Eye Tracking

Eye tracking is the ability to control the fine eye movements when following a line of print, which is especially important with reading [1]. Children with poor eye tracking control are often slow readers, have poor fluency, miss and skip words and lines, use their finger as a marker, have difficulty copying from the blackboard and often report that words appear to move on the page when reading.

When we read, our eyes don’t move smoothly across the line, instead they make a series of jumps and pauses. The small jumps between words or groups of words are called saccades. The brief pause we make while looking at the words is called a fixation. After a fixation, we move our eyes to the next word or group of words—another saccade.

The sentence below is diagrammed into a typical group of fixations and fixational pauses. The asterisks represent where the eye fixations most likely land during reading which is usually near the front of a word.

The average saccade when reading is 7 to 9 letter positions. Although the majority of words are fixated when reading many words are skipped. For example words of 2 to 3 letters in length are only fixated about 25% of the time which may help explain why small words are commonly confused such as ‘on’ and ‘of’. Whereas words of 8 letters in length are nearly always fixated. Although most saccades in English are from left to right due to the direction in which we read about 10-15% of saccades are from right to left which are known as regressions, such as those made back to the start of the next line. Studies have shown that poor readers make more regressions or backtrack through the print more so than above average readers[2].

Eye tracking skills mature with age in fact it is believed that 2/3 of the total development of eye tracking skills is complete by 9 years of age. Young children demonstrate more frequent small saccades, which take longer to initiate and are generally more inaccurate than in older children and adults.

The very precise coordination of jumps (saccades) and pauses (fixations) is controlled by our central and peripheral visual systems. Our central vision processes what we’re seeing in clear detail and defines what we’re looking at. Our peripheral, or side vision, simultaneously locates surrounding objects and let’s us know where to look. (These two systems are sometimes referred to as the "Where is it?" and "What is it?" systems). In reading, our central vision processes the word, while our side vision locates the following word and tells us where to aim our eyes next. The integration of these two systems is what allows us to efficiently move our eyes along a line of print without overshooting or undershooting, or mistakenly aiming our eyes at lines above or below. If there is not continuous, fluid, simultaneous integration between these two systems, reading will be jerky, loss of place will be common, and comprehension will be poor. Consider the following paragraph.

On the second line note how the eyes have made an overshoot from the beginning of the word 'healthy' to the beginning of the word 'seem', effectively skipping the words 'body' and 'may'. This has forced a regression back to the middle of the word 'healthy' and a second compensating saccade to the word 'body'. Note also how the eyes have skipped the small words such as 'in' and 'the'. This is actually very common with proficient readers, we train ourselves to look only at high visibility words such as 'healthy' and our brain effectively fills in the missing information. We learn to read by context rather than word by word.

Recent research has identified two possible routes in the brain which control the initiation of saccades, the sub-cortical route and the cortical route. The sub-cortical route includes the brainstem which is a more primitive part of the brain which is involved in the initiation of reflexive saccades whereas the cortical route involves saccades that are voluntarily initiated. Many children with learning difficulties have difficulty initiating and controlling their saccadic eye movements and are often affected by reflexive saccadic intrusions; not only do these unwanted saccadic intrusions affect the ability to move the eyes smoothly and accurately but it also affects the ability to maintain fixation.

Fixation stability is essential for maintaining attention on any given task. In fact it is believed that the brain areas involved in attentional orienting overlap with those areas involved in the generation of saccades. Children with ADHD have particular difficulty maintaining fixation. Studies [3] have shown that children with ADHD have a reduced ability to suppress unwanted saccades and are less able to control their fixation voluntarily, therefore they are highly distracted by visual stimuli in their peripheral vision.

Dyslexic children also have significantly worse eye movement stability during fixation of small targets than normal readers [4],[5],[6], which may help explain why they skip or confuse small words when reading.

Studies have shown that dyslexic readers also;

* Take longer to initiate saccades [7].

* Perform a greater number of saccades when reading [8].

* Have smaller saccades than normal readers when reading long words [9].

* Require a greater amount of time to fixate on words [10].

The implications of these results are important. For example when we perform a saccade new information cannot be processed because the eyes are moving so quickly across the stable visual stimulus that only a blur would be perceived, this phenomenon is known as saccadic suppression. Therefore if a child is forced to make more saccades than the average child their age when reading they will have greater periods where they are not processing new information which results in a slower reading rate. Their reading rate will slow even further considering that it takes longer to initiate saccades in the first place and the fact that their saccades are also more inaccurate.

Research has demonstrated that vision therapy is extremely valuable at improving eye tracking control. [11], [12], [13]

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[1] Pavlidis GT, Teflioudi A. "Do ophthalmokinesis and reading develop in parallel?" European Conference on Eye Movements 10, Utrecht University, The Netherlands, Sept 1999.

[2] Murray WS, Kennedy A. Spatial coding in the processing of anaphor by good and poor readers. Evidence from eye movement analysis. Quarterly Jounal of Experimental Psychology 1988, 40A, 693-718.

[3] Munoz DP, Armstrong IT, Hampton KA and Moore KD, "Altered Control of Visual Fixation and Saccadic Eye Movements in Attention-Deficit Hyperactivity Disorder." J Neurophysiol, 2003 (90): 503-514.

[4] Eden GF, Stein JF, Wood HM, Wood FB, "Differences in eye movements and reading problems in dyslexic and normal children." Vision Research 1994 May; 34(10):1345-58.

[5] Fischer B, Hartnegg K. "Stability of gaze control in dyslexia". Strabismus 2000 Jun;8(2):119-22.

[6] Raymond JE, Ogden NA, Fagan JE, Kaplan BJ. "Fixational instability and saccadic eye movements of dyslexic children with subtle cerebellar dysfunction". : Am J Optom Physiol Opt 1988 Mar;65(3):174-81.

[7] De Luca M, Di Pace E, Judica A, Spinelli D, Zoccolotti P. "Eye movement patterns in linguistic and non-linguistic tasks in developmental surface dyslexia". Neuropsychologia 1999 Nov;37(12):1407-20.

[8] De Luca M, Di Pace E, Judica A, Spinelli D, Zoccolotti P. "Eye movement patterns in linguistic and non-linguistic tasks in developmental surface dyslexia". Neuropsychologia 1999 Nov;37(12):1407-20.

[9] De Luca M, Borrelli M, Judica A, Spinelli D, Zoccolotti P. "Reading words and pseudowords: an eye movement study of developmental dyslexia". Brain Lang 2002 Mar;80(3):617-26.

[10] De Luca M, Di Pace E, Judica A, Spinelli D, Zoccolotti P. "Eye movement patterns in linguistic and non-linguistic tasks in developmental surface dyslexia". Neuropsychologia 1999 Nov;37(12):1407-20.

[11] Fischer B, Hartnegg K, "Effect of Visual Training on Saccade Control in Dyslexia." Perception, 29(5):531-542, 2000.

[12] Rounds BB, Manley CW, Norris RH "The effect of oculomotor training on reading efficiency." J Amer Optom Assoc,. 1991 (62): 92-97.

[13] Kulp MT, Schmidt PP "Effect of oculomotor and other visual skills on reading performance." Optom Visual Sci, 1996 (73): 283-292.

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