Plasticity of synchronous activity in a small neural net

Analyzing functional connectivity using a network likelihood model of ensemble neural spiking activity

Analyzing the dependencies between spike trains is an important step in understanding how neurons work in concert to represent biological signals. Usually this is done for pairs of neurons at a time using correlation-based techniques. Chornoboy, Schramm, and Karr (1988) proposed maximum likelihood methods for the simultaneous analysis of multiple pair-wise interactions among an ensemble of neurons. One of these methods is an iterative, continuous-time estimation algorithm for a network likelihood model formulated in terms of multiplicative conditional intensity functions. We devised a discrete-time version of this algorithm that includes a new, efficient computational strategy, a principled method to compute starting values, and a principled stopping criterion. In an analysis of simulated neural spike trains from ensembles of interacting neurons, the algorithm recovered the correct connectivity matrices and interaction parameters. In the analysis of spike trains from an ensemble of rat hippocampal place cells, the algorithm identified a connectivity matrix and interaction parameters consistent with the pattern of conjoined firing predicted by the overlap of the neurons' spatial receptive fields. These results suggest that the network likelihood model can be an efficient tool for the analysis of ensemble spiking activity.

Theory of the Snowflake Plot and Its Relations to Higher-Order Analysis Methods

The snowflake plot is a scatter plot that displays relative timings of three neurons. It has had rather limited use since its introduction by Perkel, Gerstein, Smith, and Tatton (1975), in part because its triangular coordinates are unfamiliar and its theoretical properties are not well studied. In this letter, we study certain quantitative properties of this plot: we use projections to relate the snowflake plot to the cross-correlation histogram and the spike-triggered joint histogram, study the sampling properties of the plot for the null case of independent spike trains, study a simulation of a coincidence detector, and describe the extension of this plot to more than three neurons.

Identification of functionally related neural assemblies

Present-day techniques of multiple-electrode together with computer-aided separation of impulses arising from different neurons permit the simultaneous recording of nerve-impulse timings in sets of neurons exceeding 20 in number. This in turn makes it feasible to search for functional groups of neurons, defined as subsets that tend to fire in near simultaneity significantly more often than would independent neurons at corresponding mean rates. A statistical technique is described that permits the detection and identification of such functional groups. The method is accretional, based on identification of associated neurons through interative application of a significance test on multiple coincidences of neuronal firings within an observational window. Examples of the operation of the method and indications as to its sensitivity are furnished through computer simulations of neural networks. The entire algorithm may be used as a screening technique to select smaller groups of neurons for cross-correlational and related finer-grained temporal analyses, or it may be used in its own right to detect and characterize functional groups that are not distinguishable by other statistical procedures.

Correlation analysis of units recorded in the cat dorsal lateral geniculate nucleus

Spike activity was simultaneously recorded from pairs of units consisting of both optic tract fibers and relay cells in the cat dLGN. The unit pairs were classified with regard to the types of receptive fields involved, the relative location of receptive fields, and their occular drive. Statistical dependencies between the discharge patterns of the simultaneously observed units were assessed by computing their auto- and crosscorrelogram during a period of maintained discharge. Two distinctly different types of statistical dependence were observed. Unit pairs having like and overlapping receptive field centers exhibited statistical dependencies manifested in their crosscorrelograms by a peak near the origin. Conversely, unit pairs having opposite and overlapping receptive field centers were characterized by a prominent valley in their crosscorrelograms near the origin. These statistical dependencies are inferred to represent functional interaction most probably occurring in the retina and characterized by a common element exerting an influence on both units.

Elaboration of a conditioned reflex in a single experiment with simultaneous recording of neural activity

In 18 of 20 experiments with click CS, and in 5 of 10 experiments with flash CS, we have elaborated a conditioned EMG response in a single session (less than 60 pairings). The US was direct electrical stimulation of the rabbit's motor cortex that evoked a movement of the forelimb. Electrical stimulation of the lateral hypothalamus was used as reinforcement. Parameters for this reinforcement were chosen to evoke a feeding reaction or self-stimulation in the freely behaving animal. The elaborated EMG response satisfied most of the required characteristics of a conditioned reflex. These included spontaneous recovery after extinction, savings of long duration, specificity to stimulus pairing, and weak efferent and afferent generalization. In most experiments with click CS, the elaborated phasic response had an amplitude of 0.1-6 mV with a latency of 12-16 msec. In form and latency the conditioned response was similar to the unconditioned startle reaction of the same animal under chloralose anesthesia, or to its unanesthetized response to a loud sound. However, the conditioned response differed from the startle reaction in that it was localized. Extracellular recordings of 2-5 neurons were simultaneously made from sensory motor cortex near the point at which the US was applied. In 7 experiments 17 neurons were followed through the entire cycle of elaboration and extinction of the conditioned response. Seven neurons showed a statistically significant (P less than 0.05) increase of the response to CS during conditioning. Latencies were 20-140 msec. Interactions between neurons were studied by computing cross-correlograms and joint PST scatter diagrams. These measures were less informative than we had hoped because of the low level of spontaneous and evoked activity, and because of the small numbers of stimulus presentations that were needed for elaboration and extinction of the conditioned reflex. In isolated presentations of the CS after elaboration of the CR, we sometimes observed neural responses to click with a latency less than 6 msec. We propose that at least one of the pathways involved in the localized conditioned startle reflex reported here goes through the sensory motor cortex.

Mutual temporal relationships among neuronal spike trains. Statistical techniques for display and analysis

We describe a statistical technique, the joint peristimulus time (PST) scatter diagram, for the analysis of data from simultaneously recorded neurons subjected to repeated stimulation. Distinguishable features in the scatter diagram are related to effects of the stimulus on the observed neurons and to functional relations among the neurons. Properties of this measure and its variants are described and practical aspects of its application to experimental data are discussed.