Receptive field structure varies with layer in the primary visual cortex

Here we ask whether visual response pattern varies with position in the cortical microcircuit by comparing the structure of receptive fields recorded from the different layers of the cat's primary visual cortex. We used whole-cell recording in vivo to show the spatial distribution of visually evoked excitatory and inhibitory inputs and to stain individual neurons. We quantified the distribution of 'On' and 'Off' responses and the presence of spatially opponent excitation and inhibition within the receptive field. The thalamorecipient layers (4 and upper 6) were dominated by simple cells, as defined by two criteria: they had separated On and Off subregions, and they had push-pull responses (in a given subregion, stimuli of the opposite contrast evoked responses of the opposite sign). Other types of response profile correlated with laminar location as well. Thus, connections unique to each visual cortical layer are likely to serve distinct functions.

Functional imaging with cellular resolution reveals precise micro-architecture in visual cortex

Neurons in the cerebral cortex are organized into anatomical columns, with ensembles of cells arranged from the surface to the white matter. Within a column, neurons often share functional properties, such as selectivity for stimulus orientation; columns with distinct properties, such as different preferred orientations, tile the cortical surface in orderly patterns. This functional architecture was discovered with the relatively sparse sampling of microelectrode recordings. Optical imaging of membrane voltage or metabolic activity elucidated the overall geometry of functional maps, but is averaged over many cells (resolution >100 microm). Consequently, the purity of functional domains and the precision of the borders between them could not be resolved. Here, we labelled thousands of neurons of the visual cortex with a calcium-sensitive indicator in vivo. We then imaged the activity of neuronal populations at single-cell resolution with two-photon microscopy up to a depth of 400 microm. In rat primary visual cortex, neurons had robust orientation selectivity but there was no discernible local structure; neighbouring neurons often responded to different orientations. In area 18 of cat visual cortex, functional maps were organized at a fine scale. Neurons with opposite preferences for stimulus direction were segregated with extraordinary spatial precision in three dimensions, with columnar borders one to two cells wide. These results indicate that cortical maps can be built with single-cell precision.

Efficacy of retinal spikes in driving cortical responses

How does a single retinal ganglion cell (RGC) affect the firing of simple cells in the visual cortex? Although much is known of the functional connections between the retina and the lateral geniculate nucleus (LGN) and between LGN and visual cortex, it is hard to infer the effect of disynaptic connections from retina to visual cortex. Most importantly, there is considerable divergence from retina to LGN, so cortical neurons might be influenced by ganglion cells through multiple feedforward pathways. We recorded simultaneously from ganglion cells in the retina and cortical simple cells in the striate cortex with overlapping receptive fields and evaluated disynaptic connections with cross-correlation analysis. In all disynaptically connected pairs, the retinal receptive field center and overlapping cortical subregion always shared the same sign (either both ON or both OFF). Connected pairs were similar in other respects, such as relative position and timing of their receptive fields, and thus obeyed the same rules of connectivity found previously for retinothalamic and thalamocortical connections. We found that a single RGC directly contributed on average to approximately 3% of the activity of its cortical target. The relative timing of pairs of spikes from the retinal cell affected their efficacy in driving the cortical cell. When two retinal spikes were closely spaced (<10 msec), the second spike was several times more likely to drive the cortical target. The relative magnitude of this disynaptic paired spike enhancement was considerably larger than has been found previously for retinogeniculate and geniculocortical connections. The amplified paired spike enhancement from retina to cortex ensures that signal transmission from retina to cortex is particularly effective when the retina fires a series of closely spaced action potentials.

Role of subplate neurons in functional maturation of visual cortical columns

The subplate forms a transient circuit required for development of connections between the thalamus and the cerebral cortex. When subplate neurons are ablated, ocular dominance columns do not form in the visual cortex despite the robust presence of thalamic axons in layer 4. We show that subplate ablation also prevents formation of orientation columns. Visual responses are weak and poorly tuned to orientation. Furthermore, thalamocortical synaptic transmission fails to strengthen, whereas intracortical synapses are unaffected. Thus, subplate circuits are essential not only for the anatomical segregation of thalamic inputs but also for key steps in synaptic remodeling and maturation needed to establish the functional architecture of visual cortex.

The spatial receptive field of thalamic inputs to single cortical simple cells revealed by the interaction of visual and electrical stimulation

Electrical stimulation of the thalamus has been widely used to test for the existence of monosynaptic input to cortical neurons, typically with stimulation currents that evoke cortical spikes with high probability. We stimulated the lateral geniculate nucleus (LGN) of the thalamus and recorded monosynaptically evoked spikes from layer 4 neurons in visual cortex. We found that with moderate currents, cortical spikes were evoked with low to moderate probability and their occurrence was modulated by ongoing sensory (visual) input. Furthermore, when repeated at 8-12 Hz, electrical stimulation of the thalamic afferents caused such profound inhibition that cortical spiking activity was suppressed, aside from electrically evoked monosynaptic spikes. Visual input to layer 4 cortical cells between electrical stimuli must therefore have derived exclusively from LGN afferents. We used white-noise visual stimuli to make a 2D map of the receptive field of each cortical simple cell during repetitive electrical stimulation in the LGN. The receptive field of electrically evoked monosynaptic spikes (and thus of the thalamic input alone) was significantly elongated. Its primary subfield was comparable to that of the control receptive field, but secondary (flanking) subfields were weaker. These findings extend previous results from intracellular recordings, but also demonstrate the effectiveness of an extracellular method of measuring subthreshold afferent input to cortex.

Saccadic eye movements modulate visual responses in the lateral geniculate nucleus

We studied the effects of saccadic eye movements on visual signaling in the primate lateral geniculate nucleus (LGN), the earliest stage of central visual processing. Visual responses were probed with spatially uniform flickering stimuli, so that retinal processing was uninfluenced by eye movements. Nonetheless, saccades had diverse effects, altering not only response strength but also the temporal and chromatic properties of the receptive field. Of these changes, the most prominent was a biphasic modulation of response strength, weak suppression followed by strong enhancement. Saccadic modulation was widespread, and affected both of the major processing streams in the LGN. Our results demonstrate that during natural viewing, thalamic response properties can vary dramatically, even over the course of a single fixation.

Laminar processing of stimulus orientation in cat visual cortex

One of the most salient features to emerge in visual cortex is sensitivity to stimulus orientation. Here we asked if orientation selectivity, once established, is altered by successive stages of cortical processing. We measured patterns of orientation selectivity at all depths of the cat's visual cortex by making whole-cell recordings with dye-filled electrodes. Our results show that the synaptic representation of orientation indeed changes with position in the microcircuit, as information passes from layer 4 to layer 2+3 to layer 5. At the earliest cortical stage, for simple cells in layer 4, orientation tuning curves for excitation (depolarization) and inhibition (hyperpolarization) had similar peaks (within 0-7 deg, n = 11) and bandwidths. Further, the sharpness of orientation selectivity covaried with receptive field geometry (r = 0.74) - the more elongated the strongest subregion, the shaper the tuning. Tuning curves for complex cells in layer 2+3 also had similar peaks (within 0-4 deg, n = 7) and bandwidths. By contrast, at a later station, layer 5, the preferred orientation for excitation and inhibition diverged such that the peaks of the tuning curves could be as far as 90 deg apart (average separation, 54 deg; n = 6). Our results support the growing consensus that orientation selectivity is generated at the earliest cortical level and structured similarly for excitation and inhibition. Moreover, our novel finding that the relative tuning of excitation and inhibition changes with laminar position helps resolve prior controversy about orientation selectivity at later phases of processing and gives a mechanistic view of how the cortical circuitry recodes orientation.

Predicting every spike: a model for the responses of visual neurons

In the early visual system, neuronal responses can be extremely precise. Under a wide range of stimuli, cells in the retina and thalamus fire spikes very reproducibly, often with millisecond precision on subsequent stimulus repeats. Here we develop a mathematical description of the firing process that, given the recent visual input, accurately predicts the timing of individual spikes. The formalism is successful in matching the spike trains from retinal ganglion cells in salamander, rabbit, and cat, as well as from lateral geniculate nucleus neurons in cat. It adapts to many different response types, from very precise to highly variable. The accuracy of the model allows a compact description of how these neurons encode the visual stimulus.

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.

Synthesis, stability, antiviral activity, and protease-bound structures of substrate-mimicking constrained macrocyclic inhibitors of HIV-1 protease

Three new peptidomimetics (1-3) have been developed with highly stable and conformationally constrained macrocyclic components that replace tripeptide segments of protease substrates. Each compound inhibits both HIV-1 protease and viral replication (HIV-1, HIV-2) at nanomolar concentrations without cytotoxicity to uninfected cells below 10 microM. Their activities against HIV-1 protease (K(i) 1.7 nM (1), 0.6 nM (2), 0.3 nM (3)) are 1-2 orders of magnitude greater than their antiviral potencies against HIV-1-infected primary peripheral blood mononuclear cells (IC(50) 45 nM (1), 56 nM (2), 95 nM (3)) or HIV-1-infected MT2 cells (IC(50) 90 nM (1), 60 nM (2)), suggesting suboptimal cellular uptake. However their antiviral potencies are similar to those of indinavir and amprenavir under identical conditions. There were significant differences in their capacities to inhibit the replication of HIV-1 and HIV-2 in infected MT2 cells, 1 being ineffective against HIV-2 while 2 was equally effective against both virus types. Evidence is presented that 1 and 2 inhibit cleavage of the HIV-1 structural protein precursor Pr55(gag) to p24 in virions derived from chronically infected cells, consistent with inhibition of the viral protease in cells. Crystal structures refined to 1.75 A (1) and 1.85 A (2) for two of the macrocyclic inhibitors bound to HIV-1 protease establish structural mimicry of the tripeptides that the cycles were designed to imitate. Structural comparisons between protease-bound macrocyclic inhibitors, VX478 (amprenavir), and L-735,524 (indinavir) show that their common acyclic components share the same space in the active site of the enzyme and make identical interactions with enzyme residues. This substrate-mimicking minimalist approach to drug design could have benefits in the context of viral resistance, since mutations which induce inhibitor resistance may also be those which prevent substrate processing.

The koniocellular pathway in primate vision

A neurochemically distinct population of koniocellular (K) neurons makes up a third functional channel in primate lateral geniculate nucleus. As part of a general pattern, K neurons form robust layers through the full representation of the visual hemifield. Similar in physiology and connectivity to W cells in cat lateral geniculate nucleus, K cells form three pairs of layers in macaques. The middle pair relays input from short-wavelength cones to the cytochrome-oxidase blobs of primay visual cortex (V1), the dorsal-most pair relays low-acuity visual information to layer I of V1, and the ventral-most pair appears closely tied to the function of the superior colliculus. Throughout each K layer are neurons that innervate extrastriate cortex and that are likely to sustain some visual behaviors in the absence of V1. These data show that several pathways exist from retina to V1 that are likely to process different aspects of the visual scene along lines that may remain parallel well into V1.

Low response variability in simultaneously recorded retinal, thalamic, and cortical neurons

The response of a cortical cell to a repeated stimulus can be highly variable from one trial to the next. Much lower variability has been reported of retinal cells. We recorded visual responses simultaneously from three successive stages of the cat visual system: retinal ganglion cells (RGCs), thalamic (LGN) relay cells, and simple cells in layer 4 of primary visual cortex. Spike count variability was lower than that of a Poisson process at all three stages but increased at each stage. Absolute and relative refractory periods largely accounted for the reliability at all three stages. Our results show that cortical responses can be more reliable than previously thought. The differences in reliability in retina, LGN, and cortex can be explained by (1) decreasing firing rates and (2) decreasing absolute and relative refractory periods.

Small peptides Do not inhibit human non-pancreatic secretory phospholipase-A(2) (Type IIA)

Seven small peptides, that are among the most potent reported inhibitors of secreted mammalian phospholipases A(2), were found not to inhibit processing of a small phospholipid substrate by human non-pancreatic secretory phospholipase A(2) (type IIa), under conditions where certain non-peptides are potent inhibitors at nanomolar concentrations.

Temporal coding of visual information in the thalamus

The amount of information a sensory neuron carries about a stimulus is directly related to response reliability. We recorded from individual neurons in the cat lateral geniculate nucleus (LGN) while presenting randomly modulated visual stimuli. The responses to repeated stimuli were reproducible, whereas the responses evoked by nonrepeated stimuli drawn from the same ensemble were variable. Stimulus-dependent information was quantified directly from the difference in entropy of these neural responses. We show that a single LGN cell can encode much more visual information than had been demonstrated previously, ranging from 15 to 102 bits/sec across our sample of cells. Information rate was correlated with the firing rate of the cell, for a consistent rate of 3.6 +/- 0.6 bits/spike (mean +/- SD). This information can primarily be attributed to the high temporal precision with which firing probability is modulated; many individual spikes were timed with better than 1 msec precision. We introduce a way to estimate the amount of information encoded in temporal patterns of firing, as distinct from the information in the time varying firing rate at any temporal resolution. Using this method, we find that temporal patterns sometimes introduce redundancy but often encode visual information. The contribution of temporal patterns ranged from -3.4 to +25.5 bits/sec or from -9.4 to +24.9% of the total information content of the responses.

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.

Dimensions of care for dementia sufferers in long-term care institutions: are they related to outcomes?

OBJECTIVES

This study empirically examined whether dimensions of care cluster in special care units (SCUs) compared with non-SCUs. The relationship between SCU status plus separate measures of the dimensions of care and outcomes for dementia sufferers was then investigated.

METHODS

Data were drawn from the Intermediate Care Facility Project. The sample (N = 510) included residents with dementia, aged 65 and older, in intermediate care facilities throughout the province of British Columbia. Canada. Longitudinal data included 6 outcomes: cognitive function, behavioral problems of agitation and social skills, physical functioning, and quality of life measured through affect and expressive language skills. Separate multiple linear regression equations were estimated, relating each of these outcomes to 5 dimensions of care: preadmission and admission procedures. staff training and education, nonuse of physical and chemical restraints, flexible care routines and resident-relevant activities, and the environment.

RESULTS

The results showed there is virtually no clustering of dimensions along SCU/non-SCU lines. Neither SCU status nor the individual dimensions were highly predictive of outcomes. Residents' affect at t1 emerged as a characteristic that was significantly correlated with other outcomes.

DISCUSSION

This Canadian research can be added to the few but growing number of rigorous studies that suggest SCUs are not homogeneous and do not necessarily provide better care than non-SCUs. Moreover, it raises questions about the benefits of "best practice" dimensions of care, regardless of SCU status.

Diverse receptive fields in the lateral geniculate nucleus during thalamocortical development

Most models of thalamocortical development in the visual system assume a homogeneous population of thalamic inputs to the cortex, each with concentric on- or off-center receptive fields. To test this, we made high-resolution spatial maps of receptive fields in the developing ferret lateral geniculate nucleus (LGN). Developing receptive fields (RFs), had a variety of shapes: some concentric, others elongated (like adult cortical receptive fields) and some with 'hot spots' of sensitivity. These receptive fields seemed to arise from convergence of multiple retinal afferents onto LGN neurons. We present a Hebbian model whereby imprecise retinogeniculate connections help refine geniculocortical connections, sharpening both thalamocortical topography and perhaps orientation selectivity.

Conformational selection of inhibitors and substrates by proteolytic enzymes: implications for drug design and polypeptide processing

Inhibitors of proteolytic enzymes (proteases) are emerging as prospective treatments for diseases such as AIDS and viral infections, cancers, inflammatory disorders, and Alzheimer's disease. Generic approaches to the design of protease inhibitors are limited by the unpredictability of interactions between, and structural changes to, inhibitor and protease during binding. A computer analysis of superimposed crystal structures for 266 small molecule inhibitors bound to 48 proteases (16 aspartic, 17 serine, 8 cysteine, and 7 metallo) provides the first conclusive proof that inhibitors, including substrate analogues, commonly bind in an extended beta-strand conformation at the active sites of all these proteases. Representative superimposed structures are shown for (a) multiple inhibitors bound to a protease of each class, (b) single inhibitors each bound to multiple proteases, and (c) conformationally constrained inhibitors bound to proteases. Thus inhibitor/substrate conformation, rather than sequence/composition alone, influences protease recognition, and this has profound implications for inhibitor design. This conclusion is supported by NMR, CD, and binding studies for HIV-1 protease inhibitors/substrates which, when preorganized in an extended conformation, have significantly higher protease affinity. Recognition is dependent upon conformational equilibria since helical and turn peptide conformations are not processed by proteases. Conformational selection explains the resistance of folded/structured regions of proteins to proteolytic degradation, the susceptibility of denatured proteins to processing, and the higher affinity of conformationally constrained 'extended' inhibitors/substrates for proteases. Other approaches to extended inhibitor conformations should similarly lead to high-affinity binding to a protease.

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.

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.

Automated purification and quantification of oligonucleotides

We have developed automated methods for the trityl-on purification and quantification of synthetic oligonucleotides. Oligonucleotide purification is by solid-phase extraction cartridges using Amberchrom CG-50 resin on an XYZ-axis robotic system. Quantification is by OD260nm using an online UV-visible spectrophotometer with sipper. The purification of 20 oligonucleotides requires 5 min of user set-up time, plus 20 min per sample of robot time. For a 15-25-mer at the 40 nmol scale of synthesis, the method gives a yield of 2.8 ODs from a load of 10.1 OD, i.e., a 28% average yield. Oligonucleotides purified by this method have proven to be successful for primers for automated DNA sequencing.

Molecular recognition of macrocyclic peptidomimetic inhibitors by HIV-1 protease

High-resolution crystal structures are described for seven macrocycles complexed with HIV-1 protease (HIVPR). The macrocycles possess two amides and an aromatic group within 15-17 membered rings designed to replace N- or C-terminal tripeptides from peptidic inhibitors of HIVPR. Appended to each macrocycle is a transition state isostere and either an acyclic peptide, nonpeptide, or another macrocycle. These cyclic analogues are potent inhibitors of HIVPR, and the crystal structures show them to be structural mimics of acyclic peptides, binding in the active site of HIVPR via the same interactions. Each macrocycle is restrained to adopt a beta-strand conformation which is preorganized for protease binding. An unusual feature of the binding of C-terminal macrocyclic inhibitors is the interaction between a positively charged secondary amine and a catalytic aspartate of HIVPR. A bicyclic inhibitor binds similarly through its secondary amine that lies between its component N-terminal and C-terminal macrocycles. In contrast, the corresponding tertiary amine of the N-terminal macrocycles does not interact with the catalytic aspartates. The amine-aspartate interaction induces a 1.5 A N-terminal translation of the inhibitors in the active site and is accompanied by weakened interactions with a water molecule that bridges the ligand to the enzyme, as well as static disorder in enzyme flap residues. This flexibility may facilitate peptide cleavage and product dissociation during catalysis. Proteases [Aba67,95]HIVPR and [Lys7,Ile33,Aba67,95]HIVPR used in this work were shown to have very similar crystal structures.

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.