Biomedical subjects

Christian Wehrhahn

Publications and source records attributed to Christian Wehrhahn.

3 recordsLinked to original sources

PubMed2006

Reversed phi revisited.

Two briefly flashed lines shown in rapid succession evoke a sensation of motion in human observers. This is examined quantitatively such that line separation, temporal offset, and contrast polarity are varied. Line pairs are presented on a gray background and are either both bright or both dark (equal contrast polarity) or one line is bright and the other is dark (opposite contrast polarity). Observers are instructed to indicate the perceived direction of motion. With foveal viewing, perceived direction is veridical for line pairs with equal contrast polarity but is reversed for line pairs with opposite contrast polarity, with spatial separations between 0 and 12 arcmin, and with temporal offsets between 8 and 33 ms. When separations, temporal offsets, or both are further increased, perception reverts to veridical. When lines of opposite contrast polarity are presented in the parafovea, reversal of perceived direction is also observed. The psychophysical results correlate well with those of recent intracellular recordings from directionally selective simple cells in the primary visual cortex of anesthetized cats (N. J. Priebe & D. Ferster, 2005).

Contrast Sensitivity↗

PubMed2005

Contextual masking of oriented lines: interactions between surface segmentation cues.

The ability of human observers to detect and discriminate a single feature of a visual image deteriorates markedly when the targeted feature is surrounded by others of a similar kind. This perceptual masking is mirrored by the suppressive effects of surround stimulation on the responses of neurons in primary visual cortex (area V1). Both perceptual and neuronal masking effects are partially relieved, however, if the targeted image feature is distinguished from surrounding features along some dimension, such as contour orientation. Masking relief is likely to play an important role in perceptual segmentation of complex images. Because dissimilar surfaces usually differ along multiple feature dimensions, we tested the possibility that those differences may influence segmentation in an invariant manner. As expected, we found that the presence of surrounding features resulted in perceptual masking and neuronal response suppression in area V1, but that either orientation or contrast polarity differences between the target and surrounding features was sufficient to partially relieve these effects. Simultaneous differences along both dimensions, however, yielded no greater relief from masking than did either difference alone. Although the averaged neuronal effects of orientation polarity cues were thus invariant, the time course over which these effects emerged after each stimulus appearance was different for the two cues. These findings refine our understanding of the functions of nonclassical receptive fields, and they support a key role for V1 neurons in surface segmentation.

Action Potentials↗

PubMed2004

Chromatic induction in humans: how are the cone signals combined to provide opponent processing?

We investigated the effect of 16 isoluminant chromatic surrounds on the perceived colour of an enclosed grey test-field at photopic (43 cd/m2) conditions. Stimuli were shown on a grey background identical to the test-field. Use of these stimuli implies that activations of receptoral (cS, cM, and cL) and postreceptoral (cM-cL, cS-(cM+cL)) mechanisms by surround colours are known quantitatively. This allows to predict shifts in colour of the test-field in terms of receptoral (adaptation) as well as postreceptoral (contrast) mechanisms assuming a standard two-stage model. Predictions are tested using matching and hue compensation procedures. Both procedures yield comparable results that are consistent with the assumption that postreceptoral mechanisms explain the observed shifts in perceived colour.

Adaptation, Physiological↗