Other studies have found an analogous hemispheric asymmetry in processing SFs, with the RH biased in discriminating LSFs and the LH biased in discriminating HSFs (Christman, Kitterle, & Hellige,
1991; Kitterle, Christman, & Hellige,
1990; Kitterle, Hellige, & Christman,
1992), as well as a direct link between LSF versus HSF discrimination and attending to global versus local objects, respectively (Flevaris, Bentin, & Robertson,
2011; Shulman, Sullivan, Gish, & Sakoda,
1986; Shulman & Wilson,
1987). Furthermore, the asymmetry occurs whether the stimuli are small or large (Robertson & Lamb,
1991), making the role of SFs more complex than a simple bottom-up analysis. To account for these and other data, Ivry and Robertson (
1998; Robertson & Ivry,
2000) developed the Double Filtering by Frequency (DFF) theory of hemispheric specialization for SF processing. According to this theory, there are two stages of SF filtering, and the two hemispheres differ in how they process SF information
after the initial visual stage. Specifically, attention first selects an SF range from the incoming spectrum that is most suited for the current task (e.g., the range of spatial scales that is most likely to contain the target; see Watt,
1988 for a model of how this might work). This SF range is then fed forward in
both cerebral hemispheres, but it is at this second stage that the hemispheres differ, with the LH acting as a relatively high-pass filter, emphasizing information from the higher SFs within the initially selected range, and the RH as a relatively low-pass filter, emphasizing the lower SFs within the range.