Attention-dependent representation of a size illusion in human V1

Abstract

One of the most fundamental properties of human primary visual cortex (V1) is its retinotopic organization, which makes it an ideal candidate for encoding spatial properties, such as size, of objects. However, three-dimensional (3D) contextual information can lead to size illusions that are reflected in the spatial pattern of activity in V1 [1]. A critical question is how complex 3D contextual information can influence spatial activity patterns in V1. Here, we assessed whether changes in the spatial distribution of activity in V1 depend on the focus of attention, which would be suggestive of feedback of 3D contextual information from higher visual areas. We presented two 3D rings at close and far apparent depths in a 3D scene. When subjects fixated its center, the far ring appeared to be larger and occupy a more eccentric portion of the visual field, relative to the close ring. Using functional magnetic resonance imaging, we found that the spatial distribution of V1 activity induced by the far ring was also shifted toward a more eccentric representation of the visual field, whereas that induced by the close ring was shifted toward the foveal representation, consistent with their perceptual appearances. This effect was significantly reduced when the focus of spatial attention was narrowed with a demanding central fixation task. We reason that focusing attention on the fixation task resulted in reduced activity in--and therefore reduced feedback from--higher visual areas that process the 3D depth cues.

Figure 1

Psychophysical experiment. Left: To directly compare 2D and 3D size judgments, a 2D ring located outside the 3D scene was adjusted to match the front and back rings. The inner and outer radii of the 2D ring could be adjusted independently. Right: The behavioral effect was quantified by dividing the adjusted size of the 2D ring by the 3D ring size. The front and back rings in the 3D scene were judged to be smaller and larger, respectively, than an equivalent 2D ring. Error bars represent S.E.M.

Figure 2

Sphere versus Ring. (A) A greater distribution of activity in V1 to the perceptually larger back sphere could result from either a positional shift in the cortical representation of the edges of the sphere (top) or an increase in the neural activity to the back sphere in combination with a saturating nonlinearity in the fMRI response (bottom). Note that the idealized activity responses are assuming that fixation is maintained at the center of the sphere. (B) These two alternative explanations can be distinguished by using a ring presented at close and far apparent depths while subjects fixate the center of the ring. Specifically, in the current experiment using rings, we will look for a shift in the location of the maximum response rather than a shift in the distribution.

Figure 3

fMRI results. (A) Cortical activation maps induced by the front and back flickering rings. The back view of the inflated left and right hemispheres from S1 is shown in the upper part. The regions in the yellow boxes are amplified and shown in the lower part. V1 is defined by retinotopic mapping and its boundaries are indicated by the white dashed lines. The green and red regions were activated by the front and back flickering rings respectively (both Ps<0.01, Bonferroni corrected). The overlap between these two regions is shown in dark yellow. Compared with the red region, the green region shifts towards the foveal representation of V1. (B) Peak fMRI signals from the six ROIs in V1 were plotted as a function of eccentricity for four individual subjects and their average. The spatial distribution of V1 activation induced by the back ring (perceptually larger) was shifted towards a more eccentric representation of the visual field in V1, while that induced by the front ring (perceptually smaller) was shifted towards the foveal representation. This resulted in peaks at cortical locations representing different eccentricities. The shift was significantly larger in the attend-to-ring experiment than the attend-to-fixation experiment. *P<0.05. Error bars represent S.E.M.

Figure 4

(A) The perceptual difference between the front and back rings for each subject plotted against the difference in the cortical position of the peak of the fMRI response in the attend-toring condition (left) and the attend-to-fixation condition (right).(B) The fMRI difference in the attend-fixation condition plotted against fixation task performance.