Sensitization by annular surrounds: Sensitization and the contrast-flash effect

SENSITIZATION SENSITIZATION

BY ANNULAR

SURROUNDS:

AND THE CONTRAST-FLASH

EFFECT

KI.NNI:TH R. ALEXANDER Center for Visual Science. University of Rochester. Rochester. New York 14627. L.S.A (Rcceirrcl 20 June 1973: irl wrisrdform

9 Norrn~ho 1973)

Abstract--In spatial sensitization. the onset of a sensitizing annulus lowers the threshold for a test probe presented concentric with a continuously-illuminated disk which fills the center of the annulus. In the contrast-flash effect. the onset of an annulus
3241

In the scotopic contrast-flash effect, the test stimulus is a disk which fills the center of a large annulus (the contrast-flash). Both test disk and contrast-flash are presented to peripheral retina. The contrast-flash is presented for 100mscc. and the test disk is typically presented for IO msec at an interval 100 msec before the annulus onset. It is found that the contrast-flash rlelates the threshold for the trst stimulus (c.g. Alpern and Rushton. 1967). There are similarities between the paradigms: both are carried out in the peripheral retina under rod-isolation techniques (i.e. green test stimulus, red background disk and annulus): each uses a IOmsec test flash: each measures the etTect of an annulus on the test flash. However. there are also differences. These includc: (I) the sizes of the stimuli; (2) the presence or absence of a large dim background field; (3) the duration of the annulus; (4) the illuminance of the annulus; and (5) the presence or absence of a steady disk on which the test stimulus falls. It is of interest to determine the conditions under which an annulus elevates or lowers thresholds. Conscqucntlc. the plan of the present series of experiments was to systematically vary the parameters differing betweon the two paradigms, to determine which paramctcr(s) is/arc the critical one(s) in determining whether the annulus does elevate or lower the threshold. METHOD Two observers participated in the study. One. DT, wore clinically-fitted contact lenses to correct a myopic condition and was a well-practice psychophysical observer. The other. CW. was emmetropic. though he had a slight vertical phoria 123

624

KENNETHR ALEXANDER

in the non-stimulated eye. and he had a slight amount of training prior to the experiments to be reported. Apparatus

All stimuli were presented via four channels of a Maxwellian view optical system identical to that used by Sturr and Teller (1973) and similar to that used by Teller and Gestrin (1969) with one major difference. In the present optical system, the non-Maxwellian bleach channel used by Teller and Gestrin was replaced by a Maxwellian channel derived from the same source as the other three Maxwellian channels. Speaker coil shutters provided temporal control of the stimuli. A small lighted fixation cross was presented at optical infinity in non-Maxwellian view at a lateral distance of 7- from the optical axis of the added Maxwellian channel. This meant that when the 0 was properfy aligned he had to rotate his eyes slightly to the left for proper fixation. A bite bar was used to hold the observer’s head in position. Stimulation was monocular. with the o’s left eye positioned such that the filament image was centered in the pupil. The diameter of the final filament image was set at 1.5 mm by placing a stop at the first filament image in each channel in the optical system. No artiticial pupil was used. Stimuli were provided by means of shim brass stops placed 1 focal length behind the Maxwellian lenses. Disks were circular holes cut in the stops. Annuli consisted of holes in the stops covered by thin pieoes of glass, with a circular opaque disk, attached to the gfass, forming the- center of the annulus. Stimufus intensities were controlled by neutral density filters (Kodak Wratten No. 96) and by a ~ntinuou~y vartable neutral density wedge (Light Filters Ltd.) placed in the test channel. Retina) iRuminances were calibrated periodicaliy by means of an SE1 photometer, and stimulus durations were calibrated with a photocell and CRO. Sfiwli

Since the present study was intended to be limited to rod vision, the properties of the stimuli were selected to favor the scotopic system (cf. Aguitar and Stiles, 1954). Test stimuli (TS) were green disks of various diameters, with wave-

length properties determined by Kodak Uratten titter No 74. The duration of the TS in all cases was 10 msec. with a presentation once every 2 sec. Other stimuli (disks. annuli and the B-field) were red, produced by Kodak Wratten fiber No. 29. All stimuli entered the eye through the center of the natural pupil and were presented 7’. from the fovea on temporal retina. A list of all the stimuli used m the experiments is presented in Table I. Choice yf’trst srcmuli

Test stimuli were chosen such that the! could be made identical in both transient sensitization and the contrastflash effect. In the transient sensitization paradigm. the TS was systematj~lly increased in diameter from 48’ to 36 Previous investigation (Alexander. 1973) had shown that spatial sensitization could be demonst~atcd using test stimuli as large as 36’ dia. However, in order to show sensitization with such large test stimuti the ihuminancc of the background disk had to be raised above that used for the smallest (4%) TS. Accordingly, in studying transient sensitization with different diameter test stimuli, two illuminances of disk and annulus were used in the present experiment: 0.1 and 1.1 log Scot. td. For the lower illuminance, dark adaptation curves of the sort described in Teller and Lindsey (1970) suggested that all test stimuli were detected by rods, though under these conditions the 36’ TS did not show much steadystate sensitization. For the higher illuminance. it was not clear from the dark adaptation curves that the small 4.8’ TS was detected by rods and not cones, so this condition was not studied. However, at this higher disk illuminance, the large 35’ TS showed a substantial ~nsit~~tion egecf and evidence indicated that detection of this TS was still mediated by rods. In studying the contrast-flash effect, Atpern et ffl. (197Oa) had used as the psychophysical task the detection of a dark line across the test disk, rather than detection of the disk itself. In earlier experiments, however. the task was detection of the test stimulus itself (Alpern 1965; Alpcrn and Rushton, 1965. 1967). In the present experiments the task used was detection of the test disk, to allow comparability with the transient sensitization studies. Preliminary investi-

Table I. Summary of the stimulus values used in the experiments

Stimuli 1. Test stimulus (a) Diameters (b) Duration (c) Dominant wavelength 2. Background disk (a) Diameter (b) Intensities (c) Dominant wavelength 3. Annulus (a) Diameters (b) Intensities (c) Durations (d) Dominant wavelength 4.

B-field (a) Diameter (b) Intensities (c) Dominant wavelength

Transient sensitization

4.8’. 24’. 36 10 msec 538 (green)

Contrast-flash

36’. 2 10 msec 538 (green)

36 0.1, 1.1 log Scot. td 633 (red)

_ __.

36--Z 0.1. 1-1 log Scot. td @I, lsec 633 (red)

36_ 2”. 2”-8” 0.1. ).l log Scot. td @I, 1 set 633 (red)

10 _ 1.1. @Olog Scot. td 633 (red)

None,

IO - 1.1, 0.0 log

633 (red) -

~~01. tti

___..-ll”-”

625

Sensitization by annular surrounds gations had shown that both line detection and disk detection yield comparable results under the conditions studied in this experiment (Alexander, 1972). Procedure

The psychophysical procedure used was the method of adjustment. Observers aligned the stimuli by means of alignment dots left in place in an unused channel of the optical system. Then the 0 fixated the fixation cross and adapted for 10 min to either the B-field, if it was used in the experiment. or to a dark field. During the adaptation he made repeated adjustments of the TS i~iuminan~ to threshold by turning the circular neutral density wedge. After each threshold determination the experimenter randomly reset the wedge. At the end of this adaptation period the 0 made five determinations of resting threshold (that is, threshold against the B-field or in darkness). Then one of the conditions was chosen at random. the 0 made five determinations of threshold, and the data were discarded. It had been found previously that such a “throw-away” condition helped reduce variability during a session. Following these preliminaries, the conditions for the session were presented in random order and the 0 made a block of five determinations of theshold for each condition. Ifa steady field was present during a condition, as in the experiments on transient sensitization, then 1 min of adaptation to the steady field preceded the threshold determinations. At the end of the session five additional determinations of resting threshold were made. The conditions were presented in counterbalanced order in a succeeding session, SO that 10 threshold determinations were averaged for the mean ofeach condition. Each session lasted I-I$ hr.

RESCLTS

Transient sensit~ation was studied using diameters of test stimuli ranging from 4%’ to 36’. Results for two subjects are shown in curves (a). (b). (c) and (g) of Figs. 1 and 2 [curves (d). (e) and (f) will be discussed in Experiment 41. Curve (a) in each figure is a replication of the paradigm used by Teller (1971). with a 4% TS. Curves (g) and (b) employed a 24’ or 36’ TS respectively, and the same disk-annu~us i~~uminan~ as for curve (a). The particular stimulus values used in generating curve (b) had not shown much spatial sensitization in the steady state (Alexander. 1973) so the illuminance of the disk and annulus was raised by 1 log unit to the point where spatial sensitization could be shown. Then transient sensitization curves were determined using the 36’ TS and the higher djsk-annulus iliuminance. The result is shown in curve (c) of Figs. 1 and 2. It should be noted that the leftmost point in each of these and subsequent functions (SOA = - 300) is equivalent to threshold in the absence of the annulus. Both 0s replicated the original transient sensitization function. showing a drop in the threshold at annulus onset [curve (a)]. though there is the same intersubject variability as found in Sturr and Teller (1973). For larger TS diameters, the intersubject variability is more marked. Observer DT shows a drop in

DT

fb)

AnnulusoffDoff 0

Annula

6b1~iu~

(d)

Annulus off6;)off

on (g ond a) Amub

-W%

(f and e)

O%M,IJIUS

on (C andb)

-e

SOA.

m set

750 SOA,

m SBC

Fig. 1. Transient sensitization functions for DT, for three sizes of TS, two disk-annulus illuminances and two annulus durations. Stimulus values were: (1) TS diameter: 4.8’ (a, d), 24’ (g). 36’ (b. c. e. f); (2) diskannuius ifluminance: 01 (a, b, d, e, g) or 1.1 (c, I) log Scot. td; (3) annulus duration: 1 set (open symbols) or 100 msec (closed symbols). For all conditions the diameter of the steady disk was 36’; diameter of the annulus was 36’-2”. The B-field illuminance was - 1.1 (a, b, d. e, g) or 0.0 (c, f) log Scot. td, Vertical bars plot f 1 SE. for standard errors greater than 0.02 log unit. In this and in a11 subsequent figures the ordinates correspond to those ofTeller (1971). in which the “log relative threshold” value of 0.0 is equivafent to -0.4 log Scot. td.

626

R. ALI.XANIXK

Kr NNI.TH

i’o-

I

x(a)

,g 2.0‘0 T : d

Wd I-O-

* ~ i

Annulus offi7aff AftM~8 0

off fAin&ts I

-250

0

SOA,

on fg and al I

* (bf

offf-loff

(fondef

Amulus OffiAMKlRls UI

(C ard

Annulus

(d)

-

ttf

I

2!xY

503

ie 750 -21x)

m set

SOA.

m set

Fig. 2. Transient sensitization functions for CW, for three sizes of TS, two disk-annulus illuminances and two annulus durations. For stimulus parameters see legend to Fig. 1.

the threshoid at annuius onset no matter what the TS diameter. For CW, the function for the 36 TS shows a threshold elevation at annulus onset, followed by a subsequent drop to a threshold levef below that found against the 36’ disk afone (e.g. - 300 m.sec SOA).’ To summarize these results: for DT. increasing the diameter of the TS preserves the transient sensitization effect. The threshold for the TS is lower after annulus onset than when the TS is presented against the 36’ disk alone. For CW. increasing the diameter of the TS in the transient sensitization paradigm changes the shape of the threshold function. The threshold at long positive SOA’s (e.g. + 500 mscc) remains lower than the threshold at -3OOmsec SOA. but with the large test stimuli there is in addition an elevation of the threshold near the time of annulus onset.

meters were identical to those used in curve (a). It can be seen that reducing the size of the stimuli in the contrast-flash effect preserves the nature of the effect for CW. The onset of the annulus elevates the threshold for the TS. The same result holds for DT, as will be seen in the next experiment.

E.~~~~i~~nt 2: srj~?~u~us si.~r in the c~~trust~~~sh ciffrct The result of changing the stimulus size in the contrast-liash effect is shown in Fig. 3. Curves (a), (b), (c) and (d) are for CW: curves (e) and (f) are for DT. Curve (a) is the threshold function obtained under conditions used by Alpern UI al. (197Oa) except that the task was disk detection. The threshold begins to rise when the TS onset precedes the annulus onset by about 1.50 m.sec and is highest near simultaneous onset of the TS and annuhts. in generating curve (b), the TS was reduced to 36 dia. and the annulus inner and outer diameters were decreased to 36’ and 2 respectively. AI1 other para-

’ SOA (Stimulus Onset Asyrtchron~) refers to the time interval

between the onsets of the test stimulus and annu~us with a negative SOA indicating that the test stimulus is pre&ted first.

Fig. 3. Contrast-gash functions for CW (left) and DT (right) with two sizes of TSand annuhts_ and with a IMeld either present or absent. Stimulus values were: (I) TS dia Z”, annuhzs dim 2”-8” (a, c, e); TS dii Xi’, annulus dia 36‘~-2”(b. d. t); (2) 8.field absent@I,b. Q; B-field pseaent of ilhtminance O@log scat. td (c, d, f). For Bu conditions the annulus iiluminance was 1-l log scat. td and the annulus duration was 100 msec.

Srnsitization

by annular

duration was shortened to 100 msec. Gcncrally the short-duration annulus resulted in less change in the threshold at annulus onset. though there are individual differences. Both OS reported that it was extremely difficult to make threshold judgments when the TS was the same size as the steady disk and the annulus duration was very short (100 mscc). which may account for the fact that curve (f) of Fig. I does not replicate well the curve for the I set annulus (c). Figure 4 plots contrast-flash threshold functions for the small stimuli at two annulus illuminances and two annulus durations. The comparisons to be made are between curves (a) and (c), (b) and (d), (e) and (g). and (f) and (h). It can be seen that lengthening the duration of the annulus does not alter the fact that the onset of the annulus elevates the threshold in the contrast-flash paradigm.

In the spatial sensitization paradigm. a dim red IO background field (B-field) is used to reduce the effects of scattered light (Westheimer. 1965). This same B-field was employed in the transient sensitization experiments. but was not present in the contrast-flash experiments of Alpern or al. (1967. 197Oa, b.c). A uniform background tield was present. however. in earlier experiments on the afterflash effect (Alpern. 1965). The etfects ofadding the B-field to the contrast-flash conditions can be seen in Fig. 3. The stimuli for the pairs of curves (a) and (c), and (b) and (d). were identical. except for one difference. The difference is that a B-field, with the same size and illuminance parameters as for transient sensitization, was added to the stimulus conditions generating curves (c) and (d). It can be seen that the addition of the B-field generally elevates thresholds and reduces the magnitude of the threshold elevation at annulus onset. but otherwise the curves all have the same shape. The results for DT are presented in curves(e) and (f). Curve (e) corresponds to curve (a) for CW, and curve (f) corresponds to curve (d) for CW. The functions for DT agree well with those for CW. The onset of the annulus results in an elevation of the threshold for the test stimulus, with or without the presence of the Bfield

In the data that have been reported on transient sensitization. the steady disk and the cycling annulus arc the same illurninancc. The etfccts of varying the common dish-annulus illuminance may be seen in Teller (1971). With all disk-annulus illuminances used (-060.9 log Scot. td) the result is the same - the threshold for the test probe is lowered by the onset of the annulus. Generally. the greater the disk-annulus illuminance. the greater the drop in the threshold at the onset of the annulus. The results of changing the annulus illuminancc in the contrast-flash effect are presented in Fig. 4 and in Aipcrn L’Ial. (1970a). The annulus elevates the threshold for the TS at all annulus illuminances used. and there is a greater elevation of the threshold with the higher annulus illuminances.

In transient

sensitization. the annulus duration is in the contrast-flash effect it is 100 msec. The result ofchanging the annulus duration in the two paradigms is shown in Figs. I, 2 and 4. Figs. 1 and 2 show the results for transient sensitization. The stimulus conditions for curves(d). (c) and (f) are comparable to those for curves (a). (b) and (c)except that the annulus 1 SK:

’

Annulus OfffAnnulus on tc ondd)

*m-h)

AMU~JS

Annutus offl;;loff

offaoff

(a and b)

-k&?kYk

627

surrounds

(e and f)

-e SOA,

m set

Fig. 4. Contrast-Rash functions for CW (left) and DT (right) for two illuminances and two durations of annulus. Stimulus values were: (I) annulus illuminance: 0.1 (b. d. f, h) or 1.1 (a. c, e. g) log Scot. td; (2) annulus duration: 100 msec (a, b. e, f) or 1 set (c. d, g. h). For all conditions the TS dia was 36’ and the annulus dia was 36’-2”. A B-field of either - 1.1 or 0.0 log Scot. td was present. Vertical bars plot + S.E. for standard errors greater than 0.02 log unit.

628

KENNETH

R. ALEXANDER

+__,-a(b) 0

i3

z-o-

-2-

f ? f

v,c,

SOA, m set

4 I.Ojs -0 !!

Ann&m Q ff&&ff A~WW

5

(d)

offl~mutus

on

(b ond c)

o-250 -+?k-kSOA,

m set

Fig. 5. Summary of transient sensitization and the contrast-flash effect for DT. Stimulus values were: (I) TS dia 48’ 60. 36’ (b, c. d), 2” (e); (2) disk dia 36’ (a, bl none tc. d. e). (3) Disk illuminance: 0.1 (a). I.! (b) log scat. td; (4) annulus dia 36’-2” (a, b, c, d), 2”-8” (e). (5) Annulus iiluminance: 0.1 (a), 1.1 (b. c. d. e) log acot. td. (6) Annulw duration: 1 set (a, b, c), 100 msec (d, e). (7) B-field ifluminance: none (e). - 1.1 (a), 0-O(b, c, d) log scat. td. Vertical bars plot + SE. for standard errors greater than 0.02 log unit,

Uhk~offhtWs

SOB,

I -m

I 0 SOA,

Fig.

6.

I 250

I xm

on (b) and Cc)

m set

I 7x)

m set

Summary of transient sensitization and the contrast-flash effect for CW. For stimulus w)ues. see legend to Fig. 5.

629

Sensitization by annular surrounds DISCUSSON

In Experiment 1 it was shown that for observer DT. increasing the diameter of the TS to 36’ in transient sensitization preserved the drop in the threshold at the onset of the annulus. For 0%‘. the transient sensitization function obtained with the 36’ TS showed both a threshold elevation at annulus onset. and a subsequent lowering of threshold below the level Sound against the peak diameter disk alone. The results of Experiment 2 demonstrated that the elevation of the threshold at annuius onset in the contrast-flash effect is preserved if the stimuli are reduced in size to a 36’ TS and 36-2’ annulus. Thus Experiments 1 and 2 show that both transient ~nsit~ation and the contrast-flash effect can be obtained using one and the same size of TS and annulus. Experiment 3 indicated that the addition of a B-field to the contrast-Hash paradigm does not alter the direction of the change in the threshold at annulus onset. Thus the B-field is not the critical parameter. Experiments 4 and 5 showed that it is possible to vary annulus duration and illuminance respectively within each paradigm and still not alter the direction of the change in the threshold at annulus onset. Annulus duration and illuminance are not the critical parameters. Experi)~e~t 6: presence ofthe steady disk By examining summary Figs. 5 and 6 one can evaluate the effects of having a steady disk at the center of the annuius. Curve (a) plots the replication of the transient sensitization function employing a 48’ TS (from Figs. 1 and 2). Curve (b) is a graph of transient sensitization with a 36’ TS and 1 log unit higher disk and annulus illuminance. Curve (e) shows the original contrast-flash function using a 2” TS and 2’-8’ annulus. Curve (d) is the contrast-flash effect with a 36’ TS and 36’-2” annulus of the iliuminance used in (b), and with a B-field added. Curve (c) shows the contrast-flash effect with the same parameters as in (d) except that the annulus duration was increased to 1 sec. For curves (b) and (c) then, the TS and annulus diameters are the same, annulus iliuminance and duration are the same, and the stimuli are presented on the same B-field. The only difference in the stimulus conditions is the presence (b) or absence (c) of a steady 36’ disk at the center of the annulus. For observer DT, the result is clear-cut: this steady disk determines whether the annulus elevates or lowers the threshold. For 0%‘. the steady disk is also an important variable. in that it determines whether the threshold at long positive SOA’s (e.g. + 500) is higher or lower than the threshold in the absence of the annulus (e.g. - 300). However, for CW the TS size is also important for when the TS is made contiguous with the annulus, the onset of the annulus elevates the threshold whether the steady disk is present or not (in agreement with the previous results of Alpern et al.. 1967, 1970b). The reason for these individual differences remains unclear.

The results show that under the conditions studied in this series of experiments. one OSthe critical parameters determining whether an annuius elevates or lowers the threshold for a test stimulus is the presence or absence of a steady disk. the same illuminance as the annulus, presented in the center of the annulus. In addition. for one 0 (CW) the size of the test stimulus is also important. The reason for this latter difference between observers with regard to the effect of TS diameter [curves (b) and (c), Figs. 1 and 71 is not clear at this time. Perhaps the difference is related to the differential effectiveness ofa given disk luminance in activating the sensitization process. Evidence suggests (e.g. Teller and Lindsey. 1970) that a given disk luminance can produce greater or lesser sensitization in different subjects. The importance of the presence or absence of the steady disk in determining the effect an annulus has on threshold will now be briefly considered in the light of two models of visual Sunctionin~-a ‘~ecfarus model for spatial sensitization, as proposed by Copenhagen (1972) and less formally by Teller, Matter, Phillips and Alexander (1971): and Aipern, Rushton and Torii’s (1970a) model of the contrast-flash effect. According to the Necrurus model, the neural units mediating psychophysical sensitization have response properties very similar to those of mudpuppy bipolar cells. Copenhagen (1972) has suggested a mechanism, based on the data of Werblin and Dowling (1969). to account for the effect of an annulus on the response of bipolar cells. Copenhagen postulates that the response ofa bipolar cell to light on its receptive field center is an S-shaped function of stimulus energy (as suggested by the data of Werblin and Dowling, 1969). The effect of light Sailing on the surround of the receptive field is to antagonize the response to the central illumination and thus to shift the intensity-response function of the unit to the right (Werbiin and Dowling, 1969). This model predicts that the addition of an annulus to a stimulus field can result in either ~nsit~ation (the energy necessary for a criterion response to a test probe decreases when an annulus is added) or desensitization (the energy for a criterion response increases when the annuius is added). The crucial variable is the point on the intensity-response function at which the annulus is added. In other words, the crucial variable is the amount of light falling on the center of the receptive field (in the psychophysical analogue, the illuminance of the steady disk). Copenhagen (1972) found support for his model from his intracellular recordings. Both he and Burkhardt (1973) have shown that the mudpuppy bipolar

ceil shows sensitization. That is, the response of a bipolar ceil to a constant-intensity test spot increases as the diameter OSthe background on which the TS is superimposed increases (and thus the intensity necessary for a fixed response decreases). Copenhagen repeated this experiment on a ganglion cell. but blocked out a fixed circular region of the

h30

KENNETH R. AL~IXANWK

ccnct‘r of the stimulus. creating an annulus with a tit& inner diameter and increasing outer diameter. In this case, when the center of the receptive field was not &rectly illuminated, the annulus resulted only in an elevation of the ganglion ceil threshold. With the assumption that the response of a bipofar cell to a brief test probe is the anaiogue of the process mediating detection ofa test probe in a psychophysical observer under these conditions, the Nectum model developed by Copenhagen (1972) suggests that in the psychophysical paradigm, the presence or absence of a steady disk at the center of an annulus should determine whether the annulus will elevate or lower the threshold for a TS. In the Alpern t’r ul. (197&t) model for the contrastflash effect, neural signals generated by the test disk and annulus converge on a summating neural unit which transmits the resultant addition of signals to a detector m~hanism. Input to the summating unit from the test disk isexcitatory: input from the annuius is inhibitory. Thus the effect of the annulus is to increase the input from the test disk needed for detection. In the contrast-flash model, the presence of a stead!, disk on which the briefly-flashed test probe is prescnted is simply to elevate the threshold for the test stimulus, as in a typical increment threshold stud>. The annulus still has its own threshold-elevating effect. Thus according to the contrast-flash model, the annulus should elevate the threshold for the TS regardless of whether or not the stead] disk is present. It is clear that the two models make different predictions and postulate different underlying me~hanisnls. both of which involve a lateral interaction. The NPC.~~VII.S model predicts that in the presence of the stead! disk, the annulus should lower thresholds: in the absence of such a disk. the annulus would elevate the threshold. The contrast-flash model predicts that thresholds should be elevated by the annulus regardless of whether or not the disk is present. The predictions of the Nccmrus model are borne out completely for DT and partially for C‘W (Figs. 5 and 6). For DT, the annulus elevates the threshold when the steady disk is absent and lowers the threshold when the disk is present. For CW. the annulus elevates the threshold when the disk is absent. but when the disk is present. the annulus first elevates and then lowers the threshold. The prediction of the contrast-flash model when the disk is absent is supported by both DT and CW (Figs. 5 and 6). For both observers, the annuius in the absence of the disk elevates the threshold, However. the prediction of the contrast-flash model when the disk is present is clearly ~ntradi~ted by the resuhs for DT. For DT. the annuius serves to lower. not elevate. the threshold, For CW, as we have seen, the annulus elevates the threshold for the TS presented ncnr the annulus on.set. as predicted by the contraststash model, but lowers the threshold for a T’S presented after annuius onset. The contrast-flash model does not make specific predictions about the latter result.

Thuh undct- ~hc condition> cnlpl~~~I WIthih stud!. the presence or absence of a stead) &sk at the center of a flashing annuhts can be seen to have a determining eflect on whether the annulus elevates the threshold for a test stimulus (Contest-flash &ectt or lowers the threshold (transient sensitization). However. the direction of the change in the threshold produced hq the annulus is not always as predicted by the models--the .Vc~rttrtr.smodel for sensitization and the Alpern. Rushton and Torii model for the contrast-flash elect. <‘iearly the models need some modification to account for these data.

-Icktlowlctly~,t~,r~~~,~-,.-This research

wts

supporred

b!

USPHS Grant EY00421 10 Davida Teller and by a NIH postdoctoral fellowship EY54529-01to the author. I wish to thank the following people for their assistance: Dr. Teller for serving as subject and for her valuable comments and criticisms: Dr. Joseph Sturr for helpful comments on the experiments: Charles Williams for serving as subject and as a very capable experimenter; Ralph Morse for invaluable assistance in constructing the optical system: and Lowry Taylor Alexander for drafting the tigures.

REFERENCES Aguilar M. and Stiles W. S. (1954) Saturation of the rod mechanism of the retina at high lcvds of stimulation. Optira Acta 1, B-65. Alexander K. R. (1972) Spatial sensit&tion and the contrast-flash effect in human vision. Ph.D. Dissertation. University of Wa~ingto~ Seattle. Washington. Alexander K. R. (1973)The effect of test stimulus size in spatial sensitization. Paper presented at the Spring meeting of the Association for Research in Vision and Ophthal-

mology. Sarasota, Florida. Alpern M. (1965)Rod-cone independents in the after-flash effect. J. Physiol., Lond. 176,462~472. Alpern M. and Rushton W. A. H. (ie65) The specificity of the cone interaction in the aster-ash effect. J. Ph):uiol.. Land. 176.4-U-482. Alpern M. and Rushton W. A. H. (1967) The nature of the rise in threshold produced by contrast-flashes. .!. Phgsiol.. Lund. 189, 519-534. Alpern M., Rushton W. A. H. and Torii S, (197Oa) The size of rod sienals. J. Phl&l.. Lmd. 206. 193-,208. Alpern M.:Rushton %. A. H. and Torii S. (1970b) The attenuation of rod signals by backgrounds. J. Phpiof.. Lmd. 206,209-227.

Alpern M.. Rushton W. A. H. and Torii S. (197Oc) The attenuation of rod signals by bleaching.% J. Pkysiot.. Lond. 207.44946

1

Borkhardt D. A. (1973) Sensitizing efftcts of background illumination in ganglion, bipolar and horizontal cells. Paper presented at the Spring meeting of the A~ati~n for Research in Vision and Op~tha~moIogy, Sarasota. Rorida. Copenhagen D. R. (1972) The rote of the interneurons in coatrolting sensitivity in the retina: a microelectrode study in Necrurus. PBD. Dissertation. University of California, Berkeley. California. Sturr J. F. and Teller D. Y, (1973) Sensitizntioa by annular surrounds: Dihoptic properties. v’ision Res. 13,909-9IX.

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Werblin F. S. and Dowling J. E. (1969) Orgamzation of the retina of the mudpuppy. iCccrurus n~uculos~s--11: intracellular recording. J. ,~~~r~~~~~~iQ~~ 32, 339-355. Westheimer G. (1965) Spatial -interaction in the human retina during scotopic vision. J. Phrsiol.. to&. 181, 881-. 894.

Resume-Dans la ~nsibili~tio~ spatiale, la presentation d’un anneau sensibihsant abuis~e Ie seuil pour un test presenti: concentrjquement avec un disque continuell~m~nt eclairi qui remplit le centre de fanneau. Dans l’effet de “contrast&ash”. la pr~~ntation de I’anneau 8&e ie s&l du stimulus test qui remplit le centre de I’anneau. Darts le present travail. on altire s~st~matiquement les parametres du stimulus qui different pour ces deux paradigmes afin de converger vers we con~guration du stimu!us tel que l’anneau laisse le seuil fixc pour un changement d’une ou deux seulement des valeurs du stimulus. On trouve que Irs variables critiques pour I’effet de l’anneau sur le seuil sent la presence ou l’absence d’un disque stable au centre de l’anneau. et pour un des sujets la taille du stimulus test. On d&cute L~S rtsultats en termes des modeies de la sensibiii~~tion et de I’effet de ..con!rast-bash”, Zu~rnrnen~~~s~~-Bej artlichen Sensib~lisjerungscxperiment~n wird die Schwelle fir einen Testreiz. der im Mitteipunkt eines homogenen Feldes dargeboten wird, durch das Ei~schalten eines Ringes erniedrigt. Beim Kontrast-Blitz-Effekt rrhiiitt das Einschalten des Ringes die Schweile fur den Testreiz. der das Ringinnere ausfiillt. In der vorliegende~ ~~ntersuchung wurden die Parameter. in denen sich die b&den Versuch~nord. nungen unterscheiden. systematisch verlndert. urn eine Konfigurat~on zu finden. in der der Ring die Schwelle fiir das Testzeichen weder erhoht noch erniedrigt. Dabei so&e das ‘I’estfeld nur in ein oder zwei Parametern gegndert werden. Es zeigte sich. dass die kritischen Variablen. die den Einfhtss eines Rings auf die Schwelle bestimmen. die ununterbrochene ~drbietun~ bzw. das Fehlen des Ringinnere~ sind; fur eine Testperson war die GrGsse des Testreizes massgebend. Diese Ergebnisse werden im Hinblick auf die Mod&e iiber die Emp~ndiichkeits~~nderung und den Kontrast-Blitz-Effekt diskutiert.

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