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What our Eyes Get up to When We Read
17 Sep, 2007 11:35 am
In this article I briefly discuss some recent research from our laboratory that investigated how we coordinate our eyes binocularly when we read. Our experiments showed that on about half of the fixations a reader makes the eyes fixate on average two letters apart. We also showed that readers construct a unified representation of the text through a process of fusion rather than suppression.
Eye movements are very valuable to scientists interested in understanding how we read. Although we may have the feeling that our eyes move very smoothly when we read, in fact, they actually make a series of very rapid, sharp movements interspersed with short periods where they are quite still. The fast movements are called saccades, and the periods when the eyes are still are called fixations (these usually last about a quarter of a second). Readers extract the visual information that they require to read during fixations, and the precise patterns of fixations and saccades that people make when they read sentences can tell us a lot about the psychological processes underlying written language comprehension. For example, when people read difficult words they often make longer fixations on those words. Also, they are more likely to make repeated fixations on tricky words or make saccades to look back and re-read portions of the text that were difficult to understand. Thus, through measuring eye movements cognitive psychologists have learnt a lot about the psychology of reading (for a very thorough review of the research in this area see the 1998 Psychological Bulletin paper by Professor Keith Rayner).
Some recent work from our laboratory that we have published concerns how we move both eyes in relation to each other when we read. Binocular coordination, as it is termed, is an aspect of reading that has not been investigated to a great degree. This is largely due to a widely held assumption that when we fixate a word during reading, we look at the same letter in the word with both the left and the right eye. In fact, our research has shown quite clearly that this is not the case on nearly half of the fixations we make when we read. Also, we have shown that when the eyes are not fixating the same letter of a word, they are on average about two letters apart (we have observed this in both adults and children). Although this difference might sound small, in fact it represents quite a substantial difference in terms of the precise representation, or in simpler terms, “picture” of the word that each eye delivers to the brain. Most readers do not normally experience double vision when they process text – instead we experience a single, clear and crisp visual representation (otherwise, how could we see the words properly to identify them, right?). Given our perceptual experience, we are now faced with a stimulating question: How is it that we can be looking at different letters with each eye and therefore obtaining a different visual representation for each eye, yet we experience a single unified word?
There are two ways that we can achieve this psychologically: One possibility is that we suppress one of the two inputs from the eye. That is to say, the visual system uses only one of the two inputs as the basis for our single perceptual experience and ignores the other. The second possibility is that the two different images from each eye may be somehow combined, or fused, during visual processing to provide a single image of the word that is being looked at. In our experiments we incorporated a sophisticated presentation manipulation whereby we were able to present just half of a target word in each sentence separately to each eye (a so-called dichoptic presentation method). For example, when reading the sentence “the man saw the cowboy in the street”, when readers fixated the word “cowboy” they would see the word cowboy quite clearly even though the letters “cowb” were presented exclusively to the left eye, and the letters “wboy” were delivered exclusively to the right eye. Note that the “wb” of each word portion overlapped so that the letters of the word were appropriately spatially arranged.
By recording readers’ binocular eye movements and examining saccadic targeting when people read sentences containing words presented in this way, we were able to show that the two word parts were fused together. Thus, our findings were consistent with the theoretical claim that we experience a single visual representation of a word due to fusion of the two different representations from each eye.
What our eyes get up to while we read, The BA Festival of Science in York, September 14
Liversedge, S.P., Rayner, K., White, S.J., Findlay, J.M., & McSorley, E. (2006). Binocular coordination of the eyes during reading. Current Biology, 16, 1726-1729.
Juhasz, B.J., Liversedge, S.P., White, S.J., & Rayner, K. (2006). Binocular coordination of the eyes during reading: Word frequency and case alternation affect fixation duration but not fixation disparity. Quarterly Journal of Experimental Psychology, 59, 1614-1625.
Liversedge, S.P., White, S.J., Findlay, J.M., & Rayner, K. (2006). Binocular Coordination of Eye Movements during Reading. Vision Research, 46, 2363-2374.
Blythe, H.I., Liversedge, S.P., Joseph, H.S.S.L., White, S.J., Findlay, J.M., & Rayner, K., (2006). The binocular coordination of eye movements during reading in children and adults. Vision Research, 46, 3898-3908.
Rayner, K. (1998). Eye movements in reading and information processing: 20 years of research. Psychological Bulletin, 124, 372–422.
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