Failure to Obtain Hemifield Differences in the Discrimination of Spatial Parameters

Abstract

Abstract: : Purpose: The two hemispheres of the human brain have different specialisations and it has often been held that these specialisations can be revealed by confining target stimuli to one or other visual hemifield. Our purpose was to test whether there do exist systematic hemifield differences in the discrimination of spatial frequency, of orientation, or of contrast. Method: In every case we presented two simultaneous Gabor patches (2 degrees in diameter) for 150 ms on each trial. The two stimuli always lay on an imaginary circle of radius 5 deg, centered on the fixation point. In the case of spatial frequency, we required the observer to report whether the more clockwise Gabor patch was of higher or lower spatial frequency than the less clockwise. In the case of orientation, we asked whether the more clockwise patch was the more tilted of the two and in the case of contrast, whether it was of higher or lower contrast. Left and right presentations were randomly intermixed, and the stimulus sequence also included trials on which one stimulus lay in one hemifield and one in the other. Several stimulus separations (2 - 8 degrees of visual angle) were tested and trials were blocked by separation. Two subjects were the authors; other subjects were naïve as to the purposes of the experiment. Results: There were no systematic right-left hemifield differences for any of the three attributes examined. Nor were thresholds systematically different according to whether the two stimuli fell in same or different hemifields. As previously reported, there was little effect of the separation of the stimuli. Conclusion: We were not able to demonstrate a left-hemifield advantage for discriminating spatial frequency, or orientation, or contrast. It is possible that the effects sometimes reported arise from attentional biases that are secondary to the primary task and that the present experiments more successfully ensured that the subject's attention was evenly distributed between hemifields.

Keywords: 586 spatial vision • 326 attention