Visibility, visual awareness, and visual masking of simple unattended targets are confined to areas in the occipital cortex beyond human V1/V2

Abstract

In visual masking, visible targets are rendered invisible by modifying the context in which they are presented, but not by modifying the targets themselves. Here, we localize the neuronal correlates of visual awareness in the human brain by using visual masking illusions. We compare monoptic visual masking activation, which we find within all retinotopic visual areas, with dichoptic masking activation, which we find only in those retinotopic areas downstream of V2. Because monoptic and dichoptic masking are equivalent in magnitude perceptually, the present results establish a lower bound for maintenance of visual awareness of simple unattended targets. Moreover, we find that awareness-correlated circuits for simple targets are restricted to the occipital lobe. This finding provides evidence of an upper boundary in the visual hierarchy for visual awareness of simple unattended targets, thus constraining the location of circuits that maintain the visibility of simple targets to occipital areas beyond V1/V2.

Fig. 1.

Stimulus layout and psychophysical results. (A) Single-trial examples of the four stimulus conditions (the layout of the stimuli varied slightly in the psychophysical experiments as described in Materials and Methods). (B) Psychophysical target visibility rating in the three target conditions.

Fig. 2.

Localization of visibility-correlated responses to the occipital lobe. (A) An individual brain model from all perspectives, including both hemispheres flat-mapped, overlaid with the functional activation from 17 subjects. The green shaded areas are those portions of the brain that did not show significant activation to Target-Only stimuli. The blue voxels exhibited significant target activation (Target-Only activation > Mask-Only activation). Yellow voxels indicate a significant difference found between Control (target-visible) and SWI (target-invisible) conditions, indicating potentially effective visual masking, and thus a correlation with perceived visibility. (B) Response time-course plots from Control versus SWI conditions in the occipital cortex (occipital masking). (C) Response time-course plots from Control versus SWI conditions in nonoccipital cortex (nonoccipital masking). (D) Response time-course plots from the nonillusory conditions (Target-Only and Mask-Only combined) in occipital versus nonoccipital cortex. (Error bars: (B, C, and D) SEM between subjects.)

Fig. 3.

Examples of retinotopy mapping from two subjects. Visual areas that have been delineated by retinotopic mapping analysis are indicated in different colors. (A) Subject 1. (B) Subject 2.

Fig. 4.

Retinotopic analysis of monoptic versus dichoptic masking. (A) The logic underlying the analysis of masking magnitude for hypothetical retinotopic areas. The Mask-Only response is bigger than the Target-Only response because masks subtend a larger retinotopic angle than targets and are moreover presented twice in each cycle for 100 ms each flash, whereas the target is single-flashed for only 50 ms. If the target response adds to the mask response in the SWI condition (no-masking percept), then the SWI response will be bigger than the Mask-Only response, whereas if the target does not add (masking percept), then the SWI response will be equal to or smaller than (as the mask itself may also be reciprocally inhibited by the target) the Mask-Only response. (B) Monoptic and dichoptic masking magnitude (%BOLD difference of Mask-Only/SWI conditions) as a function of occipital retinotopic brain area, following the analysis described in A. Negative values indicate increased activation to the SWI condition (no masking), whereas values ≥0 indicate decreased or unchanged SWI activation (masking). (C) Dichoptic masking magnitude (%BOLD difference of Mask-Only/SWI conditions) as a function of occipital retinotopic brain area within the dorsal and ventral processing streams. The strength of dichoptic masking builds up as a function of level in the visual hierarchy for both the dorsal (R² = 0.90) and ventral (R² = 0.72) processing streams.

Fig. 5.

Layout of retinotopic areas that potentially maintain awareness of simple targets. An individual brain model from all perspectives, including both hemispheres flat-mapped, overlaid with the functional activation from one typical subject. The yellow shaded areas are those portions of the brain that did not show significant dichoptic masking and thus are ruled out for maintaining visual awareness of simple targets (as in Fig. 4 B and C). The pink-colored voxels represent the cortical areas that exhibited significant dichoptic masking and thus are potential candidates for maintaining awareness of simple targets.