Frequency Slices Research Articles

Modeling inhibition of type II units in the dorsal cochlear nucleus.

Type II units in the dorsal cochlear nucleus (DCN) are characterized by vigorous but nonmonotonic responses to best frequency tones as a function of sound pressure level, and relatively weak responses to noise. A model of DCN neural circuitry was used to explore two hypothetical mechanisms by which neurons may be endowed with type II unit response properties. Both mechanisms assume that type II units receive excitatory input from auditory nerve (AN) fibers and inhibitory input from an unspecified class of cochlear nucleus interneurons that also receive excitatory AN input. The first mechanism, a lateral inhibition (LI) model, supposes that type II units receive inhibitory input from a number of narrowly tuned interneurons whose best frequencies (BFs) flank the BF of the type II unit. Tonal stimuli near BF result in only weak inhibitory input, but broadband stimuli recruit enough lateral inhibitors to greatly weaken the type II unit response. The second mechanism, a wideband inhibition (WBI) model, supposes that type II units receive inhibitory input from interneurons that are broadly tuned so that they respond more vigorously to broadband stimuli than to tones. Physiological and anatomical evidence points to the possible existence of such a class of neurons in the cochlear nucleus. The model extends an earlier computer model of an iso-frequency DCN patch to multiple frequency slices and adds a population of interneurons to provide the inhibition to model type II units (called 12-cells). The results show that both mechanisms accurately simulate responses of type II units to tones and noise. An experimental paradigm for distinguishing the two mechanisms is proposed.

A two‐pass approximation to 3-D prestack migration

A two‐pass approximation to 3-D Kirchhoff migration simplifies the migration procedure by reducing it to a succession of 2-D operations. This approach has proven very successful in the zero‐offset case. A two‐pass approximation to 3-D migration is described here for the prestack case. Compared to the one‐pass approach, the scheme presented here provides significant reduction in computation time and a relatively simple data manipulation scheme. The two‐pass method was designed using velocity independent prestack time migration (DMO‐PSI) applied in the crossline direction, followed by conventional prestack depth migration in the inline direction. Velocity analysis, an important part of prestack migration, is also included in the two‐pass scheme. It is carried out as a 2-D procedure after 3-D effects are removed from the data volume. The procedure presented here is a practical full volume 3-D prestack migration. One of its main benefits is a realistic and efficient iterative velocity analysis procedure in three dimensions. The algorithm was designed in the frequency domain and the computational scheme was optimized by processing individual frequency slices independently. Irregular trace distribution, a feature that characterizes most 3-D seismic surveys, is implicitly accounted for within the two‐pass algorithm. A numerical example tests the performance of the two‐pass 3-D prestack migration program in the presence of a vertical velocity gradient. A 3-D land survey from a fold and thrust belt region was used to demonstrate the algorithm in a complex geological setting. The results were compared with images from other 2-D and 3-D migration schemes and show improved resolution and higher signal content.

Identification of mixed-phase systems from frequency slices of the bispectrum

The identification of the parameters of a linear, time-invariant system is an important problem in signal processing and control. Power spectral methods cannot be used in the identification of mixed-phase systems, or when the output is corrupted by Gaussian noise of unknown power spectral density. Bispectral methods are effective in those cases. In this paper we present a new procedure to identify mixed-phase systems using frequency slices of the bispectral phase to obtain the group delay of the system from which we then find the system parameters. For mixed-phase moving average (MA) or minimum-phase autoregressive moving average (ARMA) systems, we show that a single slice of the bispectral phase is sufficient to identify the parameters and the order of the system, while two slices of the bispectrum are needed for mixed-phase ARMA systems. The basis of our procedure is to obtain an equivalent minimum-phase system which has the same phase as the original one. Using the modified least squares (MLS) rational approximation for the equivalent system we find the parameters of the original system. In the mixed-phase ARMA case, additional separation of the minimum- and the maximum-phase components is necessary. Examples illustrating our procedure are given.