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Biomedical subjects

M G Thomson

Publications and source records attributed to M G Thomson.

5 recordsLinked to original sources
PubMed2001

Sensory coding and the second spectra of natural signals.

Natural auditory and visual stimuli are shown to demonstrate consistent non-Gaussian signatures: their phase-only second spectra behave approximately as the inverse square root of frequency, a property which is due to a predominance of difference beats between Fourier components at slightly offset frequencies. These second spectra also demonstrate statistical scale invariance, a hypothesis which is tested explicitly by computing phase-only second spectra from bandpass-filtered data. The bandpass frequency selectivity observed experimentally in sensory cells is shown to be qualitatively consistent with a strategy of exploiting the non-Gaussian structure of natural signals.

Acoustic Stimulation↗
PubMed2001

Beats, kurtosis and visual coding.

Techniques adapted from standard higher-order statistical methods are applied to natural-image data in an attempt to discover exactly what makes 'wavelet' representations of natural scenes sparse. Specifically, this paper describes a measure known as the phase-only second spectrum, a fourth-order statistic which quantifies harmonic beat interactions in data, and uses it to show that there are statistical consistencies in the phase spectra of natural scenes. The orientation-averaged phase-only second spectra of natural images appear to show power-law behaviour rather like image power spectra, but with a spectral exponent of approximately -1 instead of -2. They also appear to display a similar form of scale-invariance. Further experimental results indicate that the form of these spectra can account for the observed sparseness of bandpass-filtered natural scenes. This implies an intimate relationship between the merits of sparse neural coding and the exploitation of non-Gaussian 'beats' structures by the visual system.

Algorithms↗
PubMed2000

Human sensitivity to phase perturbations in natural images: a statistical framework.

Fourier-phase information is important in determining the appearance of natural scenes, but the structure of natural-image phase spectra is highly complex and difficult to relate directly to human perceptual processes. This problem is addressed by extending previous investigations of human visual sensitivity to the randomisation and quantisation of Fourier phase in natural images. The salience of the image changes induced by these physical processes is shown to depend critically on the nature of the original phase spectrum of each image, and the processes of randomisation and quantisation are shown to be perceptually equivalent provided that they shift image phase components by the same average amount. These results are explained by assuming that the visual system is sensitive to those phase-domain image changes which also alter certain global higher-order image statistics. This assumption may be used to place constraints on the likely nature of cortical processing: mechanisms which correlate the outputs of a bank of relative-phase-sensitive units are found to be consistent with the patterns of sensitivity reported here.

Discrimination, Psychological↗
PubMed1999

Visual coding and the phase structure of natural scenes.

Although it is now well known that natural images display consistent statistical properties which distinguish them from random luminance distributions, this ecological approach to vision has so far concentrated on those second-order image statistics which are quantified by image power spectra, and it appears to be the image phase spectra which carry the majority of the image-intrinsic information. The present work describes how conventional nth-order statistics can be modified so that they are sensitive to image phase structure only. The modified measures are applied to an ensemble of natural images, and the results show that natural images do have consistent higher-order statistical properties which distinguish them from random-phase images with the same power spectra. An interpretation of this finding in terms of higher-order spectra suggests that these consistent properties arise from the ubiquity of edge structures in natural images, and raises the possibility that the properties of ideal relative-phase-sensitive mechanisms could be determined directly from analyses of the higher-order structure of natural scenes.

Environment↗