will j harrison

all about my active-vision research

me publications

New paper: Visual crowding is anisotropic along the horizontal meridian during smooth pursuit

My latest paper testing the interaction between eye movements and object recognition has been published at Journal of Vision. You can read the whole thing for free, and download the PDF, via the journal’s website. I co-authored this paper with PhD advisors, Roger Remington and Jason Mattingley, and this happily means all chapters of my thesis have been published in journals.

In this paper, we measured how a particular kind of eye movement affects peripheral vision. We typically move our eyes in one of two ways: 1) we make fast “saccadic” eye movements — reading this text your eyes will be rapidly jumping from word to word; and 2) we make “smooth pursuit” eye movements to track moving objects — imagine watching a bird fly through the sky.

In this movie, when you watch the dot move across the screen, you’ll be using smooth pursuit eye movements:  smooth_pursuit_demo.mov

The quality of motion in that movie isn’t great, but hopefully when you track the dot in the next movie you’ll feel your eyes are doing something quite different than before – this time you’ll be making saccades:  saccade_demo.mov

We were interested in the former type of eye movement, smooth pursuit, and how these movements affect “visual crowding”, an interesting case in which visual perception is highly limited. In the image below, stare at the dot in the centre.

crowding demo

While staring at the dot (and keeping your eyes still!), you’ll probably find it pretty easy to identify the letter “A” in the right circle, whereas the left circle looks like it’s filled with a bunch of random white lines. If you move your eyes around, you’ll see that the same letter “A” appears in both circles, and is as easy to see in the left circle as the right when you move your eyes around. This simple demonstration shows us that our ability to recognise objects (e.g. a letter) in peripheral vision depends on what other information is surrounding the object. The difficulty identifying an object when it’s surrounded by distracting information is called “crowding”.

Here’s another example. There are four concentric circles on each side of the display, and the three inner circles all have gaps in one side. While still looking at the dot in the centre, can you see where the gap is in the inner most circle on the right side of the picture? How about the left side?

concentric rings crowding demo

Something interesting you may notice when staring at the centre dot in the above image is that, not only is it difficult to identify where are the gaps in the rings on the left, but it’s difficult to tell which ring is which colour. You probably can see that there are gaps somewhere, and it’s quite obvious that there are blue and pink rings, but it’s really hard to tell which rings are pink or blue. This demonstration therefore shows us that we are not completely blind to crowded objects – we get a good impression of detail, but the detail gets “mixed up”.

So we tested whether visual crowding is altered during smooth pursuit eye movements. There are some interesting reasons why we expected crowding may be different during pursuit, but I won’t go into these — read the paper’s introduction if you’re interested. In particular, we were interested in whether crowding is different for objects positions ahead of the pursuit target versus behind the pursuit target. Image the dot in the picture below is moving rightward – observers would have to pursue the dot with their eyes, and then (on separate trials) try to identify the crowded letter behind the dot (to the left in this picture) or in front of the dot (to the right in this picture).

In short, we found that there is more crowding for objects opposite to the direction of pursuit, than for objects in the same direction as pursuit. We know that crowding got worse opposite to the direction or pursuit, not that crowding was released (ie. the target was easier to see) in the same direction as pursuit, because we included conditions in which participants did not move their eyes at all. When participants kept their eyes still, crowding was the same as crowding in the same direction as pursuit; crowding opposite to the direction of pursuit was worse than when no eye movements were made.

Why is crowding worse opposite to the direction of an eye movement? This is an open question (assuming our results can be independently verified). My hypothesis, that we put forward in the published paper, is that objects opposite to the direction of pursuit could distract you from making the eye movement, and so the visual system sort of “degrades” how visible they are. We have some evidence for this too: the change in crowding only applied to objects quite close to the pursuit target; when we moved the target farther into peripheral vision, there was no directional change in crowding. Put differently, the objects close to the thing you’re trying to pursue with your eyes, but that are in a position irrelevant to the eye movement, are not as important to you as other objects. What do you think?

And if you’re curious how saccades, the other type of eye movements, affects visual crowding, we’ve published that study too.

The full reference to my Journal of Vision paper is:

Harrison, W.J., Remington, R.W. & Mattingley, J.B. (2014). Visual crowding is anisotropic along the horizontal meridian during smooth pursuit. Journal of Vision 14(1):21, 1-16. http://www.journalofvision.org/content/14/1/21.fulldate

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