Vision II
In summary, LGN neurons project axons into the optic radiations that travel via the internal capsule to the primary visual (striate) cortex where they terminate...
(1) only in cortical area 17, (2) only in specific cortical layers (mainly layer IV), and (3) again according to cell type and retinotopic position
Describe the Lateral-Inferior fiber? (Meyer's Loop)
- "Loops" into the white matter of the temporal lobe before projecting onto the inferior bank of the calcarine sulcus (terminating in the lingual gyrus) - Carries info about the SUPERIOR FIELD of the visual field (Michael Meyers - goes after the temporal) 🔪
Describe the Pupillary Light Reflex
- An adjustment by the pupil to different levels of ambient light - Reflex is CONSENSUAL (shining a light into only one eye causes the constriction of the pupils of both eyes)
What are Cataracts
- Clouding of the eye's lens - Complaints include glare, faded colors, an increased need for additional lighting while reading - Over 1 million Cataract operations performed annually in the US - Surgery is an outpatient procedure with a very high success rate
What is Diabetic Retinopathy
- Complication of diabetes - Can cause blood vessels of retina to hemorrhage - Typically there are no symptoms before major bleeding occurs in the eye (This is why everyone with diabetes should have regular eye exams)
Describe the Medial-Superior Fibers
- Course through the white matter of the parietal lobe before projecting onto the superior band of the calcarine sulcus (terminating in the cuneus gyrus) - Carries information about the INFERIOR PORTION of the visual field
Where are the cell bodies that grey their axons into the right optic tract?
- Left nasal retina - Right temporal retina
What are Saccades
- Rapid movement of the eye between fixation points (<msec) - Used to align fovea with particular aspects of visual scene - Involves little to no perception - Eye movements allows us to scan the visual pausing to focus on parts of the scene that convey the most significant information
Images are projected in a reversed and inverted (flipped) relationship onto the retina. What does this mean
- What starts left terminates right - What starts right terminates left - What starts up terminates down - What starts down terminates up
Describe the neural mechanism of a light being shined on the right eye only
1. Action potentials from right eye reach both right and left PRETECTAL NUCLEI 2. The pretectal nuclei stimulate both sides of the EDDINGER-WESTPHAL nucleus even though the light was perceived only in the right eye 3. The right and left sides of the Eddinger-Westphal nuclei generation action potentials through the right and left oculomotor nerves, causing both pupils to constrict
Homonymous Hemianopia — What symptoms might an individual afflicted with this eye condition have?
1. Bumping into things on the side of the visual field defect 2. Reading difficulties 3. Actions such as crossing the street may be dangerous as patients fail to appreciate oncoming cars on that side 4. Driving may be problematic as they change lanes where is an oncoming car or sideswipe objects 5. Objects on a desk or table may not be seen when located to the side of the visual field loss and sometimes even the food on that side of a plate is left uneaten
The Extrastriate Cortex has outputs to 2 pathways, what are they? Describe them
1. Dorsal Stream (lateral parietal) a. Responsible for spatial aspects of vision - Relationships between objects and ourselves - Movements of object (including ourselves) through the environment - Localizing targets - Visual control of actions - "Where?" 2. Ventral Stream (inferior temporal) - High-resolution form vision - Color processing - Object recognition - i.e., "What?"
Pupillary Light Reflex Testing & Deficits - What are two types of deficits + describe them?
1. Efferent - Direct response in left eye without consensual response in right eye - Damage to visceral motor outflow to right - Oculomotor N or Edinger-Westphal nucleus 2. Afferent - Failure to elicit any response - direct or indirect - to stimulation of left if both eyes respond normally to stimulation of right - Damage to sensory input from left eye, possibly to left retina, or optic nerve
What are 3 antagonistic pairs of muscles that control eye movements
1. Lateral + medial rectus 2. Superior + inferior rectus 3. Superior + inferior oblique
What are the 4 types of pupillary reflexes?
1. Left direct pupillary reflex 2. Left consensual pupillary reflex 3. Right direct pupillary reflex 4. Right consensual pupillary reflex
Why are eye movements important?
1. Maintain foveal fixation on a moving target 2. Maintain foveal fixation on a target during head movements 3. Acquire and fixate a new visual target
There are 2 categories of center-surround receptive fields, what are they? Describe them
1. On Center Off Surround - Excited when light falls on the center of its receptive field and inhibited by light in the surround 2. Off Center On Surround - Inhibited when light falls on the center of its receptive field and excited by light in the surround Each regions responds oppositely to light
LGN neurons project axons into the optic radiations that travel via the internal capsule to the primary visual (striate) cortex where they terminate. What do they terminate? (Not sure if we need to know this)
1. Only in cortical area 17 (Primary visual cortex or V1) 2. Only in specific cortical layers (mainly layer IV) 3. According to cell type and retinotopic position
As a review, what order was the visual pathway? (Maybe fix this one?)
1. Photoreceptors 2. Bipolar cells 3. Ganglion cells
Retinal Input to the LGN (Lateral Geniculate Nucleus) Layers
1. Right LGN receives information about the left visual field - Left visual field is viewed by nasal left retina and temporal right retina 2. At the LGN, input from the two eyes is kept separate - Monocular not Binocular 3. Each LGN arranged in 6 distinct layers of cell - Projections from IPSILATERAL eye terminate in layers 2, 3 and 5 - Layers 1, 4, and 6 receive input from the CONTRALATERAL eye
What are the 2 major types of retinal ganglion cells? Describe them
1. Type M ganglion cells (Magno = Large in Latin) - Color-insensitive motion detectors - Larger receptive fields - Transient, rapidly adapting response Best suited for signaling temporal variations in, and the movement of, a stimulus 2. Type P ganglion cells (Parvo = Small in Latin) - Color-sensitive object detectors - Small receptive fields - Produces a sustained, slowly adapting response Good for signaling the presence, color + duration of a visual stimulus (Outnumber the M ganglion cells) (100:1) Different types of ganglion cells appear to play different roles in visual perception
What is Bitemporal hemianopia
A loss of sight in the right visual field of the right eye and the left visual field of the left eye Aka HETERONYMOUS HEMIANOPIA because the affected regions of the visual fields in the two eyes are not congruent
In summary, the neurons in layer IV make very specific connections with cells in other cortical layers that are appropriate for ...
Appropriate for binocular vision and are specialized to enable the detection of contrast borders
What are receptive fields?
Areas of the retina that when illuminated or darkened illicit a response in a visual sensory neuron Composed of many photoreceptors arranged in - A central disk, the "center" - A concentric ring, the "surround" (Called a center-surround receptive field)
What is the primary action and associated CN with the Superior Rectus (SR)?
CN III - Rotate the eye upward (elevation)
What is the primary action and associated CN with the Inferior Rectus (IR)?
CN III - Rotates the eye downward (depression)
What is the primary action and associated CN with the Medial Rectus (MR)?
CN III - Rotates the eye inward, toward the nose (adduction)
What is the primary action and associated CN with the inferior oblique (IO)?
CN III - Rotates the top of the eye away from the nose (extorsion) Up + out
What is the primary action and associated CN with the superior oblique (SO)?
CN IV - Rotates the top of the eye toward the nose (intorsion) Down + out
What is the primary action and associated CN with the Lateral Rectus (LR)?
CN VI - Rotates the eye outward, away from the nose (abduction)
What is the preferred stimulus for a photoreceptors? Why?
Dark rather than light - When a shadow passes across a photoreceptor, it responds by depolarizing and releasing more neurotransmitter
When the images produced by the two eyes do not absolutely match, so that the images produced are misaligned relative to one another, what is this called?
Diplopia (Double Vision)
Dorsal and Ventral stream of image recognition (When a baseball is coming towards you)
Dorsal Stream "Where" (Lateral Parietal) - "It is moving toward me at high speed" Ventral Stream "What" (Inferior Temporal) - "It is a baseball"
Patient complains that when she poured coffee into a cup, it appeared at one moment to be frozen at the bottom of the cup and then suddenly it had overfilled the cup and converted the table. She also describe her difficulty crossing the street — one moment should would perceive cars to be in the distance, and the next moment they would be right next to her
Dorsal stream = Loss of motion
Damage to the visual pathways caudal to (behind) the optic chiasm on one side will disrupt vision of the whole half field, called a Hemifield, on the SAME SIDE. T/F?
False Damage to the visual pathways caudal to (behind) the optic chiasm on one side will disrupt vision of the whole half field, called a Hemifield, on the OPPOSITE side
When a person has both eyes open, the only part of the visual field that will be lost by blindness in one eye is the monocular visual field on the OPPOSITE SIDE (to the blinded eye). T/F?
False When a person has both eyes open, the only part of the visual field that will be lost by blindness in one eye is the monocular visual field on the SAME SIDE (as the blinded eye)
A crayon is placed in front of patient and he is asked what it is. He is unable to tell you. He then picks it up and reports that it is either a crayon or a candle. When prompted to choose, he smells it and reports that it is a crayon
He's drunk out of his mind Ventral stream = Loss of ability to recognize objects (visual)
When the affected regions of the visual fields of both eyes overlap (i.e., loss of vision in the left or right visual field of both eyes), the deficit is called
Homonymous A patient could be described as having a homonymous hemianopia or a homonymous quadrantanopia, etc
Visuotopic Organization of the Striate Cortex (V1) (Come back to this)
In V1, map of contralateral hemifield established in the LGN is maintained - The fovea is represented in the posterior part of the calcarine sulcus, with more peripheral parts of the contralateral visual hemifield represented in progressively motor anterior locations - The upper visual field is represented in the inferior bank of the calcarine sulcus - The lower visual field is represented in its superior bank Remember: - What starts left terminates right - What starts right terminates left - What starts up terminated down - What starts down terminates up
What is Hemianopia
Indicates loss of sight in one half of the visual field
If the oculomotor nerve of the first eye is damaged,
It can produce no direct light reflex as the motor component is lost. However, the optic nerve still conveys the afferent signal, so that the other eye will constrict consensually to light
In simpler terms, describe the concept of Hemifield if you're talking about the left Hemifield for example
Left Hemifield projects to the right side of the brain The right side of the brain sees the left side of the visual world
Describe Left Consensual Pupillary Reflex
Left pupil's indirect response to light entering the right eye (contralateral eye)
Describe Left Direct Pupillary Reflex
Left pupil's response to light entering the left eye
A patient has come to you complaining of double vision (dipolpia). His left eye fails to adduct when he makes eye movements to the right. His left eyelid droops (Ptosis) and the pupil in his left eye is larger than the pupil in the right. The pupil in the left eye does not react to light nor does it responds when light is shown in the right eye. How would you explain this deficit in the pupillary light reflex?
Lesion in the left Edinger-Westphal nucleus or left oculomotor nerve
During a physical exam, you shine a light into a patient's left eye and you note that both pupils react only sluggishly to light. You stimulate the right eye and you find a brisk constriction of both pupils. From this information alone, what sort of neurological problem might you expect?
Lesion in the left eye or left optic nerve
What is Anopia
Loss of sight in one or both eyes
What is Quadrantanopia
Loss of sight in one quadrant of the visual field
Widespread loss of vision without damage to the most central part of the visual field representation is called
Macular sparing - macular sparing is a phenomenon often associated with lesions in the visual cortex but it can be found with lesions along the length of the visual pathways
Describe Orientation Selectivity (From Reading)
Most of the receptive fields in the retina, + LGN are circular and give their greatest response to a spot of light matched in size to the receptive field center Huber + Wiesel found that many neurons in V1 respond best to an elongated bar of light moving across their receptive fields (But the orientation of the bar is critical!) - The greatest response is given to a bar with a particular orientation; bars perpendicular to the optimal orientation generally elicit much weaker responses (neurons having this type of response are said to exhibit orientation selectivity)
What do the oblique muscles do in the eye?
Move the eye outwards as well as up and down - Each hooks around a "pulley" so that it moves the eye the opposite way to that which its name suggest - Hence, the superior oblique muslce moves the eye down and out, and the inferior oblique muscle moves it up and out Oblique = opposite
Objects in the binocular region of the LEFT visual Hemifield will be imaged on the ______ retina of the LEFT eye and on the ______ retina of the RIGHT eye
Nasal retina; temporal retina - Temporal fibers see nasally + and nasal fibers see temporally Fibers from the nasal portion of the left retina cross (decussate) to the right side at the optic chiasm Fibers from the temporal portion of the right retina remain IPSILITATERAL
Describe Direction Selectivity
Neurons fire action potentials in direction-dependent response to moving bars of light In other words, V1 (Striate Cortex) neurons respond best to moving edges of light and shadow that are presented at a certain orientation and in a specific direction Direction-selective neurons are thought to be specialized for the analysis of OBJECT MOTION
Describe Ocular Dominance Columns (Throwing up the 'Rock' - Jay Z)
Neurons in the visual cortex that respond preferentially to input from one eye or the other - Perhaps play some role in binocular vision Dominant Eye - One your brain prefers when it comes to processing visual input
The following figure shows how a direction-selective cell responds to a moving stimulus
Notice in this example that the cell responds to an elongated stimulus swept rightward across the receptive field, but much LESS with leftward movement Sensitivity to the direction of stimulus motion is a hallmark of neurons receiving input from the magnocellular layers of the LGN
Describe the concept of right and left visual Hemifields
Objects appearing on the left of the fixation point midline are in the left visual hemifield Each retina sees overlapping regions of visdual sapce that includes portions of both visual hemifields - Binocular visual field - All areas of the binocular visual field are "seen" by both eyes The monocular visual field is determined by one eye covered
What is Glaucoma
One of the leading causes of blindness in the United States - Can damage vision so gradually you may not notice any loss of vision until the disease is at an advanced stage (eye exams critical) - Increased fluid pressure in eye - Loss of peripheral vision
In the Lateral Geniculate Nucleus, the fibers from the nasal half of 1 retina and the temporal half of the other synapse on cells whose axons form the _____
Optic Radiation
In summary, all retinal ganglion cells extend axons into the _____
Optic nerve But only ganglion cell axons from the nasal retinas cross at the OPTIC CHIASM
What is a rule of thumb about visual Hemifields
Optic nerve fibers cross in the optic chiasm so, the LEFT visual Hemifield is "viewed" by the RIGHT Hemisphere + the RIGHT visual Hemifield is "viewed" by the LEFT Hemisphere (Decussation)
What are retinal ganglion cells
Process information about different stimulus properties - Different types of ganglion cells appear to play different roles in visual perception
What TF happened to receptive fields?
Retinal information is passed on to the LGN (which also has a center surround receptive fields) LGN passes it onto the Cortex Each simply cell sums input from LGN neurons with neighboring/aligned receptive fields to build an elongated receptive field that is most responsive to elongated bars or edges
Describe Right Consensual Pupillary Reflex
Right pupil's indirect response to light entering the left eye (contralateral eye)
Describe Right Direct Pupillary Reflex
Right pupil's response to light entering the right eye
What is arguably the most important stimuli in the visual world?
Seeing edges - A visual edge is the place where 2 difference areas of a visual image meet - In other words, it is the perception of a contrast between 2 adjacent areas in a visual field - Edges define the position and extent of things - Determined by organization of receptive fields
You are conducting a cranial nerve exam and find that your patient cannot performance a right and down gaze and a left and down gaze. Which cranial nerve(s) is/are not functioning?
Superior oblique + Inferior Rectus (CN IV, CN III)
If the optic nerve of the first eye is damaged
The direct light reflex is lost in the first eye, as in the consensual effect in the second eye, as it receives no message. However, the oculomotor nerve in the first eye is intact, so it's pupil will still constrict when light is shone into the other eye
Relative contribution of the rectus + oblique groups depends on what?
The horizontal position of the eyes
Horizontal movements are controlled by _____
The lateral + medial rectus
If the optic nerve of the second eye is damaged when light is shone in the (normal) first eye, what happens to the second eye?
The second eye will still show consensual constriction, since its oculomotor nerve is intact
Vertical movements require coordinated action of _____
The superior and inferior rectus muscles + the superior and inferior obliques
If the oculomotor nerve of the second eye is damaged when light is shone into the normal first eye, what happens to the second eye?
There is no consensual constriction of the second eye
In summary, axons from the two eyes are mixed in the optic tract, but in the LGN
They are sorted out again (1) by ganglion cell type, (2) by eye of origin (ipsilateral or contralateral), and (3) by retinotopic position
In summary, pupils react (i.e., constrict) equally. Comparing the direct and consensual reaction to light in both eyes is helpful in locating a lesion, remembering that the retina and optic nerve are needed for the afferent signal and that the oculomotor nerve provides the efferent component of both the direct and consensual reflexes. T/F
True
Orientation Selectivity (From Slides)
V1 neurons typically respond vigorously to a bar of light oriented at a particular angle and less strongly - or not at all - to other orientations Orientation-selective neurons are thought to be specialized for the analysis of OBJECT SHAPE
Describe the functional columns of VI (Striate Cortex)
VI (Striate Cortex) is highly organized into functional cortical columns 1. Blobs - Sensitive to color 2. Orientation columns 3. Ocular dominance columns
