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Biomedical subjects

D Purves

Publications and source records attributed to D Purves.

At least 19 recordsLinked to original sources
PubMed1998

The influence of depicted illumination on brightness.

The striking illusions produced by simultaneous brightness contrast generally are attributed to the center-surround receptive field organization of lower order neurons in the primary visual pathway. Here we show that the apparent brightness of test objects can be either increased or decreased in a predictable manner depending on how light and shadow are portrayed in the scene. This evidence suggests that perceptions of brightness are generated empirically by experience with luminance relationships, an idea whose implications we pursue in the accompanying paper.

Computer Graphics↗
PubMed1998

An empirical explanation of brightness.

In this second part of our study on the mechanism of perceived brightness, we explore the effects of manipulating three-dimensional geometry. The additional scenes portrayed here demonstrate that the same luminance profile can elicit different sensations of brightness as a function of how the objects in the scene are arranged in space. This further evidence confirms the implication of the scenes presented in the accompanying paper, namely that sensations of relative brightness-including standard demonstrations of simultaneous brightness contrast-cannot arise by computations of local contrast. The most plausible explanation of the full range of perceptual phenomena we have described is an empirical strategy that links the luminance profile in a visual stimulus with an association (the percept) that represents the profile's most probable real-world source.

Contrast Sensitivity↗
PubMed1998

The distribution of oriented contours in the real world.

In both humans and experimental animals, the ability to perceive contours that are vertically or horizontally oriented is superior to the perception of oblique angles. There is, however, no consensus about the developmental origins or functional basis of this phenomenon. Here, we report the analysis of a large library of digitized scenes using image processing with orientation-sensitive filters. Our results show a prevalence of vertical and horizontal orientations in indoor, outdoor, and even entirely natural settings. Because visual experience is known to influence the development of visual cortical circuitry, we suggest that this real world anisotropy is related to the enhanced ability of humans and other animals to process contours in the cardinal axes, perhaps by stimulating the development of a greater amount of visual circuitry devoted to processing vertical and horizontal contours.

Animals↗
PubMed1998

Unequal representation of cardinal and oblique contours in ferret visual cortex.

We have measured the amount of cortical space activated by differently oriented gratings in 25 adult ferrets by optical imaging of intrinsic signal. On average, 7% more area of the exposed visual cortex was preferentially activated by vertical and horizontal contours than by contours at oblique angles. This anisotropy may reflect the real-world prevalence of contours in the cardinal axes and could explain the greater sensitivity of many animals to vertical and horizontal stimuli.

Animals↗
PubMed1997

Similarities in normal and binocularly rivalrous viewing.

We report here a series of observations-most of which the reader can experience directly-showing that distinct components of patterned visual stimuli (orthogonal lines of a different hue) vary in perception as sets. Although less frequent and often less complete, these perceptual fluctuations in normal viewing are otherwise similar to the binocular rivalry experienced when incompatible scenes are presented dichoptically.

Humans↗
PubMed1997

The perception of transparent three-dimensional objects.

When the proximal and distal elements of wire-frame cubes are conflated, observers perceive illusory structures that no longer behave veridically. These phenomena suggest that what we normally see depends on visual associations generated by experience. The necessity of such learning may explain why the mammalian visual system is subject to a prolonged period of plasticity in early life, when novel circuits are made in enormous numbers.

Aging↗
PubMed1997

Correlated size variations in human visual cortex, lateral geniculate nucleus, and optic tract.

We have examined several components of the human visual system to determine how the dimensions of the optic tract, lateral geniculate nucleus (LGN), and primary visual cortex (V1) vary within the same brain. Measurements were made of the cross-sectional area of the optic tract, the volumes of the magnocellular and parvocellular layers of the LGN, and the surface area and volume of V1 in one or both cerebral hemispheres of 15 neurologically normal human brains obtained at autopsy. Consistent with previous observations, there was a two- to threefold variation in the size of each of these visual components among the individuals studied. Importantly, this variation was coordinated within the visual system of any one individual. That is, a relatively large V1 was associated with a commensurately large LGN and optic tract, whereas a relatively small V1 was associated with a commensurately smaller LGN and optic tract. This relationship among the components of the human visual system indicates that the development of its different parts is interdependent. Such coordinated variation should generate substantial differences in visual ability among humans.

Adult↗
PubMed1996

The extraordinarily rapid disappearance of entoptic images.

It has been known for more than 40 years that images fade from perception when they are kept at the same position on the retina by abrogating eye movements. Although aspects of this phenomenon were described earlier, the use of close-fitting contact lenses in the 1950s made possible a series of detailed observations on eye movements and visual continuity. In the intervening decades, many investigators have studied the role of image motion on visual perception. Although several controversies remain, it is clear that images deteriorate and in some cases disappear following stabilization; eye movements are, therefore, essential to sustained exoptic vision. The time course of image degradation has generally been reported to be a few seconds to a minute or more, depending upon the conditions. Here we show that images of entoptic vascular shadows can disappear in less than 80 msec. The rapid vanishing of these images implies an active mechanism of image erasure and creation as the basis of normal visual processing.

Arterioles↗
PubMed1996

Temporal events in cyclopean vision.

The majority of neurons in the primary visual cortex of primates can be activated by stimulation of either eye; moreover, the monocular receptive fields of such neurons are located in about the same region of visual space. These well-known facts imply that binocular convergence in visual cortex can explain our cyclopean view of the world. To test the adequacy of this assumption, we examined how human subjects integrate binocular events in time. Light flashes presented synchronously to both eyes were compared to flashes presented alternately (asynchronously) to one eye and then the other. Subjects perceived very-low-frequency (2 Hz) asynchronous trains as equivalent to synchronous trains flashed at twice the frequency (the prediction based on binocular convergence). However, at higher frequencies of presentation (4-32 Hz), subjects perceived asynchronous and synchronous trains to be increasingly similar. Indeed, at the flicker-fusion frequency (approximately 50 Hz), the apparent difference between the two conditions was only 2%. We suggest that the explanation of these anomalous findings is that we parse visual input into sequential episodes.

Animals↗
PubMed1996

The wagon wheel illusion in movies and reality.

Wheels turning in the movies or in other forms of stroboscopic presentation often appear to be rotating backward. Remarkably, a similar illusion is also seen in continuous light. The occurrence of this perception in the absence of intermittent illumination suggests that we normally see motion, as in movies, by processing a series of visual episodes.

Eye Movements↗
PubMed1996

Is neural development Darwinian?

Gradually, and without much debate, the idea that the developing nervous system is in some sense darwinian has become one of the canons of neurobiology. In fact, there is little evidence to support this idea.

Age Distribution↗
PubMed1996

Contextual control of conditioned responding in rats with dorsal hippocampal lesions.

The control exerted by contextual cues over classically conditioned responding was assessed for rats with electrolytic lesions of the dorsal hippocampus and sham-operated controls. In 3 experiments the rats received initial training with 2 reinforced cues, each presented in its own distinctive context, followed by a nonreinforced test in which the cues were presented in the other context. Both control and operated subjects showed context specificity, as evidenced by less vigorous responding to these cues than to cues presented on test in their original contexts. The groups did not differ in their ability to learn an explicit discrimination in which a given cue was reinforced in one context and nonreinforced in a different context (although the groups did differ on a simple autoshaping task). It is concluded that a special role for the hippocampus in the contextual control of conditioned responding still remains to be demonstrated.

Animals↗
PubMed1995

Effects of increased neural activity on brain growth.

We have measured the effects of regionally increased metabolic activity--and by inference electrical activity--on cortical growth in the developing rat brain. Cortical growth is significantly and specifically greater in regions of chronically increased activity. This effect of activity on cortical growth may help explain the permanent storage of early experience in the developing nervous system.

Aging↗
PubMed1995

Enhancement of latent inhibition in rats with electrolytic lesions of the hippocampus.

Two groups of rats--1 with electrolytic lesions of the hippocampus and 1 consisting of sham-operated controls--received flavor-aversion conditioning with 2 flavors. All subjects had received prior nonreinforced exposure to Flavor A. Latent inhibition was apparent in slower acquisition of the aversion to Flavor A than to Flavor B. Hippocampal lesions had no effect on acquisition to the nonpreexposed Flavor B but produced a marked enhancement of the latent inhibition effect. The contrast between this result and previous findings of an attenuation of latent inhibition in subjects with hippocampal lesions is discussed.

Animals↗
PubMed1995

Genotoxicity testing: current practices and strategies used by the pharmaceutical industry.

Current guidelines and recommendations for genotoxicity testing of pharmaceuticals are disparate, both in terms of the most appropriate tests to use and the protocols to follow. Recent attempts have been made to standardise genotoxicity testing procedures, coinciding with the current review of the OECD guidelines and the International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH). However, as with other aspects of non-clinical safety assessment of pharmaceuticals, guidelines have been prepared by evaluation of general chemical data due to the lack of specific information on pharmaceuticals. To address this, a project was undertaken to collect and collate information specifically pertaining to the genotoxicity testing of pharmaceuticals in order to obtain a clear understanding of international strategy and procedures in the pharmaceutical industry. It is clear that the practices and regional variations are strongly influenced by national guidelines and do not necessarily follow companies' preferences. However, there is a surprising amount of variation in approach between companies on some issues. This is evident in how companies define a genotoxin. This ranges from a positive result in an in vivo assay as indicative of a genotoxin (43%) to any positive result in vitro or in vivo (30%). Indeed many companies (particularly in Japan) will terminate development on the strength of a clear positive result in an Ames test. There is much debate within the ICH process concerning tests to detect gene mutations in mammalian cells as part of a primary test battery. This survey shows that in general, the pharmaceuticals industry has severe doubts about these assays. Thirty-seven (78%) of the 47 participating pharmaceutical companies include an in vitro test to detect gene mutation in mammalian cells as part of their routine test battery. The HPRT test using Chinese hamster cells has the most widespread use, although there is only limited use of such tests in Japan. Compound development has been affected by the results of such tests, but usually only in terms of clarification of equivocal results in other genotoxicity tests in the test battery. The majority (63%) of companies do not support its use as a primary regulatory requirement, and 83% do not consider the mouse lymphoma assay (L5178Y) an acceptable replacement for in vitro mammalian cytogenetics. In conclusion, this survey has provided valuable information on the current modus operandi of the international pharmaceutical industry for consideration in current harmonisation initiatives.

Animals↗
PubMed1995

Individual variation and lateral asymmetry of the rat primary somatosensory cortex.

We have evaluated the interindividual variability and lateral symmetry of a major cortical area by comparing the primary somatosensory cortex (S1) of adult rats. Our choice of the rat was dictated by the accuracy with which one can measure S1 and its component representations in the rodent brain; the importance of such measurements lies in understanding the rules that govern the allocation of cortical space and, ultimately, the consequences of differential allocation for behavior. With respect to interindividual differences, the major somatic representations in S1 are surprisingly variable in size. The area of the whiskerpad representation, for example, ranged from 3.72 to 6.84 mm2 in a sample of 53 rats; other components of S1 showed comparable differences among animals. With respect to lateral symmetry, the average area of each major representation was similar for the right and left hemispheres; thus, we found no consistent bias in the size of S1 or its elements in the sample as a whole. Within individual animals, however, the sizes of the major somatic representations were often quite different in the two hemispheres. The magnitude of the lateral differences averaged 7.9 +/- 0.8% (mean +/- SEM) for the whisker pad representation, 11.6 +/- 1.3% for the upper lip, 15.4 +/- 1.6% for the furry buccal pad, 13.9 +/- 1.4% for the lower jaw, and 13.3 +/- 1.2% for the forepaw. These results show that the amount of cortical space allocated to corresponding functions in individual rats--or in the two hemispheres of a particular rat--are often different. Such variations are likely to be reflected in somatosensory performance.

Animals↗
PubMed1994

Monocular preferences in binocular viewing.

Faced with an unobstructed view, both foveas can be readily aligned with a distant visual target. The minor difference in the view of the two eyes (which arises from slightly different lines of sight) presents no special problem and is, indeed, the basis of stereopsis. However, when obstructing objects are present in the foreground, the view provided by one eye becomes wholly or partially incompatible with the view of the other. We have investigated how we cope with this everyday situation by having volunteers observe distant targets through a fenestrated screen. In this circumstance, subjects naturally position themselves to view a target of interest with one eye--usually the right eye. This monocular habit in normal viewing reinforces other evidence for the unorthodox idea that visual perception arises from a union in consciousness of monocular images that are elaborated independently.

Female↗