On the functional order of binocular rivalry and blind spot filling-in

Abstract

Binocular rivalry is an important phenomenon for understanding the mechanisms of visual awareness. Here we assessed the functional locus of binocular rivalry relative to blind spot filling-in, which is thought to transpire in V1, thus providing a reference point for assessing the locus of rivalry. We conducted two experiments to explore the functional order of binocular rivalry and blind spot filling-in. Experiment 1 examined if the information filled-in at the blind spot can engage in rivalry with a physical stimulus at the corresponding location in the fellow eye. Participants' perceptual reports showed no difference between this condition and a condition where filling-in was precluded by presenting the same stimuli away from the blind spot, suggesting that the rivalry process is not influenced by any filling-in that might occur. In Experiment 2, we presented the fellow eye's stimulus directly in rivalry with the 'inducer' stimulus that surrounds the blind spot, and compared it with two control conditions away from the blind spot: one involving a ring physically identical to the inducer, and one involving a disc that resembled the filled-in percept. Perceptual reports in the blind spot condition resembled those in the 'ring' condition, more than those in the latter, 'disc' condition, indicating that a perceptually suppressed inducer does not engender filling-in. Thus, our behavioral data suggest binocular rivalry functionally precedes blind spot filling-in. We conjecture that the neural substrate of binocular rivalry suppression includes processing stages at or before V1.

Keywords: Binocular rivalry; Blind spot; Filling-in; Visual awareness.

Figure 1

(a&b) The physical stimuli, (c) experimental setup, and (d) percept categories of Experiment 1. (a) The ON condition: the inner edge of the ring was inside the blind spot (white dashed line). (b) The OFF condition: stimuli were presented away from the blind spot. Note the white dashed lines are for illustration purpose only; they were not presented during the experiment. (c) Experimental setup. (d) Possible percepts for Experiment 1. Participants reported percepts of red stimuli, hybrid stimuli, or green stimuli by holding one of three keys. The top row of this panel corresponds to the ON condition; the bottom row to the OFF condition.

Figure 2

Results for Experiment 1: proportions of reporting three percepts. Error bars are the estimated within-subject standard error following the method of Loftus and Masson (1994).

Figure 3

The (a&b) physical stimuli and (c) possible percepts of Experiment 2. Participants reported percepts of red stimuli, hybrid stimuli, or green stimuli by holding one of three keys.

Figure 4

Results of Experiment 2: (a) absolute measure and (b) relative measure of the proportion of the hybrid percept. Error bars are the estimated within-subject standard error following the method of Loftus and Masson (1994).