Biomedical subjects

M I Hofmann

Publications and source records attributed to M I Hofmann.

5 recordsLinked to original sources

PubMed1993

Preattentive discrimination of relative phase modelled by interacting Gabor or by difference-of-Gaussian filters.

A computer simulation of the human preattentive visual pathway used Gabor and difference-of-Gaussian (DOG) filters to model two-dimensional relative phase discrimination. There is a hierarchy of hexagonally packed levels: (i) an image layer; (ii) an intermediate level of on- and off-centre DOG filters; (iii) a partial set of broadband oriented Gabor-like filters with high-level DOG filters in parallel. Connections between layers use half-wave rectification and a compressive nonlinearity. Local feedback interactions between oriented Gabor filters, together with spatial averaging, allow the model to discriminate both two-dimensional relative phase and orientation differences. There were two separate simulations for the Gabor filters, one with even-symmetric filters and another with odd-symmetric. Textures were formed by superposing three high contrast sine-wave gratings with successive rotations of 60 degrees. Relative phase and global orientation were the varied parameters. Psychophysical rating data for peripheral viewing of texture pairs resemble the results from the even-symmetric simulation, showing discrimination of relative phase and orientation. In contrast, the DOG filters in the model simulate only the relative phase aspects of the data.

Discrimination, Psychological↗

PubMed1993

Texture segregation based on two-dimensional relative phase differences in composite sine-wave grating patterns.

Experiments examined the visual processing of relative phase relations between differently oriented components of a textured pattern. The textures were a super position of three 1.3 c/deg sine-wave gratings rotated 60 deg relative to one another. Global orientation and relative phase were varied. Subjects rated the segregation of pairs of textures presented in a figure/ground configuration at 10 deg retinal eccentricity. Both orientation and phase differences were used in making ratings. It was not simply the difference in relative phase that mattered. The results also depended on the values of the relative two-dimensional (2-D) phases in figure and ground. Mirror-image texture pairs (which may have a large relative phase difference) segregated poorly compared to other pairs with similar phase differences. This suggests that the peripheral visual system does not completely encode 2-D relative phase. Secondary issues include spatial frequency effects, the relevance of figure/ground borders, and better performance in the lower visual field.

Contrast Sensitivity↗

PubMed1991

Segregation of some mesh-derived textures evaluated by free viewing.

Beginning with a hexagonal mesh, a family of equiluminous geometric textures can be formed by rotating each line segment by a fixed amount. Such textures vary both in line orientation and in the sizes of the intervening spaces or "holes". In the Fourier domain the textures have the same discrete two-dimensional frequency components but the amplitudes vary. The segregation of texture pairs as figures in grounds has been assessed for free viewing by the ratings of 10 subjects. Line segment orientation information alone cannot explain the results. Linear regression techniques were used to examine the correlation between the subjects' ratings and physical differences between figures and grounds. Significant correlations were found with (a) the amplitudes of the low-frequency Fourier harmonics of the textures and (b) maximum hole size, but not with the third harmonics or line segment orientation. Patterns composed of mirror-image pairs are very poorly discriminated; in these cases line orientation information may be used.

Fourier Analysis↗

PubMed1990

Rate-determining steps in penicillopepsin-catalysed reactions.

The hydrolysis of Ac-(Ala)2-Lys-Nph-(Ala)2-amide (II) by penicillopepsin is characterized by a solvent isotope effect of 2.11, whereas the hydrolysis of Ac-Lys-Nph-amide (I) shows no solvent isotope effect. The dependence of the isotope effect on the concentration of D2O in H2O for substrate II is not linear and suggests that two or more protons are involved in its rate-determining step. We propose that for substrate I the rate-determining step is the distortion of the scissile bond towards a tetrahedral configuration, and for substrate II a conformational change induced by the occupation of the S3 pocket in the enzyme.

Amino Acid Sequence↗

PubMed1990

Detection of briefly flashed sine-gratings in dark-adapted vision.

Scotopic contrast sensitivity was measured near 20 deg retinal eccentricity for briefly flashed (10 or 20 msec) sine-wave gratings presented in darkness to dark-adapted subjects. For very low spatial frequencies (0.2-0.5 c/deg), curves of contrast sensitivity vs luminous energy show evidence of a low rod plateau and a high scotopic region, with an intervening transition at around -2 to -2.5 log scot td sec. Similar measurements made using long flashed or flickering gratings do not show a plateau. The results suggest that vision in the low rod region is impaired for brief flashes. For the briefly flashed stimuli, curves of contrast sensitivity versus spatial frequency in the low region were best fit by simple Gaussian functions with a variable centre size (sigma c = 0.5----0.25 deg), size decreasing with increasing flash energy. Difference-of-Gaussian functions with constant centre size (sigma c = 0.25 deg) provided the best fit in the high region. Overt input from the cones and grating area artefacts are excluded by appropriate tests. Calculation of photon flux into the receptive field centres suggests that signal compression in P alpha ganglion cells contributes to the low rod plateau.

Contrast Sensitivity↗