Hemispheric asymmetries in visual search

We conducted two visual search experiments, and found that target-detection accuracy and speed were better when the target was projected to the right hemisphere in the feature search condition and better when the target was projected to the left hemisphere in the feature-conjunction search condition. We propose that the highly efficient, so-called parallel search performance characteristic of feature search is enabled by a broadly distributed, global view of the visual field, and the right hemisphere is more efficient than the left in such global processing. On the other hand, the less-efficient performance characteristic of conjunction search (demonstrated by the set-size effect) involves serial shifts of focused attention, and the left hemisphere is more efficient than the right in such localised attentional processing. We suggest that hemispheric asymmetries observed in visual search are related to the attentional demands of the task, and that we adjust our attentional distribution to fit task difficulty. When the target is very distinct, a global, low-resolution attentional distribution is sufficient, and enables parallel search; but a localised, narrow-aperture attentional distribution is sometimes necessary to find targets that either require feature binding, or are very similar to other "distractor" stimuli in terms of compositional attributes.