Visual hemispheric asymmetries depend on which spatial frequencies are task relevant

Observers classified sine-wave and square-wave gratings on the basis of fundamental frequency (Are the bars wide or narrow?) or on the basis of higher harmonic frequencies (Are the bars sharp or fuzzy?). Stimuli were presented in either the left (LVF) or right (RVF) visual field. When the classification was made on the basis of the fundamental frequencies (1 or 3 c/deg), there was a LVF/right hemisphere advantage. However, when the classification was on the basis of a sharp/fuzzy distinction which involved searching for the higher harmonic frequencies, then a RVF/left hemisphere advantage was found.     

Hemispheric differences in the interference among components of compound gratings

The relationship between local/global and high/low spatial-frequency processing in hemispheric asymmetries was explored. Subjects were required to judge the orientation of a high- or low-spatial-frequency component of a compound grating presented in the left visual field (LVF) or right visual field (RVF). In Experiment 1, attention was focused on one or the other component. A signal detection analysis indicated that sensitivity (d′) to the high-spatial-frequency target was reduced more by the presence of the low-spatial-frequency component when both were presented in the LVF rather than in the RVF. In Experiment 2, subjects determined whether a target orientation was present, independent of spatial frequency at only a single level (i.e., at the high- or low-spatial-frequency level), as opposed to both or neither level. An RVF/LH (left hemisphere) advantage was found when the decision was based on the orientation of the high-frequency component. The asymmetrical influence of visual field of presentation and spatial frequency upon sensitivity is discussed in terms of hemispheric differences in the magnitude of inhibition between spatial-frequency channels and in the role of transient channel activity to capture and direct higher order attentional processes.     

Hemispheric asymmetries in the identification of band-pass filtered letters

Processing of band-pass filtered letters in the left versus right cerebral hemispheres (LH vs. RH) was examined. The present experiments constituted a partial replication of a study in which Peterzell, Harvey, and Hardyck (1989) found no hemispheric differences in accuracy or reaction time (RT) as a function of spatial frequency. However, methodological limitations of their study (e.g., the possibility that subjects were engaged in a detection, not identification, task) may have obscured possible hemispheric differences. We addressed these problems in the present study, obtaining significant hemisphere × spatial frequency interactions for RT andd', with RH advantages at low frequencies and LH advantages at high frequencies; however, these effects were not large in magnitude and were often restricted to particular dependent variables, stimulus sizes, and so forth. Hemispheric differences in response bias were also found.     

Questions of criteria: Reply to Peterzell (1997)

In a reply to our report on hemispheric differences in processing band-pass filtered letters (Christman, Kitterle, & Niebauer, 1997), Peterzell (1997) argues that our results are not attributable to hemispheric asymmetries in spatial frequency processing. Rather, Peterzell argues that factors such as response criteria and stimulus visibility can account for our results. We argue that our results are attributable (at least in part) to hemispheric asymmetries in spatial frequency processing, while at the same time we acknowledge the potential influence of other factors in the determination of hemispheric differences.     

Hemispheric differences are found in the identification,but not the detection,of low versus high spatial frequencies

The processing of sine-wave gratings presented to the left and right visual fields was examined in four experiments. Subjects were required either to detect the presence of a grating (Experiments 1 and 2) or to identify the spatial frequency of a grating (Experiments 3 and 4). Orthogonally to this, the stimuli were presented either at threshold levels of contrast (Experiments 1 and 3) or at suprathreshold levels (Experiments 2 and 4). Visual field and spatial frequency interacted when the task required identification of spatial frequency, but not when it required only stimulus detection. Regardless of contrast level (threshold, suprathreshold), high-frequency gratings were identified more readily in the right visual field (left hemisphere), whereas low-frequency gratings showed no visual field difference (Experiment 3) or were identified more readily in the left visual field (right hemisphere) (Experiment 4). Thus, hemispheric asymmetries in the processing of spatial frequencies depend on the task. These results support Sergent’s (1982) spatial frequency hypothesis, but only when the computational demands of the task exceed those required for the simple detection of the stimuli.