Visual System Visual pathways and eye movements Phase 1b lecture Dr Simon Hickman Consultant Neurologist Royal Hallamshire Hospital Sheffield Optic nerves and visual pathways • Basic anatomy of the visual pathways • Functional approach to visual perception – Cell specialisation in the optic nerves – Crossing in the optic chiasm – Primary visual cortex – “What” and “where” pathways • Eye movements – Types of eye movement – Horizontal and vertical gaze Retina www.arthursclipart.com Retinal encoding • 150 million photoreceptors and 1 million nerve fibres in the optic nerve • Individual retinal ganglion cells (RGCs) are never electrically silent –Action of the horizontal and amacrine cells to allow inhibition of adjacent areas • Inputs to ganglion cells originate from photoreceptors in circumscribed areas of the retina –Receptive field - varies in size across the retina Functional encoding in the optic nerve • Chromatic sensitivity and spatial summation: –Parvocellular RGCs • Low-contrast • High linear spatial resolution –Koniocellular RGCs • Blue-yellow colour opponency –Magnocellular RGCs • High-contrast • Low-resolution • Motion detection • Colour blind Girkin CA, Miller NR. Surv Ophthalmol 2001;45:379-405. Island of vision • “…the most helpful mental picture of the visual field is obtained when we regard it from the standpoint of visual acuity. We may imagine the field as an island* of vision surrounded by a sea of blindness” • Traquair HM. An Introduction to Clinical Perimetry. London, Henry Kimpton 1949. * The idea of the field of vision as an island in a sea of blindness derives originally from Euclid and from Heliodorus, who thought the field was cone-shaped with a circular base, outside of which nothing could be seen Island of vision • “…The coast-line is somewhat ovoid in shape , and rises steeply so that the island is surrounded by cliffs vertical at one side, sharply sloping at the other. Above the cliffs is a sloping plateau which rises more rapidly again towards the somewhat eccentrically situated summit. This is crowned by a sharp pinnacle whose sides curve steeply upwards from a narrow base.” • Fovea occupies 0.01% of the visual field (<2° visual angle), yet 10% of the retinal ganglion cells (optic nerve fibres) subserve foveal vision Normal visual field http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cm&part=A3543 Normal visual field is an island of vision 90 degrees temporally to central fixation, 50 degrees superiorly and nasally, and 60 degrees inferiorly Visual pathway Left Right Optic nerve head Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Optic radiation Primary visual cortex Hickman SJ. Neurological visual field defects. Neuro-Ophthalmology 2011;35:242-250. Optic chiasm • Functionally (and teleologically) two eyes are better than one for the exploration of the environment and for panoramic defence. • A mechanism for the purpose of fusing the images of the two eyes and preventing double vision • Deduced by Sir Isaac Newton from 1st principles – Query 15 in: – Opticks: Or, a Treatise of the Reflexions, Refractions, Inflexions and Colours of Light. 1706 Lateral geniculate nucleus • Synapse of first order retinal ganglion cell neurons to 2nd order neurons • Magnocellular layers –1 and 2 • Parvocellular layers –3 to 6 • Koniocellular layers –Interspersed Retinal image: -Vertically and horizontally inverted Primary Visual Cortex - fMRI Retinotopic map of the visual world • Approx. 90% or nerve fibres subserve central 30° of vision Visual processing • “We do not see objects as such; we see shapes, surfaces, contours, and boundaries, presenting themselves in different illuminations or contexts, changing perspective with their movement or ours. From this complex, shifting visual chaos, we have to extract invariants that allow us to infer or hypothesize objecthood.” • Oliver Sacks. The mind’s eye. Functional specialisation • Different attributes of the visual scene are processed simultaneously, in parallel, but in anatomically separate parts of the visual cortex • Firstly - vision from eye - everything is back to front and upside down - corrected in brain Dorsal and Ventral Streams Dorsal Stream “Where” Pathway Ventral Stream “What” Pathway www.arthursclipart.com • V5/MT (Dorsal occipito-parietal) – Motion detection Depth perception • Brain uses cues to perceive relative size and position of objects –Familiar size • Judge distance if know the size of an object –Occlusion • If an object is partially occluding another we presume it is in front –Linear perspective • Parallel lines converge with distance - convergence implies depth –Size perspective • The smaller object will be assumed to be more distant –Distribution of shadows and illumination • Patterns of light and dark can give the impression of depth –Motion parallax • As we move, objects closer than the object we are looking at seem to move quickly and in the direction opposite to our own movement, whereas more distant objects move more slowly and in the same direction as our movement Attention • V4 – Colour detection Colour constancy • The coloured regions appear rather different, roughly orange and brown. –They are the same colour, and in identical immediate surrounds, but the brain changes its assumption about colour due to the global interpretation of the surrounding image. Also, the white tiles that are shadowed are the same colour as the grey tiles outside the shadow. • Colour constancy and brightness constancy are responsible for the fact that a familiar object will appear the same colour regardless of the amount of light or colour of light reflecting from it. An illusion of colour or contrast difference can be created when the luminosity or colour of the area surrounding an unfamiliar object is changed. –The contrast of the object will appear darker against a black field that reflects less light compared to a white field even though the object itself did not change in colour. Similarly, the eye will compensate for colour contrast depending on the colour cast of the surrounding area http://en.wikipedia.org/wiki/Optical_illusion Face-selective regions FFA FFA STS FFA: fusiform face area OFA: occipital face area STS: superior temporal sulcus OFA FFA Face recognition processing Fox CJ, Giuseppe Iaria, Barton JJS. Disconnection in prosopagnosia and face processing. Cortex 2008;44:996-1009. How have we evolved to be able to read this? • Inferotemporal neurons have evolved for general visual recognition • All writing systems share basic topological similarities • Shapes of letters “have been selected to resemble the conglomerations of contours found in natural scenes, thereby tapping into our already-existing object recognition mechanisms” • “The brain constrains the design of an efficient writing system” • Oliver Sacks. The mind’s eye. Conscious perception • The Binding Problem Why do we move our eyes? • Clear vision of an object requires that its image be held steadily on the retina –Best vision is possible when the image lies on the fovea • Eye movements evolved to aid vision • Gaze holding • Gaze shifting Types of eye movements Class of Eye Movement Vestibular Visual Fixation Optokinetic Smooth Pursuit Nystagmus quick phases Main Function Holds images of the seen world steady on the retina during brief head rotations or translations Holds the image of a stationary object on the fovea by minimizing ocular drifts Holds images of the seen world steady on the retina during sustained head rotation Holds the image of a small moving target on the fovea; or holds the image of a small near target on the retina during linear self- motion; with optokinetic responses, aids gaze stabilization during sustained head rotation Reset the eyes during prolonged rotation and direct gaze towards the oncoming visual scene Saccades Bring images of objects of interest onto the fovea Vergence Moves the eyes in opposite directions so that images of a single object are placed or held simultaneously on the fovea of each eye Leigh RJ. Proc NANOS 2012 Cortical areas for eye movements • Primary visual cortex (V1) is the gateway for eye movements: –Without it, humans cannot make accurate visually guided eye movements • Patients with lesions affecting V5/MT report akinetopsia (loss of motion vision) and show impaired saccades and pursuit to targets moving in the contralateral visual hemifield Leigh RJ. Proc NANOS 2012 Human cortical areas important for eye movement control Liu, Volpe & Galetta. Neuro-Ophthalmology: Diagnosis and management. Philadelphia: WB Saunders Company 2001 Saccades - several rôles • Spontaneous saccade –∼20/min –Visual search of the environment • Reflexive or non-volitional saccades - short reaction times: –New visual, auditory, or tactile cues • Voluntary (intentional or volitional) saccades carry the eyes to a predetermined location corresponding to the position of a visual target or sound, ie they direct the fovea at a goal. –Can also be made in a predictive fashion when the target is moving in a regular pattern: the eye movement anticipates the change in target position. –Voluntary saccades can also be made toward imagined or remembered target locations, or in response to commands (e.g., “Look right”) • Saccades can be voluntarily suppressed to maintain steady foveal fixation Frontal Eye Field - FEF • Generating voluntary saccades • Suppression of saccades during steady fixation • Subregions of the FEF concerned with vergence and smooth pursuit • FEF is involved in triggering of the memory-guided saccade, possibly with a contribution from the parietal lobe (PEF) Leigh RJ. Proc NANOS 2012 Supplementary eye fields - SEF • Guide saccades during complex tasks: –Sequences of movements and responses when the instructional set changes • Also contribute to predictive smooth pursuit • The SEF seems to play an important role in shifting from a more automatic to volitional behaviour Leigh RJ. Proc NANOS 2012 Hypothetical scheme of the major structures that project to the brainstem saccade generator - premotor burst neurons in PPRF • LR - Lateral rectus • MR - Medial rectus • PPRF - Pontine paramedian reticular formation Liu, Volpe & Galetta. Neuro-Ophthalmology: Diagnosis and management. Philadelphia: WB Saunders Company 2001 The pursuit system • Parietal cortex enhances attention on the moving target • Frontal Eye Field contributes to initiation and maintenance of smooth pursuit • Projections descend to the brainstem and subsequently to the flocculus, paraflocculus, and vermis of the cerebellum • The flocculus projects to the ipsilateral medial vestibular nuclus, which, in turn, project to the abducens nucleus • Upward vertical smooth pursuit mediated by the y-group - a small cell group that caps the inferior cerebellar peduncle and downward via the superior vestibular nucleus Hypothetical anatomical scheme for smooth pursuit eye movements • MVN - Medial vestibular nuclei Liu, Volpe & Galetta. Neuro-Ophthalmology: Diagnosis and management. Philadelphia: WB Saunders Company 2001 Eye movements Wray SH. Eye movement disorders in clinical practice, 2014. Actions of extra-ocular muscles Muscle Primary action Secondary action Tertiary action Medial rectus Adduction — — Lateral rectus Abduction — — Superior rectus Elevation Intorsion Adduction Inferior rectus Depression Extorsion Adduction Superior oblique Intorsion Depression Abduction Inferior oblique Extorsion Elevation Abduction Wray SH. Eye movement disorders in clinical practice, 2014. Conjugate gaze Wray SH. Eye movement disorders in clinical practice, 2014. Vestibular-ocular system • Holds images of the seen world steady on the retina during brief head rotations or translations • MVN - Medial vestibular nucleus http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v7/ch038/002f.html