Biomedical subjects

I Opher

Publications and source records attributed to I Opher.

3 recordsLinked to original sources

PubMed1997

Solitary waves of integrate-and-fire neural fields.

Arrays of interacting identical neurons can develop coherent firing patterns, such as moving stripes that have been suggested as possible explanations of hallucinatory phenomena. Other known formations include rotating spirals and expanding concentric rings. We obtain all of them using a novel two-variable description of integrate-and-fire neurons that allows for a continuum formulation of neural fields. One of these variables distinguishes between the two different states of refractoriness and depolarization and acquires topological meaning when it is turned into a field. Hence, it leads to a topologic characterization of the ensuing solitary waves, or excitons. They are limited to pointlike excitations on a line and linear excitations, including all the examples noted above, on a two-dimensional surface. A moving patch of firing activity is not an allowed solitary wave on our neural surface. Only the presence of strong inhomogeneity that destroys the neural field continuity allows for the appearance of patchy incoherent firing patterns driven by excitatory interactions.

Cell Communication↗

PubMed1996

Temporal segmentation in a neural dynamic system.

Oscillatory attractor neural networks can perform temporal segmentation, i.e., separate the joint inputs they receive, through the formation of staggered oscillations. This property, which may be basic to many perceptual functions, is investigated here in the context of a symmetric dynamic system. The fully segmented mode is one type of limit cycle that this system can develop. It can be sustained for only a limited number n of oscillators. This limitation to a small number of segments is a basic phenomenon in such systems. Within our model we can explain it in terms of the limited range of narrow subharmonic solutions of the single nonlinear oscillator. Moreover, this point of view allows us to understand the dominance of three leading amplitudes in solutions of partial segmentation, which are obtained for high n. The latter are also abundant when we replace the common input with a graded one, allowing for different inputs to different oscillators. Switching to an input with fluctuating components, we obtain segmentation dominance for small systems and quite irregular waveforms for large systems.

Electrophysiology↗

PubMed1996

The importance of noise for segmentation and binding in dynamical neural systems.

Segmentation and binding are cognitive operations that underlie the process of perception. They can be understood as taking place in the temporal domain. In models of nonlinear oscillatory neurons and neuronal cell assemblies, we represent binding by phase locking of assemblies in different networks, and segmentation by phase separation of assemblies in the same network, leading to waveforms of staggered oscillations. Both processes can be facilitated if the inputs to the system, representing simultaneously activated memories, possess noisy components. In the binding problem they serve as a tagging device, driving phase locking between assemblies that belong to different networks but carry the same inputs. In the segmentation problem they allow us to overcome an inherent limitation on segmentation that, otherwise, cannot accommodate more than a few commonly excited memories.

Artifacts↗