A unitary hypothesis of mind-brain interaction in the cerebral cortex

A brief introduction to the brain-mind problem leads on to a survey of the neuronal structure of the cerebral cortex. It is proposed that the basic receptive units are the bundles or clusters of apical dendrites of the pyramidal cells of laminae V and III-II as described by Fleischhauer and Peters and their associates. There are up to 100 apical dendrites in these receptive units, named dendrons. Each dendron would have an input of up to 100000 spine synapses. There are about 40 million dendrons in the human cerebral cortex. A study of the influence of mental events on the brain leads to the hypothesis that all mental events, the whole of the World 2 of Popper, are composed of mental units, each carrying its own characteristic mental experience. It is further proposed that each mental unit, named psychon, is uniquely linked to a dendron. So the mind-brain problem reduces to the interaction between a dendron and its psychon for all the 40 million linked units. In my 1986 paper ( Proc. R. Soc. Lond . B 227, 411-428) on the mind-brain problem, there was developed the concept that the operation of the synaptic microsites involved displacement of particles so small that they were within range of the uncertainty principle of Heisenberg. The psychon-dendron interaction provides a much improved basis for effective selection by a process analogous to a quantal probability field. In the fully developed hypothesis psychons act on dendrons in the whole world of conscious experiences and dendrons act on psychons in all perceptions and memories. It is shown how these interactions involve no violation of the conservation laws. There are great potentialities of these unitary concepts, for example as an explanation of the global nature of a visual experience from moment to moment. It would seem that there can be psychons not linked to dendrons, but only to other psychons, creating what we may call a psychon world.

Interpretation of agonist affinity estimations: the question of distributed receptor states

In the receptor−transducer model of pharmacological agonism, rejection of the traditional assumption that receptor molecules are in vast excess of transducer molecules permits the receptors to become distributed among unbound, bound and complexed states. Under these conditions, agonist affinities are liable to be overestimated when the method of irreversible receptor antagonism is used. Graphical tests have been developed to detect distribution, and these were applied to experimental data from the interaction between 5-HT and phenoxybenzamine on aortic tissue. Significant receptor distribution was not detected by the method. However, in the model it was assumed that there was a linear relation between the concentration of ternary complex and pharmaco­logical effect. If this relation was replaced with a saturable one the effect of receptor distribution would be masked. The implications for phar­macologists and medicinal chemists are discussed.

The ecology and genetics of fitness in Chlamydomonas . II. The properties of mixtures of strains

Whether or not mixtures of strains possess special properties depends on whether or not their component genotypes interact with one another. This paper describes a series of experiments designed to investigate genotype-by-genotype interaction among species of the unicellular green alga Chlamydomonas when grown axenically as liquid cultures in chemically defined media. It was shown that strains varied substantially in their average effect on the overall production of mixed cultures, and that specific interactions between pairs of genotypes were also significant, though small. This conclusion was confirmed by experiments in which strains were cultured separately in growth medium that had been con­ditioned by the prior culture of another strain. However, there was no consistent general tendency for mixtures to be more productive than the average of their components in pure culture; some mixtures in some physical environments were markedly more productive than their com­ponent means, but other mixtures in other environments were markedly less productive. The average effect of a strain on the production of mixtures was uncorrelated with its performance in pure culture, showing that fitness measured in pure culture may be a poor predictor of success in mixed populations. However, the average effect of a strain in simple mixtures was correlated with its effect in more complex mixtures. Com­plex mixtures themselves displayed properties similar to, but more extreme than, those of simple mixtures; thus, in an environment where pairwise mixtures tended to outyield pure cultures, excess production tended to increase with mixture complexity. The results of these experiments seem to be consistent with those of comparable agronomic trials.

Population size and morph frequency in a long-term study of Cepaea nemoralis

Cepaea nemoralis at a site on Fyfield Down, Wiltshire, England, have been surveyed, using mark, release, recapture, for 23 years. The population has morph frequencies that do not appear to match the back­ground. At the start of the survey there was heavy predation by birds, but this soon ceased and the vegetation in the area became more uniform, probably because of the destruction of rabbits by myxomatosis. Over the study period the population fluctuated in size but morph frequencies remained almost unchanged. Variation in recruitment, rather than sur­vival, is responsible for variation in numbers. The morph frequency of captured snails differs between juveniles and adults. We suggest that this effect is due to differences in the extent to which large and small individuals move in the open (exposure). This in turn is due to changes in heating properties as the animals increase in size. If the largest selection pressure affecting morph frequency arises from visual predation, then the lack of predators, and lack of evidence of change in survival rate are consistent with the invariant morph fre­quencies. The extent to which studies of the present kind can detect selection is discussed.

ATP-sensitive K + -channel run-down is Mg 2+ dependent

ATP-sensitive K + -channel currents were recorded from isolated mem­brane patches and voltage-clamped CRI-G1 insulin-secreting cells. Inter­nal Mg 2+ ions inhibited ATP-K + channels by a voltage-dependent block of the channel current and decrease of open-state probability. The run­ down of ATP-K + channel activity was also shown to be [Mg 2+ ] i depen­dent, being almost abolished in Mg 2+ -free conditions. Substitution of Mn 2+ for Mg 2+ did not prevent run-down, nor did the presence of phos­phate-donating nucleotides, a protease or phosphatase inhibitor or replacement of Cl by gluconate.

Matched filtering in the visual system of the fly : large monopolar cells of the lamina are optimized to detect moving edges and blobs

At high levels of ambient light, large monopolar cells (LMCS) display spatially antagonistic receptive fields and a biphasic response to a brief flash of light from an axially positioned point source. In low ambient light the response becomes monophasic everywhere within the receptive field. Using the theory of matched filters, we infer that the LMCS are optimal for the detection of moving edges at high light levels, and for ‘blobs’ in low ambient light. The spatio-temporal properties predicted by the theory are in agreement with experimental observation. At high light levels, the strong temporal inhibition, the weak, diffuse lateral inhibition, and the non-separability of the receptive field in space and time are all properties that promote the sensitivity to a moving edge. At low light levels, the lack of spatial or temporal antagonism enhances the sensitivity to a blob. Our hypothesis is reinforced by the observation that flies tend to walk toward the edges of a broad, dark vertical stripe at high light levels, but uniformly toward all regions within the stripe in low ambient light.

Architecture of chicken muscles: short-fibre patterns and their ontogeny

Staining for motor endplates and chemical digestion of five major muscles of the domestic chicken shows that these confirm the short-fibre strap muscle paradigm. The individual fibres are spindle-shaped, terminating in gradually tapering ends. The motor endplates of the individual fibres align in cross-bands along the length of the fascicles. These bands are spaced much more tightly than are comparable bands in mammals; unlike the condition in mammals, many fibres are longer than twice the interband spacing. The spacings between bands differ by more than a factor of five along the length of each muscle. The proportions among bands remain relatively constant. These proportions are not affected by the degree of muscular contraction, nor do they change with ontogeny, suggesting that the arrangement is established before hatching.

The ecology and genetics of fitness in Chlamydomonas . I. Genotype-by-environment interaction among pure strains

Strains of Chlamydomonas were cultured in different macroenvironments created by manipulating levels of nitrate, phosphate and bicarbonate in liquid growth media. Cell density, measured by optical transmittance, increased in a density-regulated manner, permitting the logistic par­ameters r and K to be estimated for each genotype–environment combination. The main empirical results of a factorial experiment were as follows. (i) A large proportion of the overall genotypic variance in fitness measures was attributable to genotype-by-environment (G × E) interaction: 65 % for r and 50 % for K . Variance components for r and K were uncorrelated, but components of the interaction variance may have been correlated with corresponding components of the environmental variance, such that the relative fitness of genotypes was most strongly affected by environmental factors that have the greatest effect on average fitness. Higher-order interactions were as large as lower-order interactions, so that relative fitness was sensitive to particular combinations of environmental factors as well as to their main effects. The covariance of r with K also showed strong G × E interaction, being negative in some macroenvironments and zero in others. (ii) An ‘environmental’ decomposition of the G × E interaction vari­ance separates ‘inconsistency’, due to lack of complete correlation between genotypes over macroenvironments, from ‘responsiveness’, due to differences between environmental variances among genotypes. Inconsistency was much the larger component for both r and K , showing that the greater part of the interaction variance was created by changes in the ranking of genotypes with respect to fitness between macroenvironments. When reaction norms were defined as the linear regressions of genotypic value on mean environmental value, substantial variance among reaction norms was detected : nonlinear effects were also large. (ii) A ‘genetic’ decomposition of the G × E interaction variance separates a component due to lack of complete genetic correlation from one due to differences in genetic variance. Incomplete genetic correlation was much the larger effect, the mean correlation between genotypes in two macroenvironments being only about +0.23 for r and +0.45 for K . A very striking observation was that the genetic correlation decreased as the difference between environments increased. It declined from +0.31 (for r ; + 0.58 for K ) when one factor differed between macroenvironments to +0.18 ( + 0.40) when two factors differed, and to +0.13 ( + 0.24) when all three factors differed. Furthermore, the genetic correlation varied inversely with the difference between environmental values, approaching zero when this difference was maximal. A measure of environmental consistency was obtained by plotting the score of a genotype in a given macroenvironment on its mean score over all macroenvironments, to identify environments in which generally inferior genotypes performed relatively well and vice versa. This analysis revealed some differences between macroenvironments, but nonlinear effects were again large. (iv) The two major empirical results of this investigation were ( a ) that much of the variance in fitness among genotypes is due to G × E inter­action caused by incomplete genetic correlation, and ( b ) that genetic correlation is smaller between environments that are less similar. Both the relevance and the limitations of these findings with respect to the interpretation of diversity are discussed.

Neural network model of visual cortex for determining surface curvature from images of shaded surfaces

The visual system can extract information about shape from the pattern of light and dark surface shading on an object. Very little is known about how this is accomplished. We have used a learning algorithm to construct a neural network model that computes the principal curvatures and orientation of elliptic paraboloids independently of the illumination direction. Our chief finding is that receptive fields developed by units of such model network are surprisingly similar to some found in the visual cortex. It appears that neurons that can make use of the continuous gradations of shading have receptive fields similar to those previously interpreted as dealing with contours (i. e. ‘bar’ detectors or ‘edge’ detec­tors). This study illustrates the difficulty of deducing neuronal function within a network solely from receptive fields. It is also important to consider the pattern of connections a neuron makes with subsequent stages, which we call the ‘projective field’.

Stochastic structure and nonlinear dynamics of food webs: qualitative stability in a Lotka-Volterra cascade model

Two competing models currently offer to explain empirical regularities observed in food webs. The Lotka-Volterra model describes population dynamics; the cascade model describes trophic structure. In a real ecological community, both population dynamics and trophic structure are important. This paper proposes and analyses a new hybrid model that combines population dynamics and trophic structure: the Lotka-Volterra cascade model (LVCM). The LVCM assumes the population dynamics of the Lotka-Volterra model when the interactions between species are shaped by a refinement of the cascade model. A critical surface divides the three-dimensional parameter space of the LVCM into two regions. In one region, as the number of species becomes large, the limiting probability that the LVCM is qualitatively globally asymptotically stable is positive. In the region on the other side of the critical surface, and on the critical surface itself, this limiting probability is zero. Thus the LVCM displays an ecological phase transition: gradual changes in the probabilities of various kinds of population dynamical interactions related to feeding can have sharp effects on a community’s qualitative stability. The LVCM shows that an inverse proportionality between connectance and the number of species, and a direct proportionality between the number of links and the number of species, as observed in data on food webs, need not be directly connected with the qualitative global asymptotic stability or instability of population dynamics. Empirical testing of the LVCM will require field data on the population dynamical effects of feeding relations.