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PubMed1985

Shock controllability and opioid substrates of escape performance and nociception: differential effects of peripherally and centrally acting naltrexone.

Rats exposed to inescapable shock exhibited analgesia and a significant impairment of shock-escape learning in a shuttle box situation 24 hr later. In contrast, rats exposed to escapable shock or to no shock displayed neither effect. Naltrexone (10 mg/kg) significantly reduced the analgesia and completely eliminated the escape deficit in inescapably shocked rats but induced hyperalgesia, coupled with a marked deterioration of escape performance, in escapably shocked and nonshocked rats. The same dose of quaternary naltrexone, which has low ability to cross the blood-brain barrier, had no effect on either the antinociception or the escape deficit produced by inescapable shock, although it also induced escape impairment and hyperalgesia in rats preexposed to escapable shock or to no shock. A second experiment demonstrated that both the escape interference and the antinociceptive consequences of prior inescapable shock could be reduced partially by a much lower dose (1 mg/kg) of naltrexone but 50 times this amount of quaternary naltrexone was still without effect. These results imply that the consequences of exposure to inescapable shock are mediated by activation of central opioid processes whereas naltrexone-induced effects in escapably shocked and nonshocked animals may be peripherally mediated. The relevance of these findings to the possible role of nociception in escape performance is discussed.

Animals↗

PubMed1991

CONSPEC and CONLERN: a two-process theory of infant face recognition.

Evidence from newborns leads to the conclusion that infants are born with some information about the structure of faces. This structural information, termed CONSPEC, guides the preference for facelike patterns found in newborn infants. CONSPEC is contrasted with a device termed CONLERN, which is responsible for learning about the visual characteristics of conspecifics. In the human infant, CONLERN does not influence looking behavior until 2 months of age. The distinction between these 2 independent mechanisms allows a reconciliation of the conflicting data on the development of face recognition in human infants. Finally, evidence from another species, the domestic chick, for which a similar 2-process theory has already been put forward, is discussed. The new nomenclature is applied to the chick and used as a basis for comparison with the infant.

Arousal↗

PubMed2004

Aging and the perception of depth and 3-D shape from motion parallax.

The ability of younger and older observers to perceive 3-D shape and depth from motion parallax was investigated. In Experiment 1, the observers discriminated among differently curved 3-dimensional (3-D) surfaces in the presence of noise. In Experiment 2, the surfaces' shape was held constant and the amount of front-to-back depth was varied; the observers estimated the amount of depth they perceived. The effects of age were strongly task dependent. The younger observers' performance in Experiment 1 was almost 60% higher than that of the older observers. In contrast, no age effect was obtained in Experiment 2. Older observers can effectively perceive variations in depth from patterns of motion parallax, but their ability to discriminate 3-D shape is significantly compromised.

Adult↗

PubMed2000

Encoding of movement time by populations of cerebellar Purkinje cells.

One of the earliest computational principles attributed to the cerebellum was the measurement of time. This idea was originally suggested on anatomical grounds, and was taken up again to explain some of the deficits in cerebellar patients. The contribution of the cerebellum to eye movements, in contrast, has traditionally been discussed in the context of motor learning. This view has received support from the loss of saccade adaptation, one of the key examples of motor learning, following lesions of the posterior cerebellar vermis. However, the relationship between the properties of saccade-related vermal Purkinje cells and the behavioural deficits that follow lesions is unclear. Here we report results from single-unit recording experiments on monkeys that reconcile the seemingly unrelated concepts of timing and motor learning. We report that, unlike individual Purkinje cells, the population response of larger groups of Purkinje cells gives a precise temporal signature of saccade onset and offset. Thus a vermal population response may help to determine saccade duration. Modifying the time course of the population response by changing the weights of the contributing individual Purkinje cells, discharging at different times relative to the saccade, would directly translate into changes in saccade amplitude.

Animals↗

PubMed1979

Variables influencing phonemic discrimination performance in normal and learning-disabled children.

Twenty-five learning-disabled and 25 normal first-grade-age children took a phonemic discrimination test that manipulated word-pairs systematically according to degree of phonetic difference, position of phoneme contrast, and lexical familiarity. Results indicated that (1) the significantly lower performance of the learning-disabled to children as a group was due to the impaired performance by a small subgroup, (2) all three stimulus variables had significant effects on performance, (3) all combinations of stimulus variables interacted significantly, and (4) discrimination performance did not correlate with measures of receptive vocabulary or reading achievement for either group.

Achievement↗

PubMed2002

Appraisal of doctors: problems with terminology and a philosophical tension.

The term appraisal lacks useful definition. It is used to describe both the summative assessment which forms part of performance management, as well as an educational process sometimes referred to as formative assessment. In this analysis we trace the purpose of each of these components as applied to career grade doctors. In the first, authority for the process, and the motive for applying it, lies with management. Within a publicly funded health service the purpose of this is directed towards equality of health care, and to obtain the greatest performance available with the resources available. By contrast, appraisal which supports a doctor's self-directed learning is likely to address issues of quality. The two processes exist in tension with each other, and are mutually informative. We argue that in the appraisal of doctors the two processes and purposes should be made explicit.

Education, Medical, Continuing↗

PubMed1992

Illusory occluding contours and surface formation by depth propagation.

A novel kind of depth-spreading effect which should be distinguished in various aspects from the known interpolation, averaging, or 'filling-in' phenomena is reported. The demonstrations and experiments suggest that depth from an uncrossed disparity can be extrapolated from, not just interpolated between, illusory or real contours to form perceptually a background surface. In addition, the form of the illusory contour itself could be drastically changed in configuration and sharpness, contingently with perceptual background-surface formation. No such effects of surface and contour formation were observed in the crossed disparity case. Because the illusory contours were enhanced and perceived as illusory 'occluding contours', these effects may be closely related to the 'occlusion constraints' in the real world.

Attention↗

PubMed1993

Object and head orientation effects on symmetry perception defined by shape from shading.

A grey disc which exhibits a vertical gradient of shading usually appears convex if lighter above and concave if lighter below. This phenomenon was investigated by Howard et al (1990) who varied both the shading axis relative to gravity and the orientation of the head. Their results indicated that head-centric or retinocentric coordinates determined the depth effect rather than gravitational axes. However, several possible problems with their study were noted, not the least of which was the possible intrusion of response rather than perceptual factors in the task they used. Here, we attempted to use an indirect measure of the perception of depth from shading; rather than asking subjects whether discs looked convex or concave, we constructed ensembles of shaded discs which, in terms of depth from shading, were or were not bilaterally symmetrical about an horizontal axis. These stimulus displays were briefly flashed to prohibit the intrusion of conscious assumptions about direction of light sources. Subjects were never asked whether any discs looked concave or convex, merely whether the set of discs was or was not depth symmetrical. Results were generally consistent with those of Howard et al and supported the conclusion that depth from shading is largely a low-level and automatic mechanism.

Adult↗

PubMed1994

Shape from shading. I: Surface curvature and orientation.

The human visual system makes effective use of shading alone in recovering the shape of objects. Pictures of sculptures are readily interpreted--a situation where shading provides virtually the sole cue to shape. However, shading has been considered a poor cue to depth in comparison with retinal disparity and kinetic cues. Curvature discrimination thresholds were measured with the use of a surface-alignment task for a range of surface curvatures from 0.16 cm-1 to 1.06 cm-1. Weber fractions were around 0.1, demonstrating considerable precision in this task. Weber fractions did not vary substantially as a function of surface curvature. Rotation of the light source around the line of sight had no effect on curvature discrimination but rotation towards the viewer increased discrimination thresholds. In contrast, slant discrimination declined with rotation of the light-source vector towards the viewpoint. When a band-limited random grey-level texture was mapped onto the sphere, curvature discrimination thresholds increased gradually as a function of texture contrast, even though texture and shading provided consistent cues to depth. Adding texture also increased slant discrimination thresholds, demonstrating that texture can act as a source of noise in shape-from-shading tasks. The psychophysical findings have been used to evaluate whether current algorithms for shape from shading in computer vision could serve as models of human three-dimensional shape analysis and to highlight low-level intramodular interactions between depth cues. It is demonstrated that, in the case of surfaces defined by shading, curvature descriptions are primary and do not depend upon the prior encoding of surface orientation, and Koenderink's local-shape index is suggested as an alternative intermediate representation of surface shape in the human visual system.

Attention↗

PubMed1994

Long-range apparent motion as a result of perceptual organisation.

Five kinds of percept have been found to occur when two different stimulus objects are simultaneously presented, exchanging positions with each other in successive exposures. These five percepts can be classified as follows: (i) Appearance-Disappearance (succession), (ii) Lateral Motion, (iii) Depth Motion, (iv) Transmutation (in colour, brightness, and/or shape), and (v) Overlapping (simultaneity). Results of three experiments indicate that relative dominance among these five percepts systematically depends upon differences between the two stimulus objects in colour, luminance, shape, and size. The relative dominance depended upon the stimulus onset asynchrony (SOA) between successive presentations and on the spatial separation between the two objects. Lateral Motion became more dominant as a result of (a) an increase in the number of stimulus attributes differing between the two objects, (b) an increase in the SOA, or (c) a decrease in the distance between the two objects. Colour difference, even without a luminance difference, was one of the determinants for Lateral Motion. Depth Motion frequently occurred when the two objects differed in size and the SOA and the distances between them were relatively great. Transmutation occurred when the objects differed in colour, luminance, or shape. Perceptual modes (ii) to (iv) can be understood as different results of perceptual organisation that always maintains perceptual identity of objects and maintains perceptual constancy of their attributes as much as possible. Long-range apparent motion could be a result of such perceptual organisation.

Adult↗

PubMed1994

On the contribution of a binocular 'AND' channel at contrast threshold.

Three experiments are reported in which an attempt was made to isolate the contribution of an AND channel by measuring aftereffects following alternating monocular adaptation. The first two were designed to test Wolf and Held's proposal that the binocular AND channel does not respond at contrast threshold. In the first experiment the relative sizes of monocular and binocular contrast threshold elevation were compared with the pattern of aftereffects obtained in a study of the suprathreshold tilt aftereffect. Identical patterns of results were obtained under the two adaptation conditions. In the second experiment, the monocular and binocular contrast-reduction aftereffect reported by Blakemore et al was measured over a wide range of reference contrasts. As in the previous experiment, the monocular effect was greater than the binocular effect. This occurred at all reference contrasts. These data support the conclusion that the AND channel contributes to visual performance in the same manner, irrespective of stimulus contrast. In the final experiment an alternative explanation for existing evidence against the existence of an AND channel was assessed.

Adult↗

PubMed1995

Phenomena of illusory form: can we bridge the gap between levels of explanation?

The study of illusory brightness and contour phenomena has become an important tool in modern brain research. Gestalt, cognitive, neural, and computational approaches are reviewed and their explanatory powers are discussed in the light of empirical data. Two well-known phenomena of illusory form are dealt with, the Ehrenstein illusion and the Kanizsa triangle. It is argued that the gap between the different levels of explanation, bottom-up versus top-down, creates scientific barriers which have all too often engendered unnecessary debate about who is right and who is wrong. In this review of the literature we favour an integrative approach to the question of how illusory form is derived from stimulus configuration which provide the visual system with seemingly incomplete information. The processes that can explain the emergence of these phenomena range from local feature detection to global strategies of perceptual organisation. These processes may be similar to those that help us restore partially occluded objects in everyday vision. To understand better the Ehrenstein and Kanizsa illusions, it is proposed that different levels of analysis and explanation are not mutually exclusive, but complementary. Theories of illusory contour and form perception must, therefore, take into account the underlying neurophysiological mechanisms and their possible interactions with cognitive and attentional processes.

Arousal↗

PubMed1996

Shape constancy in pictorial relief.

Pictorial relief was measured for a series of pictures of a smooth solid object. The scene was geometrically identical (ie the perspective of the three-dimensional scene remained the same) for all pictures, the rendering different. Some of the pictures were monochrome full-scale photographs taken under different illumination of the scene. Also included were a silhouette (uniform black on uniform white) and a 'cartoon'-style rendering (visual contour and key linear features rendered in thin black line on a uniform white ground). Two subjects were naive and started with the silhouette, saw the cartoon next, and finally the full-scale photographs. Another subject had seen the object and did the experiment in the opposite sequence. The silhouette rendering is impoverished, but has considerable relief with much of the basic shape. The cartoon rendering yields well-developed pictorial relief, even for the naive subjects. Shading adds only small local details, but different illumination produces significant alterations of relief. It is concluded that shape constancy under changes in illumination is dominant throughout, but that the (small) deviations from true constancy reveal the effect of cues such as shading in a natural setting. Such a ¿perturbation analysis' appears more promising than either stimulus-reduction or cue-conflict paradigms.

Attention↗

PubMed1996

Texture laciness: the texture equivalent of transparency?

Displays with overlapping squares of different textures were produced. In some texture combinations, the overlapping area appears to be a new texture. In other combinations, however, one texture is seen through the other as if it were inscribed on a transparent sheet and held in front of the other. This phenomenon can be called texture laciness. It is claimed that texture laciness is important because in the natural world we frequently encounter overlapping textures that we perceive as separate. In examining the conditions that lead to texture laciness, it was found that (1) laciness occurs more strongly with decreasing similarity between elements of two textures, (2) when the elements of the two textures are the same, the overlapped area tends to appear as a new texture, and (3) the physical overlapping of the individual elements of two textures hinders texture decomposition, irrespective of relative positioning. It is suggested that the textures inducing laciness may be processed as surfaces rather than just collections of identical elements. Texture laciness also reveals that even if 'antitextons'-the shapes of the spaces between texture elements, which have been regarded as important in texture discrimination-are destroyed by positioning another set of texture elements in the spaces, the textures can remain sufficiently identifiable to be perceived as separate.

Adult↗

PubMed2005

Approach and applications of constrained ICA.

This paper presents the technique of constrained independent component analysis (cICA) and demonstrates two applications, less-complete ICA, and ICA with reference (ICA-R). The cICA is proposed as a general framework to incorporate additional requirements and prior information in the form of constraints into the ICA contrast function. The adaptive solutions using the Newton-like learning are proposed to solve the constrained optimization problem. The applications illustrate the versatility of the cICA by separating subspaces of independent components according to density types and extracting a set of desired sources when rough templates are available. The experiments using face images and functional MR images demonstrate the usage and efficacy of the cICA.

Algorithms↗

PubMedDate not supplied

A microgenetic/cross-sectional study of matrix completion: comparing short-term and long-term change.

A design that included both microgenetic and cross-sectional components was used to examine 135 Slovenian children's acquisition of matrix completion proficiency and compare microgenetic and age-related changes on the task. The microgenetic analyses indicated that children's errors became increasingly variable shortly before they discovered the correct strategy, that the correct strategy became dominant quite quickly following its initial use, that improvements in matrix completion performance generalized to conservation, and that amount of learning correlated positively with IQ. The microgenetic/cross-sectional comparison, which involved contrasting the changes that occurred over seven experimental sessions with the changes that occurred between ages 6 and 7 years, indicated that the two patterns of change were highly similar.

Child↗

PubMed2003

Stereoscopic surface interpolation supports lightness constancy.

The human visual system has a remarkable ability to construct surface representations from sparse stereoscopic, as well as texture and motion, information. In impoverished displays where few points are used to define regions in depth, the brain often interpolates depth estimates across intervening blank regions to create a compelling sense of a solid surface. The set of experiments described here examined stereoscopic interpolation using a novel technique based on lightness constancy. The effectiveness of this method is notable because it stands as the only technique to date that unequivocally examines the perception of interpolated surfaces, and not surfaces inferred subjectively from depth information in the stimulus. Further, these data support the growing evidence that a primary function of the stereoscopic system is to define three-dimensional surface structure.

Adult↗

PubMed2004

The effect of alternative prey on the dynamics of imperfect Batesian and Müllerian mimicries.

Both Batesian and Müllerian mimicries are considered classical evidence of natural selection where predation pressure has, at times, created a striking similarity between unrelated prey species. Batesian mimicry, in which palatable mimics resemble unpalatable aposematic species, is parasitic and only beneficial to the mimics. By contrast, in classical Müllerian mimicry the cost of predators' avoidance learning is shared between similar unpalatable co-mimics, and therefore mimicry benefits all parties. Recent studies using mathematical modeling have questioned the dynamics of Müllerian mimicry, suggesting that fitness benefits should be calculated in a way similar to Batesian mimicry; that is, according to the relative unpalatability difference between co-mimics. Batesian mimicry is very sensitive to the availability of alternative prey, but the effects of alternative prey for Müllerian dynamics are not known and experiments are rare. We designed two experiments to test the effect of alternative prey on imperfect Batesian and Müllerian mimicry complexes. When alternative prey were scarce, imperfect Batesian mimics were selected out from the population, but abundantly available alternative prey relaxed selection against imperfect mimics. Birds learned to avoid both Müllerian models and mimics irrespective of the availability of alternative prey. However, the rate of avoidance learning of models increased when alternative prey were abundant. This experiment suggests that the availability of alternative prey affects the dynamics of both Müllerian and Batesian mimicry, but in different ways.

Animals↗