Shape judgments in natural scenes: Convexity biases versus stereopsis.

Determining the relief of upcoming terrain is critical to locomotion over rough or uneven ground. Given the significant contribution of stereopsis to perceived surface shape, it should play a crucial role in determining the shape of ground surfaces. The aim of this series of experiments was to evaluate the relative contribution of monocular and binocular depth cues to judgments of ground relief. To accomplish this goal, we simulated a depth discrimination task using naturalistic imagery. Stimuli consisted of a stereoscopically rendered grassy terrain with a central mound or a dip with varying height. We measured thresholds for discrimination of the direction of the depth offset. To determine the relationship between relief discrimination and measures of stereopsis, we used two stereoacuity tasks performed under the same viewing conditions. To assess the impact of ambiguous two-dimensional shading cues on depth judgments in our terrain task, we manipulated the intensity of the shading (low and high). Our results show that observers reliably discriminated ground reliefs as small as 20 cm at a viewing distance of 9.1 m. As the shading was intensified, a large proportion of observers (30%) exhibited a strong convexity bias, even when stereopsis indicated a concave depression. This finding suggests that there are significant individual differences in the reliance on assumptions of surface curvature that must be considered in experimental conditions. In impoverished viewing environments with limiting depth cues, these convexity biases could persist in judgments of ground relief, especially when shading cues are highly salient.

Predicting the Farnsworth-Munsell D15 and Holmes-Wright-A lantern outcomes with computer-based color vision tests.

This study determined the AC1 agreement values between computer-based color vision tests and the Farnworth-Munsell D-15 (F-D15) and the Holmes-Wright Type A lantern (HWA). The computer-based tests were the United States Air Force Cone Contrast Test (OCCT), Cambridge Color Test, Innova Rabin Cone Contrast, Konan-Waggoner D15 (KWC-D15), and Color Assessment and Diagnosis (CAD). Sixty-eight color-vision-defective persons participated. The KWC-D15 had the highest AC1 with the F-D15 (${\rm AC1} = {0.88}$AC1=0.88). Both the CAD and OCCT had the highest values with the HWA (${\rm AC1} \gt {0.96}$AC1>0.96). The KWC-D15 would be the best substitute for the F-D15. Either the CAD or OCCT would be appropriate substitutes for the HWA.

Contributions of Stereopsis and Aviation Experience to Simulated Rotary Wing Altitude Estimation.

OBJECTIVE: We examined the contribution of binocular vision and experience to performance on a simulated helicopter flight task. BACKGROUND: Although there is a long history of research on the role of binocular vision and stereopsis in aviation, there is no consensus on its operational relevance. This work addresses this using a naturalistic task in a virtual environment. METHOD: Four high-resolution stereoscopic terrain types were viewed monocularly and binocularly. In separate experiments, we evaluated performance of undergraduate students and military aircrew on a simulated low hover altitude judgment task. Observers were asked to judge the distance between a virtual helicopter skid and the ground plane. RESULTS: Our results show that for both groups, altitude judgments are more accurate in the binocular viewing condition than in the monocular condition. However, in the monocular condition, aircrew were more accurate than undergraduate observers in estimating height of the skid above the ground. CONCLUSION: At simulated altitudes of 5 ft (1.5 m) or less, binocular vision provides a significant advantage for estimation of the depth separation between the landing skid and the ground, regardless of relevant operational experience. However, when binocular cues are unavailable aircrew outperform undergraduate observers, a result that likely reflects the impact of training on the ability to interpret monocular depth cues.

Perceptual enhancement as a result of a top-down attentional influence in a scene viewing task: Evidence from saccadic inhibition.

Prior research has shown that task instructions influence the locations and durations of eye fixations during scene viewing. These task-related changes in gaze patterns are likely to be associated with a top-down influence of attention. Presently we applied a saccadic-inhibition manipulation in order to detect another expected manifestation of top-down attention: perceptual enhancement. Participants viewed eight-item arrays containing photographs from two categories of scenes. Four of the photos depicted natural landscapes ("nature") and the other four depicted urban environments ("buildings"). Participants were instructed to memorize scenes from one of the categories in preparation for a later recognition memory test. During eye fixations the border around the fixated scene flickered briefly from black to white with a random interval between flickers ranging from 400 to 600 ms. We computed the likelihood of a saccade being initiated in the period following the flicker. Consistent with prior research, we found a strong saccadic inhibition effect with maximum saccadic inhibition occurring approximately 97 ms following the flicker. Importantly, the saccadic inhibition effect was stronger and longer lasting when the subject's eyes were fixated on a relevant scene compared to an irrelevant scene. These findings are consistent with perceptual enhancement as a result of top-down attention.

Spatial frequency filtering and the direct control of fixation durations during scene viewing.

The present study employed a saccade-contingent change paradigm to investigate the effect of spatial frequency filtering on fixation durations during scene viewing. Subjects viewed grayscale scenes while encoding them for a later memory test. During randomly chosen saccades, the scene was replaced with an alternate version that remained throughout the critical fixation that followed. In Experiment 1, during the critical fixation, the scene could be changed to high-pass and low-pass spatial frequency filtered versions. Under both conditions, fixation durations increased, and the low-pass condition produced a greater effect than the high-pass condition. In subsequent experiments, we manipulated the familiarity of scene information during the critical fixation by flipping the filtered scenes upside down or horizontally. Under these conditions, we observed lengthening of fixation durations but no difference between the high-pass and low-pass conditions, suggesting that the filtering effect is related to the mismatch between information extracted within the critical fixation and the ongoing scene representation in memory. We also conducted control experiments that tested the effect of changes to scene orientation (Experiment 2a) and the addition of color to a grayscale scene (Experiment 2b). Fixation distribution analysis suggested two effects on the distribution fixation durations: a fast-acting effect that was sensitive to all transsaccadic changes tested and a later effect in the tail of the distribution that was likely tied to the processing of scene information. These findings are discussed in the context of theories of oculomotor control during scene viewing.

Direct lexical control of eye movements in reading: evidence from a survival analysis of fixation durations.

Participants' eye movements were monitored in an experiment that manipulated the frequency of target words (high vs. low) as well as their availability for parafoveal processing during fixations on the pre-target word (valid vs. invalid preview). The influence of the word-frequency by preview validity manipulation on the distributions of first fixation duration was examined by using ex-Gaussian fitting as well as a novel survival analysis technique which provided precise estimates of the timing of the first discernible influence of word frequency on first fixation duration. Using this technique, we found a significant influence of word frequency on fixation duration in normal reading (valid preview) as early as 145ms from the start of fixation. We also demonstrated an equally rapid non-lexical influence on first fixation duration as a function of initial landing position (location) on target words. The time-course of frequency effects, but not location effects was strongly influenced by preview validity, demonstrating the crucial role of parafoveal processing in enabling direct lexical control of reading fixation times. Implications for models of eye-movement control are discussed.

The mask-onset delay paradigm and the availability of central and peripheral visual information during scene viewing.

We employed a variant of the mask-onset delay paradigm in order to limit the availability of visual information in central and peripheral vision within individual fixations during scene viewing. Subjects viewed full-color scene photos with instructions to search for a target object (Experiment 1) or to study them for a later memory test (Experiment 2). After a fixed interval following the onset of each eye fixation (50-100 ms), the scene was scrambled either in the central visual field or over the entire display. The intact scene was presented when the subject made an eye movement. Our results reconcile different sets of findings from prior research regarding the masking of central and peripheral visual information at different intervals following fixation onset. In particular, we found that when the entire display was scrambled, both search and memory performance were impaired even at relatively long mask-onset intervals. In contrast, when central vision was scrambled, there were subtle impairments that depended on the viewing task. In the 50-ms mask-onset interval, subjects were selectively impaired at identifying, but not in locating, the search target (Experiment 1), while memory performance (Experiment 2) was unaffected in this condition, and hence, the reliance on central and peripheral visual information depends partly on the viewing task.

Eye movements reveal solution knowledge prior to insight.

In two experiments, participants solved anagram problems while their eye movements were monitored. Each problem consisted of a circular array of five letters: a scrambled four-letter solution word containing three consonants and one vowel, and an additional randomly-placed distractor consonant. Viewing times on the distractor consonant compared to the solution consonants provided an online measure of knowledge of the solution. Viewing times on the distractor consonant and the solution consonants were indistinguishable early in the trial. In contrast, several seconds prior to the response, viewing times on the distractor consonant decreased in a gradual manner compared to viewing times on the solution consonants. Importantly, this pattern was obtained across both trials in which participants reported the subjective experience of insight and trials in which they did not. These findings are consistent with the availability of partial knowledge of the solution prior to such information being accessible to subjective phenomenal awareness.

Evidence for top-down control of eye movements during visual decision making.

Participants' eye movements were monitored while they viewed displays containing 6 exemplars from one of several categories of everyday items (belts, sunglasses, shirts, shoes), with a column of 3 items presented on the left and another column of 3 items presented on the right side of the display. Participants were either required to choose which of the two sets of 3 items was the most expensive (2-AFC) or which of the 6 items was the most expensive (6-AFC). Importantly, the stimulus display, and the relevant stimulus dimension, were held constant across conditions. Consistent with the hypothesis of top-down control of eye movements during visual decision making, we documented greater selectivity in the processing of stimulus information in the 6-AFC than the 2-AFC decision. In addition, strong spatial biases in looking behavior were demonstrated, but these biases were largely insensitive to the instructional manipulation, and did not substantially influence participants' choices.

Stimulus exposure and gaze bias: a further test of the gaze cascade model.

We tested predictions derived from the gaze cascade model of preference decision making (Shimojo, Simion, Shimojo, & Scheier, 2003; Simion & Shimojo, 2006, 2007). In each trial, participants' eye movements were monitored while they performed an eight-alternative decision task in which four of the items in the array were preexposed prior to the trial. Replicating previous findings, we found a gaze bias toward the chosen item prior to the response. However, contrary to the prediction of the gaze cascade model, preexposure of stimuli decreased, rather than increased, the magnitude of the gaze bias in preference decisions. Furthermore, unlike the prediction of the model, preexposure did not affect the likelihood of an item being chosen, and the pattern of looking behavior in preference decisions and on a non preference control task was remarkably similar. Implications of the present findings in multistage models of decision making are discussed.

EEG activity underlying successful study of associative and order information.

Two of the most well studied and ecologically relevant memory paradigms are memory for pairs ("associations") and ordered sequences ("serial lists"). Behavioral theories comprise two classes: those that use common mechanisms and those that use distinct mechanisms for study and retrieval of associations versus serial lists. We tested the common-mechanisms hypothesis by recording electroencephalographic activity related to successful study ("subsequent memory effect" [SME]) of pairs and short lists (triples) of nouns. Multivariate analysis identified four distributed patterns of brain activity: (1) right parietal activity throughout most of the study period that differentiated study of pairs from triples within subjects as well as exhibiting an SME that was significant for pairs but not for triples; (2) a left parietal and fronto-polar activity pattern that was reliable around 500 msec and later in the study trial, exhibiting an SME for pairs and a weaker, nonsignificant SME for triples; (3) a left frontal/right parietal topography in the middle of the study interval which covaried with speed and accuracy across subjects; and (4) a pattern resembling the late positive component preceded by an early potential which together covaried with accuracy in triples but slow response times for both pairs and triples. These patterns point to the relevance of three classic SME components (early, late positive, and slow components) from single-item memory to memory for structured information, but suggest that they reflect subsets of more complex spatio-temporal patterns. Our findings support common underlying mechanisms for study and recall of pairs and lists. However, existing models must be modified to account for differences in both the presence of certain study-relevant processes and in the relevance of these processes to performance measures for pairs versus serial lists.

The roles of EEG oscillations in learning relational information.

Rhythmic brain activity has been implicated in learning and memory. Many models implicate theta oscillations (4-8 Hz) specifically in learning of relational information such as pairings and ordered lists. We tested this hypothesis in humans by recording electroencephalographic activity while participants studied nouns organised into pairs or triples for a later cued recall test. If theta is critical in learning structured information, then the amount of theta activity present during study of pairs and triples should covary with subsequent memory performance (accuracy and response times). Multivariate partial least squares analysis revealed three patterns of oscillatory activity associated with task conditions in different ways: a) Within subjects, successful study of pairs but not triples was associated with elevations in oscillations at multiple frequencies including theta, b) Frontal theta oscillations, in conjunction with beta oscillations, covaried with memory performance across subjects for both pairs and triples and c) Right-lateralized gamma oscillations in conjunction with low-frequency oscillations were associated with faster responding at the expense of accuracy across subjects for both pairs and triples. These findings support models that implicate theta oscillations in learning structured information rather than item information alone but similar to prior reports, suggest that theta oscillations explain individual variability better than trial-to-trial variability in behavior.

Linking associative and serial list memory: Pairs versus triples.

Paired associates and serial list memory are typically investigated separately. An "isolation principle" (J. B. Caplan, 2005) was proposed to explain behavior in both paradigms by using a single model, in which serial list and paired associates memory differ only in how isolated pairs of items are from interference from other studied items. In the present study, 2 experiments identify a critical dissociation between the 2 paradigms, challenging this unified account. Specifically, forward and backward probes were highly correlated for pairs and less so for short lists (triples). The authors asked whether the isolation principle could quantitatively accommodate this type of dissociation. A simulation confirmed that a single model incorporating the isolation principle can adequately explain this and other dissociations, supporting the common processes view.