Detection threshold differences to crossed and uncrossed disparities

Vlrlon Res. Vol. 27, No. 9,

pp. 16834686,

1987

0042-6989/87 53.00 + 0.00

Printed in Chat Britain. All rights mcrvcd

Copyright Q 1987 Pergamon Journals Ltd

RESEARCH NOTE

DETECTION

THRESHOLD DIFFERENCES TO CROSSED AND UNCROSSED DISPARITIES

MARK L. MANNING,

DAVID

Psychology Department,

C.

FINLAY,

Rooaa A. NEILLand BARRY G. FROST

University of Newcastle, N.S.W. 2308, Australia

(Received 23 October 1986; in revisedform 12 February 1987) Abstract-Using a sample of 85 subjects measurements were made of minimum stimulus durations necessary for detection of crossed and uncrossed disparity stimuli which were presented in five positions in the visual field centre, lower. upper, right, and left field. The results indicated large detection duration differences between the two disparity conditions, with a marked superiority for crossed disparity detection at all positions. A left-right visual field anisotropy was demonstrated for crossed disparity stimuli. Dynamic random dot stereograms

Stereopsis

Detection threshold

Dynamic Random-Dot stereograms (DRDSs) provide a convenient means of studying stereopsis in the absence of monocular cues (Julesz, 1971; Neil1 et al., 1987). Despite the large number of papers reporting this technique, little attention has been paid to visual field differences (Dumford and Kimura, 1971; Breitmeyer et al., 1975; Julesz et al., 1976; Grabowska, 1983) or to the relative effectiveness of crossed and uncrossed disparity in producing depth impressions. There is, however, some evidence that crossed and uncrossed disparities are processed differently within the visual system (Mustillo, 1985). The DRDSs were presented on a pair of Tektronix 602 display units supported vertically on trolleys secured to the moving arms of a Clement-Clarke Synoptophore (Neil1 and Fenelon, 1981). The trolleys rode on ball-bearing castors so that interpupillary separation and vergence angle could be adjusted with the normal synoptophore controls. Each display could also be moved vertically allowing a vertical correction of over 4 deg. The two display images were conveyed to the viewer by half-silvered mirrors which allowed superimposition of a central fixation point (a red LED, subtending 8 arc min) in the same plane as the images. Object to eyepiece distance was 400 mm with the physical dimensions of the display field being 74.5 mm*. The display size was 256 x the size of a central 150 arc set pixel, yielding a display of approximately 10.67 deg square. Test

Visual field

Disparity

stimuli were presented at a subjectively nearinfinite distance (2.25 D eyepiece lenses allowed 0.25 D instrumental accommodation, giving an optical distance of 4 m). The stimuli were 2 deg square with a disparity for both crossed and uncrossed stimuli of 0.25 degs. Dot pairs were generated, at the rate of 75,000 pairs+, in a point-to-point mode by a hardwired dynamic stereopattern generator module consisting of (Neil1 and Kennewell, 1979); a clock circuit to synchronize all of the operations performed by the device (including dot generation rate which may be varied from 3000 to over 250,000 pairs/ set), 20 bit digital pseudo-random number generators to generate the horizontal and vertical coordinates and a disparity generator module. In this module all of the logical and arithmetic procedures were carried out. The new horizontal and original horizontal and vertical coordinates then entered the digital to analog (D/A) converter module and from there into the oscilloscopes via a set of analog switches. These switches allowed the left and right displays to be exchanged, defining the sense of the disparity (crossed or uncrossed). The hardwired dynamic stereopattern generator was controlled by a SC/MP II based microcomputer via seven digital control inputs. The first set the disparity magnitude, the second allowed the disparate region to be of either crossed or uncrossed disparity, and the third controlled onset/offset of disparity. The fourth and fifth inputs specified the height and width of

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Research Note

the disparate region and the last two specified the location of the lower extremities of the rectangular disparate region. In appearance the black background field was covered by a large number of light green points in apparent Brownian motion. The stereoscopic stimuli, when seen, appeared as either a square plane elevated in front of the background field (crossed disparity) or as a square “window” through which a plane of identical texture could be seen (uncrossed disparity). Three subjects, of an initial sample of 88, were excluded from testing due to stereoblindness. Each of the 85 stereo-perceiving subjects included in the study were presented with crossed and uncrossed disparity stimuli at each of five positions in the field: at the fixation point, or 3 deg offset to the right, left, above, or below the fixation point. The task was to maintain central fixation at all times and to identify the position of a presented stereoscopic square by pressing one of five buttons corresponding to the five possible stimulus positions. The procedure used to establish stimulus detection duration was as follows. A program randomly selected a stimulus position (and one of the two disparities) and presented a square for a duration of 10 msec. If the square was not detected it was presented again for the same duration. Failure to detect it a second time led to the duration being incremented, in this case to 20 msec. Two trials were then presented at this longer duration. The incremental procedure continued until correct responses were made by the subject. If the subject made the appropriate response on only one out of two presentations, a third stimulus was presented at that duration. When the subject could correctly detect the stimulus twice (either twice in a row or two out of three) at a given duration, the duration was taken to be the subject’s score for that visual field and disparity condition. A new position and disparity were then randomly selected and the procedure recommenced. The interval between successive stimulus presentations was four seconds and durations were 10,20,30,40,50,60,80, 100, 120, 150,200, 300, 500 and 1OOOmsec. Each subject’s data consisted of the minimum duration necessary for stimulus detection (detection duration) for the two disparity conditions at each of the five visual field positions. A disparity (2) x field position (5) analysis of variance was conducted on the detection duration data. Significant effects were found for

disparity [F(l,84) = 105.03, P < O.OOOl],position [F(4,336) = 12.39, P < O.OOOl]and for the disparity x position interaction [F( 1,84) = 8.02, P < 0.0001]. Post-hoc analyses were conducted on the disparity x field position interaction using ttests with a Bonferoni adjustment (a = 0.05, adjusted to a = 0.005). The disparity difference was found to be significant at each of the individual field positions [centre field, t(84) = 5.17, P < 0.0001; lower field, t(84) = 8.43, P c 0.0001; upper field, r(84) = 7.29, P < 0.0001; left field, t(84) = 6.37, P < 0.0001; right field, t(84) = 5.74, P < O.OOOl]. Post-hoc analysis within each disparity condition revealed, for crossed disparity only, significantly lower detection durations for stimuli presented in the left visual field compared to the right [t(84) = 3.37, P < O.OOS]and lower [t(84) = 3.03, P < O.OOS]fields. Similar right/left hemifield anisotropies have been reported (Durnford and Kimura, 1971) for a shape detection task. Others (Breitmeyer et al., 1975; Julesz et al., 1976) have noted upper/lower visual field anisotropies of detection duration for very small stimuli presented within one degree of the fixation point. Post-hoc analyses found no significant upper - lower differences for either crossed [t (84) = 0.19, P > 0.051 or uncrossed [t(84) = 1.25, P > 0.051 disparities. No significant difference was found between the centre field durations and the four surrounding positions for crossed disparity (P > 0.05 in each case). In the uncrossed condition, however, centre field durations were significantly lower than both upper [t(84) = 4.45, P < O.OOOl]and lower [t(84) = 6.04, P < O.OOOl]fields but not different to left [t(84) = 2.11, P > 0.011 or right [t(84) = 1.62, P > 0.051 fields. It is evident from the small standard errors shown on Fig. 1 that the data for crossed and uncrossed disparity are very uniform across a relatively large group of subjects, n = 85. The regularity across all field conditions provides good support for the proposal that crossed and uncrossed disparity may be processed by different pools of disparity detectors (Fischer and Poggio, 1979). These interpretations should perhaps be treated with some caution as the interrelationship between disparity- and formdetection has not been considered. The response in the individual case may be to the perception of a disparate form (square shape) or to an area

Research Note

VIWL

FIELO POSITION

Fig. 1. Detection durations for a 2 deg square dynamic random dot stereoimage for a central target and at four positions offset 3 deg (lower, upper, left and right; n = 85).

of disparity. Differences in form detection processes may render a square lying in front of a background plane more easily recognised than a square lying behind the plane. Under these circumstances, where response is delayed until a clear perception of the form takes place, longer detection thresholds would be expected (Over and Long, 1973) as would greater relative crossed/uncrossed disparity differences than in the circumstances where the response takes place to the perception of an area of disparity of no specific shape. To test which of these alternative modes of perception may apply, the data of the 10 subjects with the lowest average detection threshold were considered. The mean threshold for these subjects was 63.0 msec across all stimuli. At a dot pair presentation rate of 75,000 pairs/set, a 2-deg square stimulus with a duration of 60 msec is defined by only 157 point-pairs in the 10.67 deg square dot display. At this rate there would be insufficient points to define a stereoscopic form clearly, yet subjects still produced a very large crossed (8 = 38.6msec, 101 point-pairs) vs uncrossed (R = 87.4 msec, 229 point pairs) difference. It should be emphasised, furthermore, that the apparatus used did not create a sequence of frames to generate the disparate images but used a random-dot generator continuously presented pairs of points on the displays. As such, at very short stimulus durations, a clearly defined stereoimage would not be presented. New disparate point-pairs would gradually overlie

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non-disparate point-pairs still perceptually “present*’ due to the phosphor decay and visual persistence. In the present experiment percep tion of form is unlikely to be an explanation of these crossed/uncrossed disparity differences. Further important findings relate the spread, across the group, of threshold durations, both within and across the disparity/position conditions. Of the 85 stereo-perceivers five failed to report the stereoimage at the longest exposure duration (1000 msec). For each of the subjects, however, this was confined to one uncrossed disparity and visual field position; two did not report the upperfield stimulus, two the lowerfleld stimulus, and one the left visual field stimulus. The importance of exposure duration has been noted (e.g. Patterson and Fox, 1984) and this data demonstrates that it is possible that the width of the “normal” distribution envelope, for the detection threshold of uncrossed disparity in DRDSs, is such that a 1000 msec duration is insufficient for detection to occur in a significant number of subjects. The five subjects of the present report may have been able to detect stimuli of longer duration. Some researchers have argued (see Mustillo, 1985 for a review) for the absence of classes of disparity detectors on the basis of findings emerging from the use of fixed duration stimuli, usually of less than 100 msec (to prevent eye movements). The data reported here show clearly that most individuals achieve stereofusion, but some only with relatively long stimulus exposures. Definitions of stereo-anomaly need to consider this. This study further reports a high proportion of subjects achieving stereopsis in all positions (96.6% for crossed disparity and 90.9% for uncrossed; n = 88-including the three stereoblind subjects not included in the data analysis). This is a higher proportion than has been reported by Julesz (1971) but comparable to that more recently reported by Newhouse and Uttal (1982) using continuously viewed stereogram displays. The present study has found quite different patterns of left-right and upper-lower effects than those reported previously by Breitmeyer et al. (1975) and Julesz et al. (1976). Although minimum detection durations were used in all three studies the experiments are quite different in a number of respects. In part this is due to the fact that Breitmeyer and Julesz were examining the proposed tilted vertical horopter whereas this study was concerned with stereosensitivity per se.

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Research Note

As such, in the present study the fixation range of eccentricities than those of existing point was always set at zero disparity with studies. respect to the field, and so targets were of both crossed and uncrossed disparity with respect to Acknowledgements-This research was supported by ARGS research grant No. A28315154R. both the fixation point and the field. Breitmeyer et al. (1975) and Julesz et at. (1976) used a target that was always of crossed disparity to the field. Field and target were both varied in disparity to Breitmeyer B., Mesz B. and Kropfl W. (1975) Dynamic random-dot stereograms reveal up-down anisotropy and the fixation point giving a target to fixation left-ri@tt isotropy between cortical hemifields. Science, point disparity range of O-12’crossed disparity. N.Y. 187,269-270. Stimulus and eccentricity also varied greatly Durnford M. and Kimura D. (1971) Right hemisphere specialization for depth perception ret%ectedin visual field between this study and the earlier two. In the differences. Nature, Land. 231, 394-395. two previous studies the stimulus was a 6’ x 90 Fischer R. and Poggio G. F. (1979) Depth sensitivity of (1 x 15 pixel) bar or a 36’ x 36’ (4 x 4 pixel) binocular cortical neurons of behaving monkeys. Proc. R. square within an eccentricity of 1 deg. The sot. B 2Q4, 409-414. present study used 2 deg square stimuli (48 x 48 Grabowska A. (1983) Lateral differences in the detection of stereoscopic depth. N~ropsychoi~~ 21, 249-257. pixel) with an ~nt~~ty of 3 deg. It is possible that the more central stimuli used in the earlier Julesz B. (1971) Founrlation oj+CycropeOnPerception. Univ. of Chicago Press, Chicago. studies resulted in a higher proportion of Julesz B., Breitmeyer B. and Kropfl W. (1976) Binocularipsilateral projections negating left and right disparity-dependent upper-lower hemifield anisotropy hemifield differences. and left-right isotropy as revealed by dynamic randomThe main finding of this paper is that of the dot stereograms. Perception 5, 129-141. longer detection durations for uncrossed dis- ~ustillo P. (1985) Boxcar mechanisms m~iating crossed and uncrossed stereopsis. Psycho/. B&i. 97, 187-201. parity stimuli. A possible explanation has been Neil1R. A. and Fenelon B. (1981) The objective evaluation advanced (Fischer and Poggio, 1979)in terms of of stereopsis. Ausr. Phys. Engng Sci. Med. 4, 122-125. distinct neural pools processing the two types of Neil1 R. A., Fenelon B., Manning M. L. and Frost B. G. (1987) Evoked potentials to dynamic random dot stimuli disparity; the differing temporal properties may with varying dot density ratios of disparity to backresult from a larger number of cells processing ground. Lbcumenta Oplth. In press. crossed disparity. Also the upper/lower anisoNeil1 R. A. and Kennewell J. A. (1979) A clinical test for tropy and left/right isotropy reported earlier stereopsis. Aust. Phys. Engng Sci. Med. 2, 463-480. (Breitmeyer ef al., 1975; Julesz et al., 1976) are Newhouse M. and Uttal W. R. (1982) Distribution of stereoanomalies in the general population. Bufl. Psychol. not evident in the present study using much sot. 20,48-SO. larger stimuli over a greater ecckricity. This Patterson R. and Fox R. (1984) The effect of testing suggests that a further inv~tigation is needed methods on ste~anomaly. V&ionRes. 24,403-408. examining both stereosensitivity per se and the Over R. and Long N. (1973) Depth is visible before figure proposed tilted vertical horopter using a range in stereoscopic perception of random-dot patterns. Vision. Res. i3, 1207-1209. of stimulus sizes and extending over a greater