Detection of narrow bars when presented in the right and left visual hemifields

Three series of psychophysical experiments were performed to study the effects of additional images on the contrast thresholds for detection of narrow bars of width 1 arc min and length 1 arc degree presented randomly to the left and right sides of the fixation point with an eccentricity of 4 degrees. The additional images were bars of the same size but with lower contrast, presented to the right and left of the test bar at varying distances, i.e., 1-2 degrees. Different series used different contrast ratios for the test and additional bars. The first series revealed significant predominance of one visual hemifield over the other in performing the bar detection task, though the predominant side varied in different subjects. This predominance disappeared or changed to the opposite side in the second or third series. There was a tendency for the additional images to have asymmetrical influences on the detection threshold for the test bar: additional bars had more extensive inhibitory influences when they were on the peripheral side of the test bar. These data provide evidence for the absence of specificity in hemisphere operation in the detection task. The mechanism of description of peripheral images is discussed.