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Biomedical subjects

C W Tyler

Publications and source records attributed to C W Tyler.

At least 19 recordsLinked to original sources
PubMed2003

A survey of filamentous organisms at the Deer Island treatment plant.

The Deer Island Treatment Plant (DITP) treating both domestic and industrial wastewater, has a peak flow capacity of up to 1270 million gallons per day (MGD) (3342 m3 min(-1). The DITP contains a Pilot Plant, which consists of two identical pure oxygen activated sludge treatment trains, each with a maximum capacity of 1 MGD (2.63 m3 min(-1)) to simulate the maximum flow of the full size facility. This study documents the community of filamentous organisms living in the activatedsludge under various operational conditions in the Pilot Plant. Sixteen types of filamentous organisms were identified. The major filamentous organisms present in the Pilot Plant were Type 1701, Type 0041, Sphaerotilus natans, Microthrix parvicella, and Nocardia sp. Nocardia sp. was quite dominant in the early stages of the experimental period during high ambient temperatures (summer), indicating that temperature is one determining factor in the distribution of Nocardia sp. Thiothrix sp. was very sensitive to the dissolved oxygen (DO) level, with low DO values favoring the growth of the organisms. Microbiological observations made during the start-up and stable operation of the full-scale secondary treatment plant (maximum capacity 780 MGD) are also reported. No dominant filamentous species existed during the start-up period. Type 0803 and Type 1701, indicators of low oxygen level for the applied food to microorganism ratio, confirmed the low DO conditions of activated sludge tanks.

Data Collection↗
PubMed2003

Modular organization of adaptive colouration in flounder and cuttlefish revealed by independent component analysis.

Flounders and cuttlefish have an impressive ability to change colouration, for camouflage and, in the case of cuttlefish, for communication. We pursue the hypothesis that these diverse patterns are created by combining a small number of distinct pattern modules. Independent component analysis (ICA) is a powerful tool for identifying independent sources of variation in linear mixtures of signals. Two versions of ICA are used, one assuming that sources have independence over time, and the other over space. These reveal the modularity of the skin colouration system, and suggest how the pattern modules are combined in specific behavioural contexts. ICA may therefore be a useful tool for studying animal camouflage and communication.

Adaptation, Physiological↗
PubMed2001

Lateral sensitivity modulation explains the flanker effect in contrast discrimination.

We used a dual-masking paradigm to study how contrast discrimination can be influenced by the presence of adjacent stimuli. The task of the observer was to detect a target superimposed on a pedestal in the presence of flankers. The flankers (i) reduce the target threshold at zero pedestal contrast, (ii) shift the target threshold versus pedestal contrast (TvC) function horizontally to the left on a log-log plot at high pedestal contrasts, and (iii) reduce the size of pedestal facilitation at low pedestal contrasts. The horizontal shift at high pedestal contrasts suggests that the flanker effect is a multiplicative factor that cannot be explained by previous models of contrast discrimination. We extend the divisive inhibition model of contrast discrimination by implementing the flanker effect as a lateral multiplicative sensitivity modulation. This extended model provides a good account of the data.

Adult↗
PubMed2001

Stereoprocessing of cyclopean depth images: horizontally elongated summation fields.

The study evaluated how the detection efficiency varies with the length and width of Gabor wavelets of depth ripple in random element patterns. For local (one-cycle) wavelets, there was no anisotropy with respect to wavelet orientation, implying equal efficiency for processing shear versus compression disparities at threshold. The processing for larger patterns of depth ripple did not correspond to a fixed summation field but varied in size with spatial frequency, and in shape with orientation of depth ripples, up to four cycles of a horizontal bar at 0.5 cy/deg, or 8 degrees of visual angle. The presence of such extended summation fields in only one orientation is incompatible with the idea of local attentional processing, with a single disparity channel or with an adaptive mechanism that could accommodate to any form of disparity image. Thus, these results suggest the presence of a multichannel hypercyclopean level of processing specialized for horizontal depth contours, whose only disparity information is in their surface texture.

Attention↗
PubMed2001

Occlusions contribute to scaling in natural images.

Spatial power spectra from natural images fall approximately as the square of spatial frequency, a property also called scale invariance (scaling). Various theories for visual receptive fields consider scale invariance key. Two hypotheses have been advanced in the literature for why natural images obey scale invariance. The first is that these images have luminance edges, whose spectra fall as frequency squared. The second is that scale invariance follows from natural images being essentially a collage of independent, constant-intensity regions, whose sizes follow a power-law distribution. Recently, an argument by example was made against the first hypothesis. Here we refute that argument and show that the first hypothesis is consistent with the scaling under a wide variety of distributions of sizes. There are two reasons for this: first, for every frequency, the log-log slope of the rotationally averaged power spectrum of an image is the weighted mean of the log-log slopes from the independent regions of the image formed by objects occluding one another. Second, the log-log slopes of the spectrum envelope for a constant-intensity region are 0 and -3 for frequencies corresponding to periods much larger and much smaller than the region's size, respectively. Therefore, it is not surprising that natural images have log-log slopes between -1.5 and -3, with a mean near -2.

Depth Perception↗
PubMed2001

The symmetry magnification function varies with detection task.

Detection of the presence of bilateral symmetry was investigated at various retinal eccentricities for static and dynamic noise reflected around a vertical axis. At a low detection criterion (60% correct), peak duration sensitivities were high and varied little (<0.2 log units) from 0 masculine eccentricity to 10 masculine eccentricity for either static or dynamic targets. Duration thresholds for symmetry in dynamic noise fields were significantly higher (about 100 ms) than those for static symmetry detection (about 40 ms), despite the fact that the information was refreshed many times during the threshold presentation period. The spatial summation width for symmetry processing was evaluated with randomization around the axis of symmetry. The estimated summation width for static symmetry detection was approximately constant with eccentricity for short duration stimuli. For long duration stimuli, the summation width was substantially greater in central vision but decreased with eccentricity, the first known visual function to exhibit such reverse magnification behavior (Tyler, 1999). These findings suggest that static and dynamic symmetry detection are supported by different neural mechanisms and that these mechanisms are relatively invariant across the retina, unlike known mechanisms of spatial processing.

Attention↗
PubMed2000

Signal detection theory in the 2AFC paradigm: attention, channel uncertainty and probability summation.

Neural implementation of classical High-Threshold Theory reveals fundamental flaws in its applicability to realistic neural systems and to the two-alternative forced-choice (2AFC) paradigm. For 2AFC, Signal Detection Theory provides a basis for accurate analysis of the observer's attentional strategy and effective degree of probability summation over attended neural channels. The resulting theory provides substantially different predictions from those of previous approximation analyses. In additive noise, attentional probability summation depends on the attentional model assumed. (1) For an ideal attentional strategy in additive noise, summation proceeds at a diminishing rate from an initial level of fourth-root summation for the first few channels. The maximum improvement asymptotes to about a factor of 4 by a million channels. (2) For a fixed attention field in additive noise, detection is highly inefficient at first and approximates fourth-root summation through the summation range. (3) In physiologically plausible root-multiplicative noise, on the other hand, attentional probability summation mimics a linear improvement in sensitivity up to about ten channels, approaching a factor of 1000 by a million channels. (4) Some noise sources, such as noise from eye movements, are fully multiplicative and would prevent threshold determination within their range of effectiveness. Such results may require reappraisal of previous interpretations of detection behavior in the 2AFC paradigm.

Attention↗
PubMed2000

Relative contributions of sustained and transient pathways to human stereoprocessing.

It has been proposed [Hubel & Livingstone (1987) Journal of Neuroscience, 7, 3378-3415] that stereopsis is mediated solely by magnocellular pathway in primates. This hypothesis was evaluated for humans in psychophysical experiments with dynamic random-noise stimuli, based on the sustained/transient relationship of behavior mediated by the two divisions of the LGN [Merigan & Maunsell (1993) Annual Review of Neuroscience, 16, 369-402]. The stereoscopic limits show that stereoscopic system is more sensitive to sustained random-dot stimuli than to transient ones. Quantitative modeling of the result implied a weak role for magnocellular input, suggests that human stereopsis is more strongly influenced by parvocellular input through the LGN.

Adolescent↗
PubMed1999

What pattern the eye sees best.

The visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to physiological summation within the extent of the largest spatial filter and due to probability summation between the outputs of linear, independent filters beyond that range. It is generally assumed that this improvement is isotropic to the patch configuration. In contrast, the existence of long-range facilitation that is configuration-specific suggests that the visibility of a local contrast is dependent on the spatial configuration of the stimuli. We measured contrast thresholds for circular and elongated Gabor patches with a static carrier. The patch envelope orientation was either the same as the bar orientation (collinear) or orthogonal to it. Contrast sensitivity was highest for elongated configurations that were collinear with the grating bars, and reached maximal efficiency at a length of about four grating cycles (eight bar widths), but a width of only one cycle.

Contrast Sensitivity↗
PubMed1999

Bayesian adaptive estimation of psychometric slope and threshold.

We introduce a new Bayesian adaptive method for acquisition of both threshold and slope of the psychometric function. The method updates posterior probabilities in the two-dimensional parameter space of psychometric functions and makes predictions based on the expected mean threshold and slope values. On each trial it sets the stimulus intensity that maximizes the expected information to be gained by completion of that trial. The method was evaluated in computer simulations and in a psychophysical experiment using the two-alternative forced-choice (2AFC) paradigm. Threshold estimation within 2 dB (23%) precision requires less than 30 trials for a typical 2AFC detection task. To get the slope estimate with the same precision takes about 300 trials.

Bayes Theorem↗
PubMed1999

Nonlinearities of near-threshold contrast transduction.

The existence of analytic threshold nonlinearities was probed with 2AFC incremental threshold functions for both local and extended test patterns on stationary matched pedestals of the same and opposite sign. In contrast to the facilitation effect with same-sign pedestals, sensitivity with opposite-sign pedestals first deteriorated up to the mask detection level, abruptly improved and then deteriorated again. Analytic solutions for the transducer function with additive noise were derived to account for the incremental data in all conditions. The results for positive difference-of-Gaussian (DoG) stimuli (whose increment made the central spot lighter) and for 10 c deg-1 Gabor stimuli were consistent with accurate hard-threshold behavior with best-fitting d' powers from 17 to 358. The 10 c deg-1 data further implied that contrast gain control was operating throughout the subthreshold range. The results for negative DoGs (whose increment corresponds to the darkening of the central spot) and 2 c deg-1 Gabor profiles were consistent with mild nonlinearities having d' powers of 1.6-3. Significant differences between the nonlinearities for positive and negative DoGs indicate that only a small portion, if any, of the near-threshold nonlinearity could be attributed to uncertainty. Our analysis suggests that, with low spatial frequency gratings, detection was based on those bars that become darker; with high-frequency gratings, on the bars that become brighter.

Adult↗
PubMed1999

Spatial pattern summation is phase-insensitive in the fovea but not in the periphery.

We measured the effect of phase coherence on the detectability of a chain of Gabor patches. The stimuli were elongated patterns consisting of 1-8 vertical Gabor patches, aligned vertically and either in phase or 180 deg out of phase from their immediate neighbors. In the fovea, the phase configuration had no effect on detection threshold, suggesting a full-wave rectification operation mediating foveal spatial summation. At 5 deg in the periphery, detection thresholds for the in-phase configuration were lower than those for the alternating-phase configuration, to an extent compatible with a half-wave rectification operation mediating peripheral spatial summation.

Computer Simulation↗
PubMed1999

Distraction of attention and the slope of the psychometric function.

The influential uncertainty model [J. Opt. Soc. Am. A 2, 1508 (1985)] attributes nonlinear contrast sensitivity near threshold to the inability of the observer to discriminate between the signal from stimulated locations and the noise from nonstimulated locations. We introduce an alternative interpretation, the distraction model, to describe the behavior of an observer who knows exactly which location was stimulated but may miss the test stimulus because attention has been distracted by irrelevant (noise) signals. For any stimulus sample, the observer is assumed to be certain of whether this sample is relevant or irrelevant to the stimulus. The non-linear effects predicted by the distraction model without uncertainty are similar to those predicted by the uncertainty model.

Attention↗
PubMed1998

How much of the visual object is used in estimating its position?

Localization accuracy for a wide Gaussian bar (sigma 1 degree) was essentially invariant with sampling density over a wide range. Luminance contrast, on the contrary, had a profound effect on localization accuracy. This difference suggests that the position is estimated from a few (3-4) samples from all those available in the image. A subsequent experiment confirmed that limiting the display to four samples did not impair localization accuracy. Computer simulations show that the result cannot be explained by the peak or centroid rule for position. The results imply that the visual system uses only a few samples to interpolate the luminance profiles, regardless of how many samples are available in the image.

Contrast Sensitivity↗
PubMed1998

Long-range twinkle induction: an achromatic rebound effect in the magnocellular processing system?

After viewing a blank region surrounded by a dynamic noise stimulus, viewers report the perception of prolonged dynamic twinkle in the unstimulated blank region. This twinkle aftereffect may be induced over long ranges in the visual field, up to 10 degrees from the edge of the noise in central vision. Our previous studies of the properties of this aftereffect suggested mediation by the magnocellular processing system. We therefore evaluated the properties predicted by the magnocellular hypothesis by varying the coloring, the temporal and the spatial frequency of the stimulus. No aftereffect could be induced by an equiluminant color stimulus or by luminance noise below the temporal frequency of 5 Hz. The aftereffect obtained by luminance noise above 5 Hz was stronger for larger inducing elements. These results are consistent with known properties of the magnocellular processing system.

Color Perception↗