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C Blakemore

Publications and source records attributed to C Blakemore.

At least 19 recordsLinked to original sources
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↗
PubMed1979

The development of stereoscopic mechanisms in the visual cortex of the cat.

It is argued that those neural systems (such as that responsible for stereoscopic vision) that have the greatest precision of operation are the most likely, during their developmental construction, to take advantage of infromation supplied by their own input. There is evidence that binocularly driven neurons in the kittens's visual cortex do indeed become modified in their synaptic organization during early visual experience in a manner that enhances the specificity of binocular interaction and ensures that the ranges of positional and orientational disparities of the receptive fields, within limits, become matched to the nature of the actual stimulation encountered by the animal.

Aging↗
PubMed1979

Projections to the visual cortex in the golden hamster.

Retrograde transport of horseradish peroxidase (HRP) was used to determine the origins of afferent connexions to the visual cortex (areas 17, 18a and 18b) in the hamster. The distribution of neurons projecting to the visual cortex from other cortical areas, from the thalamus and from the brainstem was studied using a computer technique for three-dimensional reconstruction. There is a topographically organized projection from the dorsal lateral geniculate nucleus to area 17, but probably to no other of the areas studied. The lateral posterior nucleus of the thalamus (LP) projects to area 18a and weakly to area 17. The lateral nucleus (L) projects to area 18b and also, probably, weakly to area 17. The cortical projections from LP and L are also organized topographically but relatively grossly compared with the geniculo-cortical pathway. There are reciprocal association projections between area 17 and areas 18a and 18b. Areas 18a projects weakly to 18b. The main commissural connexions of the posterior neocortex are between the area 17/18a boundary zones in the two hemispheres, with little between the bodies of area 17. Labelled neurons were found bilaterally in the locus coeruleus, more ipsilaterally than contralaterally, after multiple injections into the visual cortex: single, small injections sometimes resulted in the labelling of a single cell body in the locus coeruleus.

Animals↗
PubMed1979

Representation of reality in the perceptual world.

What contribution can neurobiologists make to the philosophical issues involved in the Theory of Knowledge? The sensory physiologist or psychologist must start with the assumption (albeit philosophically naive) that the biological function of sense organs is to act as transducers of genuine events in the outside world, and thus to contribute to an internal description of external reality. Although visual perception seems to its introspecting owner to be a unitary process, involving an integrated and complete description of all the properties of the visual scene, there is now good evidence that sensory processing of messages from the eyes involves a great deal of filtering of information and anatomically segregated analysis of such features as shape, colour, movement and distance. Sensory neurobiology can say little about the way in which conscious perceptions are synthesized from the heap of features into which the sensory signals are shattered, about how the inherent ambiguity of messages from sensory neurons is overcome, or about how the ultimate percepts are externalized. Finally, the evidence that the nature of the sensory world varies from species to species forces us to re-examine the theory of solipsism in biological terms.

Adaptation, Physiological↗
PubMed1978

Physiological basis of anisometropic amblyopia.

In the visual cortex of kittens that have received their only visual experience while wearing a high-power lens before one eye, most neurons are dominated by input from the normal eye. Moreover, contrast sensitivity and resolving power are lower for stimulation through the originally defocused eye, mimicking psychophysical results from human anisometropic amblyopes.

Amblyopia↗
PubMed1978

Modification of the kitten's visual cortex by exposure to spatially periodic patterns.

Kittens were dark-reared except for exposure to three types of spatially periodic, vertically striped pattern: 1. single, widely spaced black bars; 2. wide areas of regular vertical grating separated by large blank patches; 3. a uniform, continuous grating with a spatial frequency of 0.5 c/deg. In each case there was a bias towards vertical in the distribution of preferred orientations of cells recorded in the visual cortex. The contrast sensitivity of individual neurones for gratings of different spatial frequencies was analysed quantitatively. In kittens exposed to a uniform grating of 0.5 c/deg, many cells were maximally sensitive close to 0.5 c/deg, as they are in normal cats. The occipital potential evoked by vertical gratings higher in frequency than 0.3 c/deg was consistently greater in amplitude than that for horizontal, and a vertical grating of 0.5 c/deg produced the maximum activity. These results are compared with those of Maffei and Fiorentini (1974); the differences between our results and theirs may be attributable to the degree of variability in spatial frequency and orientation during rearing, and to the duration of exposure.

Animals↗
PubMed1978

The physiological effects of monocular deprivation and their reversal in the monkey's visual cortex.

1. 1127 single units were recorded during oblique penetrations in area 17 of one normal, three monocularly deprived and four reverse sutured monkeys. 2. In all animals most cells outside layer IV c were orientation-selective, and preferred orientation usually shifted from cell to cell in a regular progressive sequence. 3. The presence in layer IV c of non-oriented, monocularly driven units, organized in alternating right-eye and left-eye 'stripes' (LeVay, Hubel & Wiesel, 1975) was confirmed. 4. Early monocular deprivation (2--5 1/2 weeks) caused a strong shift of ocular dominance towards the non-deprived eye. However, even outside layer IV c, neural background and some isolated cells could still be driven from the deprived eye in regularly spaced, narrow columnar regions. In layer IV c the non-deprived eye's stripes were almost three times wider, on average, than the deprived. 5. Later monocular deprivation (11--16 months) had no detectable influence on layer IV c but seemed to cause a small shift in ocular dominance outside IV c. Deprivation for 6 1/4 months in an adult had no such effect. 6. After early reverse suturing (at 5 1/2 weeks) the originally deprived eye gained dominance over cells outside layer IV c just as complete as that originally exercised by the eye that was first non-deprived. 7. The later reverse suturing was delayed, the less effective was recapture by the originally deprived eye. Reversal at 8 weeks led to roughly equal numbers of cells being dominated by each eye; fewer cells became dominated by the newly open eye after reverse suturing at 9 weeks and most of them were non-oriented; reversal at 38 1/2 weeks had no effect. 8. Binocular cells, though rare in reverse sutured animals, always had very similar preferred orientations in the two eyes. The columnar sequences of preferred orientation were not interrupted at the borders of ocular dominance columns. 9. Even within layer IV c there was evidence for re-expansion of physiologically determined ocular dominance stripes. After early reverse suture, stripes for the two eyes became roughly equal in width. Possible mechanisms for these changes are discussed.

Action Potentials↗
PubMed1977

An attempt to assess the effects of monocular deprivation and strabismus on synaptic efficiency in the kitten's visual cortex.

The relative effectiveness of the two eyes in exciting cells in the visual cortex was assessed, using both natural stimulation and electrical stimulation of the optic discs. It is is argued that supramaximal electrical stimulation of the optic nerve could possibly reveal 'subliminal' synaptic inputs even after monocular deprivation or artificial strabismus has caused a loss of natural input from that eye, if such 'silent' synaptic input still survives. However, in kittens monocularly deprived for various periods of time or made artificially strabismic, there was usually excellent agreement between the relative visual excitability in the two eyes and their relative electrical excitability. In one animal, monocularly deprived continuously until 23 weeks of age, we examined the effect of reversibly turning off signals from the normal eye by pressure blinding. There was no evidence of a very rapid return of sensitivity to either electrical or natural stimulation of the deprived eye.

Animals↗
PubMed1977

Uptake of horseradish peroxidase by geniculo-cortical axons in the golden hamster: analysis by computer reconstruction.

Micro-injections of horseradish peroxidase (HRP) were made into the visual cortex of the golden hamster. The "projection lines" of labelled neurons in the dorsal lateral geniculate nucleus (LGNd) were three-dimensionally reconstructed, using a computer graphics technique. The lines run rostrally and medially from their origins at the lateral surface of the nucleus. Using an anatomically determined retinotopic map of the LGNd, the positions of all labelled cells near the lateral surface were converted into equivalent visual field co-ordinates and displayed on a physiologically determined retinotopic map of the primary visual cortex. Comparison between the scatter of these equivalent retinotopic loci and an actual reconstruction of the injection site revealed that: 1. there was general agreement between the independent retinotopic maps of LGNd and visual cortex; 2. there was greater retinotopic scatter of labelled LGNd cells than could be accounted for by the area of tissue injury in the cortex; 3. the retinotopic scatter matched more closely the total visible halo of HRP staining in the grey matter; 4. HRP can be taken up from a cytoarchitectonic field into which it diffuses after injection into a neighbouring area; 5. HRP is probably not taken up by undamaged axons in the white matter. These results are compared with those obtained in other animals and other systems. No general rules emerge, but the possibility of uptake from wide areas of diffusion must be considered when interpreting results of HRP injection.

Animals↗