Perceptual assumptions and projective distortions in a three-dimensional shape illusion

We examine a shape illusion, in which the balconies of a building appear to tilt up or down, depending on the viewpoint. The balconies are actually level parallelogram shapes, but appear as tilted rectangles. We measured the illusory tilts observed when parallelogram shapes are viewed above the line of sight, using three-dimensional stimuli consisting of parallelograms of various tilts viewed at different orientations. Under perspective projection, parallelism and orthogonality are not preserved. However, perspective distortions alone cannot account for the perceived tilts measured in these experiments, since observers perceived illusory tilts even for stimuli in the frontoparallel plane. We introduce a model, based on the theory that observers assume ambiguously projected three-dimensional angles to be equal to 90 degrees, but revise their predictions on the basis of observation. In the model, perceived tilt is predicted as a weighted sum of the tilts predicted by the assumptions that the shape is rectangular, and that the shape is level (i.e. that the angle between the shape and the vertical backboard is equal to 90 degrees). We prove that it is mathematically impossible for a planar rectangle to share a projection with a nonrectangular parallelogram. A less restrictive assumption that just the two leading internal angles are equal to 90 degrees is suggested as an alternative, and it is further proven that this new configuration of angles leads to a unique perceived tilt. The relative weights in the model reflect the amount that each prediction is revised, and are shown to vary systematically with stimulus orientation. For some observers a better fit was found by replacing the level-tilt assumptions with an assumption that physical tilt was equal to the projected tilt.     

Searching in the center: pigeons (Columba livid) encode relative distance from walls of an enclosure

Pigeons (Columba livia) searched for food hidden in the center of a square enclosure. On occasional tests without food, the enclosure was (a) unchanged from training (control tests), (b) moved to different corners of the testing room (corner tests), or (c) doubled in size (expansion tests). The birds showed localized search in the center of the enclosure on control and corner tests. On expansion tests, some birds searched near the center of the enclosure, suggesting relative-distance encoding. Other birds searched at locations that maintained the training distance from walls, suggesting absolute-distance encoding. These results are consistent with previous studies on chicks (Gallus gallus) in similar enclosures and contrast with previous results on pigeons' responses to expansions of discrete landmark arrays.     

Visual identification and spatial location in Alzheimer's disease

Twelve patients with Alzheimer's disease (AD) and 15 healthy elderly control subjects were shown sets of luminance-defined letters, texture-defined letters, luminance-defined squares, and texture-defined squares. They were asked to name the letters or point to the target square on each page. The stimuli were graded into four levels of difficulty based on the amount of contrast between the figure and the background. Performance was measured in terms of the maximum level of difficulty at which the participant correctly identified or located the three figures. Contrary to expectations, no significant difference was found between the performance of AD patients and control subjects on texture discrimination tasks vs. luminance discrimination tasks. However, results indicate that AD patients are impaired in performing a task requiring them to locate a texture-defined target of known shape in a noisy background field. By contrast, AD patients show no significant deficit in a task requiring them to locate a texture-defined shape in a known location. This argues that the observed deficit in the location task is not due to a failure in the system that discriminates target texture from background texture (since both location and identification tasks require the same textural discriminations), but rather to an impairment of the system responsible for "finding things" (i.e., locating known targets at unknown locations). This observation suggests that AD patients may suffer selective damage to the dorsal "Where" pathway, which is responsible for localizing objects in space.     

Circadian phase-shifted rats show normal acquisition but impaired long-term retention of place information in the water task

It is thought that circadian rhythms may influence learning and memory processes. However, research supporting this view does not dissociate a mnemonic impairment from other performance deficits. Furthermore, published reports do not specify the type of memory system influenced by the circadian system. The present study assessed the effects of phase shifting on acquisition and expression of place navigation in the water maze, a task sensitive to hippocampal dysfunction. The results showed that phase-shifting circadian rhythms in rats impaired the expression of place information on a retention test but not initial acquisition or encoding of place information. These results suggest that disruption of circadian rhythms may impair consolidation of previously encoded hippocampal place information.     

The depth and selectivity of suppression in binocular rivalry

Binocular rivalry occurs when the two eyes are presented with incompatible stimuli and the perceived image alternates between the two stimuli. The aim of this study was to find out whether the periodic perceptual loss of a monocular stimulus during binocular rivalry is mirrored by a comparable loss of contrast sensitivity. We presented brief test stimuli to one eye while its conditioning stimulus was dominant or suppressed. The test stimuli were varied widely across four stimulus domains--namely, the relative stimulation of medium- and long-wavelength-sensitive cones, duration, spatial frequency, and grating orientation. The result in each case was the same. Suppression depended slightly or not at all on the type of test stimulus, and contrast sensitivity during suppression was around 64% of that during dominance. The effect of suppression on sensitivity is therefore very weak, relative to its effect on the perceived image. Furthermore, suppression was largely independent of the similarity between the conditioning and the test stimuli, indicating that our results are better explained by eye suppression than by stimulus suppression. A model is presented to account for the small, monocular sensitivity loss during suppression: It assumes that test detection precedes conditioning stimulus perception in the visual pathway.     

Images of God and the imaginary in the face of loss: a quantitative research on Vietnamese immigrants living in Canada

ABSTRACT

This study examines the relationships between images of God, the imaginary and experiences of loss among Vietnamese immigrants living in Canada. One hundred twenty-nine participants completed four sets of questionnaires: a demographic questionnaire, a Questionnaire on God Image, the Brief Symptom Inventory, the Davidson Trauma Scale. In the second phase, 32 participants were administered the AT.9 test (an archetypal test with nine elements). In this test, participants are asked to draw, using nine symbolic elements, and then write a story based on their drawing. Results showed that a positive perception of God negatively correlated with somatisation, anxiety and hostility, whereas negative God image was significantly related to high levels of emotional distress. Results also showed that the synthetic categories of the imaginary were associated with a positive perception of God; and were negatively associated with somatisation, intrusion and hyper-vigilance. Further discussions on the main findings will address cultural implications in the fields of Mental Health, Religion and Culture.