Abstract
Texture discontinuities are a fundamental cue by which the visual system segments objects from their background. The neural mechanisms supporting texture-based segmentation are therefore critical to visual perception and cognition. In the present experiment we employ an EEG source-imaging approach in order to study the time course of texture-based segmentation in the human brain. Visual Evoked Potentials were recorded to four types of stimuli in which periodic temporal modulation of a central 3° figure region could either support figure-ground segmentation, or have identical local texture modulations but not produce changes in global image segmentation. The image discontinuities were defined either by orientation or phase differences across image regions. Evoked responses to these four stimuli were analyzed both at the scalp and on the cortical surface in retinotopic and functional regions-of-interest (ROIs) defined separately using fMRI on a subject-by-subject basis. Texture segmentation (tsVEP: segmenting versus non-segmenting) and cue-specific (csVEP: orientation versus phase) responses exhibited distinctive patterns of activity. Alternations between uniform and segmented images produced highly asymmetric responses that were larger after transitions from the uniform to the segmented state. Texture modulations that signaled the appearance of a figure evoked a pattern of increased activity starting at ∼143 ms that was larger in V1 and LOC ROIs, relative to identical modulations that didn't signal figure-ground segmentation. This segmentation-related activity occurred after an initial response phase that did not depend on the global segmentation structure of the image. The two cue types evoked similar tsVEPs up to 230 ms when they differed in the V4 and LOC ROIs. The evolution of the response proceeded largely in the feed-forward direction, with only weak evidence for feedback-related activity.
Highlights
The boundaries between objects and their supporting backgrounds, or between surfaces at different depths, create discontinuities in feature maps of orientation, relative alignment, motion, disparity, color and spatial scale
The early stages of the scene segmentation process have been extensively studied in humans and primates using textured stimuli into which discontinuities in single visual features have been embedded
A common finding in the single-unit literature has been that the response to an isolated feature presented within the classical receptive field is suppressed by the addition of texture in the cell’s non-classical surround [6,7,8,9]
Summary
The boundaries between objects and their supporting backgrounds, or between surfaces at different depths, create discontinuities in feature maps of orientation, relative alignment, motion, disparity, color and spatial scale. In the present study we exploited a refined version of this ROI-based EEG source-imaging approach [31,32] with stimuli that modulated at a much slower rate in order to examine the time course and source distribution of the tsVEP generated by orientation and alignment cues. This approach allowed us to assess how texture-based segmentation proceeds through both retinotopic visual areas, as well as, areas of lateral cortex previously implicated in object processing [33,34], and to evaluate differences in these responses due to the defining texture cues. This study provides the first quantitative measurements of the relative strength and precise timing of segmentation-related activity as it is propagated throughout the human visual cortical hierarchy
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