No abstract

Amorphous packings of nonspherical particles such as ellipsoids and spherocylinders are known to be hypostatic: The number of mechanical contacts between particles is smaller than the number of degrees of freedom, thus violating Maxwell’s mechanical stability criterion. In this work, we propose a general theory of hypostatic amorphous packings and the associated jamming transition. First, we show that many systems fall into a same universality class. As an example, we explicitly map ellipsoids into a system of “breathing” particles. We show by using a marginal stability argument that in both cases jammed packings are hypostatic and that the critical exponents related to the contact number and the vibrational density of states are the same. Furthermore, we introduce a generalized perceptron model which can be solved analytically by the replica method. The analytical solution predicts critical exponents in the same hypostatic jamming universality class. Our analysis further reveals that the force and gap distributions of hypostatic jamming do not show power-law behavior, in marked contrast to the isostatic jamming of spherical particles. Finally, we confirm our theoretical predictions by numerical simulations.

1. Post‐stimulus histograms were obtained from ‘sustained’ and ‘transient’ retinal ganglion cells for receptive field plots using a light spot with square‐wave modulation of intensity, and of variable intensity and area. Fundamental differences in their receptive field organization in time and space were revealed. 2. In ‘sustained’ cells, excitation consists of ‘transient’ and ‘sustained’ components and the ratio of transient/sustained components remains constant at a given retinal locus for a wide range of intensities. The transient component becomes proportionally larger towards the periphery of the receptive field. This rule is also applicable for the inhibitory and disinhibitory surround. In ‘transient’ cells, however, there is no true ‘sustained’ component, but some cells produce a double peaked transient post‐stimulus histogram at the R.F. centre when high flux stimuli are used, while others show a single peak transient response. The magnitude and shape of transient responses changes with intensity as well as with location in the receptive field. 3. The sensitivity gradients of ‘sustained’ and ‘transient’ cells show consistent differences in shape. The mean slope of the sensitivity gradients of a sample of ‘sustained’ cells was 10 times that of a sample of ‘transient’ cells. The sensitivity gradient of ‘sustained’ cells shows a distinct surround region where the inhibitory mechanism is more sensitive, while that of ‘transient’ cells usually does not, owing to an extensive ‘tail’ on the sensitivity gradient of the centre mechanism, which overlaps the surround. 4. Ricco's Law also holds for the centre mechanism of ‘transient’ cells. Non‐linear summation occurs at supra‐threshold levels, and when the surround mechanisms are involved. 5. Supra‐optimal stimuli give a saturation of the response in both ‘transient and ‘sustained’ cells. This saturation is associated with a decrease of latency in ‘transient’ cells, but not in ‘sustained’ cells. 6. The latency of retinal ganglion cells is determined by both stimulus and background flux. The effect of the background is negligible except at low values of stimulus flux, where its effect may be analysed primarily in terms of its effect on the incremental threshold. 7. The latency to stimulation with a standard small spot (25–27′) at the receptive field centre is shorter for ‘sustained’ cells than for ‘transient’ cells; this latency difference being related to the greater sensitivity of the ‘sustained’ cells to stimuli of this size. Differences in conduction time along ‘transient’ and ‘sustained’ pathways to the lateral geniculate nucleus (LGN) and cortex were estimated, and it is concluded that despite the latency difference noted above, a response to a stimulus which is optimal for a ‘transient’ cell reaches the cortex faster than the response to a stimulus which is optimal for a ‘sustained’ cell. 8. The above results together with previous evidence available suggest that for most stimuli, centre and surround mechanisms are activated simultaneously and algebraically su...

No abstract

We have carried out near-IR/optical observations to examine star formation toward a bright-rimmed cometary globule (BRC37) facing the exciting star(s) of an H ii region (IC1396) containing an IRAS source, which is considered to be an intermediate-mass protostar. With slitless spectroscopy we detected ten Hα emission stars around the globule, six of which are near the tip of the globule and are aligned along the direction to the exciting stars. There is evidence that this alignment was originally toward an O9.5 star, but has evolved to align toward a younger O6 star when that formed. Near-IR and optical photometry suggests that four of these six stars are low-mass young stellar objects (YSOs) with masses of ∼0.4 M . Their estimated ages of ∼1 Myr indicate that they were formed at the tip in advance of the formation of the IRAS source. Therefore, it is likely that sequential star formation has been taking place along the direction from the exciting stars toward the IRAS source, due to the UV impact of the exciting star(s). Interestingly, one faint, Hα emission star, which is the closest to the exciting star(s), seems to be a young brown dwarf that was formed by the UV impact in advance of the formation of other YSOs at the tip.

The swap Monte Carlo algorithm combines the translational motion with the exchange of particle species, and is unprecedentedly efficient for some models of glass former. In order to clarify the physics underlying this acceleration, we study the problem within the mean field replica liquid theory. We extend the Gaussian ansatz so as to take into account the exchange of particles of different species, and we calculate analytically the dynamical glass transition points corresponding to the swap and standard Monte Carlo algorithms. We show that the system evolved with the standard Monte Carlo algorithm exhibits the dynamical transition before that of the swap Monte Carlo algorithm. We also test the result by performing computer simulations of a binary mixture of the Mari-Kurchan model, both with standard and swap Monte Carlo. This scenario provides a possible explanation for the efficiency of the swap Monte Carlo algorithm. Finally, we discuss how the thermodynamic theory of the glass transition should be modified based on our results.

In order to identify the pretectal nucleus which contains pupillomotor cells in the rat, cells were sought which were sensitive to changes in luminance level at the eye. Two types were found: Luminance detectors which showed a graded increase in firing with increase in luminance, and darkness detectors which showed a graded increase in firing rate with graded dimming of luminance intensity. All luminance detectors were located in the olivary pretectal nucleus, whereas darkness detectors were located in the posterior pretectal nucleus. Consensual pupil responses were recorded in conscious normal and sympathectomised rats using an infra-red sensitive T.V. pupillometer. Pupil diameter varied 2mm in an approximately linear fashion over six log units range in luminance intensity. Sympathectomy produced a general constriction of the pupil, but the overall response to light was unaffected. The changes in pupil size occurred over the same range of luminance that the firing rates of both luminance and darkness detectors changed. The olivary pretectal nucleus may therefore be involved in pupilloconstruction in the light, and the posterior pretectal nucleus, with pupillodilation in the dark.

No abstract