< Back to previous page

Publication

Integration Processes at Multiple Levels of the Visual Cortical Hierarchy Revealed by Emergent Intermodulation Frequencies

Book - Dissertation

The visual system integrates visual inputs in a context-sensitive fashion and constructs a coherently organized whole. Although this is one of the most prominent aspects of human vision, the underlying neural mechanisms are still not fully understood. How integrated representations arise at the neural level remains elusive. To provide additional insight on this issue, in this thesis, we investigated the neural correlates of different types of Gestalts in which the whole requires the integration of visual information about the parts. Specifically, we applied high-density electroencephalography (EEG) by collecting steady state visual evoked potentials (SSVEPs; Regan, 1966) in combination with the technique of “frequency tagging” (Regan & Heron, 1969) to different Gestalts. SSVEPs are the periodic neural responses to a periodic visual stimulation and the “frequency tagging” technique allows us to detect the neural activities corresponding to separate parts of the input images modulated at different frequencies (i.e., physically given fundamental frequencies). Most importantly, nonlinear intermodulation (IM) frequencies (e.g., summation of two input frequencies or differences between them and their harmonics), which emerge as a result of the interactions at the neural level, may appear in response to global configurations in Gestalt formation. In three studies, we used specific IM components as objective neural markers for different types of Gestalt formation. In Chapter 2, we applied this technique to the Kanizsa illusory square in which a global configuration arises through spatial integration, and to a stimulus variation in which this is not the case. We showed strong nonlinear IM components, resulting from the integration processes specific to long-range interactions between the pacman inducers, giving rise to the illusory surface. In Chapter 3, we applied this technique to the wallpaper patterns in which a global configuration (reflection or rotational symmetry) arises through spatial integration of dots, and to a stimulus variation in which this is not the case. We found enhancement of IM components over occipital cortex in the reflection symmetry compared to two control patterns (rotational symmetry and random) which revealed spatial integration processes driven specifically by reflection symmetry. In Chapter 4, we extended the previous studies, in which we investigated only spatial integration, and identified the neural responses of spatiotemporal integration processes. We disassociated two distinct levels of interactions in which one was associated with the human quality of multiple moving point-light displays, while another one was associated with their motion synchrony. Taken together, this thesis investigated an old question with a new technique. A common question which we asked in all studies was: Does perceiving a coherent whole involve neural grouping of parts into a global entity? By applying frequency tagging to different Gestalts, we obtained specific neural responses corresponding to a coherent whole, which can be used as objective neural markers of the integration of the parts at multiple levels of the visual cortical hierarchy.
Publication year:2017
Accessibility:Closed