The dynamics of interocular suppression

When the two eyes are presented with dissimilar images, the brain has to select one percept for awareness while suppressing the other. Interocular suppression describes the loss of Visibility of one image in favour of its competitor, and can be seen as a mechanism for understanding how, why, and Where percept selection occurs within the Visual system. This thesis addresses how and where interocular suppression takes place. By comparing the time courses of interocular With intraocular suppression, that is, Visibility loss due to conflicting images presented to only one eye, the major goal of the thesis is to show that interocular suppression occurs in two stages along the visual pathway. Four lines of experimental evidence are presented. When Viewing a monocular conditioning stimulus, the abrupt onset of a brief stimulus to the opposite eye results in a switch in perception to the new stimulus. This phenomenon is known as flash suppression. The first study (Chapter 4) investigated flash suppression under monocular and dichoptic viewing conditions to provide the intraocular and interocular time courses, respectively. This was carried out by probing Visual sensitivity to a test stimulus before, during, and after the appearance of the flash stimulus. The time course measured was the variation of threshold across time. The intraocular time course had a single elevation, a transient peak occurring Close to the time the flash stimulus was introduced. The interocular time course, on the other hand, had two elevations: the first peak was similar to that of the intraocular time course, and the second was a sustained peak starting about 100 ms later. The second study (Chapter 5) used visual masking as a technique for eliciting intraocular and interocular suppression, through monocular and dichoptic masking, respectively. In the dichoptic masking condition, one eye was presented with a masking stimulus for 100 ms. After a varying inter-stimulus interval, a brief test stimulus was presented to the other eye. The contrast threshold of the test stimulus was measured for each inter-stimulus interval. For monocular masking, both masking and test stimuli were presented to the same eye. The results showed a two-staged time course for interocular suppression, which was not apparent in intraocular suppression. Additionally, interocular suppression was more prolonged than intraocular suppression. The third study (Chapter 6) measured suppression using a different approach to the first and second studies. The experiment investigated crossorientation interactions using a stream of rapidly-changing grating orientations displayed to one eye and an independent stream to the other eye. One orientation was nominated as the target, and participants pressed a key when they saw the target. Using a reverse correlation technique, probability densities of two orientations were found. The first, 61, preceded the key-press by the reaction time, and the second, 02, preceded 61 by several hundreds of milliseconds. Analysis of the data examined the cross-orientation interactions between 6] and 92 for grating streams presented to the same eye (intraocular effect), and to different eyes (interocular effect). Despite the differences in experimentation methods between this and the masking study, the prolonged interocular suppression time course was once again apparent in the cross-orientation experiments