed adjacent to M and are presented either continuously throughout the T-M display sequence or else synchronously with T. a reduction of metacontrast etTects at the previously optimal SOA’s occurs. Such a reduction does not occur when F is presented synchronously with M. In the framework of current neurophvsiological interpretations of metacontrast. these results indicate that sustained channels inhibit transient channels.
I. INTRODUCTION
Recent neurophysiological interpretations of visual masking (Breitmeyer and Ganz, 1976) indicate that inhibitory interactions between transient and sustained neurons (Singer and Bedworth, 1973) can account for a variety of masking phenomena. In particular, according to several current models of metacontrast. the typical U-shaped function relating masking magnitude to stimulus onset asynchrony (SOA) between target and adjacent mask is taken as an indicator of inhibition of slow, target-activated sustained channels by fast, mask-activated transient channels (Breitmeyer and Ganz. 1976; Weisstein, Ozoc and Szoc 1975) or by fast transient channels activated by the target-mask sequence (Matin. 1975). The assumption of latency differences between transient and sustained channels is based on neurophysiological findings (Dow, 1974) as well as psychophysical findings (Breitmeyer, 1975; Lupp, Hauske and Wolf, 1976; Vassilev and Mitov, 1976). Besides establishing the existence of transient-onsustained neural inhibition, Singer and Bedworth’s (1973) study also tentatively pointed to a reciprocal sustained-on-transient inhibition. Exploiting fovea1 and parafoveal stroboscopic motion phenomena, von Griinau (1978) recently obtained results consistent with this latter form of inhibition. The present study ’ This research was carried out while the author was on leave of absence from the Department of Psychology, University of Houston, from July. 1976to July. 1977. during which time he was a fellow of the Alexander von Humboldt Foundation. Bonn-Bad Godesberg, West Germany. He thanks the Foundation for its generous support. Thanks also go to Lothar Spillmann who provided excellent research facilities and support. and to Anne Ransom who served as a subject. This research was supported by Deutsche Forschung Gemeinschaft SFB 70. ’ Reorint reauests should be sent to the author at the followmg address: Department of Psychology, University of Houston, Houston, TX 77004. U.S.A.
exploits systematic modifications of fovea1 metacontrast effects to demonstrate psychophysically not only transient-on-sustained inhibition but also the reciprocal sustained-on-transient inhibition. Fovea1 metacontrast generally has been found to be weaker and less robust than parafoveal metacontrast (Alpern. 1953: Kolers and Rosner. 1960: Saunders. 1977; Stewart and Purcell, 1970). This is most likely due to at least two factors. The first is that the concentration of sustained neurons relative to transient neurons is highest in the central part of the retina. and decreases towards the periphery (Fukuda and Stone, 1974; Gouras. 1968; Hoffmann. Stone and Sherman. 1972). The second is that the relative magnitude of the responses of sustained neurons increases as one approaches the center of the retina, whereas for transient neurons it mcreases as one moves to the peripheral retina (Cleland and Levick, 1974). These factors should lead not only to a decrease in the effective transient-on-sustained channel inhibition in the fovea, i.e. to weaker fovea1 metacontrast, but also to a maximal fovea1 sustained-ontransient inhibition. Exploiting this fact, the present study employs a fovea1 vernier resolution task which is known to yield U-shaped metacontrast functions when the briefly presented vernier target is followed by spatially adjacent masking bars (Westheimer and Hauske. 1975). To demonstrate sustained-on-transient inhibition. the following rationale is employed. The SOA or range of SOA’s at which optimal metacontrast masking occurs is assumed to be a measure of the latency difference between slow sustained channels activated by the vernier target and fast transient channels activated by the adjacent mask bars. Thus, at these SOA’s the fast transient channels activated by the delayed mask can maximally inhibit the slow sustained channels activated by the target. Such a situation would prevail if one restricted the stimulus display to a small vernier target fT’)and two larger adjacent mask bars (M) as shown in Fig. I. However. suppose now that
BRLXOG BREITXE>ER mg in 20 msec steps from S to I1Y mjzc. were used. -\I1 sttmuli were front-tllumlnated m a 4IodeI N-1000 Sclentllic Prototype tachlstoscope. and kere LIeued bmoculsrl) at a distance of I25 cm At the eqeplece of the tachlsroscope the luminance of the uhtte porttons of the vrsual dtspla) was I> ft-I..
Procedure
T
37.0 1
Ftg. I. Spattal Ia)-out of stimulus display. Dimensions are in mmutes of arc. T: target. M. mask. F: flank. See text
for other detalis. two additional Ranking bars (F) in turn are placed immediately adjacent to the mask bars. If the sustained activity generated by the vernier target. Thus wtth Ranking bars of appropriate spatio-temporal activit) generated by the mask bars, one should in turn obtain less inhibition, i.e. disinhibition of the sustained activity generated by the t’ernier target. Thus with Ranking bars of appropriate spatio-temporal relation to the target-mask stimulus sequence. so that sustained activity generated by the flanking bars can inhibit the transient activity generated by the mask bars, one should obtain decreases in masking of the vernier target at those target-mask SOA’s at which optimal target masking is otherwise obtained in absence of flanking bars. 2. GENERAL METHOD Subjects Two experienced psychophysical observers were used. One uias the author (BB). a 30-year-old male with normal vision: the other (AR), was a Z-year-old female research assoctate at Freiburg University with normal vision. AR
was unaware of the purposes of the experiment. Srllrluli and apparatus The stimuli used in this study are shown rn Fig. 1. The dimensions of the stimuli. where indicated, are in minutes of arc. Two coitnear vertical bars and two colinear horizontal bars formed an imaginary cross, the center of which the subjects were instructed to fixate during the experimental sessions. At this central location was presented one of two vernier targets: either the one as shown with the upper portion slightly shifted right relative to the lower one or else one with the upper part shifted an equai amount to the left relative to the lower portion. The width of the target bar segments was 3’. the vernier offset was 0.3’. All stimuli were drawn with black Indian ink on white index cards. The black-to-white contrast was approximately 0.9. For BB. target and mask bars were each presented for 5 msec; for AR, they were each presented for 8 msec. For both subjects. seven target-mask SOA’s vary-
Two ezperlments were run. In Experiment I. -hen the flanking bars Were used. the) here presented contmuousl> at one of t%o spatial separations relauce to the mask. In Expertment ?. the Ranking bars. when used. were presented either s)nchronouslq with the target or else wrth the mask bars In both expertments a total of three conditions were employed. In one condition only the tsr_ger-mask sequence was presented. in the other two condmons. flanking bars m specified spatial (Experiment I) and temporal (Expertmrnt 1) relation to the mask bars v.ere also presented. Data were collected over several Reeks: first for Expertment I. then for Expertment 1. Wlrhm a dzul> session. three ZY trzal blocks were run per subject. one bloch devoted to each of the three conditions. In each block. four trials were devoted to each of the seven SO.A’s. On two of the trials the vermer target with the upper portion displaced to the right aas used: on the remaining t*o maIs. the vernier target with the upper portion displaced to the l:ft. Wlthin a block of ZS tr!sls target and SO.4 uere cross-randomized. Also within a daily session. order of blocks was randomized. Trials were imtiated by the subject. After a trial the subject \tas required to respond either “right” or --left”, depending on whether he beiieked the upper portion of the vernier target to be displaced to the right or left. For each condition of Euperlment 1. 61 trials per SOA were run m 16 nonconsecutive daqs; for each condltton in Expertment f. 1S trials per SOA were run In I.! nonconsecuttve da>s. The proportion of response errors was taken as an index of the mazking effect with higher lfrror proportions correspondmg :c*greater maskmg effects 3. ELPERI%IEST I
In the current experiment. masking effects are measured in the absence of flanking bars (Condition 1) or in the presence of a stationary and continuousl> present set of flanking bars separated from the mask bars either by 4’ (Condition 2) or else by 25’ (Condition 3). In Condition I, one should expect to obtam the usual U-shaped metacontrast masking function with optimal masking at intermediate SOXs. The effect of the continuously presented and stationar? flanking bars should be primarii? to activate sustained channels (Enroth-Cugell and Robson. 1966: Kulikowski and Tolhurst. 1973; Tolhurst. 1975). Provided that. as in Condition 2, the spatial separation of flank and mask bars is sufficientI> small (4’). the inhibitory effect. if any. that this sustained channel activity has on the transient channel activity of the mask should produce less target inhibition, i.e. target disinhibition. This disinhibition should manifest itself In terms of a noticeable reduction in masking effects at intermediate target-mask SOA’s. However. if, as in Condition 3. the spatial separation between flank and mask bars is sufficiently large (20’). little. if an>. such disinhibit~on should be obtained. The results of Experiment 1 are shown separateI> for each subject and each condition in Fig. 2. It should be noted that Condition I (no flank) yields the typical U-shaped metacontrast function with optimal masking at SOA’s ranging from 40 to 85msec.
1403
Metacontrast masking of vernier acuity targets
1
o--o CONOiTlON1
2 --* CONDITION 3 o---q CONOITION
z .1 z z E
l
0,
0
LO 60 80 100 120 STIMLILUS ONSETASYNCHRONY(MSEC)
2G
Fig. 7. The proportion of incorrect identification of the vernier target as a function of SOA and experimental condition. Higher proportions correspond to greater masking.
However. in Condition 2. where the flanking bars were in relatively close spatial proximity to the mask bars. the masking effect is comparatively small in that range of SOA values. In fact. the effect of the nearby flank bars is not only to decrease masking at intermediate SOA’s but also to increase masking at the smallest SOA. Although the latter result is unexpected and somewhat puzzling the former result was expected on the hypothesis that sustained channels can inhibit transient channels. Under viewing conditions used in this experiment. the sustained-on-transient inhibition must be a relatively local mechanism since. compared to Condition 1, little, if any, disinhibition or reduction of masking at intermediate SOA’s was obtained in Condition 3, where the flanking bars were separated by 20’ from the mask bars. However, more precise and complete determinations of the spatial parameters affecting this disinhibition require further study.
sient channei response which is of shorter latency-by an amount equal to the optimal masking SOA-than the sustained channel response. For instance. when the target and mask are presented synchronously, the short latency transient activity generated by the mask cannot effectively inhibit the longer latency sustained activity generated by the target. The same holds when the mask follows the target at large SOA’s. Here the transient activity generated by the mask occurs after the sustained activity generated by the target. Only at intermediate target-mask delays which synchronize mask transient activity with target sustained activity can one expect to obtain optimal target masking. Given also the reasonable interpretation that (cl the target disinhibition results obtained in the prior experiment were produced by a reciprocal, sustained-ontransient inhibition, one zan make specific predictions about how variations in mask-flank temporal separation affect the strength of target disinhibition. In this experiment. when Rank bars were used. only two types of variations of mask-flank temporal separations are employed, as shown in Fig 3. In Condition 2 of the experiment the flank bars, separated by 2.8’ from the mask bars. were presented synchronously with the target. In Condition 3 the same flank bars were presented synchronously with the mask bars. Condition I was the control condition in which only vernier target and mask bars were presented. Condition 1 was expected to yield the usual U-shaped metacontrast function. This follows from assumptions a and b above. However. analyzing Condition 2. one notes that for any trial the temporal separation between mask and flank bars is the same as the temporal separation between vernier target and mask. From assumptions a and b. above. it follows that one should obtain a U-shaped metaconttast function. i.e. the faster transient activity ol’ the mask inhibits optimally at some positive SOA the slower sustained activity of the target. In this argument we neglect. for the time being the reciprocal sustainedon-transient inhibition that the target has on the mask (however. see Discussion). But. taking assumption c in conjunction with assumption b, this predicts a decrease in the metacontrast effect at otherwise optimal SOA’s. That is to say. by temporally locking the flanks bars to the target. one is assuring that at this otherwise optimal masking SOA the sustained activity generated by the flank bars (and target) is optimally
B
A
ON
-S -Bms-
OFF Ic-
4 EXPERIMENT
2
Whereas the previous experiment investigated the elects of variation of spatial separation between mask and continuously presented flank bars on target disinhibition. the current experiment investigates the effects of variations of temporal separation between mask and flank bars. Recall that the theoretical interpretation of U-shaped metacontrast functions relies on (a) transient-on-sustained inhibition and (b) a tran-
8 -128 msec-4 STIMULUS CONOlTlON 1 2 3
A
8
r T&F T
M
M M&F
Fik 3. Temporal sequence of the two successive stimuli (A and B) presented in Experiment 2. The nature of stimuli
A and B varies for the different conditions as shown.
1404
BRLVO G. BRLITME~ ER
superimposed m time on the transient activity generated by the mask. Thus one should optimize the simultaneous occurrence of two reciprocal and antagonistic inhibitory process. On the one hand. one should optimize the mask transient-on-target sustained inhibition giving rise to the target masking effect. but on the other hand one should optimize also the flank sustained-on-mask transient inhibition giving rise to the target disinhibition effect. Consider now Condition 3. Here mask and flank are temporally locked. By an argument similar to the one above. it can be seen that the sustained activity generated by the flank occurs later than the transient activity generated by the mask. and hence little if any disinhibitory effects should be measured. The results for each condition and for each subject are shown separately in Fig. 4. Condition I, the control condition consisting only of the vernier target and mask bars. yielded the typical U-shaped metacontrast function peaking around 40-60 msec. Under Condition 3. U-shaped functions, similar in form and magnitude. were obtained, indicating that the flanking bars. when synchronized with the mask bars. manifested no disinhibitory effect. However. under Condition 1. when the flanking bars were presented synchronously with the target, a reduction of masking. i.e. target disinhibition. occurred at that SOA range in which masking was optimal under Conditions 1 and 3. Thts pattern of results was expected on the basis of a sustained-on-transient neural inhibition.
o----d
---• o-.-o
l
CONDITI”UN 1 CONOITION 2 CONOITION 3
;
. D
0
80 100 120 20 LO 60 STIMULUS ONSET ASYNCHRONY iMSEC)
=
Fig. 4. The proportion of Incorrect identification of the vernier target as a function of SOA and experimental condition.
However. as m Expenment I. Condition 2. t\hlch >lelds disinhlbltion of target bisibihtk at intermedIate SOA’s. increased mask effectiveness at the smallest SOA value. This result *as unexpected. and cannot be explained as a simple addltl\e combmatton of masking effects of mask and flank bars at small target-mask SOA’s. A control expenment &as run m which only the first stimulus (stimulus A of Fig. 3) of each condition was presented The results are shown for each subJect bk the set of three points m the lower right of Fig. 4. Here. under each condition target error rate is roughly 0 I. That is, the flank alone presented synchronously with the targst in Condition 2 had no striking maskmg effect on the target. OnI> in combination with the mask at short target-mask SOA’s. and therefore at short flankmask SOA’s. did the flank bars enhance the effect of the mask bars. This result is puzzling and ma! indicate a potentiailq interesting spatio-temporal interaction in the cisual system. I.
DISCLSSIO\
Disinhibition in backward masking has been reported previously in metacontrast paradigms using spatially non-overlapping disk and ring stimuli (Dember and Purcell. 1967) as well as uniform flash paradigms where the test flash was spatially superimposed on a larger uniform conditioning (mask) flash (Robinson. 1966. 1968). The results of both experiments reported here mdicate that the presence of sustained-on-transient inhibinon in the human visual system ma) be one mechanism for producing such disinhibition phenomena. Experiment I showed that the continua;. i.e. sustained. presence of a set of flanking bars diminished the ability of mask bars to inhibit visibility of a vernier target. Since the sustained presence of the flank bars under steady fixation presumably activated only sustained channels, this reduction of mask effectiveness is very likely due to the inhibition of mask transient channels by flank sustained channels. l\lioreover. this inhibition. at least under the viewing conditions used in Experiment I, must be a spatial& local one. The results of Experiment 2 corroborate the general fmding of Experiment 1 and its interpretation. In Experiment 2, when a flashed instead of continuously presented set of flank bars was used. a reduction in the effectiveness of the mask occurred only when the temporal asynchrony between flank and mask hars was such that. due to inherent latency differences between transient and sustained channels. the activities of the mutually antagonistic channels overlapped maximally in time. This. or a similar mutual antagonism. has been inferred from a variet) of other studies. Recall that the electrophyslological findings of Singer and Bedworth (1973) point conclusively to a transient-on-sustained neural inhibition; however. from their data these investigators also were able to infer a reciprocal sustained-on-transient neural inhibition. Recent psychophysical findings also seem to corroborate this interpretation of reciprocal inhibition between transient and sustained channels. Von Griinau’s (1978) findings on fovea1 and near-fovea1 stroboscopic motion are consistent with this view. Stroboscopic motion
1405
Metacontrast masking of vernier acurty targets appears weaker in the fovea where sustained channeIs are dominant. and may inhibit the transient motion sienals.’ In a complicated set of experiments using stimuli similar to the ones used in the present study. Storer and Banffy (1977) also found a reduction of masking effectiveness in a metacontrast design when mask stimuli where flanked by neighboring ones. Although Stofer and Banffy (1977) interpret this finding in terms of an interaction between flank offset and mask onset. i.e. transient events. they nevertheless did obtain. as in Experiment 1. a reduction of mask effectiveness in the susraine~f presence of the Rank stimuli. Nelson (1974) has recently offered an intriguing hypothesis for one possible functional interpretation of this mutual antagonism between transient and sustained channels. He assumes that sustained channels subserve a focal percepr system whereas the transient channels subserve a tlisrrihrrrrd percept system: the systems are mutually antagonistic. Although Nelson I19741 does not make the following claim. if one were to identify the distributed percept system with the ambient visual system. the focal and ambient visual systems (Schneider, 1969: Trevarthen. 1968) may be interrelated in a mutually antagonistic manner. The importance of a mutual, reciprocal inhibition between sustained and transient channels also has implications for some current theories of visual masking. Weisstein or al. (1975) have incorporated sustainedart-transient inhibition in their recent model of masking as a possible mechanism for paracontrast effects. Whereas both Breitmeyer and Ganz (1976) and Matin (1975) propose different mechanisms for paracontrast. neither explicitly incorporates sustained-~~.transient inhibition in their models to explain any other masking effects. Future models-perhaps versions of Bridgeman’s (1971) model based on a Hartline-Ratliff neural net (Ratliff. 1965) and modified to include reciprocal inhibitory interactions between transient and sustamed channels-may be more adequate from a qualitative as well as quantitative viewpoint than current theories.
of temporal response properties of transient and sustained channels. Won Res. IS. I41 I-1412. Breitmeyer B. G. and Ganz L. (19761 Implications of sustained and transient channels for theories of visual pattern masking. saccadic suppression. and information processing. Ps_&tol. Rrr. 83. l-36. Bridgeman B. (I9711 Metaconttast and lateral lnh~bition. Psycho/. Ret. 78. 5’8-539. Cleland B. G. and Levick W. R. (19741 Brisk and sluggish
concentrically organized ganglion cells in the cat’s retina. J. Physiol.. Lond. 240. 421-456. Dember W. N. and Purcell D. G. (1967) Recovery of masked visual targets b! inhibition of the masking stimulus. Science 157. 133S-1336. Dow B. M. (1974) Functional classes of ceils and their laminar distribution in monkey visual cortex. J. Krurophysiol. 37. 927-946. Enroth-Cuaell C. and Robson J. G. (19661 The contrast sensitivity of retmal ganglion cells ok the cat. f. Plt~sioL. Lond. 187. 517-552. Fukuda Y. and Stone 1. (1974) Retinal distribution and central projection of Y-. X-. and W-cells of the cat’s retina, J. Nrurophysiof. 37. 749-772. Georgeson M. (1976) Antagonism between channels for pattern and movement in human vision. Nature. Lmd. 259, 413-415. Gouras. P. (1968) Identification of cone mechanisms in monkey ganglion cells. J. PhysioL, Land. 199. S33-547. Griinau. M. von (1978) Interaction between sustained and transient channels: Form inhibits motion in the human visual system. vision Rex 1%. 197-201. Hoffmann K. P.. Stone J. and Sherman S. M. (19721 Relay of receptive-field properties in dorsal lateral geniculate nucleus of the cat. f. .~~urop~~~io~. 35. 518-531. Kolers I? and Rosner B. S. (19601 On visual masking (metacontrast): Dichoptic observations. 4rir. J. Pychol. 73. 2-21. Kulikowski J. J. and Tolhurst D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. Physiot. Lmd. 232. 149-162. LUDD U.. Hauske G. and Wolf W. 119761 Perceotual latenties to sinusoidal gratings. Vision Rut. 16. 969-972. MacKay D. M. and MacKay V. (19761 Antagonism between visual channels for pattern and movement” Mawr. Lond. 263. 3 12-3 I4 Matin E. (1975) The two-transient (masking) paradigm. Ps.wltoi. Rrr. 8f. 451-461. Nelson J. I. (19741 Motion sensitivity in peripheral riston. Frrccption 3. 151-152. Ratliff F. (1965) Mach buncls: Qu~tff~~fa~j~~ wdirs on mwrut rteraork\ in r/w wrirtu. Holden-Day. San Francisco. Robinson D. N. (19661 Disinhibition of visually masked stimuli. Scirncr 154. 1S7- 158. Robinson D. N. (1968) Visual disinhibitron with binocular and interocular presentations. J. opr. Sot ._(m. 58. 254-257. Saunders J. E. (1977) Fovea1 and spatial properties or brightness and metacontrast. Kiort Rr\. 17. 375-378. Schneider G. E. (19691 Two visual systems. Scietrce 163. 895-902. Singer W. and Bedworth N. (19731 Inhibitory interaction between X and Y units in the cat lateral geniculatc nucleus. Brairr Rrs. 49. 291-307. Stewart A. L. and Purcell D. G. (19701 U-shaped masking functions in visual backward masking: Effects of iarget configuration and retinal position. Pcrcrpr. P\.I chophys. 7. 253-256. Storer A. and Banfly S. (1977) Relation of split apparent motion to metacontrast. J. up. Psycho/: Hum. Percept. Perform. 3, 258-277. Toihurst D. J. (19751 Sustained and transient channels in human vision. Visiou Re. 15. IISI-IISS. Trevarthen C. 8. (1968) Two mechanisms of vision in
’ Georgeson (19761 also has interpreted a streaming motion after-effect observed after prolonged viewing of a stationary grating pattern as evidence for mutual antagonism between transient and sustained channels. However. this interpretation of the after-effect. as noted by MacKay and MacKay (1976). is highly questionable.
Vassilev A. and Mitov D. (1976) Perception time and spatial frequency. CTsiort Rrr. 16. 719-730. Weisstein N.. Ozoc G. and Szoc R. (19751 A comparison and elaboration of two models of metacontrast. Psyltol. Rec. 82. 325-343. Westheimer G. and Hauske G. (197s) Temporal and spatial interference with Vernier acuit). k’isiori Rrs. 15. 1137-l 141.
REFERESCES
Alpern M. (19.53) Metacontrast. J. opt. SOC. Attt. 43. 648-657. Breitmeyer B. G. (1975) Simple reaction time as a measure
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