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9/12/12 Neural Integra0on Neural Integra0on • The oculomotor system can hold gaze steady when the eyes are pointed at eccentrically located targets. • More than visual fixa0on is at play since eccentric gaze can be maintained in complete darkness • Motoneurons in the oculomotor system encode posi0on and velocity informa0on required to move the eye and hold it in eccentric loca0ons • Premotor inputs are velocity coded – Saccade burst cells encode saccade velocity – Cells in cortex, DLPN, flocculus encode smooth-‐pursuit eye velocity – Ves0bular afferents and secondary ves0bular neurons encode head and eye velocity – Purkinje cells in cerebellum encode gaze velocity 1 9/12/12 Ra0onale for Neural Integra0on Re0nal Error Posi0on Saccades Re0nal Error Velocity Head accelera0on Head velocity Smooth-‐pursuit Ves0bulo-‐ocular reflex Re0nal Error Velocity Optokine0c system Velocity Command ???? Motor nuclei Position & Velocity Command Eye Plant Neural Integra0on • Mathema0cal integra0on of velocity informa0on is required to generate posi0on informa0on • In a network of neurons, if a par0cular cell inhibits its neighbors and in turn is inhibited by them, then this will result in sustained ac0vity aVer ini0al s0mula0on without further input. • Lorente De No’s system of reverbera0ng collaterals 2 9/12/12 NI Func0on (From Leigh and Zee 1999) Schema0c for Neural Integra0on 3 9/12/12 NI Func0on • NI func0on is usually determined by its 0me-‐ constant as measured in darkness. • The 0me constant is the 0me taken for the eye to driV back 63% from an eccentric posi0on. • A perfect NI has an infinite 0me-‐constant. • A real (normal) NI in humans has a 0me constant of about 20 -‐ 70sec. Neural Integra0on (cont) • Nucleus Prepositus Hypoglossi func0ons as the horizontal neural integrator. • Inters00al Nucleus of Cajal func0ons as the ver0cal and torsional neural integrator. 4 9/12/12 Neural Integra0on (cont) • Cerebellum is also important for neural integra0on. Floccular lesions also causes gaze-‐ evoked nystagmus. CBM + + NI • Gene0c disorders of certain calcium channels specific to the cerebellum results in mice with deficient neural integra0on What happens if you lesion the NI? • A – No driV • Post lesion data indicates gaze-‐evoked nystagmus • AVer bilateral lesion, neural integrator func0on is lost From Cannon and Robinson 1987 5 9/12/12 Leaky Neural Integrator in a Pa0ent Pathology – Acquired Pendular Nystagmus (Movie from Leigh&Zee 4th edi0on) 6
