Steven B. Lowen

Efficent Generation of Fractional Brownian Motion for Simulation of Infrared Focal-plane Array Calibration Drift

Infrared focal-plane arrays suffer from a type of 1/f noise which leads to slow drifts in detected radiation levels following calibration. This noise can be modeled by fractional Brownian motion (FBM), with an empirical Hurst parameter (H) in the range 0 < H < 1 / 2. For such noise we examine the statistics of both the maximum deviation from the calibration point during a fixed time, and the time to reach a fixed deviation from the calibration point. We employ analytical and numerical means; for the latter, we provide a new algorithm for generating a discrete-time version of FBM with 0 < H ≤ 1 / 2 which is fast (order N log N), and exact. Statistics of the maximum deviation show the same qualitative behavior for different values of H, and rapidly approach a limit as the length N increases. Results for first passage times, in contrast, vary markedly with H, but not with N.

Intrinsic oscillations in spike trains indicate non-renewal statistics due to convergence of inputs in dorsal cochlear nucleus neurons

The occurrence of intrinsic oscillations (IOs) in a units discharge is reflected by a prominent peak in the power spectrum (i.e., Fourier transform of the autocorrelation function) of spike trains obtained from single-unit discharge, at a frequency independent of stimulus spectral characteristics. IOs have been reported by researchers in the dorsal cochlear nucleus (DCN) of both the cat and the Mongolian gerbil. It has been hypothesized that IOs are related to inter-spike interval (ISI) regularity (e.g., [Hear. Res. 58 (1992) 153]). This hypothesis is tested in this paper. Responses to multiple presentations of 50-300 ms duration tone bursts, at and near the units best frequency (BF) at 20-60 dB re threshold were recorded from DCN units of barbiturate-anesthetized (30 units), as well as decerebrate (53 units) Mongolian gerbils. IOs in the recordings were then compared with the IOs in simulations of spiking-neuron models. The models were selected because: (1) their ISI regularity characteristics follow those of experimental data and (2) their IO properties are completely determined by their ISI regularity. Such comparison reveals that Ghoshals hypothesis fails for a fraction of the units. These results suggest a re-evaluation of the purported relationship between IOs, ISI regularity, and SAM response. Alternate hypotheses are proposed here using computational models that are based on convergence of multiple neural inputs onto the unit under study. These models produce non-renewal statistics that resemble those of the experimental data, as is evident from IO-based analysis.

Effects of citalopram and escitalopram on fMRI response to affective stimuli in healthy volunteers selected by serotonin transporter genotype

This study was designed to assess whether functional magnetic resonance imaging (fMRI) following antidepressant administration (pharmaco-fMRI) is sufficiently sensitive to detect differences in patterns of activation between enantiomers of the same compound. Healthy adult males (n=11) participated in a randomized, double-blind, cross-over trial with three medication periods during which they received citalopram (racemic mixture), escitalopram (S-citalopram alone), or placebo for 2 weeks. All participants had high expression serotonin transporter genotypes. An fMRI scan that included passive viewing of overt and covert affective faces and affective words was performed after each medication period. Activation in response to overt faces was greater following escitalopram than following citalopram in the right insula, thalamus, and putamen when the faces were compared with a fixation stimulus. For the rapid covert presentation, a greater response was observed in the left middle temporal gyrus in the happy versus fearful contrast following escitalopram than following citalopram. Thus, the combination of genomics and fMRI was successful in discriminating between two very similar drugs. However, the pattern of activation observed suggests that further studies are indicated to understand how to optimally combine the two techniques.

Bupropion Sustained released versus Placebo for seasonal affective Disorder

Background: The majority of seasonal affective disorder (SAD) studies have evaluated the use of light or selective serotonin reuptake inhibitors (SSRI). The purpose of the present study was to evaluate bupropion sustained-released (SR), a non-SSRI antidepressant, for the treatment of SAD.