Material-illumination ambiguities and the perception of solid objects

The appearance of objects depends on their material, shape, and on the illumination conditions. Conversely, object appearance provides us with cues about the illumination and the material. This so-called inverse problem is basically underdetermined and therefore we expect that material and illumination perception are confounded. To gain insight into the relevant mechanisms, we rendered a set of artificial spheres for vastly different canonical light fields and reflectance functions. We used four physics-based bidirectional reflectance distribution functions (BRDFs) representing glossy, pitted, velvety, and matte material. The six illumination conditions were collimated illumination from four directions, hemispherical diffuse illumination, and fully diffuse (Ganzfeld) illumination. In three sub-experiments we presented pairs of stimuli and asked human observers to judge whether the material was the same, whether the illumination was the same, and for a subset in which either the illumination or the material was the same to judge which of the two was constant. We found that observers made many errors in all sub-experiments. In experiment 2 the illumination direction was chosen at random. Using an interactive interface, we asked human observers to match the illumination direction of a sphere of one of the four materials with that of a Lambertian sphere. We found systematical material-dependent deviations from veridical performance. Theoretical analysis of the radiance patterns suggests that judgments were based mainly on the position of the shadow edge. In conclusion, we found no evidence for 'material constancy' for perception of smooth rendered spheres despite vast quantitative and qualitative differences in illumination and in BRDF between the stimuli. Although human observers demonstrated some 'illumination constancy', they made systematic errors depending on the material reflectance, suggesting that they used mainly the location of the shadow edge. Our results suggest that material perception and light-field perception are basically confounded.     

Detection of the sign of expansion as a function of field size and eccentricity

Small optical flow fields are involved in object and shape recognition. These tasks depend on high resolution and low speed of the flow-field elements. Large optical flow fields are involved in tasks such as orientation and navigation, which require flow-field elements to move at high speed. From the above, we would expect to find different behavior of subjects for different parameter ranges of field size and speed of the elements. In this paper, we address the question of whether such different behavior exists for a task that is associated neither with object recognition nor with navigation. We obtained detection thresholds for expansion in the presence of translation for a wide range of field sizes. The same paradigm was used to investigate whether subjects made efficient use of peripheral information. We found viewing-distance invariance, meaning that subjects’ performance scaled with stimulus size. Subjects performed very similarly with and without foveal information.     

Visual discrimination of spectral distributions

Human observers seem to be able to use different features that classify materials with a large degree of accuracy. In this paper, we look at human perception of statistical properties of the spectral distribution in a scene. We investigated whether human observers can discriminate just as accurately between coloured textures that have a spectral distribution due either to shading only or to both shading and specular reflectance as between uniform colours. Thresholds for the discrimination of coloured textures are about 15 times as high as thresholds for the discrimination of uniform colours, provided there is a sharp transition between the two colours. However, the coloured texture thresholds are only 1.5 times higher when we introduce a gradual transition between the two colours. There are also distinct qualitative differences in discrimination thresholds for different base colours. These differences cannot be predicted from discrimination thresholds for uniform colours. Human observers are surprisingly good at discriminating between a material edge and a shadow edge in complex scenes. Statistical differences in the orientation of the colour distributions in colour space might be used to accomplish this. In a second experiment we investigated how well observers can discriminate between two linear distributions in colour space that have the same base colour but different orientations. When we vary the line-length in R, G, B space, thresholds cannot be predicted completely by the conservation of the average distance between the two distributions. This means that observers use not only the maximum colour difference in the stimulus to do the task, but other cues are also involved.     

Detection of spatial discontinuities in first-order optical flow fields

We investigated the extent to which the human visual system can detect discontinuities in firstorder optical flow fields. We constructed two types of spatial discontinuities: a circular split field with a straight edge and a disk with annular surround. We used two different first-order optical flow components: an expansion and a rotation. We found an intriguing difference in the detection thresholds for straight and circular discontinuities. Whereas straight discontinuities yielded thresholds of 10%— 50% difference in expansion or rotation, circular discontinuities could, at first, only be detected at extreme differences (>>100%). After a learning period, thresholds for such stimuli decreased, but they remained significantly higher than thresholds for the straight edge. Thresholds rose for stimuli that formed a gradual transition between a circular and a straight edge, and they decreased with increasing eccentricity of the circular discontinuity. Results suggest that symmetry in the stimulus, defined by the coincidence of the center of expansion or rotation and the center of the circular discontinuity, was responsible for the difference in thresholds for circular and straight discontinuities.     

Does monocular visual space contain planes?

The issue of the existence of planes—understood as the carriers of a nexus of straight lines—in the monocular visual space of a stationary human observer has never been addressed. The most recent empirical data apply to binocular visual space and date from the 1960s (Foley, 1964). This appears to be both the first and the last time this basic issue was addressed empirically. Yet the question is of considerable conceptual interest. Here we report on a direct empirical test of the existence of planes in monocular visual space for a group of sixteen experienced observers. For the majority of these observers monocular visual space lacks a projective structure, albeit in qualitatively different ways. This greatly reduces the set of viable geometrical models. For example, it rules out all the classical homogeneous spaces (the Cayley-Klein geometries) such as the familiar Luneburg model. The qualitatively different behavior of experienced observers implies that the generic population might well be inhomogeneous with respect to the structure of visual space.     

Teacher- and school-level predictors of teacher efficacy and burnout: identifying potential areas for support

Although several studies relate low teacher efficacy and high burnout to the quality of instruction and students' academic achievement, there has been limited research examining factors that predict teacher efficacy and burnout. The current study employed a longitudinal, multilevel modeling approach to examine the influence of teacher- and school-level factors on the development of both teacher efficacy and burnout. Data were collected 3 times across 2 academic years from 600 teachers at 31 elementary schools. The results indicated that both teacher efficacy and burnout increased over time. Teacher preparedness and perceptions of teacher affiliation and leadership were significantly associated with both the intercept and growth of teacher efficacy and burnout; however, school-level factors were generally unrelated to both outcomes. Implications for screenings and teacher-targeted interventions are discussed.     

Effort responses to suboptimal reward cues are related to striatal dopaminergic functioning

Reward cues have been found to increase the investment of effort in tasks even when cues are presented suboptimally (i.e. very briefly), making them hard to consciously detect. Such effort responses to suboptimal reward cues are assumed to rely mainly on the mesolimbic dopamine system, including the ventral striatum. To provide further support for this assumption, we performed two studies investigating whether these effort responses vary with individual differences in markers of striatal dopaminergic functioning. Study 1 investigated the relation between physical effort responses and resting state eye-blink rate. Study 2 examined cognitive effort responses in relation to individually averaged error-related negativity. In both studies effort responses correlated with the markers only for suboptimal, but not for optimal reward cues. These findings provide further support for the idea that effort responses to suboptimal reward cues are mainly linked to the mesolimbic dopamine system, while responses to optimal reward cues also depend on higher-level cortical functions.     

Organizing the voice questionnaire for transgender persons

Abstract

Background: The validated Transsexual Voice Questionnaire Male to Female (TVQ^MtF) and the adapted TVQ Female to Male (FtM) (TVQ^FtM) are both 30-item-long questionnaires used to evaluate self-perception of voice in transgender persons. They are part of a series of questionnaires completed by transgender persons during follow-up of cross-sex hormone therapy (CSHT).

Aim: The aim of this study was to examine if these questionnaires can be organized.

Methods: The TVQ^MtF or the TVQ^FtM was filled out at the start of CSHT, by 145 trans women and 83 trans men. Data were analyzed by factor analyses on both the questionnaires.

Results: The factor analyses resulted in a three-factor solution for both the TVQ^MtF and the TVQ^FtM. The three factors were labeled as anxiety and avoidance, vocal identity, and vocal function.

Conclusion: Both the questionnaires can be organized into three factors. This could contribute to making shorter versions of the questionnaires. Shorter versions would be useful to hormone prescribing physicians to track down more quickly voice problems in trans people undergoing virilizing or feminizing hormone treatment.