Human Neurophysiology Laboratory Manual. Laboratory 4. Vision

Transcription

1 Department of Electrical and Computer Engineering University of Cyprus Human Neurophysiology Laboratory Manual Laboratory 4 Vision Nicosia 2006

2 2006 Constantinos Pitris Department of Electrical and Computer Engineering University of Cyprus The reproduction of this material, in full or partially, is prohibited without the written consent of the author.

3 Goal Laboratory 3 Vision Understand the neurologic exam as it relates to vision and the eye Examine and understand the some of the details of function in the visual system Session 1 Neurologic Exam (Vision and Extraocular Muscles) During the course of this laboratory we will perform different tests which are usually included in the neurologic physical exam. The goal is to demonstrate how each step of the neurologic exam is performed. In addition, we will briefly discuss which neuroanatomic systems are tested by each part of the neurologic exam, so that its functional and practical relevance will be better appreciated. For a more detailed discussion of the neurologic exam, as well as a clinically oriented approach to neuroanatomy with over 100 real clinical cases presented in an interactive format, please see Neuroanatomy Through Clinical Cases, Sinauer Associates, Inc., Publishers. The neurologic exam as a diagnostic tool gained mythical proportions in the pre-ct/mri era when great clinicians could pinpoint a lesion in the nervous system with often astounding accuracy. Today, with the availability of modern imaging techniques, the neurologic exam remains a critical way station in the clinical decision-making process. Does the patient who just collapsed on the street have cardiac disease or an intracranial bleed? Is the patient with leg weakness and numbness suffering from degenerative joint disease or from impending spinal cord compression? Does the patient with nausea and vomiting need a gastroenterology consult, a head CT, or emergency interventions to lower dangerously elevated intracranial pressure? These, and many similar questions that frequently arise for health care providers in all subspecialties can quickly be answered by a carefully performed neurologic exam. Most clinicians include certain important tests in the neurologic exam, although there are variations in personal style, emphasis and order of the tests. Here, we will adopt a fairly widely accepted format for the neurologic exam, consisting of the following six subdivisions: 1. Mental Status 2. Cranial Nerves 3. Motor Exam 4. Reflexes 5. Coordination and Gait 6. Sensory Exam In this laboratory we will examine the neurologic exam for Cranial nerves II, III, IV and VI. II - Visual acuity, visual fields and ocular fundi II,III - Pupillary reactions III,IV,VI - Extra-ocular movements, including opening of the eyes For the following experiments you will need 4 willing volunteers. You can use family, friends and significant others. Experiment 1 Cranial Nerve II - Visual acuity, visual fields Test Visual Acuity Repeat the following steps for each volunteer. If the volunteer wears glasses, repeat the test without (uncorrected vision) and with glasses (corrected vision) 1. Hold a Rosenbaum pocket card at about 35 cm "reading" distance from the volunteer s eyes. (You can find a printed version of the card at the end of this laboratory) 2. Have the volunteer cover one eye at a time.

4 3. Ask him/her to read progressively smaller letters until they can go no further. 4. Record the smallest line the volunteer can read successfully (20/20, 20/30, etc.) 5. Repeat with the other eye. 6. Fill in the following table. Volunteer Number Volunteer Age Left Eye Right Eye Uncorrected Corrected Uncorrected Corrected Visual acuity is reported as a pair of numbers (20/20) where the first number is how far the patient is from the chart and the second number is the distance from which the "normal" eye can read a line of letters. For example, 20/40 means that at 20 feet the patient can only read letters a "normal" person can read from twice that distance. Screen Visual Fields by Confrontation 1. Stand about 60 cm in front of the volunteer and have them look into your eyes. 2. Hold your hands extend out and forward, half way between you and the volunteer, and wiggle a finger on one hand. 3. Ask the patient to indicate which side they see the finger move while they keep looking straight at your eyes. Move your hands closer if the volunteer can not see the fingers moving. (Note: you should be able to see your fingers move too while looking straight at the volunteers eyes.) 4. Repeat two or three times to test both temporal fields. 5. Wiggle fingers on both hands occasionally to test for visual extinction on double simultaneous stimulation. In visual extinction, a form of hemineglect, patients do not report seeing the fingers wiggle on the affected (usually left) side of the visual field, although they can see them when they are presented on that side alone. 6. Fill in the following table. Note any asymmetries. Volunteer Number Volunteer Age Left Visual Field Can see stimulus? Extend of Arm Right Visual Field Can see stimulus? Extend of Arm Notes (Asymmetry, hemineglect, etc) 1 Yes / No Yes / No 2 Yes / No Yes / No 3 Yes / No Yes / No 4 Yes / No Yes / No What is Being Tested? Damage anywhere in the visual pathway from the eye to the visual cortex can cause specific deficits in the visual fields of one or both eyes. Importantly, some visual information from each eye crosses to the opposite side at the optic chiasm. Therefore, lesions in front of the optic chiasm (eye, optic nerve)

5 cause visual deficits in one eye, while lesions behind the optic chiasm (optic tract, thalamus, white matter, visual cortex) cause visual field deficits that are similar for both eyes. Visual hemineglect or extinction is usually caused by contralateral parietal lesions, and less often by frontal or thalamic lesions. Neglect is usually more robust in lesions of the right hemisphere. Experiment 2 Cranial Nerves II&III - Pupillary Responses Test Pupillary Reactions to Light 1. Dim or turn off the room lights so that the pupils dilate. 2. Ask the volunteer to look into the distance. 3. Record the size of the pupils in mm at this state (dilated.) 4. Shine a bright light obliquely into each pupil in turn. 5. Look for both the direct (same eye) and consensual (other eye) reactions. 6. Record pupil size in mm and any asymmetry or irregularity. Volunt. Numb. Volunt. Age Dilated Left Pupil Size Right Pupil Size Notes Direct Constr. Consensual Constr. Dilated Direct Constr. Consensual Constr. (Asymmetry, irregularities, etc) Note: you can estimate pupil diameter by comparing to the pupil sizes on the vision chart at the end of this laboratory. It will be easier if you cut off the chart and hold it close to the volunteers face. What are your observations regarding direct and consensual reaction? Test Pupillary reactions to swinging llight 1. Dim or turn off the room lights so that the pupils dilate. 2. Ask the volunteer to look into the distance. 3. Swing a bright light obliquely from one eye to the other 4. How do the pupils react? 5. What do you expect would happen if one eye was blind?

6 Test Pupillary Reactions to Accommodation 1. Stand about 50 cm in front of the volunteer. 2. Hold your index finger up at nose level and ask him/her to keep looking at it while you move it closer. 3. Move your finger to about 10cm from the volunteer's nose. (Yes, they will become cross-eyed!) 4. Record the pupillary diameter in each eye. Volunteer Number Volunteer Age Accommodation Left Pupil Size Accommodation Right Eye Before During Before During What is Being Tested? Direct response (pupil illuminated). The direct response is impaired in lesions of the ipsilateral optic nerve, the pretectal area, the ipsilateral parasympathetics traveling in CN III, or the pupillary constrictor muscle of the iris. Consensual response (contralateral pupil illuminated). The consensual response is impaired in lesions of the contralateral optic nerve, the pretectal area, the ipsilateral parasympathetics traveling in CN III, or the pupillary constrictor muscle. Accommodation (response to looking at something moving toward the eye). Accommodation is impaired in lesions of the ipsilateral optic nerve, the ipsilateral parasympathetics traveling in CN III, or the pupillary constrictor muscle, or in bilateral lesions of the pathways from the optic tracts to the visual cortex. Accommodation is spared in lesions of the pretectal area. Interesting factoid: Pupils with a diminished response to light but a normal response to accommodation (Argyll-Robertson Pupils) are a sign of neurosyphilis. Experiment 3 Cranial Nerves III, IV $ VI - Extraocular Movements (Graduate Students) You will probably want to review extraocular eye movement before proceeding. There is a very nice animated tutorial at: Test Smooth Pursuit Movements 1. Stand or sit 3 to 6 feet in front of the patient. 2. Ask the patient to follow your finger with their eyes without moving their head. 3. Check gaze in the six cardinal directions using an H pattern of motion (i.e. move your finger from the center out (as if you were checking visual fields and then up and down, drawing a big H.) 4. Record whether your volunteer can follow your fingers in all directions and whether the pursuit was smooth.

7 5. Pause during upward and lateral gaze to check for nystagmus at the top, bottom, left or right. (Nystagmus is a rhythmic oscillation of the eyes. Horizontal nystagmus is described as being either "leftward" or "rightward" based on the direction of the fast component.) Volunt. Numb. Volunt. Age Full Motion? Left Eye Smooth Motion? Nystagmus Full Motion? Right Eye Smooth Motion? Nystagmus Notes Test Saccadic Movements Saccades are eye movements used to rapidly refixate from one object to another. 1. Test saccades by holding two widely spaced targets in front of the volunteer (such as your thumb on one hand and index finger on the other.) 2. Ask the volunteer to look back and forth between the targets. 3. Are both eyes moving fast and in unison? Test optokinetic nystagmus (OKN) 1. Move a strip with parallel stripes on it in front of the volunteer's eyes. First slowly (~3 sec per direction) and then faster (~1 sec per direction) 2. Ask him/her to keep their head straight and watch the center of the strip as the stripes go by. 3. Observe the motion of their eyes. Can you see OKN? What do you observe at the two different sppeds? What is being tested? Careful testing can often identify abnormalities in individual muscles or in particular cranial nerves (oculomotor, trochlear, or abducens) in their course from the brainstem to the orbit, in the brainstem nuclei, or finally, in the higher-order centers and pathways in the cortex and brainstem that control eye movements. Spontaneous nystagmus can indicate toxic or metabolic conditions such as drug overdose or alcohol intoxication, or peripheral or central vestibular dysfunction.

8 Session 2 Fun with Vision Experiment 1 Compare Shades of Gray 1. Go to the website: 2. Choose the brightness difference between the two big squares to be 255 (i.e. the left will be completely black and the right completely white.) 3. Using the appropriate buttons, make the right inner square darker or lighter so that the intensity matches the left inner square: 4. When done, press the button below to compare the difference between the left and right inner square gray levels. Record the difference between the two. 5. Repeat the experiment for the background differences in the table below and record the values. 6. Plot the values in the space below. Background Level Difference Left Inner square value Left Inner square value Difference What are your conclusions regarding the apparent difference in brightness? How is this effect explained?

9 Experiment 2 Fun with Blindspots The visual system calculates visual information through both brute force with millions of neurons, and through a number of short cuts, extrapolations, and self regulating mechanisms. These non-linearities in the visual system can be illustrated with visual illusions. Here, we will illustrate some of the features of the visual system that are related to those which we are currently investigating. The visual system attempts to extrapolate information based upon what is known. A particularly prominent example of this is the Filling-in of missing information at the blind spot. The blind spot is where the ganglion cell axons gather together and leave the retina. Since there are no photoreceptors here, it forms a spot of no visual input. This spot is actually rather huge. That means that if you close one eye, there is a hole in your visual world that is an cm across at arms length!!! I don't know about you, but when I try this with my computer monitor, I would swear that there is no such "hole" in the visual world. That is because, the visual system is very good at "repairing" itself, and has compensated for this blind spot since early development. Here are some demonstrations of this amazing feat, as well as demonstrations on other regulatory and compensatory mechanisms. Fixate your eyes on the black square with your face about 15 cm from the page. When you close your right eye, the left spot should disappear, while when you close your left eye, the right spot will disappear. You may have to move your head forward and back a bit to find your blind spot. Note that the fill-in area is yellow - since the surround of the blind spot is yellow. That is the visual system's best guess as to what should be there! Repeat with the figure below. What color is the fill-in area now? What does that tell you about global vs. local scene information?

10 Use your blind spot to make the sun disappear! Close your right eye, and fix your gaze on the X in the figure below. At the right point, the sun should disappear. Note that the sky seems to fill in where the sun used to be Extrapolation is great, but can't make up for reality. Note that pretty much only the R and D of READ are visible in the blind spot

11 Experiment 3 Map your Blindspot 1. Go to the website 2. Close your left eye and position your self so that when looking at the cross the black spot disappears. This is your blindspot. 3. Measure the distance of your right eye from the screen. Distance = 4. Start the application and move the cursor in and out of your blindspot, clicking on the screen every time the cursor just appears. Be sure to keep looking at the cross. 5. Do this several times, moving the mouse in several directions. When you have enough crosses on the screen to be able to draw an outline, measure the diameter of the blind spot on the screen. Diameter of Blind spot = 6. Given the above two measurements, what is the size of the blind spot on the retina? How did you calculate that value?

12 Experiment 4 Fixated images fade from view Fixate on the center of this gray circle without blinking too much! Note that much of the surface disappears and become white, leaving only the center black dot and a grayish immediate surround. This works best with one eye closed, but will also work with both eyes open. How long does it take before the shaded region disappears? Time = Edges affect Fill-in and Fading. Fixation of the figure below results in relatively little fading, although it may appear to contain a relatively uniform level of gray.

13 Experiment 5 Adaptation to color. First fixate on the left X for at least 30 seconds, and then fixate on the right X. You should see a negative afterimage of the image on the left. Stimulation of a neural pathway results in its adaptation, meaning that it is excited less over time. This adaptation can result in negative afterimages. Here, fixation on the X results in adaptation of the redgreen color pathway. When looking at a blank (white) screen, the visual pathways that were strongly stimulated by the image are adapted and discharge at a reduced rate. The brain interprets this as the opposite colors. In this particular slide, most of the adaptation takes place in the retina. A good way to test for this is to examine the binocularity of the effect. Fixate the left image with your left eye only. Now shift to the right X and note the after image. If you quickly switch eyes, you will note that the right eye has only a very week after-image in comparison to the left. The vast majority of the effect is monocular, and therefore occurs before the information from the two eyes is combined in the primary visual cortex. What is the mechanism of this adaptation?

14 Experiment 6 Psychedelic Motion! (Actually adaptation to motion) The visual system contains neurons that are very sensitive to motion (in area MT, for example). You can adapt these to give a motion after-effect with the following program. Run the program and fixate on the middle of the twirling figure for about 10 seconds, then look at your hand, computer keyboard, or other objects to make them squirm. Click on the link below to start the program for motion adaptation (requires java) How long does the motion last? Time = What is the mechanism of this adaptation? (Graduate students only)

15 Appendix Visual Acuity Chart

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17 OKN Strip On the following page there are two parts of an OKN strip. 1. Cut along the dotted lines. 2. Glue the center to make it longer. 3. Hold by the ends to move in front of volunteer s eyes.

18 Hold here Glue Here Hold here