Biomedical subjects

Kenneth Knoblauch

Publications and source records attributed to Kenneth Knoblauch.

7 recordsLinked to original sources

PubMed2006

Modeling psychometric functions in R.

We demonstrate some procedures in the statistical computing environment R for obtaining maximum likelihood estimates of the parameters of a psychometric function by fitting a generalized nonlinear regression model to the data. A feature for fitting a linear model to the threshold (or other) parameters of several psychometric functions simultaneously provides a powerful tool for testing hypotheses about the data and, potentially, for reducing the number of parameters necessary to describe them. Finally, we illustrate procedures for treating one parameter as a random effect that would permit a simplified approach to modeling stimulus-independent variability due to factors such as lapses or interobserver differences. These tools will facilitate a more comprehensive and explicit approach to the modeling of psychometric data.

Computer Simulation↗

PubMed2005

Frequency and phase contributions to the detection of temporal luminance modulation.

Observers detected a temporally modulated luminance pattern embedded in dynamic noise. A Gabor function with a carrier frequency, in separate conditions of 0, 1.56, or 3.12 Hz, modulated signal contrast. Classification images were constructed in the time, temporal frequency, and temporal phase domains. As stimulus frequency increased, amplitudes of the phase images decreased and amplitudes of the frequency images increased, indicating a corresponding shift in the observers' criteria. The reduced use of phase attenuated time-domain images from signal-absent trials, but physical interactions between signal and noise components tended to preserve time-domain images from signal-present trials. The results illustrate a frequency-dependent strategy shift in detection that may reflect a degree of stimulus uncertainty in the time domain.

Adaptation, Ocular↗

PubMed2004

Difference scaling of gloss: nonlinearity, binocularity, and constancy.

Gloss is an attribute of visual appearance that originates from the geometrical distribution of the light reflected by the surface. We used the maximum likelihood difference scaling (MLDS) procedure (L.T. Maloney & J. N. Yang, 2003) to estimate gloss scales over an extended range. Observers' judgments were obtained for a series of 10 black, coated samples for two directions of illumination, in binocular and monocular vision. The results showed a nonlinear relation between gloss percept and instrumental specular gloss values. Sensitivity is higher at extreme scale values than in the middle. In binocular vision, the sensitivity to gloss is higher than in monocular vision exclusively for high gloss levels. Lastly, we found that gloss difference scales, when expressed in terms of the samples rather than the photometric characteristics, vary little with the direction of illumination. Gloss scaling thus seems to be independent of the geometrical variations of the luminous flux at the surface of the sample. By analogy with the term "color constancy," we call this property "gloss constancy."

Humans↗

PubMed2004

Quantitative analysis of connectivity in the visual cortex: extracting function from structure.

It is generally agreed that information flow through the cortex is constrained by a hierarchical architecture. Lack of precise data on areal connectivity leads to indeterminacy of existing models. The authors introduce two quantitative parameters (SLN and FLN) that hold the promise of resolving such indeterminacy. In the visual system, using a very incomplete database, provisional hierarchies are in line with the recent proposal of higher functions of area V1 and suggest a hitherto unsuspected central function of the frontal eye field.

Nerve Net↗

PubMed2002

Early specification of the hierarchical organization of visual cortical areas in the macaque monkey.

The laminar organization of cortico-cortical projection neurons (expressed by the percentage of supragranular projecting neurons - SLN%) characterizes cortical pathways as feedforward (FF) or feedback (FB) and determines the hierarchical ranking of cortical areas. There is evidence of a developmental reduction in SLN% of pathways to area V1. Here, by analyzing pre- and postnatal projections to area V4, we have been able to address whether developmental reductions of SLN% impact on information processing in the immature cortex. FB pathways to area V4 exhibit 28-84% reduction of SLN%. This contrasts with the FF projections, which show little or no SLN% reduction. However, SLN% values in the immature cortex allocated cortical areas to the same hierarchical levels as in the adult. The developmental reduction of SLN% is a widespread phenomenon in the neocortex and is a distinctive feature of FB pathways. Two mechanisms contribute to developmental changes in SLN%: (i) delayed ingrowth of axons into the cortical target from infragranular layer neurons and (ii) prolonged developmental reduction of the divergence of projections from supragranular layer neurons. The present results show that FF and FB projections exhibit different developmental processes and patterns of connections linking cortical areas and their hierarchical relations are established prenatally, independently of regressive phenomena.

Animals↗

PubMedDate not supplied

Perceptual classification of chromatic modulation.

We measured the regions of the equiluminant plane that are exploited by observers during a Yes/No detection task. The signal was a 640-ms Gaussian modulation (sigma(t) = 160 ms) of a Gaussian spatial patch (sigma(s) = 2.4 deg) presented in chromatically bivariate uniform noise. One component of the noise was along the direction axial with the signal in color space, the other perpendicular. Four signal directions were tested: along cardinal LM and S axes and two intermediate directions to which the cardinal axes were equally sensitive. The distribution of noise chromaticities from each trial was correlated with the observers' responses and the presence and absence of the signal to build a classification image of the distribution of chromaticities on which the decision of the observer was based. The images show a narrowly selective peak in the signal direction flanked by regions with a broader selectivity. These results raise the possibility that detection judgments are mediated by both linear and nonlinear mechanisms with peak sensitivities between the cardinal directions.

Adult↗

PubMedDate not supplied

Chromatic contrast detection in spatial chromatic noise.

The spectral properties of chromatic-detection mechanisms were investigated using a noise-masking paradigm. Contrast-detection thresholds were measured for a signal with a Gaussian spatial profile, modulated in the equiluminant plane in the presence of spatial chromatic noise. The noise was distributed within a sector in the equiluminant plane, centered on the signal direction. Each stimulus consisted of two adjacent fields, one of which contained the signal, separated horizontally by a gap with the same average chromaticity as the uniform background. Observers were asked to judge on which side of the central fixation point the signal was displayed via a two-alternative, forced-choice (2AFC) paradigm. Contrast thresholds were measured for four color directions and three sector widths at increasing levels of the average energy of the axial component of the noise. Results show that contrast thresholds are unaffected by the width of the noise sector, as previously found for temporally modulated stimuli (D'Zmura & Knoblauch, 1998). The results are consistent with the existence of spectrally broadband linear-detection mechanisms tuned to the signal color direction and support the hypothesis of the existence of higher-order color mechanisms with sensitivities tuned to intermediate directions in color space.

Acoustic Stimulation↗