Biomedical subjects

H Longbotham

Publications and source records attributed to H Longbotham.

3 recordsLinked to original sources

PubMed1993

Instantaneous VEP signal frequency analysis.

Various methods have been used in the analysis of visually evoked potential signals. Due to the high noise content of the ssVEP signal, the signal is usually filtered using averaging or an order statistic filter, and then analyzed for frequency content. A frequency analysis technique often employed is the short-time Fourier transform (STFT) which yields information about both the frequency content and the spatial localization simultaneously. In 1946, D. Gabor modified the STFT by using a Gaussian window instead of a Hanning, Hamming, etc. for optimal simultaneous location in time and frequency. The common problem with the windowed DFT/FFT is the ambiguity of the estimate of frequency at a point (frequency localization). This paper will introduce a frequency analysis technique utilizing the WMMR (weighted majority with minimum range) filter, which, while in the time domain, will determine the frequency and spatial properties of a ssVEP signal within one period. This technique has been shown to be robust to impulsive noise of up to 40% and also robust to independent and identically distributed (i.i.d.) noise and DC shifts, for frequency content and spatial localization analysis for sinusoidal signals. The WMMR technique will be compared to the Gabor STFT by application to steady state visual evoked potential data.

Animals↗

PubMed1993

Application of Gabor transform as texture discriminator of masses in digital mammograms.

It has been demonstrated that the class of the Gabor functions possesses the property of simultaneously minimizing uncertainty in spatial position and spatial frequency. A technique of detecting masses in digital mammogram has been investigated using Gabor Transform as a texture discriminator. The digital mammograms is "transformed" via Gabor function into a frequency spectrum representation that provides a means of discrimination between features base on the frequency differences. In addition, Gabor function are used as a model of the receptive fields of neurons in the visual cortex. The application of this technique is demonstrated using computer generated images and digital mammograms.

Breast Neoplasms↗

PubMed1993

Computer modeling of laser damage to the eye.

The effects of laser damage in the eye have been studied with an electrophysiological measure, the Visual Evoked Potentials (VEP). The VEP is generated by placing surface electrodes at the back of a subject's head, and having the subject watch a pattern of alternating light and dark bars. The spatial frequency of the bar pattern, measured in cycles per degree, increases as the bars become finer. The bar pattern changes in time, as the light bars become dark, and visa versa. The rate of this change is the temporal frequency, measured in Hertz. The VEP amplitudes referred to in this paper are a sum of the first three even harmonics of the temporal frequency (since the pattern change two times for every temporal cycle). As spatial frequency increases, a point will be reached where the subject will no longer be able to resolve the bars, and the VEP amplitude will become indistinguishable from noise. This spatial frequency is a measure of visual acuity. A computer model has been developed to simulate vision loss from lesions on the retina. It has been checked against experimental data from Rhesus monkeys with burns made under laboratory conditions. Vision was checked before and after the burns with VEP at a temporal frequency of six Hertz. This model divides the visual field into concentric rings centered about the visual axis. Each ring contributes to the VEP by an amount that depends on its eccentricity, or radius in degrees, as well as the spatial frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗