A role in Poggendorff illusions for eye movement planning
In moving to consider possible physiological processes responsible for these proposed axial effects, we must first re-admit to the discussion of these figures an agency that has hovered on the margins of debate, only to be robustly rejected from the mainstream - the pre-attentive signalling of eye movements. I will now present some informal evidence to add to the case that at the very least eye movements cannot be excluded from any full account of these figures.
Poggendorff illusions are especially puzzling in relation to eye movements. It seems established that the misalignments observed cannot depend on errors in realised saccades, since they appear even when the figures are fixed as after-images on the retina, moving when the eye moves, so that scanning is impossible (Evans and Marsden 1966). Nonetheless they do seem to be affected by whereabouts in a figure we fixate and how we scan (Novak 1966; Prytulak 1973; Virsu 1971; Wenderoth et al. 1978). It has therefore been suggested that they might involve neural activation associated with the pre-attentive planning of saccades.
For example, some years ago I noticed that when the Poggendorff without parallels, with arms at 22.5 degrees from vertical, is adorned just with blurry blobs as to the right, misalignment seems enhanced.
Misalignment in the Poggendorff without parallels can appear enhanced just by adding blurry blobs to mark the approximate vertical axis.
That seemed an interesting example of the way that different figures can arouse the cardinal axis effect at different levels, and I did some experiments to measure the effect of the blobs, with a small sample of observers. (The result showed an increase of average misalignment from about 0.5 degrees without the blobs to around 2 degrees once they were added). One exceptionally acute observer however noted that the effect depended where in the figure he fixated. That chimed with a similar observation made earlier by another observer in a different study, to which I had paid little attention at the time. I therefore went back to ten other of the observers of the blurry blob effect and asked them to note the extent of misalignment first when fixating one of the blobs, and then when fixating a line end, and to report any variation in illusion. Eight reported increased effect with fixation on a blob (the other two saw no difference). The enhancement seems to me quite strong if I fixate one or other blob, but strongest if I scan between the two blobs, switching from one to the other every two seconds or so. It is weakest if I scan between the line ends, or fixate the centre of the figure.
For me there is a stronger effect still with full Poggendorff figures. In the figure to the right, try scanning between the angles, noting that you can still sense the extent of misalignment of the test segments as you do so. To my eye illusion is dramatically reduced when I scan obliquely between the acute angles.
Deliberate scanning between the blobs, switching fixation between blobs at about one second intervals, strikingly reduces misalignment for the author when the blobs are within the acute angles, as to the right.
These are informal observations, but they tie up with a subjective report in a study from about a century ago. Two researchers, after training themselves (with a heroic 3,200 alignments) to defeat Poggendorff misalignments by deliberate visual tracking up the line of the oblique, both independently observed "that even after the illusion had been overcome, it was possible to take a kind of general view of the figure, and to see the illusion again." (Cameron and Steele 1905).
The idea that some kind of effect associated with prominent saccade targets is at work also seems consistent to my eye with appearances in the figure below.
Even without deliberate scanning, misalignment appears to the author reduced when the axis between acutes is emphasised, at (b) though not to the extent of reversal of effect, as at (c) in the absence of parallels. At (e), with vertical emphasis added to the test figure, misalignment is enhanced. The effects seem consistent with a role for eye movement planning.
Here the strength of misalignment seems affected, in finely balanced versions of the Poggendorff figure, just by adjusting the salience of saccade targets. In these examples no deliberate scanning is involved. In the figure at (a), with the obtuse angles weighted, the usual Poggendorff shift appears. Weighting the acute angles at (b) seems to leave the test lines pretty much aligned. Yet this is not because at (b) the figure has in effect been reduced just to opposed acute angles. At this orientation these tend to give a small reversed misalignment, in the opposite direction to the standard Poggendorff effect, as at figure (c). Then at (d), to my eye the test lines are pretty much aligned, whereas when the figure is given a salient vertical emphasis at (e), once again a small reversed misalignment appears, (upper bar shifted left, lower bar right), of just about the width of the test bar.