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Biomedical subjects

A Cowey

Publications and source records attributed to A Cowey.

At least 145 records · Page 8Linked to original sources
PubMed1985

Rearrangements in the retino-geniculate projections of rats following ablation of the superior colliculus in infancy.

The superior colliculus was bilaterally or unilaterally ablated at different early postnatal ages in rats. When adult, each rat received a unilateral eye injection of Horseradish peroxidase to reveal the crossed and uncrossed retinal terminal fields within the dorsal lateral geniculate nucleus. Collicular ablation in the first seven days after birth, but not thereafter, produced a small hole or vacancy within the contralateral retinal terminal field which was occupied by an aberrant ipsilateral retinal terminal field. These rearrangements in the retino-geniculate projections occurred in the caudal quarter of the nucleus dorso-laterally just beneath the optic tract, solely ipsilateral to the ablated colliculus. Possible causes of the formation of these rearrangements are discussed, and similarities with other aberrant retinal projections following early damage to the visual system are considered.

Aging↗
PubMed1985

The ganglion cell and cone distributions in the monkey's retina: implications for central magnification factors.

The distribution of cones and ganglion cells was determined in whole-mounted monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being up to three times greater in nasal retina. A similar but smaller asymmetry occurred with cones. The total number of ganglion cells varied from 1.4 to 1.8 X 10(6), agreeing well with counts of optic nerve axons. The variation of ganglion cell density with eccentricity indicates the magnification factor (MF) of the retina. This was compared with MF at the dorsal lateral geniculate nucleus and at striate cortex, revealing that the relative representation of the fovea increases substantially in both thalamus and cortex.

Animals↗
PubMed1984

Effects of damage to superior colliculi and pre-tectum on movement discrimination in rhesus monkeys.

Eight rhesus monkeys (Macaca mulatta) were trained to detect an instantaneous lateral displacement of a small spot of light. The smallest movement for 79% correct performance was then determined. Severing the splenium of the corpus callosum, which has to be done to reveal the superior colliculi, had no effect on the movement threshold. But when the superior colliculi were damaged in addition, there was an initial substantial impairment in the detection of movement and a smaller but permanent elevation in the threshold. Only if the rostral superior colliculi and pretectum were spared, was there no change in threshold, and there was evidence that the pretectal rather than collicular damage may be more important in relation to movement discrimination. Tests with human observers performing the same task showed that the threshold can be elevated by imprecise fixation and accomodation, suggesting that the consequences of mid-brain damage on movement detection may be wholly or in large part attributed to oculomotor disorders.

Accommodation, Ocular↗
PubMed1984

Characterization by Golgi impregnation of neurons that accumulate 3H-GABA in the visual cortex of monkey.

3H-GABA was injected into restricted regions of visual areas 1 and 2 (cortical areas 17 and 18) on the lateral surface of the occipital lobe in monkeys. The injected tissue was processed for Golgi impregnation and gold toning. Sections containing Golgi-impregnated neurons were re-embedded, sectioned at 1 micron, and prepared for autoradiography to reveal neurones that had selectively accumulated 3H-GABA. Golgi-impregnated pyramidal, spiny stellate and aspiny nonpyramidal neurons were examined for 3H-GABA accumulation. Out of 47 aspiny non-pyramidal neurons 16 were labelled by 3H-GABA. The other cell types did not accumulate the amino acid. Twelve of the labelled neurons were drawn. Eight were bitufted neurons with their dendrites oriented predominantly radially, three were small multipolar neurons, and one could be reconstructed only partially. One neuron had a locally arborizing axon in layer III. Two bitufted, Golgi-impregnated neurons in layer II and upper III of area 18 were labelled from GABA injection radially beneath in layer VI, providing evidence for earlier suggestions that in the monkey's visual cortex the cells in the upper layers which project radially and accumulate 3H-GABA are aspiny non-pyramidal cells. The results indicate the existence of different types of putative GABA-ergic interneurons.

Animals↗
PubMed1984

Hemispheric asymmetry for a visuo-spatial task in monkeys.

The hypothesis that hemispheric asymmetries for visuo-spatial ability exist in rhesus monkeys was tested using a task for which the right hemisphere is specialized in man. The task required discrimination between two squares, one containing a central dot and the other a dot displaced upwards from centre. The animals' thresholds were determined before and after left-sided (LH-lesion) or right-sided (RH-lesion) unilateral occipital lobectomy combined with splenial transection, and the percentage change in threshold calculated. The LH-lesion monkeys were all worse than the RH-lesion monkeys on this measure, indicating the existence of a functional asymmetry.

Animals↗
PubMed1984

Retinal ganglion cells that project to the dorsal lateral geniculate nucleus in the macaque monkey.

Horseradish peroxidase was deposited in the optic nerve to retrogradely label and reveal the dendritic form of all classes of ganglion cell, or it was injected into the dorsal lateral geniculate nucleus to reveal only those classes projecting to the thalamus. The results were compared with those of the accompanying paper in which the ganglion cells projecting to the midbrain are selectively revealed. Two major classes of ganglion cells are described, the P alpha and P beta cells. For both classes dendritic field size increases with eccentricity from the fovea and there is no overlap in the two classes at any given eccentricity. Cell body size shows a similar mean difference but with a slight overlap. Both cell bodies and dendritic fields are larger along the temporal horizontal meridian than the nasal horizontal meridian, for P alpha and for P beta cells, but these differences are reduced when naso-temporal differences in ganglion cell density are taken into account, that is, size correlates closely with density. Injections restricted to the parvocellular layers of the lateral geniculate nucleus labelled almost exclusively P beta cells, whereas injections confined to the magnocellular layers labelled almost exclusively P alpha cells. As midbrain injections label no P beta cells and few P alpha cells it can be shown that about 80% of ganglion cells are P beta cells projecting to parvocellular lateral geniculate nucleus, and that about 10% are P alpha cells projecting to magnocellular layers. The coverage factor, that is the number of cells covering each point on the retina, varied from 1.9-2.3 for P beta cells, and from 2-7 for P alpha cells. Comparing the results with those of comparable investigations on cats and rabbits shows a much clearer segregation of the terminal targets of different classes of ganglion cell in monkeys, the greatest difference being the absence in the monkey of a projection to the geniculate from gamma- and epsilon-like cells. Further, axons which branch and innervate both thalamus and midbrain are rare in monkeys but common in other mammals. Comparing the results with those from physiological investigations suggests that the P beta cells correspond to colour-opponent cells, whereas P alpha cells correspond to the achromatic broad-band magnocellular cells.

Animals↗
PubMed1984

Retinal ganglion cells that project to the superior colliculus and pretectum in the macaque monkey.

Horseradish peroxidase was injected into the superior colliculus or pretectum or both in order to label, by retrograde axoplasmic transport, the retinal ganglion whose cells axons innervate the dorsal midbrain. The dendrites of ganglion cells were sufficiently well-labelled to reveal their overall morphological characteristics. It was therefore possible to compare the number and form of ganglion cells projecting to the midbrain with the total population of ganglion cells as revealed by optic nerve injections, and with ganglion cells labelled by injections in the lateral geniculate nucleus. We found that not more than 10% of all retinal ganglion cells project to the superior colliculus in the macaque monkey. This percentage varies little over the retina, being about 6% of all ganglion cells near the fovea and increasing slightly with eccentricity. The superior colliculus does not receive a projection from P beta cells and only a few P alpha cells terminate there. The majority of cells which project to the superior colliculus have a small- to medium-sized cell body and sparsely branched dendritic tree. We have called them P gamma and P epsilon cells by analogy with the gamma cells and epsilon cells in the cat's retina. Anatomically the P gamma and P epsilon cells are heterogeneous, which would be consistent with the physiological heterogeneity found for ganglion cells which project to the midbrain in monkeys.

Animals↗
PubMed1983

Projection lines and the ipsilateral retino-geniculate pathway in the hooded rat.

The organization of the hooded rat's dorsal lateral geniculate nucleus was studied with anatomical techniques, with particular regard to the representation of temporal retina and the binocular field. The ipsilateral and contralateral retinal terminal fields were examined in three stereotaxic planes following injections of horseradish peroxidase into the eye. Projections arising from the temporal crescent of the retina were studied with silver staining techniques for anterograde degeneration products. Following discrete retinal lesions there was clear evidence that the temporal retina projects in a topographic fashion both ipsilaterally and contralaterally. The orientation of the lines of projection in the dorsal lateral geniculate nucleus was assessed by retrograde labelling of cells after cortical implants of horseradish peroxidase. Although both the lines of projection and the ipsilateral terminal field extend rostro-caudally in the dorsal lateral geniculate nucleus, their paths are oblique rather than parallel. Their intersection appears to correspond to the representation in this nucleus of conjugate retinal points. This was confirmed by administering horseradish peroxidase by iontophoresis in either the binocular or monocular representation of the primary visual cortex, while one eye received an injection of [3H]proline. Only those cortical injections in the binocular region gave rise to labelled projection lines passing through the autoradiographically-labelled ipsilateral terminal field. The rat's dorsal lateral geniculate nucleus displays none of the cytoarchitectural lamination which is so prominent in the primate and cat. Even after labelling the input to the nucleus from one eye, there is still no obvious laminar relationship between the terminal fields from the two eyes. Despite the absence of lamination, the current results suggest that the principle of apposing the representation of conjugate retinal points in the dorsal lateral geniculate nucleus is the same in the rat as in cat and monkey.

Animals↗
PubMed1983

Retrograde transport of gamma-amino[3H]butyric acid reveals specific interlaminar connections in the striate cortex of monkey.

Several lines of evidence suggest that gamma-aminobutyric acid is an inhibitory neurotransmitter in the cerebral cortex. To study the intracortical projection of neurons that selectively accumulate this amino acid, we injected radioactive gamma-aminobutyric acid into the upper layers of the striate cortex of monkeys along tracks at an oblique angle to the pia. Sections from the injected area were then processed by a combination of autoradiography and Golgi impregnation to reveal the distribution of labeled neurons and their morphological characteristics. Labeled neurons always occurred around the injection site in each layer. In addition, a consistent radial pattern of perikaryal labeling was observed in layers IVc-VI below the injection track in layers I-IVa. The closer the injection track was to the pia the deeper the peak density of labeled cells appeared. After injection in layers IVa and the lower part of III, the highest number of labeled neurons was in layer IVc; after injection in the upper part of layer III, most labeled neurons were in layer V; and, after injection in layers I and II, the proportion of labeled neurons increased in the lower part of layer V and in layer VI. All these neurons in the infragranular layers are presumably labeled by retrograde axonal transport via the labeled fiber bundles that extended from upper to lower layers. Thirty-four Golgi-stained neurons of various types were also examined for retrograde labeling. Two were labeled, and both were aspiny stellate cells in layer V. The arrangement of these putative GABAergic neurones, with axons that ascend from lower to upper layers in a regular pattern and arborize locally, would enable them to mediate inhibition within cortical columns and between neighboring columns.

Animals↗
PubMed1982

Sensory and non-sensory visual disorders in man and monkey.

The posterior third of the cerebral cortex in monkeys consists of a patchwork of visual areas in each of which there is a 'map' of the retina. The details of the 'map' vary considerably from one area to another and one notable variation concerns the optimal visual feature to which the cells respond. Orientation, disparity, colour and movement are emphasized in separate areas that appear to be concerned with sensory analysis. Their existence and the possibility that brain damage is occasionally restricted chiefly to one such area may explain the rare highly selective visual sensory impairments that can follow posterior cerebral damage in man. Other areas are notable for having little or no retinotopic representation. Here the cells may have huge receptive fields and complex trigger features. When such regions are removed, the animal's visual sensory abilities are intact but its recognition of patterns and objects is not. This condition resembles human visual agnosia.

Agnosia↗
PubMed1982

Effects on visual acuity of neonatal or adult tectal ablation in rats.

Tectal ablation in neonatal rats leads to retrograde degeneration of retinal ganglion cells whereas similar damage in adults does not. We show here that the behavioral effects are comparably different. When rats with neonatal tectal ablation are tested as adults they are impaired in learning a discrimination between vertical and horizontal stripes and their visual acuity for square-wave gratings is slightly but significantly reduced.

Aging↗
PubMed1982

The role of frontal eye-fields and superior colliculi in visual search and non-visual search in rhesus monkeys.

Rhesus monkeys were tested on a visual search task in which they had to find and retrieve a peanut from a display of visually similar but inedible objects. The speed with which they did so was measured. Animals in which the superior colliculi or frontal eye-fields had been removed took longer to find the peanut than two operated control groups. Animals with collicular lesions had longer latencies than those with frontal eye-fields removed. These two groups were also tested on a second task, non-visual search, in which a peanut was concealed in each of 25 identical holes. The animals' task was to retrieve all 25 peanuts as quickly as possible. The group with frontal eye-fields removed made significantly more return errors, i.e. returning to a hole already sampled, than the control group but, in contrast to the first task, the animals with collicular lesions were not impaired. The results are related to the physiological properties of frontal eye-fields and superior colliculi and to the effects of frontal cortical brain damage in man. It is suggested that the frontal eye-fields are concerned with internally organized, i.e. voluntary, eye scanning whereas the superior colliculi are concerned with the detection and location of targets which are then fixated involuntarily.

Animals↗
PubMed1982

A sensitive period for ganglion cell degeneration and the formation of aberrant retino-fugal connections following tectal lesions in rats.

Rats of 0, 3, 5, 10 and 30 days of age received unilateral tectal lesions. After surviving for 150 days the retinal ganglion cell layer of the contralateral eye was examined for evidence of a ganglion cell loss. The retino-fugal projections of the eye contralateral to the lesion were studied in autoradiographs. In the animals operated at 0 days of age, 33% of the ganglion cells had degenerated but in animals operated at 5 days of age, 67% of the ganglion cells had degenerated. The animals operated at 30 days of age show no significant cell loss. An aberrant retinal projection to the lateral posterior nucleus of the thalamus was found only in animals operated at 0 and 3 days of age. The retinal projection to the thalamus was investigated in normal rats of 0, 3, 5 and 10 days of age using the anterograde transport of horseradish peroxidase. There was a conspicuous projection to the lateral posterior nucleus in animals of 0 and 3 days of age, but in the 5-day-old rat the retinal projection to the lateral posterior nucleus was very small and similar to the adult pattern. We conclude that transecting the tectal terminals of retinal ganglion cells causes the ganglion cells to degenerate, unless they are old enough to have formed sustaining collaterals. In addition, the tectal lesion removes a major tectal input to the lateral posterior nucleus and, if carried out within the first few days, leads to the preservation of the normally transient retinal projection to the lateral posterior nucleus, presumably by reducing competition between axon terminals.

Animals↗
PubMed1982

The axo-axonic interneuron in the cerebral cortex of the rat, cat and monkey.

The synaptic connections of a specific type of identified cortical interneuron, the axo-axonic cell, were studied using Golgi methods. In the light-microscope axo-axonic cells were demonstrated in certain layers of the primary and secondary visual cortex of rat, cat and monkey, in the motor cortex of cat and in the subiculum and pyriform cortex of rat. The dendrites originating from the oval soma were oriented radially in a lower and upper spray within a cylinder about 100-150 microns wide. Electron-microscopy of Golgi impregnated, gold-toned axo-axonic cells showed predominantly but not exclusively asymmetrical synaptic contacts on their dendrites and spines, few synaptic contacts on the perikarya some of which were asymmetrical, and no synaptic contacts on the axon initial segment. The axon usually arborized within the vicinity of the cell's own dendritic field in an area 100-200 microns in diameter. In the kitten motor cortex the axon of a neuron in layer III descended to layer VI, providing a columnar arborization. The axon formed specialized, 10-50 microns long terminal segments invariably oriented parallel with the axon initial segment of pyramidal cells. All 85 identified symmetrical-type synaptic contacts, deriving from 31 specialized terminal segments, were found exclusively on the axon initial segment of pyramidal neurons. Rare, lone boutons of axo-axonic cells also made synaptic contact only with axon initial segments, confirming the exclusive target specificity of these cells. In identified gold-toned boutons, flattened pleomorphic vesicles were present. Electron-microscopy showed that axons ending in specialized terminal segments may originate from myelinated fibres, indicating that Golgi impregnation has revealed only part of the axon. Counting of axon terminal segments, each of which was in contact with the axon initial segment of a pyramidal neuron, revealed 166 pyramidal neurons receiving input from a partially reconstructed axo-axonic cell in the motor cortex of the kitten, and 67 from another cell in the visual cortex of the cat. The convergence of five axo-axonic cells onto one pyramidal cell was demonstrated in the striate cortex of the cat by counting all synaptic contacts on three initial segments. Cells from a one-month-old kitten were compared with those of the adult. The axon of the developing neurons was more diverse, having many growth cones and filopodia which made no specialized membrane contacts. However, the developing specific terminal segments formed synapses only with axon initial segments. It is concluded that the presence of axo-axonic cells in all the species and cortical areas we have examined suggests their association with the structural design of pyramidal cells, wherever the latter occur, and with their participation in the information processing of pyramidal cells. Axo-axonic cells are uniquely endowed with the means of simultaneously influencing the action potential at the site of origin in groups of pyramidal cells...

Animals↗
PubMed1981

Combined Golgi and electron microscopic study on the synapses formed by double bouquet cells in the visual cortex of the cat and monkey.

The morphology of certain Golgi-stained cells was examined in the striate and peristriate cortex of the cat and in the striate cortex of the rhesus monkey. Neurons in layer III were selected on the basis of their characteristic vertical axon bundles, which are 20-150 microns in diameter and traverse layers II-V. Selected neurons were examined under the electron microscope to characterize their synapses and to establish their postsynaptic targets. It was found that double bouquet cells form symmetrical or type II synapses. In the cat the postsynaptic membrane specialization was more extensive than in the monkey. After removing the Golgi precipitate from boutons of two cells in the cat, small pleomorphic and flattened vesicles were found in the boutons. Earlier suggestions that double bouquet cells make synapses preferentially with spines of apical dendrites could not be confirmed. Out of 66 boutons in area 17 of the cat, 86.4% formed synapses with dendritic shafts, many of them belonging to nonpyramidal cells, 9% with perikarya of nonpyramidal cells, and only 4.6% with spines. Out of 19 synapses examined in area 18, 74% were contacting dendritic shafts and the rest contacted spines. In the monkey 60% of a total of 35 double bouquet cell synapses made synapses with dendritic shafts. A different type of double bouquet cell with densely spiny dendrites is also described in layer IV of the monkey striate cortex. This neuron formed asymmetrical synapses. It is suggested that layer III double bouquet cells with vertical axon bundles are probably inhibitory and act on other nonpyramidal cells and certain parts of pyramidal cells.

Animals↗
PubMed1981

The morphological correlates of X- and Y-like retinal ganglion cells in the retina of monkeys.

The morphology of the ganglion cells of the monkey's retina was revealed by filling the cells with horseradish peroxidase from their cut axons in the optic nerve. This procedure gave much more consistent results than the Golgi method, was much quicker and filled dendrites just as extensively. Quantitative measures of the dendritic tree of two types of ganglion cell, P alpha and P beta, suggest that they correspond to the physiologically defined Y- and X-cells, respectively.

Animals↗