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

x a

tan(a/2) = x/da = 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: 100-600Hz vs spinal motorNeurons: 50-100Hz

Types of Eye Movement

Information Gathering StabilizingVoluntary (attention) Reflexive

Saccades vestibular ocular reflex (vor)new location, high velocity (700 deg/sec), body movements

ballistic(?)Smooth pursuit optokinetic nystagmus (okn)object moves, velocity, slow(ish) whole field image motionMostly 0-35 deg/sec but maybe up to100deg/sec

Vergencechange point of fixation in depthslow, disjunctive (eyes rotate in opposite directions)(all others are conjunctive)Note: link between accommodation and vergenceFixation: period when eye is relatively stationary between saccades.

Retinal structure

Accomodation:tension on zonule fibres

AccelerationDepth-dept gain, Precision in natural vision

VelocityOcular 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 thatallows prediction.

Step-ramp allows separation of pursuit (slip) and saccade (displacement)

“main sequence”: duration = c Amplitude + bMin saccade duration approx 25 msec, max approx 200msec

Demonstration of “miniature” eye movements

It is almost impossible to hold the eyes still.

DriftMicro-saccadesTremor

Significance??

What’s involved in making a saccadic eye movement?

Behavioral goal: make a sandwichSub-goal: get peanut butterVisual search for pb: requires memory for eg color of pb or locationVisual search provides saccade goal - attend to target locationPlan saccade to location (sensory-motor transformation)Coordinate with hands/headCalculate velocity/position signalExecute saccade/

Brain Circuitry for Saccades

Oculomotor nuclei

V1: striate cortex

Basal ganglia

1. Neural activity related to saccade2. Microstimulation generates saccade3. 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 AreaTarget selection for saccades: cells fire before saccade to attended object

Posterior Parietal Cortex

reaching

grasping

Intra-Parietal Sulcus: areaof multi-sensory convergence

Visual stability

-Saccades/smooth pursuit

-Planning/ Error checking-relates to behavioral

goals

Supplementary eye fields: SEF

FEF:-Voluntary controlof 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

HV

Pulse-Step signal for a saccade

Brain areas involved in making a saccadic eye movementBehavioral 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 FEFPlan saccade - FEF, SEFCoordinate with hands/headExecute saccade/ control time of execution: basal ganglia (substantia nigra pars reticulata, caudate) Calculate velocity/position signal oculomotor nucleiCerebellum?

Relation between saccades and attention.

Saccade is always preceded by an attentional shiftHowever, 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) showactivity long before movement onset, and provide an excitatory input to EBNs.

RF reticular formation VN vestibular nuclePN , pontine nucleii

CerebellumOV oculomotor vermis VPF ventral paraflocculus FN fastigial nucleus