Microtube theory of sensory transduction

A theory of marginotomy has been proposed to explain the limitation of the cell doubling potential of clones of normal somatic cells. Marginotomy of DNA is the shortening of the replica with respect to the template. Two possible mechanisms of marginotomy are discussed. The first mechanism is a DNA-polymerase, postulated to have a catalytically inactive zone lying between the catalytic centre and the outer edge of the enzyme molecule. The terminal template segment which is equal to the length of the non-catalytic marginal zone of the DNA-polymerase will not appear in the replica. The second mechanism of marginotomy results from a requirement for an initiating RNA-primer for activity of some DNA-polymerases. In this case the final DNA replica will be shorter than the template by the length of RNA-primer.Marginotomy causes the appearance, in the daughters of dividing cells, of more and more shortened end-genes, the so-called telogenes, with every new mitosis. The telogenes function as the starting points of end-replicons in chromosomes and also as “buffers”, being sacrificed during successive mitoses. After the exhaustion of telogenes the cells become aged and are eliminated due to the loss of some vitally important genes localized in end-replicons. Marginotomy is therefore responsible for the loss with age of various cell clones of the body, including some endocrine cell clones. Therefore marginotomy may be the primary cause of various disorders of age of the ageing of multicellular organisms.     

NMDA receptors in sensory information processing.

During the past year electrophysiological studies, particularly in the visual and somatosensory systems, have begun to uncover the specific roles played by NMDA receptors in the processing of sensory information. Many of the features of NMDA-receptor-mediated sensory responses reflect known properties of the receptor.     

On the information received by sensory receptors

It is hypothesized that a sensory neuron, a neuron issuing from a sensory receptor, encodes the rate at which entropy or uncertainty is removed at the receptor level. This hypothesis is tested for the case of the entropy associated with the magnitude of a signal (stimulus) applied at the sensory receptor. A simple mathematical model of the process is presented and a number of well-known stimulus-response relationships are seen to emerge. For example, the adaptation of a receptor may be seen to occur as a consequence of reduced uncertainty regarding stimulus intensity. A general equation relating stimulus and response is developed, and this equation will simplify, depending upon the ratio of signal power to noise power, to either a logarithmic or a power law.     

Central processing of sensory information in electric fish

Summary Comparative studies of neural mechanisms underlying the perception of natural stimulus patterns and the control of adaptive behavioral responses have revealed organizational principles that are shared by a wide spectrum of animals. Mechanisms of perception and motor control are commonly executed in a distributed network of neurons that lack pontifical elements. Individual neurons even at an organizational level as high as the optic tectum may still have very general response characteristics, and the recruitment of individual neurons reveals little about the nature of the stimulus situation outside. Only the joint evaluation of messages from large populations of such neurons yields unambiguous pictures of the outside world. Stimulus variables are commonly mapped continuously within a stratum of neurons so that their variation over time can be monitored by mechanisms similar to motion detection in a retina. The ordered representation of a stimulus variable within an array of broadly tuned elements allows for a degree of stimulus resolution that by far exceeds that of individual elements in the array. Neural systems are burdened by their evolutionary history and suffer from imperfections that are overcome by a patchwork of compensations. The existence of multiple neuronal representations of sensory information and multiple circuits for the control of behavioral responses should provide the necessary freedom for evolutionary tinkering and the invention of new designs.     

Excitatory local interneurons enhance tuning of sensory information

Neurons in the insect antennal lobe represent odors as spatiotemporal patterns of activity that unfold over multiple time scales. As these patterns unspool they decrease the overlap between odor representations and thereby increase the ability of the olfactory system to discriminate odors. Using a realistic model of the insect antennal lobe we examined two competing components of this process -lateral excitation from local excitatory interneurons, and slow inhibition from local inhibitory interneurons. We found that lateral excitation amplified differences between representations of similar odors by recruiting projection neurons that did not receive direct input from olfactory receptors. However, this increased sensitivity also amplified noisy variations in input and compromised the ability of the system to respond reliably to multiple presentations of the same odor. Slow inhibition curtailed the spread of projection neuron activity and increased response reliability. These competing influences must be finely balanced in order to decorrelate odor representations.     

Quantification of sensory information transmission using timeseries decorrelation techniques

To estimate the information transmitted across a neuronal sensory system one has to deal with serial dependence among consecutive samples of the stimulus and the response signal. Common methods usually require a huge amount of data, or are restricted to Gaussian stimuli. Here, we describe stimulus and response as stochastic processes, i.e. as sequences of random variables, in the same coordinate system. Stimulus-response pairs of these random variables must not be considered independently because otherwise the transinformation is overestimated. To account for the linear fraction of the serial dependence, we present two decorrelation techniques based on coordinate transformation. They provide a representation of the processes with uncorrelated random variables and yield a more precise estimate of the transinformation.     

Code models of sensory information from skin analyzer periphery

Experimental studies on the relations between the afferent flows in A-, A-, and C-fibers of a skin nerve for different stimulations of skin receptors provided the basis for modeling of various digital and graphic codes of known and unknown sensations.S. M. Kirov Medical Institute, Russian Ministry of Health, Nizhnii Novgorod. Translated from Neirofiziologiya, Vol. 24, No. 2, pp. 141–145, March–April, 1992.     

Language evolution and information theory

This paper places models of language evolution within the framework of information theory. We study how signals become associated with meaning. If there is a probability of mistaking signals for each other, then evolution leads to an error limit: increasing the number of signals does not increase the fitness of a language beyond a certain limit. This error limit can be overcome by word formation: a linear increase of the word length leads to an exponential increase of the maximum fitness. We develop a general model of word formation and demonstrate the connection between the error limit and Shannon's noisy coding theorem.     

Investigations into the organization of information in sensory cortex

One might take the exploration of sensory cortex in the first decades of the last century as the opening chapter of modern neuroscience. The combined approaches of (i) measuring effects of restricted ablation on functional capacities, both in the clinic and the laboratory, together with (ii) anatomical investigations of cortical lamination, arealization, and connectivity, and (iii) the early physiological probing of sensory representations, led to a fundamental body of knowledge that remains relevant to this day. In our time, there can be little doubt that its organization as a mosaic of columnar modules is the pervasive functional property of mammalian sensory cortex [Brain 120 (1997) 701]. If one accepts the assertion that columns and maps must improve the functioning of the brain (why else would they be the very hallmark of neocortex?), then the inevitable question is: exactly what advantages do they permit? In this review of our recent presentation at the workshop on Homeostasis, plasticity and learning at the Institut Henri Poincaré, we will outline a systematic approach to investigating the role of modular, map-like cortical organization in the processing of sensory information. We survey current evidence concerning the functional significance of cortical maps and modules, arguing that sensory cortex is involved not solely in the online processing of afferent data, but also in the storage and retrieval of information. We also show that the topographic framework of primary sensory cortex renders the encoding of sensory information efficient, fast and reliable.     

A calculation of the probability of spontaneous biogenesis by information theory.

The Darwin-Oparin-Haldane “warm little pond” scenario for biogenesis is examined by using information theory to calculate the probability that an informational biomolecule of reasonable biochemical specificity, long enough to provide a genome for the “protobiont”, could have appeared in 10⁹ years in the primitive soup. Certain old untenable ideas have served only to confuse the solution of the problem. Negentropy is not a concept because entropy cannot be negative. The role that negentropy has played in previous discussions is replaced by “complexity” as defined in information theory. A satisfactory scenario for spontaneous biogenesis requires the generation of “complexity” not “order”. Previous calculations based on simple combinatorial analysis over estimate the number of sequences by a factor of 10⁵. The number of cytochrome c sequences is about 3·8 × 10⁶¹. The probability of selecting one such sequence at random is about 2·1 ×10^?65. The primitive milieu will contain a racemic mixture of the biological amino acids and also many analogues and non-biological amino acids. Taking into account only the effect of the racemic mixture the longest genome which could be expected with 95 % confidence in 10⁹ years corresponds to only 49 amino acid residues. This is much too short to code a living system so evolution to higher forms could not get started. Geological evidence for the “warm little pond” is missing. It is concluded that belief in currently accepted scenarios of spontaneous biogenesis is based on faith, contrary to conventional wisdom.     

The role of inhibition and adaptation in sensory information processing

Some common features of neural transformations along sensory pathways are discussed. The emphasis is on spatial mapping in the visual system, but close parallels exist in temporal visual mapping as well as other sensory systems. The role played by lateral inhibition in sequential transformations is investigated by direct computation and by mathematical analysis.     

Age-related trends in using sensory information for movement organization

In the experimental studies of movement organization, two seemingly contradictory age-related trends are usually observed: on the one hand, the relative use of sensory information in motor control is reduced, but on the other hand, the role of sensory information in movement organization grows up in the course of development. Analysis of the experimental data suggests that these two trends can be considered to be consequences of the growing ability to build the internal models of the processes occurring in the external world and the body motor system.