Asymmetric representations of upper and lower visual fields in egocentric and allocentric references.

Two spatial reference systems, i.e., the observer-centered (egocentric) and object-centered (allocentric) references, are most commonly used to locate the position of the external objects in space. Although we sense the world as a unified entity, visual processing is asymmetric between upper and lower visual fields (VFs). For example, the goal-directed reaching responses are more efficient in the lower VF. Such asymmetry suggests that the visual space might be composed of different realms regarding perception and action. Since the peripersonal realm includes the space that one can reach, mostly in the lower VF, it is highly likely that the peripersonal realm might mainly be represented in the egocentric reference for visuomotor operation. In contrast, the extrapersonal realm takes place away from the observer and is mostly observed in the upper VF, which is presumably represented in the allocentric reference for orientation in topographically defined space. This theory, however, has not been thoroughly tested experimentally. In the present study, we assessed the contributions of the egocentric and allocentric reference systems on visual discrimination in the upper and lower VFs through measuring the manual reaction times (RTs) of human subjects. We found that: (a) the influence of a target's egocentric location on visual discrimination was stronger in the lower VF; and (b) the influence of a target's allocentric location on visual discrimination was stronger in the upper VF. These results support the hypothesis that the upper and lower VFs are primarily represented in the allocentric and egocentric references, respectively.