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Hybrid model of the context dependent vestibulo-ocular reflex: implications for vergence-version interactions.

Ranjbaran M, Galiana HL - Front Comput Neurosci (2015)

Bottom Line: By simply assigning proper nonlinear neural computations at the premotor level, the model replicates all reported experimental observations.This work sheds light on potential underlying neural mechanisms driving the context dependent AVOR and explains contradictory results in the literature.Moreover, context-dependent behaviors in more complex motor systems could also rely on local nonlinear neural computations.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, McGill University Montreal, QC, Canada.

ABSTRACT
The vestibulo-ocular reflex (VOR) is an involuntary eye movement evoked by head movements. It is also influenced by viewing distance. This paper presents a hybrid nonlinear bilateral model for the horizontal angular vestibulo-ocular reflex (AVOR) in the dark. The model is based on known interconnections between saccadic burst circuits in the brainstem and ocular premotor areas in the vestibular nuclei during fast and slow phase intervals of nystagmus. We implemented a viable switching strategy for the timing of nystagmus events to allow emulation of real nystagmus data. The performance of the hybrid model is evaluated with simulations, and results are consistent with experimental observations. The hybrid model replicates realistic AVOR nystagmus patterns during sinusoidal or step head rotations in the dark and during interactions with vergence, e.g., fixation distance. By simply assigning proper nonlinear neural computations at the premotor level, the model replicates all reported experimental observations. This work sheds light on potential underlying neural mechanisms driving the context dependent AVOR and explains contradictory results in the literature. Moreover, context-dependent behaviors in more complex motor systems could also rely on local nonlinear neural computations.

No MeSH data available.


Related in: MedlinePlus

(A) Envelope fit on the decay rate of conjugate nystagmus velocity. (B) Vergence nystagmus associated with conjugate nystagmus in response to step rotation stimuli in the dark (prediction).
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Figure 10: (A) Envelope fit on the decay rate of conjugate nystagmus velocity. (B) Vergence nystagmus associated with conjugate nystagmus in response to step rotation stimuli in the dark (prediction).

Mentions: Raphan et al. (1979) also studied the decay rate of nystagmus velocity. As commonly done in the literature, they evaluated the dynamics of the VOR slow phase velocity by removing fast phases and replacing the gaps by interpolation: the reconstructed envelope was deemed to represent VOR dynamics, fitted with exponentials. The main result is that nystagmus decay appears much slower than the underlying slow-phase system (≈ 15 s vs. 4–6 s canal), hence the term velocity storage in the VOR coined by Raphan et al. However, an envelope fit ignores the contribution of initial conditions introduced at the start of each slow phase segment, biasing estimates of the slow phase time constant. To illustrate, Figure 10 provides the hybrid model response to a step of −250 degree/s in head velocity, with a canal time constant of 6 s and a conjugate slow-phase time constant of 1.2 s (see Table 1). The slow phase central time constant is intentionally low to highlight the effects, but these hold whenever there is nystagmus, especially at very low frequencies like steps. In Figure 10A, the envelope of slow phase velocities decays with a time constant of 5.55 s, despite slow phase central dynamics of 1.2 s. Such a response is often seen in unilateral vestibular patients. As discussed for sinusoidal rotations in Galiana (1991), ignoring the effect of nystagmus results in biasing the estimated conjugate VOR dynamics. There is a plateau-like response in the initial nystagmus velocity also seen by Raphan et al. (1979) at higher head speeds. Here it is caused by nonlinearities in the canal sensitivity, now exceeded by the input range. In addition, with the hybrid model, we predict the appearance of vergence nystagmus (Figure 10B) during step rotations in the dark. In order to extend velocity storage beyond both canal and central time constants, it is sufficient to incorporate the resting rates of sensors and central components (Galiana, 1991); at this time we only include modulations at all sites about resting rates, so only the central time constants can be masked during nystagmus.


Hybrid model of the context dependent vestibulo-ocular reflex: implications for vergence-version interactions.

Ranjbaran M, Galiana HL - Front Comput Neurosci (2015)

(A) Envelope fit on the decay rate of conjugate nystagmus velocity. (B) Vergence nystagmus associated with conjugate nystagmus in response to step rotation stimuli in the dark (prediction).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4321407&req=5

Figure 10: (A) Envelope fit on the decay rate of conjugate nystagmus velocity. (B) Vergence nystagmus associated with conjugate nystagmus in response to step rotation stimuli in the dark (prediction).
Mentions: Raphan et al. (1979) also studied the decay rate of nystagmus velocity. As commonly done in the literature, they evaluated the dynamics of the VOR slow phase velocity by removing fast phases and replacing the gaps by interpolation: the reconstructed envelope was deemed to represent VOR dynamics, fitted with exponentials. The main result is that nystagmus decay appears much slower than the underlying slow-phase system (≈ 15 s vs. 4–6 s canal), hence the term velocity storage in the VOR coined by Raphan et al. However, an envelope fit ignores the contribution of initial conditions introduced at the start of each slow phase segment, biasing estimates of the slow phase time constant. To illustrate, Figure 10 provides the hybrid model response to a step of −250 degree/s in head velocity, with a canal time constant of 6 s and a conjugate slow-phase time constant of 1.2 s (see Table 1). The slow phase central time constant is intentionally low to highlight the effects, but these hold whenever there is nystagmus, especially at very low frequencies like steps. In Figure 10A, the envelope of slow phase velocities decays with a time constant of 5.55 s, despite slow phase central dynamics of 1.2 s. Such a response is often seen in unilateral vestibular patients. As discussed for sinusoidal rotations in Galiana (1991), ignoring the effect of nystagmus results in biasing the estimated conjugate VOR dynamics. There is a plateau-like response in the initial nystagmus velocity also seen by Raphan et al. (1979) at higher head speeds. Here it is caused by nonlinearities in the canal sensitivity, now exceeded by the input range. In addition, with the hybrid model, we predict the appearance of vergence nystagmus (Figure 10B) during step rotations in the dark. In order to extend velocity storage beyond both canal and central time constants, it is sufficient to incorporate the resting rates of sensors and central components (Galiana, 1991); at this time we only include modulations at all sites about resting rates, so only the central time constants can be masked during nystagmus.

Bottom Line: By simply assigning proper nonlinear neural computations at the premotor level, the model replicates all reported experimental observations.This work sheds light on potential underlying neural mechanisms driving the context dependent AVOR and explains contradictory results in the literature.Moreover, context-dependent behaviors in more complex motor systems could also rely on local nonlinear neural computations.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, McGill University Montreal, QC, Canada.

ABSTRACT
The vestibulo-ocular reflex (VOR) is an involuntary eye movement evoked by head movements. It is also influenced by viewing distance. This paper presents a hybrid nonlinear bilateral model for the horizontal angular vestibulo-ocular reflex (AVOR) in the dark. The model is based on known interconnections between saccadic burst circuits in the brainstem and ocular premotor areas in the vestibular nuclei during fast and slow phase intervals of nystagmus. We implemented a viable switching strategy for the timing of nystagmus events to allow emulation of real nystagmus data. The performance of the hybrid model is evaluated with simulations, and results are consistent with experimental observations. The hybrid model replicates realistic AVOR nystagmus patterns during sinusoidal or step head rotations in the dark and during interactions with vergence, e.g., fixation distance. By simply assigning proper nonlinear neural computations at the premotor level, the model replicates all reported experimental observations. This work sheds light on potential underlying neural mechanisms driving the context dependent AVOR and explains contradictory results in the literature. Moreover, context-dependent behaviors in more complex motor systems could also rely on local nonlinear neural computations.

No MeSH data available.


Related in: MedlinePlus