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.

Structure of the human sensorimotor system. I: Morphology and cytoarchitecture of the central sulcus

We have studied the morphology of the central sulcus and the cytoarchitecture of the primary sensorimotor cortex in 20 human brains obtained at autopsy. Although the surface appearance of the central sulcus varies greatly from brain to brain (and between hemispheres of individual brains), its deep structure is remarkably consistent. The fundus of the central sulcus is divided into medial and lateral limbs by a complex junction midway between the sagittal and Sylvian fissures. Based on functional imaging studies, this junction appears to be a structural hallmark of the sensorimotor representation of the distal upper extremity. We also identified and measured area 4 (primary motor cortex) and area 3 (primary somatic sensory cortex) in Nissl-stained sections cut orthogonal to the course of the central sulcus. Although the positions of the cytoarchitectonic boundaries in the paracentral lobule showed considerable interindividual variation, the locations of the borders of areas 4 and 3 along the course of the sulcus were similar among the 40 hemispheres examined. In addition to describing more thoroughly this portion of the human cerebral cortex, these observations provide a basis for evaluating lateral symmetry of the human primary sensorimotor cortex.

Structure of the human sensorimotor system. II: Lateral symmetry

We have evaluated the lateral symmetry of the human central sulcus, brainstem and spinal cord using quantitative histological and imaging techniques in specimens from 67 autopsy cases. Our purpose was to determine whether the preferred use of the right hand in the majority of humans is associated with grossly discernible asymmetries of the neural centers devoted to the upper extremities. In the accompanying report, we described a consistent set of morphological features in the depths of the central sulcus that localize the sensorimotor representation of the distal upper extremity. Measurements of the cortical surface in this region, and indeed throughout the entire central sulcus, showed no average lateral asymmetry. Cytoarchitectonic measurements of area 4 and area 3 confirmed this similarity between the left and right hemispheres. The medullary pyramids, which contain the corticospinal tracts, were also symmetrical, as were the cross-sectional areas of white and gray matter in the cervical and lumbar enlargements of the spinal cord. Finally, we found no lateral difference in the size and number of motor neurons in the ventral horns at these levels of the cord. Based on these several observations, we conclude that the preferred use of the right hand in humans occurs without a gross lateral asymmetry of the primary sensorimotor system.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Bone alkaline phosphatase and prostate-specific antigen in the monitoring of prostate cancer

A retrospective study has been made on the interrelationship of serum bone alkaline phosphatase (BAP), measured by the Ostase-RIA, and prostate-specific antigen (PSA) in 156 patients with M0 and M1 prostate cancer. BAP is a more sensitive and more specific method of determining osteoblast activity than total alkaline phosphatase (TAP). The main difference between these two assays is seen when the TAP is in the range of normal to twice-normal. BAP shows a low intraindividual variation in M0 disease, and was within normal limits in 18 patients following radical prostatectomy with a PSA < 0.1 ng/ml. A raised BAP was observed in 86.4% of M1 disease at diagnosis before treatment. The change of BAP was concordant with PSA in 69% of 49 cases of M1 disease, although there are marked differences in the rates of change of the two markers. A nadir of PSA < or = 10 ng/ml after androgen blockade in M1 disease was associated with a high probability of a normal BAP.

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.

Manual asymmetry and handedness

Volumetric measurements show that right-handed individuals have larger right hands than left hands. In contrast, the hands of left-handers are much more nearly symmetrical. Based on what is known about trophic interactions between neurons and targets, these findings predict a corresponding asymmetry of the relevant parts of the sensorimotor system in right-handers. The lack of an opposite-hand asymmetry among left-handers further implies that right- and left-handed phenotypes do not arise according to the same developmental rules.

Neural activity and the development of the somatic sensory system

Present thinking about the role of neural activity in the developing brain is based largely upon observations in the visual system. Attempts to generalize these findings in the somatic sensory system, however, have yielded perplexing results. Unlike the visual system, recent evidence suggests that activity plays a relatively minor role in establishing structural patterns in the primary somatic sensory cortex. Activity levels in the primary somatic sensory cortex are nonetheless highest in those regions that grow most during postnatal development, implying that activity promotes differential cortical growth.

Differential metabolic and electrical activity in the somatic sensory cortex of juvenile and adult rats

We have examined relative levels of metabolic and electrical activity across layer IV in the primary somatic sensory cortex (S1) of the rat in relation to regions of differential postnatal cortical growth. Each of several indices used--mitochondrial enzyme histochemistry, microvessel density, Na+/K+ pump activity, action potential frequency, and deoxyglucose uptake--indicate regional variations of metabolic and electrical activity in this part of the brain in both juvenile (1-week-old) and adult (10-12-week-old) animals. At both ages, areas of the somatic sensory map related to special sensors such as whiskers and digital pads showed evidence of the most intense activity. Thus, mitochondrial enzyme staining, blood vessel density, and Na+/K+ ATPase activity were all greatest in the barrels and barrel-like structures within S1, and least in the adjacent interbarrel cortex and the cortex surrounding S1. Multiunit recordings in and around the posteromedial barrel subfield of anesthetized animals also showed that the average ratio of evoked to spontaneous activity was greater in barrels than in the surrounding, metabolically less active cortex. Furthermore, autoradiograms of labeled deoxyglucose accumulation in awake behaving animals indicated systematic differences in neural activity across S1 barrels and barrel-like structures showed more deoxyglucose accumulation than interbarrel, nonbarrel, or peri-S1 cortex. These regional differences in neural activity correspond to regional differences in neocortical growth (Riddle et al., 1992). The correlation of greater electrical activity, increased metabolism, and enhanced cortical growth during postnatal maturation suggests that neural activity foments the elaboration of circuitry in the developing brain.

Development of blobs in the visual cortex of macaques

We have examined the area of the primary visual cortex and the number and size of blobs within it in 10 neonatal and 11 adult rhesus monkeys. The average area of the primary visual cortex (V1) increases from 919 mm2 in newborns to 1,069 mm2 in adult animals (16%). The number of blobs decreases per unit area from an average of 5.2/mm2 at birth to 4.3/mm2 in maturity (18%). As a consequence, the number of blobs remains approximately the same during maturation, at about 4,800/hemisphere. These observations correct a preliminary report on a subset of the animals studied here (Purves and LaMantia: Proc Natl Acad Sci 87:5765, '90), in which it appeared that blob number might increase between birth and maturity. As in other regions of the developing postnatal brain, we found no net loss of modular circuitry.

Iterated patterns of brain circuitry (or how the cortex gets its spots)

The prominence of repeating patterns of circuitry in the mammalian brain has led to the general view that iterated modular units reflect a fundamental principle of cortical function. Here we argue that these intriguing patterns arise not because the functional organization of the brain demands them, but as an incidental consequence of the rules of synapse formation.

Growth of the rat somatic sensory cortex and its constituent parts during postnatal development

We have compared the size and arrangement of the primary somatic sensory cortex (SI) and its constituent parts in juvenile (1 week old) and mature (10-12 weeks old) rats using succinic dehydrogenase histochemistry and digital image analysis. Our goal was to determine whether some regions of the maturing cortex grow more than others. To this end, we examined (1) the growth of barrels and the surrounding (interbarrel) cortex, (2) the growth of the major somatic representations within SI, and (3) the overall growth of SI compared to the neocortex as a whole. With respect to the first of these issues, SI barrels and barrel-like structures grow more than the intervening cortex. The growth of these elements varies according to region: barrels in the head representation more than double in size, whereas the barrel-like structures in the paw representations increase by only about half this amount. The growth of the major somatic representations within SI is also heterogeneous, the representation of the head enlarging to a greater extent than the representations of the paws. Thus, the ratio of the total area of head representation to the combined paw representation is 15% greater in adults than in juveniles. Finally, the primary somatic sensory cortex grows to a somewhat greater extent than the neocortex as a whole. These observations demonstrate that postnatal cortical growth is not uniform; it varies among cortical barrels and the immediately surrounding (interbarrel) cortex, among the representations of different body parts, and between SI and the rest of the neocortex. As an explanation of this differential growth, we suggest that the neuropil of metabolically (and/or electrically) more active cortical regions grows to a greater extent during maturation than that of less active regions.

Diagnosis of the acute ischaemic heart by ion elution from myocardium

The post-mortem diagnosis of acute myocardial ischaemia may be difficult to establish in the absence of morphological changes in the myocardium or recent coronary thrombosis. Ischaemic cell injury leads to potassium (K) efflux and sodium (Na) influx and, if the blood is still circulating, the K:Na ratio of the tissue falls. In this study, the K:Na ratio was measured by eluting the ions from samples of myocardium and assaying the eluate. The method yields similar results to those obtained by a previous method, in which myocardial samples were homogenized. The K:Na ratios on samples of horizontal slices through the heart were plotted on maps of the slices. A low K:Na ratio corresponded to, but extended beyond, areas where there was morphological evidence of ischaemia. The method is simple and may be of use in routine practice.