Abstract

            Perception across the visual field is not homogeneous.  Performance is best on the horizontal meridian and is often better in the lower visual field.  Performance is particularly poor on the vertical meridian above the point of fixation (the north effect), and sometimes below the point of fixation (the south effect). Visual field inhomogeneities have been explored with stationary gratings – light and dark bars whose luminance profile varies sinusoidally – where spatial frequency (SF) refers to the number of bars in a finite region of space.  Visual field inhomogeneities become more pronounced as SF increases.  This study explores visual field inhomogeneities with moving stimuli at 2 SFs.  Participants performed a 2-alternative forced-choice direction discrimination task for stimuli presented in the near periphery, while maintaining central fixation on a computer display.  The results indicate a strong north effect at both SFs and surprisingly, a strong south effect at low SF.  The relative magnitude of the north and south effects are the reverse of those observed for stationary stimuli.  One possible explanation is that the magnitude of the inhomogeity is a function of the perceived contrast (visibility) of the stimuli.  We believe that people could not see the low SF gratings as readily and thus that they will not be able to localize the stimuli as well in that condition.  We are currently testing this hypothesis.