Biomedical subjects

W M Usrey

Publications and source records attributed to W M Usrey.

11 recordsLinked to original sources

PubMed2001

Rules of connectivity between geniculate cells and simple cells in cat primary visual cortex.

Hundreds of thalamic axons ramify within a column of cat visual cortex; yet each layer 4 neuron receives input from only a fraction of them. We have examined the specificity of these connections by recording simultaneously from layer 4 simple cells and cells in the lateral geniculate nucleus with spatially overlapping receptive fields (n = 221 cell pairs). Because of the precise retinotopic organization of visual cortex, the geniculate axons and simple-cell dendrites of these cell pairs should have overlapped within layer 4. Nevertheless, monosynaptic connections were identified in only 33% of all cases, as estimated by cross-correlation analysis. The visual responses of monosynaptically connected geniculate cells and simple cells were closely related. The probability of connection was greatest when a geniculate center overlapped a strong simple-cell subregion of the same sign (ON or OFF) near the center of the subregion. This probability was further increased when the time courses of the visual responses were similar. In addition, the connections were strongest when the simple-cell subregion and the geniculate center were matched in position, sign, and size. The rules of connectivity between geniculate afferents and simple cells resemble those found for retinal afferents to geniculate cells. The connections along the retinogeniculocortical pathway, therefore, show a precision that goes beyond simple retinotopy to include many other response properties, such as receptive-field sign, timing, subregion strength, and size. This specificity in wiring emphasizes the need for developmental mechanisms (presumably correlation-based) that can select among afferents that differ only slightly in their response properties.

Action Potentials↗

PubMed2000

Synaptic interactions between thalamic inputs to simple cells in cat visual cortex.

We performed experiments in the cat geniculocortical pathway, in vivo, to examine how presynaptic spikes interact to influence the firing of postsynaptic targets. In particular, we asked (1) how do multiple spikes from a single presynaptic neuron interact to influence the firing of a postsynaptic target (homosynaptic interactions), (2) how do spikes from two different presynaptic neurons interact (heterosynaptic interactions), and (3) what is the time course of homosynaptic and heterosynaptic interactions? We found that both homosynaptic and heterosynaptic interactions increase the likelihood of driving a postsynaptic spike, although with different time courses. For two spikes traveling down a single geniculate axon, the second spike is more effective than the first for approximately 15 msec. For two spikes on separate axons, the interaction is faster ( approximately 7 msec duration, approximately 2.5 msec time constant). Thus changes in firing rate are perhaps best relayed by homosynaptic interactions, whereas heterosynaptic interactions may help detect coincident spikes from different thalamic inputs.

Action Potentials↗

PubMed2000

Visual physiology of the lateral geniculate nucleus in two species of new world monkey: Saimiri sciureus and Aotus trivirgatis.

1. Visual responses were recorded from neurones in the magnocellular and parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus in two species of New World monkeys - the diurnal squirrel monkey (Saimiri sciureus) and the nocturnal owl monkey (Aotus trivirgatis). Recording sites were reconstructed in postmortem tissue and comparisons were made between the response properties of magnocellular and parvocellular neurones. 2. Receptive fields were characterized with both white noise and drifting gratings. We found that most of the differences between magnocellular and parvocellular neurones that have been described in the macaque monkey hold for the squirrel monkey and owl monkey. In squirrel monkey and owl monkey, receptive fields of magnocellular neurones were larger than those of parvocellular neurones at similar eccentricities. Although visual responses in the owl monkey were significantly slower than in the squirrel monkey, in both species magnocellular neurones differed from parvocellular neurones in that their responses (1) had higher contrast gains, (2) tended to peak at higher temporal frequencies (but with considerable overlap), (3) had shorter response latencies, and (4) were more transient. 3. The strength of a neurone's receptive-field surround was assessed by comparing neuronal responses to gratings of optimal spatial frequency with responses to gratings of low spatial frequency. Using this approach, receptive-field surrounds were found to be equally strong on average for magnocellular and parvocellular neurones. 4. Spatial summation, as measured by a null test, was linear for all magnocellular and parvocellular cells tested; that is, Y cells were not observed in either species. Finally, most magnocellular neurones showed a contrast gain control mechanism, although this was not seen for parvocellular neurones.

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PubMed1999

Synchronous activity in the visual system.

Synchronous activity among ensembles of neurons is a robust phenomenon observed in many regions of the brain. With the increased use of multielectrode recording techniques, synchronous firing of ensembles of neurons has been found at all levels in the mammalian visual pathway, from the retina to the extrastriate cortex. Here we distinguish three categories of synchrony in the visual system, (a) synchrony from anatomical divergence, (b) stimulus-dependent synchrony, and (c) emergent synchrony (oscillations). Although all three categories have been well documented, their functional significance remains uncertain. We discuss several lines of evidence both for and against a role for synchrony in visual processing: the perceptual consequences of synchronous activity, its ability to carry information, and the transmission of synchronous neural events to subsequent stages of processing.

Animals↗

PubMed1999

Specificity and strength of retinogeniculate connections.

Retinal ganglion cells and their target neurons in the principal layers of the lateral geniculate nucleus (LGN) of the thalamus have very similar, center-surround receptive fields. Although some geniculate neurons are dominated by a single retinal afferent, others receive both strong and weak inputs from several retinal afferents. In the present study, experiments were performed in the cat that examined the specificity and strength of monosynaptic connections between retinal ganglion cells and their target neurons. The responses of 205 pairs of retinal ganglion cells and geniculate neurons with overlapping receptive-field centers or surrounds were studied. Receptive fields were mapped quantitatively using a white-noise stimulus; connectivity was assessed by cross-correlating the retinal and geniculate spike trains. Of the 205 pairs, 12 were determined to have monosynaptic connections. Both the likelihood that cells were connected and the strength of connections increased with increasing similarity between retinal and geniculate receptive fields. Connections were never found between cells with <50% spatial overlap between their centers. The results suggest that although geniculate neurons often receive input from several retinal afferents, these multiple afferents represent a select subset of the retinal ganglion cells with overlapping receptive-field centers.

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PubMed1998

Paired-spike interactions and synaptic efficacy of retinal inputs to the thalamus.

In many neural systems studied in vitro, the timing of afferent impulses affects the strength of postsynaptic potentials. The influence of afferent timing on postsynaptic firing in vivo has received less attention. Here we study the importance of afferent spike timing in vivo by recording simultaneously from ganglion cells in the retina and their targets in the lateral geniculate nucleus of the thalamus. When two spikes from a single ganglion-cell axon arrive within 30 milliseconds of each other, the second spike is much more likely than the first to produce a geniculate spike, an effect we call paired-spike enhancement. Furthermore, simultaneous recordings from a ganglion cell and two thalamic targets indicate that paired-spike enhancement increases the frequency of synchronous thalamic activity. We propose that information encoded in the high firing rate of an individual retinal ganglion cell becomes distributed among several geniculate neurons that fire synchronously. Because synchronous geniculate action potentials are highly effective in driving cortical neurons, it is likely that information encoded by this strategy is transmitted to the next level of processing.

Action Potentials↗

PubMed1998

Coding of visual information by precisely correlated spikes in the lateral geniculate nucleus.

Correlated firing among neurons is widespread in the nervous system. Precisely correlated spiking, occurring on a millisecond time scale, has recently been observed among neurons in the lateral geniculate nucleus with overlapping receptive fields. We have used an information-theoretic analysis to examine the role of these correlations in visual coding. Considerably more information can be extracted from two cells if temporal correlations between them are considered. The percentage increase in information depends on the degree of correlation; the average increase is approximately 20% for strongly correlated pairs. Thus, precise temporal correlation could be used as an additional information channel from thalamus to visual cortex.

Action Potentials↗

PubMed1996

Precisely correlated firing in cells of the lateral geniculate nucleus.

Simple cells within layer IV of the cat primary visual cortex are selective for lines of a specific orientation. It has been proposed that their receptive-field properties are established by the pattern of connections that they receive from the lateral geniculate nucleus (LGN) of the thalamus. Thalamic inputs, however, represent only a small proportion of the synapses made onto simple cells, and others have argued that corticocortical connections are likely to be important in shaping simple-cell response properties. Here we describe a mechanism that might be involved in selectively strengthening the effect of thalamic inputs. We show that neighbouring geniculate neurons with overlapping receptive fields of the same type (on-centre or off-centre) often fire spikes that are synchronized to within 1 millisecond. Moreover, these neurons often project to a common cortical target neuron where synchronous spikes are more effective in evoking a postsynaptic response. We propose that precisely correlated firing within a group of geniculate neurons could serve to reinforce the thalamic input to cortical simple cells.

Action Potentials↗

PubMed1996

Specificity in the axonal connections of layer VI neurons in tree shrew striate cortex: evidence for distinct granular and supragranular systems.

Pyramidal neurons in layer VI of striate cortex are the source of descending projections to the lateral geniculate nucleus (LGN) as well as a major source of axon terminals in the layers of striate cortex that receive LGN projections. This study examines how the connections of layer VI neurons are arranged with respect to the functionally distinct classes of neurons that compose their cortical and subcortical targets. By placing injections of biocytin into layer VI of tree shrew striate cortex, we identified two sublayers that differ in their intracortical and thalamic connections. Neurons in the upper part of layer VI, layer VIa, terminate in cortical layer IV, whereas those in the lower part of layer VI, layer VIb, terminate throughout the supragranular layers, layers I-III. The selectivity of layer VI subdivisions for the granular and supragranular layers is also evident in their descending projections. Neurons in layer VIa terminate preferentially in the LGN layers that supply layer IV (LGN layers 1, 2, 4, and 5), whereas neurons in layer VIb terminate in the LGN layers that supply layers I-III (LGN layers 3 and 6) and in the pulvinar nucleus. Additional subclasses of layer VIa neurons were identified based on the restriction of their terminal fields to narrow subtiers within layer IV. By influencing the activity of distinct populations of cortical neurons and the thalamic neurons that supply them, layer VI neurons could exert a powerful influence on the flow of activity in functionally distinct cortical circuits.

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PubMed1992

Lateral geniculate projections to the superficial layers of visual cortex in the tree shrew.

Our recent studies of tree shrew striate cortex have focused on the organization of lateral geniculate projections to layer IV and the projections from IV to layer III. Although these pathways play an important role in determining the response properties of layer III neurons, there are additional pathways from the lateral geniculate nucleus (LGN) that terminate directly in layer III. Previous studies provided evidence that these projections originate from layers 6 and 3 of the LGN and terminate in different subdivisions of layer III. In this study we used injections of biocytin to examine the projections of layers 6 and 3 to the cortex in more detail. Consistent with earlier work, we found that LGN layer 6 projects heavily to lower IIIc, while LGN layer 3 terminates densely in layer IIIb and sparsely throughout layers IIIa-I. In addition, we found that neurons in layers 6 and 3 have collateral projections: neurons in LGN layer 6 project to the bottom of layer IVb and sparsely to I-IIIb; neurons in LGN layer 3 project sparsely to layers V and VI and to the middle of IV. These patterns of projections are significant in the light of our studies of the connections from cortical layer IV to layer III. LGN projections to the superficial layers are organized into parallel pathways that exert selective influence over different populations of neurons in layers I-III and on the layer IV neurons that supply them.

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PubMedDate not supplied

The sublaminar organization of corticogeniculate neurons in layer 6 of macaque striate cortex.

We examined the laminar distribution of corticogeniculate neurons in the macaque striate cortex labeled by axonal transport following injections of retrograde tracers into the lateral geniculate nucleus (LGN). Large injections of retrograde tracers involving all layers of the LGN resulted in a distinctive bilaminar distribution of labeled cells in cortical layer 6. One tier of labeled neurons was located along the layer 5-6 border and a second was located near the bottom of the layer, leaving the middle of layer 6 largely free of labeled neurons. Following injections of tracers that were restricted to the magnocellular layers of the LGN, almost all of the labeled neurons were located in the lower tier. In contrast, following injections of retrograde tracers confined to the parvocellular layers of the LGN, labeled cells were found in both tiers, with the greatest number in the upper tier. Thus, layer 6 of macaque striate cortex consists of three distinct sublayers only two of which are the source of descending projections to the LGN: an upper tier that projects exclusively to the parvocellular layers and a lower tier that projects to both magnocellular and parvocellular layers.

Affinity Labels↗