Population coding of motion patterns in the early visual system.

Using extracellular recordings and computational modeling, we study the responses of a population of turtle (Pseudemys scripta elegans) retinal ganglion cells to different motion patterns. The onset of motion of a bright bar is signaled by a rise of the population activity that occurs within less than 100 ms. Correspondingly, more complex stimulus movement patterns are reflected by rapid variations of the firing rate of the retinal ganglion cell population. This behavior is reproduced by a computational model that generates ganglion cell activity from the spatio-temporal stimulus pattern using a Wiener model complemented by a non-linear contrast gain control feedback loop responsible for the sharp transients in response to motion onset. This study demonstrates that contrast gain control strongly influences the temporal course of retinal population activity, and thereby plays a major role in the formation of a population code for stimulus movement patterns.

Multidimensional encoding strategy of spiking neurons.

Neural responses in sensory systems are typically triggered by a multitude of stimulus features. Using information theory, we study the encoding accuracy of a population of stochastically spiking neurons characterized by different tuning widths for the different features. The optimal encoding strategy for representing one feature most accurately consists of narrow tuning in the dimension to be encoded, to increase the single-neuron Fisher information, and broad tuning in all other dimensions, to increase the number of active neurons. Extremely narrow tuning without sufficient receptive field overlap will severely worsen the coding. This implies the existence of an optimal tuning width for the feature to be encoded. Empirically, only a subset of all stimulus features will normally be accessible. In this case, relative encoding errors can be calculated that yield a criterion for the function of a neural population based on the measured tuning curves.

Dielectric and transport properties of a supercooled symmetrical molten salt.

The liquid-glass transition of the restricted primitive model for a symmetrical molten salt is studied using mode-coupling theory. The transition at high densities is predicted to obey the Lindemann criterion for melting, and the charge-density peak found in neutron-scattering experiments on ionic glass formers is qualitatively reproduced. Frequency-dependent dielectric functions, shear viscosities, and dynamical conductivities of the supercooled liquid are presented. Comparing the latter to the diffusion constant, we find that mode-coupling theory reproduces the Nernst-Einstein relation. The Stokes-Einstein radius is found to be approximately equal to the particle radius only near the high-density glass transition.