Motion Minima for Different Directions in Color Space

AbstractWe have used the minimum-motion stimulus of Cavanagh, MacLeod and Anstis (1987) [Journal of the Optical Society of America A, 4, 1428–1438] to examine how signals along different directions in color space interact in motion perception. Stimuli were pairs of counterphasing gratings combined 90 deg out of phase in both space and time and modulated along different color-luminance axes. The axis for one of the gratings was fixed, while the axis for the second was varied so as to null perceived motion in the stimulus. The motion nulls show that observers are sensitive to motion signals carried by each of the cardinal directions of color space [an achromatic axis and L-M and S-(L + M) chromatic axes], but that signals along different cardinal axes are not combined to yield a net direction of motion. Pairing an achromatic and chromatic grating resulted in a motion null regardless of the relative or overall contrast of the two gratings, while the null directions for intermediate axes shifted depending on contrast. This result points to the special status of the luminance and chromatic axes. However, our results do not reveal a special pair of axes within the equiluminant plane. When contrasts along the cardinal axes are scaled for equal multiples of their respective detection thresholds, the L-M and S chromatic contrasts contribute roughly equally to the perceived motion, but are many times weaker than luminance contrast. Moreover, sensitivity to luminance motion is little affected by the presence of chromatic contrast, whereas sensitivity to chromatic motion is strongly masked by either luminance or chromatic contrast. These asymmetric interactions suggest that the motion of the luminance and chromatic components is encoded in qualitatively different ways. © 1997 Elsevier Science Ltd

Inferring the neural representation of faces from adaptation aftereffects

The aftereffects of adaptation to faces have been studied widely, in part to characterize the coding schemes for representing different facial attributes. Often these aftereffects have been interpreted in terms of two alternative models of face processing: 1) a norm-based or opponent code, in which the facial dimension is represented by relative activity in a pair of broadly-tuned mechanisms with opposing sensitivities; or 2) an exemplar code, in which the dimension is sampled by multiple channels narrowly-tuned to different levels of the stimulus. Evidence for or against these alternatives is based on the different patterns of aftereffects they predict (e.g. whether there is adaptation to the norm, and how adaptation increases with stimulus strength). However, these models make many and often implicit assumptions about the channels themselves and how they are combined. We re-evaluated these models to explore how their output depends on factors such as the number, selectivity, and decoding strategy of the channels to clarify the fundamental differences between these coding schemes and the adaptation effects most diagnostic for discriminating between them. We show that the distinction between norm and exemplar codes has less to do with the number of channels and more on how the channel outputs are decoded to represent the stimulus. We also compare how these models depend on assumptions about the stimulus (e.g. broadband vs. punctate) and the impact of noise. These analyses point to the fundamental distinctions between different coding strategies and the patterns of visual aftereffects that are best for revealing them

Response normalization and blur adaptation:data and multi-scale model

Adapting to blurred or sharpened images alters perceived blur of a focused image (M. A. Webster, M. A. Georgeson, & S. M. Webster, 2002). We asked whether blur adaptation results in (a) renormalization of perceived focus or (b) a repulsion aftereffect. Images were checkerboards or 2-D Gaussian noise, whose amplitude spectra had (log-log) slopes from -2 (strongly blurred) to 0 (strongly sharpened). Observers adjusted the spectral slope of a comparison image to match different test slopes after adaptation to blurred or sharpened images. Results did not show repulsion effects but were consistent with some renormalization. Test blur levels at and near a blurred or sharpened adaptation level were matched by more focused slopes (closer to 1/f) but with little or no change in appearance after adaptation to focused (1/f) images. A model of contrast adaptation and blur coding by multiple-scale spatial filters predicts these blur aftereffects and those of Webster et al. (2002). A key proposal is that observers are pre-adapted to natural spectra, and blurred or sharpened spectra induce changes in the state of adaptation. The model illustrates how norms might be encoded and recalibrated in the visual system even when they are represented only implicitly by the distribution of responses across multiple channels

Self‐guided clinical cases for medical students based on postmortem CT scans of cadavers

In the summer of 2009, we began full body computed tomography (CT) scanning of the pre‐embalmed cadavers in the University of Michigan Medical School (UMMS) dissection lab. We theorized that implementing web‐based, self‐guided clinical cases based on postmortem CT (PMCT) scans would result in increased student appreciation for the clinical relevance of anatomy, increased knowledge of cross‐sectional anatomy, and increased ability to identify common pathologies on CT scans. The PMCT scan of each cadaver was produced as a DICOM dataset, and then converted into a Quicktime movie file using Osirix software. Clinical cases were researched and written by the authors, and consist of at least one Quicktime movie of a PMCT scan surrounded by a novel navigation interface. To assess the value of these clinical cases we surveyed medical students at UMMS who are currently using the clinical cases in their coursework. Students felt the clinical cases increased the clinical relevance of anatomy (mean response 7.77/10), increased their confidence finding anatomical structures on CT (7.00/10), and increased their confidence recognizing common pathologies on CT (6.17/10). Students also felt these clinical cases helped them synthesize material from numerous courses into an overall picture of a given disease process (7.01/10). These results support the conclusion that our clinical cases help to show students why the anatomy they are learning is foundational to their other coursework. We would recommend the use of similar clinical cases to any medical school utilizing cadaver dissection as a primary teaching method in anatomy education. Clin. Anat. 24:655–663, 2011. © 2011 Wiley‐Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87056/1/21143_ftp.pd

Tracking the Orbital and Super-orbital Periods of SMC X-1

The High Mass X-ray Binary (HMXB) SMC X-1 demonstrates an orbital variation of 3.89 days and a super-orbital variation with an average length of 55 days. As we show here, however, the length of the super-orbital cycle varies by almost a factor of two, even across adjacent cycles. To study both the orbital and super-orbital variation we utilize lightcurves from the Rossi X-ray Timing Explorer All Sky Monitor (RXTE-ASM). We employ the orbital ephemeris from Wojdowski et al. (1998) to obtain the average orbital profile, and we show that this profile exhibits complex modulation during non-eclipse phases. Additionally, a very interesting ``bounceback'' in X-ray count rate is seen during mid-orbital eclipse phases, with a softening of the emission during these periods. This bounceback has not been previously identified in pointed observations. We then define a super-orbital ephemeris (the phase of the super-orbital cycle as a function of date) based on the ASM lightcurve and analyze the trend and distribution of super-orbital cycle lengths. SMC X-1 exhibits a bimodal distribution of these lengths, similar to what has been observed in other systems (e.g., Her X-1), but with more dramatic changes in cycle length. There is some hint, but not conclusive evidence, for a dependence of the super-orbital cycle length upon the underlying orbital period, as has been observed previously for Her X-1 and Cyg X-2. Using our super-orbital ephemeris we are also able to create an average super-orbital profile over the 71 observed cycles, for which we witness overall hardening of the spectrum during low count rate times. We combine the orbital and super-orbital ephemerides to study the correlation between the orbital and super-orbital variations in the system.Comment: 10 pages, using emulateapj style. To be published in the Astrophysical Journa

Prenatal Exposure to Tetrachloroethylene-Contaminated Drinking Water and the Risk of Congenital Anomalies: A Retrospective Cohort Study

BACKGROUND: Prior animal and human studies of prenatal exposure to solvents including tetrachloroethylene (PCE) have shown increases in the risk of certain congenital anomalies among exposed offspring. OBJECTIVES: This retrospective cohort study examined whether PCE contamination of public drinking water supplies in Massachusetts influenced the occurrence of congenital anomalies among children whose mothers were exposed around the time of conception. METHODS: The study included 1,658 children whose mothers were exposed to PCE-contaminated drinking water and a comparable group of 2,999 children of unexposed mothers. Mothers completed a self-administered questionnaire to gather information on all of their prior births, including the presence of anomalies, residential histories and confounding variables. PCE exposure was estimated using EPANET water distribution system modeling software that incorporated a fate and transport model. RESULTS: Children whose mothers had high exposure levels around the time of conception had an increased risk of congenital anomalies. The adjusted odds ratio of all anomalies combined among children with prenatal exposure in the uppermost quartile was 1.5 (95% CI: 0.9, 2.5). No meaningful increases in the risk were seen for lower exposure levels. Increases were also observed in the risk of neural tube defects (OR: 3.5, 95% CI: 0.8, 14.0) and oral clefts (OR 3.2, 95% CI: 0.7, 15.0) among offspring with any prenatal exposure. CONCLUSION: The results of this study suggest that the risk of certain congenital anomalies is increased among the offspring of women who were exposed to PCE-contaminated drinking water around the time of conception. Because these results are limited by the small number of children with congenital anomalies that were based on maternal reports, a follow-up investigation should be conducted with a larger number of affected children who are identified by independent records.National Institute of Environmental Health (5 P42 ES007381); National Institutes of Healt

Social recognition and social attraction in group-living fishes

This work was supported by a grant from the Australian Research Council, Discovery Project DP190100660, focusing on the social behavior of animals.Social aggregation is a widespread and important phenomenon among fishes. Understanding the questions of why and how aggregations form and are subsequently maintained is a central goal for behavioral ecologists. Research in this field has shown that aggregations are typically structured, non-random associations. This indicates that fish are able to differentiate between potential group-mates and that this ability mediates their association preferences, and, ultimately, the composition of their groups. In this review, we examine the characteristics that influence the expression of social attraction among fishes, before going on to describe the recognition mechanisms that underpin social attraction. Finally, we highlight a number of outstanding questions in the field with a view to generating a more complete understanding of social aggregation in fishes.Publisher PDFPeer reviewe

Interactions between chromatic adaptation and contrast adaptation in color appearance

AbstractColor appearance depends on adaptation processes that adjust sensitivity both to the average color in the stimulus (through light or chromatic adaptation) and to the variations in color (through contrast adaptation). We explored how these different forms of adaptation interact, by examining how the state of chromatic adaptation depends on the time-varying color contrasts in the stimulus, and conversely, how adaptation to the mean determines the stimulus contrasts underlying contrast adaptation. Light adaptation levels remain very similar whether observers adapt to a static chromaticity or to large temporal modulations in cone excitation that vary at rates of 0.5 Hz or higher. This suggests that up to the sites of light adaptation, the response to moderate contrasts is effectively linear and that the adaptation effectively averages over several seconds of the stimulus. For slower flicker rates color is differentially biased by the last half-cycle of the flicker, and perceived contrast may be altered by response polarization. This polarization selectively saturates responses to moderate (but not low) contrasts along the color direction complementary to the mean color bias, implying that the response changes occur within multiple mechanisms tuned to different chromatic axes. Chromatic adaptation often adjusts only partially to the mean color of the stimulus, and thus leaves a residual bias in the color appearance of the field. Contrast adaptation reduces perceived contrast relative to this residual color, and not relative to the stimulus that appears achromatic. Similarly, contrast discrimination thresholds appear lower around the residual color than around the achromatic point. Thus under biased states of chromatic adaptation alternative measures of ‘zero contrast’ can be dissociated, suggesting that they do not depend on a common null point within the channels encoding chromatic contrast

Ecological and Social Factors Constrain Spatial and Temporal Opportunities for Mating in a Migratory Songbird

Many studies of sexual selection assume that individuals have equal mating opportunities and that differences in mating success result from variation in sexual traits. However, the inability of sexual traits to explain variation in male mating success suggests that other factors moderate the strength of sexual selection. Extrapair paternity is common in vertebrates and can contribute to variation in mating success and thus serves as a model for understanding the operation of sexual selection. We developed a spatially explicit, multifactor model of all possible female-male pairings to test the hypothesis that ecological (food availability) and social (breeding density, breeding distance, and the social mate's nest stage) factors influence an individual's opportunity for extrapair paternity in a socially monogamous bird, the black-throated blue warbler, Setophaga caerulescens. A male's probability of siring extrapair young decreased with increasing distance to females, breeding density, and food availability. Males on food-poor territories were more likely to sire extrapair young, and these offspring were produced farther from the male's territory relative to males on food-abundant territories. Moreover, males sired extrapair young mostly during their social mates' incubation stage, especially males on food-abundant territories. This study demonstrates how ecological and social conditions constrain the spatial and temporal opportunities for extrapair paternity that affect variation in mating success and the strength of sexual selection in socially monogamous species