Laura Geppetti

Application of non-linear filters based on the median filter to experimental and simulated multiunit neural recordings

Two non-linear, high-pass filters based on the median filter are proposed and tested as substitutes for linear filtering in applications involving multiunit neural recordings. The first, the median-based high-pass (MH) filter, operates by subtracting the output from the input of the median filter; it is aimed at preserving the shape of the impulses. The second, the negative median-based high-pass (NMH) filter, sets at zero the positive values in the output of the MH filter; it is aimed at transforming the impulses into monophasic waves placed on a flat baseline. When applied to experimental recordings and to a template action potential, the two median-based filters clearly outperformed two corresponding procedures based on a linear filter (moving-average filter). They did not produce appreciable distortions of the impulses, whereas their two counterparts induced or enlarged lateral lobes, as is the rule for linear high-pass filters. The recording display was much improved and impulse identification was made easier. When the two filters were applied to simulated recordings and the mean output was estimated by averaging and cross-correlation, a certain degree of performance deterioration was assessed in conditions of sustained activity and/or noise, with a resulting growing similarity to the mean output of the two corresponding, moving-average-based filters.

Cross-correlation-based evaluations of the impulse transmission in a nerve: simulation and experimental studies

Bidirectional impulse transmission can be evaluated by cross-correlation analysis when two recording points are simultaneously available on a nerve. This method was tested here on digitized experimental recordings--from the cerebro-buccal connective of Aplysia--and on computer-simulated recordings with predetermined signal and noise content. The data were processed as such, or after being subjected to one of two preliminary treatments aiming at improving the sensitivity and discrimination power of the method. In the first treatment--Positive Value Saving, PVS--digitized values that were larger than the mean level were left unmodified, while the others were replaced by the mean value itself; in the second treatment--Positive Peak Saving, PPS--the values left unmodified were those which were larger than the mean level and represented a relative maximum. PVS tended to eliminate the negative deflections of the extracellular spikes; PPS tended to transform each spike into a single value equal to the spike amplitude. The cross-correlation histograms obtained yielded a clear separation of the impulses travelling in one and the opposite direction of propagation, and provided their subdivision and quantitative estimation according to propagation velocity. In the conditions adopted, spikes comparable in size to the noise range could be revealed. PVS improved sensitivity and discrimination power; PPS provided a very sharp discrimination between impulses with similar propagation velocity, at the expense of a loss of sensitivity.

Control of feeding behaviour in Aplysia depilans by haemolymphatic glucose

Glucose solutions (160 mg/kg and 320 mg/kg body weight) were injected into the haemolymphatic sinuses of Aplysia depilans through an implanted cannula, and the effects on feeding behaviour were noted. The feeding parameter examined was the ingestion velocity of a standard weed ribbon (Ulva): it was evaluated from the time taken to ingest each of ten successive centimetres of the ribbon. The ingestion velocity was found to decrease in a dose‐dependent manner following glucose injection. No irregularities or alterations in the shape of the feeding curves were induced. The effect became progressively larger during the 60 minutes following injection, and appeared to level off afterwards.

Polyphasic responses evoked by multiaction interneurones in the buccal ganglia of Aplysia

Abstract In the buccal ganglia of Aplysia depilans and A. fasciata, the small S1 cells were found to affect synaptically not only the A, B, and C cells, as already reported, but also the middle-sized and large neurones which are placed in a central position in the caudal face of the ganglion, and which are labelled “D cells” here. The responses of the D cells were quite varied: mono-, di-, tri-, and tetra-phasic responses were found. Most D cells were specialized towards a limited number of types of these responses. By using standard criteria, the responses proved to be monosynaptic and chemically mediated. Four monophasic responses were found to occur: a fast and a slow depolarization, and a fast and a slow hyperpolarization. The polyphasic responses appeared to originate from associations of the four monophasic PSPs at the various synaptic contacts. All the types of response which were possible according to such a mechanism were found in the experiments. The modifications of the phases with imposed pola...

Fractionated analysis of paired-electrode nerve recordings

Multi-unit activity recorded from two electrodes positioned at a distance on a nerve may be analysed by cross-correlation, but units similar in direction and velocity of propagation cannot be distinguished and separately evaluated by this method. To overcome this limit, we added two features, represented by the impulse amplitudes of the paired recordings, to the dimension given by the impulse delay. The analysis was fractionated according to the new dimensions. In experimental recordings from the locomotor appendage of the lobster Homarus americanus, the fractionated analysis proved capable of identifying the contributions of single active units, even if these were superimposed and indiscernible in the global cross-correlation histogram. Up to 5 motor and 10 sensory units could be identified. The shape of the paired impulses was evaluated by an averaging procedure. Analogous evaluations on simulated recordings made it possible to estimate the influences exerted on performance by variations in noise level and in the number and firing rate of active units. The global signal could be resolved into single units even under the worst conditions. Accuracy in evaluating the amount of unit activity varied, exceeding 90% in about half of the cases tested; a similar performance was attained by evaluation of the impulse shapes.

Composition of the antennal nerve ofHomarus americanusat different points along the flagellum

The antennal flagellum nerve of Homarus americanus was investigated as to structure, number and size of axons, and propagation velocity. Frequency distributions of axon diameters, evaluated at four equidistant levels on the flagellum, ranged with continuity between 0.25 and 14.7 µm, with maximum at 0.5-1.5 µm. Axons 0.5-1.5 µm in diameter were more abundant at the distal level, indicating sensory specialization near the tip. The total axon numbers increased from about 9000 at the distal level to about 45 000 at the base. Axons of different size followed different patterns of increase in number from tip to base; these patterns were examined in relation to structural features of the flagellum, and to hypotheses of association with known or unidentified receptors. Propagation velocities were distributed with continuity, in the range between 3.39 and 0.24 m/s; velocity-diameter correspondences were outlined.

A morphological study on the amount and origin of axons in the pleuroabdominal connectives of Aplysia fasciata

Abstract The pleuro‐abdominal connectives (PACs) in Aplysia fasciata (Poiret) observed at EM showed irregular compartments in which different calibre fibres were intermingled with abundant glia laminae. The estimated total numbers of axons were 1388 in the left and 1832 in the right PAC, with similar size distribution. Co2+ backfills were applied to the severed PACs to highlight the populations of central neurones projecting into these nerves from either side. After left abdominal PAC stump backfills, an average of 51.5 ipsilateral and 9.2 contralateral neurones were stained; correspondingly, right PAC backfills yielded 32.5 and 13.1 neurones. Aided by a preliminary examination of the abdominal ganglion of A. fasciata, a comparison with A. californica was carried out and correspondences of neurones and clusters were proposed. In all, the stained abdominal neurones outnumbered the ones described as projecting into the PACs in A. californica, and a part appeared as newly identified. Backfills applied to pleural left or right PAC stump stained less numerous neurones, all ipsilateral, in the pleural ganglion (average 30.6 and 16.5, respectively) and pedal ganglion (3.3 and 3.9). Many stained fibres could not be traced to a soma; although often crossing the ganglia and entering a peripheral nerve, they appeared to assure long‐distance connections with the periphery.

Activity inAplysia buccal ganglia characterized by signal processing techniques

SummaryTwo procedures aimed at a synthetic, quantitative characterization of the integrative activity in a small system of nerve cells are proposed, and applications to the buccal ganglion of the molluskAplysia are described. Both methods are based on cross-correlation evaluation. The first method processes intracellular signals simultaneously recorded from pairs of neurones, and reveals and measures the occurrence of shared synaptic input. The second method processes extracellular multiunit signals recorded by two cuff electrodes placed around one nerve, and evaluates the impulse traffic, subdivided according to direction and speed of propagation. The outcome of both methods may be used to represent the state of the system, and may provide a basis to further studies on its dynamics.