The projection of the visual field to the lateral geniculate and medial interlaminar nuclei in the cat

Early monocular deprivation reduces the capacity for neural plasticity in the cat visual system

Obstruction of vision to one eye during early postnatal development elicits neural modifications in the visual system that can last a lifetime. Research in rodents has revealed that an early and transient monocular deprivation (MD) can produce an enduring alteration to the framework of neural connections within visual cortex. This lasting trace of early MD enables an enhanced effect of a second MD imposed on the same eye in adulthood. In the current study, we examined whether the modification of plasticity potential was bidirectional by assessing whether the effect of early and brief MD attenuated the impact of a subsequent MD when applied to the fellow eye. Results were clear in showing that animals with an early MD exhibited a smaller response to later visual deprivation of the fellow eye. Compared to controls, animals with a history of MD exhibited less atrophy of neurons, and a smaller loss of neurofilament labeling within the dorsal lateral geniculate nucleus. The shift in cortical ocular dominance elicited by MD was also smaller in animals with a prior MD. These results indicate that early MD elicits abiding and eye-specific neural modifications that can selectively alter plasticity potential in the visual system.

Comparative analysis of structural modifications induced by monocular retinal inactivation and monocular deprivation in the developing cat lateral geniculate nucleus

During a critical period of postnatal life, monocular deprivation (MD) by eyelid closure reduces the size of neurons in layers of the dorsal lateral geniculate nucleus (dLGN) connected to the deprived eye and shifts cortical ocular dominance in favor of the non-deprived eye. Temporary inactivation of the non-deprived eye can promote superior recovery from the effects of long-term MD compared to conventional occlusion therapy. In the current study, we assessed the modification of neuron size in the dLGN as a means of measuring the impact of a brief period of monocular inactivation (MI) imposed at different postnatal ages. The biggest impact of MI was observed when it occurred at the peak of the critical period. Unlike the effect of MD, structural plasticity following MI was observed in both the binocular and monocular segments of the dLGN. With increasing age, the capacity for inactivation to alter postsynaptic cell size diminished but was still significant beyond the critical period. In comparison to MD, inactivation produced effects that were about double in magnitude and exhibited efficacy at older ages. Notwithstanding the large neural alterations precipitated by MI, its effects were remediated with a short period of binocular experience, and vision through the previously inactivated eye fully recovered. These results demonstrate that MI is a potent means of modifying the visual pathway and does so at ages when occlusion is ineffective. The efficacy and longevity of inactivation to elicit plasticity highlight its potential to ameliorate disorders of the visual system such as amblyopia.

Dark Rearing Promotes the Recovery of Visual Cortical Responses but Not the Morphology of Geniculocortical Axons in Amblyopic Cat

Monocular deprivation (MD) of vision during early postnatal life induces amblyopia, and most neurons in the primary visual cortex lose their responses to the closed eye. Anatomically, the somata of neurons in the closed-eye recipient layer of the lateral geniculate nucleus (LGN) shrink and their axons projecting to the visual cortex retract. Although it has been difficult to restore visual acuity after maturation, recent studies in rodents and cats showed that a period of exposure to complete darkness could promote recovery from amblyopia induced by prior MD. However, in cats, which have an organization of central visual pathways similar to humans, the effect of dark rearing only improves monocular vision and does not restore binocular depth perception. To determine whether dark rearing can completely restore the visual pathway, we examined its effect on the three major concomitants of MD in individual visual neurons, eye preference of visual cortical neurons and soma size and axon morphology of LGN neurons. Dark rearing improved the recovery of visual cortical responses to the closed eye compared with the recovery under binocular conditions. However, geniculocortical axons serving the closed eye remained retracted after dark rearing, whereas reopening the closed eye restored the soma size of LGN neurons. These results indicate that dark rearing incompletely restores the visual pathway, and thus exerts a limited restorative effect on visual function.

Characterization of White Matter Tracts by Diffusion MR Tractography in Cat and Ferret that Have Similar Gyral Patterns

The developmental relationships between gyral structures and white matter tracts have long been debated, but it is still difficult to discern whether they influence each other's development or are causally related. To explore this topic, this study used cats and ferrets as models for species that share similar gyral folding patterns and imaged with diffusion magnetic resonance imaging to compare white matter innervations in homologous gyri and other brain regions. Adult cat and ferret brains were analyzed via diffusion spectrum imaging tractography and homologous regions of interest were compared. Although similar genetic lineage and gyral structures would suggest analogous white matter tracts, tractography reveals significantly differing white matter connectivity in both the visual and auditory cortices. Similarities in connectivity were concentrated primarily in the highly conserved cerebellar region. These results correlate well with existing histological and functional studies of both species. Our results indicate that, while the 2 species may share similar gyral structures, they utilize different white matter connectivity; suggesting that while species may share similar gyral structures, they can develop different underlying white matter connectivity.

Focal Gain Control of Thalamic Visual Receptive Fields by Layer 6 Corticothalamic Feedback

The projections between the thalamus and primary visual cortex (V1) are a key reciprocal neural circuit, relaying retinal signals to cortical layers 4 & 6 while being simultaneously regulated by massive layer 6 corticothalamic feedback. Effectively dissecting the influence of this corticothalamic feedback circuit in higher mammals remains a challenge for vision research. By pharmacologically increasing the focal gain of visually driven layer 6 responses of cat V1 in a controlled fashion, we examined the effects of such focal cortical changes on the response amplitudes and spatial structure of the receptive fields (RFs) of individual dorsal lateral geniculate nucleus (dLGN) cells. We found that enhancing visually driven cortical feedback could facilitate or suppress the overall responses of dLGN cells, and such an effect was linked to the orientation preference of the cortical neuron. Related to these selective retinotopic gain changes, enhanced feedback induced the RFs of dLGN cells to expand, contract or shift their spatial focus. Our results provide further evidence for a functional mechanism through which the cortex can selectively gate visual information flow from the thalamus back to the visual cortex.

CAS: Cell Annotation Software - Research on Neuronal Tissue Has Never Been so Transparent

CAS (Cell Annotation Software) is a novel tool for analysis of microscopic images and selection of the cell soma or nucleus, depending on the research objectives in medicine, biology, bioinformatics, etc. It replaces time-consuming and tiresome manual analysis of single images not only with automatic methods for object segmentation based on the Statistical Dominance Algorithm, but also semi-automatic tools for object selection within a marked region of interest. For each image, a broad set of object parameters is computed, including shape features and optical and topographic characteristics, thus giving additional insight into data. Our solution for cell detection and analysis has been verified by microscopic data and its application in the annotation of the lateral geniculate nucleus has been examined in a case study.

Spatial scale of receptive fields in the visual sector of the cat thalamic reticular nucleus

Inhibitory projections from the visual sector of the thalamic reticular nucleus to the lateral geniculate nucleus complete the earliest feedback loop in the mammalian visual pathway and regulate the flow of information from retina to cortex. There are two competing hypotheses about the function of the thalamic reticular nucleus. One regards the structure as a thermostat that uniformly regulates thalamic activity through negative feedback. Alternatively, the searchlight hypothesis argues for a role in focal attentional modulation through positive feedback, consistent with observations that behavioral state influences reticular activity. Here, we address the question of whether cells in the reticular nucleus have receptive fields small enough to provide localized feedback by devising methods to quantify the size of these fields across visual space. Our results show that reticular neurons in the cat operate over discrete spatial scales, at once supporting the searchlight hypothesis and a role in feature selective sensory processing.The searchlight hypothesis proposes that the thalamic reticular nucleus regulates thalamic relay activity through focal attentional modulation. Here the authors show that the receptive field sizes of reticular neurons are small enough to provide localized feedback onto thalamic neurons in the visual pathway.

Receptive field properties of cat perigeniculate neurons correlate with excitatory and inhibitory connectivity to LGN relay neurons

The cat perigeniculate nucleus (PGN) is a visual sector of the thalamic reticular nucleus that consists of GABAergic neurons. It receives excitatory axon-collateral input from relay neurons of the dorsal lateral geniculate nucleus (LGN) to which it provides inhibitory input. Thus, it is usually argued that the PGN works as feedback inhibition to the LGN. At the single neuron level, however, this circuit can also provide lateral inhibition. Which inhibition dominates in the visual circuit of the thalamus has yet to be well characterized. In this study, we conducted cross-correlation analysis of single spike trains simultaneously recorded from PGN and LGN neurons in anesthetized cats. For 12 pairs of functionally connected PGN and LGN neurons with overlapped receptive fields (RF), we quantitatively compared RF properties including the spatial frequency (SF) and temporal frequency (TF) tunings of each neuron. We found the SF and TF tunings of PGN neurons and LGN neurons were similar when there was only excitatory input from the LGN neuron to the PGN neuron, but different when the PGN neuron returned inhibitory inputs back, suggesting the circuit between PGN and LGN neurons works as lateral inhibition for these properties.

Intracellular, In Vivo, Dynamics of Thalamocortical Synapses in Visual Cortex

Seminal studies of the thalamocortical circuit in the visual system of the cat have been central to our understanding of sensory encoding. However, thalamocortical synaptic properties remain poorly understood. We used paired recordings, in the lateral geniculate nucleus (LGN) and primary visual cortex (V1), to provide the first in vivo characterization of sensory-driven thalamocortical potentials in V1. The amplitudes of EPSPs we characterized were smaller than those previously reported in vitro Consistent with prior findings, connected LGN-V1 pairs were only found when their receptive fields (RFs) overlapped, and the probability of connection increased steeply with degree of RF overlap and response similarity. However, surprisingly, we found no relationship between EPSP amplitudes and the similarity of RFs or responses, suggesting different connectivity models for intracortical and thalamocortical circuits. Putative excitatory regular-spiking (RS) and inhibitory fast-spiking (FS) V1 cells had similar EPSP characteristics, showing that in the visual system, feedforward excitation and inhibition are driven with equal strength by the thalamus. Similar to observations in the somatosensory cortex, FS V1 cells received less specific input from LGN. Finally, orientation tuning in V1 was not inherited from single presynaptic LGN cells, suggesting that it must emerge exclusively from the combined input of all presynaptic LGN cells. Our results help to decipher early visual encoding circuits and have immediate utility in providing physiological constraints to computational models of the visual system.SIGNIFICANCE STATEMENT To understand how the brain encodes the visual environment, we must understand the transfer of visual signals between various regions of the brain. Therefore, understanding synaptic dynamics is critical to our understanding of sensory encoding. This study provides the first characterization of visually evoked synaptic potentials between the visual thalamus and visual cortex in an intact animal. To record these potentials, we simultaneously recorded the extracellular potential of presynaptic thalamic cells and the intracellular potential of postsynaptic cortical cells in input layers of primary visual cortex. Our characterization of synaptic potentials in vivo disagreed with prior findings in vitro This study will increase our understanding of thalamocortical circuits and will improve computational models of visual encoding.

A new model of strabismic amblyopia: Loss of spatial acuity due to increased temporal dispersion of geniculate X-cell afferents on to cortical neurons

Although the neural locus of strabismic amblyopia has been shown to lie at the first site of binocular integration, first in cat and then in primate, an adequate mechanism is still lacking. Here we hypothesise that increased temporal dispersion of LGN X-cell afferents driven by the deviating eye onto single cortical neurons may provide a neural mechanism for strabismic amblyopia. This idea was investigated via single cell extracellular recordings of 93 X and 50 Y type LGN neurons from strabismic and normal cats. Both X and Y neurons driven by the non-deviating eye showed shorter latencies than those driven by either the strabismic or normal eyes. Also the mean latency difference between X and Y neurons was much greater for the strabismic cells compared with the other two groups. The incidence of lagged X-cells driven by the deviating eye of the strabismic cats was higher than that of LGN X-cells from normal animals. Remarkably, none of the cells recorded from the laminae driven by the non-deviating eye were of the lagged class. A simple computational model was constructed in which a mixture of lagged and non-lagged afferents converge on to single cortical neurons. Model cut-off spatial frequencies to a moving grating stimulus were sensitive to the temporal dispersion of the geniculate afferents. Thus strabismic amblyopia could be viewed as a lack of developmental tuning of geniculate lags for neurons driven by the amblyopic eye. Monocular control of fixation by the non-deviating eye is associated with reduced incidence of lagged neurons, suggesting that in normal vision, lagged neurons might play a role in maintaining binocular connections for cortical neurons.

Binocular eyelid closure promotes anatomical but not behavioral recovery from monocular deprivation

Deprivation of patterned vision of frontal eyed mammals early in postnatal life alters structural and functional attributes of neurones in the central visual pathways, and can produce severe impairments of the vision of the deprived eye that resemble the visual loss observed in human amblyopia. A traditional approach to treatment of amblyopia has been the occlusion of the stronger fellow eye in order to force use of the weaker eye and thereby strengthen its connections in the visual cortex. Although this monocular treatment strategy can be effective at promoting recovery of visual acuity of the amblyopic eye, such binocular visual functions as stereoscopic vision often remain impaired due in part to the lack of concordant vision during the period of unilateral occlusion. The recent development of binocular approaches for treatment of amblyopia that improve the possibility for binocular interaction have achieved success in promoting visual recovery. The full and rapid recovery of visual acuity observed in amblyopic kittens placed in complete darkness is an example of a binocular treatment whose success may in part derive from a restored balance of visually-driven neural activity. In the current study we examined as an alternative to dark rearing the efficacy of binocular lid suture (BLS) to stimulate anatomical and visual recovery from a preceding amblyogenic period of monocular deprivation. In the dorsal lateral geniculate nucleus (dLGN) of monocularly deprived kittens, darkness or BLS for 10days produced a complete recovery of neurone soma size within initially deprived layers. The growth of neurone somata within initially deprived dLGN layers after darkness or BLS was accompanied by an increase in neurotrophin-4/5 labeling within these layers. Although anatomical recovery was observed in both recovery conditions, BLS failed to promote any improvement of the visual acuity of the deprived eye no matter whether it followed immediately or was delayed with respect to the prior period of monocular deprivation. Notwithstanding the lack of visual recovery with BLS, all animals in the BLS condition that were subsequently placed in darkness exhibited a substantial recovery of visual acuity in the amblyopic eye. We conclude that the balanced binocular visual input provided by BLS does not stimulate the collection of neural events necessary to support recovery from amblyopia. The complete absence of visually-driven activity that occurs with dark rearing evidently plays an important role in the recovery process.

Shrinkage of X cells in the lateral geniculate nucleus after monocular deprivation revealed by FoxP2 labeling

The parallel processing of visual features by distinct neuron populations is a central characteristic of the mammalian visual system. In the A laminae of the cat dorsal lateral geniculate nucleus (dLGN), parallel processing streams originate from two principal neuron types, called X and Y cells. Disruption of visual experience early in life by monocular deprivation has been shown to alter the structure and function of Y cells, but the extent to which deprivation influences X cells remains less clear. A transcription factor, FoxP2, has recently been shown to selectively label X cells in the ferret dLGN and thus provides an opportunity to examine whether monocular deprivation alters the soma size of X cells. In this study, FoxP2 labeling was examined in the dLGN of normal and monocularly deprived cats. The characteristics of neurons labeled for FoxP2 were consistent with FoxP2 being a marker for X cells in the cat dLGN. Monocular deprivation for either a short (7 days) or long (7 weeks) duration did not alter the density of FoxP2-positive neurons between nondeprived and deprived dLGN layers. However, for each deprived animal examined, measurement of the cross-sectional area of FoxP2-positive neurons (X cells) revealed that within deprived layers, X cells were smaller by approximately 20% after 7 days of deprivation, and by approximately 28% after 7 weeks of deprivation. The observed alteration to the cross-sectional area of X cells indicates that perturbation of this major pathway contributes to the functional impairments that develop from monocular deprivation.

The role of thalamic population synchrony in the emergence of cortical feature selectivity

In a wide range of studies, the emergence of orientation selectivity in primary visual cortex has been attributed to a complex interaction between feed-forward thalamic input and inhibitory mechanisms at the level of cortex. Although it is well known that layer 4 cortical neurons are highly sensitive to the timing of thalamic inputs, the role of the stimulus-driven timing of thalamic inputs in cortical orientation selectivity is not well understood. Here we show that the synchronization of thalamic firing contributes directly to the orientation tuned responses of primary visual cortex in a way that optimizes the stimulus information per cortical spike. From the recorded responses of geniculate X-cells in the anesthetized cat, we synthesized thalamic sub-populations that would likely serve as the synaptic input to a common layer 4 cortical neuron based on anatomical constraints. We used this synchronized input as the driving input to an integrate-and-fire model of cortical responses and demonstrated that the tuning properties match closely to those measured in primary visual cortex. By modulating the overall level of synchronization at the preferred orientation, we show that efficiency of information transmission in the cortex is maximized for levels of synchronization which match those reported in thalamic recordings in response to naturalistic stimuli, a property which is relatively invariant to the orientation tuning width. These findings indicate evidence for a more prominent role of the feed-forward thalamic input in cortical feature selectivity based on thalamic synchronization.

While the visual system is selective for a wide range of different inputs, orientation selectivity has been considered the preeminent property of the mammalian visual cortex. Existing models of this selectivity rely on varying relative importance of feedforward thalamic input and intracortical influence. Recently, we have shown that pairwise timing relationships between single thalamic neurons can be predictive of a high degree of orientation selectivity. Here we have constructed a computational model that predicts cortical orientation tuning from thalamic populations. We show that this arrangement, relying on precise timing differences between thalamic responses, accurately predicts tuning properties as well as demonstrates that certain timing relationships are optimal for transmitting information about the stimulus to cortex.

An examination of linking hypotheses drawn from the perceptual consequences of experimentally induced changes in neural circuitry

Because targeted early experiential manipulations alter both perception and the response properties of particular cells in the striate cortex, they have been used as evidence for linking hypotheses between the two. However, such hypotheses assume that the effects of the early biased visual input are restricted to just the specific cell population and/or visual areas of interest and that the neural populations that contribute to the visual perception itself do not change. To examine this assumption, we measured the consequences for vision of an extended period of early monocular deprivation (MD) on a kitten (from 19 to 219 days of age) that began well before, and extended beyond, bilateral ablation of visual cortical areas 17 and 18 at 132 days of age. In agreement with previous work, the lesion reduced visual acuity by only a factor of two indicating that the neural sites, other than cortical areas 17 and 18, that support vision in their absence have good spatial resolution. However, these sites appear to be affected profoundly by MD as the effects on vision were just as severe as those observed following MD imposed on normal animals. The pervasive effects of selected early visual deprivation across many cortical areas reported here and elsewhere, together with the potential for perception to be mediated at a different neural site following deprivation than after typical rearing, points to a need for caution in the use of data from early experiential manipulations for formulation of linking hypotheses.

Neurons in the thalamic reticular nucleus are selective for diverse and complex visual features

All visual signals the cortex receives are influenced by the perigeniculate sector (PGN) of the thalamic reticular nucleus, which receives input from relay cells in the lateral geniculate and provides feedback inhibition in return. Relay cells have been studied in quantitative depth; they behave in a roughly linear fashion and have receptive fields with a stereotyped center-surround structure. We know far less about reticular neurons. Qualitative studies indicate they simply pool ascending input to generate non-selective gain control. Yet the perigeniculate is complicated; local cells are densely interconnected and fire lengthy bursts. Thus, we employed quantitative methods to explore the perigeniculate using relay cells as controls. By adapting methods of spike-triggered averaging and covariance analysis for bursts, we identified both first and second order features that build reticular receptive fields. The shapes of these spatiotemporal subunits varied widely; no stereotyped pattern emerged. Companion experiments showed that the shape of the first but not second order features could be explained by the overlap of On and Off inputs to a given cell. Moreover, we assessed the predictive power of the receptive field and how much information each component subunit conveyed. Linear-non-linear (LN) models including multiple subunits performed better than those made with just one; further each subunit encoded different visual information. Model performance for reticular cells was always lesser than for relay cells, however, indicating that reticular cells process inputs non-linearly. All told, our results suggest that the perigeniculate encodes diverse visual features to selectively modulate activity transmitted downstream.

Dorsal lateral geniculate substructure in the long-evans rat: a cholera toxin B subunit study

The pigmented rat is an increasingly important model in visual neuroscience research, yet the lamination of retinal projections in the dLGN has not been examined in sufficient detail. From previous studies it was known that most of the rat dLGN receives monocular input from the contralateral eye, with a small island receiving predominantly ipsilateral projections. Here we revisit the question using cholera toxin B subunit, a tracer that efficiently fills retinal terminals after intra-ocular injection. We imaged retinal termini throughout the dLGN at 0.5 μm resolution and traced areas of ipsilateral and contralateral terminals to obtain a high resolution 3D reconstruction of the projection pattern. Retinal termini in the dLGN are well segregated by eye of origin, as expected. We find, however, that the ipsilateral projections form multiple discrete projection zones in three dimensions, not the single island previously described. It remains to be determined whether these subdomains represent distinct functional sublaminae, as is the case in other mammals.

How thalamus connects to spiny stellate cells in the cat's visual cortex

In the cat's visual cortex, the responses of simple cells seem to be totally determined by their thalamic input, yet only a few percent of the excitatory synapses in layer 4 arise from the thalamus. To resolve this discrepancy between structure and function, we used correlated light and electron microscopy to search individual spiny stellate cells (simple cells) for possible structural features that would explain the biophysical efficacy of the thalamic input, such as synaptic location on dendrites, size of postsynaptic densities, and postsynaptic targets. We find that thalamic axons form a small number of synapses with the spiny stellates (188 on average), that the median size of the synapses is slightly larger than that of other synapses on the dendrites of spiny stellates, that they are not located particularly proximal to the soma, and that they do not cluster on the dendrites. These findings point to alternative mechanisms, such as synchronous activation of the sparse thalamic synapses to boost the efficacy of the thalamic input. The results also support the idea that the thalamic input does not by itself determine the cortical response of spiny stellate cells, allowing the cortical microcircuit to amplify and modulate its response according to the particular context and computation being performed.

Evidence regarding the integrity of the posterior medial lateral suprasylvian visual area in the cat

Among the areas of lateral suprasylvian visual cortex in cats defined by Palmer et al. (J Comp Neurol [1978] 177:237-256), PMLS (posterior lateral suprasylvian area) has been the most studied. Although PMLS has strong and well-documented connections with area 17, it is unclear whether these connections extend to its upper visual field representation. We asked what cortical areas send input to the upper field representation in PMLS by making tracer injections in areas 17, 19, and posterior suprasylvian cortex. Tracer injections made in area 17's upper field representation in 15 cats failed to label the corresponding region in PMLS. Instead, they showed that area 17 is strongly connected with the posterior bank of the posterior suprasylvian sulcus (pSS), a region attributed by Palmer et al. to area 21a. Injections in area 19 had the same outcome. We consider this posterior upper field representation plus the lower field representation in PMLS to belong to a single area, LS (lateral suprasylvian visual area). Our data suggest that the upper field representation in PMLS belongs to a different area, most likely AMLS (anterior medial lateral suprasylvian area).

High-resolution fMRI mapping of ocular dominance layers in cat lateral geniculate nucleus

In this work, we exploited the superior capability of high-resolution functional magnetic resonance imaging (fMRI) for functional mapping of ocular dominance layer (ODL) in the cat lateral geniculate nucleus (LGN). The stimulus-evoked neuronal activities in the LGN ODLs associated with contralateral- and ipsilateral-eye visual inputs were successfully differentiated and mapped using both blood-oxygenation-level dependent (BOLD)-weighted and cerebral blood volume (CBV)-weighted fMRI methods. The morphology of mapped LGN ODLs was in remarkable consistency with histology findings in terms of ODL shape, orientation, thickness and eye-dominance. Compared with the BOLD signal, the CBV signal provides higher reproducibility and better spatial resolvability for function mapping of LGN because of improved contrast-to-noise ratio and point-spread function. The capability of fMRI for non-invasively imaging the functional sub-units of ODL in a small LGN overcomes the limitation of conventional neural-recording approach, and it opens a new opportunity for studying critical roles of LGN in brain function and dysfunction at the fine scale of ocular dominance layer.

Cortico-geniculate feedback linking the visual fields surrounding the blind spot in the cat

Previous studies have shown that neurons in layer 6 of V1 are activated by visual stimuli that induce perceptual filling-in at the blind spot (BS). As the main target of layer 6 neurons is the lateral geniculate nucleus (LGN), we speculate that the cortico-geniculate projection is involved in mediating filling-in at BS. To begin to test that hypothesis, we examined whether there is an anatomical basis for integration of visual signals from both sides of BS by cortico-geniculate feedback neurons in V1. We injected an anterograde tracer into a site adjacent to the region representing BS. We observed that numerous axons traverse the neuron-free gap that retinotopically corresponds to BS within LGN. This indicates that visual signals from one side of BS are conveyed to the opposite side via a feedback connection. Cortico-geniculate feedback projection may integrate visual signals from around BS and contribute to perceptual filling-in at BS.

Selective targeting of the dendrites of corticothalamic cells by thalamic afferents in area 17 of the cat

Pyramidal cells of layer 6 in cat visual cortex are the source of the corticothalamic projection, and their recurrent collaterals provide substantially more excitatory synapses in layer 4 than does the thalamic input. They have predominantly simple receptive fields and can be driven monosynaptically by electrically stimulating thalamic relay cells. Layer 6 cells could thus provide a significant disynaptic amplification of the thalamic input to layer 4, particularly since their synapses facilitate, unlike the thalamic afferents whose synapses depress. However, purely geometric considerations of the relation of their dendritic trees to the thalamic input indicate that they should form a far smaller number of synapses with thalamic afferents than do the simple cells of layer 4. We thus analyzed quantitatively the thalamic input to identified corticothalamic cells by labeling the thalamic afferents and corticothalamic cells in vivo. We made a correlated light and electron microscopic study of 73 "contacts" between thalamic afferents and five corticothalamic cells. The electron microscope revealed that only 24 of the contacts identified at light microscope level were indeed synapses and, contrary to geometric predictions, virtually all were located on spines on the basal dendrites. Our quantitative estimates indicate that the corticothalamic cells form even fewer synapses with the thalamic afferents than predicted by geometric considerations and only 1/10 as many as do the layer 4 simple cells. These data strongly suggest it is the collective computation of cortical neurons, not the monosynaptic thalamic input, that determines the output of the corticothalamic cells.

Dual chemoarchitectonic lamination of the visual sector of the thalamic reticular nucleus

The chemoanatomical organization of the visual sector of the cat's thalamic reticular nucleus (TRN)-that is at the dorsal lateral geniculate nucleus (dLGN) and at the pulvinar nucleus (Pul)-was investigated with two novel cytoarchitectonic markers. The Wisteria floribunda agglutinin (WFA) binding reaction visualized the extracellular perineuronal net (PN) and the SMI 32 immunoreaction stained intracellular neurofilaments. Two distinct layers of the TRN could be detected, particularly by WFA- but also by SMI 32-staining. The outer tier outlined a canopy of labeling placed a bit detached from the diencephalon dorsolaterally, while the inner TRN tier is very tightly attached to the thalamic lamina limitans externa. The labeled neurons showed typically fusiform morphology with dendrites orienting in the plane of TRN. Additionally, these chemoarchitectural reactions identified a chain of structures in the ventral diencephalon connected to the TRN tiers. One stained string is formed by the subthalamic nucleus bound laterally to the peripeduncular nucleus extending further dorsolateral into the outer TRN tier. The other chain laced up the field of Forel, the zona incerta, the ventral LGN, the perigeniculate nucleus (PGN) and the previously-overlooked peripulvinar nucleus (PPulN) and so formed the inner TRN tier. In the third most distanced TRN tier, in the perireticular nucleus, a very few WFA-binding presenting neuron were found. In addition to the PN possessing TRN neurons, WFA-reactive presumable interneurons were also labeled within the visual thalamus. Following tracer injections into the feline Pul, two stripes of cells were retrogradely labeled in the neighboring visual TRN sector. The location of these reticular neurons coincided precisely with the chemoanatomically identified inner and outer TRN tiers. On the analogy of the PGN-TRN duality at the dLGN, the chemoanatomical and tract tracing findings strongly suggest a similar dual organization in the pulvinoprojecting TRN portion.

The proportion of synapses formed by the axons of the lateral geniculate nucleus in layer 4 of area 17 of the cat

The connection between the dorsal lateral geniculate nucleus (dLGN) and area 17 of the cat is a classical model for studying thalamocortical relations. We investigated the proportion of asymmetric synapses in layer 4 of area 17 of cats formed by axons of the dLGN, because this is an important morphological parameter in understanding the impact of dLGN axons on their target neurons. Although the present consensus is that this proportion is small, the exact percentage remains in doubt. Most previous work estimated that the thalamus contributes less than 10% of excitatory synapses in layer 4, but one estimate was as high as 28%. Two issues contribute to these widely different estimates, one being the tracers used, the other being the use of biased stereological approaches. We have addressed both of these issues. Thalamic axons were labeled in vivo by injections of biotinylated dextran amine into the A lamina of the dLGN of anesthetized cats. After processing, the brain was cut serially and prepared for light and electron microscopy. The density of asymmetric synapses in the neuropil and the density of synapses formed by labeled dLGN boutons were measured by using an unbiased sampling method called the physical disector. Our counts indicate that, in the fixed cat brain, there are 5.9 x 10(8) +/- 0.9 x 10(8) asymmetric synapses per cubic millimeter of layer 4 in area 17, and the dLGN input provides only 6% of all asymmetric synapses in layer 4. The vast majority of synapses of layer 4 probably originate from other neurons in area 17.

The W cell pathway to cat primary visual cortex

The thalamic input to area 17 in the cat can be divided into at least three parallel pathways, the W, X, and Y. Although the latter two are some of the best studied synaptic connections in the brain, the former remains poorly understood both in structure and in function. By combining light and electron microscopy, we have reconstructed in 3-D single W axons and described quantitatively the synapses that they form. We have also made a structural comparison of reconstructed synapses from the three visual pathways. Thalamic axons were labeled in vivo by injections of biotinylated dextran amine into the dLGN. W axons originating from C laminae injections arborized in layers 1, 2/3, and 5. Axons that traversed layer 1 supplied a few descending collaterals to layer 2/3, but the most extensive innervation in layer 2/3 was provided by axons ascending from the white matter. Most W boutons formed a single synapse, dendritic spines being the most common target, with dendritic shafts forming the remaining targets. In layer 1, the area of the postsynaptic density of spine synapses (0.16 microm(2)) was significantly larger than that of layers 2/3 (0.11 microm(2)) and 5 (0.09 microm(2)). Synapses from X and Y axons in layer 4 were similar in size to synapses formed by W boutons in layer 1. In layer 1, the main targets of the W axons are likely the apical dendrites of pyramidal cells, so that both proximal and distal regions of pyramidal cell dendritic trees can be excited by the W pathway.

Monocular deprivation provokes alteration of the neuronal cytoskeleton in developing cat lateral geniculate nucleus

Monocular deprivation early in development produces considerable change in the organization of connections within the central mammalian visual system. In the dorsal lateral geniculate nucleus, the soma, dendrites, and axon terminal fields of deprived cells become considerably smaller than nondeprived counterparts. We have examined the possibility that subcellular events enabling structural modification of deprived neurons include modification of proteins comprising the cytoskeleton. We examined the integrity of the cytoskeleton by measuring the response of a subset of its proteins to varying durations of monocular deprivation. Loss of all three neurofilament subunits (light, medium, and heavy) within deprived layers was observed to parallel changes in neuron gross structure. Monocular deprivation initiated beyond early life produced neither a change in structure nor a loss of neurofilament labeling.

Interhemisphere connections of eye dominance columns in the cat visual cortex in conditions of impaired binocular vision

Data from studies of interhemisphere connections in fields 17 and 18 of cats reared in conditions of impaired binocular vision (monocular deprivation, uni- and bilateral strabismus) are presented. Monosynaptic connections between neurons were studied by microiontophoretic application of horseradish peroxidase into cortical eye dominance columns and the distributions of retrograde labeled callosal cells were analyzed. Spatial asymmetry and eye-specific interhemisphere neuron connections persisted in conditions of monocular deprivation and strabismus. Quantitative changes in connections were less marked in monocular deprivation than strabismus. In cats with impaired binocular vision, as in intact animals, the widths of callosal-receiving zones were greater than the widths of the callosal cell zones, which is evidence for the non-reciprocity of interhemisphere connections in cortical areas distant from the projection of the vertical meridian. Morphofunctional differences between cells mediating connections in the opposite directions are proposed.

Functional consequences of neuronal divergence within the retinogeniculate pathway

The neuronal connections from the retina to the dorsal lateral geniculate nucleus (dLGN) are characterized by a high specificity. Each retinal ganglion cell diverges to connect to a small group of geniculate cells and each geniculate cell receives input from a small number of retinal ganglion cells. Consistent with the high specificity of the connections, geniculate cells sharing input from the same retinal afferent are thought to have very similar receptive fields. However, the magnitude of the receptive-field mismatches, which has not been systematically measured across the different cell types in dLGN, seems to be in contradiction with the functional anatomy of the Y visual pathway: Y retinal afferents in the cat diverge into two geniculate layers (A and C) that have Y geniculate cells (Y(A) and Y(C)) with different receptive-field sizes, response latencies, nonlinearity of spatial summation, and contrast sensitivity. To better understand the functional consequences of retinogeniculate divergence, we recorded from pairs of geniculate cells that shared input from a common retinal afferent across layers and within the same layer in dLGN. We found that nearly all cell pairs that shared retinal input across layers had Y-type receptive fields of the same sign (i.e., both on-center) that overlapped by >70%, but frequently differed in size and response latency. The receptive-field mismatches were relatively small in value (receptive-field size ratio <5; difference in peak response <5 ms), but were robustly correlated with the strength of the synchronous firing generated by the shared retinal connections (R(2) = 0.75). On average, the percentage of geniculate spikes that could be attributed to shared retinal inputs was about 10% for all cell-pair combinations studied. These results are used to provide new estimates of retinogeniculate divergence for different cell classes.

Neuronal connections of eye-dominance columns in the cat cerebral cortex after monocular deprivation

Plastic changes in intrahemisphere neuronal connections of the eye-dominance columns of cortical fields 17 and 18 were studied in monocularly deprived cats. The methodology consisted of microintophoretic administration of horseradish peroxidase into cortical columns and three-dimensional reconstruction of the areas of retrograde labeled cells. The eye dominance of columns was established, as were their coordinates in the projection of the visual field. In field 17, the horizontal connections of columns receiving inputs from the non-deprived eye via the crossed-over visual tracts were longer than the connections of the "non-crossed" columns of this eye and were longer than in normal conditions; the connections of the columns of the deprived eye were significantly reduced. Changes in the spatial organization of horizontal connections in field 17 were seen for the columns of the non-deprived eye (areas of labeled cells were rounder and the density of labeled cells in these areas were non-uniform). The longest horizontal connections in deprived cats were no longer than the lengths of these connections in cats with strabismus. It is suggested that the axon length of cells giving rise to the horizontal connections of cortical columns has a limit which is independent of visual stimulation during the critical period of development of the visual system.

Auditory cortex projections target the peripheral field representation of primary visual cortex

The purpose of the present study was to identify projections from auditory to visual cortex and their organization. Retrograde tracers were used to identify the sources of auditory cortical projections to primary visual cortex (areas 17 and 18) in adult cats. Two groups of animals were studied. In the first group, large deposits were centered on the lower visual field representation of the vertical meridian located along the area 17 and 18 border. Following tissue processing, characteristic patterns of cell body labeling were identified in extrastriate visual cortex and the visual thalamus (LGN, MIN, & LPl). In auditory cortex, of the four tonotopically-organized regions, neuronal labeling was identified in the supragranular layers of the posterior auditory field (PAF). Little to no labeling was evident in the primary auditory cortex, the anterior auditory field, the ventral posterior auditory field or in the remaining six non-tonotopically organized regions of auditory cortex. In the second group, small deposits were made into the central or peripheral visual field representations of primary visual cortex. Labeled cells were identified in PAF following deposits into regions of primary visual cortex representing peripheral, but not central, visual field representations. Furthermore, a coarse topography was identified in PAF, with neurons projecting to the upper field representation being located in the gyral portion of PAF and neurons projecting to the lower field representation located in the sulcal portion of PAF. Therefore, direct projections can be identified from tonotopically organized auditory cortex to the earliest stages of visual cortical processing.

First order connections of the visual sector of the thalamic reticular nucleus in marmoset monkeys (Callithrix jacchus)

The thalamic reticular nucleus (TRN) supplies an important inhibitory input to the dorsal thalamus. Previous studies in non-primate mammals have suggested that the visual sector of the TRN has a lateral division, which has connections with first-order (primary) sensory thalamic and cortical areas, and a medial division, which has connections with higher-order (association) thalamic and cortical areas. However, the question whether the primate TRN is segregated in the same manner is controversial. Here, we investigated the connections of the TRN in a New World primate, the marmoset (Callithrix jacchus). The topography of labeled cells and terminals was analyzed following iontophoretic injections of tracers into the primary visual cortex (V1) or the dorsal lateral geniculate nucleus (LGNd). The results show that rostroventral TRN, adjacent to the LGNd, is primarily connected with primary visual areas, while the most caudal parts of the TRN are associated with higher order visual thalamic areas. A small region of the TRN near the caudal pole of the LGNd (foveal representation) contains connections where first (lateral TRN) and higher order visual areas (medial TRN) overlap. Reciprocal connections between LGNd and TRN are topographically organized, so that a series of rostrocaudal injections within the LGNd labeled cells and terminals in the TRN in a pattern shaped like rostrocaudal overlapping "fish scales." We propose that the dorsal areas of the TRN, adjacent to the top of the LGNd, represent the lower visual field (connected with medial LGNd), and the more ventral parts of the TRN contain a map representing the upper visual field (connected with lateral LGNd).

Auditory cortex mapmaking: principles, projections, and plasticity

Maps of sensory receptor epithelia and computed features of the sensory environment are common elements of auditory, visual, and somatic sensory representations from the periphery to the cerebral cortex. Maps enhance the understanding of normal neural organization and its modification by pathology and experience. They underlie the derivation of the computational principles that govern sensory processing and the generation of perception. Despite their intuitive explanatory power, the functions of and rules for organizing maps and their plasticity are not well understood. Some puzzles of auditory cortical map organization are that few complete receptor maps are available and that even fewer computational maps are known beyond primary cortical areas. Neuroanatomical evidence suggests equally organized connectional patterns throughout the cortical hierarchy that might underlie map stability. Here, we consider the implications of auditory cortical map organization and its plasticity and evaluate the complementary role of maps in representation and computation from an auditory perspective.

High-resolution neurometabolic coupling in the lateral geniculate nucleus

The relationships between neural and metabolic processes in activated brain regions are central to the interpretation of noninvasive imaging. To examine this relationship, we have used a specialized sensor to measure simultaneously tissue oxygen changes and neural activity in colocalized regions of the cat's lateral geniculate nucleus (LGN). Previous work with this sensor has shown that a decrease or increase in tissue oxygen can be elicited by selective control of the location and extent of neural activation in the LGN. In the current study, to evaluate the temporal integration and homogeneity of neurometabolic coupling, we have determined the relationship between multiunit extracellular neural activity and tissue oxygen responses to visual stimuli of various durations and contrasts. Our results show that the negative but not the positive oxygen response changes in an approximately linear manner with stimulus duration. The relationship between the negative oxygen response and neural activity is relatively constant with stimulus duration. Moreover, both negative and positive oxygen responses saturate at high stimulus contrast levels. Coupling between neural activity and negative oxygen responses is well described by a power law function. These results help elucidate differences between the initial negative and subsequent positive metabolic responses and may be directly relevant to questions concerning brain mapping with functional magnetic resonance imaging.

Cytoskeleton alteration correlates with gross structural plasticity in the cat lateral geniculate nucleus

Monocular deprivation during early development causes rearrangement of neural connections within the visual cortex that produces a shift in ocular dominance favoring the non-deprived eye. This alteration is manifested anatomically within deprived layers of the lateral geniculate nucleus (LGN) where neurons have smaller somata and reduced geniculocortical terminal fields compared to non-deprived counterparts. Experiments using monocular deprivation have demonstrated a spatial correlation between cytoskeleton alteration and morphological change within the cat LGN, raising the possibility that subcellular events mediating deprivation-related structural rearrangement include modification to the neuronal cytoskeleton. In the current study we compared the spatial and temporal relationships between cytoskeleton alteration and morphological change in the cat LGN. Cross-sectional soma area and neurofilament labeling were examined in the LGN of kittens monocularly deprived at the peak of the critical period for durations that ranged from 1 day to 7 months. After 4 days of deprivation, neuron somata within deprived layers of the LGN were significantly smaller than those within non-deprived layers. This structural change was accompanied by a spatially coincident reduction in neurofilament immunopositive neurons that was likewise significant after 4 days of deprivation. Both anatomical effects reached close to their maximum by 10 days of deprivation. Results from this study demonstrate that alteration to the neuronal cytoskeleton is both spatially and temporally linked to the gross structural changes induced by monocular deprivation.

Dependence of response properties on sparse connectivity in a spiking neuron model of the lateral geniculate nucleus

We present a large-scale anatomically constrained spiking neuron model of the lateral geniculate nucleus (LGN), which operates solely with retinal input, relay cells, and interneurons. We show that interneuron inhibition and sparse connectivity between LGN cells could be key factors for explaining a number of observed classical and extraclassical response properties in LGN of monkey and cat. Among them are 1) weak orientation tuning, 2) contrast invariance of spatial frequency tuning in the absence of cortical feedback, 3) extraclassical surround suppression, and 4) orientation tuning of extraclassical surround suppression. The model also makes two surprising predictions: 1) a possible pinwheel-like spatial organization of orientation preference in the parvo layers of monkey LGN, much like what is seen in V1, and 2) a stimulus-induced trend (bias) in the orientation and phase preference of surround suppression, originating from the stimulus discontinuity between center and surround gratings rather than from specific circuitry.

Complex motion selectivity in PMLS cortex following early lesions of primary visual cortex in the cat

In the cat, the analysis of visual motion cues has generally been attributed to the posteromedial lateral suprasylvian cortex (PMLS) (Toyama et al., 1985; Rauschecker et al., 1987; Rauschecker, 1988; Kim et al., 1997). The responses of neurons in this area are not critically dependent on inputs from the primary visual cortex (VC), as lesions of VC leave neuronal response properties in PMLS relatively unchanged (Spear & Baumann, 1979; Spear, 1988; Guido et al., 1990b). However, previous studies have used a limited range of visual stimuli. In this study, we assessed whether neurons in PMLS cortex remained direction-selective to complex motion stimuli following a lesion of VC, particularly to complex random dot kinematograms (RDKs). Unilateral aspiration of VC was performed on post-natal days 7–9. Single unit extracellular recordings were performed one year later in the ipsilateral PMLS cortex. As in previous studies, a reduction in the percentage of direction selective neurons was observed with drifting sinewave gratings. We report a previously unobserved phenomenon with sinewave gratings, in which there is a greater modulation of firing rate at the temporal frequency of the stimulus in animals with a lesion of VC, suggesting an increased segregation of ON and OFF sub-regions. A significant portion of neurons in PMLS cortex were direction selective to simple (16/18) and complex (11/16) RDKs. However, the strength of direction selectivity to both stimuli was reduced as compared to normals. The data suggest that complex motion processing is still present, albeit reduced, in PMLS cortex despite the removal of VC input. The complex RDK motion selectivity is consistent with both geniculo-cortical and extra-geniculate thalamo-cortical pathways in residual direction encoding.

Retinogeniculate connections: A balancing act between connection specificity and receptive field diversity

Retinogeniculate connections are one of the most striking examples of connection specificity within the visual pathway. In almost every connection there is one dominant afferent cell per geniculate cell, and both afferent and geniculate cells have very similar receptive fields. The remarkable specificity and strength of retinogeniculate connections have inspired comparisons of the lateral geniculate nucleus (LGN) with a simple relay that connects the retina with the visual cortex. However, because each retinal ganglion cell diverges to innervate multiple cells in the LGN, most geniculate cells must receive additional inputs from other retinal afferents that are not the dominant ones. These additional afferents make weaker connections and their receptive fields are not as perfectly matched with the geniculate target as the dominant afferent. We argue that these 'match imperfections' are important to create receptive field diversity among the cells that represent each point of visual space in the LGN. We propose that the convergence of dominant and weak retinal afferents in the LGN multiplexes the array of retinal ganglion cells by creating receptive fields that have a richer range of positions, sizes and response time courses than those available at the ganglion cell layer of the retina.

Does the development of the perigeniculate nucleus support the notion of a hierarchical progression within the visual pathway?

The development of the visual pathway has been extensively studied. However, despite of the importance of the perigeniculate nucleus within this pathway, there is a lack of information concerning its development. The present study examined the dendritic development of perigeniculate nucleus cells using single cell injections in 400-500 microm thick fixed brain slices from kittens of different ages between postnatal day 0 and postnatal day 125. A total of 189 perigeniculate nucleus cells were reconstructed from serial sections for qualitative and quantitative analysis. Cells during the first month were characterized by an abundance of branch points and appendages. There was a significant (P>0.05), albeit variable, increase in the number of branch points and appendages up to about postnatal day 12 after which the numbers were rapidly reduced over the next two weeks. Similarly, appendage numbers significantly increased over the first two weeks until postnatal day 17 and then fell to near adult levels by postnatal day 34. The majority of branch points and appendages occur within 100-200 microm of the soma (10-30% of the dendritic diameter). The data indicate that perigeniculate nucleus dendritic maturation lags shortly behind that of the retina but may precede that of its dorsal thalamic target, the lateral geniculate nucleus. Thus, it may be that the earlier maturation of the perigeniculate nucleus and its inhibitory input is a necessary requirement for the proper development of retinogeniculate and corticothalamic topographic maps within the dorsal lateral geniculate nucleus and perigeniculate nucleus.

The suppressive field of neurons in lateral geniculate nucleus

The responses of neurons in lateral geniculate nucleus (LGN) exhibit powerful suppressive phenomena such as contrast saturation, size tuning, and masking. These phenomena cannot be explained by the classical center-surround receptive field and have been ascribed to a variety of mechanisms, including feedback from cortex. We asked whether these phenomena might all be explained by a single mechanism, contrast gain control, which is inherited from retina and possibly strengthened in thalamus. We formalized an intuitive model of retinal contrast gain control that explicitly predicts gain as a function of local contrast. In the model, the output of the receptive field is divided by the output of a suppressive field, which computes the local root-mean-square contrast. The model provides good fits to LGN responses to a variety of stimuli; with a single set of parameters, it captures saturation, size tuning, and masking. It also correctly predicts that responses to small stimuli grow proportionally with contrast: were it not for the suppressive field, LGN responses would be linear. We characterized the suppressive field and found that it is similar in size to the surround of the classical receptive field (which is eight times larger than commonly estimated), it is not selective for stimulus orientation, and it responds to a wide range of frequencies, including very low spatial frequencies and high temporal frequencies. The latter property is hardly consistent with feedback from cortex. These measurements thoroughly describe the visual properties of contrast gain control in LGN and provide a parsimonious explanation for disparate suppressive phenomena.

Separate spatial scales determine neural activity-dependent changes in tissue oxygen within central visual pathways

The relationship between oxygen levels and neural activity in the brain is fundamental to functional neuroimaging techniques. We have examined this relationship on a fine spatial scale in the lateral geniculate nucleus (LGN) and visual cortex of the cat using a microelectrode sensor that provides simultaneous colocalized measurements of oxygen partial pressure in tissue (tissue oxygen) and multiunit neural activity. In previous work with this sensor, we found that changes in tissue oxygen depend strongly on the location and spatial extent of neural activation. Specifically, focal neural activity near the microelectrode elicited decreases in tissue oxygen, whereas spatially extended activation, outside the field of view of our sensor, yielded mainly increases. In the current study, we report an expanded set of measurements to quantify the spatiotemporal relationship between neural responses and changes in tissue oxygen. For the purpose of data analysis, we develop a quantitative model that assumes that changes in tissue oxygen are composed of two response components (one positive and one negative) with magnitudes determined by neural activity on separate spatial scales. Our measurements from visual cortex and the LGN are consistent with this model and suggest that the positive response spreads over a distance of 1-2 mm, whereas the negative component is confined to a few hundred micrometers. These results are directly relevant to the mechanisms that generate functional brain imaging signals and place limits on their spatial properties.

Intravitreal macrophage activation enables cat retinal ganglion cells to regenerate injured axons into the mature optic nerve

In mature mammals, retinal ganglion cells (RGCs) are generally unable to regenerate injured axons into the optic nerve. Here, we report that an intravitreal injection of either of two macrophage activators, oxidized galectin-1 or zymosan, strongly enhanced the regeneration of transected RGC axons beyond an optic nerve crush site in adult cats. Using WGA-HRP as an anterograde tracer, we found that injection of either macrophage activator caused many axons to grow into the distal optic nerve when evaluated 14 days later, with the strongest effects seen after injecting 100 ng of galectin-1. Elongation continued for at least another 2 weeks. Control eyes injected with saline contained very few labeled axons extending across the crush site. Elevation of intracellular cAMP levels using forskolin also enhanced regeneration beyond the crush site to some extent, but this treatment did not augment the effect of galectin-1 any further. These results indicate that RGCs of adult cats are capable of reverting to an active growth state and at least partially overcoming an inhibitory CNS environment as a result of intravitreal macrophage activation.

Impact of Cortical Network Activity on Short-term Synaptic Depression

Repetitive stimulation of synaptic connections in the cerebral cortex often induces short-term synaptic depression (STD), a property directly related to the probability of transmitter release and critical for the computational properties of the network. In order to explore how spontaneous activity in the network affects this property, we first studied STD in cortical slices that were either silent or that displayed spontaneous rhythmic slow oscillations resembling those recorded during slow wave sleep in vivo. STD was considerably reduced by the occurrence of spontaneous rhythmic activity in the cortical network. Once the rhythmic activity started, depression decreased over time in parallel with the duration and intensity of the ongoing activity until a plateau was reached. Thalamocortical and intracortical synaptic potentials studied in vivo also showed stronger depression in a silent than in an active cortical network, and the depression values in the active cortical network in vivo were indistinguishable from those found in active slices in vitro. We suggest that this phenomenon is due to the different steady states of the synapses in active and in silent networks.

Functional impact of primary visual cortex deactivation on subcortical target structures in the thalamus and midbrain

The functional relationships between the primary visual cortex and its major subcortical target structures have long been a subject of interest. We studied these relationships by using localized cooling deactivation to silence portions of primary visual cortex and measuring 2-deoxyglucose (2DG) uptake to assess neural activity in subcortical and midbrain targets. We focused analysis on the largest subcortical targets of primary visual cortex: the superior colliculus (SC), the dorsal lateral geniculate nucleus of the thalamus (dLGN), and the lateral division of the lateral posterior nucleus of the thalamus (LPL). We found that localized cooling of different regions of primary visual cortex caused specific decreases in 2DG uptake in target structures such that the location of 2DG decrease varied according to joint retinotopy, and the magnitude of the decreases in target structures was associated with the amount of cooled cortex. In addition, we found that the impact of cortical cooling was more profound on the SC than on the dLGN. The functional impact of cortical deactivations on the LPL was weak for small deactivations but approximated the impact on the SC when deactivations were large. We discuss these findings in terms of neural circuits and in terms of drivers and modulators.