Browsing by Subject "Contour processing"

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    Meta- and paracontrast reveal differences between contour- and brightness-processing mechanisms
    (Elsevier, 2006) Breitmeyer, B. G.; Kafalıgönül, Hulusi; Öğmen, H.; Mardon, L.; Todd, S.; Ziegler, R.
    We investigated meta- and paracontrast masking using tasks requiring observers to judge the surface brightness or else the contours of target stimuli. The contour task revealed strongest metacontrast at SOAs shorter than those obtained for the brightness task. Paracontrast revealed related temporal differences between the tasks. Additionally, the paracontrast results support the existence not only of prolonged inhibitory effects but also of facilitatory effects. The combined results comport with the existence of cortical mechanisms for: (i) fast contour processing, (ii) slow surface-brightness processing, (iii) prolonged inhibition, and (iv) facilitation.
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    Metacontrast masking and stimulus contrast polarity
    (Elsevier, 2008) Breitmeyer, B.; Tapia, E.; Kafalıgönül, Hulusi; Öğmen, H.
    A recent report [Becker, M. W., & Anstis S. (2004). Metacontrast masking is specific to luminance polarity. Vision Research, 44, 2537–2543] of a failure to obtain metacontrast with target and mask stimuli of opposite contrast polarity is reexamined in an experiment that systematically varies not only stimulus contrast polarity but also target size and target-mask onset asynchrony (SOA). The results show that (a) although, as previously shown [Breitmeyer, B. G. (1978a). Metacontrast with black and white stimuli: Evidence of inhibition of on and off sustained activity by either on or off transient activity. Vision Research, 18, 1443–1448], metacontrast is weaker with stimuli of opposite contrast polarity, (b) substantial metacontrast can be obtained with targets and masks of opposite contrast polarity, especially (c) when the target is small. We conclude that Becker and Anstis’s failure to obtain metacontrast with stimuli of opposite contrast polarity is due to their use of a single, relatively large, SOA value.