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Cerebral blood flow changes during retrieval of traumatic memories before and after psychotherapy: a SPECT study
- JULIO F. P. PERES, ANDREW B. NEWBERG, JULIANE P. MERCANTE, MANOEL SIMÃO, VIVIAN E. ALBUQUERQUE, MARIA J. P. PERES, ANTONIA G. NASELLO
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- Journal:
- Psychological Medicine / Volume 37 / Issue 10 / October 2007
- Published online by Cambridge University Press:
- 09 February 2007, pp. 1481-1491
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Background
Traumatic memory is a key symptom in psychological trauma victims and may remain vivid for several years. Psychotherapy has shown that neither the psychopathological signs of trauma nor the expression of traumatic memories are static over time. However, few studies have investigated the neural substrates of psychotherapy-related symptom changes.
MethodWe studied 16 subthreshold post-traumatic stress disorder (PTSD) subjects by using a script-driven symptom provocation paradigm adapted for single photon emission computed tomography (SPECT) that was read aloud during traumatic memory retrieval both before and after exposure-based and cognitive restructuring therapy. Their neural activity levels were compared with a control group comprising 11 waiting-list subthreshold PTSD patients, who were age- and profile-matched with the psychotherapy group.
ResultsSignificantly higher activity was observed in the parietal lobes, left hippocampus, thalamus and left prefrontal cortex during memory retrieval after psychotherapy. Positive correlations were found between activity changes in the left prefrontal cortex and left thalamus, and also between the left prefrontal cortex and left parietal lobe.
ConclusionsNeural mechanisms involved in subthreshold PTSD may share neural similarities with those underlying the fragmented and non-verbal nature of traumatic memories in full PTSD. Moreover, psychotherapy may influence the development of a narrative pattern overlaying the declarative memory neural substrates.
Nonlinear shear instabilities of alongshore currents on plane beaches
- J. S. Allen, P. A. Newberger, R. A. Holman
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- Journal:
- Journal of Fluid Mechanics / Volume 310 / 10 March 1996
- Published online by Cambridge University Press:
- 26 April 2006, pp. 181-213
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Evidence for the existence in the nearshore surf zone of energetic alongshore propagating waves with periods O(100 s) and wavelengths O(100 m) was found from observations by Oltman-Shay et al. (1989). These oscillations have wavelengths that are much too short to be surface gravity waves at the observed frequencies. The existence and properties of the wave-like motions were found to be related to the presence, strength and direction of an alongshore current in the surf zone. Based on a linear stability analysis of a mean alongshore current with offshore scale O(100 m), Bowen & Holman (1989) described these fluctuations as unstable waves associated with a shear instability. Good agreement of wavelengths and wave speeds from observations and from predictions based on the most unstable linear mode was obtained by Dodd et al. (1992). The nonlinear dynamics of finite-amplitude shear instabilities of alongshore currents in the surf zone are studied here utilizing numerical experiments involving finite-difference solutions to the shallow water equations for idealized forced dissipative initial-value problems. Plane beach (i.e. constant slope) geometry is used with periodic boundary conditions in the alongshore direction. Forcing effects from obliquely incident breaking surface waves are approximated by an across-shore-varying steady force in the alongshore momentum equation. Dissipative effects are modelled by linear bottom friction. The solutions depend on the dimen-sionless parameter Q, which is the ratio of an advective to a frictional time scale. The steady frictionally balanced, forced, alongshore current is linearly unstable for Q less than a critical value Qc. The response of the fluid is studied for different values of ΔQ = Qc - Q. In a set of experiments with the alongshore scale of the domain equal to the wavelength 2π/k0 of the most unstable linear mode, disturbances that propagate alongshore in the direction of the forced current with propagation velocities similar to the linear instability values are found for positive ΔQ. The disturbances equilibrate with constant amplitude for small ΔQ and with time-varying amplitudes for larger ΔQ. For increasing values of ΔQ the behaviour of this fluid system, as represented in a phase plane with area-averaged perturbation kinetic energy and area-averaged energy conversion as coordinates, is similar to that found in low-dimensional nonlinear dynamical systems including the existence of non-trivial steady solutions, bifurcation to a limit cycle, period-doubling bifurcations, and irregular chaotic oscillations. In experiments with the alongshore scale of the domain substantially larger than the wavelength of the most unstable linear mode, different behaviour is found. For small positive ΔQ, propagating disturbances grow at wavelength 2π/k0. If ΔQ is small enough, these waves equilibrate with constant or spatially varying amplitudes. For larger ΔQ, unstable waves of length 2π/k0 grow initially, but subsequently evolve into longer-wavelength nonlinear propagating steady or unsteady wave-like disturbances with behaviour dependent on ΔQ. The eventual development of large-scale nonlinear propagating disturbances appears to be a robust feature of the flow response over plane beach geometry for moderate, positive values of ΔQ and indicates the possible existence in the nearshore surf zone of propagating finite-amplitude shear waves with properties not directly related to results of linear theory.
Chaos in a model of forced quasi-geostrophic flow over topography: an application of Melnikov's method
- J. S. Allen, R. M. Samelson, P. A. Newberger
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- Journal:
- Journal of Fluid Mechanics / Volume 226 / May 1991
- Published online by Cambridge University Press:
- 26 April 2006, pp. 511-547
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We demonstrate the existence of a chaotic invariant set of solutions of an idealized model for wind-forced quasi-geostrophic flow over a continental margin with variable topography. The model (originally formulated to investigate mean flow generation by topographic wave drag) has bottom topography that slopes linearly offshore and varies sinusoidally alongshore. The alongshore topographic scales are taken to be short compared to the cross-shelf scale, allowing Hart's (1979) quasi-two-dimensional approximation, and the governing equations reduce to a non-autonomous system of three coupled nonlinear ordinary differential equations. For weak (constant plus time-periodic) forcing and weak friction, we apply a recent extension (Wiggins & Holmes 1987) of the method of Melnikov (1963) to test for the existence of transverse homoclinic orbits in the model. The inviscid unforced equations have two constants of motion, corresponding to energy E and enstrophy M, and reduce to a one-degree-of-freedom Hamiltonian system which, for a range of values of the constant G = E − M, has a pair of homoclinic orbits to a hyperbolic saddle point. Weak forcing and friction cause slow variations in G, but for a range of parameter values one saddle point is shown to persist as a hyperbolic periodic orbit and Melnikov's method may be applied to study the perturbations of the associated homoclinic orbits. In the absence of time-periodic forcing, the hyperbolic periodic orbit reduces to the unstable fixed point that occurs with steady forcing and friction. The method yields analytical expressions for the parameter values for which sets of chaotic solutions exist for sufficiently weak time-dependent forcing and friction. The predictions of the perturbation analysis are verified numerically with computations of Poincaré sections for solutions in the stable and unstable manifolds of the hyperbolic periodic orbit and with computations of solutions for general initial-value problems. In the presence of constant positive wind stress τ0 (equatorward on eastern ocean boundaries), chaotic solutions exist when the ratio of the oscillatory wind stress τ1 to the bottom friction parameter r is above a critical value that depends on τ0/r and the bottom topographic height. The analysis complements a previous study of this model (Samelson & Allen 1987), in which chaotic solutions were observed numerically for weak near-resonant forcing and weak friction.