Biomedical subjects

Lynn A Olzak

Publications and source records attributed to Lynn A Olzak.

4 recordsLinked to original sources

PubMed2006

Contextual effects on fine orientation discrimination tasks.

We examined the influence of context on fine orientation discrimination performance using sinusoidal grating patterns. Discrimination performance was impaired in the presence of modulated surrounds of the same spatial frequency, orientation, and contrast as the center. When center and surround were out-of-phase, separated by a gap of mean luminance, or very different in spatial frequency, performance remained at control levels. When center and surround were in-phase but mismatched in mean luminance, suppression was reduced or eliminated and performance was equivalent to luminance-mismatched control conditions. We speculate that lateral interactions in fine orientation discrimination tasks do not occur between objects that are perceptually distinct.

Adult↗

PubMed2005

Contextual effects in fine spatial discriminations.

The context in which a pattern is viewed can greatly affect its apparent contrast, a phenomenon commonly attributed to pooled contrast gain control processes. A low-contrast surround may slightly enhance apparent contrast, whereas increasing the contrast of the surround leads to a monotonic decline in contrast appearance. We ask here how the presence of a patterned surround affects the ability to perform fine, suprathreshold orientation, contrast, and spatial frequency discriminations as a function of surround contrast and phase. Our results revealed an unexpected dip in performance when center and surround were in phase and similar in contrast. These results suggest that additional processes, perhaps those involved in scene segregation, play a role in contextual effects on discrimination.

Adaptation, Ocular↗

PubMed2003

Dual nonlinearities regulate contrast sensitivity in pattern discrimination tasks.

Many current psychophysical models propose that visual processing in cortex is hierarchical, with nonlinearities sandwiched between linear stages of processing. In earlier publications, we proposed a model of this type to account for masking effects found with spatial frequency and orientation discriminations. Our model includes two nonlinear mechanisms that regulate contrast sensitivity in early cortical mechanisms. The first is a local within-pathway nonlinearity that accelerates at low contrasts but is compressive at high. The second is a pooled nonlinear gain control process that operates over a broad range of neurons with different tuning characteristics. Here, we test predictions of the model for spatial frequency discriminations. The model predicts that at low contrasts, adding a grating mask oriented parallel to test gratings will improve discrimination performance via operation of the within-pathway nonlinearity, analogous to the "dipper effect" found with contrast discriminations. Adding an orthogonally oriented mask is predicted to have no effect at low contrasts, where pooled gain control processes contribute little to performance. At high contrasts, the model predicts that performance will asymptote and become independent of contrast with either parallel or orthogonal masks. The results confirm model predictions.

Adult↗

PubMed2002

Orientation-selective summing mechanisms revealed in visual search.

We investigated properties of the neural mechanisms that mediate detection of complex grating targets in an orientation-based visual search task. Targets and distractors were composed of small patches of compound sinusoidal gratings. Components were chosen to differ enough in spatial frequency to stimulate separate and independent mechanisms at the primary cortical layer of processing. The orientations of the components were both vertical in distractor patches. In the uncrossed condition, both components of the target tilted either 3 degrees left or right. In the crossed condition, one component of the target tilted left and the other tilted right. Search was faster and more accurate in the uncrossed condition, ruling out mediation either by V1-like tuned mechanisms or by a higher-level mechanism that signals differences in orientation. Results were consistent with two classes of mid-level summing mechanisms. We argue that mid-level mechanisms such as these may be the neural substrate for conceptual orientation feature maps.

Adult↗