Visual experience as a determinant of the response characteristics of cortical receptive fields in cats

Impaired DNA-binding affinity of novel PAX6 mutations

Mutations in human PAX6 gene are associated with various congenital eye malformations including aniridia, foveal hypoplasia, and congenital nystagmus. These various phenotypes may depend on the mutation spectrums that can affect DNA-binding affinity, although this hypothesis is debatable. We screened PAX6 mutations in two unrelated patients with congenital nystagmus, and measured DNA-binding affinity through isothermal titration calorimetry (ITC). To elucidate phenotypic differences according to DNA-binding affinity, we also compared DNA-binding affinity among the previously reported PAX6 missense mutations within the linker region between two subdomains of the paired domain (PD). We identified two novel mutations of PAX6 gene: c.214 G > T (p.Gly72Cys) and c.249_250delinsCGC (p.Val84Alafs*8). Both were located within the linker region between the two subdomains of the PD. ITC measurement revealed that the mutation p.Val84Alafs*8 had no DNA-binding affinity, while the p.Gly72Cys mutation showed a decreased binding affinity (Kd = 0.58 μM) by approximately 1.4 times compared to the wild type-PAX6 (Kd = 0.41 μM). We also found that there was no close relationship between DNA-binding affinity and phenotypic differences. Our results suggest that the DNA-binding affinity alone might be insufficient to determine PAX6-related phenotypes, and that other modifier genes or environmental factors might affect phenotypes of the PAX6 gene.

Review : Metabolic Imaging: A Window on Brain Development and Plasticity

Various biochemical and physiological processes that undergo maturational changes during human brain development can be now studied in vivo using PET. The distribution of local cerebral glucose utilization shows regional alterations in the first year of life in agreement with behavioral, neurophysiological, and anatomical changes known to occur during development of the infant. Measurement of the absolute rates of glucose utilization with PET reveals that during the major portion of the first decade, the human brain has a higher energy (glucose) demand compared with both the newborn and adult brains. With adolescence, glucose utilization rates decline to reach adult values by age 16-18 years. This nonlinear course of cerebral glucose 'metabolic' maturation is also seen in a number of animal models and coincides with the develop mental course of transient synaptic exuberance associated with enhanced brain plasticity and efficient learn ing. Evidence of brain reorganization detected with PET is discussed in children with unilateral brain injury and early sensory deprivation. NEUROSCIENTIST 5:29-40, 1999

Multiple representations of space underlying behavior

We argue that a map is meaningless unless we have a process for using it. Thus, in this paper, we not only offer the world graph as a representation of relationships among situations the animal has encountered and may encounter again, but we also offer algorithms for how the information encoded in the world graph may be used by the animal in determining its behavior. Each node of the graph encodes a recognizable situation in the animal's world, but a given place may well be encoded in a number of different nodes. Nodes not only require algorithms for the recognition of the situation; they store information about drive reduction associated with the encoded situation. We note that the use of graphs as a basis for exploring some search space is well known in artificial intelligence (AI), but we stress the importance of the animal's exploration of its environment for growing the graph, as distinct from the mathematically described potential nodes frequent in AI search spaces. To explore a number of hypotheses about the way information in the world graph is used to guide the animal's movement, we recall a number of classical experiments on maze exploration by animals, and use them to argue for the nonlocal hypothesis (selection of a path does not depend only upon information about the immediate environment of the animal) and the competing nodes hypothesis (more than one drive may enter into the determination of the animal's behavior at any time). An important feature of the model is that it yields exploration and latent learning without the introduction of an exploratory drive. We also note that the performance exhibited by the model appears to be state-dependent when the animal operates under high drive levels. The drive-interaction matrix is offered as a subject for future research.

We complement the presentation of the world graph model, its drive dynamics, and how these are constrained by experiments on maze behavior, with a brief analysis of maps in the brain. We distinguish egocentric maps–which we relate to the many visual systems–from allocentric maps. We offer a somewhat unconventional view of short-term and long-term memory. We examine cooperative computation in somatotopically organized networks, relating this to visually guided behavior in the frog, and to the interaction of colliculus and cortex in the control of eye movements. We examine, but do not advocate, the hypothesis that the hippocampus is a cognitive map. We do stress that if it is a cognitive map, it must be seen as a chart of the local neighborhood, rather than the whole atlas; and we note that the cognitive map hypothesis would lead one to expect the region to exhibit activation of place cells before the animal leaves the previous place.

Metabolic maturation of the brain: a study of local cerebral glucose utilization in the developing cat

Previously, using positron emission tomography (PET), we showed that local cerebral metabolic rates for glucose (lCMRglc) in children undergo dynamic maturational trends before reaching adult values. In order to develop an animal model that can be used to explore the biological significance of the different segments of the lCMRglc maturational curve, we measured lCMRglc in kittens at various stages of postnatal development and in adult cats using quantitative [14C]2-deoxyglucose autoradiography. In the kitten, very low lCMRglc levels (0.14 to 0.53 mumol min-1 g-1) were seen during the first 15 days of life, with phylogenetically older brain regions being generally more metabolically mature than newer structures. After 15 days of age, many brain regions (particularly telencephalic structures) underwent sharp increases of lCMRglc to reach, or exceed, adult rates by 60 days. This developmental period (15 to 60 days) corresponds to the time of rapid synaptic proliferation known to occur in the cat. At 90 and 120 days, a slight decline in lCMRglc was observed, but this was followed by a second, larger peak occurring at about 180 days, when sexual maturation occurs in the cat. Only after 180 days did lCMRglc decrease to reach final adult values (0.21 to 2.04 mumol min-1 g-1). In general, there was good correlation between the metabolic maturation of various neuroanatomical regions and the emergence of behaviors mediated by the specific region. At least in the kitten visual cortex, which has been extensively studied with respect to developmental plasticity, the "critical period" corresponded to that portion of the lCMRglc maturational curve surrounding the 60-day metabolic peak. These normal maturational lCMRglc data will serve as baseline values with which to compare anatomical and metabolic plasticity changes induced by age-related lesions in the cat.

The role of visual experience in the development of cat striate cortex

By the third postnatal week, intrinsic developmental programs have established a framework within the cat visual system; this will be used to guide the course of subsequent experience-dependent development. Key elements in this framework are precociously mature cells in visual cortex area 17. These orientation-selective cells are predominantly first-order neurons, they are concentrated in layers IV and VI of area 17, most of them are activated monocularly, many may receive their direct excitatory input from lateral geniculate nucleus X cells, and the distribution of their preferred orientations is biased toward horizontal and vertical. Between the third and the sixth postnatal week, most of the remaining cells in area 17 develop orientation selectivity; this extension of orientation selectivity is blocked or delayed if kittens are deprived of normal patterned visual stimulation. Furthermore, exposure to a limited range of stimulus orientations can lead to an increase in the proportion of orientation-selective cells, and the range of orientation preferences that the cells acquire is restricted by the range of orientations to which the animal is exposed. This occurs with no apparent change in the physiology or morphology of intrinsically selective area 17 cells. Thus selective exposure may have its effect by influencing the connections between the intrinsically selective cells and higher-order neurons in area 17. Experience-dependent changes in the visual system may function to "fine-tune" sensory processing and thus optimize the system's response to the dominant features of the environment. This experience-dependent process could help the young animal to focus its "attention" on those features of its environment that are critical to its survival.

Imprinting and cortical plasticity: a comparative review

Results of research on imprinting and developmental neurobiology of the visual cortex are compared to evaluate the evidence for or against a frequently hypothesized linkage of the two phenomena. The comparison reveals striking similarities. In both paradigms a sensitive period exists. Once this sensitive period is over, the storage of early influences from the environment remains stable throughout life. Storage of "natural" stimuli is facilitated by a certain preorganisation of the receiving brain areas. It is stated that the two phenomena are not directly linked, but are two expressions of a developmental process, which may be common for the organisation of the connectivity of single cells as well as for complex neuronal networks as they are likely to be involved in imprinting. This process is basically self-organizing, but can be influenced by superimposed controls. Differences of the stability of storage of external influences might be explained by the difference in the overall amount of morphological alterations, which is large in the young and small in the adult animal. This holds for both the modifiability in the visual cortex and imprinting.

A Boolean complete neural model of adaptive behavior

A multi-layered neural assembly is developed which has the capability of learning arbitrary Boolean functions. Though the model neuron is more powerful than those previously considered, assemblies of neurons are needed to detect non-linearly separable patterns. Algorithms for learning at the neuron and assembly level are described. The model permits multiple output systems to share a common memory. Learned evaluation allows sequences of actions to be organized. Computer simulations demonstrate the capabilities of the model.

A neurophysiological model for anomalous correspondence based on mechanisms of sensory fusion

Normal retinal correspondence is not stable. The arguments for the plasticity of correspondence in normal binocular vision have been given into previous papers (Nelson, 1975, 1977). In this paper, both laboratory research and the clinical strabismus literature are reviewed to show similarities between normal and abnormal binocular vision. In particular, it is argued that sensory fusion (Panum's areas) and anomalous retinal correspondence (AC) obey similar principles, and so a sensory fusional model of AC may be developed. Recent advances in the neurophysiology of binocular vision are reviewed, but current laboratory knowledge cannot account for many phenomena known clinically unless certain postulates are made. Two hypothesized intracortical interactions among binocular disparity detectors, termed disparity domain inhibition and spatial domain facilitation, play key roles in extending the neurophysiology of binocular vision to an account of both normally - and clinically - observed plasticities of correspondence. The fusional model of retinal correspondence developed here from postulated domain interactions contrasts with the older concept of fixed corresponding points, an approach which has failed to provide a unified foundation for the treatment of normal and abnormal binocular vision.

Development of receptive field properties of retinal ganglion cells in kittens raised with a convergent squint

The effects on retinal ganglion cell receptive fields of rearing kittens with convergent squint, surgically induced on the 12th post-natal day, were investigated by utilizing the extracellular single unit recording technique. The data revealed that responses of cross-eyed cat ganglion cells to contrast reversal stimuli were severely depressed and the retinal region exhibiting the best responses varied according to the degree of convergent misalignment of the eyes displayed by each animal. Receptive field sizes of X-type (but not Y-type) units located within 10 degrees of the area centralis of cross-eyed cats were significantly larger than those in normally reared cats. Finally, the encounter rate for units exhibiting non-linearity of spatial summation (Y-type) were much lower in cross-eyed cats. The results suggest developmental alterations in the retinal neurophysiology of common cats reared with a large convergent squint.

Visually evoked potentials from occipital and precentral cortex in visually deprived humans

The visually evoked potential (VEP) was recorded from the scalp overlying precentral and occipital cortex in three monocularly deprived humans. The subjects had defects of the globe from birth that effectively created a condition of diffuse light rearing in one eye. In subject, BER, a 19-year-old with a recently removed congenital cataract, the occipital response evoked from the deprived eye was reduced by 53% compared with the response evoked from the good eye. In subject GUZ, age 20 with a congenital cataract, the occipital response evoked from the deprived eye was actually larger by 20% than was the response evoked from the good eye. For UTZ, age 5 with whitish cellular debri in the anterior vitreous of the eye, the occipital response evoked from the deprived eye was only 4% smaller than the response evoked from his good eye. In all three subjects, the precentral response evoked from the deprived eye was reduced in amplitude compared with the precentral response evoked from the good eye. In terms of relative effect of the deprivation upon the VEP from the two recording sites, the precentral VEP was altered to a greater extent than was the occipital VEP. These findings indicate that diffuse light rearing can affect the pathways projecting to precentral cortex independently of the specific visual pathways.

Neurophysiological predictions of a two-pathway informon theory of neural conditioning

According to the informon theory there must be variable and fixed synapses in a neurone for conditioning to occur. For a variable synapse to behave like an informon pathway its conductivity needs to depend only on the average values of its presynaptic potential and of the internal state of the neurone. Eight predictions are made about the detailed functioning of such a synapse. In a minimal hypothesis all fixed synapses are inhibitory; but sign reversals are considered. Let one unit A in the receptive field of a neurone drive it through a fixed synapse, and all other units, e.g. B, drive variable synapses; then the theory predicts that the conductivity of the B synapse becomes proportional to the mutual information function between the signals at A and B; so inputs which tend to occur with the A signal become connected positively to the neurone. Applied to visual pathways this principle leads to the formation of edge and grating detectors. If X and Y cells excite variable and fixed synapses respectively, simple and complex cells should be driven by both X and Y cells, the latter being inhibitory. The two-pathway theory resolves two apparent conflicts between experimental facts.

Innate and environmental factors in the development of the kitten's visual cortex

1. This is a study of the receptive fields of 771 cells recorded in the visual cortex of twenty-five kittens reared normally or subjected to various kinds of visual deprivation or environmental manipulation. 2. Kittens deprived of patterned visual experience, by dark rearing or diffuse occlusion of the eyes, have a majority of cirtical neurones with little or no specificity for the orientation or axis of movement of visual stimuli. However, in such deprived animals, especially those younger than 3 weeks, there are a number of genuinely orientation selective cells. They are broadly "turned" (by adult standards), they are almost always of the simple type, are heavily dominated by one eye, and are found mainly in the deeper layers of the cortex, especially layer IV. 3...

The development of synapses in the visual system of the cat

Synapses have been counted by electron microscopy and neurones by light microscopy through the depth of the visual cortex in a series of cats from 37 days gestation to adulthood. A few definite synapses are present as early as three weeks before birth, but there is then a latent period of four weeks before synapses increase rapidly in number 8-37 days after birth. The synapses occur just above and just below the cell plate at first, but in the adult cat they become evenly distributed in the depth of the cortex. The gradual separation of neurones by neuropil during development precedes a parallel increase in the density of synapses by about one week. The average number of synapses associated with one neurone rises to a peak of about 13,000 at seven weeks after birth. The densities of synapses and of neurones subsequently fall to slightly lower values in adult cats as the glial cells continue to develop. The timing of synaptic development in the visual cortex has been compared quantitatively with that in the L. G. N. and qualitatively with synaptogenesis in the retina. Synapses develop in the L. G. N. and cortex in a parallel fashion, and the L. G. N. precedes the cortex by a short interval of about two days. In the cell plate of the retina a few receptor synapses are present nine days before birth. Inner plexiform synapses are aslo present at this time, but ribbon-containing synapses do not appear until birth. Very few receptors possess outer segments with discs at birth, but five days later disc-bearing outer segments have developed. Thus synaptic development starts before afferent impulses can enter the visual system, but the main increase in synapses in the L. G. N. and cortex takes place four weeks after the start of synapse formation while the visual system is being used.

Behavioral deficits in cats following early selected visual exposure to contours of a single orientation

The ability of adult cats, whose early visual experience was confined to contours of a single orientation (either vertical or horizontal), to resolve gratings of different orientations was studied by operant methods. Following selective visual exposure during part or all of the first 4 months of life, the cats were trained on a simultaneous discrimination between gratings of various orientations and blank fields of the same mean luminance. The spatial frequency of the gratings was systematically altered in order to obtain an estimate of acuity based upon extrapolation to chance levels of performance. Selectively deprived cats performed as well as normally reared cats on gratings having the same orientation as that of the stripes they saw as kittens, but their performance on gratings orthogonal to these was poorer. The deficits in acuity for gratings perpendicular to the experienced orientation varied between 0.26 and 0.87 of an octave. On the other hand, control cats whose early visual experience alternated between vertical and horizontal stripes, or who were reared in an environment containing randomly oriented contours, failed to show any difference in their acuity for vertical and horizontal gratings. The acuity deficits shown by the selectively deprived animals are long-standing since they remain unchanged even after 30 months of normal visual exposure. It is argued that these perceptual deficits are a consequence of the changes in cortical physiology that other investigators have described in cats who had undergone similar early visual deprivation. Taken together, these findings provide a basis for explaining a number of human perceptual disorders.