Texture analysis describes a variety of image-analysis techniques that quantify the variation in surface intensity or patterns, including some that are imperceptible to the human visual system. Texture analysis may be particularly well-suited for lesion segmentation and characterization and for the longitudinal monitoring of disease or recovery. We begin this review by outlining the general procedure for performing texture analysis, identifying some potential pitfalls and strategies for avoiding them. We then provide an overview of some intriguing neuro-MR imaging applications of texture analysis, particularly in the characterization of brain tumors, prediction of seizures in epilepsy, and a host of applications to MS.

http://www.ajnr.org/content/ajnr/31/5/809.full.pdf

Texture analysis: a review of neurologic MR imaging applications.

In many networks of oscillatory neurons, synaptic interactions can promote the entrainment of units into phase-coupled groups. The detection of synchrony in experimental data, especially if the data consist of single-trial runs, can be problematic when, for example, phase entrainment is of short duration, buried in noise, or masked by amplitude fluctuations that are uncorrelated among the oscillating units. In the present study, we tackle the problem of detecting neural interactions from pairs of oscillatory signals in a narrow frequency band. To avoid the interference of amplitude fluctuations in the detection of synchrony, we extract a phase variable from the data and utilize statistical indices to measure phase locking. We use three different phase-locking indices based on coherence, entropy, and mutual information between the phase variables. Phase-locking indices are calculated over time using sliding analysis windows. By varying the duration of the analysis windows, we were able to inspect the data at different levels of temporal resolution and statistical reliability. The statistical significance of high index values was evaluated using four different surrogate data methods. We determined phase-locking indices using alternative methods for generating surrogate data and found that results are sensitive to the particular method selected. Surrogate methods that preserve the temporal structure of the individual phase time series decrease substantially the number of false positives when tested on a pair of independent signals.

https://www.physiology.org/doi/pdf/10.1152/jn.00853.2003

Statistical Method for Detection of Phase-Locking Episodes in Neural Oscillations

Receptive fields of simple cells in the cat visual cortex have recently been discussed in relation to the 'theory of communication' proposed by Gabor (1946). A number of investigators have suggested that the line-weighting functions, as measured orthogonal to the preferred orientation, may be best described as the product of a Gaussian envelope and a sinusoid (i.e. a Gabor function). Following Gabor's theory of 'basis' functions, it has also been suggested that simple cells can be categorized into even- and odd-symmetric categories. Based on the receptive field profiles of 46 simple cells recorded from cat visual cortex, our analysis provides a quantitative description of both the receptive-field envelope and the receptive-field 'symmetry' of each of the 46 cells. The results support the notion that, to a first approximation, Gabor functions with three free parameters (envelope width, carrier frequency and carrier phase) provide a good description of the receptive-field profiles. However, our analysis does not support the notion that simple cells generally fit into even- and odd-symmetric categories.

The structure and symmetry of simple-cell receptive-field profiles in the cat's visual cortex

This paper is a survey of research in discrete expansions over the last 10 years, mainly of functions in L 2 (R). The concept of an orthonormal basis {fn}, allowing every function f ∈ L 2 (R) to be written f = cnfn for suitable coefficients {cn}, is well understood. In separable Hilbert spaces, a generalization known as frames exists, which still allows such a representation. However, the coefficients {cn} are not necessarily unique. We discuss the relationship between frames and Riesz bases, a subject where several new results have been proved over the last 10 years. Another central topic is the study of frames with additional structure, most important Gabor frames (consisting of modulated and translated versions of a single function) and wavelets (translated and dilated versions of one function). Along the way, we discuss some possible directions for future research.

/pdf/frames-riesz-bases-and-discrete-gabor-wavelet-expansions-a1sedycdft.pdf

Frames, Riesz bases, and discrete Gabor/wavelet expansions

Unauthorized network access and spoofing attacks at wireless access points (WAPs) have been traditionally addressed using bit-centric security measures and remain a major information technology security concern. This has been recently addressed using RF fingerprinting methods within the physical layer to augment WAP security. This paper extends the RF fingerprinting knowledge base by: 1) identifying and removing less-relevant features through dimensional reduction analysis (DRA) and 2) providing a first look assessment of device identification (ID) verification that enables the detection of rogue devices attempting to gain network access by presenting false bit-level credentials of authorized devices. DRA benefits and rogue device rejection performance are demonstrated using discrete Gabor transform features extracted from experimentally collected orthogonal frequency division multiplexing-based wireless fidelity (WiFi) and worldwide interoperability for microwave access (WiMAX) signals. Relative to empirically selected full-dimensional feature sets, performance using DRA-reduced feature sets containing only 10% of the highest ranked features (90% reduction), includes: 1) maintaining desired device classification accuracy and 2) improving authorized device ID verification for both WiFi and WiMAX signals. Reliable burst-by-burst rogue device rejection of better than 93% is achieved for 72 unique spoofing attacks and improvement to 100% is demonstrated when an accurate sample of the overall device population is employed. DRA-reduced feature set efficiency is reflected in DRA models requiring only one-tenth the number of features and processing time.

Authorized and Rogue Device Discrimination Using Dimensionally Reduced RF-DNA Fingerprints

It is suggested that it may be possible to transmit speech and music in much narrower wavebands than was hitherto thought necessary, not by clipping the ends of the waveband, but by condensing the information. Two possibilities of more economical transmission are discussed. Both have in common that the original waveband is compressed in transmission and re-expanded to the original width in reception. In the first or "kinematical" method a temporary or permanent record is scanned by moving slits or their equivalents, which replace one another in continuous succession before a "window." Mathematical analysis is simplest if the transmission of the window is graded according to a probability function. A simple harmonic oscillation is reproduced as a group of spectral lines with frequencies which have an approximately constant ratio to the original frequency. The average departure from the law of proportional conversion is in inverse ratio to the time interval in which the record passes before the window. Experiments carried out with simple apparatus indicate that speech can be compressed into a frequency band of 800 or even 500 c/s without losing much of its intelligibility. There are various possibilities for utilizing frequency compression in telephony by means of the "kinematical" method. In a second method the compression and expansion are carried out electrically, without mechanical motion. This method consists essentially in using non-sinusoidal carriers, such as repeated probability pulses, and local oscillators producing waves of the same type. It is shown that one variety of the electrical method is mathematically equivalent to the kinematical method of frequency conversion.

D. Gabor

Papers

Theory of communication. Part 3: Frequency compression and expansion