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T B Lawton

Publications and source records attributed to T B Lawton.

7 recordsLinked to original sources
PubMed1994

On the role of X and simple cells in human contrast processing.

We investigated the potential role of retinal X and cortical simple cells in determining human psychophysical detection performance under contrast masking conditions. Since both X and simple cells exhibit a null phase, the phase of a background mask should affect the visibility of a test grating processed by such cells. Sinusoidal test gratings of either 1 or 7 c/deg were presented as a sustained or transient increment against a background mask of the same size and spatial frequency at either 0 or 90 deg phase. For background contrasts from 0.5% up to 40%, psychophysical contrast sensitivity was phase-independent for all conditions. Therefore, either (1) contrast threshold is mediated by cells with non-linear spatial summation properties, such as Y or complex cells, or (2) the masking effect of the background occurs after a phase-insensitive combination or pooling of simple cell responses in the cortex.

Contrast Sensitivity↗
PubMed1992

Image enhancement filters significantly improve reading performance for low vision observers.

As people age, so do their photoreceptors; many photoreceptors in central vision stop functioning when a person reaches their late sixties or early seventies. Low vision observers with losses in central vision, those with age-related maculopathies, were studied. Low vision observers no longer see high spatial frequencies, being unable to resolve fine edge detail. We developed image enhancement filters to compensate for the low vision observer's losses in contrast sensitivity to intermediate and high spatial frequencies. The filters work by boosting the amplitude of the less visible intermediate spatial frequencies. The lower spatial frequencies. These image enhancement filters not only reduce the magnification needed for reading by up to 70%, but they also increase the observer's reading speed by 2-4 times. A summary of this research is presented.

Adult↗
PubMed1989

Improved reading performance using individualized compensation filters for observers with losses in central vision.

By boosting the amplitudes of the intermediate spatial frequencies more than the amplitude of the lower spatial frequencies, reading performance improved significantly when observers with losses in central vision read words that were filtered. Words that were filtered using an image enhancement function based on an observer's losses in visual function relative to a normal observer (1) reduced the magnification (30-70% less magnification was needed) and (2) increased the reading rate (2-3 times), measured in words per minute. The greater the loss in central visual function, the more individualized compensation filters reduced the magnification needed for word recognition. Individualized compensation filters improved the clarity and visibility of words for low vision observers. This study also found that the shape of the enhancement function was important to determine the optimum compensation filter for improving reading performance. In addition, the individualized compensation filters can be implemented by inexpensive hardware, for example in a closed circuit television (CCTV), to provide a significantly more effective low vision aid for observers with losses in central vision to read text, than is provided using only magnification.

Adult↗
PubMed1989

Outputs of paired Gabor filters summed across the background frame of reference predict the direction of movement.

A cortical neural network that computes the visibility of shifts in the direction of movement is proposed that computes: 1) the magnitude of the position difference between the test and background patterns, 2) localized contrast differences at different spatial scales analyzed by computing temporal gradients of the difference and sum of the outputs of paired even- and odd-symmetric bandpass filters convolved with the input pattern, and 3) using global processes that pool the output from paired even- and odd-symmetric simple and complex cells across the spatial extent of the background frame of reference to determine the direction a test pattern moved relative to a textured background. Evidence is presented that magnocellular pathways are used to discriminate the direction of movement. Since magnocellular pathways are used to discriminate the direction of movement, this task is not affected by small pattern changes such as jitter, short presentations, blurring, and different background contrasts that result when the veiling illumination in a scene changes.

Artificial Intelligence↗
PubMed1985

Spatial-frequency spectrum of patterns changes the visibility of spatial-phase differences.

This study showed that spatial-frequency components over a 4-octave range affected the visibility of spatial-phase differences. Contrast thresholds were measured for discrimination between two (+45- and -45-deg) spatial phases of a sinusoidal test grating added to a background grating. The background could contain one or several sinusoidal components, all in 0-deg phase. Phase differences between the test and the background were visible at lower constrasts (1) when test and background frequencies were harmonically related than when they were not, (2) when test and background frequencies were within 1 octave than when they were farther apart, (3) when the fundamental frequency of the background was low than when it was high, and (4) for some discriminations more than for others, after practice. The visibility of phase differences was not affected by additional components in the background if the fundamental and difference frequencies of the background remained unchanged. Observers' reports of their strategies gave information about the types of attentive processing that were used to discriminate phase differences. Attentive processing facilitated phase discrimination for multifrequency gratings spanning a much wider range of spatial frequencies than would be possible by using only local preattentive processing. These results were consistent with the visibility of phase differences being processed by some combination of even- and odd-symmetric simple cells tuned to a wide range of different spatial frequencies.

Humans↗
PubMed1984

The effect of phase structures on spatial phase discrimination.

This study was concerned with the discrimination of multifrequency gratings which differed only in the relative phase of one sinewave component (the test frequency) relative to the other components (the background frequencies). The dependent variable was the contrast of the test frequency required to discriminate between the two gratings. This study found that increasing the test frequency's relative phase difference from 10 to 90 deg significantly increased an observer's contrast sensitivity. However, no overall change in contrast thresholds was measured for test gratings having a relative phase difference between 90 to 180 deg. Moreover, when the mean relative phase was changed from 0 to 45 deg, contrast thresholds did not change when discriminating between gratings having a 90 deg relative phase difference. The type of processing most likely to account for these results in discussed.

Discrimination, Psychological↗