Biomedical subjects

Serge O Dumoulin

Publications and source records attributed to Serge O Dumoulin.

4 recordsLinked to original sources

PubMed2006

What image properties regulate eye growth?

The growth of the eye, unlike other parts of the body, is not ballistic. It is guided by visual feedback with the eventual aim being optimal focus of the retinal image or emmetropization . It has been shown in animal models that interference with the quality of the retinal image leads to a disruption to the normal growth pattern, resulting in the development of refractive errors and defocused retinal images . While it is clear that retinal images rich in pattern information are needed to control eye growth, it is unclear what particular aspect of image structure is relevant. Retinal images comprise a range of spatial frequencies at different absolute and relative contrasts and in different degrees of spatial alignment. Here we show, by using synthetic images, that it is not the local edge structure produced by relative spatial frequency alignments within an image but rather the spatial frequency composition per se that is used to regulate the growth of the eye. Furthermore, it is the absolute energy at high spatial frequencies regardless of the spectral slope that is most effective. Neither result would be expected from currently accepted ideas of how human observers judge the degree of image "blur" in a scene where both phase alignments and the relative energy distribution across spatial frequency (i.e., spectral slope) are important.

Animals↗

PubMed2006

Modulation of V1 activity by shape: image-statistics or shape-based perception?

It is current dogma that neurons in primary visual cortex extract local edges from the scene from which later visual areas reconstruct more meaningful shapes. Recent neuroimaging studies, however, have shown V1 modulations by the degree of structure in the image (shape). These V1 modulations due to the level of shape coherence have been explained in one of two possible ways: due to changes in image statistics or shape-based perceptual influences from higher visual areas. Here we compare both hypotheses using stimuli composed of Gabor arrays constructed to form circular shapes that can be successively degraded by manipulating the orientations of individual Gabors while maintaining local and global statistics. In a first experiment, we confirm that V1 responses are inversely correlated with the degree of structure in the image. In a second experiment, stimulus predictions are compared based on the degree of circular shape or change in the image statistic varied (orientation variance) in the image. We find that these V1 modulations to shape change are correlated with low-level changes in orientation contrast rather than shape perception per se.

Adult↗

PubMed2003

Automatic volumetric segmentation of human visual retinotopic cortex.

Previous identification of early visual cortical areas in humans with phase-encoded retinotopic mapping techniques have relied on an accurate cortical surface reconstruction. Here a 3D phase-encoded retinotopic mapping technique that does not require a reconstruction of the cortical surface is demonstrated. The visual field sign identification is completely automatic and the method directly supplies volumes for a region-of-interest analysis, facilitating the application of cortical mapping to a wider population. A validation of the method is provided by simulations and comparison to cortical surface-based methodology.

Adult↗

PubMed2003

Cortical specialization for processing first- and second-order motion.

Distinct mechanisms underlying the visual perception of luminance- (first-order) and contrast-defined (second-order) motion have been proposed from electrophysiological, human psychophysical and neurological studies; however a cortical specialization for these mechanisms has proven elusive. Here human brain imaging combined with psychophysical methods was used to assess cortical specializations for processing these two kinds of motion. A common stimulus construction was employed, controlling for differences in spatial and temporal properties, psychophysical performance and attention. Distinct cortical regions have been found preferentially processing either first- or second-order motion, both in occipital and parietal lobes, producing the first physiological evidence in humans to support evidence from psychophysical studies, brain lesion sites and computational models. These results provide evidence for the idea that first-order motion is computed in V1 and second-order motion in later occipital visual areas, and additionally suggest a functional dissociation between these two kinds of motion beyond the occipital lobe.

Adult↗