Biomedical subjects

A Cowey

Publications and source records attributed to A Cowey.

At least 127 records · Page 7Linked to original sources

PubMed1989

Residual target detection as a function of stimulus size.

In the visual field defects of 5 patients with damage in the postgeniculate part of the primary visual system, residual detectability of an achromatic intensity target was tested as a function of target diameter, which was 10, 27, 44, 69 and 116 min of arc of visual angle. The set of 5 targets was presented at 3 different intensity ranges on a background of low photopic luminance. Within this size range residual detectability was a V-shaped function of target size with a dip at 44 min of arc. This result is discussed in relation to the centre/surround organization of receptive and perceptive fields.

Adult↗

PubMed1989

Interlaminar and lateral excitatory amino acid connections in the striate cortex of monkey.

The intrinsic excitatory amino acid pathways within the striate cortex of monkeys were studied by autoradiographic detection of retrogradely labeled somata following microinjections of D-3H-aspartate (D-3H-Asp) into different layers. The labeled amino acid was selectively accumulated by subpopulations of neurons and, to a small extent, by glial cells, the latter mainly in the supragranular layers. Immunocytochemical detection of neurons containing GABA showed that, apart from a few cells exclusively in layer I, GABAergic neurons do not accumulate D-3H-Asp. Several lines of evidence suggest that D-3H-Asp uptake occurred only at nerve terminals; thus, the pattern of perikaryal labeling allowed the delineation of interlaminar and lateral projections. Neurons in layer I probably project laterally, and layer I receives wide-ranging projections from layer IVB and layer V from cells up to 1300 microns laterally. Some neurons in layer II send a focused projection to lower layer VI. Some neurons in layers II/III project up to 1 mm laterally within their own layer, but relatively few neurons can be labeled in these projections. Similarly, in layers II/III few neurons can be retrogradely labeled from layers V and upper VI, and this projection is organized such that cells closer to the pia project deeper in layer V/VI. The connections of layer IVA could not be revealed separately because of the difficulty of confining injections to this thin sublamina. Neurons in layer IVB project up to 1300 microns within IVB itself. A small number of cells from IVB also project to layers III, IVC-alpha, V, and VI with much more restricted lateral spread. Neurons in upper IVC-alpha send axons to layer IVB with at least 600-800 microns lateral spread. Neurons in lower IVC-alpha/upper IVC-beta project to layer III with at least 300-500 microns lateral spread. The bottom 50-80 microns of layer IVC-beta contains neurons with a very focused projection, apparently exclusively to the layer III/IVA border region. Both layers IVC alpha and beta have rich connections within themselves, the beta sublayer having more restricted lateral connections. Some neurons in layer IVC-beta give a laterally restricted small input to layers IVC-alpha and IVB. Both IVC-alpha and -beta project to layers V and VI, and these projections are spread at least 400 microns laterally. Neurons in layer V project to all layers, but the projection to layers I-III and within layer V itself spread much further laterally than the projections to layers IV and VI.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acids↗

PubMed1988

Segregation of functionally distinct axons in the monkey's optic tract.

The classical neuro-ophthalmologic literature describes the organization of the primate's optic tract as containing a single topographic representation of the complete contralateral visual hemifield. In contrast, cats have separate visual field representations for the optic axons of the functionally distinct retinal ganglion cell classes. As the line of decussation for each ganglion cell class in the cat occupies a different location on the retinal surface, whereas in primates they are all superimposed, such a species difference might be expected. We report that implants of horseradish peroxidase placed in either the deep or superficial extremes of the monkey's optic tract produce retrograde labelling of distinct retinal ganglion cell classes, and produce anterograde labelling confined to distinct laminae of the lateral geniculate nucleus. Hence, the optic tract of the primate cannot contain a single representation of the contralateral visual hemifield; rather, independent visual field representations for the functionally distinct optic axons must exist. Their anatomical segregation may account for the clinical observation of selective impairments of distinct visual abilities following partial interruption of the optic tract in man.

Animals↗

PubMed1988

The involvement of the temporal lobes in colour discrimination.

Monkeys with ablation of lateral striate, prestriate or inferotemporal cortex were compared with unoperated controls in their ability to discriminate Munsell colours, or greys, of increasing difficulty. Whereas lateral striate or prestriate lesions centred on visual area V4 mildly impaired only the most difficult discriminations, inferotemporal ablation resulted in a severe impairment in the acquisition of colour discriminations. However, the ability to discriminate greys was much less affected.

Animals↗

PubMed1988

Contrast sensitivity in rats with increased or decreased numbers of retinal ganglion cells.

Contrast thresholds were measured electrophysiologically on striate cortex in normal rats and in rats in which either the superior colliculi were removed bilaterally or unilaterally at 5 days of age, or one eye was removed on the day of birth. Despite the fact that the collicular ablation leads to the degeneration of more than half the retinal ganglion cells, contrast sensitivity was normal in this group, with the possible exception of sensitivity at very low spatial frequencies below 0.1 c/deg. The result is strong evidence that retinal ganglion cells which project to thalamus as well as to mid-brain escape the degenerative effects of neonatal mid-brain lesions. The contrast sensitivity of neonatally operated one-eyed rats was significantly and substantially better than that of normal rats tested monocularly. The increased sensitivity was greatest in the cortex ipsilateral to the remaining eye. This supernormal sensitivity is presumably related to the increase in the number of ganglion cells in the remaining eye, especially those projecting ipsilaterally from the temporal retina and which show a five-fold expansion of their terminal zone in the thalamus.

Animals↗

PubMed1988

The lengths of the fibres of Henle in the retina of macaque monkeys: implications for vision.

In Golgi preparations of retinae from macaque monkeys the lengths of the fibres of Henle from photoreceptors, and Müller's fibres were measured. It was shown that the lengths of Müller's fibres provide a good estimate of the lengths of adjacent fibres of Henle of photoreceptors. The fibres form a radiate pattern with respect to the fovea. They are longest at the fovea and their length decreases in a systematic way with distance from the fovea. The implications of the fibre length are considered with respect to the relationship between the ganglion cell distribution and central magnification factors. We show that even when the functional offset introduced by the fibres of Henle and by bipolar and ganglion cells is taken into account there is not a constant proportional relationship between ganglion cells and central magnification factors. The representation of the central few degrees of the visual field on the striate cortex is greater than would be predicted on the basis of the ganglion cell density for the central retina.

Animals↗

PubMed1987

On the role of posterior parietal and prefrontal cortex in visuo-spatial perception and attention.

Monkeys with bilateral removal of caudal prefrontal cortex (area 8) or dorso-lateral parietal cortex (areas 5 and 7) or the inferior temporal cortex (area TE) were presented with two versions of a go-left/go-right visuo-spatial discrimination. In the first task they had to displace either the left or right of two identical plaques according to whether both plaques were black or white respectively. There were no performance differences among groups. In the second task, the two plaques were always red and the appropriate response was indicated by whether a spatially remote third plaque was black or white. Both the prefrontal and parietal groups were impaired relative to the inferotemporal group on this task. The results indicate that whether an impairment occurs on a task that is thought to test the perception of egocentric space may depend on whether the animal has to notice and attend to a remote cue, and that an attentional disorder may also explain impairments reported on tests of allocentric perception where the critical cue is spatially remote from the response site.

Animals↗

PubMed1987

Effects on visual search of lesions of the superior colliculus in infant or adult rats.

The superior colliculus was removed from rats at either one or five days of age or in maturity. Four months later they were tested on two versions of a visual search task. Experiment 1 required animals to retrieve food pellets concealed in a depression in the top of identical narrow pillars arranged in an arena. Rats with lesions of the superior colliculus, regardless of the age at operation, showed a large number of 'return' errors compared with sham-operated controls. Return errors were defined as occasions on which the animal returned to pillars that had previously been visited on that trial, before every pillar had been visited at least once. Experiment 2 compared the ability of infant- and adult-operated animals to detect and locate a single, baited white pillar in an array of black ones. There were no group differences in response latencies to targets presented in the rostral visual field (within 40 degrees of the midline). However, animals operated on in adulthood or at 5 days of age were slower than both sham-operated animals and animals operated on at one day of age in their responses to more peripheral targets. The latter two groups were indistinguishable.

Age Factors↗

PubMed1987

Contrast sensitivity and visual acuity of the pigmented rat determined electrophysiologically.

The contrast sensitivity function of the rat was assessed by investigating the relationship between the amplitude of visually evoked cortical potentials (VECP) and the spatial frequency and contrast of grating stimuli. Pattern reversal VECPs were recorded in Area 17 in the region of representation of the central binocular visual field. Transient responses were obtained with a 1 Hz contrast reversal. The mean contralateral monocular CSF peaked around 0.1 c/deg, with a threshold sensitivity of 20-25, low frequency attenuation and a high frequency cut-off of 1.18 c/deg. The mean binocular CSF showed a cut-off of 1.20 c/deg, which matches several behavioural measurements of visual acuity. The greater binocular sensitivity in the low frequency range (0.04 c/deg) could be tentatively attributed to the greater influence of the population of large ganglion cells that reaches its maximal density in the ipsilateral projection.

Animals↗

PubMed1987

The crossed projection from the temporal retina to the dorsal lateral geniculate nucleus in the rat.

The crossed projection from the temporal crescent in the rat's retina was studied by producing a discrete retinal lesion in one eye and examining the dorsal lateral geniculate nucleus and superior colliculus contralateral to the lesion for anterograde degeneration products. The position of this crossed degeneration was described in relation to the uncrossed retinal termination in the same structures by injecting the opposite eye with [3H]proline and processing the tissue for autoradiography. The location of the retinal lesion in relation to the temporal crescent was identified by injecting the dorsal lateral geniculate nucleus ipsilateral to the lesioned eye with a fluorescent tracer, to retrogradely label the ipsilaterally projecting retinal ganglion cells in the lesioned eye. Retinal lesions that were histologically verified to be restricted to the temporal crescent produced crossed degeneration in the superior colliculus at its rostral border, in accord with this projection's published visual topography. These same lesions consistently yielded a very circumscribed and sparse amount of degeneration in the contralateral dorsal lateral geniculate nucleus at its dorsomedial border, abutting the optic tract dorsally and the lateroposterior nucleus medially. The degeneration bore no consistent relationship to the position of the uncrossed retinal terminal field, which is situated further ventrally in the dorsal lateral geniculate nucleus; rather, this crossed temporal projection terminated in the outer shell of the nucleus along its medial border. This crossed temporal retinogeniculate projection, together with the crossed projection from nasal retina, forms a continuous map of the complete contralateral retina in the outer shell of the dorsal lateral geniculate nucleus, likely to arise from a population of retinal ganglion cells possessing small soma sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

A case study of cortical colour "blindness" with relatively intact achromatic discrimination.

A patient is described whose most striking visual disorder was a grossly impaired ability to discriminate between different colours (hues) that were matched for brightness. In contrast his ability to discriminate between different neutral greys presented in the same fashion was much less abnormal, even though the greys were perceptually difficult. Although visual acuity was reduced and visual fields were constricted, and the patient's memory was moderately impaired, these associated symptoms could not themselves be the cause of his unusual colour vision. The patient had the symptoms of cerebral achromatopsia, and the relative preservation of his form vision (when his reduced acuity is taken into account) and his achromatic vision supports the view that the many different visual cortical areas recently demonstrated in the brains of monkeys, and presumed to exist in man, have a perceptual specialisation that matches their physiological differences.

Adult↗

PubMed1987

On the role of cortical area V4 in the discrimination of hue and pattern in macaque monkeys.

Cortical visual area V4 in macaque monkeys has a large proportion of neurons that are sensitive to the wavelength or to the color of light. We tested its role in hue discrimination by removing it in macaque monkeys trained to discriminate small differences in hue. Hue discrimination thresholds were permanently elevated in 4 macaque monkeys in which V4 was removed bilaterally. In contrast, there was no impairment in achromatic intensity thresholds tested in an identical manner. However, the discrimination of pattern and orientation was also conspicuously impaired, indicating that area V4 is not concerned solely with processing information about wavelength. The multiple defect is consistent with evidence that V4 provides the major cortical visual input to the temporal lobe, where a large range of visual properties is registered. The performance of monkeys with V4 ablation was compared with that of unoperated control monkeys and monkeys with removal of cortex in the banks and floor of the rostral superior temporal sulcus (STS). Removal of STS had only slight effects on pattern discrimination and none of hue discrimination. To control for the possible effects of inadvertent damage to the visual radiations when removing V4, the lateral striate cortex was partially ablated bilaterally in a control monkey. This had no effect on any discrimination, despite producing more retrograde damage to the lateral geniculate nuclei than in any monkey with V4 ablation. The visual disorder following removal of visual area V4 strikingly resembles the clinical disorder of mild cerebral achromatopsia with associated apperceptive agnosia for objects and patterns.

Animals↗

PubMed1986

The effects of pretectal and superior collicular lesions on binocular vision.

Monkeys with mid-brain lesions involving the pre-tectum and superior colliculi often have an odd stare, as if the visual axes were parallel and the animal was looking into the distance. Such a visuomotor abnormality could lead to double-vision for objects close to the animal. The experiments reported here were designed to test this hypothesis of diplopia in monkeys with combined bilateral ablations of the colliculi and frontal eye-fields (area 8). These animals performed better on a task of visually-guided reaching, and in a series of visual pattern discriminations, when they viewed stimuli monocularly rather than binocularly. Monkeys with other cortical lesions showed no such monocular superiority. We propose that an abnormality of vergence eye movements provides a simple explanation for some of the so-called perceptual impairments that follow damage to the mid-brain visual pathways in monkeys.

Animals↗

PubMed1986

The relationship between GABA immunoreactivity and labelling by local uptake of [3H]GABA in the striate cortex of monkey.

An antiserum to GABA was used in the macaque monkey to determine whether neurons that accumulate exogenously applied [3H]GABA in vivo are also immunoreactive for GABA. Following the injection of [3H]GABA into different laminae of striate cortex in two untreated animals and in one animal treated with amino-oxyacetic acid, selective accumulation of the labelled amino acid was demonstrated in perikarya by autoradiography. Radiographically labelled neurons (n, 519) and their unlabelled neighbours were tested in consecutive 0.5 micron thick sections by immunocytochemistry for GABA immunoreactivity. Injection of [3H]GABA did not increase the number of neurons showing GABA immunoreactivity. On the contrary many of the cells that accumulated [3H]GABA were immunonegative. These neurons were mostly located in layers IVC and VA following [3H]GABA injection into layers II-III, and in layers upper III and II following injection into layers V and VI. A comparison of the position of these neurons with known local projection patterns in the striate cortex of monkey suggests that GABA-immunonegative neurons may nevertheless become labelled by [3H]GABA if most of their local axon terminals fall within the injection site. The interlaminar projection of GABA-immunopositive neurons, which probably contain endogenous GABA, could be deduced from the position of the [3H]GABA injection site that leads to their autoradiographic labelling. Although the present study confirmed our previous results on the interlaminar connections of neurons that accumulate [3H]GABA, it demonstrated that [3H]GABA labelling alone may not be a sufficient criterion to assess the GABAergic nature of neurons in the striate cortex of monkey.

Animals↗

PubMed1986

Large retinal ganglion cells in the rat: their distribution and laterality of projection.

The distributions of the ipsilaterally and contralaterally projecting large ganglion cells in the retina of the rat were determined, using the retrograde transport of Horseradish peroxidase (HRP) following injections into one optic tract. Labelled large retinal ganglion cells occur throughout the contralateral retina and throughout the temporal crescent of the ipsilateral retina, but there is a noticeable decrease in their density in the contralateral retina's temporal crescent. This retinal region was identified in these same retinae by injecting a retrogradely transported fluorescent tracer into the optic tract opposite that receiving the HRP. The density of large retinal ganglion cells increases in both the contralateral retina and the ipsilateral temporal crescent in the upper temporal periphery such that, together, these two populations of large cells combine to produce a peak density centred on the retinal representation of the visual field's vertical midline. This peak density of large retinal ganglion cells must therefore be further peripheral than the peak density for the total population of retinal ganglion cells, since all evidence indicates that the latter is positioned nasal to the vertical midline's representation. This was verified in one rat, in which the density distribution of the total population of retinal ganglion cells was determined and compared with the distribution of the large cell population. The results suggest that the rat possesses a specialized retinal focus of large ganglion cells for viewing the visual field directly in front of the animal.

Animals↗

PubMed1986

The nature of the visual discrimination impairment after neonatal or adult ablation of superior colliculi in rats.

The superior colliculi were removed in rats at either one or five days of age or in adulthood. Seven months later they were tested on four successively presented two-choice intensity discriminations. The intensity difference between the discriminanda was reduced across the four problems to encourage choice by comparison. The purpose was to establish whether impoverished scanning is a feature of rats with collicular lesions and whether the age at which the lesion is incurred is important. The number of door-push and approach errors made in reaching criterion were used as measures of performance and the number of head-scans during acquisition was counted. The results provide no evidence that either one- or five-day operated rats exhibit sparing or recovery of the ability to scan discriminanda since all operated animals were impaired. Furthermore, novel retinal projections, present in one-day operated animals, fail to mediate such sparing. Finally, the results did not demonstrate a selective increase in approach errors following collicular lesions and were therefore inconsistent with the view that the impairment is one of visually-guided locomotion. It is concluded that visual discrimination learning is impaired following collicular lesions in circumstances where scanning of discriminanda is required for efficient performance.

Age Factors↗

PubMed1986

Synaptic relationships of a type of GABA-immunoreactive neuron (clutch cell), spiny stellate cells and lateral geniculate nucleus afferents in layer IVC of the monkey striate cortex.

The precise stimulus specificity of striate cortical neurons is strongly influenced by processes involving gamma-aminobutyric acid (GABA). In the visual cortex of the monkey most afferents from the lateral geniculate nucleus terminate in layer IVC. We identified a type of smooth dendritic neuron (clutch cell) that was immunoreactive for GABA, and whose Golgi-impregnated dendrites and axon were largely restricted to layer IVC beta. The slightly ovoid somata were 8-12 micron by 12-15 micron and the dendritic field was often elongated, extending 80-200 micron in one dimension. The axonal field was 100-150 micron in diameter and densely packed with large bulbous boutons. Although mainly located in IVC beta both the dendritic and axonal processes entered IVC alpha. Fine structural features of GABA-immunoreactive and-impregnated clutch cells and impregnated spiny stellate cells were compared. Clutch cells had more cytoplasm, more densely packed mitochondria and endoplasmic reticulum, and made type II as opposed to type I synapses. A random sample of 159 elements postsynaptic to three clutch cells showed that they mainly terminated on dendritic shafts (43.8-58.5%) and spines (20.8-46.3%), rather than somata (10-17%). The majority of the postsynaptic targets were characteristic of spiny stellate cells. This was directly demonstrated by studying synaptic contacts between an identified GABA positive clutch cell and the dendrites and soma of an identified spiny stellate cell. The termination of clutch cells mainly on dendrites and spines of spiny stellate cells suggests that they interact with other inputs to the same cells. Following an electrolytic lesion in the ipsilateral lateral geniculate nucleus we examined the distribution of degenerating terminals on three identified spiny stellate neurons in layer IVC beta. Out of eight synapses from the lateral geniculate nucleus one was on a dendritic shaft, the rest on spines. Only a small fraction of all synapses on the cells were from degenerating boutons. A clutch cell within the area of dense terminal degeneration was not contacted by terminals from the lateral geniculate nucleus. The results show that layer IVC in the monkey has a specialized GABAergic neuron that terminates on spiny stellate cells monosynaptically innervated from the lateral geniculate nucleus. Possible functions of clutch cells may include inhibitory gating of geniculate input to cortex; maintenance of the antagonistic subregions in the receptive fields; and the creation from single opponent of double colour opponent receptive fields.

Animals↗

PubMed1985

A comparison of the effects of superior collicular ablation in infant and adult rats.

Rats sustaining damage to the superior colliculus in infancy or adulthood and sham-operated controls were tested on two tasks requiring visual orienting to 'unexpected' and 'expected' stimuli, respectively. Experiment 1 measured the distracting effect of novel visual and auditory stimuli presented while the animals traversed a familiar runway. Control rats oriented to the distracting stimulus and had lengthened running times on the trials where it was presented. Rats with lesions of the superior colliculus were less distracted by the appearance of unexpected stimuli than sham operated controls but the severity of this effect depended on modality, and on age at operation. Experiment 2 examined the same animals' ability to detect and respond to a small and expected light that appeared at randomly determined positions around the perimeter of a chamber while the rat was at the centre. Filmed records and analyses of response latencies and errors showed that at low stimulus intensities, the active exploration shown particularly by infant-operated animals resulted in performance that was superior to that of adult-operated animals and, at least in terms of error rate, sham-operated controls. It is concluded that unusual behavioural strategies can explain all the differences between the animals operated at different ages. There was no evidence that novel projections from the retina were responsible.

Aging↗