Perinatal-lesion-induced reorganization of cerebral functions revealed using reversible cooling deactivation and attentional tasks.

We tested the concept that lesions of primary visual cortical areas 17 and 18 sustained on the day of birth induce a redistribution of cerebral operations underlying the ability to disengage visual attention and then redirect it to a new location. In cats, these operations are normally highly localizable to posterior middle suprasylvian (pMS) cortex. Three stimulation paradigms were used: (i) movement of a high contrast visual stimulus into the visual field; (ii) illumination of a static light-emitting diode (LED) stimulus; and (iii) a control static auditory stimulus. To test for the redistribution of critical neural operations, cryoloops were implanted bilaterally in the pMS sulcus and in contact with ventral posterior suprasylvian (vPS) cortex. Separate and combined deactivations of pMS and vPS cortices in cats with early lesions of primary visual cortex showed that full, unilateral deactivation of pMS cortex only partially impaired the ability to detect and orient to stimuli moved into the contracooled hemifield. Much more complete impairment required the additional deactivation of ipsilateral vPS cortex. Bilateral pMS deactivation alone, or in combination with bilateral vPS deactivation, largely reversed the unilateral contracooled neglect. For the orienting to static, illuminated LED stimuli, unilateral deactivation of pMS cortex was sufficient to fully impair orienting to stimuli presented in the contracooled hemifield. Bilateral pMS deactivation induced an almost complete visual-field-wide neglect of stimuli. On its own, unilateral deactivation of vPS cortex was without effect on either task, although bilateral vPS deactivations introduced inconsistencies into the performance. Termination of cooling reversed all deficits. Finally, neither the initial lesion of areas 17 and 18 nor cooling of either the MS or vPS cortex alone, or in combination, interfered with orienting to sound stimuli. Overall, our results provide evidence that at least one highly localizable visual function of normal cerebral cortex is remapped across the cortical surface following the early lesion of primary visual cortical areas 17 and 18. Moreover, the redistribution has spread the essential neural operations from the visual parietal cortex to a normally functionally distinct type of cortex in the visual temporal system.

On variability in the density of corticocortical and thalamocortical connections.

Variability is an important but neglected aspect of connectional neuroanatomy. The quantitative density of the 'same' corticocortical or thalamocortical connection may vary by over two orders of magnitude between different injections of the same tracer. At present, however, the frequency distribution of connection densities is unknown. Therefore, it is unclear what kind of sampling strategies or statistical methods are appropriate for quantitative studies of connectivity. Nor is it clear if the measured variability represents differences between subjects, or if it is simply a consequence of intra-individual differences resulting from experimental technique and the exact placement of tracers relative to local spatial and laminar variation in connectivity. We used quantitative measurements of the density of a large number of corticocortical and thalamocortical connections from our own laboratories and from the literature. Variability in the density of given corticocortical and thalamocortical connections is high, with the standard deviation of density proportional to the mean. The frequency distribution is close to exponential. Therefore, analysis methods relying on the normal distribution are not appropriate. We provide an appendix that gives simple statistical guidance for samples drawn from exponentially distributed data. For a given corticocortical or thalamocortical connection density, between-individual standard deviation is 0.85 to 1.25 times the within-individual standard deviation. Therefore, much of the variability reported in conventional neuroanatomical studies (with one tracer deposited per animal) is due to within-individual factors. We also find that strong, but not weak, corticocortical connections are substantially more variable than thalamocortical connections. We propose that the near exponential distribution of connection densities is a simple consequence of 'patchy' connectivity. We anticipate that connection data will be well described by the negative binomial, a class of distribution that applies to events occurring in clumped or patchy substrates. Local patchiness may be a feature of all corticocortical connections and could explain why strong corticocortical connections are more variable than strong thalamocortical connections. This idea is supported by the columnar patterns of many corticocortical but few thalamocortical connections in the literature.

Transgeniculate signal transmission to middle suprasylvian cortex in intact cats and following early removal of areas 17 and 18: a morphological study.

Removal of cat areas 17 and 18 early, but not late, in postnatal development results in the sparing of certain reflexive and nonreflexive visually guided behaviors. These spared behaviors are accompanied by an expansion of geniculocortical projections to middle suprasylvian (MS) cortex. However, little is known about the types of visual signals relayed along these pathways. The purpose of our study was to reveal the morphologies of the neurons participating in the rewired circuits and, by relating them to the morphologies of functionally characterized neurons described by others, infer the types of visual signals transmitted via the lateral geniculate nucleus (LGN) to MS cortex. To do this, we retrogradely labeled LGN neurons from MS cortex with fluorescent microspheres, and subsequently intracellularly filled them with Lucifer Yellow. We then classified well-filled neurons according to a battery of morphological parameters (such as soma size and shape, and dendritic field-form and specializations), and compared them with already defined structure/function relationships. By doing this, we found that the large majority of visual thalamic relay neurons to MS cortex of both normal cats and cats that incurred removal of areas 17 and 18 were types I and IV. These results indicate that visual Y and W signals, respectively, are relayed directly from LGN to MS cortex in both types of cats. Following the early lesions, some of the MS-projecting type I neurons were found in layers A and A1, where they are never found in intact cats. Thus, some layer A and A1 type I neurons redirect axons to MS cortex following early removal of areas 17 and 18. For the type IV MS-projecting neurons in early lesioned cats, the somas were hypertrophied and they had more profuse and broader dendritic arbors than equivalent neurons in intact cats. These results suggest that dynamic interactions take place between inputs and outputs of LGN neurons during development that influence final LGN neuron morphology. Moreover, they suggest that signals transferred to MS cortex by type IV neurons may be modified by early lesions of areas 17 and 18. Overall, these results contribute to our understanding of the types of behaviors that may be spared by early lesions of areas 17 and 18.

Thalamic and cortical projections to middle suprasylvian cortex of cats: constancy and variation.

We investigated the constancy and variability in the numbers of thalamic and cortical neurons projecting to cat middle suprasylvian (MS) visual cortex. Retrograde pathway tracers were injected at a single anatomically and physiologically defined locus in MS cortex. Counts of labeled neurons showed that the visual thalamic projections to MS cortex consistently arose from a fixed set of nuclei in relatively constant proportions. In contrast, counts of cortical neurons revealed that transcortical inputs to MS cortex were much more variable. This differential variability may be linked to the developmental program, which affords greater influence of experiential factors on cortical pathway development than on thalamocortical pathway development. These results have implications for the development of models of cerebral connectivity that include measures of pathway variability.

Age dependent modification of cytochrome oxidase activity in the cat dorsal lateral geniculate nucleus following removal of primary visual cortex.

The purpose of the present study was to assess changes in the levels of cytochrome oxidase (CO) activity in the dorsal lateral geniculate nucleus (dLGN) of the adult cat following removal of primary visual cortical areas 17 and 18 on the day of birth (PI), P28, or in adulthood (> or = 6 months). Cytochrome oxidase activity was measured in histological sections 9 or more months after the cortical ablation. Control measures obtained from intact cats show that CO activity is normally highest in the A-laminae of dLGN, and slightly lower in the C-complex. Following visual cortex ablations incurred at any age, CO activity levels are reduced in the A-laminae. This reduction is most profound following ablations incurred on P28 or in adulthood. In contrast, CO activity in the C-complex of dLGN is at nearly normal levels following ablations on P1 or P28, but not in adulthood. These findings contribute to our understanding of the role played by the dLGN in the transfer of visual signals along retino-geniculo-extrastriate pathways that expand following early removal of areas 17 and 18. Moreover, they have implications for our understanding of spared behavioral functions attributed to the extrastriate cortex in cats which incurred early damage of areas 17 and 18.

Amplification of thalamic projections to middle suprasylvian cortex following ablation of immature primary visual cortex in the cat.

The purpose of the present study was to identify expansions in thalamic projections to middle suprasylvian (MS) cortex that could be linked to the sparing of visually guided behaviors that follow the removal of visual cortex early in postnatal life. Injections of retrograde tracers were made into the medial bank of the middle suprasylvian sulcus in intact, adult cats and in adult cats that had incurred ablations of areas 17 and 18 on the day of birth (P1), P28, or > or = 6 months of age, and the numbers of labeled neurons in the thalamus were counted. In the thalamus of the intact cat, the greatest number of labeled neurons are located in the lateral division of the lateral posterior nucleus and there are intermediate numbers in the medial division of the lateral posterior nucleus (LPm); and smaller numbers within the medial interlaminar nucleus, the C-complex of the dorsal lateral geniculate nucleus (dLGN), the geniculate wing, and the pulvinar nucleus. Following the removal of areas 17 and 18 at different ages, thalamic projections to MS cortex exhibit an age-dependent reorganization. Removals on P1, induce twice the normal number of neurons in LPm and three times the normal number of neurons in the C-complex of dLGN to project to MS cortex. Removals on P28 induce five times the normal number of neurons in the C-complex to project to MS cortex. In addition, removals at both ages resulted in projections from the A-laminae to MS cortex becoming permanently established. No changes in the pattern or number of neurons that project to MS cortex were detected when areas 17 and 18 were removed in adulthood. These results show that pathways through the C-complex of dLGN and through LPm expand substantially following ablation of immature areas 17 and 18. These expanded pathways are linked to Y- and W-functional streams of visual signals that are relayed from the retina to extrastriate cortex either directly through dLGN or indirectly via the superior colliculus and LPm. These signals may be critical for the sparing of neural operations following ablation of areas 17 and 18 early in development.

Tritiated uridine uptake in the retinal ganglion cell layer of the cat during normal development and after visual cortex ablation.

The short-term metabolic response of immature retinal ganglion cells to destruction of their target cells in the dorsal lateral geniculate nucleus (dLGN) was assessed in newborn cats. Retrograde degeneration of virtually all dLGN cells was induced by ablation of the 13 contiguous areas of visual cortex on the day of birth. The metabolic response of retinal ganglion cells to this loss of target cells in dLGN was determined by exposing the ganglion cell layer to tritiated uridine, a precursor of RNA. Control measurements were made from unoperated littermates. Following sectioning and processing of the retinae from both groups of kittens for autoradiography, silver grain densities overlying the cellular profiles in the ganglion cell layer were calculated. These calculations revealed levels of uridine incorporation at Postnatal Day 4 in both groups of kittens significantly higher than at either Postnatal Day 2 or 7, but no significant differences between the two groups on any day examined. These results show that the level of RNA synthesis in retinal ganglion cells increases temporarily during the first postnatal week and that this synthesis is unaffected by the death of target cells in the dLGN. The temporary increase may be related to the establishment of synaptic connections on retinal ganglion cells by their afferent bipolar and amacrine neurons in the inner nuclear layer.

Pyruvate carboxylase as a model for oligosubstituted enzyme-ligand conjugates in homogeneous enzyme immunoassays.

Theoretical models suggest that the detection capabilities of homogeneous enzyme immunoassays can be improved by the use of oligosubstituted enzyme-ligand conjugates rather than the traditionally used multisubstituted ones. The natural form of pyruvate carboxylase contains four covalently bound biotins (one per subunit) and it can be considered as an oligosubstituted enzyme-biotin conjugate. The enzyme is nearly completely inhibited in the presence of the natural binder for biotin, avidin. When the enzyme is incubated with avidin and free biotin, a competition occurs between the free biotin and the prosthetic group of the enzyme for the avidin. Steep dose-response curves are obtained by relating the observed inhibition to the free biotin concentration. By variation of the amount of avidin or enzyme in the assay, the detection limits of the system can be altered allowing for sensitive determinations over a wide range of biotin concentrations. Such data from real sample analysis of several vitamin supplements are reported.

Neocortical connections in fetal cats.

The major extrinsic projections to and from visual and auditory areas of cerebral cortex were examined in fetal cats between 46 and 60 days of gestation (E46-E60) using axonal transport of horseradish peroxidase either alone or in combination with tritiated proline. Projections to visual cortex from the dorsal lateral geniculate nucleus and lateral-posterior/pulvinar complex exist by E46, and those from the contralateral hemisphere, claustrum, putamen, and central lateral nucleus of the thalamus are present by E54-E56. In addition, cells in the medial geniculate nucleus project to auditory cortex by E55. At E54-E56 efferent cortical projections reach the contralateral hemisphere, claustrum, putamen, lateral-posterior/pulvinar complex and reticular nucleus of the thalamus. Cells in visual cortex also project to the dorsal and ventral lateral geniculate nuclei, pretectum, superior colliculus and pontine nuclei, and cells in auditory cortex project to the medial geniculate nucleus. Except for interhemispheric projections, all pathways demonstrated are ipsilateral, and projections linking cerebral cortex with claustrum, dorsal lateral geniculate nucleus and lateral-posterior/pulvinar complex are reciprocal. The reciprocal projections formed with the dorsal lateral geniculate nucleus, lateral-posterior/pulvinar complex and the claustrum show a greater degree of topological organization compared to the projections formed with the contralateral hemisphere and superior colliculus, which show little or no topological order. Therefore, the results of the present study show that the major extrinsic projections of the cat's visual and auditory cortical areas with subcortical structures are present by the eighth week of gestation, and that the origins and terminations of many of these projections are arranged topologically.

Contralateral corticofugal projections to cat visual thalamus.

Contralateral corticofugal projections from visual cortical areas to thalamic nuclei were demonstrated in the cat using anterograde transport of tritiated proline. Thalamic nuclei receiving projections from contralateral visual cortex include both subdivisions of the lateral-posterior nucleus, the posterior nucleus of Rioch, and the posterior nuclear complex.