Eye movements and visual stability Kandel et al Ch 29, end of Wolfe Ch 8 Kandel Ch 39 for more info.

Is a set of Reichardt detectors is sensitive to motion in one direction and only in a particular speed? It seems like an inefficient design since a great number of neurons will be required to encode motion in all possible directions and speed, unless each of them can actually encode for a small range of speed, although that might lower the sensitivity to speed change. Or the visual cortices simply have enough neurons to do so. I also want to know if information about the motion of objects stays in MT/MST etc or is it transferred to other areas to bind with other properties of objects that allow recognition of moving objects. Again the binding problem. The reason I asked the binding question again is because the integration between properties of object in the dorsal stream (eg. motion signals in MT) and properties in the ventral stream (eg. orientation and shape for object recognition) must differ somehow from integration within ventral stream for 'what' information. Referring to Campbell and Robson's experiment, the lecturer mentioned that if the cells adapt to seeing an upward motion, they will notice a "downward" motion after the motion stops. Is this the same kind of adaptation that sees an opposite color when the color disappears (like in the rotating color circle that we saw two weeks ago)?

Why do we move our eyes? - Image stabilization - Information acquisition

Visual Acuity matches photoreceptor density

Why do we move our eyes? 1. To bring objects of interest onto high acuity region in fovea.

Cone Photoreceptors are densely packed in the central fovea

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

Why do we move our eyes? 1. To bring objects of interest onto high acuity region in fovea. 2. Cortical magnification suggests “enhanced” processing of image in the central visual field.

Oculomotor Muscles Muscles innervated by oculomotor, trochlear, and abducens (cranial) nerves from the oculomotor nuclei in the brainstem. Oculo-motor neurons: Hz vs spinal motor Neurons: Hz

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.

Retinal structure Accomodation:tension on zonule fibres

Acceleration Depth-dept gain, Precision in natural vision Velocity Ocular following - Miles Acuity – babies

otoliths Rotational (semi-circular canals) translational (otoliths)

Latency of vestibular-ocular reflex=10msec VOR

It is almost impossible to hold the eyes still. Demonstration of VOR and its precision – sitting vs standing

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 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 V1: striate cortex Basal ganglia 1. Neural activity related to saccade 2. Microstimulation generates saccade 3. Lesions impair saccade Dorso-lateral pre-frontal

target selection signals to muscles (forces) inhibits SC saccade decision saccade command (where to go) monitor/plan movements Function of Different Areas H V

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

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.

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

Pulse-Step signal for a saccade

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.

Superior colliculus

Smooth pursuit & Supplementary Brain Circuitry for Pursuit Velocity signal Early motion analysis

Gaze shifts: eye plus head

Visual Stability Efference copy or corollary discharge

Figure 8.18 The comparator

Brainstem circuits for saccades. Omnipause neurons (OPN) in the nucleus raphe interpositus (RIP) tonically inhibit excitatory burst neurons (EBN) located in the paramedian pontine reticular formation (PPRF). When OPNs pause, the EBNs emit a burst of spikes, which activate motor neurons (MN) in the abducens nucleus (VI) innervating the lateral rectus muscle. The burst also activates interneurons (IN) which activate motor neurons on the oculomotor nucleus (III) on the opposite side, innervating the medial rectus. Inhibitory burst neurons (IBN) show a pattern of activity similar to EBNs, but provide inhibitory inputs to decrease activation in the complementary circuits and antagonist muscles. Long-lead burst neurons (LLBN) show activity long before movement onset, and provide an excitatory input to EBNs.

RF reticular formation VN vestibular nucle PN, pontine nuclei i Cerebellum OV oculomotor vermis VPF ventral paraflocculus FN fastigial nucleus