Biomedical subjects

S C Fullenkamp

Publications and source records attributed to S C Fullenkamp.

6 recordsLinked to original sources

PubMed1999

Target detection against narrow band noise backgrounds.

We studied the detectability of narrow band random noise targets embedded in narrow band random noise backgrounds as a function of differences in center frequency, spatial frequency bandwidth and orientation bandwidth between target and the immediately adjacent background. Unlike most target detection experiments the targets were not added to the background; they replaced the underlying background texture. Simulations showed that target detection probabilities could be accounted for by a simple transformation on the summed outputs of a two layer filter model similar to the complex channels model proposed by Graham, Beck and Sutter (Graham, N., Beck, J., & Sutter, A. (1992). Vision Research, 32, 719-743). Subsequently, the model was tested on the detection of camouflaged vehicle targets with encouraging results.

Contrast Sensitivity↗

PubMed1996

A model for inhibitory lateral interaction effects in perceived contrast.

The apparent contrast of a centrally viewed Gabor target patch was measured by contrast matching in the presence of Gabor flanking patches positioned on a ring of radius r from the center of the target patch. Central patch apparent contrast was determined as a function of the number of flanking patches, the radius r of the ring, and the contrasts of both central and flanking patches. The apparent contrast of the central patch was reduced by the presence of the flanking patches for all experimental conditions. A two layer non-linear model for contrast perception accounts quite well for the data. The first layer performs a power function transformation on the contrast signals from the patches. The second layer takes the outputs from the first layer and divides them by one plus the square root of spatially weighted responses of nearby first layer mechanisms.

Contrast Sensitivity↗

PubMed1993

Spatial interactions in apparent contrast: individual differences in enhancement and suppression effects.

The apparent contrast of a small grating patch embedded in the center of an annular grating filled surround was studied as a function of the surround to central patch contrast ratio, the width of the annulus, and the contrast level of the central patch. Data from ten observers show large individual differences in apparent contrast, but these differences can be interpreted in terms of the relative strength of two interacting mechanisms. One mechanism mediates enhancement of the central patch. The other mediates suppression.

Contrast Sensitivity↗

PubMed1991

Spatial interactions in apparent contrast: inhibitory effects among grating patterns of different spatial frequencies, spatial positions and orientations.

Suppression of the apparent contrast of a small 4 cycle wide suprathreshold sine wave grating patch by a high contrast sine wave grating surround pattern was studied as a function of the spatial frequency, orientation and spatial extent of the surround. The data are consistent with the existence of a complex network of inhibitory interconnections among mechanisms that mediate contrast perception. These connections must extend over spatial distances equivalent to more than 12 cycles of the central grating patch.

Adaptation, Physiological↗

PubMed1991

A transducer model for contrast perception.

Multiple channel models of visual function proposed to date have been either threshold models or suprathreshold models. The transition between detection and perception of contrast which involves the gradual disappearance of spatial response pooling has not been addressed. We propose a model that allows us to simulate contrast detection, contrast perception and the gradual disappearance of spatial response pooling with contrast. The model successfully predicts thresholds and perceived contrast functions for several multiple component stimuli.

Calibration↗

PubMed1988

Perceived contrast and stimulus size: experiment and simulation.

Perceived contrast functions were determined for three different Gabor patch sizes using magnitude estimation and verified by contrast matching. While thresholds show a significant decrease with decreasing patch size, perceived contrasts are equal and independent of patch size for contrasts above 0.06. Contrast matching was also used to study the apparent contrast of two other spatially limited stimuli; the sum of two orthogonal 4 c/deg sine waves multiplied by a gaussian envelope and the sum of spatially adjacent positive and negative gaussians. Models of contrast perception, based on tuned Gabor spatial filters, were formulated and tested for agreement with our experimental data. A model that pools filter responses across spatial frequencies and orientations was found to be more in agreement with our data than a model that simply uses the response of a single, maximally excited, mechanism to mediate contrast perception. Optimum filter bandwidth was found to be about 1.1 octaves.

Contrast Sensitivity↗