Flanked objects are difficult to identify using peripheral vision. This is visual crowding. Crowding is the perceptual bottleneck for reading (Pelli et al., 2007) and object recognition (Pelli & Tillman, 2008). Theories for crowding had been suggested, but the underlying neural mechanism remained unclear. Some perceptual manipulations had been shown to break crowding, hinting the cortical locus of crowding. In the first part of the current study, we ask whether illusory contour formation survives crowding of illusion inducers. We measured the contrast thresholds for discriminating the rotation angle of a target Kanizsa inducer with and without flankers. When the other three Kanizsa inducers were rotated to misalign with the target, we observed strong crowding indicated through contrast threshold elevation. Interestingly, however, subjects were able to judge the shape (thin vs. fat) of the illusory Kanizsa square even when the individual inducers were flanked. Internal representation of the percept in the inducer-aligned condition was further studied through the classification image technique. Classification images indicated that illusory contours were formed and used in the perceptual decision task in both the non-flanked and flanked conditions. These findings suggest that crowding happens after illusory contour formation.
In the second part, we ask if Chinese character orientation affects crowding. Fifty commonly used traditional Chinese characters were selected as stimuli. We measured the contrast threshold for identifying upright and inverted target surrounded by either upright or inverted flankers. At 5? eccentricity, we observed an interaction effect between target and flanker orientation on crowding: upright flankers produced a stronger crowding effect than inverted flankers for inverted but not upright target. Follow-up experiments showed that the observation was not due to a rise of detection sensitivity for upright flankers or a change of spatial extent between upright and inverted target. Yet, adding an enclosure to flankers eliminated the flanker orientation effect, suggesting that the flanker orientation effect may be attributed to the facilitation of texture perception in the periphery. At 10? eccentricity, upright target was generally more immune from crowding, further supporting that crowding takes place after Chinese character orientation has been coded.
From previous fMRI studies, we know that lateral occipital complex (LOC) is the cortical site that first responds to Kanizsa?s illusory contours (Murray et al., 2002) and the fusiform gyrus is sensitive to the inversion of Chinese character (Kao, Chen, & Chen, 2009). Taken together with our findings, we infer that crowding happens at or after the two cortical locations. / published_or_final_version / Psychology / Master / Master of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/174487 |
| Date | January 2011 |
| Creators | Lau, Siu-fung., 劉兆鋒. |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Source | http://hub.hku.hk/bib/B47753079 |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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