A first- and second-order motion energy analysis of peripheral motion illusions leads to further evidence of "feature blur" in peripheral vision

Abstract

Background: Anatomical and physiological differences between the central and peripheral visual systems are well documented. Recent findings have suggested that vision in the periphery is not just a scaled version of foveal vision, but rather is relatively poor at representing spatial and temporal phase and other visual features. Shapiro, Lu, Huang, Knight, and Ennis (2010) have recently examined a motion stimulus (the "curveball illusion") in which the shift from foveal to peripheral viewing results in a dramatic spatial/temporal discontinuity. Here, we apply a similar analysis to a range of other spatial/temporal configurations that create perceptual conflict between foveal and peripheral vision.

Methodology/principal findings: To elucidate how the differences between foveal and peripheral vision affect super-threshold vision, we created a series of complex visual displays that contain opposing sources of motion information. The displays (referred to as the peripheral escalator illusion, peripheral acceleration and deceleration illusions, rotating reversals illusion, and disappearing squares illusion) create dramatically different perceptions when viewed foveally versus peripherally. We compute the first-order and second-order directional motion energy available in the displays using a three-dimensional Fourier analysis in the (x, y, t) space. The peripheral escalator, acceleration and deceleration illusions and rotating reversals illusion all show a similar trend: in the fovea, the first-order motion energy and second-order motion energy can be perceptually separated from each other; in the periphery, the perception seems to correspond to a combination of the multiple sources of motion information. The disappearing squares illusion shows that the ability to assemble the features of Kanisza squares becomes slower in the periphery.

Conclusions/significance: The results lead us to hypothesize "feature blur" in the periphery (i.e., the peripheral visual system combines features that the foveal visual system can separate). Feature blur is of general importance because humans are frequently bringing the information in the periphery to the fovea and vice versa.

Figure 1. Motion energy analysis for a solid dropping disk.

A) A series of frames depicting a solid dropping disk. B&C) First-order motion plots in the x-t and y-t planes. D&E) Projections of DC-removed and rectified second-order dropping disk movie in the x-t and y-t planes. F–I) Fourier analysis of the first-order and second-order motion energy of the solid dropping disk in the fx-ft and fy-ft planes.

Figure 2. Motion energy analysis for the peripheral escalator illusion (See Movie S1).

A) A single frame of the peripheral escalator illusion. The background is a stationary gradient. The three blurred columns shift horizontally back and forth. The columns are perceived as drifting horizontally when viewed in the fovea, but obliquely when viewed in the periphery. B&C) First-order motion plots in the x-t and y-t planes. D&E) Projections of DC-removed and rectified second-order peripheral escalator movie in the x-t and y-t planes. F–I) Fourier analysis of the first-order and second-order motion energy of the peripheral escalator movie in the fx-ft and fy-ft planes.

Figure 3. Motion energy analysis for the peripheral acceleration illusion (See Movies S2).

A) A single frame of the peripheral acceleration illusion. Ovals drift from left to right across the screen. Inside each oval is an internal gradient that moves faster than the oval and in the same direction as the oval. When viewed foveally, observers can separate the ovals and internal grating; when viewed peripherally, the ovals appear to accelerate, and the interior of the oval appears fixed. B&C) First-order motion plots in the x-t and y-t planes. D&E) Projections of DC-removed and rectified second-order peripheral acceleration movie in the x-t and y-t planes. F–I) Fourier analysis of the first-order and second-order motion energy of the peripheral acceleration movie in the fx-ft and fy-ft planes.

Figure 4. Motion energy analysis for the peripheral deceleration illusion (See Movie S3).

A) A single frame in the peripheral deceleration illusion. Ovals drift from left to right across the screen. Inside each oval is an internal gradient that moves in the direction opposite to the motion of the oval. When viewed foveally, observers can separate the ovals and internal grating; when viewed peripherally, the speed of the ovals is determined by the internal motion, yet it is difficult to see the motion of the internal gradient. In the supplementary movie, the observer can adjust the speed of the internal grating. When the gradient is faster than the ovals, a shift from foveal to peripheral viewing will make the ovals appear to accelerate; when the gradient is slower than the ovals, a shift from foveal to peripheral viewing will make the ovals appear to decelerate and even reverse direction (creating the paradoxical view that the ovals are moving slowly leftward, yet somehow get to the far right side of the screen). B&C) First-order motion plots in the x-t and y-t planes. D&E) Projections of DC-removed and rectified second-order peripheral deceleration movie in the x-t and y-t planes. F–I) Fourier analysis of the first-order and second-order motion energy of the peripheral deceleration movie in the fx-ft and fy-ft planes.

Figure 5. The rotational reveals illusion (see Movies S4).

A ring of ovals moves counter-clockwise. Within each oval is an internal gradient that moves clockwise. When viewed foveally, the ring appears to rotate counter-clockwise, but when viewed peripherally, the ring appears to rotate clockwise. When the background is near the same luminance as the white or black of the internal gradient, the ring appears jumbled when viewed peripherally.

Figure 6. The Disappearing Squares illusion (see Movies S5).

A) A 16×12 array of Kanizsa pacmen rotate in opposite directions so as to continually assemble/disassemble arrays of Kanizsa squares. B) Two observers adjusted the radius of the circles to encompass the range of visible squares as a function of rotation rate of the pacmen. The results for each observer are indicated by the squares and filled circles. As the rotation rate increases, the peripheral range over which the illusory squares can be seen decreases.