Human perception of flicker-fused letters that are luminance balanced.

The Talbot-Plateau law specifies what combinations of flash frequency, duration, and intensity will yield a flicker-fused stimulus that matches the brightness of a steady stimulus. It has proven to be remarkably robust in its predictions, and here we provide additional support though the use of a contrast discrimination task. However, we also find that the visual system can register flicker-fused letters when the combination of frequency and duration is relatively low. The letters are recognized even though they have the same physical luminance as background. We hypothesize that the letters elicit synchronous oscillations that encode for stimulus attributes, which prevents the letter from blending into the background.

Evaluating integration of letter fragments through contrast and spatially targeted masking.

Four experiments were conducted to gain a better understanding of the visual mechanisms related to how integration of partial shape cues provides for recognition of the full shape. In each experiment, letters formed as outline contours were displayed as a sequence of adjacent segments (fragments), each visible during a 17-ms time frame. The first experiment varied the contrast of the fragments. There were substantial individual differences in contrast sensitivity, so stimulus displays in the masking experiments that followed were calibrated to the sensitivity of each participant. Masks were displayed either as patterns that filled the entire screen (full field) or as successive strips that were sliced from the pattern, each strip lying across the location of the letter fragment that had been shown a moment before. Contrast of masks were varied to be lighter or darker than the letter fragments. Full-field masks, whether light or dark, provided relatively little impairment of recognition, as was the case for mask strips that were lighter than the letter fragments. However, dark strip masks proved to be very effective, with the degree of recognition impairment becoming larger as mask contrast was increased. A final experiment found the strip masks to be most effective when they overlapped the location where the letter fragments had been shown a moment before. They became progressively less effective with increased spatial separation from that location. Results are discussed with extensive reference to potential brain mechanisms for integrating shape cues.

Progression of atherosclerosis versus arterial stiffness with age within and between arteries in systemic lupus erythematosus.

The progression of atherosclerosis versus arterial stiffness with age within and between arteries has not been defined. Systemic lupus erythematosus (SLE) is a human model of accelerated arterial disease that may permit this determination. 76 SLE patients (69 women, age 37 ± 12 years) and 26 age-and-sex-matched controls (22 women, age 34 ± 11 years) underwent transesophageal echocardiography and carotid ultrasonography for assessment of atherosclerosis [plaques and intima-media thickening (IMT)] and arterial stiffness [increased pressure-strain elastic modulus (PSEM)] of the descending thoracic aorta and carotid arteries. Since IMT is highly associated with plaques, IMT was used as a marker of atherosclerosis to assess its progression in relation with age and PSEM. Aortic and carotid plaques, IMT, and PSEM were greater in patients than in controls (all p ≤ 0.05). Within the aorta and within the carotid arteries, the average percent increases per decade of age for IMT versus PSEM were similar in patients (8.55% versus 9.33% and 3.39% versus 2.46%, respectively) and controls (5.53% versus 6.60% and 4.75% versus 3.49%, respectively) (all p ≥ 0.58). However, in SLE patients, the average percent increases per decade of age for IMT and PSEM were higher in the aorta than in the carotid arteries (8.55% and 9.33% versus 3.39% and 2.46%, respectively, both p ≤ 0.03). In patients with SLE, atherosclerosis and arterial stiffness progress with age parallel to each other within arteries, but divergently between arteries with different anatomy and hemodynamics.

Lambl's Excrescences: Association with Cerebrovascular Disease and Pathogenesis.

BACKGROUND: Lambl's excrescences (LEx) are detected by transesophageal echocardiography (TEE) and are characterized as thin, elongated, and hypermobile structures located at the leaflets' coaptation point of the heart valves. The association of LEx with cerebrovascular disease (CVD) is still undefined and yet patients with LEx and suspected CVD receive unproven effective antiplatelet or anticoagulant therapy or even undergo valve surgery. Also, the association of LEx with aging and atherogenic, inflammatory, or thrombogenic parameters has not been reported. METHODS: Seventy-seven patients with systemic lupus erythematosus (SLE) (71 women, age 37 ± 12 years) and 26 age- and sex-matched healthy controls (22 women, age 34 ± 11 years) prospectively underwent routine history and physical exam, transcranial Doppler, brain MRI, TEE, carotid duplex, and clinical and laboratory evaluations of atherogenesis, inflammation, platelet activity, coagulation, and fibrinolysis. Subjects without stroke/TIA on enrollment (with and without LEx) had a median follow-up of 57 months. RESULTS: On enrollment, 33 (43%) of 77 patients had CVD manifested as acute stroke/TIA (23 patients), cerebromicroembolism by transcranial Doppler (17 patients), or cerebral infarcts by MRI (14 patients). Mitral or aortic valve LEx were equally frequent in healthy controls (46%) as in patients with and without any CVD (39 and 43%), stroke/TIA (35 and 43%), cerebromicroembolism (41 and 42%), or cerebral infarcts (36 and 43%) (all p ≥ 0.72). Also, other mechanisms for CVD other than LEx such as Libman-Sacks vegetations, patent foramen ovale or interatrial septal aneurysm, aortic or carotid atherosclerosis, or thrombogenesis were found in ≥94% of patients with CVD. In addition, 36 subjects with and 44 without LEx had similar low incidence of stroke/TIA (1 (1.3%) and 2 (2.5%), respectively, p = 1.0) during follow-up. Finally, LEx were not associated with aging, atherogenic risk factors, atherosclerosis, inflammation, or thrombogenesis. CONCLUSIONS: In this study, LEx are similarly prevalent in healthy controls and SLE patients, are not associated with CVD, and are not associated with pathogenic risk factors. Therefore, the study findings suggest that LEx may not be cardioembolic substrates, may not represent pathologic valve structures, and may not require therapy.

Evaluating the Talbot-Plateau law.

The Talbot-Plateau law asserts that when the flux (light energy) of a flicker-fused stimulus equals the flux of a steady stimulus, they will appear equal in brightness. To be perceived as flicker-fused, the frequency of the flash sequence must be high enough that no flicker is perceived, i.e., it appears to be a steady stimulus. Generally, this law has been accepted as being true across all brightness levels, and across all combinations of flash duration and frequency that generate the matching flux level. Two experiments that were conducted to test the law found significant departures from its predictions, but these were small relative to the large range of flash intensities that were tested.

Evaluating the contribution of shape attributes to recognition using the minimal transient discrete cue protocol.

Subjects were tested for their ability to identify objects that were represented by an array of dots that marked the major contours, usually only the outer boundary. Each dot was briefly flashed to make its position known, and a major variable was the time interval that was required to flash all the dots for a given shape. Recognition declined as the total time for display of the dot inventory was increased. Each shape was shown to a given subject only once and it was either recognized -- named - or not. Although the recorded response was binary, a large number of subjects was tested, which made it possible to derive regression functions and thus specify an intercept and slope for each shape. Shapes differed substantially in their slopes, which is likely due to the amount of redundant information provided by neighboring dots. Indices of shape attributes were also derived, specifically Attneave's indices of complexity, mean curvature, inflection count, and symmetry. Three of the four shape attributes were significantly related to intercept and slope levels, but none made a substantial contribution. This suggests that these attributes are not essential properties that define shapes and allow for recognition.

Recognition of letters displayed as successive contour fragments.

Shapes can be displayed as parts but perceived as a whole through feedforward and feedback mechanisms in the visual system, though the exact spatiotemporal relationships for this process are still unclear. Our experiments examined the integration of letter fragments that were displayed as a rapid sequence. We examined the effects of timing and masking on integration, hypothesizing that increasing the timing interval between frames would impair recognition by disrupting contour linkage. We further used different mask types, a full-field pattern mask and a smaller strip mask, to examine the effects of global vs local masking on integration. We found that varying mask types and contrast produced a greater decline in recognition than was found when persistence or mask density was manipulated. The study supports prior work on letter recognition and provides greater insight into the spatiotemporal factors that contribute to the identification of shapes.

Libman-Sacks endocarditis and associated cerebrovascular disease: The role of medical therapy.

BACKGROUND: Libman-Sacks endocarditis in patients with systemic lupus erythematosus (SLE) is commonly complicated with embolic cerebrovascular disease (CVD) or valve dysfunction for which high-risk valve surgery is frequently performed. However, the role of medical therapy alone for Libman-Sacks endocarditis and associated acute CVD remains undefined. OBJECTIVE: To determine in this cross-sectional and longitudinal study if conventional anti-inflammatory and anti-thrombotic therapy may be an effective therapy in SLE patients with Libman-Sacks endocarditis and associated acute CVD. METHODS AND MATERIALS: 17 SLE patients with Libman-Sacks endocarditis detected by two-and-three-dimensional transesophageal echocardiography (TEE) and complicated with acute CVD [stroke/TIA, focal brain injury on MRI, or cognitive dysfunction] were treated with conventional anti-inflammatory and anti-thrombotic therapy for a median of 6 months and then underwent repeat TEE, transcranial Doppler, brain MRI, and neurocognitive testing for re-assessment of Libman-Sacks endocarditis and CVD. RESULTS: Valve vegetations decreased in number, diameter, and area (all p ≤0.01); associated valve regurgitation significantly improved (p = 0.04), and valve thickening did not progress (p = 0.56). In 13 (76%) patients, valve vegetations or valve regurgitation resolved or improved in number and size or by ≥1 degree, respectively, as compared to 4 (24%) patients in whom vegetations or valve regurgitation persisted unchanged or increased in size or by ≥1 degree (p = 0.03). Also, cerebromicroembolism, lobar and global gray and white matter cerebral perfusion, ischemic brain lesion load, and neurocognitive dysfunction resolved or significantly improved (all p ≤0.04). CONCLUSION: These preliminary data suggest that combined conventional anti-inflammatory and antithrombotic therapy may be an effective treatment for Libman-Sacks endocarditis and its associated CVD and may obviate the need for high-risk valve surgery.

Evaluating spatiotemporal integration of shape cues.

Prior work has shown that humans can successfully identify letters that are constructed with a sparse array of dots, wherein the dot pattern reflects the strokes that would normally be used to fashion a given letter. In the present work the dots were briefly displayed, one at a time in sequence, varying the spatial order in which they were shown. A forward sequence was spatially ordered as though one were passing a stroke across the dots to connect them. Experiments compared this baseline condition to the following three conditions: a) the dot sequence was spatially ordered, but in the reverse direction from how letter strokes might normally be written; b) the dots in each stroke of the letter were displayed in a random order; c) the sequence of displayed dots were chosen for display from any location in the letter. Significant differences were found between the baseline condition and all three of the comparison conditions, with letter recognition being far worse for the random conditions than for conditions that provided consistent spatial ordering of dot sequences. These findings show that spatial order is critical for integration of shape cues that have been sequentially displayed.

Masking the Integration of Complementary Shape Cues.

Retinal and cortical mechanisms provide for persistence of visual information across intervals of many hundreds of milliseconds, which supports the integration of partial shape cues. The present experiments displayed unknown shapes in a match recognition task, wherein a target shape was quickly followed by a comparison shape; the task was to specify whether the comparison shape was the same or different from the target. The target and comparison shapes were displayed as sparse dots that marked boundary locations. The first experiment successively displayed the target shape as two complementary subsets and found that the probability of correct match remained above chance with up to 500 ms of subset separation. The second experiment demonstrated masking of the target by a random pattern of dots when the target and mask were displayed simultaneously, but with much less or no masking when the two were separated by 100 ms. The third experiment displayed the target subsets with 200 ms of separation and found that match recognition was disrupted when the random-dot mask was displayed midway between the two subsets. Much less masking of an intact target was produced with that amount of temporal separation, which suggests that mechanisms for integration of shape cues have a special vulnerability to masking. The third experiment also found very little impairment of match recognition when the mask was displayed simultaneous with one of the subsets. We hypothesize that there is embedding of the subset pattern within the mask pattern, but additional display of the other subset effectively disembeds the buried partial shape cues.

Additional evidence that contour attributes are not essential cues for object recognition.

It is believed that certain contour attributes, specifically orientation, curvature and linear extent, provide essential cues for object (shape) recognition. The present experiment examined this hypothesis by comparing stimulus conditions that differentially provided such cues. A spaced array of dots was used to mark the outside boundary of namable objects, and subsets were chosen that contained either contiguous strings of dots or randomly positioned dots. These subsets were briefly and successively displayed using an MTDC information persistence paradigm. Across the major range of temporal separation of the subsets, it was found that contiguity of boundary dots did not provide more effective shape recognition cues. This is at odds with the concept that encoding and recognition of shapes is predicated on the encoding of contour attributes such as orientation, curvature and linear extent.

Wavelet modelling of collinearity judgment error.

Prior research has found that participants manifest complex profiles of error when they are asked to judge collinearity of stimulus elements. These studies used harmonic analysis to model the data, and found large departures from accurate collinear judgments, with the amplitude and specific angular position of departures from valid judgment varying from one participant to the next. The models appeared to be a composite of many large-, medium- and small-scale departures, but this could not be established with any certainty because of the global nature of harmonic model components. Wavelet modelling is better suited to answer the question of whether the error profile is produced by independent sources that vary in size and location. Here, we examined judgments of collinearity of dot pairs across 360 degrees of angular position. A priori and post hoc wavelet modelling strategies were used to identify independent sources of error that could not be attributed to chance, and some new statistical protocols were applied. We found evidence of error sources at several levels of scale, and these results were confirmed by the application of cross-validation methods that make no assumption about the nature of the error probability distribution.

New encoding concepts for shape recognition are needed.

Models designed to explain how shapes are perceived and stored by the nervous system commonly emphasize encoding of contour features, especially orientation, curvature, and linear extent. A number of experiments from my laboratory provide evidence that contours deliver a multitude of location markers, and shapes can be identified when relatively few of the markers are displayed. The emphasis on filtering for orientation and other contour features has directed attention away from full and effective examination of how the location information is registered and used for summarizing shapes. Neural network (connectionist) models try to deal with location information by modifying linkage among neuronal populations through training trials. Connections that are initially diffuse and not useful in achieving recognition get eliminated or changed in strength, resulting in selective response to a given shape. But results from my laboratory, reviewed here, demonstrate that unknown shapes that are displayed only once can be identified using a matching task. These findings show that our visual system can immediately encode shape information with no requirement for training trials. This encoding might be accomplished by neuronal circuits in the retina.

Evaluating persistence of shape information using a matching protocol.

Many laboratories have studied persistence of shape information, the goal being to better understand how the visual system mediates recognition of objects. Most have asked for recognition of known shapes, e.g., letters of the alphabet, or recall from an array. Recognition of known shapes requires access to long-term memory, so it is not possible to know whether the experiment is assessing short-term encoding and working memory mechanisms, or has encountered limitations on retrieval from memory stores. Here we have used an inventory of unknown shapes, wherein a string of discrete dots forms the boundary of each shape. Each was displayed as a target only once to a given respondent, with recognition being tested using a matching task. Analysis based on signal detection theory was used to provide an unbiased estimate of the probability of correct decisions about whether comparison shapes matched target shapes. Four experiments were conducted, which found the following: a) Shapes were identified with a high probability of being correct with dot densities ranging from 20% to 4%. Performance dropped only about 10% across this density range. b) Shape identification levels remained very high with up to 500 milliseconds of target and comparison shape separation. c) With one-at-a-time display of target dots, varying the total time for a given display, the proportion of correct decisions dropped only about 10% even with a total display time of 500 milliseconds. d) With display of two complementary target subsets, also varying the total time of each display, there was a dramatic decline of proportion correct that reached chance levels by 500 milliseconds. The greater rate of decline for the two-pulse condition may be due to a mechanism that registers when the number of dots is sufficient to create a shape summary. Once a summary is produced, the temporal window that allows shape information to be added may be more limited.

Visual encoding of partial unknown shape boundaries.

Prior research has found that known shapes and letters can be recognized from a sparse sampling of dots that mark locations on their boundaries. Further, unknown shapes that are displayed only once can be identified by a matching protocol, and here also, above-chance performance requires very few boundary markers. The present work examines whether partial boundaries can be identified under similar low-information conditions. Several experiments were conducted that used a match-recognition task, with initial display of a target shape followed quickly by a comparison shape. The comparison shape was either derived from the target shape or was based on a different shape, and the respondent was asked for a matching judgment, i.e., did it "match" the target shape. Stimulus treatments included establishing how density affected the probability of a correct decision, followed by assessment of how much positioning of boundary dots affected this probability. Results indicate that correct judgments were possible when partial boundaries were displayed with a sparse sampling of dots. We argue for a process that quickly registers the locations of boundary markers and distills that information into a shape summary that can be used to identify the shape even when only a portion of the boundary is represented.

Spatial and temporal proximity as factors in shape recognition.

Prior research from this laboratory examined minimal stimulus conditions that allow for recognition of objects. Using briefly flashed dots that marked the outer border of objects, it was found that timing differentials within and among successive dot pairs affected recognition, with significant declines being seen by the addition of temporal separations in the millisecond range. These experiments were done with dot pairs that had close spatial proximity, which leaves open the possibility that the effects could be attributed to strictly local neural encoding processes. The present research reports that spatial separation of pair members resulted in declines in recognition that were similar to those produced with close spacing of pair members. Both for close and separated dot pairs, recognition was best when they were displayed with near simultaneity, which likely generated synchronized spikes in the retina. These results provide cognitive evidence in support of proposals that synchronous neural activity is part of the image encoding process. The physiological literature is surveyed and discussed in an effort to delineate the issues, and a tentative model of retinal response to these stimulus conditions is offered.

The integration window for shape cues is a function of ambient illumination.

Minimal discrete shape cues, i.e., dots that marked positions on the outer boundary of namable objects, were divided into two subsets, which were shown very quickly with a variable delay between subsets. Recognition of a given object required integration of the information provided by the two subsets, and previous research had found that recognition declined as the delay between subsets was increased. The present experiment found the decline in recognition to be linear for each of several levels of ambient illumination, dropping rapidly under photopic test conditions, and with the slope being progressively less steep with transition into the scotopic range. The change in the duration of information persistence may be related to the density of information that is provided under various lighting conditions, and a requirement that the information be buffered against noise or "packaged" to accommodate successive saccades.

Recognition of objects displayed with incomplete sets of discrete boundary dots.

Most extant theories of shape perception assume or assert that various contour attributes, and in particular, the orientation, curvature and linear extent of the contours provide essential object recognition cues. The present study examined this proposal using discrete dots that marked locations on the outer boundary of namable objects, providing shape-patterns similar to silhouettes. For each shape, the display initially provided only a sampling of the total number of dots in the boundary, and the number of dots was periodically increased until the participant named the object. There were three treatment conditions in which the initial display as well as the periodic increments consisted of continuous arrays (strings) of dots, randomly positioned dots, or evenly spaced dots. Analysis showed objects were recognized with the fewest percentage of dots with the evenly spaced condition, and participants needed the greatest percentage with the contiguous array condition. In many cases objects could be identified when very few evenly spaced dots were shown, thereby providing large spacing between the dots. It seems unlikely that known neural mechanisms could extract contour attributes, e.g., orientation, curvature, and linear extent, from such sparse stimulus patterns, which provides a challenge to the proposition that these are essential shape cues.

Simultaneity in the millisecond range as a requirement for effective shape recognition.

Neurons of the visual system are capable of firing with millisecond precision, and synchrony of firing may provide a mechanism for "binding" stimulus elements in the image for purposes of recognition. While the neurophysiology is suggestive, there has been relatively little behavioral work to support the proposition that synchrony contributes to object recognition. The present experiments examined this issue by briefly flashing dots that were positioned at the outer boundary of namable objects, similar to silhouettes. Display of a given dot lasted only 0.1 ms, and temporal proximity of dot pairs, and among dot pairs, was varied as subjects were asked to name each object. In Exp 1, where the display of dots pairs was essentially simultaneous (0.2 ms to show both), there was a linear decline in recognition of the shapes as the interval between pairs increased from 0 ms to 6 ms. Compared with performance at 0 ms of delay, even the 2 ms interval between pairs produced a significant decrease in recognition. In Exp 2 the interval between pairs was constant at 3 ms, and the interval between pair members was varied. Here also a linear decline was observed as the interval between pair members increased from 0 ms to 1.5 ms, with the difference between 0 ms and 0.5 ms being significant. Thus minimal transient discrete cues can be integrated for purposes of shape recognition to the extent that they are synchronously displayed, and coincidence in the millisecond and even submillisecond range is needed for effective encoding of image data.

Information persistence evaluated with low-density dot patterns.

After more than a century of study, we do not yet fully understand how shapes and patterns are encoded and identified. Greater progress might result from quantifying stimulus information, thus allowing manipulation of the degree to which a shape or pattern can elicit recognition. The present work used discrete dot patterns that are seen as letters of the alphabet. By adjusting the density of the dots in each pattern, one can determine the probability that it will be recognized. The experiments displayed low-density dot patterns to human respondents, assessing the interval across which non-redundant information provided by two compatible subsets would combine to elicit recognition. This provided a measure of the time required for decay of information persistence. Viewed in the context of prior work, the evidence indicates that the retina mediates initial visibility of the stimulus trace, but the longer-duration persistence required for memory retrieval is mediated by visual cortex.