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Luminance contrast and the visual span during visual target localization
Displays 34 (2013) 27–32
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Luminance contrast and the visual span during visual target localization Harold H. Greene a,⇑, James M. Brown b, Bryce A. Paradis a a b
Department of Psychology, University of Detroit Mercy, Detroit, MI. USA Department of Psychology, University of Georgia, Athens, GA. USA
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 1 August 2012 Accepted 12 November 2012 Available online 29 November 2012 Keywords: Human experimental psychology Visual perception Saccades Luminance contrast Visual span
a b s t r a c t A concern for designers of monocular and binocular devices is the ability of users to search for, and localize target items embedded in noisy displays. Twenty-two participants searched (12 under monocular conditions) to localize a target embedded in random gray dot displays. The target was defined by a variation in pattern that did not differ in average contrast from the rest of each display. Displays were presented at .54 and .04 Michelson contrast. Across binocular and monocular viewings, fixation counts increased with decreasing contrast, but the gradient was steeper for monocular viewing. With decreasing contrast, fixations were longer, and the amplitudes of saccades used to localize the target decreased. The findings highlight for monocular vs. binocular target localization, the importance of considering separately, how many fixations are needed to localize the target, and how close to fixation the target must be for it to be noticed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Some display devices are set up to be viewed monocularly, or binocularly (e.g., telescopes, microscopes, video spectacles, helmet-mounted displays). Other devices allow users a choice of viewing preference (e.g., television sets, computers, telephones, and cameras). Irrespective of the set up for viewing, a common concern for designers is the ability of users to search for, and localize target items known to be embedded in display noise. During visual target localization (VTL), the average time to localize a target embedded in noise is longer for low luminance-contrast displays than for medium- or high-contrast displays. Whereas average times reveal when a target was localized, they say nothing about how it was localized. Eye movement recordings provide access to how a target is localized ([9]; see also [26] for a review). Efforts aimed at determining how a target is localized contribute to the prediction and modeling of VTL performance. In human observers, the optics of the eye, image processing mechanisms in the retina and visual cortex, attention, and decision-making mechanisms influence VTL times (e.g., [5]). Longer times for low luminance-contrast displays are reliably associated with longer fixation durations and increased numbers of fixations [18,19,21]; see also [29]. With respect to fixation durations, an explanation inspired by a similar one for an ideal observer’s search times is useful (see Fig. 1 in [5]). Given the inherent randomness of light, even an ideal observer (i.e., one limited only by random fluctuations in the light stimulus) is susceptible to longer fixations for low contrast ⇑ Corresponding author. Tel.: +1 313 578 0456; fax: +1 313 578 0507. E-mail address: [email protected] (H.H. Greene). 0141-9382/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.displa.2012.11.005
stimuli. The mean number of photons sampled at a location on the retina over a given time is distributed with some mean value k (an indicator of luminance at said location). Low contrast in this context is effected by similar values of k at two locations in a visual scene. Longer sampling periods (i.e. with larger numbers of samples in the sampling distribution) lead to less noisy distributions, and hence, a greater ability to discriminate among similar mean values of k (see also [5]). In effect, given the nature of random distributions, longer fixations are expected for low contrast displays, even at high luminance levels. The reason is that the sampling error associated with k will usually be larger for shorter sampling time periods, hence, reducing the sensitivity to small luminance differences (i.e. low contrast). Sampling error decreases with longer sampling periods (i.e., with longer fixation durations), and the extra time facilitates low contrast discriminability. Mathematical descriptions of how fixation durations might change as a function of luminance contrast are beyond the scope of the present study (but see [3,23] for insights). In point, luminance contrast effects on fixation duration are reasonably explained by low-level factors (i.e. image input mechanisms in the visual system). The number of eye fixations made to localize an inconspicuous target correlates very strongly with the time taken to localize the target [31]; see also [8]. The relationship may be explained by the visual span. Whereas spatial processing is increasingly coarser at greater eccentricities from eye fixation, the visual span is the area (typically in angular units) within which objects in a context-free display can be recognized [20]. Spatial processing limitations necessitate the execution of multiple fixations to localize inconspicuous targets in a display. Comparisons of visual span size, from single-fixation target detection tasks, to visual search
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Fig. 1. A sample random gray dot display at .54 Michelson contrast. The target is the fragmented 1° 1° cross to the right of center. The squares (which were not presented to the participants) illustrate the distribution of possible target locations. Participants searched for the target under .54 and .04 contrast levels.
performance, have shown that at least 80% of saccade amplitude variance [11] and search time variance [5] are associated with the size of the visual span. Effectively, with a smaller visual span, shorter saccades are utilized, more fixations are required to cover a display, and localization times are longer. Indeed, the increase in the numbers of fixations for low luminance contrast displays [19,21] is consonant with a narrowing of the visual span. The primary goal of the present study was to place saccade amplitudes in the context of the visual span. Whereas the effects of luminance contrast are reliably replicable for fixation duration (i.e., when the eyes move in search of a target) and for fixation counts (i.e., how often the eyes move in search of a target), findings for saccade amplitudes (i.e. how far the eyes move in search of a target) have been inconsistent. Näsänen et al. [19] instructed observers to localize alphanumeric characters embedded in rectangular arrays of alphanumeric characters. They found a small but significant decrease in saccade amplitudes when luminance contrast was very low. In a similar study, Näsänen and Ojanpää [18] found no effect of contrast on saccade amplitudes. Finally, Ojanpää and Näsänen [21] reported mixed findings. For VTL with small (0.17°) and large (1.26°) characters, saccade amplitudes were not affected by luminance contrast levels. However, for medium-sized (0.37°) characters, there was (surprisingly) a small but significant increase in saccade amplitudes when luminance contrast was very low. The inconsistencies of saccade amplitudes in the context of the visual span have not previously been discussed. Compared to fixation counts, saccade amplitude fluctuation as a function of span manipulation provides researchers with a spatial metric of visual span size. Consistent with a narrowing span, increased semantic processing load appears to reduce saccade amplitudes during text reading (see [25]) and visual search [10,13]. Hence, if VTL in a lowluminance display is associated with a smaller span (as suggested repeatedly by fixation count results), it is reasonable to expect this situation to be reliably reflected in saccade amplitudes. We suspect that the inconsistent findings of luminance contrast on saccade amplitudes (see findings of [18,19,21] may be driven by their use of a rather noisy saccade amplitude distribution, as explained next. The visual span [20] must not be confused with a related term in the VTL literature, the perceptual span [14]. The perceptual span is a region from which useful information may
be obtained (deliberately, or pre-attentively) during an eye fixation. It is made up of a central decision region (within which objects are recognized) and an outer preview region that influences the launching of saccades [27,28]; see also [4,25,26]. The visual span corresponds to the decision region in the perceptual span (see [7,22]). The amplitude distribution of saccades in VTL contains saccades that are reflective of the size of the preview region (see [2,7,9,10,15–17,30]). However, given the positive skew in the distribution (e.g., unpublished data from our laboratory; see also Fig. 2 in [12], the distribution also contains some long saccades beyond the preview region. These long saccades are probably made to orient attention to far-away clusters of distractors within which the target may be found. They positively skew the amplitude distribution. Some saccades within the distribution are very small in amplitude, and may be corrective saccades (made to reposition the fovea within an object of fixation). Lastly, a target is localized (by acknowledgment) when it appears within the decision region/visual span (i.e. when a saccade moves the foveal area to the target). Henceforth, this saccade amplitude is referred to as Target Localization Saccade amplitude (TLSA). Clearly, with all these contributors to the saccade amplitude distribution, the mean amplitude is a rather noisy estimator of visual span size. Using the mean amplitude was a limitation of earlier studies (e.g., [18,19,21]. We propose that the appropriate saccade amplitude estimator of the visual span is the TLSA. If, as we suspect, saccade amplitudes can be related to visual span size, a decrease in TLSA should be reliably apparent for low luminance contrast displays. The secondary goal of the study was to provide corroborating evidence of longer fixation durations and increased numbers of fixations for low luminance contrast VTL. The search task in our study was presented under binocular and monocular conditions. This manipulation has practical implications for a number of reasons. First, for some devices, users have the option of buying a monocular, or binocular display setup (e.g., telescopes, microscopes, video spectacles, helmet mounted displays). Second, humans do engage in monocular or binocular visual search of displays (e.g. television sets, telephones, computer screens), either by choice, or because of blindness). Third, monocular contrast sensitivity is typically lower than binocular contrast sensitivity in non-clinical adults (e.g., [6]). Direct comparisons of monocular and binocular performances are not sufficiently addressed in the visual search literature. The assumption of lower contrast sensitivity for monocular viewing allowed us further tests of luminance contrast effects. Compared to binocular viewing, longer fixation durations, higher fixation counts, and smaller TLSAs were hypothesized for monocular viewing. 2. Method 2.1. Participants Twenty-two female students at the University of Detroit Mercy participated in the experiment. By self-report, all had normal or corrected-to-normal binocular visual acuity. None was aware of the purpose of the experiment. Contrast sensitivity of participants was not assessed. 2.2. Stimulus and apparatus Stimuli were presented on a 17-inch color monitor controlled by an Intel processor. The computer system’s graphics adapter was used at a refresh rate of 60 Hz, and a resolution of 1280 960 pixels. Each stimulus was a 21° wide 17° high random gray dot image with an embedded target. Paired luminance values in a stimulus image were either 15.51 cd/m2 and 14.49 cd/
H.H. Greene et al. / Displays 34 (2013) 27–32
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Fig. 2. Fixation scatter plots across all participants in the .04 contrast condition. In both panels, the squares depict the size and possible locations of the 1° 1° target. The left panel shows the locations of fifth fixations. The choice of fixation number five was arbitrary. Fifth fixations are randomly scattered across the display. The right panel depicts target localizing fixations. In this panel, fixations are clustered around the target locations, suggesting that the target was indeed localized.
m2, or, 15.51 cd/m2 and 4.70 cd/m2. Luminance values were obtained with a Konica Minolta Luminance Meter (LS-110). A closeup lens was used with the light meter to magnify the image areas sampled, and the only light source in the room at the time of measurement was the monitor. Image contrast was expressed as Michelson contrast (Mc) thus: Mc = (Lhigh Llow)/(Lhigh + Llow), where Lhigh and Llow were the higher and lower luminance values paired in an image. The two image contrasts formed were .04 (i.e., low contrast) and .54 (i.e., medium contrast). High contrast stimulus images (.98) were used in practice trials to familiarize participants with the localization task. Fig. 1 depicts a sample stimulus display at .54 contrast. The squares shown in Fig. 1 illustrate the 17 target positions used in the experiment. These squares were not presented to participants. The target in all displays was a fragmented 1° 1° cross with the same average contrast as the rest of the display. A chinrest was used to minimize head movement, and eye positions were sampled at 500 Hz by an Eyelink II system controlled by EYETRACK software (see http://www.psych.umass.edu/ eyelab/software/eyelabmuseum/).
2.3. Procedure Participants sat 55 cm from the monitor. The experiment utilized a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) factorial design. Ten of the 22 participants were fitted with a left eye patch for monocular viewing. All participants were fitted with the head-mounted Eyelink II eye tracking headband. They were instructed to search for the target cross in each stimulus presentation, and to terminate the presentation with a mouse click response as soon as the target was localized. Eye drift correction was performed before each trial to ensure minimal discrepancy between actual and eye-tracker-reported point of regard. To familiarize the participants with the target localization task, 10 trials were presented with the target at randomly chosen positions in the .98 contrast stimulus image. Following the practice trials, two blocks of stimulus contrast conditions (.04 and .54) were presented in counter-balanced order per participant. The only light sources in the laboratory were the stimulus presentation monitor and the experimenter’s monitor. Participants were dark adapted for about 10 min by the time they started target localization in the counter-balanced stimulus contrast conditions. The target (unbeknownst to the participants) was presented 3 times at random in
17 positions on the image, for a total of 51 trials per block. A maximum of 30,000 ms was allotted in each trial to localize the target. 3. Results 3.1. Qualitative analysis A mouse click response was always made within the 30,000 ms search window. In the interest of minimizing self-conscious/artificial behavior during the task, and minimizing spurious saccades, participants were not asked to verify the location of the target after it was localized. Before drawing conclusions from performance in a VTL experiment, it is wise to verify that participants were able to localize the (always present) target [9]. Experimenter-monitoring of eye movements on the experimenter‘s monitor indicated that participants were indeed localizing the target in the .54 contrast condition. It was more difficult to verify this in the .04 contrast condition because the resolution on the experimenter’s monitor was much poorer than that on the participants’ display. Towards verifying in a formal manner that the target was indeed localized in the .04 contrast condition, the (arbitrarily chosen) fifth fixation for each trial, and target localizing fixations across all participants have been plotted in Fig. 2. The squares in the figure illustrate the size (1° 1°) and possible locations of the target. Target localizing fixation locations across monocular and binocular viewing sufficiently matched the possible target locations (see right panel of Fig. 2). Hence, we are satisfied that mouse click responses were executed when the target was localized. 3.2. Saccade amplitudes A saccade was signaled if the velocity of the tracked eye exceeded 30° s 1, or eye acceleration exceeded 8000° s 2. Were TLSAs shorter under lower contrast conditions as predicted by a visual span account of search processes? To start, saccades less than .50° were considered corrective (i.e., used to reposition the eyes within a fixated region of interest), and were excluded from analysis. The .50° criterion was chosen because it is smaller than the size of the target (i.e. 1° 1°). Hence, very small saccades within the target were not counted in the analysis. Fig. 3A shows that probability distributions for target-localizing saccade amplitudes after the elimination of corrective saccades (i.e., TLSAs) were severely skewed in the positive direction. For this reason, median
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H.H. Greene et al. / Displays 34 (2013) 27–32
(a)
Fig. 5. Average durations of fixations made to localize the target.
(b) Median TLSA (Deg)
3.0
Binocular viewing Monocular viewing
2.5
3.3. Fixation counts
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2.31
2.0 1.7
1.5
1.31
1.0 0.0
0.1
0.2
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Contrast Fig. 3. Probability distributions for amplitudes of target-localizing saccades across Viewing and Contrast conditions (a). Given the skew of the distributions, median saccade amplitudes were utilized to describe their central tendencies (b).
A fixation was indicated if the velocity of the tracked eye did not exceed 30° s 1, and eye acceleration did not exceed 8000° s 2 . Previous findings indicate that on average, more fixations are used to localize a target under lower contrast conditions [18,19,21]. Median fixation counts were subjected to a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) mixed factorial ANOVA. Viewing was a between subjects factor. There was a main effect of Stimulus Contrast. About 14 more fixations were made in the .04 contrast condition than the .54 contrast condition (18 vs. 4), F(1, 20) = 222.55, p < .01, g2p = .92. The main effect of Viewing was significant with twice as many fixations made in the monocular viewing condition (14 for monocular vs. 7 for binocular), F(1, 20) = 38.69, p < .01, g2p = .66. The Viewing Stimulus Contrast interaction was significant, F(1, 20) = 45.50, p < .01, g2p = .70. Fig. 4 shows that fewer fixations were always made in the higher contrast (.54) condition. However, the difference between monocular and binocular viewing was minimal at the higher contrast level. 3.4. Fixation duration
Fig. 4. Average number of fixations made to localize the target.
amplitudes were used to summarize the distributions. Median TLSAs were subjected to a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) mixed factorial ANOVA. Viewing was a between subjects factor. There was a main effect of Stimulus Contrast. Amplitudes were about 1° shorter in the .04 contrast condition than the .54 contrast condition (1.50° vs. 2.54°), F(1, 20) = 17.56, p < .01, g2p = .47. Although slightly shorter saccades were executed during monocular viewing (see the small effect size below, and Fig. 3B), the main effect of Viewing was not statistically significant (1.81° vs. 2.23°), F(1, 20) = 2.73, p = .11, g2p = .12. Finally, the Viewing Stimulus Contrast interaction was not significant, F(1, 20) = .01, p = .91, g2p = .001.
Fixation duration was any duration of eye fixation after initial saccades, but before final fixations were terminated by mouseclick responses. Fixation durations were expected to be longer under the lower contrast condition [18,19,21]. Median fixation durations were subjected to a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) mixed factorial ANOVA with Viewing a between subjects factor. There was a main effect of Stimulus Contrast with fixation durations being about 100 ms longer in the .04 contrast condition than the .54 contrast condition (319 ms vs. 206 ms), F(1, 20) = 121.53, p < .01, g2p = .86. The main effect of Viewing was not significant (275 ms for monocular vs. 250 ms for binocular), F(1, 20) = 2.56, p = .13, g2p = .11. Fig. 5 shows that although durations were slightly longer under the lower contrast condition, the Viewing Stimulus Contrast interaction was not statistically significant, F(1, 20) = 2.16, p = .16, g2p = .10. 4. Discussion 4.1. The visual and perceptual spans The visual span is the area around eye fixation within which objects are recognized. The visual span control hypothesis posits that eye movements are controlled by the size of the visual span. We reasoned that the lower the fidelity of a target signal (i.e., its contrast), the greater the processing time (e.g., fixation duration) and effort (e.g., increased number of fixations) needed to obtain that
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signal, which should also be reflected in the amplitude of the saccade that localized the target. Fixation durations were longer in the lower contrast condition, as predicted (see also [18,19,21] for similar findings). Effort increases because the visual span is smaller for lower fidelity targets (e.g., [21]). Hence TLSAs were expected, and were found to be smaller at the lower contrast level. For a robust account of the link between eye movement behavior and visual span properties, the task in the experiment was performed under binocular and monocular viewing conditions. Longer fixation durations, shorter TLSAs and increased fixation counts were expected for monocular viewing. However, whereas there appeared to be small effects of monocular viewing for TLSA and fixation duration in the predicted directions, only the fixation count effects were statistically significant. Specifically, fixation counts were significantly higher under monocular viewing, but only at the lower contrast level. The primary contribution of the present work was the use of TLSAs as an index of span size during visual target localization. We were concerned about previously inconsistent effects of luminance contrast on saccade amplitudes (see [18,19,21]). Saccade amplitudes potentially provide a spatial metric of visual span size. This potential allowed us to provide a possible solution to the previously unsolved problem of how saccade amplitudes relate to visual span size. We suspected that previously inconsistent effects may have been driven by too broad a definition of saccade amplitudes for visual span estimation. We have argued that during VTL, if all saccades are considered (e.g., [18,19,21]), the distribution is rather noisy for estimating visual span size. The visual span is a central region within a wider perceptual span (see [7,22]). Whereas target recognition is limited by the size of the visual span, effective stimulus processing is limited by the size of the wider perceptual span [27,28]; see also [4,25,26]. Effective stimulus processing includes obtaining useful preview information about stimuli beyond fixation, but not necessarily target recognition information (e.g. [28]). We have proposed that whereas overall saccade amplitudes may reasonably estimate perceptual span dynamics (e.g., [9]), a more appropriate account of the visual span from saccade amplitudes is obtainable from the amplitude of the saccade that localized the target (i.e., what we have termed TLSA). The results of the present study were consistent with expectation for the lower contrast conditions: increased fixation counts, and shorter TLSAs. Indeed, TLSA decreased with decreasing contrast across binocular and monocular viewing. Hence we contend that saccade amplitudes (specifically TLSAs), like fixation counts, provide a window on visual span properties. Notwithstanding the primary conclusion above, the difference in Viewing Stimulus Contrast relationships for TLSA and fixation counts is insightful. Both measures generally indicated a smaller visual span at the low contrast level. However, differences are evident between monocular and binocular viewing at the higher contrast level. Whereas contrast differentially influenced fixation counts (revisit the interaction effect shown in Fig. 4), this was not the case for TLSAs (revisit Fig. 3b). If TLSA and fixation counts index the same span, one would expect the relationships to be the same. During VTL, each fixation (except the target recognition fixation) is associated with an ensuing saccade of some amplitude. Reasonably, non-TLSAs primarily reflect saccadic movements within the bounds of the preview region [2,7,9,10,15–17,30]. We suggest that the fixation count measure, somewhat like the overall saccade amplitude measure, does not solely index the visual span; it also indexes the wider perceptual span. A wider perceptual span predictably brings the target within useful preview processing range with fewer exploratory fixations than a narrower span. The results of the present study suggest that under monocular viewing, the perceptual span fluctuates more rapidly with contrast changes than under binocular viewing. However, contrast-induced fluctua-
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tions of the visual span may not be (very) sensitive to ocular conditions. 4.2. Application Item visibility under low contrast conditions is a potential problem for displays in devices such as telescopes, microscopes, computer monitors, and mobile telephones, for which target localization is a common goal. For such devices, perceived contrast reduction may occur because of image quality metrics (e.g., telescopes, microscopes, helmet-mounted displays), or from ambient illumination (e.g., computer monitors, mobile telephones). Studies that address contrast effects on eye movement indices are useful for informing display designers, and users. The fact that effect sizes were smaller for TLSAs than for fixation counts does not minimize the importance of the corroborating evidence of visual span manipulation from these two measures. In most applications, it may suffice to simply say that the visual span is smaller, without reference to a spatial metric. However, one may envision situations where a spatial metric may be useful (e.g. for modeling human limitations, towards improving display designs). The work of Greene [7] illustrates a simple case of the utility of a spatial metric like TLSA for the simulation of human VTL. In that study, Monte Carlo simulations of VTL were conducted with arbitrary saccade amplitude limits for visual span and perceptual span boundaries. A better approach would have been to use biologically-guided saccade information in the simulation process. In the present study, visual performance occurred under photopic display, and scotopic ambient conditions. An application for comparison is the use of night vision devices. One such device is the Integrated Helmet and Display Sight System (IHADSS). The system is monocular (video imagery to the right eye only), and is used by AH-64 attack helicopter aviators (see [24]). Another device is the Aviator’s Night Vision Imaging System (ANVIS). This device is set up for binocular viewing, and it is the most widely used helmet-mounted night vision setup for aviators [1]. The findings of the present study suggest that under low contrast conditions, the perceptual span decreases faster under monocular viewing (as indicated by fixation count results) than under binocular viewing. The visual span however (i.e. the area within which targets are recognized), may decrease similarly for monocular and binocular viewing. A concern for designers of display devices is the ability of users to search for, and localize target items known to be embedded in display noise. The study describes a methodology for accessing the size of the area within which a target is identifiable during visual target localization. The findings highlight for monocular vs. binocular target localization, the importance of considering separately, how many fixations are needed to localize the target, and how close to fixation the target must be for it to be noticed. Acknowledgments This work was made possible by University of Detroit Mercy IRF equipment grants. We thank the formal and informal reviewers for their insights and suggestions. We also thank research assistants Carmela A.M. Martin, Britanny M. Polisuk, Ebony N. Fails, and Perrier T. Greene for their contributions to the project. References [1] M.M. Bayer, C.E. Rash, J.H. Brindle, Introduction to helmet-mounted displays, in: C.E. Rash, M.B. Russo, T.R. Letowski, E.T. Schmeisser (Eds.), Helmetmounted displays: Sensation, perception, and cognitive issues, U.S. Army Aeromedical Research Laboratory, 2009, pp. 47–108. [2] J.H. Bertera, K. Rayner, Eye movements and the span of the effective stimulus in visual search, Perception & Psychophysics 62 (2000) 576–585.
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Contents lists available at SciVerse ScienceDirect
Displays journal homepage: www.elsevier.com/locate/displa
Luminance contrast and the visual span during visual target localization Harold H. Greene a,⇑, James M. Brown b, Bryce A. Paradis a a b
Department of Psychology, University of Detroit Mercy, Detroit, MI. USA Department of Psychology, University of Georgia, Athens, GA. USA
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 1 August 2012 Accepted 12 November 2012 Available online 29 November 2012 Keywords: Human experimental psychology Visual perception Saccades Luminance contrast Visual span
a b s t r a c t A concern for designers of monocular and binocular devices is the ability of users to search for, and localize target items embedded in noisy displays. Twenty-two participants searched (12 under monocular conditions) to localize a target embedded in random gray dot displays. The target was defined by a variation in pattern that did not differ in average contrast from the rest of each display. Displays were presented at .54 and .04 Michelson contrast. Across binocular and monocular viewings, fixation counts increased with decreasing contrast, but the gradient was steeper for monocular viewing. With decreasing contrast, fixations were longer, and the amplitudes of saccades used to localize the target decreased. The findings highlight for monocular vs. binocular target localization, the importance of considering separately, how many fixations are needed to localize the target, and how close to fixation the target must be for it to be noticed. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Some display devices are set up to be viewed monocularly, or binocularly (e.g., telescopes, microscopes, video spectacles, helmet-mounted displays). Other devices allow users a choice of viewing preference (e.g., television sets, computers, telephones, and cameras). Irrespective of the set up for viewing, a common concern for designers is the ability of users to search for, and localize target items known to be embedded in display noise. During visual target localization (VTL), the average time to localize a target embedded in noise is longer for low luminance-contrast displays than for medium- or high-contrast displays. Whereas average times reveal when a target was localized, they say nothing about how it was localized. Eye movement recordings provide access to how a target is localized ([9]; see also [26] for a review). Efforts aimed at determining how a target is localized contribute to the prediction and modeling of VTL performance. In human observers, the optics of the eye, image processing mechanisms in the retina and visual cortex, attention, and decision-making mechanisms influence VTL times (e.g., [5]). Longer times for low luminance-contrast displays are reliably associated with longer fixation durations and increased numbers of fixations [18,19,21]; see also [29]. With respect to fixation durations, an explanation inspired by a similar one for an ideal observer’s search times is useful (see Fig. 1 in [5]). Given the inherent randomness of light, even an ideal observer (i.e., one limited only by random fluctuations in the light stimulus) is susceptible to longer fixations for low contrast ⇑ Corresponding author. Tel.: +1 313 578 0456; fax: +1 313 578 0507. E-mail address: [email protected] (H.H. Greene). 0141-9382/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.displa.2012.11.005
stimuli. The mean number of photons sampled at a location on the retina over a given time is distributed with some mean value k (an indicator of luminance at said location). Low contrast in this context is effected by similar values of k at two locations in a visual scene. Longer sampling periods (i.e. with larger numbers of samples in the sampling distribution) lead to less noisy distributions, and hence, a greater ability to discriminate among similar mean values of k (see also [5]). In effect, given the nature of random distributions, longer fixations are expected for low contrast displays, even at high luminance levels. The reason is that the sampling error associated with k will usually be larger for shorter sampling time periods, hence, reducing the sensitivity to small luminance differences (i.e. low contrast). Sampling error decreases with longer sampling periods (i.e., with longer fixation durations), and the extra time facilitates low contrast discriminability. Mathematical descriptions of how fixation durations might change as a function of luminance contrast are beyond the scope of the present study (but see [3,23] for insights). In point, luminance contrast effects on fixation duration are reasonably explained by low-level factors (i.e. image input mechanisms in the visual system). The number of eye fixations made to localize an inconspicuous target correlates very strongly with the time taken to localize the target [31]; see also [8]. The relationship may be explained by the visual span. Whereas spatial processing is increasingly coarser at greater eccentricities from eye fixation, the visual span is the area (typically in angular units) within which objects in a context-free display can be recognized [20]. Spatial processing limitations necessitate the execution of multiple fixations to localize inconspicuous targets in a display. Comparisons of visual span size, from single-fixation target detection tasks, to visual search
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Fig. 1. A sample random gray dot display at .54 Michelson contrast. The target is the fragmented 1° 1° cross to the right of center. The squares (which were not presented to the participants) illustrate the distribution of possible target locations. Participants searched for the target under .54 and .04 contrast levels.
performance, have shown that at least 80% of saccade amplitude variance [11] and search time variance [5] are associated with the size of the visual span. Effectively, with a smaller visual span, shorter saccades are utilized, more fixations are required to cover a display, and localization times are longer. Indeed, the increase in the numbers of fixations for low luminance contrast displays [19,21] is consonant with a narrowing of the visual span. The primary goal of the present study was to place saccade amplitudes in the context of the visual span. Whereas the effects of luminance contrast are reliably replicable for fixation duration (i.e., when the eyes move in search of a target) and for fixation counts (i.e., how often the eyes move in search of a target), findings for saccade amplitudes (i.e. how far the eyes move in search of a target) have been inconsistent. Näsänen et al. [19] instructed observers to localize alphanumeric characters embedded in rectangular arrays of alphanumeric characters. They found a small but significant decrease in saccade amplitudes when luminance contrast was very low. In a similar study, Näsänen and Ojanpää [18] found no effect of contrast on saccade amplitudes. Finally, Ojanpää and Näsänen [21] reported mixed findings. For VTL with small (0.17°) and large (1.26°) characters, saccade amplitudes were not affected by luminance contrast levels. However, for medium-sized (0.37°) characters, there was (surprisingly) a small but significant increase in saccade amplitudes when luminance contrast was very low. The inconsistencies of saccade amplitudes in the context of the visual span have not previously been discussed. Compared to fixation counts, saccade amplitude fluctuation as a function of span manipulation provides researchers with a spatial metric of visual span size. Consistent with a narrowing span, increased semantic processing load appears to reduce saccade amplitudes during text reading (see [25]) and visual search [10,13]. Hence, if VTL in a lowluminance display is associated with a smaller span (as suggested repeatedly by fixation count results), it is reasonable to expect this situation to be reliably reflected in saccade amplitudes. We suspect that the inconsistent findings of luminance contrast on saccade amplitudes (see findings of [18,19,21] may be driven by their use of a rather noisy saccade amplitude distribution, as explained next. The visual span [20] must not be confused with a related term in the VTL literature, the perceptual span [14]. The perceptual span is a region from which useful information may
be obtained (deliberately, or pre-attentively) during an eye fixation. It is made up of a central decision region (within which objects are recognized) and an outer preview region that influences the launching of saccades [27,28]; see also [4,25,26]. The visual span corresponds to the decision region in the perceptual span (see [7,22]). The amplitude distribution of saccades in VTL contains saccades that are reflective of the size of the preview region (see [2,7,9,10,15–17,30]). However, given the positive skew in the distribution (e.g., unpublished data from our laboratory; see also Fig. 2 in [12], the distribution also contains some long saccades beyond the preview region. These long saccades are probably made to orient attention to far-away clusters of distractors within which the target may be found. They positively skew the amplitude distribution. Some saccades within the distribution are very small in amplitude, and may be corrective saccades (made to reposition the fovea within an object of fixation). Lastly, a target is localized (by acknowledgment) when it appears within the decision region/visual span (i.e. when a saccade moves the foveal area to the target). Henceforth, this saccade amplitude is referred to as Target Localization Saccade amplitude (TLSA). Clearly, with all these contributors to the saccade amplitude distribution, the mean amplitude is a rather noisy estimator of visual span size. Using the mean amplitude was a limitation of earlier studies (e.g., [18,19,21]. We propose that the appropriate saccade amplitude estimator of the visual span is the TLSA. If, as we suspect, saccade amplitudes can be related to visual span size, a decrease in TLSA should be reliably apparent for low luminance contrast displays. The secondary goal of the study was to provide corroborating evidence of longer fixation durations and increased numbers of fixations for low luminance contrast VTL. The search task in our study was presented under binocular and monocular conditions. This manipulation has practical implications for a number of reasons. First, for some devices, users have the option of buying a monocular, or binocular display setup (e.g., telescopes, microscopes, video spectacles, helmet mounted displays). Second, humans do engage in monocular or binocular visual search of displays (e.g. television sets, telephones, computer screens), either by choice, or because of blindness). Third, monocular contrast sensitivity is typically lower than binocular contrast sensitivity in non-clinical adults (e.g., [6]). Direct comparisons of monocular and binocular performances are not sufficiently addressed in the visual search literature. The assumption of lower contrast sensitivity for monocular viewing allowed us further tests of luminance contrast effects. Compared to binocular viewing, longer fixation durations, higher fixation counts, and smaller TLSAs were hypothesized for monocular viewing. 2. Method 2.1. Participants Twenty-two female students at the University of Detroit Mercy participated in the experiment. By self-report, all had normal or corrected-to-normal binocular visual acuity. None was aware of the purpose of the experiment. Contrast sensitivity of participants was not assessed. 2.2. Stimulus and apparatus Stimuli were presented on a 17-inch color monitor controlled by an Intel processor. The computer system’s graphics adapter was used at a refresh rate of 60 Hz, and a resolution of 1280 960 pixels. Each stimulus was a 21° wide 17° high random gray dot image with an embedded target. Paired luminance values in a stimulus image were either 15.51 cd/m2 and 14.49 cd/
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Fig. 2. Fixation scatter plots across all participants in the .04 contrast condition. In both panels, the squares depict the size and possible locations of the 1° 1° target. The left panel shows the locations of fifth fixations. The choice of fixation number five was arbitrary. Fifth fixations are randomly scattered across the display. The right panel depicts target localizing fixations. In this panel, fixations are clustered around the target locations, suggesting that the target was indeed localized.
m2, or, 15.51 cd/m2 and 4.70 cd/m2. Luminance values were obtained with a Konica Minolta Luminance Meter (LS-110). A closeup lens was used with the light meter to magnify the image areas sampled, and the only light source in the room at the time of measurement was the monitor. Image contrast was expressed as Michelson contrast (Mc) thus: Mc = (Lhigh Llow)/(Lhigh + Llow), where Lhigh and Llow were the higher and lower luminance values paired in an image. The two image contrasts formed were .04 (i.e., low contrast) and .54 (i.e., medium contrast). High contrast stimulus images (.98) were used in practice trials to familiarize participants with the localization task. Fig. 1 depicts a sample stimulus display at .54 contrast. The squares shown in Fig. 1 illustrate the 17 target positions used in the experiment. These squares were not presented to participants. The target in all displays was a fragmented 1° 1° cross with the same average contrast as the rest of the display. A chinrest was used to minimize head movement, and eye positions were sampled at 500 Hz by an Eyelink II system controlled by EYETRACK software (see http://www.psych.umass.edu/ eyelab/software/eyelabmuseum/).
2.3. Procedure Participants sat 55 cm from the monitor. The experiment utilized a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) factorial design. Ten of the 22 participants were fitted with a left eye patch for monocular viewing. All participants were fitted with the head-mounted Eyelink II eye tracking headband. They were instructed to search for the target cross in each stimulus presentation, and to terminate the presentation with a mouse click response as soon as the target was localized. Eye drift correction was performed before each trial to ensure minimal discrepancy between actual and eye-tracker-reported point of regard. To familiarize the participants with the target localization task, 10 trials were presented with the target at randomly chosen positions in the .98 contrast stimulus image. Following the practice trials, two blocks of stimulus contrast conditions (.04 and .54) were presented in counter-balanced order per participant. The only light sources in the laboratory were the stimulus presentation monitor and the experimenter’s monitor. Participants were dark adapted for about 10 min by the time they started target localization in the counter-balanced stimulus contrast conditions. The target (unbeknownst to the participants) was presented 3 times at random in
17 positions on the image, for a total of 51 trials per block. A maximum of 30,000 ms was allotted in each trial to localize the target. 3. Results 3.1. Qualitative analysis A mouse click response was always made within the 30,000 ms search window. In the interest of minimizing self-conscious/artificial behavior during the task, and minimizing spurious saccades, participants were not asked to verify the location of the target after it was localized. Before drawing conclusions from performance in a VTL experiment, it is wise to verify that participants were able to localize the (always present) target [9]. Experimenter-monitoring of eye movements on the experimenter‘s monitor indicated that participants were indeed localizing the target in the .54 contrast condition. It was more difficult to verify this in the .04 contrast condition because the resolution on the experimenter’s monitor was much poorer than that on the participants’ display. Towards verifying in a formal manner that the target was indeed localized in the .04 contrast condition, the (arbitrarily chosen) fifth fixation for each trial, and target localizing fixations across all participants have been plotted in Fig. 2. The squares in the figure illustrate the size (1° 1°) and possible locations of the target. Target localizing fixation locations across monocular and binocular viewing sufficiently matched the possible target locations (see right panel of Fig. 2). Hence, we are satisfied that mouse click responses were executed when the target was localized. 3.2. Saccade amplitudes A saccade was signaled if the velocity of the tracked eye exceeded 30° s 1, or eye acceleration exceeded 8000° s 2. Were TLSAs shorter under lower contrast conditions as predicted by a visual span account of search processes? To start, saccades less than .50° were considered corrective (i.e., used to reposition the eyes within a fixated region of interest), and were excluded from analysis. The .50° criterion was chosen because it is smaller than the size of the target (i.e. 1° 1°). Hence, very small saccades within the target were not counted in the analysis. Fig. 3A shows that probability distributions for target-localizing saccade amplitudes after the elimination of corrective saccades (i.e., TLSAs) were severely skewed in the positive direction. For this reason, median
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(a)
Fig. 5. Average durations of fixations made to localize the target.
(b) Median TLSA (Deg)
3.0
Binocular viewing Monocular viewing
2.5
3.3. Fixation counts
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2.31
2.0 1.7
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1.31
1.0 0.0
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0.2
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Contrast Fig. 3. Probability distributions for amplitudes of target-localizing saccades across Viewing and Contrast conditions (a). Given the skew of the distributions, median saccade amplitudes were utilized to describe their central tendencies (b).
A fixation was indicated if the velocity of the tracked eye did not exceed 30° s 1, and eye acceleration did not exceed 8000° s 2 . Previous findings indicate that on average, more fixations are used to localize a target under lower contrast conditions [18,19,21]. Median fixation counts were subjected to a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) mixed factorial ANOVA. Viewing was a between subjects factor. There was a main effect of Stimulus Contrast. About 14 more fixations were made in the .04 contrast condition than the .54 contrast condition (18 vs. 4), F(1, 20) = 222.55, p < .01, g2p = .92. The main effect of Viewing was significant with twice as many fixations made in the monocular viewing condition (14 for monocular vs. 7 for binocular), F(1, 20) = 38.69, p < .01, g2p = .66. The Viewing Stimulus Contrast interaction was significant, F(1, 20) = 45.50, p < .01, g2p = .70. Fig. 4 shows that fewer fixations were always made in the higher contrast (.54) condition. However, the difference between monocular and binocular viewing was minimal at the higher contrast level. 3.4. Fixation duration
Fig. 4. Average number of fixations made to localize the target.
amplitudes were used to summarize the distributions. Median TLSAs were subjected to a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) mixed factorial ANOVA. Viewing was a between subjects factor. There was a main effect of Stimulus Contrast. Amplitudes were about 1° shorter in the .04 contrast condition than the .54 contrast condition (1.50° vs. 2.54°), F(1, 20) = 17.56, p < .01, g2p = .47. Although slightly shorter saccades were executed during monocular viewing (see the small effect size below, and Fig. 3B), the main effect of Viewing was not statistically significant (1.81° vs. 2.23°), F(1, 20) = 2.73, p = .11, g2p = .12. Finally, the Viewing Stimulus Contrast interaction was not significant, F(1, 20) = .01, p = .91, g2p = .001.
Fixation duration was any duration of eye fixation after initial saccades, but before final fixations were terminated by mouseclick responses. Fixation durations were expected to be longer under the lower contrast condition [18,19,21]. Median fixation durations were subjected to a 2 Viewing (Monocular vs. Binocular) 2 Stimulus Contrast (.04 vs. .54) mixed factorial ANOVA with Viewing a between subjects factor. There was a main effect of Stimulus Contrast with fixation durations being about 100 ms longer in the .04 contrast condition than the .54 contrast condition (319 ms vs. 206 ms), F(1, 20) = 121.53, p < .01, g2p = .86. The main effect of Viewing was not significant (275 ms for monocular vs. 250 ms for binocular), F(1, 20) = 2.56, p = .13, g2p = .11. Fig. 5 shows that although durations were slightly longer under the lower contrast condition, the Viewing Stimulus Contrast interaction was not statistically significant, F(1, 20) = 2.16, p = .16, g2p = .10. 4. Discussion 4.1. The visual and perceptual spans The visual span is the area around eye fixation within which objects are recognized. The visual span control hypothesis posits that eye movements are controlled by the size of the visual span. We reasoned that the lower the fidelity of a target signal (i.e., its contrast), the greater the processing time (e.g., fixation duration) and effort (e.g., increased number of fixations) needed to obtain that
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signal, which should also be reflected in the amplitude of the saccade that localized the target. Fixation durations were longer in the lower contrast condition, as predicted (see also [18,19,21] for similar findings). Effort increases because the visual span is smaller for lower fidelity targets (e.g., [21]). Hence TLSAs were expected, and were found to be smaller at the lower contrast level. For a robust account of the link between eye movement behavior and visual span properties, the task in the experiment was performed under binocular and monocular viewing conditions. Longer fixation durations, shorter TLSAs and increased fixation counts were expected for monocular viewing. However, whereas there appeared to be small effects of monocular viewing for TLSA and fixation duration in the predicted directions, only the fixation count effects were statistically significant. Specifically, fixation counts were significantly higher under monocular viewing, but only at the lower contrast level. The primary contribution of the present work was the use of TLSAs as an index of span size during visual target localization. We were concerned about previously inconsistent effects of luminance contrast on saccade amplitudes (see [18,19,21]). Saccade amplitudes potentially provide a spatial metric of visual span size. This potential allowed us to provide a possible solution to the previously unsolved problem of how saccade amplitudes relate to visual span size. We suspected that previously inconsistent effects may have been driven by too broad a definition of saccade amplitudes for visual span estimation. We have argued that during VTL, if all saccades are considered (e.g., [18,19,21]), the distribution is rather noisy for estimating visual span size. The visual span is a central region within a wider perceptual span (see [7,22]). Whereas target recognition is limited by the size of the visual span, effective stimulus processing is limited by the size of the wider perceptual span [27,28]; see also [4,25,26]. Effective stimulus processing includes obtaining useful preview information about stimuli beyond fixation, but not necessarily target recognition information (e.g. [28]). We have proposed that whereas overall saccade amplitudes may reasonably estimate perceptual span dynamics (e.g., [9]), a more appropriate account of the visual span from saccade amplitudes is obtainable from the amplitude of the saccade that localized the target (i.e., what we have termed TLSA). The results of the present study were consistent with expectation for the lower contrast conditions: increased fixation counts, and shorter TLSAs. Indeed, TLSA decreased with decreasing contrast across binocular and monocular viewing. Hence we contend that saccade amplitudes (specifically TLSAs), like fixation counts, provide a window on visual span properties. Notwithstanding the primary conclusion above, the difference in Viewing Stimulus Contrast relationships for TLSA and fixation counts is insightful. Both measures generally indicated a smaller visual span at the low contrast level. However, differences are evident between monocular and binocular viewing at the higher contrast level. Whereas contrast differentially influenced fixation counts (revisit the interaction effect shown in Fig. 4), this was not the case for TLSAs (revisit Fig. 3b). If TLSA and fixation counts index the same span, one would expect the relationships to be the same. During VTL, each fixation (except the target recognition fixation) is associated with an ensuing saccade of some amplitude. Reasonably, non-TLSAs primarily reflect saccadic movements within the bounds of the preview region [2,7,9,10,15–17,30]. We suggest that the fixation count measure, somewhat like the overall saccade amplitude measure, does not solely index the visual span; it also indexes the wider perceptual span. A wider perceptual span predictably brings the target within useful preview processing range with fewer exploratory fixations than a narrower span. The results of the present study suggest that under monocular viewing, the perceptual span fluctuates more rapidly with contrast changes than under binocular viewing. However, contrast-induced fluctua-
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tions of the visual span may not be (very) sensitive to ocular conditions. 4.2. Application Item visibility under low contrast conditions is a potential problem for displays in devices such as telescopes, microscopes, computer monitors, and mobile telephones, for which target localization is a common goal. For such devices, perceived contrast reduction may occur because of image quality metrics (e.g., telescopes, microscopes, helmet-mounted displays), or from ambient illumination (e.g., computer monitors, mobile telephones). Studies that address contrast effects on eye movement indices are useful for informing display designers, and users. The fact that effect sizes were smaller for TLSAs than for fixation counts does not minimize the importance of the corroborating evidence of visual span manipulation from these two measures. In most applications, it may suffice to simply say that the visual span is smaller, without reference to a spatial metric. However, one may envision situations where a spatial metric may be useful (e.g. for modeling human limitations, towards improving display designs). The work of Greene [7] illustrates a simple case of the utility of a spatial metric like TLSA for the simulation of human VTL. In that study, Monte Carlo simulations of VTL were conducted with arbitrary saccade amplitude limits for visual span and perceptual span boundaries. A better approach would have been to use biologically-guided saccade information in the simulation process. In the present study, visual performance occurred under photopic display, and scotopic ambient conditions. An application for comparison is the use of night vision devices. One such device is the Integrated Helmet and Display Sight System (IHADSS). The system is monocular (video imagery to the right eye only), and is used by AH-64 attack helicopter aviators (see [24]). Another device is the Aviator’s Night Vision Imaging System (ANVIS). This device is set up for binocular viewing, and it is the most widely used helmet-mounted night vision setup for aviators [1]. The findings of the present study suggest that under low contrast conditions, the perceptual span decreases faster under monocular viewing (as indicated by fixation count results) than under binocular viewing. The visual span however (i.e. the area within which targets are recognized), may decrease similarly for monocular and binocular viewing. A concern for designers of display devices is the ability of users to search for, and localize target items known to be embedded in display noise. The study describes a methodology for accessing the size of the area within which a target is identifiable during visual target localization. The findings highlight for monocular vs. binocular target localization, the importance of considering separately, how many fixations are needed to localize the target, and how close to fixation the target must be for it to be noticed. Acknowledgments This work was made possible by University of Detroit Mercy IRF equipment grants. We thank the formal and informal reviewers for their insights and suggestions. We also thank research assistants Carmela A.M. Martin, Britanny M. Polisuk, Ebony N. Fails, and Perrier T. Greene for their contributions to the project. References [1] M.M. Bayer, C.E. Rash, J.H. Brindle, Introduction to helmet-mounted displays, in: C.E. Rash, M.B. Russo, T.R. Letowski, E.T. Schmeisser (Eds.), Helmetmounted displays: Sensation, perception, and cognitive issues, U.S. Army Aeromedical Research Laboratory, 2009, pp. 47–108. [2] J.H. Bertera, K. Rayner, Eye movements and the span of the effective stimulus in visual search, Perception & Psychophysics 62 (2000) 576–585.
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