Neuronal coincidence detection by voltage-sensitive electrical synapses

Coincidence detection is important for functions as diverse as Hebbian learning, binaural localization, and visual attention. We show here that extremely precise coincidence detection is a natural consequence of the normal function of rectifying electrical synapses. Such synapses open to bidirectional current flow when presynaptic cells depolarize relative to their postsynaptic targets and remain open until well after completion of presynaptic spikes. When multiple input neurons fire simultaneously, the synaptic currents sum effectively and produce a large excitatory postsynaptic potential. However, when some inputs are delayed relative to the rest, their contributions are reduced because the early excitatory postsynaptic potential retards the opening of additional voltage-sensitive synapses, and the late synaptic currents are shunted by already opened junctions. These mechanisms account for the ability of the lateral giant neurons of crayfish to sum synchronous inputs, but not inputs separated by only 100 μsec. This coincidence detection enables crayfish to produce reflex escape responses only to very abrupt mechanical stimuli. In light of recent evidence that electrical synapses are common in the mammalian central nervous system, the mechanisms of coincidence detection described here may be widely used in many systems.

Implications of a possible clustering of highest-energy cosmic rays

Recently, a possible clustering of a subset of observed ultra-high energy cosmic rays above ≃40 EeV (4 × 1019 eV) in pairs near the supergalactic plane was reported. We show that a confirmation of this effect would provide information on the origin and nature of these events and, in case of charged primaries, imply interesting constraints on the extragalactic magnetic field. Possible implications for the most common models of ultra-high energy cosmic ray production in the literature are discussed.

Cosmic microwave background theory

A long-standing goal of theorists has been to constrain cosmological parameters that define the structure formation theory from cosmic microwave background (CMB) anisotropy experiments and large-scale structure (LSS) observations. The status and future promise of this enterprise is described. Current band-powers in ℓ-space are consistent with a ΔT flat in frequency and broadly follow inflation-based expectations. That the levels are ∼(10−5)2 provides strong support for the gravitational instability theory, while the Far Infrared Absolute Spectrophotometer (FIRAS) constraints on energy injection rule out cosmic explosions as a dominant source of LSS. Band-powers at ℓ ≳ 100 suggest that the universe could not have re-ionized too early. To get the LSS of Cosmic Background Explorer (COBE)-normalized fluctuations right provides encouraging support that the initial fluctuation spectrum was not far off the scale invariant form that inflation models prefer: e.g., for tilted Λ cold dark matter sequences of fixed 13-Gyr age (with the Hubble constant H0 marginalized), ns = 1.17 ± 0.3 for Differential Microwave Radiometer (DMR) only; 1.15 ± 0.08 for DMR plus the SK95 experiment; 1.00 ± 0.04 for DMR plus all smaller angle experiments; 1.00 ± 0.05 when LSS constraints are included as well. The CMB alone currently gives weak constraints on Λ and moderate constraints on Ωtot, but theoretical forecasts of future long duration balloon and satellite experiments are shown which predict percent-level accuracy among a large fraction of the 10+ parameters characterizing the cosmic structure formation theory, at least if it is an inflation variant.

Cosmic velocity fields and their interpretation

We review the current status of our knowledge of cosmic velocity fields, on both small and large scales. A new statistic is described that characterizes the incoherent, thermal component of the velocity field on scales less than 2h−1 Mpc (h is H0/100 km·s−1·Mpc−1, where H0 is the Hubble constant and 1 Mpc = 3.09 × 1022 m) and smaller. The derived velocity is found to be quite stable across different catalogs and is of remarkably low amplitude, consistent with an effective Ω ∼ 0.15 on this scale. We advocate the use of this statistic as a standard diagnostic of the small-scale kinetic energy of the galaxy distribution. The analysis of large-scale flows probes the velocity field on scales of 10–60 h−1 Mpc and should be adequately described by linear perturbation theory. Recent work has focused on the comparison of gravity or density fields derived from whole-sky redshift surveys of galaxies [e.g., the Infrared Astronomical Satellite (IRAS)] with velocity fields derived from a variety of sources. All the algorithms that directly compare the gravity and velocity fields suggest low values of the density parameter, while the POTENT analysis, using the same data but comparing the derived IRAS galaxy density field with the Mark-III derived matter density field, leads to much higher estimates of the inferred density. Since the IRAS and Mark-III fields are not fully consistent with each other, the present discrepancies might result from the very different weighting applied to the data in the competing methods.