Spatial summation in lateral geniculate nucleus and visual cortex.

We have compared the spatial summation characteristics of cells in the primary visual cortex with those of cells in the dorsal lateral geniculate nucleus (LGN) that provide the input to the cortex. We explored the influence of varying the diameter of a patch of grating centred over the receptive field and quantitatively determined the optimal summation diameter and the degree of surround suppression for cells at both levels of the visual system using the same stimulus parameters. The mean optimal summation size for LGN cells (0.90 degrees) was much smaller than that of cortical cells (3.58 degrees). Virtually all LGN cells exhibited strong surround suppression with a mean value of 74%+/-1.61% SEM for the population as a whole. This potent surround suppression in the cells providing the input to the cortex suggests that cortical cells must integrate their much larger summation fields from the low firing rates associated with the suppression plateau of the LGN cell responses. Our data suggest that the strongest input to cortical cells will arise from geniculate cells representing areas of visual space located at the borders of a visual stimulus. We suggest that analysis of response properties by patterns centred over the receptive fields of cells may give a misleading impression of the process of the representation. Analysis of pattern terminations or salient borders over the receptive field may provide much more insight into the processing algorithms involved in stimulus representation.

Comparison of the laminar distribution of input from areas 17 and 18 of the visual cortex to the lateral geniculate nucleus of the cat.

The feedback from area 18 of the cat visual cortex to the lateral geniculate nucleus has been investigated by labeling and reconstructing seventeen axons of known receptive field position and eye preference. The distribution of boutons from each axon was quantified with respect to the compartments of the geniculate complex, and the results were compared with an equivalent analysis of fourteen area 17 axons. Area 18 axons form large, sparse arborizations that extend up to 1.9 mm laterally (1170 +/- 85 microm; mean +/- SEM), with a core of relatively dense innervation spanning on average 600 +/- 70 microm (mean +/- SEM). Thus, they have the potential to influence the transmission of visual information from well beyond their own classical receptive fields. In this respect, they are surprisingly similar to the axons from area 17, despite the fact that the two cortical areas have very different retinotopy. However, there are important differences between the pathways. Area 18 axons project more heavily to the C layers and medial interlaminar nucleus. Whereas the input from both areas to the A layers is biased toward the layer appropriate to the eye preference of each axon, the area 18 input to magnocellular layer C is not. The distribution of area 18 boutons favors the bottom of their preferred A layer, and the area 17 boutons favor the top. These differences mirror those seen in the afferent pathways, suggesting that each cortical area preferentially targets the cells from which it receives input. Finally, their greater diameter suggests that area 18 axons provide the earliest feedback signal in the corticogeniculate loop.

Feedback connections to the lateral geniculate nucleus and cortical response properties.

The cerebral cortex receives sensory input from the periphery by means of thalamic relay nuclei, but the flow of information goes both ways. Each cortical area sends a reciprocal projection back to the thalamus. In the visual system, the synaptic relations that govern the influence of thalamic afferents on orientation selectivity in the cortex have been studied extensively. It now appears that the connectivity of the corticofugal feedback pathway is also fundamentally linked to the orientation preference of the cortical cells involved.

Immunoprecipitation and virus neutralization assays demonstrate qualitative differences between protective antibody responses to inactivated hepatitis A vaccine and passive immunization with immune globulin.

Antibodies to hepatitis A virus (anti-HAV) were measured in children from two separate vaccine trials (n = 70) 4 weeks after a dose of inactivated hepatitis A vaccine (VAQTA). The geometric mean titers (GMTs) of anti-HAV were 49.3 and 45.2 mIU/mL by immunoassay, while reciprocal GMTs of neutralizing anti-HAV were 6.5 and 15.0 by an 80% radioimmunofocus inhibition test (RIFIT) and 55.6 and 92.0 by antigen reduction assay (HAVARNA). The GMT of antibody detected by radioimmunoprecipitation (RIPA) was > or =401. These data establish serologic correlates of protection against disease and show that RIPA is most sensitive for detection of early vaccine-induced antibody. Sera collected from adults (n = 20) 7 days after administration of immune globulin contained similar antibody levels by immunoassay (45.1 mIU/mL) and slightly higher GMTs of neutralizing antibody (27.5 by RIFIT and 146 by HAVARNA) but negligible precipitating antibody (GMT, 5.6). These results are best explained by differences in the affinity of antibodies for virus following active versus passive immunization.

Functional morphology of the feedback pathway from area 17 of the cat visual cortex to the lateral geniculate nucleus.

Two approaches were adopted to study the pattern of connectivity between the cat visual cortex and lateral geniculate nucleus. Fourteen individual cortico-geniculate axons were labeled and reconstructed after intracellular or extracellular injection of biocytin into regions of known receptive-field position and ocular dominance preference, and the distribution of boutons from multi-axon clusters was mapped in three dimensions and compared with the locations of strategically placed geniculate recordings made in the same tissue. The results show that the feedback has an accurate retinotopic component but that individual axons are both more extensive and more selective than described previously. Area 17 feedback axons terminate primarily in layers A and A1, but the distribution of terminal boutons is strongly biased (3:1 ratio) toward the layer that matches their eye preference. Thus, those driven by the contralateral eye preferentially target layer A, and those driven by the ipsilateral eye target layer A1. Each axon also innervates the perigeniculate nucleus (PGN), but the pattern is otherwise variable, suggesting that there are different axonal classes. The terminal fields of individual axons are much larger than described previously, with a maximum spread of 500-1500 microns. Nevertheless, the projection from a given location in area 17 has a center of maximum terminal density 400-500 microns across, which is in retinotopic correspondence with the aggregate receptive field of the cortical cells of origin. The surrounding zone of relatively sparse boutons, however, must permit corticofugal cells to influence visual processing well beyond the regions over which their own responses summate. It follows that any geniculate cell receives corticofugal input covering an equally extensive area of visual space.

A hepatitis A virus deletion mutant which lacks the first pyrimidine-rich tract of the 5' nontranslated RNA remains virulent in primates after direct intrahepatic nucleic acid transfection.

Cell culture-adapted variants of hepatitis A virus (HAV) in which the first pyrimidine-rich tract (pY1; nucleotides 99 to 138) of the 5' nontranslated region has been deleted (delta 96-137 or delta 96-139) replicate as well as parental virus in cultured cells (D.R. Shaffer, E.A. Brown, and S.M. Lemon, J. Virol. 68:5568-5578, 1994). To determine whether viruses with such large deletion mutations are able to replicate and to produce acute hepatitis in primates, we reconstructed the delta 96-137 deletion in the genetic background of a virulent virus which differs from the wild type by only one mutation in the 2B-coding region (HM175/8Y). Full-length synthetic delta 96-137/8Y RNA was injected into the livers of two HAV-seronegative marmosets (Saguinus mystax). Both animals developed serum liver enzyme elevations and inflammatory changes in serial liver biopsies within 3 to 4 weeks of inoculation which were comparable in magnitude to those observed previously following intrahepatic inoculation of marmosets with HM175/8Y RNA. Sequencing of RNA from virus shed in feces demonstrated the presence of the delta 96-137 deletion. These results indicate that the pY1 sequence of HAV is not required for efficient viral replication in hepatocytes in situ or for production of acute hepatic injury following intrahepatic RNA transfection in primates.

Effects of brain stem parabrachial activation on receptive field properties of cells in the cat's lateral geniculate nucleus.

1. The lateral geniculate nucleus is the primary thalamic relay for the transfer of retinal signals to the visual cortex. Geniculate cells are heavily innervated from nonretinal sources, and these modify retinogeniculate transmission. A major ascending projection to the lateral geniculate nucleus arises from cholinergic cells in the parabrachial region of the brain stem. This is an important pathway in the ascending control of arousal. In an in vivo preparation, we used extracellular recordings to study the effects of electrical activation of the parabrachial region on the spontaneous activity and visual responses of X and Y cells in the lateral geniculate nucleus of the cat. 2. We studied the effects of two patterns of parabrachial activation on the spontaneous activity of geniculate cells. Burst stimulation consisted of a short pulse at high frequency (16 ms at 250 Hz). Train stimulation was of longer duration at lower frequency (e.g., 1 s at 50 Hz). The firing rate of almost all geniculate cells was enhanced by either pattern of stimulation. However, the burst pattern of stimulation elicited a short, modulated response with excitatory and inhibitory epochs. We found that the different epochs could differentially modulate the visual responses to drifting gratings. Thus the temporal alignment of the brain stem and visual stimuli was critical with burst stimulation, and varied alignments could dramatically confound the results. In comparison, the train pattern of stimulation consistently produced a relatively flat plateau of increased firing, after a short initial period of more variable effects. We used the less confounding pattern of train stimuli to study the effects of parabrachial activation on visual responses. 3. Our main emphasis was to examine the parabrachial effects on the visual responses of geniculate cells. For most visual stimuli, we used drifting sine wave gratings that varied in spatial frequency; these evoked modulated responses from the geniculate cells. Parabrachial activation enhanced the visual responses of almost all geniculate cells, and this enhancement included both increased depth of modulation and greater response rates. 4. Our results were incorporated quantitatively into a difference-of-Gaussians model of visual receptive fields in order to study the parabrachial effects on the spatial structure of the receptive field. This model fit our data well and provided measures of the response amplitude and radius of the receptive field center (Kc and Rc, respectively) and the response amplitude and radius of the receptive field surround (Ks and Rs, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Brain-stem modulation of the response properties of cells in the cat's perigeniculate nucleus.

Transmission through the lateral geniculate nucleus is facilitated following activation of the cholinergic input from the brain stem, which is thought to reflect activity patterns seen during arousal. One of the underlying mechanisms is the suppression of inhibitory circuits local to the lateral geniculate nucleus. However, evidence exists that some visually driven inhibitory inputs to geniculate nucleus. preserved or even enhanced under conditions of arousal, and during electrical activation of the parabrachial region of the brain stem. We have therefore reexamined the effect of brain-stem activation on the visual responses of one group of local inhibitory inputs to geniculate relay cells, those emanating from the adjacent perigeniculate nucleus. We recorded single perigeniculate cells in anesthetized, paralyzed cats. Axons innervating the lateral geniculate and perigeniculate nuclei from the parabrachial region of the brain stem were electrically activated, and the effect of this activation was assessed on both spontaneous and visually evoked responses. Visual stimulation consisted of sinusoidally modulated sine-wave gratings of varying spatial and temporal frequency. For the great majority of perigeniculate cells (32 of 40), brain-stem activation inhibited spontaneous activity, while one cell was excited, three showed a mixed effect and four were unaffected. Nevertheless, the responses of most cells (30 of 40) were facilitated when brain-stem activation was paired with certain spatio-temporal patterns of visual stimulation. Spatial tuning curves were constructed for 17 cells and temporal tuning curves for 14, before and during parabrachial activation. The responses of any one cell could be facilitated, unchanged, or suppressed, depending on the visual stimulus used. In some cases, this substantially modified the cell's spatial and temporal tuning properties. We conclude that activation of the brain stem disinhibits geniculate relay cells in the absence of visual stimulation, but it has the potential to enhance either the magnitude or specificity of visually driven inhibition arising from the perigeniculate nucleus.

Removal/neutralization of hepatitis A virus during manufacture of high purity, solvent/detergent factor VIII concentrate.

Recent reports have suggested an increased risk of type A viral hepatitis in hemophilic patients treated with high purity factor VIII concentrates prepared using ion exchange chromatography coupled with solvent/detergent treatment for inactivation of viruses. To determine the capacity for removal or inactivation of hepatitis A virus during the factor VIII manufacturing process, human plasma and various factor VIII production intermediates were spiked with cell culture-propagated virus and subjected to scaled down conditions mimicking the manufacture of solvent/detergent factor VIII. The combination of antibody-mediated neutralization, cryoprecipitation, anion exchange chromatography, and lyophilization in the absence of sucrose resulted in a minimal reduction of 5.5 to 8.55 log10 in the infectivity of hepatitis A virus.

Attenuation phenotype of a cell culture-adapted variant of hepatitis A virus (HM175/p16) in susceptible New World owl monkeys.

The virulence of a clonally isolated, cell culture-adapted hepatitis A virus (HM175/p16) was assessed in 4 seronegative owl monkeys inoculated intravenously with 2.8 x 10(4) radioimmunofocus-forming units of virus. The virus was highly attenuated, even though its complete nucleotide sequence contains only 19 mutations from the wild-type genome. Only 3 monkeys developed antibodies to hepatitis A virus (only 2 within 96 days of virus inoculation). One monkey had viremia and significantly elevated serum aminotransferase levels. In this animal, maximum viremia and fecal shedding of virus occurred 30-33 days after inoculation. In contrast, in earlier studies of a related cell culture-adapted but still hepatovirulent virus (HM175/S18), viremia was documented in 6 of 6 animals and peak viremia and fecal shedding of virus occurred 18 or 19 days after intravenous inoculation of about one-tenth as much virus.

Effects of vasoactive intestinal polypeptide on the response properties of cells in area 17 of the cat visual cortex.

1. Vasoactive intestinal polypeptide (VIP) was iontophoretically applied to a population of 90 single cells in the primary visual cortex (area 17) of the cat. Response magnitude, response selectivity, spontaneous activity, and the ratio between the visual response and spontaneous activity (signal-to-noise ratio) of the cells were assessed quantitatively before and during drug application. 2. VIP had little effect in the absence of visual stimulation, with only 29/90 (32%) of the cells showing a change of even 1 sp/s in their spontaneous activity. In contrast it had a clear effect on the visual responses of the majority (73/90, 81%) of the cells tested. 3. VIP produced a substantial change (i.e., > or = 40%) in optimal response magnitude for 57 of the affected cells. Of these 65% were facilitated, usually with no change or an improvement in signal-to-noise ratio and direction selectivity. The remaining cells were inhibited, with more variable effects on their visual response characteristics, and were found predominantly in the superficial laminae. 4. The effects of VIP bore a remarkable resemblance to those reported previously for the muscarinic action of acetylcholine (ACh). VIP and a muscarinic cholinergic agonist, either ACh or acetyl-beta-methacholine (MeCh), were therefore applied in turn to a group of 40 cells. In 23 cases VIP and the muscarinic agonist were also applied simultaneously. 5. The effects of VIP and the cholinergic agonist matched in 92% of the cases where both drugs were effective. That is to say, cells that were facilitated by VIP were facilitated also by ACh or MeCh, and vice versa. In many instances there was a clear similarity in the pattern as well as the direction of the effects produced by the two substances. The result of simultaneous application was generally additive. 6. These data suggest that VIP and ACh activate very similar postsynaptic mechanisms, and share a closely related function at the level of individual cortical cells. Thus VIP may facilitate the responses of both the excitatory and the inhibitory components of the cortical circuit, leading to an overall increase in responsiveness and selectivity. In contrast to the cholinergic input from the basal forebrain, however, the VIP-positive cortical cells are likely to exert a very localized influence, over a circumscribed region of the cortex, in response to the presence of an effective visual stimulus.

Export of north american ozone pollution to the north atlantic ocean.

Measurement of the levels of ozone and carbon monoxide (a tracer of anthropogenic pollution) at three surface sites on the Atlantic coast of Canada allow the estimation of the amount of ozone photochemically produced from anthropogenic precursors over North America and transported to the lower troposphere over the temperate North Atlantic Ocean. This amount is greater than that injected from the stratosphere, the primary natural source of ozone. This conclusion supports the contention that ozone derived from anthropogenic pollution has a hemisphere-wide effect at northern temperate latitudes.

Orientation sensitive elements in the corticofugal influence on centre-surround interactions in the dorsal lateral geniculate nucleus.

In a previous study, we have shown that the corticofugal projection to the dLGN enhances inhibitory mechanisms underlying length tuning. This suggests that the inhibitory influences deriving from the corticofugal feedback should exhibit characteristics that reflect the response properties of orientation-tuned layer VI cells. Here we report data obtained from experiments using a bipartite visual stimulus, with an inner section over the dLGN cell receptive field centre and an outer section extending beyond it. For both X and Y cells there was a modulation of the strength of the surround antagonism of centre responses that was dependent on the orientation alignment of contours in the two components of the stimulus. Layer VI cells showed maximal responses when the two components were aligned to the same orientation; dLGN cells showed a minimal response. Varying the orientation alignment of the inner and outer components of the stimulus in a randomised, interleaved fashion showed that bringing the stimulus into alignment resulted in a 24.28% increase in the surround antagonism of the centre response. Blocking cortical activity showed this effect of alignment to be strongly dependent on corticofugal feedback. This effect of orientation alignment appears to apply for any absolute orientation of the alignment condition and supports the view that an entire subset of cortical orientation columns generate the feedback influencing any given dLGN cell. This mechanism makes dLGN cells sensitive to the orientation domain discontinuities in elongated contours moving across their receptive field.

Antigenic and genetic variation in cytopathic hepatitis A virus variants arising during persistent infection: evidence for genetic recombination.

Variants of hepatitis A virus (pHM175 virus) recovered from persistently infected green monkey kidney (BS-C-1) cells induced a cytopathic effect during serial passage in BS-C-1 or fetal rhesus kidney (FRhK-4) cells. Epitope-specific radioimmunofocus assays showed that this virus comprised two virion populations, one with altered antigenicity including neutralization resistance to monoclonal antibody K24F2, and the other with normal antigenic characteristics. Replication of the antigenic variant was favored over that of virus with the normal antigenic phenotype during persistent infection, while virus with the normal antigenic phenotype was selected during serial passage. Viruses of each type were clonally isolated; both were cytopathic in cell cultures and displayed a rapid replication phenotype when compared with the noncytopathic passage 16 (p16) HM175 virus which was used to establish the original persistent infection. The two cytopathic virus clones contained 31 and 34 nucleotide changes from the sequence of p16 HM175. Both shared a common 5' sequence (bases 30 to 1677), as well as sequence identity in the P2-P3 region (bases 3249 to 5303 and 6462 to 6781) and 3' terminus (bases 7272 to 7478). VP3, VP1, and 3Cpro contained different mutations in the two virus clones, with amino acid substitutions at residues 70 of VP3 and 197 and 276 of VP1 of the antigenic variant. These capsid mutations did not affect virion thermal stability. A comparison of the nearly complete genomic sequences of three clonally isolated cytopathic variants was suggestive of genetic recombination between these viruses during persistent infection and indicated that mutations in both 5' and 3' nontranslated regions and in the nonstructural proteins 2A, 2B, 2C, 3A, and 3Dpol may be related to the cytopathic phenotype.

Cholinergic enhancement of direction selectivity in the visual cortex of the cat.

Acetylcholine and acetyl-beta-methacholine were applied iontophoretically to single cells in the feline striate cortex. The directional bias of the visual response to an optimally oriented stimulus was assessed quantitatively, before and during drug application. For the great majority of the cells that were affected by the drugs, selectivity was either unchanged (28/60, 47%) or increased (21/60, 35%). In particular, directional bias increased for 36% (12/33) of the cells that were facilitated by acetylcholine or acetyl-beta-methacholine and 43% (nine out of 21) of those that were inhibited, as compared with 9% (three out of 33) and 24% (five out of 21) for which the bias decreased. Six additional cells, of which three showed a reduced selectivity, were apparently excited by the drugs, in that the background discharge level was greatly increased with a concomitant decrease in signal-to-noise ratio. It is known that cholinergic input has the potential to enhance cortical function, by facilitating both the excitatory and the inhibitory components of the neuronal circuit. Our data show that this combination of effects can precipitate an enhancement of selectivity as well as of response magnitude.

Dependence of retinogeniculate transmission in cat on NMDA receptors.

1. We have examined the possibility that N-methyl-D-aspartate (NMDA) receptors may be involved in the visual response of relay cells in the cat dorsal lateral geniculate nucleus (dLGN). The selective NMDA receptor antagonists D-2-amino-5-phosphonovalerate (APV) and 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) have been iontophoretically applied to X and Y cells in the dLGN and their effects on the visual response to a light spot flashed within the receptive field center determined. 2. The antagonist effects were assessed at ejection current levels producing a selective blockade of the responses to iontophoretically applied NMDA with respect to those elicited by the non-NMDA receptor agonists quisqualate and kainate. These selective effects were determined in an experimental paradigm where the visual response and responses to NMDA and the non-NMDA receptor agonists were compared in the same test run. The data refer to a total population of 52 cells (28 X, 24 Y). 3. Application of APV abolished or greatly reduced the visual responses of both X and Y cells. The mean percentage reduction in the visual response for the X cells studied was 59 +/- 10% (SE; n = 7) and for the Y cells 66 +/- 8% (SE; n = 11). Both the early onset transient and the sustained component of the visual response to the flashed stimulus were equally affected. 4. The antagonist CPP produced a similar pattern of effect to APV, substantially reducing or abolishing the visual response in both X and Y cells.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo replication and reversion to wild type of a neutralization-resistant antigenic variant of hepatitis A virus.

Six seronegative owl monkeys were intravenously inoculated with an antigenic variant (S18) of hepatitis A virus that is highly adapted to growth in cell culture and resists neutralization by monoclonal antibodies due to replacement of aspartic acid 70 of capsid protein VP3 with histidine. Each developed hepatitis 22-33 days after inoculation. Virus in feces, serum, and liver was quantified by radioimmunofocus assay. Viremia developed 7-11 days after inoculation, in parallel with fecal shedding of virus, and persisted for a mean of 20.5 days. Although the antigenic variant was recovered from feces or liver of three animals, virus in liver at the time of enzyme elevations was predominantly wild-type antigenic phenotype. Virus was not recovered from liver 96 days after challenge. These studies further define virologic events in hepatitis A and show that in vivo replication of an antigenic variant was restricted compared with that of wild-type virus.

The contribution of the non-N-methyl-D-aspartate group of excitatory amino acid receptors to retinogeniculate transmission in the cat.

The N-methyl-D-aspartate receptor antagonist 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid and the non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione have been iontophoretically applied to cells in the cat dorsal lateral geniculate nucleus and their effects on the visual response compared. The objective was to examine the possibility of both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors being involved in the transfer of the retinal input to X and Y cells in the dorsal lateral geniculate nucleus. The results show that selective blockade of either N-methyl-D-aspartate receptors or non-N-methyl-D-aspartate receptors can block the visual response of both X and Y cells. Overall, the most potent reductions of visual responses across the population of cells studied were obtained with the N-methyl-D-aspartate receptor antagonist with X cells showing a slightly greater reduction on average (80%) than Y cells (66%). The relatively smaller overall reductions in visual responses obtained with the non-N-methyl-D-aspartate receptor blockade reflected the lower levels of blockade that were compatible with selectivity using iontophoretic applications of 6-cyano-7-nitroquinoxaline-2,3-dione. It is concluded that N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors are critically involved in the visual response of both "on" and "off" centre X and Y cells.