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Paper   IPM / Cognitive Sciences / 7595
School of Cognitive Sciences
  Title:   The role of feature-dependent backward masking in perceptual asynchrony
  Author(s): 
1.  B. Bahrami
2.  R. Rajimehr
  Status:   In Proceedings
  Proceeding: ECVP
  Year:  2002
  Supported by:  IPM
  Abstract:
Reducing the visibility of a target object by presenting another object (mask) afterwards is called 'backward masking'. Whether masking acts holistically or corresponding features mask one another is not clearly understood. The issue of perceptual asynchrony has been frequently investigated by sequentially changing the features of some visual stimulus and asking the subjects to decide about those changes. Feature-dependent backward masking and its possible asymmetric interactions could have affected those experiments. In this study, 8 white letters were briefly flashed in a clock-face configuration around fixation point and a colour (red or green) or orientation (horizontal or vertical grating) mask later appeared in one of the eight locations and subjects decided whether the masked letter was an 'S' or not. Error rates for various stimulus onset asynchronies (SOAs) were determined for colour and orientation masks differentially. We showed that high-level backward masking by orientation is strongest in SOAs around 80 to 100 ms and declines rapidly in higher SOAs; however, masking by colour is more persistent (p < 0.05), extending to SOAs as long as 160 ms. We conclude that colour and orientation impose asymmetric masking trends and this asymmetry could have acted as a source of uncontrolled bias in the experiments on perceptual asynchrony. [Authors wish to thank Hossein Esteky and Bruno Breitmeyer for their valuable remarks and helpful comments throughout the project.] .


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