Biology subjects

Kohler, P. J.

Publications and source records attributed to Kohler, P. J..

3 recordsLinked to original sources

bioRxiv2018

Functional organization of motion and disparity sensitivity in human visual cortex

Vision with two eyes makes perception of weak visual contrast signals easier and, due to the lateral separation of the eyes, allows for the triangulation of depth relationships. While binocular summation of contrast signals affords the observer increased sensitivity, binocular summation of spatial cues related to changes in depth is associated with decreased sensitivity to the corresponding retinal image displacements. Perceptual models of contrast and motion-in-depth sensitivity have explained this divergence in sensitivity by proposing that probabilistic neural noise limits summing and differencing operations on small signals. Because these models do not scale well for highly suprathreshold visual signals typical of the natural environment, we approached the question of how dynamic binocular image differences are coded using direct neural measurements. Here we use Steady-State Visual Evoked Potentials in human participants to show that inter-ocular differences in retinal image motion that produce elevated perceptual thresholds generate strongly suppressed evoked response amplitudes compared to motion that is matched between the two eyes. This suppression is strongly dependent on the availability of well-defined spatial references in the image and is highly immature in 5-month-old infants. Because the suppression is of equal strength for horizontal and vertical directions of motion, it is not specific to the perception of motion in depth. Relational image cues play a critical role in early to intermediate perceptual processing stages, and these results suggest that a succession of spatial and inter-ocular differencing operations condition the visual signal representation, prior to the extraction of motion-in-depth.\n\nSignificance StatementThe present work underscores the importance of relational spatial cues in both the motion and disparity domains for binocular visual coding. Relative motion and relative disparity cues not only support fine-grain displacement sensitivity but strongly influence suprathreshold responsiveness. Extraction of these cues supports a powerful binocular interaction within the motion pathway that is suppressive in nature and poorly developed in infants. This suppressive interaction is present for both horizontal and vertical directions of motion and is thus not specific to motion-in-depth, as previously believed, but is rather hypothesized to be a pre-processing step, with motion-in-depth being computed at a later or separate stage.

neuroscience↗

bioRxiv2017

From image formation to image representation: the human visual system preserves the hierarchy of 2-dimensional pattern regularity

Symmetries are present at many scales in images of natural scenes, due to a complex interplay of physical forces that govern pattern formation in nature. The importance of symmetry for visual perception has been known at least since the gestalt movement of the early 20th century. Since then, symmetry has been shown to contribute to the perception of shapes[1, 2] scenes[3] and surface properties[4], as well as the social process of mate selection[5]. In the two spatial dimensions relevant for images, the four fundamental symmetries, reflection, rotation, translation and glide reflection, can be combined in 17 distinct ways, the \"wallpaper\" groups[6-8]. The 17 wallpaper obey a hierarchy of complexity, determined by mathematical group theory, where simpler groups are subgroups of more complex ones[9]. Here we use Steady-State Visual Evoked Potentials (SSVEPs) to measure responses to the complete set of wallpaper groups, and present a complete description of the neural basis of symmetry. We find that activity in human visual cortex is remarkably consistent with the hierarchical relationships between the wallpaper groups. Specifically, the amplitudes of symmetry-specific responses in individual participants (n=25) preserve these relationships at an above-chance level in 88.3% (53 out of 60) of cases. Visual cortex thus encodes all the fundamental symmetries using a representational structure that closely approximates the subgroup relationships from group theory. Given that most participants had no knowledge of group theory, the ordered structure of visual responses to wallpaper groups is likely learned implicitly from regularities in the visual environment.

neuroscience↗

bioRxiv2017

Image segmentation based on relative motion and relative disparity cues in topographically organized areas of human visual cortex

The borders between objects and their backgrounds create discontinuities in image feature maps that can be used to recover object shape. Here we used functional magnetic resonance imaging (fMRI) to study the sensitivity of visual cortex to two of the most important image segmentation cues: relative motion and relative disparity. Relative motion and disparity cues were isolated using random-dot kinematograms and stereograms, respectively. For motion-defined boundaries, we found a strong retinotopically organized representation of a 2-degree radius motion-defined disk, starting in V1 and extending though V2 and V3. In the surrounding region, we observed phase-inverted activations indicative of suppression, extending out to at least 6 degrees of retinal eccentricity. For relative disparity, figure responses were only robust in V3, while suppression was observed in all early visual areas. When attention was captured at fixation, figure responses persisted while suppression did not, suggesting that suppression is generated by attentional feedback from higher-order visual areas. Outside of the early visual areas, several areas were sensitive to both types of cues, most notably hV4, LO1 and V3B, making them additional candidate areas for motion- and disparity-cue combination. The overall pattern of extra-striate activations is consistent with recent three-stream models of cortical organization.

neuroscience↗