Distance determined by the angular declination below the horizon.

A biological system is often more efficient when it takes advantage of the regularities in its environment. Like other terrestrial creatures, our spatial sense relies on the regularities associated with the ground surface. A simple, but important, ecological fact is that the field of view of the ground surface extends upwards from near (feet) to infinity (horizon). It forms the basis of a trigonometric relationship wherein the further an object on the ground is, the higher in the field of view it looks, with an object at infinity being seen at the horizon. Here, we provide support for the hypothesis that the visual system uses the angular declination below the horizon for distance judgement. Using a visually directed action task, we found that when the angular declination was increased by binocularly viewing through base-up prisms, the observer underestimated distance. After adapting to the same prisms, however, the observer overestimated distance on prism removal. Most significantly, we show that the distance overestimation as an after-effect of prism adaptation was due to a lowered perceived eye level, which reduced the object's angular declination below the horizon.

Improving visually guided action and perception through use of prisms.

BACKGROUND: Severe loss of vision can occur as a result of insults to the visual system. Depending on the level of insult, the patient can experience symptoms ranging from simple loss of visual acuity to more-complex visual motor and perceptual disorders, like visual-field loss and accompanying spatial disorientation, all of which can result in significant restrictions to the patient's mobility. This report describes the therapeutic use of ophthalmic prisms to treat patients with distinct visual impairments. CASE REPORTS: The management of four patients, each with visual impairment of unrelated origins (age-related macular degeneration, brain injury, Friedreich's ataxia, stroke) are discussed, with special emphasis on the positive impact of prismatic prescriptions. With each case, we also discuss our reasoning for prescribing the particular prisms and their possible roles. CONCLUSIONS: Our clinical reports reinforce the notion that prisms can be effectively used in visual rehabilitation. The modulatory role of prisms is considered in terms of the neurology of the brain and its functions.

Sensory eye dominance.

BACKGROUND: Sensory eye dominance is revealed in tasks like the Red Lens test and binocular rivalry. To understand its neural basis, we used a new protocol based on binocular rivalry to quantify its consequent interocular imbalance. Then we investigated whether the extent or sign of interocular imbalance is correlated with the difference in monocular contrast responses at threshold and suprathreshold and with the observer's motor eye dominance. METHODS: To evaluate sensory eye dominance, the stimulus intensity in each eye during rivalry was adjusted to achieve equal predominance. The difference in stimulus intensity constitutes the interocular imbalance. Standard procedures were used to measure monocular spatial contrast sensitivity, suprathreshold brightness judgment, and motor eye dominance. RESULTS: There was no positive correlation between interocular imbalance (sensory eye dominance) and motor eye dominance. No systematic correlation was found between interocular imbalance and monocular contrast sensitivities at 1 and 3 cycles/degree. Correlation coefficient between interocular imbalance and monocular suprathreshold brightness judgment was close to significant, suggesting that a difference in monocular brightness percept might (in part) account for interocular imbalance. But this explanation is only partial, since the difference in the monocular brightness percept was too small to account for the interocular imbalance. CONCLUSIONS: Interocular imbalance is a sensory eye dominance that cannot be equated with motor eye dominance. It manifests largely as a binocular phenomenon, which bears little relationship with the monocular neural mechanisms of contrast detection and brightness perception.

Perceiving binocular depth with reference to a common surface.

A common surface is a spatial regularity of our terrestrial environment. For instance, we walk on the common ground surface, lay a variety of objects on the table top, and display our favorite paintings on the wall. It has been proposed that the visual system utilizes this regularity as a reference frame for coding objects' distances. Presumably, by treating the common surface as such--i.e. an anticipated constant--the visual system can reduce its coding redundancy, and divert its resources to representing other information. For intermediate-distance space perception, it has been found that absolute distance judgment is most accurate when a common ground surface is available. Here we explored if the common surface also serves as the reference frame for the processing of binocular-disparity information, which is a predominant cue for near-distance space perception. We capitalized on an established observation where the perceived slant of a surface with linear binocular-disparity gradient is underestimated. Clearly, if the visual system utilizes this incorrectly represented slant surface as a reference frame for coding the objects' locations, the perceived depth separation between the objects will be adversely affected. Our results confirm this, by showing that the depth judgment of objects (two laterally separated vertical lines) on, or in the vicinity of, the surface is underestimated. Furthermore, we show that the impact of the common surface on perceived depth separation most likely occurs at the surface-representation level where the visual surface has been explicitly delineated, rather than at the earlier disparity-processing level.

Deaths following acute diarrhoeal diseases among hospitalised infants in Kuala Lumpur.

The risk factors and modes of death following acute diarrhoeal illness in children admitted to University Hospital, Kuala Lumpur between 1982 and 1997 were studied retrospectively. Among 4,689 cases of acute gastroenteritis admitted, ten deaths were noted. The case mortality rate was 2.1/1000 admissions. All deaths were infants below one year, with eight females and two males. Acute renal failure and acute pulmonary oedema were common preceding events. Female sex, infants less than twelve months, the presence of hyper or hyponatraemia and moderate to severe dehydration on admission were risk factors for deaths.

Binocular rivalry and visual awareness: the role of attention.

When the right eye and the left eye view dissimilar scenes, the observer does not experience a stable superimposed percept of the images presented to the two eyes, but instead perceives an alternation between the images seen by each eye. A critical question confronting this robust and intriguing phenomenon of binocular rivalry is how the visual system selects the image to be perceived (dominant). The current main-stream literature emphasizes a bottom-up explanation in which the rivalry stimulus with the higher contour strength has the advantage, and becomes dominant in rivalry. Nevertheless, some workers in the past have favored an attention-selection explanation for binocular rivalry. We investigated the role of attention in binocular rivalry by employing novel psychophysical paradigms which capitalized on several established phenomena (e.g. the Cheshire Cat effect, attention cueing, pop-out effect). Our results revealed two major aspects of attention modulation in binocular rivalry. We found that a dominant image is less likely to be suppressed when voluntary attention is directed to it. This suggests the role of voluntary attention in retaining the dominant image in visual awareness. Second, a rivalry stimulus is more likely to become dominant if accompanied by a pop-out cue (in the same eye and proximity). Since a pop-out cue attracts involuntary attention to its location/eye, this result suggests that cue-mediated involuntary attention can promote the ability of a rivalry stimulus to reach visual awareness.

Perceptual organization of apparent motion in the Ternus display.

A typical Ternus display has three sequentially presented frames, in which frame 1 consists of three motion tokens, frame 2 (blank) defines the interstimulus interval, and frame 3 has similar motion tokens with their relative positions shifted to the right. Interestingly, what appears to be a seemingly simple arrangement of stimuli can induce one of two distinct apparent-motion percepts in the observer. The first is an element-motion perception where the left-end token is seen to jump over its two neighboring tokens (inner tokens) to the right end of the display. The second is a group-motion perception where the entire display of the three tokens is seen to move to the right. How does the visual system choose between these two apparent-motion perceptions? It is hypothesized that the choice of motion perception is determined in part by the perceptual organization of the motion tokens. Specifically, a group-motion perception is experienced when a strong grouping tendency exists among the motion tokens belonging to the same frame. Conversely, an element-motion perception is experienced when a strong grouping tendency exists between the inner motion tokens in frames 1 and 3 (i.e. the two tokens that overlap in space between frames). We tested this hypothesis by varying the perceptual organization of the motion tokens. Both spatial (form similarity, 3-D proximity, common surface/common region, and occlusion) and temporal (motion priming) factors of perceptual organization were tested. We found that the apparent-motion perception of the Ternus display can be predictably affected, in a manner consistent with the perceptual organization hypothesis.

Illusory-contour formation affected by luminance contrast polarity.

We report a new type of illusory contour (Illusory-O) whose formation is contingent upon the contrast polarity of its juxtaposed inducing elements being similar, i.e. both elements must either be positive or negative in contrast sign. To test the hypothesis that this contingency is primarily dictated by factors that determine amodal surface completion (occlusion) between the inducing elements we conducted a series of experiments employing known spatial properties of the amodal completion mechanism, to show that spatial conditions unfavorable to occlusion lead to a concurrent weakening of the Illusory-O formation. For instance, we found that when the juxtaposed inducing elements (solid rectangles) were spatially misaligned, or when their spatial separation increased, our observers rated the perception of the Illusory-O as reduced. We also showed that, in addition to using solid-form inducing elements, the Illusory-O can be induced by line terminals, as long as these lines respect the requirements of the amodal completion mechanism such as similar contrast polarity and spatial alignment. Then we demonstrated that the role of the amodal completion mechanism is not limited to our particular arrangement of inducing elements by showing that the formation of the illusory Necker cube also relies on similar contrast polarity. Finally, to explain why some illusory contours like the Illusory-O are dependent on contrast polarity while others (e.g. Kanizsa square) are not, we propose that the key rests upon the visual system's presumption of occlusion. That is, in forming the illusory contour, if the visual system infers that it is a byproduct of the inducing elements being occluded, then having inducing elements of similar contrast polarity becomes a prerequisite. This assumption can be traced to the occurrence in the real world where partially occluded objects usually have visible parts (on both ends) with similar contrast polarity. Along this line of thinking, we suggest a plausible neural circuitry that may be implemented to form both contrast polarity sensitive and insensitive types of illusory contours.

Terrain influences the accurate judgement of distance.

Mathematically, three-dimensional space can be represented differently by the cartesian, polar, and other coordinate systems. However, in physical sciences, the choice of representation system is restricted by the need to simplify a machine's computation while enhancing its efficiency. Does the brain, for the same reasons, 'select' the most cost-efficient way to represent the three-dimensional location of objects? As we frequently interact with objects on the common ground surface, it might be beneficial for the visual system to code an object's location using a ground-surface-based reference frame. More precisely, the brain could use a quasi-two-dimensional coordinate system (x(s), y(s)) with respect to the ground surface (s), rather than a strictly three-dimensional coordinate system (x, y, z), thus reducing coding redundancy and simplifying computations. Here we provide support for this view by studying human psychophysical performance in perceiving absolute distance and in visually directed action tasks. For example, when an object was seen on a continuous, homogeneous texture ground surface, the observer judged the distance to the object accurately. However, when similar surface information was unavailable, for example, when the object was seen across a gap in the ground, or across distinct texture regions, distance judgement was impaired.

Visual suppression and its effect upon color and luminance sensitivity.

Psychophysical increment thresholds were compared for periods of phenomenological dominance or suppression produced by different stimulation of the two eyes. Three experimental procedures were used; binocular rivalry, permanent suppression and flash suppression. The amount of suppression produced by each procedure was evaluated under conditions intended to accentuate color or luminance system contribution to the detection of a spectral flash. All three procedures resulted in a different pattern of color and luminance suppression. Binocular rivalry suppressed color sensitivity more than luminance and within color, blue (439 nm) sensitivity was more suppressed than red (613 nm). Permanent suppression resulted in a similar pattern of suppression but only blue color sensitivity was reliably more suppressed than luminance sensitivity. Flash suppression produced distinctly different results such that blue color sensitivity was reliably less suppressed than luminance or red color sensitivity, which were not different from each other. Taken together these results provide clues as to where and when the physiological processes mediating visual suppression may be found in the nervous system.