Effect of time-correlation of input patterns on the convergence of on-line learning

We studied the effects of time correlation of subsequent patterns on the convergence of on-line learning by a feedforward neural network with backpropagation algorithm. By using chaotic time series as sequences of correlated patterns, we found that the unexpected scaling of converging time with learning parameter emerges when time-correlated patterns accelerate learning process.Comment: 8 pages(Revtex), 5 figure

Prolonged calcium influx after termination of light-induced calcium release in invertebrate photoreceptors

© The Authors, 2009 . This article is distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License. The definitive version was published in Journal of General Physiology 134 (2009): 177-189, doi:10.1085/jgp.200910214.In microvillar photoreceptors, light stimulates the phospholipase C cascade and triggers an elevation of cytosolic Ca2+ that is essential for the regulation of both visual excitation and sensory adaptation. In some organisms, influx through light-activated ion channels contributes to the Ca2+ increase. In contrast, in other species, such as Lima, Ca2+ is initially only released from an intracellular pool, as the light-sensitive conductance is negligibly permeable to calcium ions. As a consequence, coping with sustained stimulation poses a challenge, requiring an alternative pathway for further calcium mobilization. We observed that after bright or prolonged illumination, the receptor potential of Lima photoreceptors is followed by the gradual development of an after-depolarization that decays in 1–4 minutes. Under voltage clamp, a graded, slow inward current (Islow) can be reproducibly elicited by flashes that saturate the photocurrent, and can reach a peak amplitude in excess of 200 pA. Islow obtains after replacing extracellular Na+ with Li+, guanidinium, or N-methyl-D-glucamine, indicating that it does not reflect the activation of an electrogenic Na/Ca exchange mechanism. An increase in membrane conductance accompanies the slow current. Islow is impervious to anion replacements and can be measured with extracellular Ca2+ as the sole permeant species; Ba can substitute for Ca2+ but Mg2+ cannot. A persistent Ca2+ elevation parallels Islow, when no further internal release takes place. Thus, this slow current could contribute to sustained Ca2+ mobilization and the concomitant regulation of the phototransduction machinery. Although reminiscent of the classical store depletion–operated calcium influx described in other cells, Islow appears to diverge in some significant aspects, such as its large size and insensitivity to SKF96365 and lanthanum; therefore, it may reflect an alternative mechanism for prolonged increase of cytosolic calcium in photoreceptors.This work was supported by National Science Foundation grant 0639774

Do you see what I see? Optical morphology and visual capability of ‘disco’ clams ( Ctenoides ales

The ‘disco’ clam Ctenoides ales (Finlay, 1927) is a marine bivalve that has a unique, vivid flashing display that is a result of light scattering by silica nanospheres and rapid mantle movement. The eyes of C. ales were examined to determine their visual capabilities and whether the clams can see the flashing of conspecifics. Similar to the congener C. scaber, C. ales exhibits an off-response (shadow reflex) and an on-response (light reflex). In field observations, a shadow caused a significant increase in flash rate from a mean of 3.9 Hz to 4.7 Hz (P=0.0016). In laboratory trials, a looming stimulus, which increased light intensity, caused a significant increase in flash rate from a median of 1.8 Hz to 2.2 Hz (P=0.0001). Morphological analysis of the eyes of C. ales revealed coarsely-packed photoreceptors lacking sophisticated structure, resulting in visual resolution that is likely too low to detect the flashing of conspecifics. As the eyes of C. ales are incapable of perceiving conspecific flashing, it is likely that their vision is instead used to detect predators

Immunohistochemistry of Diverging and Converging Neurotransmitter Systems in Mollusks

Volume: 181Start Page: 484End Page: 49

Computations that neurons perform in networks: lessons learned from a Sixteenth Century shoemaker

Cognitive and other neural processes emerge from the interactions be- tween neurons. Major advances have been made in studying networks in which the interac- fions occur instantaneously by means ofgraded synapses (Guckenheimer and Rowat, I 997). In other networks, the interaction between neurons involves time-delayed signals (action potentials or spikes) that activate synapses on other neurons discontinuously in a pulse-like manner. These interactions can also be treated as being graded if, when appropriate, the information transmitted between neurons can be measured as the average number of spikes per unit time (Freeman, 1992); i.e., the amount ofinformation carried by individual spikes is relatively low. We refer to both ofthese types ofinteractions as "graded." There is a large armamentarium of mathematical and dynamical systems tools for studying the computa- tions that such neurons perform. There is also a complementary connection between these tools and biological experimentation. The subject of the present paper is on networks in which averaging can not be done. The generation of spikes in these neurons is significantly affècted by the temporal order of spikes sent to them by other neurons. Two input spike trains, having the saine average spikes per unit time but different temporal spacing between the spikes, produce different outputs in target neurons; i.e., the amount of information carried by individual spikes is relatively high. We refer to these networks as "spike-activated." By comparison to graded networks, there is little formal or experimental work on the general principles underlying these networks. There are many nonlinear physiological processes in spike-activated networks that need to be considered. We have begun by focusing on a single nonlinearity analysis, the threshold transition between spiking and nonspiking behavior, and use linear perturbation to examine it. The fmdings indicate that there may be an epistemological distinction be- tween graded networks and spike-activated networks. This is reminiscent of the distinction between endophysics and exophysics whose resolutions requires an external observer hav- ing information about a system and its external universe (Rössler, 1989). Interestingly, the philosophical roots of our approach and the study of dynamics more generally may be traceable to Jacob Böhme (1575-1624), a mystic and contemporary of Descartes. Böhme influenced many philosophers and scientists, and may have provided Isaac Newton the metaphorical insight into his laws of physics (Mpitsos, 1995; Yates, 1972, 1979)