Visual motion-evoked potentials in man

Visual motion-evoked potentials were recorded from the human scalp. The stimulus chosen for most detailed study was sudden reversal of the motion of a patterned field, on the hypothesis that this was likely to activate only mechanisms selectively sensitive to the direction of stimulus motion. A large proportion of the experiments were designed to test this hypothesis; and in fact they supported it. In addition to motion-reversal VEPs, VEPs to the onset and the offset of pattern motion, and to the appearance and disappearance of patterns were recorded and analysed. The relationships between these different types of VEP were investigated. Also, the dependence of the motion-onset, -offset end -reversal VEPs on certain stimulus parameters was studied. Are motion-reversal VEPs produced by direction-selective mechanisms? That direction-selective mechanisms were at least partly responsible for the motion-reversal VEPs was confirmed, since an adapting stimulus moving in the same direction as the motion before reversal produced an effect on the VEP different to that produced by en adapting stimulus moving in the opposite direction. Further investigation indicated that direction-selective mechanisms were probably the sole contributors to the motion-reversal VEPs, since control experiments failed to support any of the most likely alternative ways in which direction-insensitive mechanisms might theoretically have contributed to the motion-reversal VEPs. In particular, considerable attention was devoted to the possibility that mechanisms sensitive to contrast but insensitive to direction of motion might have been activated by a brief increase in the effective contrast of the stimulus pattern at the moment of reversal, and thereby have contributed to the VEP. Such an increase in the effective contrast could in theory have been caused by the brief slowing down which inevitably occurred at the moment of reversal, but several experiments refuted this interpretation. In particular, the VEPs were virtually independent of the time taken for reversal, but were very dependent on the velocity before and after reversal, reducing almost to zero at very high or very low velocities. A sudden step-displacement or change of the pattern at the moment of reversal suppressed the VEP. This effect was not caused by interference with the time-course of slow movement at reversal, since suppression occurred even when the step-displacement took place outside the period of slow movement. A psychophysical effect has been observed which may be connected with this phenomenon. Involuntary eye movements ere apparent~ not implicated in the production of the VEPs, since periodic and aperiodic stimulation yield similar results. Certain other ways in which VEP components might have arisen, even in the absence of eye movements or imperfections in the stimulus motion, have been investigated; but there has been no indication of the occurrence of such components. So the motion-reversal VEPs probably arose almost entirely from direction-selective mechanisms. Component analysis of VEPs The VEPs to the reversal and to the offset of motion apparently comprised three separate component peaks. In this respect they were similar to pattern-appearance VEPs, and the distribution over the scalp of any one of the components was the same for all three kinds of VEP (e.g. the first peak of the motion-reversal VEP had the some scalp-distribution as the first peak of the motion-offset VEP and the first peak of the pattern-appearance VEP). This implied that the corresponding components originated in the same cortical areas, and a correlation analysis of the amplitudes of the various components of motion-reversal VEPs and pattern-appearance VEPs for different subjects supported this conclusion. Now there is convincing evidence (Jeffreys, 1971) that the first component of pattern-appearance VEPs originates in striate cortex end the later components in extrastriate cortex. It is therefore concluded that the first peaks of motion-reversal and. motion-offset VEPs are likewise probably from striate cortex, and the later peaks from extrastriate. The VEP to motion-onset was very different from the above VEPs, however, and appeared to be more closely related to the pattern-disappearance VEP. It is possible that the same mechanisms underlie these two kinds of VEP. Although motion-reversal VEPs appear to be the product of direction-selective mechanisms alone, it is far from certain that this is true of motion-onset and -offset VEPs. Nevertheless, there is evidence that the latter kinds of VEP may share generating mechanisms with the former; since the motion-reversal VEP was, under many conditions though not all, a good approximation to the sum of the motion-onset and -offset VEPs recorded under similar stimulus conditions. The effects of varying stimulus parameters Motion-reversal VEPs were found to be largely independent of brightness except at the lowest levels, but the latency did tend to increase slightly as the brightness was reduced. Despite the discovery (MaCKay & Rietveld, 1968) that the proximity of a stationary reference line enhances the VEP to the onset of motion of a stimulus line,' it appears that the sharp contours comprising the edge of the visual field did not influence the VEPs to the onset, offset or reversal of pattern motion, since replacing the sharp contours by blurred ones did not affect the VEPs. The onset, offset and reversal VEPs did not depend greatly on the direction of motion. Superimposing a steady motion did, however, markedly modify the VEPs. The effects of using patterns other than visual noise were investigated. Checkerboards and visual noise produced similar results, but line rasters produced very different VEPs

Deconfined Fermions but Confined Coherence?

The cuprate superconductors and certain organic conductors exhibit transport which is qualitatively anisotropic, yet at the same time other properties of these materials strongly suggest the existence of a Fermi surface and low energy excitations with substantial free electron character. The former of these features is very difficult to account for if the material possesses three dimensional coherence, while the latter is inconsistent with a description based on a two dimensional fixed point. We therefore present a new proposal for these materials in which they are categorized by a fixed point at which transport in one direction is not renormalization group irrelevant, but is intrinsically incoherent, i.e. the incoherence is present in a pure system, at zero temperature. The defining property of such a state is that single electron coherence is confined to lower dimensional subspaces (planes or chains) so that it is impossible to observe interference effects between histories which involve electrons moving between these subspaces.Comment: 31 pages, REVTEX, 3 eps figures, epsf.tex macr

Interchain coherence of coupled Luttinger liquids at all orders in perturbation theory

We analyze the problem of Luttinger liquids coupled via a single-particle hopping \tp and introduce a systematic diagrammatic expansion in powers of \tp. An analysis of the scaling of the diagrams at each order allows us to determine the power-law behavior versus \tp of the interchain hopping and of the Fermi surface warp. In particular, for strong interactions, we find that the exponents are dominated by higher-order diagrams producing an enhanced coherence and a failure of linear-response theory. Our results are valid at any finite order in \tp for the self-energy.Comment: 4 pages, 3 ps figures. Accepted for publication in Phys. Rev. Let

Wrinkling of a bilayer membrane

The buckling of elastic bodies is a common phenomenon in the mechanics of solids. Wrinkling of membranes can often be interpreted as buckling under constraints that prohibit large amplitude deformation. We present a combination of analytic calculations, experiments, and simulations to understand wrinkling patterns generated in a bilayer membrane. The model membrane is composed of a flexible spherical shell that is under tension and that is circumscribed by a stiff, essentially incompressible strip with bending modulus B. When the tension is reduced sufficiently to a value \sigma, the strip forms wrinkles with a uniform wavelength found theoretically and experimentally to be \lambda = 2\pi(B/\sigma)^{1/3}. Defects in this pattern appear for rapid changes in tension. Comparison between experiment and simulation further shows that, with larger reduction of tension, a second generation of wrinkles with longer wavelength appears only when B is sufficiently small.Comment: 9 pages, 5 color figure

The FRII Broad Line Seyfert 1 Galaxy: PKSJ 1037-2705

In this article, we demonstrate that PKSJ 1037-2705 has a weak accretion flow luminosity, well below the Seyfert1/QSO dividing line, weak broad emission lines (BELs) and moderately powerful FRII extended radio emission. It is one of the few documented examples of a broad-line object in which the time averaged jet kinetic luminosity, $\bar{Q}$, is larger than the total thermal luminosity (IR to X-ray) of the accretion flow, $L_{bol}$. The blazar nucleus dominates the optical and near ultraviolet emission and is a strong source of hard X-rays. The strong blazar emission indicates that the relativistic radio jet is presently active. The implication is that even weakly accreting AGN can create powerful jets. Kinetically dominated ($\bar{Q}>L_{bol}$) broad-line objects provide important constraints on the relationship between the accretion flow and the jet production mechanism.Comment: To appear in ApJ November 1, 2008, v687n1 issu

Inhomogeneous Dust Collapse in 5D Einstein-Gauss-Bonnet Gravity

We consider a Lemaitre - Tolman - Bondi type space-time in Einstein gravity with the Gauss-Bonnet combination of quadratic curvature terms, and present exact solution in closed form. It turns out that the presence of the coupling constant of the Gauss-Bonnet terms alpha > 0 completely changes the causal structure of the singularities from the analogous general relativistic case. The gravitational collapse of inhomogeneous dust in the five-dimensional Gauss-Bonnet extended Einstein equations leads to formation of a massive, but weak, timelike singularity which is forbidden in general relativity. Interestingly, this is a counterexample to three conjecture viz. cosmic censorship conjecture, hoop conjecture and Seifert's conjecture.Comment: 8 Latex Pages, 2 EPS figure

Temperature dependence of the interlayer magnetoresistance of quasi-one-dimensional Fermi liquids at the magic angles

The interlayer magnetoresistance of a quasi-one-dimensional Fermi liquid is considered for the case of a magnetic field that is rotated within the plane perpendicular to the most-conducting direction. Within semi-classical transport theory dips in the magnetoresistance occur at integer amgic angles only when the electronic dispersion parallel to the chains is nonlinear. If the field direction is fixed at one of the magic angles and the temperature is varied the resulting variation of the scattering rate can lead to a non-monotonic variation of the interlayer magnetoresistance with temperature. Although the model considered here gives a good description of some of the properties of the Bechgaard salts, (TMTSF)2PF6 for pressures less than 8kbar and (TMTSF)2ClO4 it gives a poor description of their properties when the field is parallel to the layers and of the intralayer transport.Comment: 10pages, RevTeX + epsf, 3 figure

Valuing labour

The British construction labour process rests on casual, self-employment, output-based pay, rigid trade divisions, low levels of training and a sharp divide between operative and professional/technical skills. Skill shortages beset the industry and their solution focuses not on employment regulation and a comprehensive industry-wide training scheme but on importing the necessary skilled labour. The paper shows how qualitatively differently construction labour is valued in Britain compared with other leading European countries. These rely on higher skill levels, based on knowledge gained through the training process and on a more stable and collectively negotiated structure of training provision and employment. In Britain, in contrast, labour is not valued according to the knowledge it incorporate but according to an individual's ability to fulfil the task in hand, Training is geared to meeting individual employers' immediate needs, qualifications are not a prerequisite for entry, and labour is rewarded for its product not for its potential. The paper pinpoints the key features if the British system that give rise to concern and concludes by outlining the ways in which the British system needs to change for any sustainable development of the construction process

Physical nature of the central singularity in spherical collapse

We examine here the nature of the central singularity forming in the spherically symmetric collapse of a dust cloud and it is shown that this is always a strong curvature singularity where gravitational tidal forces diverge powerfully. An important consequence is that the nature of the naked singularity forming in the dust collapse turns out to be stable against the perturbations in the initial data from which the collapse commences.Comment: Latex file, 11 pages, 2 figures, Updated version to match the published version in PR