Intra-analyzer temporary connections in the visual system]

The possibility of setting up intraanalyser temporary connections integrating the activity of various elements within the central part of the visual analyser at a neuronal level was studied in acute experiments on unanaesthetized immobilized cats. In the given model of temporary connection the unit activity in the lateral geniculate bodies (LGB) was investigated. Electrical stimulation of the superior colliculi was used as a conditioned stimulus, and illumination of the receptive fields of the studied LGB neurones, as an unconditioned one. In the process of conditioning, 10 to 11% of learning elements were revealed in LGB. The possibility is discussed of integration of learning elements into a definite microsystem achieving the process of temporary connection closing in the visual analyser in the course of conditioning.     

Patterning of retinotectal connections in the vertebrate visual system.

Recent work on the retinotectal projection clearly establishes the roles of neuronal activity and position-based cues in the patterning of nerve connections. In some species, the high degree of spatial order has been shown to emerge from a continued process of terminal growth and refinement. The future challenge is now to determine how multiple cues work together to guide the sculpting of the final pattern.     

Evolutionary evaluation of reciprocity of connections in the turtle tectofugal visual system

In two turtle species—Emys orbicularis and Testudo horsfieldi—by the method of anterograde and retrograde traicing at the light and electron microscopy level, the existence is proven of direct descending projections from the thalamic nucleus of the tectofugal visual system n. rotunds (Rot) to the optic tectum. After injection of tracers into Rot alone and into Rot with involvement of the tectothalamic tract (Trtth), occasional labeled fibers with varicosities and terminals are revealed predominantly in the deep sublayers of SGFS of the rostral optic tectum, while in the lower amount—in other tectal layers. After the tracer injections into the optic tectum, a few retrogradely labeled neurons were found mainly in the Rot ventral parts and within Trtth. Their localization coincides with that of GABA-immunoreactive cells. Electron microscopy showed the existence of many retrogradely labeled dendrites throughout the whole Rot; a few labeled cell bodies were also present there, some of them being also GABA-immunoreactive. These results allow us to conclude about the existence of reciprocal connections between the optic tectum and Rot in turtles, these connections being able to affect processing of visual information in tectum. We suggest that reciprocity of tectothalamic connections might be the ancestral feature of the vertebrate brain; in the course of amniote evolution the functional significance of this feature can be decreased and even lost in parallel with a rise of the role of direct corticotectal projections.     

Figure-ground discrimination by relative movement in the visual system of the fly

The visual system of the fly performs various computations on photoreceptor outputs. The detection and measurement of movement is based on simple nonlinear multiplication-like interactions between adjacent pairs and groups of photoreceptors. The position of a small contrasted object against a uniform background is measured, at least in part, by (formally) 1-input nonlinear flicker detectors. A fly can also detect and discriminate a figure that moves relative to a ground texture. This computation of relative movement relies on a more complex algorithm, one which detects discontinuities in the movement field. The experiments described in this paper indicate that the outputs of neighbouring movement detectors interact in a multiplication-like fashion and then in turn inhibit locally the flicker detectors. The following main characteristic properties (partly a direct consequence of the algorithm's structure) have been established experimentally: a) Coherent motion of figure and ground inhibit the position detectors whereas incoherent motion fails to produce inhibition near the edges of the moving figure (provided the textures of figure and ground are similar). b) The movement detectors underlying this particular computation are direction-insensitive at input frequencies (at the photoreceptor level) above 2.3 Hz. They become increasingly direction-sensitive for lower input frequencies. c) At higher input frequencies the fly cannot discriminate an object against a texture oscillating at the same frequency and amplitude at 0° and 180° phase, whereas 90° or 270° phase shift between figure and ground oscillations yields maximum discrimination. d) Under conditions of coherent movement, strong spatial incoherence is detected by the same mechanism. The algorithm underlying the relative movement computation is further discussed as an example of a coherence measuring process, operating on the outputs of an array of movement detectors. Possible neural correlates are also mentioned.     

Making connections in food webs

Patterns in food web structure have provided an important, though contentious, testing ground for ideas about the population dynamics and energetics of multispecies systems. One of the most debated of these patterns is the apparent decrease in food web connectance as the number of species in a web Increases. Several contrasting mechanisms that might determine food web connectance have been suggested. These mechanisms, in combination with new, food web data, suggest that the conventional pattern, and explanations for it, may well be open to dispute. The true nature of the relationship between connectance and species number has implications for the explanation of other web patterns and for theories of food web structure, but a general explanation remains elusive.     

A recurrent system incorporating characteristics of the visual system: a model for the function of backward neural connections in the visual system

A recurrent system is constructed in order to investigate the role of the backward neural connections found in the primate visual system. The system incorporates a layer to perform localized spatial frequency analysis of input images, a function which has been assumed to take place in the primary visual cortex. The function of the system is examined by simulation. The results show that the system can separate an object pattern from its background, irrespective of its precise position. The acceptable displacement range for input images is determined from the width of the window function used to calculate the local Fourier transform. A multilayer version of the above recurrent system is also constructed.     

Encoding of motion in real time by the fly visual system.

Direction-selective cells in the fly visual system that have large receptive fields play a decisive role in encoding the time-dependent optic flow the animal encounters during locomotion. Recent experiments on the computations performed by these cells have highlighted the significance of dendritic integration and have addressed the role of spikes versus graded membrane potential changes in encoding optic flow information. It is becoming increasingly clear that the way optic flow is encoded in real time is constrained both by the computational needs of the animal in visually guided behaviour as well as by the specific properties of the underlying neuronal hardware.     

A special class of nonlinear interactions in the visual system of the fly

Flies can detect a small object in front of a randomly contrasted background if the object undergoes small motions. The effect was investigated in fixed flying flies under open-loop conditions. The results suggest that nonlinear inhibitory interactions underly this elementary case of figure-ground discrimination.     

Formation of plastic connections of the visual system during ontogenesis of the rabbit

Analysis of evoked potentials and unit activity in the visual cortical projection area of rabbits revealed a definite succession of forming of interneuronal connections in ontogeny. In early postnatal period, the neuronal reactions were characterized by stable responses with one excitatory phase corresponding to initially negative surface evoked potential. Similarity of reactions of neurones situated in the same vertical column was observed and explained by functioning of a system of rigid connections of the thalamic relay nuclei afferents with cortical pyramidal neurones. Beginning from the third week of postnatal life of rabbits the neuronal reactions assumed a distinctly expressed phasic character, and variability of responses was seen along the vertical line. The changes revealed correlated with formation of a system of interneurones providing a possibility of plastic neuronal interaction. A study of the influence of preliminary cortical stimulation of the associative areas showed that intercentral cooperation mediated by cortical interneurones providing a systemic analysis of visual information began to form from the third week of postnatal life and reached the definitive level at later stages of development.     

The development of connections in the tecto-thalamo-telencephalic visual system of the brain in 13-day-old chick embryos

Light and electron microscopic studies have been made on degenerative changes in the nervous tissue induced by experimental destruction of the median brain bulb at the 5th day of incubation, in parts of the tecto-thalamo-telencephalic visual system in 13-day chick embryos (in the visual tectum, round nucleus of the thalamus and ectostriatum of the telencephalon). It was shown that to this period tecto-thalamic connections are already formed in the visual system, whereas thalamo-telencephalic connections are, presumably, indirect ones.     

Making connections: boundaries and insulators in Drosophila

In eukaryotes, enhancers must often exert their effect over many tens of kilobases of DNA with a choice between many different promoters. Given this situation, elements known as chromatin boundaries have evolved to prevent adventitious interactions between enhancers and promoters. The amenability of Drosophila to molecular genetics has been crucial to the discovery and analysis of these elements. Since these elements are involved in such diverse processes and show little or no sequence similarity between them, no single molecular mechanism has been identified that accounts for their activity. However, over the past approximately 5 years, evidence has accumulated suggesting that boundaries probably function through the formation of long-distance chromatin loops. These loops have been proposed to play a crucial role in both controlling enhancer-promoter interactions and packing DNA.     

On the neuronal basis of figure-ground discrimination by relative motion in the visual system of the fly

A new class of large-field tangential neurones (Figure Detection (FD-) cells) has been found and analysed in the lobula plate, the posterior part of the third visual ganglion, of the fly by combined extra-and intracellular recording as well as Lucifer Yellow injection. The FD-cells are likely to play a prominent role in figure-ground discrimination. Together with the Horizontal Cells, the output elements of the neuronal network underlying the optomotor course control reaction, they seem to be appropriate to account for the characteristic yaw torque response to relative motion. The FD-cells might thus compensate for the deficits of the Horizontal Cells with respect to figureground discrimination (see Egelhaaf, 1985a).The FD-cells are directionally selective for either front-to-back (FD 1, FD 4) or back-to-front motion (FD 2, FD 3). Their excitatory receptive fields cover part of (FD 1, FD 2, FD 3) or the entire horizontal extent (FD 4) of the visual field of one eye. Their most important common property in the context of figureground discrimination is that they are more sensitive to relatively small objects than to spatially extended patterns. Their response to a small figure is much reduced by simultaneous large-field motion in front of the ipsi-as well as the contralateral eye. This large-field inhibition is either directionally selective or bidirectional, depending on the FD-cell under consideration. The main dendritic arborization of all FD-cells resides in the lobula plate. Their axonal projections lie in either the ipsi-or contralateral posterior optic foci and, thus, in the same area as the terminals of the Horizontal Cells. The FD-cells are, therefore, appropriate candidates for output elements of the optic lobes involved in figure-ground discrimination.     

On the neuronal basis of figure-ground discrimination by relative motion in the visual system of the fly

It has been concluded in the preceding papers (Egelhaaf, 1985a, b) that two functional classes of output elements of the visual ganglia might be involved in figure-ground discrimination by relative motion in the fly: The Horizontal Cells which respond best to the motion of large textured patterns and the FD-cells which are most sensitive to small moving objects. In this paper it is studied by computer simulations (1) in what way the input circuitry of the FD-cells might be organized and (2) the role the FD-cells play in figure-ground discrimination. The characteristic functional properties of the FD-cells can be explained by various alternative model networks. In all models the main input to the FD-cells is formed by two retinotopic arrays of small-field elementary movement detectors, responding to either front-to-back or back-to-front motion. According to their preferred direction of motion the FD-cells are excited by one of these movement detector classes and inhibited by the other. The synaptic transmission between the movement detectors and the FD-cells is assumed to be non-linear. It is a common property of all these model circuits that the inhibition of the FD-cells induced by large-field motion is mediated by pool cells which cover altogether the entire horizontal extent of the visual field of both eyes. These pool cells affect the response of the FD-cells either by pre- or postsynaptic shunting inhibition. Depending on the FD-cell under consideration, the pool cells are directionally selective for motion or sensitive to motion in either horizontal direction. The role the FD-cells and the Horizontal Cells are likely to play in figure-ground discrimination can be demonstrated by computer simulations of a composite neuronal model consisting of the model circuits for these cell types. According to their divergent spatial integration properties they perform different tasks in figure-ground discrimination: Whereas the Horizontal Cells mainly mediate information on wide-field motion, the FD-cells are selectively tuned to efficient detection of relatively small targets. Both cell classes together appear to be sufficient to account for figure-ground discrimination as it has been shown by analysis at the behavioural level.