First, the discussed study confirmed the previous observation (i.e., a higher prevalence of unstable binocular fixation in dyslexic subjects).
9–11,24,26,53 Studies have shown that dyslexic children show a more unstable coordination of both eyes (higher variability of fixation disparity) when measured with both subjective and objective methods. Jaschinski et al.
26 reported an increased variability of FD in children with reading disability and writing impairment when using psychophysical methods. In the study by Brenk-Krakowska et al.,
9 unstable fixation was found in dyslexic adults using the Wesson card. Dyslexic subjects exhibited higher motor instability but showed no difference in sensory instability. Similar findings were obtained in this study, that is, higher motor instability in dyslexic subjects (almost 70% of dyslexics) and no sensory instability. Probably the lack of central fusion lock in the Wesson card creates a difficulty in maintaining vergence stability
54 and provokes higher motor instability. In our study, no more than 17% of controls showed motor instability with the Wesson card. However, the instability may not reflect the actual condition during reading. Devices with a good central fusion lock, such as the modified Mallett test, are better indicators of the actual binocular condition when fixating the target.
49 In our study, while detecting fixation disparity with the modified Mallett test, more than half of the dyslexic subjects exhibited sensory instability, yet only less than 25% of dyslexics detected some motor instability. Due to problems with oculomotor adjustments (reflected in motor instability during Wesson card measurements), sensory instability occurred during measurements using the device with a good central fusion lock (modified Mallett test). This fusion lock might reflect a higher demand on the dyslexic's sensory fusion processes during a reading task. Variability in FD has also been observed in children using objective measurement methods
10,24 but conversely, Evans et al.
11 did not find any differences in the subjects' sensory or motor responses. The procedure of instability assessment (motor—movement of dichoptic nonius targets or sensory—fading away of the one of the dichoptic nonius targets) relies on the subject's subjective response. As these changes are quite discrete and dynamic, this procedure might be a challenge considering children's attention, because they may not be aware of them. Further studies are necessary in order to compare the variability of FD obtained with psychometric clinical tests and objective eyetracking methods. It is known that FD measured objectively (eyetracking systems) and subjectively (dichoptic nonius lines) differs.
55
The lack of a central fusion lock may also influence the extent of FD. Some authors suggest that dyslexic adults have at least some tendency toward exo-FD in the Wesson card test.
9 We also found that the mean FD values in dyslexic subjects were higher and shifted in the exo-direction. We suspect that the weak fusion lock may have caused a difficulty in maintaining vergence stability, and thus a greater amount of exo-FD could occur.
Poor binocular coordination should not be treated as a major cause of reading problems because FD and/or poor vergences were observed mainly in saccadic tasks performed with a text stimuli, but not in the simple dot scanning.
21,56 One can argue that poor binocular coordination during reading was caused by a phonologic disorder (i.e., problems with symbol decoding could have impaired the control of eye movements and vergences). As was mentioned by Kirkby et al.,
56 it is possible that some differences in visual characteristics between a sentence and a dot stimuli could cause fusional difficulties, resulting in more difficult ocular scanning during text reading as compared with dot scanning. In our study, a simple nontext target scanning was performed and slower RTs were observed in dyslexic subjects under binocular viewing conditions. This observation demonstrates that unstable binocular fixation in dyslexia may disturb motor and oculomotor performance, not only while reading a text but also in a simpler nontext task.
How can one explain the lack of FD in dyslexic subjects during simple fixation or dot scanning found by some researchers?
53,56 One possibility may be a compensatory mechanism where motor disabilities are compensated by attentional resources.
57,58 Similar compensation was found in subjects with dyslexia during body balance measurements. Their body balance was worse when an additional concurrent task was employed during quiet standing.
29,59 It is possible that dyslexic individuals' oculomotor deficits could be difficult to detect in simple scanning paradigm because of the aforementioned compensation. However, when a more complex task involving attentional resources is required, the actual oculomotor deficits could be detected. In our study, eye movements were simple but the subjects had to show an additional motor response adequate to the location of the stimulus in space. Thus, the paradigm used in the current study was more complex and it is possible that oculomotor deficits were well compensated.