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C Blakemore

Publications and source records attributed to C Blakemore.

At least 55 records · Page 3Linked to original sources
PubMed1995

Analysis of connectivity in the cat cerebral cortex.

The mammalian cerebral cortex is innervated by a large number of corticocortical connections. The number of connections makes it difficult to understand the organization of the cortical network. Nonetheless, conclusions about the organization of cortical systems drawn from examining connectional data have often been made in a speculative and informal manner, unsupported by any analytic treatment. Recently, progress has been made toward more systematic ways of extracting organizing principles from data on the network of connections between cortical areas of the monkey. In this article, we extend these approaches to the cortical systems of the cat. We collated information from the neuroanatomical literature about the corticocortical connections of the cat. This collation incorporated 1139 reported corticocortical connections between 65 cortical areas. We have previously used an optimization technique (Scannell and Young, 1993) to analyze this database in order to represent the connectional organization of cortical systems in the cat. Here, we report the connectional database and analyze it in a number of further ways. First, we employed rules from Felleman and Van Essen (1991) to investigate hierarchical relations among the areas. Second, we compared quantitatively the results of the optimization method with the results of the hierarchical method. Third, we examined quantitatively whether simple connection rules, which may reflect the development and evolution of the cortex, can account for the experimentally identified corticocortical connections in the database. The results showed, first, that hierarchical rules, when applied to the cat visual system, define a largely consistent hierarchy. Second, in both auditory and visual systems, the ordering of areas by hierarchical analysis and by optimization analysis was statistically significantly related. Hence, independent analyzes concur broadly in their ordering of areas in the cortical hierarchies. Third, the majority of corticocortical connections, and much of the pattern of connectivity, were accounted for by a simple "nearest-neighbor-or-next-door-but-one" connection rule, which may suggest one of the mechanisms by which the development of cortical connectivity is controlled.

Animals↗
PubMed1994

Interocular control of neuronal responsiveness in cat visual cortex.

Neurons in the cat primary visual cortex are selective for particular contour orientations but their responsiveness can vary under certain conditions. After prolonged stimulation (adaptation), the contrast sensitivity of cortical cells is reduced and the 'gain' (the strength of response as a function of contrast) falls. The response to an optimal contour is also reduced when a different stimulus is superimposed on the receptive field in the same eye. Here we report that the sudden appearance of an inappropriate stimulus in one eye can interocularly suppress the activity of cortical neurons if they are already responding to an optimally oriented stimulus in the other eye. In strabismic cats, whose cortical neurons lack binocular facilitation, even contours of similar orientation shown to the two eyes trigger such suppression. This interocular control of cortical responsiveness could serve to veto signals from one eye under conditions that would otherwise cause double vision and perceptual confusion.

Animals↗
PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. I. Correlation among cell morphology, intrinsic electrophysiological properties, and axon targets.

Previous work has established two structure/function correlations for pyramidal neurons of layer 5 of the primary visual cortex of the rat. First, cells projecting to the superior colliculus have thick apical dendrites with a florid terminal arborization in layer 1, whereas those projecting to the visual cortex of the opposite hemisphere have thinner apical dendrites that terminate below layer 1, without a terminal tuft (e.g., Hallman et al.: J Comp Neurol 272:149, '90). Second, intracellular recording combined with dye injection has revealed two classes of cells: the first has a thick, tufted apical dendrite and fires a distinctive initial burst of two or more impulses, of virtually fixed, short interspike interval, in response to current injection; and the other, with a slender apical dendrite lacking a terminal tuft, tends to have a longer membrane time constant and higher input resistance, and does not fire characteristic bursts (e.g., Larkman and Mason: J Neurosci 10:1407, '90). The present study combined intracellular recording in isolated slices of rat visual cortex and injection of carboxyfluorescein, to reveal soma-dendritic morphology, with prior injection of rhodamine-conjugated microspheres into the superior colliculus or contralateral visual cortex to label neurons according to the target of their axons. This permitted a complete correlation of morphology, intrinsic electrophysiological properties, and identity of the projection target for individual pyramidal cells. Neurons retrogradely labeled from the opposite visual cortex were found in all layers except layer 1 while those labeled from the superior colliculus lay exclusively in layer 5. Within layer 5 interhemispheric cells were more concentrated in the lower half of the layer but extensively overlapped the distribution of corticotectal cells. Every cell studied that projected to the superior colliculus was of the bursting type and had a thick apical dendrite with a terminal tuft. Every cell in this study projecting to the opposite visual cortex was a "nonburster" and had a slender apical dendrite with fewer oblique branches that ended without a terminal tuft, usually in the upper part of layer 2/3. Interhemispheric cells also had rounder, less conical somata and generally had fewer basal dendrites than corticotectal neurons. Many cells with the physiological and morphological characteristics of interhemispheric cells were not back-labeled from the opposite visual cortex, implying that pyramidal cells of this type can have other projection targets (e.g., other cortical sites in the ipsilateral hemisphere).(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties.

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. III. Differential maturation of axon targeting, dendritic morphology, and electrophysiological properties.

This paper describes the early morphological and physiological development of pyramidal neurons in layer 5 of the rat visual cortex in relation to the targets chosen by their axons. Cells were prelabeled by retrograde transport from the superior colliculus or the contralateral visual cortex and intracellularly injected either in fixed slices or after recording in living slices. In the adult, corticotectal cells have thick apical dendrites with an extensive terminal arborization extending into layer 1, and fire characteristic bursts of action potentials when injected with a depolarizing current; interhemispheric cells have slender apical dendrites that terminate without a terminal tuft, usually in layer 2/3, and they display a more regular firing pattern (Kasper et al.: J Comp Neurol, this issue, 339:459-474). At embryonic day E18 (when axons of the two classes of cells are already taking different routes towards their targets) and E21, pyramidal-like cells throughout the cortical plate all have similar soma-dendritic morphology, with spindle-shaped cell bodies and few, short basal dendrites but apical dendrites that all end in distinct tufts in the marginal zone. At postnatal day P3, after the axons of both cell classes have reached their targets, all pyramidal neurons in layer 5 still have distinct terminal arborizations in layer 1, though they vary in complexity and extent. The somata are now more mature (round to ovoid in shape), and the basal dendritic tree has extended. As early as P5, all cells studied could be clearly classified as tufted or untufted (considerably earlier than previously reported; Koester and O'Leary: J Neurosci 12:1382, '92), and these features correlated precisely with the projection target, as in the adult. Measurement showed that although interhemispheric cells lose their terminal tufts, in general the trunks of their apical dendrites do not withdraw but continue to grow, at roughly the same rate as those of corticotectal cells. The two classes of layer 5 pyramidal neurons differentiate from each other in three distinct phases: pathway selection by axons precedes the loss of the apical tuft by interhemispheric cells, and these morphological characteristics are established 10 days before the onset of burst-firing in corticotectal cells. These three steps may be guided by different molecular signals.

Action Potentials↗
PubMed1994

Functional organization of corticocortical projections from area 17 to area 18 in the cat's visual cortex.

We used anatomical and physiological methods to study the functional organization of the association projection from area 17 to area 18 in the cat's visual cortex. Neurons in area 17 projecting to area 18 (revealed by retrograde transport of fluorescent tracer) tend to be clustered over regions of layer 4 receiving input from the ipsilateral eye (visualized by anterograde transneuronal tracing). Since the contralateral input overlaps these ipsilateral patches, the association cells lie preferentially in regions that are likely to be binocularly innervated. Indeed, almost all cells recorded electrophysiologically within the association clusters were strongly binocular, whereas between the clusters, many neurons were dominated by the contralateral eye. There is sufficient jitter in the retinotopic organization of area 17 for the discontinuous distribution of association cells to provide a continuous representation of the visual field. Cells in each association cluster in the rostral part of area 17 project divergently to innervate a zone extending up to 3 mm wide, anteroposteriorly, in the superficial layers of area 18. The receptive fields of cells at any point in area 18 are larger than for the corresponding point in area 17. Neurons recorded at two points in area 18, separated by a distance equal to the limit of anatomical divergence of the projection from area 17, have receptive fields that overlap by an amount similar to the region of visual field covered by the receptive fields of cells in a single association cluster in area 17 at a similar retinotopic position. Thus, area 18 receives a full and strongly binocular representation of the visual field not only from the lateral geniculate nucleus but also from area 17. The divergence of the area 17 to 18 projection compensates for the difference in receptive field size by ensuring that the receptive fields of each cluster of projecting neurons overlap fairly precisely those of the recipient neurons in area 18.

Amidines↗
PubMed1994

Postnatal development and plasticity of corticocortical projections from area 17 to area 18 in the cat's visual cortex.

We used retrogradely transported fluorescent tracers to study the development of projections from area 17 to area 18 in normal and monocularly deprived kittens. In newborn animals, cells in area 17 that were labeled from small, discrete injections in area 18 were concentrated around the retinotopically corresponding zone, but distributed with lower density over a very wide surrounding area. Hence, the total convergence and divergence of the projection were initially enormous, but they decreased dramatically, mainly during the first postnatal month, through elimination of the sparse, widespread distribution of projections. Injections of two different tracers close together in area 18 produced very few double-labeled cells in area 17 at any age, implying that most individual axons arborize over very small territories even at birth. In normal kittens the peak density of association cells in the upper layers, corrected for the overall expansion of the cortex, doubled over the first postnatal month and then declined gradually over the following several months, presumably because of continuing selection and elimination. As shown in previous work (Price and Blakemore, 1985a), area 17 to 18 cells in newborn kittens were distributed in two continuous bands in supragranular and infragranular layers. During normal maturation, elimination of projections results in the formation of distinct clusters; these lie preferentially in the upper layers above patches of ipsilateral eye input to layer 4 (Price et al., 1994). Monocular deprivation, which causes the terminal patches representing the deprived eye to become much smaller than normal, did not stop the normal decrease in overall convergence/divergence or the appearance of clusters of association cells, but the clusters were distinctly larger than normal in both hemispheres. Monocular deprivation also prevented the normal reduction in density of association cells within clusters after 1 month of age. Comparison with results from binocularly deprived animals, where clusters also form but association cell density is low, suggests that the size of clusters and the density of association cells retained depend on the overall level of cortical activity.

Aging↗
PubMed1994

Interocular suppression in the visual cortex of strabismic cats.

Strabismic humans usually experience powerful suppression of vision in the nonfixating eye. In an attempt to demonstrate physiological correlates of such suppression, we recorded from the primary visual cortex of cats with surgically induced squint and studied the responses of neurons to drifting gratings of different orientation, spatial frequency, and contrast in the two eyes. Only 1 of 50 apparently monocular cells showed any evidence of remaining, subliminal excitatory input from the "silent" eye when the two eyes were stimulated with gratings of similar orientation, and even among the small proportion of cells that remained binocularly driven, very few exhibited facilitation when stimulated binocularly. The majority of cells from both exotropes and esotropes, even those that could be independently driven through either eye, displayed nonspecific interocular suppression: stimulation of the nondominant eye with a drifting grating of any orientation depressed the response to an optimal grating being presented to the dominant eye. This phenomenon exhibited a gross nonlinearity in that it was dependent on the temporal sequence of stimulus presentation: stimulation of the nondominant eye caused significant suppression only if the neuron was already responding to an appropriate stimulus in the dominant eye, but not when onset of stimulation in the two eyes was simultaneous. Interocular suppression was always independent of the relative spatial phase of the two grating stimuli, and usually broadly tuned for the spatial frequency of the suppressive stimulus. Suppression may depend on inhibitory interaction between neighboring ocular dominance columns, combined with the loss of conventional disparity-selective binocular interactions for matched stimuli in the two eyes. The similarity of interocular suppression in strabismic cats and that caused by orthogonal gratings in the two eyes in normal cats (Sengpiel and Blakemore, 1994; Sengpiel et al., 1994) suggests that strabismic suppression and binocular rivalry depend on similar neural mechanisms.

Animals↗
PubMed1993

Effects of neonatal ablation of area 18 on corticocortical projections from area 17 to extrastriate visual areas in cats.

Using retrogradely transported fluorescent dyes, we investigated the distributions of corticocortical neurones in area 17 projecting to area 19 and to the lateral suprasylvian visual area in a normal cat and two cats in which area 18 had been ablated at 4 days old. Cells projecting to area 19 were distributed in a much more continuous manner in the lesioned cats than in the normal one, whereas those projecting to the lateral suprasylvian cortex were located in discrete patches in all three cats. Neonatal ablation of area 18 can thus prevent the normally occurring developmental removal of neurones or their axons projecting from area 17 to area 19, but not the removal occurring in the projection from area 17 to the lateral suprasylvian cortex.

Animals↗
PubMed1993

Single-fibre EPSPs in layer 5 of rat visual cortex in vitro.

Simultaneous intracellular recordings were made from pairs of synaptically connected layer 5 pyramidal neurones less than 150 microns apart, in slices of rat visual cortex. Action potentials were evoked in one cell of a pair by depolarizing current pulses, while spike-triggered averaging was employed to reveal connections in the other cell and vice versa. Only 4 excitatory postsynaptic potentials (EPSPs) (0.7-1.2 mV) were recorded out of 270 cell pairs tested, equivalent to a connection probability of less than 0.02. Therefore, single-fibre EPSPs between pyramidal cells in layer 5 are less frequent, but individually larger in amplitude, than those found previously between pyramidal cells in layer 2/3 of rat visual cortex.

Animals↗
PubMed1993

Plasticity of an aberrant geniculocortical pathway in neonatally lesioned cats.

Geniculocortical afferents to the lateral suprasylvian (LS) visual area come only from the C layers in normal adult cats. After neonatal lesions of areas 17 and 18, inputs to the LS area also arise from the A layers. We studied the plasticity of this aberrant pathway. We made lesions in areas 17 and 18 of newborn kittens, some of which were then monocularly deprived. In lesioned undeprived cats, retrograde tracing confirmed projections to the LS area from both the A and A1 as well as the C geniculate layers. In lesioned deprived cats, geniculate afferents from the A layers to the LS area came mainly from the layer receiving a normal visual input. We conclude that the development of the abnormal pathway from the A layers to the LS area is influenced by patterned visual activity.

Animals↗
PubMed1993

Modulation of EPSP shape and efficacy by intrinsic membrane conductances in rat neocortical pyramidal neurons in vitro.

1. Intracellular recordings were made from pyramidal neurons in layers II/III and V of rat visual cortical slices. Distal and proximal excitatory postsynaptic potentials (EPSPs) were evoked using extracellular bipolar electrodes placed on the slice horizontal to each cell, near the apical and basal dendrites respectively. Experiments were conducted in the presence of 2-amino-5-phosphonopentanoate, picrotoxin and, in most cases, 2-hydroxy-saclofen. 2. For layer II/III pyramidal neurons, voltage undershoots following distal and proximal EPSPs (n = 7 pairs) and injected somatic pulses were rarely apparent. In layer V pyramidal neurons substantial voltage undershoots were recorded following distal and proximal EPSPs (n = 27 pairs) and injected somatic pulses, with undershoot being greatest for apical inputs (P = 0.001). The greater undershoots following apical EPSPs were also apparent in semilogarithmic plots of voltage decay where the slope of decay for apical EPSPs was quicker than the voltage decay following pulses of current injected at the soma. There was no significant difference in the shapes of distal and proximal EPSPs in layer II/III or layer V pyramidal cells under control conditions. 3. Pharmacological agents were used to reduce voltage undershoots. The most successful of these was alinidine, a putative blocker of the slow inward rectifier (IH) conductance. In the presence of bath-applied 100 microM alinidine, undershoots were significantly reduced and it became possible to distinguish the relative origins of EPSPs on the basis of their shape. Distally generated EPSPs (n = 14) had rise times and half-widths that were 2.8 and 1.5 times longer respectively than those evoked proximally (n = 10; P = 0.001 for both parameters). 4. These results confirm previous theoretical simulations of somatic recordings in passive model neurons where distal EPSPs display slower rise times and longer half-widths than proximal EPSPs. The present results suggest that, at least in pyramidal neurons of layer V, distal synaptic inputs can be specifically modulated by intrinsic membrane conductances.

Animals↗
PubMed1992

EPSPs in rat neocortical pyramidal neurones in vitro are prolonged by NMDA receptor-mediated currents.

We investigated the influence of 2-amino-5-phosphonovalerate (APV), a selective antagonist of the N-methyl-D-aspartate (NMDA) receptor, on the time course of small excitatory postsynaptic potentials (EPSPs) in pyramidal neurones in layer 2/3 of adult rat visual cortex in vitro. Time constants of the voltage decay following the EPSPs (T(s)) and after brief (2 ms) pulses of current injected at the soma (T(p)) were determined from semilogarithmic plots of averages of 100-250 trials. The mean T(s)/T(p) ratio decreased from 1.53 +/- 0.29 (S.D.) to 1.10 +/- 0.08 on addition of 50 microM APV to the bathing medium (P less than 0.001; n = 23), but there was no significant change in EPSP peak amplitude or rise-time. These results suggest that the time course of many small EPSPs, even at negative membrane potentials and in the presence of Mg2+, can be prolonged by NMDA receptor-mediated currents.

2-Amino-5-phosphonovalerate↗
PubMed1991

Lack of regional specificity for connections formed between thalamus and cortex in coculture.

The mammalian cerebral cortex consists of many structurally and functionally specialized areas, with characteristic input from particular nuclei of the thalamus. Some localized external influence, such as the arrival of fibres from the appropriate thalamic nucleus before or around the time of birth, could trigger the emergence of committed cortical fields from an undifferentiated 'protocortex. The guidance of axons from each thalamic nucleus to its appropriate target area in the cortex could, then, be crucial in the regulation of cortical differentiation. Recently, Yamamoto et al. and Bolz et al. have demonstrated that cocultured explants of rat lateral geniculate nucleus and visual cortex can form layer-specific interconnections. We have now tested the possibility that each cortical area exerts a selective trophic influence on axons from its appropriate thalamic nucleus, and vice versa, by coculturing explants of different regions of the thalamus and cortex taken at various stages of development. Although thalamo-cortical and cortico-thalamic connections formed in vitro can be remarkably normal in many respects, they lack regional specificity.

Animals↗
PubMed1991

Sensitive and vulnerable periods in the development of the visual system.

In advanced mammals the visual system consists of a number of parallel channels for the efficient processing of different aspects of the visual scene. Much of the basic anatomical structure of the visual pathway is constructed before birth. A wave of maturation sweeps through the system, from the eye to the visual cortex, the correct formation of connections depending on precisely timed interactions between axons and their targets. Competition between growing axons (apparently dependent on spontaneous impulse activity in those axons), cell death (partly influenced by competition between those cells' axons), axon withdrawal, trophic interactions--these and other mechanisms play a part in constructing the visual pathway and laying down basic 'maps' of the visual field before birth. Disturbances in such processes might underlie disorders of the genesis of the nervous system. At the level of the visual cortex, synaptic plasticity continues after birth and may permit cortical neurons to refine their processing capacities on the basis of information provided by the visual environment. This makes the young animal vulnerable to disturbances of visual experience early in life, which can cause virtually irreversible deficits in stereoscopic vision, visual resolution and sensitivity to contrast (amblyopia) in adult life.

Animals↗