When two eyes see different grays, with Alan Ho

Free fuse the two columns of gray squares in (a), so that the left eye sees column L and the right eye sees column R (or vice versa: it doesnt matter), to give a single vertical column of gray squares. Note the perceived brightness of the squares. On a light background (a), the middle square (arrowed) is the same to both eyes and probably looks lighter than the squares above and below. However, on a black background (b) the middle square probably looks darker than those above and below. Squares in (a) and (b) are actually identical, only the backgrounds differ.

What s going on? The stimulus presents a light square to one eye and a dark square to the other eye, so that the two luminances always sum to a constant. So if your eyes (and your web browser) were linear, all fused squares would look the same brightness. However, the visual system is non-linear, and systematically overweights the square with the higher contrast (not luminance), favouring dark squares on a light background in (a), and favouring light squares on a dark background in (b). Careful measurements have shown that the weighting function is quadratic for light squares (spatial increments) but square, or winner take all, for dark squares (spatial decrements), as shown in the graphs.

Adaptation to Flicker and Spatial Frequency, with Sae Kaneko & Debbie Giaschi

[quicktime width=”500″ height=”400″]http://quote.ucsd.edu/anstislab/files/2012/11/adaptflicker.mov[/quicktime]

Run the movie & fixate the red cross. Both gratings are the same, but following adaptation to spatially uniform flicker, the upper grating looks apparently finer. Reason: Adaptation of transient pathways that are tuned to high temporal and low spatial frequency.

At first, this ambiguous motion stimulus looks like four pairs of dots, each rotating about their common center, but after a while it perceptually reorganizes into two large squares (with a dot at each corner) floating over each other. These local and global forms of “common fate” often alternate; on a 30s trial, local motion is usually seen first, followed by global motion. And across a series of trials, global motion is gradually seen more often. This suggests two adaptation (or learning) processes with different time constants.

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Above: These cubes are readily organised perceptually into two large squares

Conversely, the lovers gazing into each other’s eyes are seen not as a large female square and large male square but as locally moving pairs.

Each pair of spots is phase shifted by 45 degrees from its neighbors. The blue circles tend to constrain the pairs to remain local

Without the circles, one perceives two intertwined global octagons.

with ROB VAN LIER and MARK VERGEER

Gaze steadily at the cross, ALWAYS! Without moving your eyes, note the colors of the squares (red, green, blue, yellow). Every so often, different colors will flash up briefly. These colors are afterimages–not on the screen, but in your head!

The adapting plaid, below, consists of a blue/yellow vertical grating, superimposed on a red/cyan horizontal grating. After adapting to this plaid, vertical black test lines make the afterimage look yellow/blue, while horizontal test lines make the after image look cyan/pink. Thus one and the same adapting pattern gives differently colored afterimages.

Conclusion: the visual system averages after image colors within but not across luminance test contours.

Below, second-order test stripes defined only by motion give the same BY and RC afterimages.  These are not first-order test contours defined by luminance, but are second-order contours defined only by motion.  But the horizontal bars still look blue/yellow and the vertical bars look red/green.

Above:  The “+” test field looks red and green, while the “Tic-Tac-Toe” test field looks blue and yellow.  All from one and the same adapting field.

What’s the difference?

a:  the rightward motion of a simple black line can be decomposed into 2 vectors, along and across the line.
b: The vector along the line is not seen, so the line is perceived as moving at right angles to its own length.
c:  The rightward motion of a dotted line comprises the same two vectors. However, the black/white contrast along the line is much stronger than the second-order black/gray or white/gray contrast across the line.
d: So now the vector along the line is perceptually weighted more than the vector across the line.
Move a  pencil point slowly to left and right across the dots, and they will appear to move up and down.  Reason: Each pair of black/white dots contains oblique components.  This is a minimal version of Pinna’s expanding/ rotating diamond shapes.

Boogie Woogie Illusion
with Patrick Cavanagh

Gaze at the two black fixation marks in turn.
Bottom left (control condition): The surround is darker than the drifting checkered lines, which are seen normally.
Top right: The surround luminance lies between the luminances of the spots (checkers) that constitute the lines. Now the spots appear to trickle non-rigidly along the lines, as though the spots are moving faster than the lines. Explanation: When the spot luminances straddle the surround luminance, the contrast along the lines (between adjacent spots) is higher than across the lines (between the surround and the spots). The motion with the higher contrast wins, and motion along the lines predominates over the sideways motion of the lines.

[quicktime width=”600″ height=”400″]http://quote.ucsd.edu/anstislab/files/2012/11/cogs1-6_1.mov[/quicktime]
Movie #1: Silvanus P. Thompson, in “Light visible & invisible” (1897) noted that when a cogwheel pattern moves along a CW path, it shows an illusory counter-CW rotation. (I admit that these movies do not work all that well. Try jiggling a paper version.)
[quicktime width=”500″ height=”400″]http://quote.ucsd.edu/anstislab/files/2012/11/cogs2-4.mov[/quicktime]
Movie #2: Low contrast cogs seem to rotate CW. I don’t know why.
[quicktime width=”500″ height=”400″]http://quote.ucsd.edu/anstislab/files/2012/11/cogs3.mov[/quicktime]
Movie #3: Hedgehogs with outward-pointing teeth seem to rotate CW.
[quicktime width=”500″ height=”400″]http://quote.ucsd.edu/anstislab/files/2012/11/cogs4-2.mov[/quicktime]
Movie #4: Simulation & explanation. When only the outer tips of hedgehog teeth are visible, they rotate strongly CW. When only the inner teeth of cogwhels are visible, they rotate strongly CCW.

Each chopstick moves along a clockwise (CW) circular path.  their intersection also appears to move CW, although it really moves CCW.  Conclusion: The CW motion of the tips propagates along the lines to the central intersection. (To avoid interactions, look at each display in turn while covering the others with your hand.)

Similarly, the two black ring stiuli are the same except fo the small whute gaps.  Left: When the gaps float, always lying at ‘6 & 12 o’clock’, one perceives two rings circling each other.  Right: When the gaps rotate with the rings, one perceives a single rotating figure-8.  So local gaps control the global percept.

On the left: Try to track the central intersection of the chopsticks, or the X-junction where the rings intersect.  You cannot!  So pursuit eye movements are not simple dumb clockwork servos, but are controlled by top-down object parsing.

Below: HIRO ITO recorded eye movements tracking the chopstick stimulus with and without a surrounding frame.

With the frame present, CW motion is seen correctly and eye movements (red trace) are accurate.  Without the frame, the intersection appears to move CCW and eyes can no longer track it properly. Tracking errors are 10x greater without the frame!  (Blue spot was not present in original stimulus).

07 PortholeChop2

Diamond Illusion
with Isao Watanabe and Patrick Cavanagh

Below:  All the diamonds are identical. However, each row of diamonds looks darker  than the row above it. This illusion is related to Craik-O’Brien-Cornsweet edges.

Flicker Augmented Contrast
With Alan Ho

Top row:  A gray cross looks apparently dark gray on a white surround, and light gray on a white surround.

Middle row: But a black/white flickering cross looks almost black on a white surround, and almost white on a black surround.
Reason: The visual system selects the phase that has the higher contrast on the surround, and ignores the other phase.
Bottom row:  Two bottom crosses are identical, flickering between red & yellow. Black surround emphasises the light yellow, and white surround emphasises the dark red, so the two crosses look different colors.

The background flickers between light green and dark blue in both halves.  But white bars emphasis the dark blue, while black bars emphasise the light green.  Bars do not induce any colors since they themselves are achromatic.  But they select the background color that is more salient, i.e. has the higher luminance contrast.

Note: The afterimages in both halves are an identical pink color, so afterimages simply depend upon total chromatic stimulation with no salience-based selection.