Interactions between Surround Suppression and Interocular Suppression in Human Vision

Several types of suppression phenomena have been observed in the visual system. For example, the ability to detect a target stimulus is often impaired when the target is embedded in a high-contrast surround. This contextual modulation, known as surround suppression, was formerly thought to occur only in the periphery. Another type of suppression phenomena is interocular suppression, in which the sensitivity to a monocular target is reduced by a superimposed mask in the opposite eye. Here, we explored how the two types of suppression operating across different spatial regions interact with one another when they simultaneously exert suppressive influences on a common target presented at the fovea. In our experiments, a circular target grating presented to the fovea of one eye was suppressed interocularly by a noise pattern of the same size in the other eye. The foveal stimuli were either shown alone or surrounded by a monocular annular grating. The orientation and eye-of-origin of the surround grating were varied. We found that the detection of the foveal target subjected to interocular suppression was severely impaired by the addition of the surround grating, indicating strong surround suppression in the fovea. In contrast, when the interocular suppression was released by superimposing a binocular fusion ring onto both the target and the dichoptic mask, the surround suppression effect was found to be dramatically decreased. In addition, the surround suppression was found to depend on the contrast of the dichoptic noise with the greatest surround suppression effect being obtained only when the noise contrast was at an intermediate level. These findings indicate that surround suppression and interocular suppression are not independent of each other, but there are strong interactions between them. Moreover, our results suggest that strong surround suppression may also occur at the fovea and not just the periphery

Results of Experiments 1 and 2.

<p>Individual detection thresholds of the target grating for each surround condition are shown for (A) Experiment 1 and (B) Experiment 2. Averaged results (n=5) for the two experiments are shown in panel (C). Horizontal axis represents different surround conditions: NS, no surround; MP, monoptic parallel surround; MO, monoptic orthogonal surround; DP, dichoptic parallel surround; and DO, dichoptic orthogonal surround. Suppression factors, defined as the ratio of with-surround to no-surround thresholds, for each with-surround condition of Experiments 1 and 2 are shown in panel (D). An iconic depiction of the stimuli for each condition is illustrated below the horizontal axis (note that the black ring is not shown in iconic depictions). Error bars represent standard error of the mean.</p

Results of Experiment 3.

<p>Detection thresholds (upper panels) and suppression factors (lower panels) are plotted as functions of noise contrast for each surround condition. Results for three participants are arranged in columns; the last column shows the average result. An iconic depiction of the stimuli for each condition and the corresponding symbol are shown above the plots. Error bars represent standard error of the mean.</p

Experimental design.

<p>(A) Stimuli and sequence of events for a typical trial in Experiment 1. Observers judged which interval contained the center target grating, which was masked by a noise pattern superimposed with a black ring in the opposite eye. The target grating was either presented alone (no-surround condition) or surrounded by an annular grating (with-surround conditions). The surround and target gratings were either parallel or orthogonal to each other, and they were presented to either the same eye (monoptically) or the opposite eye (dichoptically). The figure shows the monoptic parallel condition (MP). Other conditions are not shown. The cosine curve depicts the contrast modulation of the center target in time. The five black points in each display were always presented to aid binocular alignment, and the central one served as the fixation point. (B) Typical stimuli displayed in the target interval of trials in Experiment 2. The only difference with regard to Experiment 1 was that the black ring was presented binocularly and superimposed on both the target and the noise pattern. With such a design, the ring was expected to promote summation of inputs from two eyes, thereby releasing interocular suppression.</p