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Slow Eye Movements in Congenital Nystagmus
The role of the standard deviation of eye position (SDp) during foveations with respect to visual acuity has been discussed in the past ten years [10,16]. Fostered also by a remarkable increase in some CN patients’ visual acuity, obtained with botulinum toxin treatment, which didn’t correspond to large extensions n foveation time, pointed us to characterize in more details such foveation variability. A slow sinusoidal-like oscillation of the baseline (baseline oscillation or BLO) was found superimposed to nystagmus waveforms [9,11,12] and its relation with the SDp was estimated [7].
Presence of similar slow pendular waveforms, superimposed to nystagmus, was also reported by Gottlob et al. [27]. In addition, in eye movement recordings presented by Dell’Osso et al. [15,16] it is possible to recognize slow oscillations superimposed to the nystagmus. Akman et al. [5], using dynamical systems analysis to quantify the dynamics of the nystagmus in the region of foveation, found that the state-space fixed point, or steady state, is not unique. Physiologically this means that the control system does not appear to maintain a unique gaze position at the end of each fast phase. Similarly, Evans [24] reported that some of the analyzed patients fail to coordinate target with fovea position (approximately 50% of patients). Kommerell [35] noticed that in CN patients, tracking moving targets, the eye recording presented a slow eye movement superimposed to the stimulus trajectory in addition to nystagmic cycles.
Nystagmus and the slow oscillation could modify visual acuity. Currie et al. [14] evaluated acuity for optotypes in healthy subjects using moving light sources to simulate retinal image motion that occurs in nystagmus. Their results are that acuity depends on both foveation duration and position variability, although the presence of other sensory defects (e.g. astigmatism) must be taken into account. Moreover, they found that an addition of low-frequency (1.22 Hz) waves to the light stimuli, i.e. slow oscillation, caused a worsening of visual acuity.
In order to estimate the slow sinusoidal oscillations, a common least mean square (LMS) fitting technique could be used. For each signal block the highest peak of the power spectrum of the eye movement signal in the range 0.1–1.5 Hz can be considered as an estimator of the BLO frequency. The high frequency limit result from the lowest frequency commonly associated to nystagmus (accordingly to Bedell and Loshin, 1991, and Abadi and Dickinson, 1986), while the low frequency limit depends on the signal length corresponding to each gaze position (in our tests approximately 10 s).
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Figure 5a and 5b. Examples of acquired signals showing the presence of the slow eye movement added up to the nystagmus oscillations.
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Conclusion
Eye movement recording methodology is most commonly used as a research tool by neurologists, neurophysiologists, psychophysicists, psychologists/ psychiatrists, ophthalmologists, and optometrists [18,21,25]. Eye movement recording and estimation of concise parameters, such as waveform shape, nystagmus amplitude, frequency, direction of beating, foveation periods and eye position variability, are a strong support for an accurate diagnosis, for patient follow-up and for therapy evaluation [8].
Regarding the last parameter, the slow eye movement, described as baseline oscillation, explains most of the eye position variability during foveations (SDp) [7], which in turn was found exponentially well related to visual acuity [10]. According to the procedure described above, baseline oscillation parameters can be estimated for any CN eye movement recordings. In a case study by Pasquariello et al. carried out on 96 recordings, almost 70% of the recordings had BLO amplitude greater than 1° (appreciatively the fovea angular size); in the remaining 30% the amplitude of the BLO was smaller and didn’t affect significantly visual acuity.
In that study a high correlation coefficient (R2 = 0.78) was also found in the linear regression analysis of BLO and nystagmus amplitude, suggesting a strong level of interdependence between the two. The regression line slope coefficient was about 0.5, which implies that BLO amplitude on average is one half of the correspondent nystagmus amplitude.
Specifically, since BLO amplitude resulted directly related to nystagmus amplitude, its presence is particularly evident in the signal tracts away from the null zone (i.e., not in the position in which nystagmus amplitude is lesser).
The origin of such baseline oscillation is unknown. Some authors assert that slow movement can be recorded only in subjects with severely reduced visual experience from birth (like CN patients) [27]. However, the high value of the correlation coefficient between BLO and nystagmus amplitude found in this study suggests that the two phenomena are somewhat linked together. Therefore the origin of the BLO could be searched analyzing within the same ocular motor subsystems considered for nystagmus.
The baseline oscillation highlights the presence of a slow ‘periodic’ component in the eye movement signal. The sine function is a rather good estimator of this slow periodic component added to nystagmus; the basic shape of the baseline is indeed a sinusoid, sometimes and randomly disrupted by phase inversions, interruptions (as short as hundreds of milliseconds, lasting to even 1 second) and other non linear components. To the periodic component represented
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by BLO the small, additional random movements should be added, in order to assess the whole variability of eye position during fixation [7].
Figure 6. The relationship between Baseline Oscillation and Nystagmus amplitude.
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