Search PubMed⌕ Search

Biomedical subjects

A Longtin

Publications and source records attributed to A Longtin.

16 recordsLinked to original sources
PubMed1997

Spike train patterning and forecastability.

Theories of neural coding rely on a knowledge of correlations between firing events. These correlations are also useful to validate biophysical models for the neural activity. We present a methodology for validating models based on the assessment of linear and non-linear correlations between variables derived from the spike train. The firing pattern of an electroreceptor is analyzed in this framework. We show that a purely stochastic model fails to capture the essential correlations between interspike intervals, even though it reproduces the interval histogram and certain spike train spectral features. However, a biophysical model, based on the Fitzhugh-Nagumo equations with noise, does exhibit many of the correlations seen in the data, including those between successive firing phases.

Action Potentials↗
PubMed1996

Encoding with bursting, subthreshold oscillations, and noise in mammalian cold receptors.

Mammalian cold thermoreceptors encode steady-state temperatures into characteristic temporal patterns of action potentials. We propose a mechanism for the encoding process. It is based on Plant's ionic model of slow wave bursting, to which stochastic forcing is added. The model reproduces firing patterns from cat lingual cold receptors as the parameters most likely to underlie the thermosensitivity of these receptors varied over a 25 degrees C range. The sequence of firing patterns goes from regular bursting, to simple periodic, to stochastically phase-locked firing or "skipping." The skipping at higher temperatures is shown to necessitate an interaction between noise and a subthreshold endogenous oscillation in the receptor. The basic period of all patterns is robust to noise. Further, noise extends the range of encodable stimuli. An increase in firing irregularity with temperature also results from the loss of stability accompanying the approach by the slow dynamics of a reverse Hopf bifurcation. The results are not dependent on the precise details of the Plant model, but are generic features of models where an autonomous slow wave arises through a Hopf bifurcation. The model also addresses the variability of the firing patterns across fibers. An alternate model of slow-wave bursting (Chay and Fan 1993) in which skipping can occur without noise is also analyzed here in the context of cold thermoreception. Our study quantifies the possible origins and relative contribution of deterministic and stochastic dynamics to the coding scheme. Implications of our findings for sensory coding are discussed.

Action Potentials↗
PubMed1994

Bistability and the dynamics of periodically forced sensory neurons.

Many neurons at the sensory periphery receive periodic input, and their activity exhibits entrainment to this input in the form of a preferred phase for firing. This article describes a modeling study of neurons which skip a random number of cycles of the stimulus between firings over a large range of input intensities. This behavior was investigated using analog and digital simulations of the motion of a particle in a double-well with noise and sinusoidal forcing. Well residence-time distributions were found to exhibit the main features of the interspike interval histograms (ISIH) measured on real sensory neurons. The conditions under which it is useful to view neurons as simple bistable systems subject to noise are examined by identifying the features of the data which are expected to arise for such systems. This approach is complementary to previous studies of such data based, e.g., on non-homogeneous point processes. Apart from looking at models which form the backbone of excitable models, our work allows us to speculate on the role that stochastic resonance, which can arise in this context, may play in the transmission of sensory information.

Acoustic Stimulation↗
PubMed1990

Evaluation of pupil constriction and dilation from cycling measurements.

Pupil cycling was produced using an electronic circuit so that the retina was illuminated in Maxwellian view only when pupil area exceeded an adjustable area threshold, Aref. The maximum (Amax) and minimum (Amin) amplitude of the oscillations varied linearly with Aref. These observations are described by a delay-differential equation. The Aref-dependent changes in Amax, Amin were used, respectively, to quantitate dilation and constriction. A comparison of the predicted and observed period of pupil cycling suggests that the latency times for light onset and offset are the same. Measurements of Amax, Amin provide a method for determining the average pupil light response.

Adult↗
PubMed1990

Complex dynamics and noise in simple neural networks with delayed mixed feedback.

This paper briefly reviews the role of mixed feedback, neural delays, and neural noise in the genesis of complex oscillations in neurological feedback systems. The results are concretely discussed within the context of recurrent inhibition in the mammalian hippocampus, and a hybrid version of the pupil light reflex with externally imposed electronic feedback.

Animals↗
PubMed1989

Insight into the transfer function, gain, and oscillation onset for the pupil light reflex using nonlinear delay-differential equations.

Analogies are drawn between a physiologically relevant nonlinear delay-differential equation (DDE) model for the pupil light reflex and servo control analytic approaches. This DDE is shown to be consistent with the measured open loop transfer function and hence physiological insight can be obtained into the gain of the reflex and its properties. A Hopf bifurcation analysis of the DDE shows that a limit cycle oscillation in pupil area occurs when the first mode of the characteristic equation becomes unstable. Its period agrees well with experimental measurements. Beyond the point of instability onset, more modes become unstable corresponding to multiple encirclings of (-1, 0) on the Nyquist plot. These modes primarily influence the shape of the oscillation. Techniques from dynamical systems theory, e.g. bifurcation analysis, can augment servo control analytic methods for the study of oscillations produced by nonlinear neural feedback mechanisms.

Blinking↗
PubMed1989

Modelling autonomous oscillations in the human pupil light reflex using non-linear delay-differential equations.

Neurophysiological and anatomical observations are used to derive a non-linear delay-differential equation for the pupil light reflex with negative feedback. As the gain or the time delay in the reflex is increased, a supercritical Hopf bifurcation occurs from a stable fixed point to a stable limit cycle oscillation in pupil area. A Hopf bifurcation analysis is used to determine the conditions for instability and the period and amplitude of these oscillations. The more complex waveforms typical of the occurrence of higher order bifurcations were not seen in numerical simulations of the model. This model provides a general framework to study the different types of dynamical behaviors which can be produced by the pupil light reflex, e.g. edge-light pupil cycling.

Feedback↗
PubMed1988

Irregular pupil cycling as a characteristic abnormality in patients with demyelinative optic neuropathy.

We used an infrared videopupillometer combined with an electronic circuit that regulated the retinal light level as a function of pupil area to assess the regularity of pupil cycling in normal subjects and in patients with known abnormalities in the pupil light reflex pathways. The light stimulus was turned on whenever pupil area exceeded a preset value. Two types of abnormalities were observed for patients with demyelinative optic neuropathy: a failure of the pupil to cycle despite a preserved pupillary response to a single light pulse; and, for those patients in whom cycling was possible, a characteristic intermittent irregularity in the amplitude of pupil cycling. These abnormalities were not seen in normal subjects or in patients with ischemic optic neuropathy, surgical lesions involving the optic chiasm, Adie's syndrome, or Horner's syndrome.

Adolescent↗
PubMed1986

A new model of the acoustic reflex.

A system-type model of the acoustic reflex in man is proposed with the intention of sheding light on certain of its nonlinear behaviors. This model is the first to incorporate into the multipath structure of the reflex arc the adaptation and recovery processes. Parameter distribution in the parallel pathways is based on the current knowledge on the stapedius muscle and on motoneuron pool organization. A piecewise linear system is used in modeling adaptation at onset and recovery at offset. The model is calibrated at 2000 Hz, a frequency for which all the important parameters are available. Two nonlinear behaviors of the adaptation rate are explained: the frequency and intensity dependence, related respectively to the frequency dependence of the feedback gain and to the sigmoidal shape of the closed-loop stimulus-response curve. Underlying physiological mechanisms are discussed, along with other plausible nonlinear models, and extensions of the model to other stimuli are suggested.

Acoustic Stimulation↗
PubMed1984

Multicompartment model of lung dynamics.

A mathematical model was developed to simulate the function of the lungs. The lungs are represented by 24 compartments each corresponding to a generation of the Weibel model A. In the model it is assumed that gases are transported in the lungs by convection and diffusion from one compartment to the other. Furthermore, the clearance of gases from the lungs by the blood perfusion is taken into account. The driving force of the inhalation and exhalation processes is the filling and emptying of the alveolar volume which follows a sinusoidal pattern. Mathematically the model is represented by two sets (one for inhalation, the other for exhalation) of 24 first-order coupled ordinary differential equations which were numerically integrated by means of a computer. The model predicts quite well the buildup of gases in the lungs and the washout of gases from the lungs.

Computers↗