Introduction. We move our eyes to help us see better. Eye movements

Transcription

1 The Physiology of the Senses Transformations For Perception and Action Lecture 11 - Eye Movements Tutis Vilis Introduction We move our eyes to help us see better. Eye movements 1) place the image of the object of interest on the part of the retina with the highest acuity, the fovea. Try to read without moving your eyes. How much of this sentence can you make out while looking at this.? 2) keep the image in the eye stationary in spite of movement of the object or movements of one's head. Try shaking this page from side to side. Can you make out what it says? 11.1 revised

2 There are 5 types of eye movements. Each 1) Saccades 2) Vergence 3) Pursuit 1) serves a unique function & 2) has properties particularly suited to that function. If an image appears to the side, eye movements called saccades rotate both eyes so that the image now falls on the fovea. Likewise saccades are used to point the fovea at each word in this sentence. If you look (i.e. direct the foveas) from a far object to a near one, vergence eye movements are generated; convergence when looked from far to near and divergence when looking from near to far. When an object that we are looking at moves, the image is kept still on the retina by means of a pursuit eye movement (e.g. tracking a ball or your moving finger). 4) Vestibular ocular reflex (VOR) If we rotate our head, an eye movement very similar to pursuit is elicited whose function is also to keep the image still on the retina. However, in spite of the fact that the movement looks similar, it is generated by a different neural circuit, the VOR. The VOR does not need a visual stimulus. It works in the dark. Rotate your head with your eyes closed. Feel your eyes move with your finger tips. 5) Optokinetic Reflex (OKR) The VOR does not work well for slow prolonged movements. In this case vision, through the OKR, assists the VOR. The OKR is activated when the image of the world slips on a large portion of the retina and produces a sense of self motion (e.g. when sitting in a car stopped at a light and a car beside you starts to move, you sometimes feel like you are moving). 11.2

3 The eyes are rotated by 6 extraocular muscles Superior Oblique Superior Rectus Left Eye Nasal Medial Rectus Lateral Rectus Temporal Inferior Rectus Inferior Oblique They act as 3 agonist/antagonist pairs When your eye points forward 60% of its motorneurons are active. To look elsewhere, one of a pair contracts & the other relaxes You need 3 pairs to allow rotations in all three directions: horizontal, vertical & torsional MR: adduction SR: elevation & small intorsion SO: intorsion & small depression Nasal Temporal LR: abduction IR: depression & small extorsion IO: extorsion & small elevation -Problem: What pairs of muscles must you activate to 1) look up with no torsion and 2) to intort the left eye? 11.3

4 Describe the direct path of the VOR This is a short tri-synaptic reflex with synapses at 1) vestibular nucleus (vn), 2) motoneurons (VI), and 3) lateral rectus muscle (lr) The medial rectus(mr) of the right eye is also activated by a projection via motoneurons in the III nerve nucleus. 3. lr mr III c eye rotation d drift phasic b 2. VI 1. vn head a The direct path, by itself, is not enough. Why? Recall that function of the VOR is to keep the eyes still in space, by rotating the eyes in the opposite direction to head. During a head rotation to the right (a), a phasic response (b) is seen in vestibular afferents (because vestibular afferents sense how fast you rotate), in the motoneurons and the muscles. This rotates the eyes to the left (c). After the rotation you want the eyes to remain pointing to the left. But the eyes would drift back to center (d) because muscles need a tonic input to keep the eye rotated. 11.4

5 Where does the required tonic input come from? lr mr eye rotation no tonic III motoneurons 3 normal mlf phasic & tonic VI direct tonic 2 PPH vn vestibular afferent phasic 1 indirect head The tonic command originates via the indirect path through the n. prepositus hypoglossi (PPH) This nucleus converts the phasic vestibular input into a tonic signal, probably via a reverberating neural circuit, a form of short term memory. Motoneurons thus normally exhibit the combined phasic & tonic input from: 1) a phasic command (which rotates the eye) from the direct path and 2) a tonic command (which holds it there) from the indirect path. 11.5

6 Describe the generation of saccadic eye movements Saccades redirect foveas to objects of interest, e.g. words in this sentence. Saccades are very fast (velocities of 500deg/s). These high velocities are generated by a phasic burst of ap s( up to 1000 ap/sec). lr mr eye rotation no tonic normal III mlf motoneurons phasic & tonic VI direct tonic phasic PPH indirect This burst of activity originates in the (paramedian pontine reticular formation, near the nucleus of the sixth cranial nerve (VI).) The generates conjugate (both eyes) ipsilateral rotations. As in the VOR, there are two paths: 1) a direct path which mediates the phasic command to move the eyes. 2) an indirect path via PPH which generates the tonic command to hold the eyes in an eccentric position

7 Lesions in the saccadic system -Problem: Match the lesions a, b, c with the eye movements ( 1, 2 or 3) a) if indirect pathway or PPH is lesioned b) if VI n is partially damaged c) if direct pathway is lesioned. normal eye rotation lr mr VI b direct III c mlf PPH indirect a 11.7

8 What initiates a saccade? lr mr As we have seen, to make a saccade to A, activity at location A in the SC must occur. III But before a saccade can begin, a hill of activity at the center must be removed. This hill of activity keeps the eyes fixating the current location. It does this through omnipause neurons which inhibit burst neurons in the. Thus to make a saccade, a visual stimulus produces activity at A, and at the same time the activity at the center is removed. Then neurons are released from inhibition and driven by the collicular activity at A. VI PPH - + omnipause neurons Center disengage A engage 11.8

9 What stops a saccade? lr mr Saccades are very fast and of a short duration, (lasting roughly 50 ms for a 20 deg saccade). Visual feedback would arrive too late to stop the saccade. It takes 50 ms for visual information to reach the visual cortex. III Proprioceptive information would likewise take too long. Because saccades are not guided by sensory feedback they are called ballistic movements. Like guided missiles, the saccadic system uses an internal sense of eye position to guide and stop saccades. VI PPH - + An internal estimate of eye position is generated by the PPH. One hypothesis is that this eye position signal is used to guide the activity of the hill in the SC back to the central foveal representation. When the hill returns to the foveal representation, omnipause neurons are reactivated, burst activity stops and so does the saccade. Center A -Problem: Suppose the is damaged so that the burst it generates is half of normal. Given the above circuit, what effect do you think this damage will have on a saccade.? 11.9

10 What is nystagmus? A) Normal nystagmus is seen during large head rotations: The VOR generates the slow phases. This helps keep your eye on a target. When eye approaches the edge of oculomotor range, a saccade (quick phase) is generated in the opposite direction to new target The frequency of saccades increases when you look in the direction of rotation, because saccades interrupt the VOR more often. head VOR VOR VOR saccade oculomotor range Nystagmus is also seen with lesions. Two main types are: a) imbalance in the VOR This looks like normal nystagmus (but drive is not from head motion but from an imbalance) Recall that the VOR is a push-pull system: afferents have tonic drive which normally cancel bilaterally. Unilateral lesions (vestibular organ, afferents, nuclei etc) disrupt this balance. b) Tonic activity too small This is often caused by lesion of the PPH (tone generator) This is also caused by lesions of cerebellar flocculus (normally tunes up PPH) In contrast to a) here slow phase i) shows an exponential (not linear) drift to a position of rest (often centre) ii) its direction switches when the patient looks in the opposite direction 11.10

11 How are vertical (& torsional) saccades generated? Pulse Step Horizontal PPH Vertical & Torsional rimlf INC rimlf INC III IV - As in horizontal saccades, you need both a pulse & step of activity - the pulse (both for up and down) is generated by the rimlf (rostral interstitial n. of the MLF) in the mesencephalic reticular formation near III & IV n. VI PPH - the step (tone) by adjacent INC (interstitial n. of Cajal) the pulse & step combine on the MN of the 4 vertical muscles in the III & IV n

12 The cerebellum calibrates saccades (and all other eye movements) Normally saccades in the two eyes are equal (conjugate) and reasonably accurate. Left Target Immediately after a palsy to the right MR, saccades in the right eye are too small. Patient sees double. Then if the unaffected left eye is patched, saccade amplitude adapts over a few days. They increase in both eyes In the right eye (unpatched weak) saccades become normal. In the left eye (patched normal) saccades become too large. - i.e.. a larger command is sent to motoneurons of both eyes If neither eye is patched only the weak right eye adapts. Saccades only in the right eye changes and become normal. - i.e. a larger command is sent to only motoneurons of the affected muscle. left eye right eye palsy left eye patched This recalibration arises from the cerebellum Palsies persist if: 1) they exceed the repair capabilities of the cerebellum (eg total paralysis) or 2) the cerebellar "repair shop" is damaged. right eye compensated Problem: If the saccade size in only one eye can be changed, what is wrong with the diagram on page 7. Suggest how this circuit can be changed

13 Where do the saccadic commands to the brainstem pprf & rimlf comes from? The eye's ganglion cells project to both the visual cortex (via LGN) and superior colliculus Superior colliculus responds to flashing/moving stimuli in the peripheral retina > elicits a contralateral saccade (via the contra lateral pprf or rimlf) so that fovea can examine the stimulus. frontal eye fields direct a saccade to a remembered target ie continue to fire if eyes are closed - target (for saccades) held in "working memory" In summary SC > direct short latency saccades to an unexpected flash - involuntary eye LGN & rimlf superior colliculus contra visual cortex FEF > direct longer latency saccades to a remembered target - voluntary FEF eye FEF & rimlf contra SC LGN V

14 Vergence eye movements These are the slowest to develop during childhood. These movements are linked to accommodation reflex (the reflex that changes the eye s lens properties). On viewing a near object the eyes 1) converge to eliminate retinal disparity (seeing double) 2) accommodate (lens becomes more round) to eliminate blur. Either blur or retinal disparity will generate vergence The strongest response is elicited when the image is blurred and there is retinal disparity. blur brainstem accomodation region (pretectum) lens adjustment retinal disparity Visual Cortex vergence eye movement 11.14

15 Pursuit eye movements The sequence of structures that are used to generate pursuit eye movements Eye V1 MT MSTl Cerebellum Brainstem If a target starts to move 1) a pursuit movement is generated after a short delay 2) a saccade is often used to catch up to the target target pursuit saccade 3) finally if the pursuit is perfect, your eye tracks the moving object -Problem: What drives the eye movement when pursuit is perfect ( i.e. in phase 3)? The stimulus that initiates pursuit needs not be slip of the image on the retina. pursuit eg 1) tracking the center of the wheel when all you see is 2 small lights on the rim in a dark room. eg 2) a moving auditory stimulus -Problem: Speculate how pursuit of this wheel is generated

16 How do you sense that an object is moving? Often it is because its image slips on the retina. For example try looking at your left hand which is kept still while moving the right hand. You sense motion of the right hand because of its retinal slip. However you can have retinal slip with no sense of motion and also sense motion with no retinal slip. -Problem: Give an example in which each of the following situations can occur. 1) The image slip on the retina but you sense no motion. 2) The image of an object is stationary on the eye and you sense motion. 3) The image of an object is stationary on the eye while the eye is moving and you sense no motion. -Problem: Examine the above answers and suggest some criterion that the brain might use for sensing that an object is indeed moving. Fixation Lastly consider what happens between eye movements. During fixation, the eye's fovea is examining the image. Drift Surprisingly the eye is not still. If one looks closely one can see miniature movements, a fraction of a degree in size, consisting of rapid saccades and a slow drift. Saccade The figure show how these movements move the retina's cones across the image of the letter "A". One might think that these tiny movements would impair vision. However they appear to be essential for vision. If the image of the "A" is artificially stabilised on the retina, it disappears

17 Summary: the function and characteristics of the five eye movement types. OLD SYSTEMS: Purpose to keep the images in the background still on the entire retina when head moves (e.g. reading signs while walking) VOR: fast, has only 3 synapses but decays during prolonged rotation (cupula adapts) calibrated by the cerebellar flocculus Optokinetic system helps the VOR during prolonged rotations slow to get going, but that s all right because the VOR is fast visual input projects via the n. optic tract & accessory optic system to the vestibular n. tested by a large full field visual stimulus (e.g. optokinetic drum) which elicits the sensation of being rotated NEW SYSTEMS: Purpose to bring the image of a selected object onto the fovea and keep it there. Saccades: are very fast and accurate. You have no voluntary control over their speed. (Ask subject to look back and forth between two fingers. If you can see the eye move during the saccade, its speed is abnormal.) If instead of a large saccade you see several small saccades, damage is likely in the motorneurons, or cerebellar vermis. Pursuit: used to keep you fovea on a moving target: e.g. a ball involuntary slow, needs visual feedback, requires many synapses disrupted by lesions of the cerebellar flocculus or cortical MT/ MST area Vergence: points both foveae at a near target prevents diplopia (double vision) disconjugate (in opposite directions) 11.17