CONTRAST

SENSITIVITY DURING EYE MOVEMENTS

SACCADIC

FRANCESC. VOLKMANN Department of Psychology. Clark Science Center. Smith College. Northampton.

MA 01063, U.S.A.

LOIUZIX A. RIGGS Hunter Laboratory of Psychology, Brown University, Providence, RI 02912, U.S.A. KEITH D. WHITE Department

of Psychology, University of Florida, Gainesville, FL 32611, U.S.A and ROBERT K. MOORE

Hunter Laboratory of Psychology. Brown University, Providence, RI 02912. U.S.A. (Receiced 30 June 1917: in revised form 29 Nowmber 1977) Abstract-The experiment measured contrast sensitivity of three human observers to sinusoidal gratings presented in 10 msec exposures. The gratings were presented to the steadily fixating eye and during 6” horizontal saccades. Experimental conditions of viewing in a Ganzfeld reduced possible effects of contour masking. The use of horizontal gratings minimized retinal smear. Results showed a significant suppression of sensitivity (more than 0.6 log unit of contrast) to low spatial frequency gratings presented during saccades. The magnitude of saccadic suppression decreased as spatial frequency of the gratings increased. We conclude that optical and neural effects combine in normal viewing to produce saccadic suppression. ~inim~ing the optically originating effects of contour masking and retinal image smear failed to eliminate the considerable impairment of vision that occurs during a saccadic eye movement. Key Words--contrast

sensitivity; eye movements; saccadic suppression; spatial frequency.

INTRODUCTTON The rn~n~t~~e and time-course

ofsaccadic oppression

Early observers noted that vision is a phenomenally

clear and continuous process, even though it may often be interrupted by saccadic eye movements that sweep the image across the retina as we shift our gaze from one object to another (Dodge, 1900, 1905; Hok, 1903; ~oodworth, 1906). They concluded that vision of the smeared retinal image is decreased during saccades in most conditions of normai viewing, so that the smeared image is not noticed at all. More recent quantitative investigations have shown that vision is not completely absent during saccades, but that visual thresholds are elevated significantly to stimuli presented during or in close temporal proximity to saccades (for reviews, see Matin, 1974; Volkmann, 1976). The magnitude of this elevation and its time course vary with specific stimulus conditions. Typically, for the light-adapted eye, thresholds are found to be raised when stimuli are presented at any time within a 100-150 msez period, beginning about 50 msec before the onset of the saccade. Threshold elevation reaches a maximum of approximately 0.5 log unit of relative luminance to stimuli delivered in mid-saccade. This threshold elevation has come to be called “saccadic suppression”. It has been investigated with a variety of stimuli ranging from punctate or diffuse flashes of Iight on a dark background (see e.g. Latour, 1962, i966; Brooks and Fuchs, 1975) to points of tight, alphabet letters, or words flashed as

a pattern of added brightness on a steady light field (see e.g. Volkmann, 1962; Uttal and Smith, 1968). The present experiment provides an evaluation of the magnitude and time course of saccadic suppression using a basic visual function, that of contrast sensitivity. Contrast thresholds are determined as a function of spatial frequency for both the saccading and fixating eye. Mechanisms

of saccadic

suppression

Saccadic suppression has been attributed to each of a number of possible mechanisms and to various combinations of them (Matin, 1974; Volkmann, 1976; Curnming, in press). A primary theoretical issue is whether or not saccadic suppression is attributable at least in part to an active, centrally originating neural event such as a corollary discharge (von Helmholtz, 1867; Sperry, 19%; von Holsf 1954) which operates to impair vision during saccades (Volkmann, Schick and Riggs, 1968) Possible mechanisms of sup pression which do not rest on such a neural event include (a) visual impairment resulting from the smear of the stimulus on the rapidly moving retina (Dodge, 1900, 1905; Woodworth, 1906; Mitrani, Mateeff and Yakimoff, 1970a); (b) visual effects of shearing forces set up within the retina by a saccade (Richards, 1969), and (c) visual masking effects which, though central and neural in their operation, originate solely as a result of spatial and temporal relations between stimulus patterns at the retinal level (Alpem, 1953,

1193

1969: Kahneman. 1968; Weisstein, 1972; L&ton, 1973; Breitmeyer and Gang, 1976: Weisstein. Harris, Berbaum, Tangney and Williams, 1977). Evaluation of these altemative~interpretations is. of course. incomplete. Crnrral i~hihifion is the most difficult to evaluate since it can only be measured in isotation by experiments which take particular care to eliminate alI other sources of visual impairment. Rrrinal anrar is important in impairing vision during saccades (Dodge, 1900, 1905; Mitrani et al.. 197Oa; Wurtz and Campbell. 1977: Moore. Volkmann and Riggs, in preparation). Suppression atso occurs. however. when smear is minimized by presenting the stimulus to the saccading eye in an exposure lasting only a few microseconds (Volkmann, 1962; Latour, I966). Retirzaf shear has not been extensively investigated. although some evidence for it exists (Richards, 1969). If this variable were important. for example, ma& tude of suppression should vary with saccade size. a finding about which clear agreement is not avaiiabfe (for review, see Volkmann. 1976). Visual masking has been shown to play a role in saccadic suppression (MacKay, 1970; Mitrani et al., 197Ob, 1971; Matin, Clymer and Matin. 19f.2; Davidson. Fox and Dick. 1973; Mitrani. Yakimoff and Mateeff. 1973; Vaughan, 1973; Matin, 1974, 1975; Brooks and Fuchs. 1975. 1977: Mitrani. Radii-Weiss. Yakimoff, Mateeff and Boikov. 1975; Breitmeyer and Ganz. 1976; Mateeff. Yakimoff and Mitrani. 1976: White. 1976). More generally, events occurring in the peripheral retina are known to affect visual responses near the fovea (e.g. Mcllwain, 1964, 1966; Fischer, Kriiger and Droll, 1975). The rapid motion of a contoured visual field across the retina of either the fixating or the moving eye is known to set up spatial and temporal sequences of stimulation that can produce metacontrast or other masking effects (MacKay, 1970; flrooks and Fuchs, 1975; Breitmeyer and Ganz, 1976). We can be almost certain that effects such as these are prominent under conditions of normal viewing in a brightly illuminated. contoured environment. and that they operate effectively to impair visibility of stimuli which are present during shifts of gaze. Indeed, some investigators have suggested that in normal viewing, masking conditions are brought about primarily by saccades; within an evolutionary context, the adaptive value to- the organism of such masking effects may include the suppression of vision during saccades (Matin. 1974, 1975; Breitmeyer and Ganz, 1976). Althou~ masking effects are important in decreasing vision during sacfades under certain conditions. the extension of a masking hypothesis to account for saccadic suppression under all conditions is not warranted. Riggs, Merton and Morton (1974) demonstrated a small but significant elevation of threshold to visual phosphenes produced eiectrically during saccades executed in complete darkness, when no contour, and thus no masking effects which depend upon contour, could be present in the field. However, Brooks and Fuchs (1975) did not find saccadic sup pression to stimuli flashed during saccades in the dark. The two outcomes may be due in part to differences in experimental conditions. For example. differ1973: Scheerer.

tnt psychophysical methods were used. Brooks md Fuchs employed a yes;no detection task, in a variant of the method of limits in which observers were given

5-7 stimulations in which to detecr a flash. Riggs r’ al. used a two-alternative forced-choice procedure. which Green and Swets (1966) have advocated as 9 means of maintaining a symmetric response criterioi. The present experiment minimizes contour in theviewing field and optical smear in the test pattern while maintaining photopic levels of luminance. Thus suppression due to contour shifts across the retina

should be eliminated reduced.

or at leasr very substantially

.MF.THOD The experiment

was designed to meet four sets of re-

quirements: (1) To present sinusoidal grating stimuli to the macufa at known times before, during. and after saccades. in such a way as to minimizer optical smear. ;utd to monitor the saccades and the stimulus exposures. (2) To measure contrast sensitivity to the gratings while maintaining a space-average fumirianfe in the stimulus field equal to the luminance of the fixation field. @To minimize masking effects produced by contour in the visual field while maintaining a fairly high photopic level of luminance. (4) To minimize stimulus context effects and to assist the observer in ~~ntain~n~ a symmetric criterion by presenting the gratings accord&g COa.method bf constant stimuli and by obtaining judgments -wording to a two-alternative forced-choice psychophysical procedure. Viewing conditions The observer sat with his or her head in a verticalfy oriented cylindrical Ganzfeld about 3Ocm in diameter. The matte white surface (Nextel velvet coating, IOL-AI0 white. 3 ;M Co.) of the interior was illuminated from above _and below the fiefd of view, at a luminance of about 30 ft-L. Forehead and chin rests held the head in posit@n, ed the left eye was covered with a patch. With the right eye the observer viewed two thin vertical fixation-guides located 6” apart in a 9.5” square fietdrna@ed in hue and luminance to the Gantfeld These fix&ion guides, shown in Fig 1. were viewed at an accornmodativ~ distance of 1III. The observer was instructed to bxate the midpoint of the blank area between one or the other St of vertical guidelines (the “fixation gap”); this gap subtended I” 30’. He or she was instructed to make a 6’ horizontaf saczade by f&king quickly from the gap between one set of vertical tines to the gap between the other set These viewing conditions had the effect of minimiring contour in the visual field. The outer edges Of the Gandeld were not visible in the peripheral visual fields. The edges of the 9.5’ viewing window were scarcely +%pc_table.Being close to the eye. they were out of focus and blurred-into the surrounding Ganzfeld. A small mirror, through which the fixation and stimulus fields were viewed, was located as close as tcm from the eye. so that its ed@s blurred into the surrounding field (see Fig. 2). fr si’touid be emphasized that the color and huninance d the re&eted f%lds were carefully matched to those of the suwnding field, so that the entire field of view blended iato~a contourless Ganzfeld. The fixation guides were in sharp FOEU$but not present in the fovea. Having the observer fixate a gap rather than a fixation point or spot minim&d the masking that such a point might otherwise produce (Weisstein et a!.. 1973). We cannot say. of course. that alt m&king-effects have been completely eliminated. Sprc&ca&y, an observer might occasion&fly notice the shadow of&e no* (if attending to it). Small differences in luminance which occurred in different parts of the Ganzfeld were outside of the

observer’s field of view.

1195

Contrast sensitivity during saccadic eye movements Recording display

of saccades.

trzggering of stimulus exposures

and

Figure 2 diagrams the experimental arrangements for stimulation and recording. The bitemporal electrooculogram (EOG). after appropriate amplification. was displayed on a storage oscilloscope to monitor the horizontal saccades used in the experiment. The EOG could also be used to trigger the timing circuits of an Iconix three-channel tachistoscope. the lamps of which provided light for the fixation field and the stimulus exposures. By varying the delay set into the trigger circuit. the experimenter could cause the stimulus field to replace the fixation field in a 10 msec exposure at various times during or after a saccade. To present the stimulus before a saccade the experimenter activated the tachistoscope manually just after delivering a signal which told the observer to look from one fixation gap to the other. The time of exposure of the stimulus in relation to saccade onset was displayed, along with the EOG. on the storage oscilloscope. In addition. a digital clock was activated on each trial by either the stimulus exposure or the saccade. whichever occurred first. and stopped by the second of these events; this provided a direct readout of the temporal relation in msec between the saccade and the stimulus exposure.

EYE’ BEAMSPLITTERS J Fig. 2. Block diagram of apparatus. At left are shown the systems for recording the electro-oculogram (EOG) and using it to trigger the lamps of a three-channel tachistoscope to present stimulus exposures in various temporal relations to saccades. At right is the optical system for replacing the steady light fixation field with blank stimuli or with gratings of variable contrast. while maintaining space-average luminance equal to that of the fixation held. For further explanation see text.

Stimuli

Spatial characteristics. The stimuli consisted of horizontal gratings with a sinusoidal luminance profile. Four spatial frequencies were used: 0.21, 0.65, 1.85 and 4.5 cideg of visual angle. The inclusion of the lower spatial frequencies minimized contour in the stimulus field. and also minimized possible etfects of changes in accommodation which might occur during saccades. The inclusion of the higher values permitted investigation at frequencies near those at which the human contrast sensitivity function reaches a maximum (Schade. 1956; Campbell and Robson, 1968). Still higher spatial frequencies were avoided because of possible failures of accommodation and increasing retinal smear in cases where saccades were not executed in a perfectly horizontal path (see Discussion). The grating stimuli filled the entire 9.5” square field during their 10 msec exposures. ensuring stimulation of the entire macular region regardless of where the eye might be in its excursion from one set of fixation guides to the

Fig. I. Fixation guides used in the present experiment. In moving-eye conditions observers fixated in the middle of the gap in the left or right guide, and executed 6’ horizontal saccades to the middle of the other gap. In fixating-eye conditions. observers fixated the middle of the field between the two guides.

other. The gratings were always oriented horizontally to minimize smear of the stimulus on the horizontally moving retina. The procedure of the experiment also called for the use of blank film as a stimulus. This film matched the grating stimuli in both spectral transmission characteristics and space-average luminance. Any given stimulus exposure might consist of presentation of either a grating of a psrtitular low contrast, or of a matched blank held consisting of the film. Conrrasr and luminance characteristics. Grating contrast. defined as C = (L,,,, - L,,,)i(L,,, + L,,,). constituted the principal independent variable in the present experiment. The maximum contrast attainable at each spatial frequency was first determined. The grating image projected and enlarged by the optical system was scanned vertically with a horizontally oriented l/30 mm x 5 mm slit mounted on a photomultiplier photometer (Photovolt Corporation). Attenuation of contrast, while maintaining constant spaceaverage luminance, was achieved by mixing light from the stimulus channel with that from a uniform balance channel. This arrangement is shown at the right-hand side of Fig. 2. The space-average luminance of the stimulus channel alone was first adjusted to match that of the fixation channel by means of a circular Inconel neutral density wedge (Bausch & Lomb). Lower values of contrast were produced by changing the wedge setting to decrease the amount of light coming through the stimulus channel; at the same time a second similar wedge in the balance channel was rotated to increase proportionally the amount of light from that source, so that constant space-average luminance was maintained equal to that of the fixation field and of the Ganzfeld. In this situation the contrast is equal to the maximum contrast attainable multiplied by the proportion of light coming from the stimulus channel. The optical system on the right-hand side of Fig. 2 shows how light from the three tachistoscope channels was brought to the eye. so that either the fixation marks or the stimulus could be viewed at an optical distance of 1m. Temporal characteristics. A stimulus exposure required that the fixation field be turned off for 10 msec, and replaced during that interval by the stimulus and balance fields acting together. A minor transient accompanied the beginning and end of this substitution. We monitored the system with a very fast photo-diode and adjusted the _ lcomx timing circuits to produce the smallest possible

1196

FRANCE C.

transient. With proper adjustment. observers were unable to tell when the stimulus replaced ?he fixation field, regardless of whether the stimulus consisted of a blank film or a grating of sufficiently low contrast. Psychophysical methods and experimental procedures

VOLK.MANN

er

ai.

proximateiy -IO I-hr sessions each. One of the observers wore spectacles for optimal correction of vision at the required 1 m distance. RESULlS

The observer began each moving-eye trial by fixating the gap between the designated (right or left) set of vertical fixation guides. Then, on signal. the observer executed a horizontal saccade to the other fixation gap. and on another signal about I seelater, saccaded back again to the first gap. Before. during. or after one saccade of the pair. a sinusoidai gra?ing of preset contrast and spatial frequency replaced the fixation field in a 10 msec exposure. In the same temporal relation to the other saccade of the pair a blank field of equal average luminance replaced the fixation fieid. also in a 10 msec exposure. The observer was instructed to indicate which of the saccades was accompanied by the grating exposure. This two-alternative forced-choice procedure applied to each triat within a psychophysical session designed according to a constant stimulus method. Contrast and time of stimulation in relation to a saccade varied from trial to trial in a predetermined order. and spatial frequency varied in blocks of trials such that each of the four spatial frequencies occurred in 25% of the trials of a given session. Each saccade. and the temporal relation of the stimulus exposure to it, was monitored, and the time of the stimulus recorded. along with the observer’s judgment. The procedure during fixating-eye trials was identical to that described above, except the observer was instructed to fixate the middle of the field between the left and right fixation guides. Two women and one man served as observers for ap

’ This is obviously true despite an unfortunate technical limitation, namely that the highest grating contrast used at that spatial frequency was 0.35. Note that higher contrasts would have been needed to bring the proportion of correct judgments up to the criterion level of 0.75 for the moving eye data at 0.21 c/deg. ’ Calling the observed proportions PO. we uansformed them to corrected proportions P, according to the formula P, = 2 (PO - 0.50) (see e.g. Harrison and Harrison, 1951; Blackwell, 1953). The corrected proportions were then transformed to r-scores. A linear regression using the method of least-squares was applied between these z-scores and log contrast for each condition and spatiaf frequency. (Average value r2 = 0.94.) Within the data for any one regression we have adopted a number of conventions. If P, assumed a negative value, that datum, and any data for lower contrasts, were discarded. Similarly, if P, reached 1.00 then that datum, and any data for higher contrasts, were discarded The tfiresholds plotted in Fig, 4 are the con?rasts predicted to yield 0.75 correct performance in the forced-choice procedure. These manipulations can be defended for all the points on Fig 4 with the possible exception of ?he one for the moving eye at the lowest spatial frequency. That point exhibits an extrapolated contrast threshoid of about 0.55, which lies outside any actually obtainable contrast in this experiment. At face value, that threshold is higher by a lactor of six than the correspond@ threshold for the 6xating eye at that spatial frequency. While such exnapolation is admittedly ques?ionabIe, we note from Fig. 3 that the moving-eye data reach reliabfe above-chance performanfe levels at the highest (0.35) contrast vaiue used and that the corresponding levels of performance for the &sitting eye are at contrast values that are lower by a factor of at least four. fn summary, we conclude that a? a spatial frequency of 0.21 c/deg saccadic suppression reduces contrast sensitivity by about 0.6-0.8 log unit.

Figure 3 shows psychophysical ‘functions for the moving eye and for the fixating eye at four d&rent values of spatial frequency. Stimuli for the moving eye were triggered by the saeeade, and occUrred within 50 msec of its onset, Group data are -shown

since the data of the three observers did not differ substantially. Each of the small &phs shows the proportion of correct forced-choice judgments as a function of contrast for a given spatial frequency. The number of judgments per plotted point {N) varies, but on the average is 75. The vertical bars &ociated with each point indicate 95% confidence limits for the plotted proportion. The graphs of Fig. 3 show that the eye is typicrtify less sensitive to grating contrast when the grating is presented in close temporal proximity to a saccade than when it is presented during steady fixation. At the lowest value of spatial frequency (0.21 c/deg) there is no overlap at ail between the 95% confidence -intervals for the moving eye and ftxating eye data.‘. As the spatial frequency increases, however, there is a progressively increasing overlap of these intervals until, at 4.5 c/de& the overlap is substantial. Figure 4 shows, for the fixating-eye and the moving eye, threshold contrasts required for de?ection at each of the four spatial frequencies. We derived these contrasts from linear regressions’ fitted to the data depicted in Fig. 3. The curve for the fixating eye shows a relatively small variation of contrast sensitivity over the range of spatial frequencies covered in this experiment. Its shape is simihu to ?hat

contrast Fig. 3. Psychophysical judgments of gratings exposed during or immediately foffowing a 6’ saccade, and of the same gratings exposed during steady fixation, for four values of spatial frequency of‘the gratings. Each curve shows proportion of-correct forced-choice response-$ on a probabi@y

scale as a function of grating contest, on a logarithm?c scale, for the combined data of three observers. Vertical Lines indicate 95% confidence limits for each point.

Contrast sensitivity during saccadic eye

o Flxotlng 0 Moving

Spat101

eye eye

frequency

Fig. 4. Contrast sensitivity as a function of spatial frequency for the saccading and fixating eye. Plotted points are contrast thresholds derived from functions fitted to the combined data of three observers. For further details see text.

reported by Kelly (1977) for 10 msec exposures of a grating. The curve for the moving eye shows a larger variation; in particular, contrast sensitivity showed maximal impairment of from 0.6 to 0.8 log unit at 0.21 c/deg (the lowest spatial frequency) and impairments of about 0.33, 0.27 and 0.25 log units at the spatial frequencies of 0.65, 1.85 and 4.3 c/deg, respectively. Time course of saccadic

suppression

of contrast sensi-

ticity Figure 5 shows the time course of saccadic suppression in this experiment. We fitted functions to data of the type shown in Fig. 3, for each of eight temporal relations between the stimulus exposure and the saccade, and derived from them a threshold value of con-

-100

0

Time of

100 200 stimulus

In relatlon

-ice

0

to onset

loo 200

of saccode,

msec Fig. 5. Time course of the saccadic suppression of contrast sensitivity. Each curve shows changes in contrast threshold as a function of the time at which the stimulus occurred in relation to the onset of a saccade, for a given spatial frequency of the grating. Horizontal dashed lines indicate threshold contrast under conditions of steady fixation. Group data from three observers. For further explanation see text,

1197

movements

trast, as described above. Each of the small graphs in Fig. j Shows contrast threshold as a function of the temporal relation between the stimulus exposure and the Saccade, for a given spatial frequency. The horizontal dashed fines indicate contrast threshold for the steadily fixating eye. Each point on the curves is a running average (see Guilford, 1956) representing the midpoint of a 100 msec interval, although samples were taken every j0 msec. This smoothing procedure helps to counteract the effects of the smah numbers of observations which occurred in some of the time bins at the extreme ends of the scale. Although this m&in&s deviations att~butable to chance, it also, of course, reduces the apparent magnitude of any real decrement in sensitivity that accompanies the saccade. fie extent of this reduction is related to the coarseness of the sampling interval. Consequently, the smoothed data of Fig. 5 represent conservative estimates of the temporal fluctuations in contrast sensitivity associated with the saccade. Contrast sensitivity begins to decrease before saccade onset, reaches a minimum during the saccade, and appears to oscillate after the saccade before approaching the value determined for the steadily fixating eye. DSCBSION

lnterpretation

of the present results

At lower spatial frequencies, the elevation of contrast thresholds associated with saccades is similar in magnitude and duration to values typically reported for various other visual tasks (see Introduction; Matin, 1974; Volkrnann et al., 1968). However, the magnitude of suppression decreases as spatial frequency increases. This finding is inconsistent with the explanations of suppression in terms of retinal smear. errors of acco~odation and contour masking. Any image smeared across the retina has both horizontal and vertical components; the horizontal component was minimized by using horizontally oriented test gratings and horizontal saccades. It is probable, however, that the observers’ saccades included some irreducible vertiti component. Such a component, if present to any significant extent, should have resulted in a huger amount of suppression at the higher spatial frequencies since a relatively larger fraction of a cycle would have been smeared by that component. Our opposing results show no main effect attributable to such a source. Similarly, explanations in terms of errors of accommodation accompanying a saccade appear to be ruled out, since they also would predict greater suppression at higher spatial frequencies. Finally, the results show that suppression occurs under conditions of minimal contour masking. Maximum suppression occurs for gratings of low spatial frequency, which introduce minimal contour into the stimulus field; and the same minimal contour fixation field was used throughout These results, along with the results of experiments cited in the Introduction, suggest to us that saccadic suppression is produced by neural events, either alone or in conjunction with optical factors (see also Volkmann, Riggs, Moore and White, 1978; Bridgeman. 1977). These combine to favor maximally clear fovea1 vision when the stimulus occurs several hundred msec after the saccade.

Impticarions jbr normal h3Gng

Upem M. (1949) Movements

Under conditions of normal viewing in a contoured environment_ spatial and temporal aspects of contour maSkbIg are important determiners of saccadic suppression. We saccade to a stimulus of interest. This StimulUS acts to impair our vision of stimuli in nearby of the same retinal locations at some time during or just prior to the saccade (Alpern. 1969: Breitmeyer and Ganz. 1976: Matin. 1974: see also Introduction). Such masking effects should decrease in strength or probability with decreasing contour information in the visual field (e.g. decreasing spatial frequency). Saccadic suppression is also consistent with the effects of smearing of the stimulus image during the saccade. Because of the relatively long integration time of the eye. retina1 smear has the effect of diminishing local contrast (see also Matin. 1974). This factor must play an important role during ordinary saccades, especially when sharply focussed and complex contours having high spatial frequency components are present in the field. The effect of retinal smear should diminish, however, as saccade amplitude decreases or in a low spatial frequency environment. If the present analysis is correct. both contour masking and retinal smear would be expected to produce more suppression of stimuli having moderate to high spatial frequencies than of those having low spatial frequencies. Suppression of lower and lower spatial frequencies must increasingly originate from another source. The neural component of suppression would appear to constitute such a source: under the conditions of the present experiment, where contour masking and retina1 smear were reduced to a minimum. suppression was found to be maximum for stimuli of low spatial frequencies. Under naturally occurring conditions of minimal contour and contrast. the contributions of contour masking and retinal smear must be similarty reduced. Saccadic suppression which occurs under these conditions must then be increasingly attributable to a neural component. Over the wide range of contrast and contour information present in normal viewing. saecadic suppression must be composed of varying contributions of contour masking, retinal smear and neural inhibition. Under all conditions the system acts to impair vision during the saccade and to provide clear vision of the stimulus of interest in the fixational pause that follows the saccade. ,fcknowledgemenrs-This research was supported by Grant No. GB-41103 ~~NS7~01135) from the National Science Foundation to the first two authors. and was conducted at Brown University while the first author was on sabbatical leave from Smith Cobge. Portions were prmttd at the Association for Research in Vision and Ophthalmology, Sarasota, FL, 1975. We are indebted to John Volkfor help in apparatus construction. to Thomas I&&r, Jillian Van Nostrand and Martha Romeskie for serving as o&rvers, and to Jerrilpn Peters for help with data analysis. The gratings were provided to US as 2 x 2 transparencies by C. Stromeyer. mann

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