Representation of three-dimensional visual space in the cerebral cortex

This article reviews two issues relevant to the topic of how three-dimensional space is represented in the cerebral cortex. The first is the question of how individual neurons encode information that might contribute to stereoscopic estimation of visual depth. Particular attention is given to the current understanding of the neural representation of motion through three-dimensional space and to the complexities that arise in interpreting neuronal responses to this complex stimulus parameter. The second issue considered is the disorderlines that exists in the retinotopic mapping of the visual field in some cortical visual areas. Several extrastriate areas have been found to contain maps of the contralateral visual hemifield that are disorderly in the sense that the representation of various parts of the visual field are often misplaced or grossly over-or under-represented. It is suggested that this disorderlines may in some cases represent adaptations to facilitate certain types of visual functions.     

Representation of response categories in visual cortex

It is known that single neurons in visual area V4 are selective for visual features and that their responses are modulated by selective attention. Surprisingly, a new study by Mirabella et al. in this issue of Neuron found that V4 neurons also signal the behavioral response category of the attended feature.     

Representation of visual stimuli in inferior temporal cortex.

In primates, inferior temporal (IT) cortex is crucial for the processing and storage of visual information about form and colour. This article reviews the properties of IT neurons and considers how these properties may underlie the perceptual and mnemonic functions of IT cortex. The available evidence suggests that the processing of the facial image by IT cortex is similar to its processing of other visual patterns. Faces and other complex visual stimuli appear to be represented by the pattern of responses over a population of IT neurons rather than by the responses of specific 'feature detectors' or 'grandmother' cells. IT neurons with adult-like stimulus properties are present in monkeys as young as six weeks old.     

A model of the ventral visual system based on temporal stability and local memory

The cerebral cortex is a remarkably homogeneous structure suggesting a rather generic computational machinery. Indeed, under a variety of conditions, functions attributed to specialized areas can be supported by other regions. However, a host of studies have laid out an ever more detailed map of functional cortical areas. This leaves us with the puzzle of whether different cortical areas are intrinsically specialized, or whether they differ mostly by their position in the processing hierarchy and their inputs but apply the same computational principles. Here we show that the computational principle of optimal stability of sensory representations combined with local memory gives rise to a hierarchy of processing stages resembling the ventral visual pathway when it is exposed to continuous natural stimuli. Early processing stages show receptive fields similar to those observed in the primary visual cortex. Subsequent stages are selective for increasingly complex configurations of local features, as observed in higher visual areas. The last stage of the model displays place fields as observed in entorhinal cortex and hippocampus. The results suggest that functionally heterogeneous cortical areas can be generated by only a few computational principles and highlight the importance of the variability of the input signals in forming functional specialization.     

Organization of local interneurons in optic glomeruli of the dipterous visual system and comparisons with the antennal lobes

The lateral protocerebrum of the fly's brain is composed of a system of optic glomeruli, the organization of which compares to that of antennal lobe glomeruli. Each optic glomerulus receives converging axon terminals from a unique ensemble of optic lobe output neurons. Glomeruli are interconnected by systems of spiking and nonspiking local interneurons that are morphologically similar to diffuse and polarized local interneurons in the antennal lobes. GABA-like immunoreactive processes richly supply optic glomeruli, which are also invaded by processes originating from the midbrain and subesophageal ganglia. These arrangements support the suggestion that circuits amongst optic glomeruli refine and elaborate visual information carried by optic lobe outputs, relaying data to long-axoned neurons that extend to other parts of the central nervous system including thoracic ganglia. The representation in optic glomeruli of other modalities suggests that gating of visual information by other sensory inputs, a phenomenon documented from the recordings of descending neurons, could occur before the descending neuron dendrites. The present results demonstrate that future studies must consider the roles of other senses in visual processing.     

Representation of color stimuli in awake macaque primary visual cortex

We investigated the responses of single neurons in primary visual cortex (area V1) of awake monkeys to chromatic stimuli. Chromatic tuning properties, determined for homogeneous color patches presented on a neutral gray background, varied strongly between cells. The continuum of preferred chromaticities and tuning widths indicated a distributed representation of color signals in V1. When stimuli were presented on colored backgrounds, chromatic tuning was different in most neurons, and the changes in tuning were consistent with some degree of sensitivity of the neurons to the chromatic contrast between stimulus and background. Quantitatively, the average response changes matched the magnitudes of color induction effects measured in human subjects under corresponding stimulus conditions.     

Statistics and geometry of orientation selectivity in primary visual cortex

Orientation maps are a prominent feature of the primary visual cortex of higher mammals. In macaques and cats, for example, preferred orientations of neurons are organized in a specific pattern, where cells with similar selectivity are clustered in iso-orientation domains. However, the map is not always continuous, and there are pinwheel-like singularities around which all orientations are arranged in an orderly fashion. Although subject of intense investigation for half a century now, it is still not entirely clear how these maps emerge and what function they might serve. Here, we suggest a new model of orientation selectivity that combines the geometry and statistics of clustered thalamocortical afferents to explain the emergence of orientation maps. We show that the model can generate spatial patterns of orientation selectivity closely resembling the maps found in cats or monkeys. Without any additional assumptions, we further show that the pattern of ocular dominance columns is inherently connected to the spatial pattern of orientation.     

Characterization of local geometry of protein surfaces with the visibility criterion

Experimentally determined protein tertiary structures are rapidly accumulating in a database, partly due to the structural genomics projects. Included are proteins of unknown function, whose function has not been investigated by experiments and was not able to be predicted by conventional sequence-based search. Those uncharacterized protein structures highlight the urgent need of computational methods for annotating proteins from tertiary structures, which include function annotation methods through characterizing protein local surfaces. Toward structure-based protein annotation, we have developed VisGrid algorithm that uses the visibility criterion to characterize local geometric features of protein surfaces. Unlike existing methods, which only concerns identifying pockets that could be potential ligand-binding sites in proteins, VisGrid is also aimed to identify large protrusions, hollows, and flat regions, which can characterize geometric features of a protein structure. The visibility used in VisGrid is defined as the fraction of visible directions from a target position on a protein surface. A pocket or a hollow is recognized as a cluster of positions with a small visibility. A large protrusion in a protein structure is recognized as a pocket in the negative image of the structure. VisGrid correctly identified 95.0% of ligand-binding sites as one of the three largest pockets in 5616 benchmark proteins. To examine how natural flexibility of proteins affects pocket identification, VisGrid was tested on distorted structures by molecular dynamics simulation. Sensitivity decreased approximately 20% for structures of a root mean square deviation of 2.0 A to the original crystal structure, but specificity was not much affected. Because of its intuitiveness and simplicity, the visibility criterion will lay the foundation for characterization and function annotation of local shape of proteins.     

The local and non-local components of the local field potential in awake primate visual cortex

The Local Field Potential (LFP) is the analog signal recorded from a microelectrode inserted into cortex, typically in the frequency band of approximately 1 to 200 Hz. Here visual stimuli were flashed on in the receptive fields of primary visual cortical neurons in awake behaving macaques, and both isolated single units (neurons) and the LFP signal were recorded from the same unipolar microelectrode. The fall-off of single unit activity as a visual stimulus was moved from near the center to near the edge of the receptive field paralleled the fall-off of the stimulus-locked (evoked) LFP response. This suggests that the evoked LFP strongly reflects local neuronal activity. However, the evoked LFP could be significant even when the visual stimulus was completely outside the receptive field and the single unit response had fallen to zero, although this phenomenon was variable. Some of the non-local components of the LFP may be related to the slow distributed, or non-retinotopic, LFP signal previously observed in anesthetized animals. The induced (not time-locked to stimulus onset) component of the LFP showed significant increases only for stimuli within the receptive field of the single units. While the LFP primarily reflects local neuronal activity, it can also reflect neuronal activity at more distant sites, although these non-local components are typically more variable, slower, and weaker than the local components.     

Tendency of the brain to sharp images and geometry of visual illusions

Usually, to shift the gaze, our will is needed, which the brain realizes by giving an order to the corresponding eye muscles. However, under some conditions, the eye movement and the shift of the gaze to perspective can be accomplished by non-forced methods, without any active interference of our consciousness.     

Apoptosis in the developing visual system

Programmed cellular death is a widespread phenomenon during development of the nervous system. Two classes of molecules are particularly important in the context of apoptosis control in the nervous system: intracellular effectors homologous to the Caenorhabditis elegans Ced-3, -4, and -9 proteins, which in mammals correspond to the proteases of the caspase family, Apaf-1, and the members of the Bcl-2 protein family, and neurotrophic factors. Retinal ganglion cells lend a convenient model system with which to investigate apoptosis in central neurons during development as well as after injury. In this review, we discuss the role of these molecules in the control of programmed cellular death in the retinotectal system. Transgenic animal models and expression studies have shown that caspases, Bcl-2, Bax, and possibly Bcl-X are necessary players for the control of programmed cellular death in retinal ganglion cells. Bax and caspase 3 expression in retinal ganglion cells is upregulated after injury, and inhibition of Bax or caspase 3 increases the survival of injured retinal ganglion cells. Neurotrophins can support the survival of injured retinal ganglion cells, but this effect is transient. The physiological role of neurotrophins in the development of the retinocollicular system seems more related to the topographic refinement of retinocollicular projections, a process that is mediated, at least partially, by selective elimination of retinal ganglion cells making inappropriate topographic projections.     

Adaptation of the visual system

We recorded the total pulse response of the optic nerve in frogs to varying degrees of increase and decrease of light from the original adapting level. On the basis of these data, we plotted curves of dependence of the magnitude of response on the logarithm of relative value of increase and decrease of light (the amplitude characteristic — AC). The AC is steepest in the zone of adapting background and sloped on either side of it. It follows that under stationary conditions of illumination, the eye is capable of finely differentiating light intensity only within a narrow range (one logarithmic unit). After adaptation to a new level of illumination, the AC shifts along the scale of light intensity in such a way that the steepest portion corresponds to the adapting brightness. Increase in steepness of the AC occurs precisely during the process of adaptation. The contrast sensitivity of the human visual system is greatest near the adapting level and declines on either side of it. It follows that in man steepness of the visual system AC is greatest in the zone of the adapting background. Both increase and decrease of intensity of the adapting background are accompanied by a decline of contrast sensitivity, which rises again during the process of adaptation to a new level. Thanks to adaptive shift of the steep portion of the AC along the scale of light intensity, a visual system having a high contrast sensitivity only within a narrow "working" range is capable of finely differentiating light intensity in significantly changing conditions of illumination.Institute of Problems of Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 1, No. 1, pp. 81–89, July–August, 1969.     

Analysis of local helix geometry in three B-DNA decamers and eight dodecamers

Local variations in B-DNA helix structure are compared among three decamers and eight dodecamers, which contain examples of all ten base-pair step types. All pairwise combinations of helix parameters are compared by linear regression analysis, in a search for internal relationships as well as correlations with base sequence. The primary conclusions are: (1) Three-center hydrogen bonds between base-pairs occur frequently in the major groove at C-C, C-A, A-A and A-C steps, but are less convincing at C-C and C-T steps in the minor groove. The requirements for large base-pair propeller are (1) that the base-pair should be A.T rather than G.C, and (2) that it be involved in a major groove three-center hydrogen bond with the following base-pair. Either condition alone is insufficient. Hence, a large propeller is expected at the leading base-pair of A-A and A-C steps, but not at A-T, T-A, C-A or C-C steps. (2) A systematic and quantitative linkage exists between helix variables twist, rise, cup and roll, of such strength that the rise between base-pairs can hardly be described as an independent variable at all. Two typical patterns of behavior are observed at steps from one base-pair to the next: high twist profile (HTP), characterized by high twist, low rise, positive cup and negative roll, and low twist profile (LTP), marked by low twist, high rise; negative cup and positive roll. Examples of HTP are steps G-C, G-A and Y-C-A-R, where Y is pyrimidine and R is purine. Examples of LTP steps are C-G, G-G, A-G and C-A steps other than Y-C-A-R. (3) The minor groove is especially narrow across the two base-pairs of the following steps: A-T, T-A, A-A and G-A. (4) In general, base step geometry cannot be correlated solely with the bases that define the step in question; the two flanking steps also must be taken into account. Hence, local helix structure must be studied in the context, not of two base-pairs: A-B, but of four: x-A-B-y. Calladine's rules, although too simple in detail, were correct in defining the length of sequence over which a given perturbation is expressed. Whereas ten different two-base steps are possible, allowing for the identity of complementary sequences, there are 136 different four-base steps. Only 33 of these 136 four-base steps are represented in the decamer and dodecamer structures solved to date, and hence it is premature to try to set up detailed structural algorithms. (5) The sugar-phosphate backbone chains of B-DNA place strong limits on sequence-induced structural variation, damping down most variables within four or five base-pairs, and preventing purine-purine anti-anti mismatches from causing bulges in the double helix. Hence, although short-range sequence-induced deformations (or deformability) are observed, long-range deformations propagated down the helix are not to be expected.     

Axon targeting in the Drosophila visual system

The neuronal wiring of the Drosophila melanogaster visual system is constructed through an intricate series of cell-cell interactions. Recent studies have identified some of the gene regulatory and cytoskeletal signaling pathways responsible for the layer-specific targeting of Drosophila photoreceptor axons. Target selection decisions of the R1-R6 subset of photoreceptor axons have been found to be influenced by the nuclear factors Brakeless and Runt, and target selection decisions of the R7 subset of axons have been found to require the cell-surface proteins Ptp69d, Lar and N-cadherin. A role for the visual system glia in orienting photoreceptor axon outgrowth and target selection has also been uncovered.