Why eye movements are hard to measure. 18mm 0.3mm = 1 deg visual angle xa tan(a/2) = x/d a = 2 tan - 1 x/d Visual Angle d 1 diopter = 1/focal length in meters 55 diopters = 1/.018 A small eye rotation translates into a big change in visual angle
Oculomotor Muscles Muscles innervated by oculomotor, trochlear, and abducens (cranial) nerves from the oculomotor nuclei in the brainstem. Oculo-motor neurons: 100-600Hz vs spinal motor Neurons: 50-100Hz
Types of Eye Movement Information GatheringStabilizing Voluntary (attention)Reflexive Saccadesvestibular ocular reflex (vor) new location, high velocity (700 deg/sec), body movements ballistic(?) Smooth pursuitoptokinetic nystagmus (okn) object moves, velocity, slow(ish) whole field image motion Mostly 0-35 deg/sec but maybe up to100deg/sec Vergence change point of fixation in depth slow, disjunctive (eyes rotate in opposite directions) (all others are conjunctive) Note: link between accommodation and vergence Fixation: period when eye is relatively stationary between saccades.
Acceleration Depth-dependent gain, Precision in natural vision Velocity Ocular following - Miles Acuity – babies Rotational or translational
Saccade latency approx 200 msec, pursuit approx 100 – smaller when there is a context that allows prediction. Step-ramp allows separation of pursuit (slip) and saccade (displacement)
“main sequence”: duration = c Amplitude + b (also V = a Amp+d) Min saccade duration approx 25 msec, max approx 200msec
Demonstration of “miniature” eye movements It is almost impossible to hold the eyes still. Drift Micro-saccades Tremor Significance??
What’s involved in making a saccadic eye movement? Behavioral goal: make a sandwich Sub-goal: get peanut butter Visual search for pb: requires memory for eg color of pb or location Visual search provides saccade goal - attend to target location Plan saccade to location (sensory-motor transformation) Coordinate with hands/head Calculate velocity/position signal Execute saccade/
Brain Circuitry for Saccades Oculomotor nuclei Basal ganglia 1. Neural activity related to saccade 2. Microstimulation generates saccade 3. Lesions impair saccade Dorso-lateral pre-frontal (memory) H V monitor/plan movements
LIP: Lateral Intra-parietal Area Target selection for saccades: cells fire before saccade to attended object Posterior Parietal Cortex reaching grasping Intra-Parietal Sulcus: area of multi-sensory convergence Visual stability
- Saccades/smooth pursuit -Planning/ Error checking -relates to behavioral goals Supplementary eye fields: SEF FEF: -Voluntary control of saccades. -Selection from multiple targets -Relates to behavioral goals.
Monkey makes a saccade to a stimulus - some directions are rewarded. Cells in caudate signal both saccade direction and expected reward. Hikosaka et al, 2000
Motor neurons for the eye muscles are located in the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), and reach the extraocular muscles via the corresponding nerves (n. III, n. IV, n. VI). Premotor neurons for controlling eye movements are located in the paramedian pontine reticular formation (PPRF), the mesencephalic reticular formation (MRF), rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), the interstitial nucleus of Cajal (IC), the vestibular nuclei (VN), and the nucleus prepositus hypoglossi (NPH). Motor neurons Pre-motor neurons Oculomotor nucleus Trochlear Abducens H V
Brain areas involved in making a saccadic eye movement Behavioral goal: make a sandwich (learn how to make sandwiches) Frontal cortex. Sub-goal: get peanut butter (secondary reward signal - dopamine - basal ganglia) Visual search for pb: requires memory for eg color of pb or location (memory for visual properties - Inferotemporal cortex; activation of color - V1, V4) Visual search provides saccade goal. LIP - target selection, also FEF Plan saccade - FEF, SEF Coordinate with hands/head Execute saccade/ control time of execution: basal ganglia (substantia nigra pars reticulata, caudate) Calculate velocity/position signal oculomotor nuclei Cerebellum?
Relation between saccades and attention. Saccade is always preceded by an attentional shift However, attention can be allocated covertly to the peripheral retina without a saccade. Pursuit movements also require attention.
Smooth pursuit & Supplementary Brain Circuitry for Pursuit Velocity signal Early motion analysis
Experiments with partial and complete paralysis of extra-ocular muscles Stevens et al – partial paralysis – world jumps during an em Matin – complete paralysis – no motion Resolution: Bayesian cue combination.
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