Browsing by Author "Breitmeyer, B."

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    Effects of contrast polarity in paracontrast masking
    (Springer New York, 2009) Kafalıgönül, Hulusi; Breitmeyer, B.; Öğmen, H.
    The visibility of a target stimulus can be suppressed (inhibition) or increased (facilitation) during paracontrast masking. Three processes have been proposed to be involved in paracontrast masking: brief inhibition, facilitation, and prolonged inhibition (Breitmeyer et al., 2006). Brief inhibition is observed when the mask precedes the target at short stimulus onset asynchronies (SOAs) ranging from -10 to -30 msec, whereas prolonged inhibition is effective up to very large SOAs of -450 msec. Facilitation, enhancement in target visibility, can be observed at SOA values between -20 and -110 msec. We further investigated these processes by changing target-mask spatial separation and the contrast polarity of the mask. Our results show that (1) facilitation weakens when spatial separation between the target and mask is increased or when they have opposite contrast polarity, and (2) brief inhibition turns into facilitation for the opposite-polarity mask, whereas prolonged inhibition does not change significantly. These results suggest a fast inhibition mechanism realized in the contrast-specific center-surround antagonism of classical receptive fields for brief inhibition and a slower, higher level cortical processing that is indifferent to contrast polarity for prolonged inhibition.
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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.