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Distribution or nuclear space-valued stochastic differential equations (SDEs) (diffusions as well as discontinuous equations) are discussed as stochastic models for the behavior of voltage potentials of spatially distributed neurons. A propagation of chaos result is obtained for an interacting system of Hilbert space-valued SDEs.
Impulse trains simulating the maintained discharges of retinal ganglion cells were generated by digital realizations of the integrate-and-fire model. If the mean rate were set by a "bias" level added to "noise," the variability of firing would be related to the mean firing rate as an inverse square root law; the maintained discharges of retinal ganglion cells deviate systematically from such a relationship. A more realistic relationship can be obtained if the integrate-and-fire mechanism is "leaky"; with this refinement, the integrate-and-fire model captures the essential features of the data. However, the model shows that the distribution of intervals is insensitive to that of the underlying variability. The leakage time constant, threshold, and distribution of the noise are confounded, rendering the model unspecifiable. Another aspect of variability is presented by the variance of responses to repeated discrete stimuli. The variance of response rate increases with the mean response amplitude; the nature of that relationship depends on the duration of the periods in which the response is sampled. These results have defied explanation. But if it is assumed that variability depends on mean rate in the way observed for maintained discharges, the variability of responses to abrupt changes in lighting can be predicted from the observed mean responses. The parameters that provide the best fits for the variability of responses also provide a reasonable fit to the variability of maintained discharges.
The characteristics of the BVP neuron model response to periodic pulse stimuli are investigated. Temporal patterns of the output of the model are analyzed as a function of the stimulus intensity and period. The BVP model exhibits the same chaotic behavior, and a Cantor function-like graph of the response frequency (mean firing rate) as in electrophysiological experiments. This shows that the BVP model describes the complicated response characteristics of the neuron at least qualitatively.
Multichannel brain evoked potentials can be represented as trajectories V(t) in n-dimensional voltage space, analogous to three-channel Lissajous trajectories (3-CLT). Equations of motion are developed based on an arbitrary number of dipole generators at arbitrary locations within the brain, and the properties of 3-CLT are generalized to the higher dimensional case. The trajectory is shown to be limited to k < n dimensions, and k channels are found to be sufficient for predicting the potential difference between any two points on the head, provided that an empirically determined set of linear functionals is known. A method for choosing the "best" m-channel montage (m < or = k) is described, by associating with each montage an alternating m-tensor on Rk. Planar analysis of the voltage trajectories is generalized to the k-dimensional case, in which m-planes are compared using a mapping between the Grassman manifold and real projective space.
Visual laterality studies are reviewed in the framework of an afferent hierarchical information-processing model. According to this model, the hemispheres are viewed as separate information-processing systems, each with its own components and resources, as well as a common pool of resources. Information is shared across interhemispheric pathways. The magnitude and direction of visual field differences are determined by two factors: the processing efficiency of the components in one hemisphere relative to the other and interhemispheric transmission to components that are functionally localized to only one hemisphere. Rules are derived for weighing the relative contribution of these to visual field differences. As designed, the model can accommodate dynamic features characteristic of competing efferent (attentional) models without sacrificing its basic structure. The model's adequacy and usefulness for interpreting and guiding research on normal and brain-damaged people is discussed.
Male stutterers and fluent speakers were compared on their performance of a task requiring tapping keys as rapidly and as accurately as possible to reproduce different finger movement sequences demonstrated on a visual display panel. Although overall finger tapping rate was the same in the two groups, indicating no difficulty by stutterers in performing simple motor movements, stutterers achieved fewer correct sequences and made more errors than fluent speakers. In addition, their response initiation times were slower. Once a correct response was initiated, however, the time to execute the sequence was similar to that of fluent speakers. Replicating earlier work, the two groups were not found to differ on a repetitive sequential finger tapping task with respect to correct sequences or total presses, although the probability of error was greater for the stutterers. The data were interpreted as indicating that in stutterers sequential response mechanisms are lateralized normally as they are in fluent speakers; these left-hemisphere sequential response mechanisms in stutterers appear unusually susceptible to interference, possibly from on-going right-hemisphere activity; stutterers have special difficulty in organizing and/or initiating new response sequences, but once the sequence is initiated, they can perform the sequence as rapidly (but with greater probability of error) as do fluent speakers; and stuttering reflects not a simple motor problem per se, but a higher level organizational problem of a cognitive nature.
Subjects viewed words, pictures, crosshairs, or a large X flanked by two smaller xs on either side while their brain activity was monitored using positron emission tomography (PET). When activation from the pictures, crosshairs, or Xs condition was subtracted from activation in the words condition, the left angular gyrus and Broca's area were found to be activated. In the comparison of words and pictures, additional language areas were activated. These results provide support for the classical neurological model of reading. The results also suggest that a "word form area" is near the margin of the left angular gyrus.
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Models based on spatial-frequency channels and local features provide alternative explanations for suprathreshold pattern discrimination. We compared psychophysical discrimination data with the predictions of the Wilson and Gelb channel model and three local-feature models. The features were peak-valley local contrast, peak-peak local contrast, and luminance gradients. We measured visual sensitivity for discriminating compound gratings (F + 3F or F + 5F, in peaks-add or peaks-subtract phases) whose component contrasts were yoked together so that a contrast increment in one component was accompanied by an equal decrement in the other. The Wilson and Gelb model accounted for the results with peaks-add gratings, but failed to predict those with peaks-subtract gratings. None of the local-feature models explained the results by themselves. Most of the data fell close to an envelope composed of the lowest thresholds of the three feature-detector models, although there were important exceptions. Our findings are consistent with the view that suprathreshold pattern discrimination is mediated by mechanisms responsive to spatially localized features and that more than one type of feature is used.
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An experiment demonstrated a complete hemispheric processing reversal in 10 male, dysphonetic dyslexic children that occurred during a dichotic listening test of their verbal working memory. Requiring a written response to dichotic digits produced a right hemisphere/left ear superiority in the dysphonetic dyslexics whereas normal subjects and other dyslexics maintained a left hemisphere/right ear advantage. This reversal was unaffected by changes in task difficulty. A second experiment assessed the influence on producing the reversal of concurrent manual interference with left hemisphere verbal processing (responding orally vs. manually) and selective right hemisphere priming (Forward Writing vs. Backward Writing). The dysphonetic children reverted to a strong left hemisphere superiority when recalling the dichotic digits orally. Backward writing produced no ear advantage in either direction. The findings suggest that dysphonetic dyslexia may be related to (1) left hemisphere processing demands that exceed capacity, (2) easily activated right hemisphere processing strategies and (3) failure to coordinate linguistic processing interhemispherically. The results supported a novel hybrid conceptualization of dyslexia consisting of a synthesis of selective activation, and dual processor-limited capacity, theories.
The pharmacokinetic responses of the cholinergic enteric neurone to treatment with acetylcholinesterases, tetrodotoxin, some chloride salts of divalent cations, botulinum toxin, beta-bungarotoxin and changes in the concentration of calcium ions in the external medium and repetitive stimulation are presented. The numerical results obtained reproduce quantitatively the effects of toxins and salts of divalent cations acting at different levels of acetylcholine release from the nerve-terminal. The addition of cholinergic agonists potentiates the action of acetylcholine and increases the amplitude of the generated excitatory postsynaptic potential. A decrease in the concentration of extracellular Ca2+ ions reduces the amplitude of the excitatory postsynaptic potential and significantly increases synaptic transmission time. The effect of tetrodotoxin is the blockade propagation of the action potential along the nerve axon and, as a consequence, acetylcholine release from the vesicular store. All these effects have been shown to be dose-dependent. The repetitive stimulation of the neurone reproduces the effects of accumulation and potentiation. The possible applications of the model for the analysis of the enteric nervous system function are discussed.
Euphoric properties of cocaine lead to the development of chronic abuse, and appear to involve the acute activation of central DA neuronal systems. This is based upon known effects of cocaine on DA neurons, and the role played by DA in reward states and self-stimulation behavior. With chronic cocaine use, neurotransmitter and neuroendocrine alterations occur. DA depletion is hypothesized to result from overstimulation of these neurons and excessive synaptic metabolism of the neurotransmitter. DA depletion may underlie dysphoric aspects of cocaine abstinence, and cocaine urges. Neurochemical disruptions caused by cocaine are consistent with the concept of "physical" rather than "psychological" addiction. Possible pharmacological interventions in cocaine addiction are outlined and the psychological approach to these patients is discussed.
We applied spectral analysis through multivariant autoregressive model fitting [1] to RR interval (RRI) and respiratory (RES) oscillation obtained during quiet sleep in an infant with congenital central hypoventilation syndrome (Ondine's curse), a child with obstructive sleep apnea, and two healthy children. Power spectra, impulse response and noise contribution ratio between RRI and RES oscillation were calculated by using a minicomputer PFU-1200 (FACOM) to determine the structure of the feedback system between RRI and RES within the central nervous system. We found that the respiratory noise contribution ratio to RRI was significantly smaller in Ondine's curse (37 +/- 7.7%, at 0.23 Hz) than in obstructive sleep apnea (90 +/- 6.7%, at 0.39 Hz) and healthy subjects. We postulate that the result shows disturbance of the central autonomic control of breathing and heart rate in Ondine's curse.
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It is assumed that recovery curves may be produced either by a dynamic process caused by a feedback mechanism or by a linear compensation process which is independent of previous compensation. Based on this assumption, 3 types of recovery functions are derived. Two behavior functions correspond to each of these types of recovery curves, one starting at a low level and the other at a high level. For each of the resulting 6 types of behavior curves, examples of real data have been reported in literature. Elementary parameter estimates are provided and substantial interpretations of the parameters are given. Behavior measures are derived for the data from an illustrative example, and 1 of the 6 behavior curves is fitted to this data. By additionally taking into account a physical repair function which is assumed to cause a certain delay in recovery, it is shown that an even better fit can be achieved.
Based on a bidirectional model and the temporal relation of the signals, directed coherence is defined to describe the EEG correlation according to the direction of information transmission in the frequency domain. The interhemispheric directed coherences of EEG pairs in the prefrontal, frontal, central, parietal and occipital cortices were investigated. Statistically, significantly greater right-left directed coherences were found in the parietal and occipital regions than that of the left-right direction for alpha activity. The results indicate different information processing for different frequency bands between the left and right hemispheres, and this can not be derived from the examination of original coherence. This may suggest that the measure of directed coherence can provide more information than that of original coherence.