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Phys Ch 51
Visual Pathways
 Optic radiation also called geniculocalcarine tract
 Fibers that synapse in dorsal lateral geniculate nucleus of thalamus
and go to primary visual cortex called geniculocalcarine fibers
 Visual fibers also pass
o From optic tracts to suprachiasmatic nucleus of hypothalamus
to control circadian rhythms
o Into pretectal nuclei in midbrain to elicit reflex movements of
eyes to focus on objects of importance and activate pupillary
light reflex
o Into superior colliculus to control rapid directional
movements of eyes
o Into ventral lateral geniculate nucleus of thalamus and
surrounding basal regions of brain to help control some of
body’s behavioral functions
 Visual pathways divided into old system (to midbrain and base of
forebrain) and new system (responsible for direct transmission of visual signals into visual cortex; perception of
virtually all aspects of visual form, colors, and other conscious vision)
 Optic nerve fibers of new visual system terminate in dorsal lateral geniculate nucleus (lateral geniculate body)
o Lateral geniculate body relays visual info from optic tract to visual cortex by way of optic radiation; relay
function translates to exact point-to-point transmission with high degree of spatial fidelity from retina to
visual cortex
o Signals from 2 eyes kept apart in dorsal lateral geniculate nucleus
o Nucleus composed of 6 nuclear layers (layers II, III, and V receive signals from lateral half of ipsilateral
retina; layers I, IV, and VI receive signals from medial half of retina of opposite eye)
o Respective retinal areas of 2 eyes connect with neurons superimposed over one another in paired
layers; similar parallel transmission preserved all the way to visual cortex
o Dorsal lateral geniculate nucleus gates transmission of signals to visual cortex; nucleus receives gating
control signals from corticofugal fibers (returning in backward direction from primary visual cortex to
lateral geniculate nucleus) and reticular areas of mesencephalon
 Both inhibitory and when stimulated, can turn off transmission through selected portions of
dorsal lateral geniculate nucleus
 Both help highlight visual info that is allowed to pass
o Doral lateral geniculate nucleus divided by
 Layers I and II – magnocellular layers because they contain large neurons; receive input almost
entirely from large type Y retinal ganglion cells
 System provides rapidly conducting pathway to visual cortex
 Only transmits black-and-white info
 Point-to-point transmission poor because there aren’t many Y ganglion cells, and their
dendrites spread widely in retina
 Layers III-VI – parvocellular layers; contain large numbers of small to medium-sized neurons;
receive input almost entirely from type X retinal ganglion cells that transmit color and convey
accurate point-to-point spatial info only at moderate velocity (not high)
Organization and Function of Visual Cortex
 Primary visual cortex is terminus of direct visual signals from eyes
o Signals from macular area of retina terminate near occipital pole, and
signals from more peripheral retina terminate at or in concentric half
circles anterior to pole but still along calcarine fissure on medial
occipital lobe
o Upper portion of retina represented superiorly and lower portion
inferiorly
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Primary visual cortex also called visual area I or striate cortex because of
grossly striated appearance
Secondary visual areas (visual association areas) – lateral, anterior, superior,
and inferior to primary visual cortex
o Most areas fold outward over lateral surfaces of occipital and parietal
cortex
o Secondary signals transmitted to these areas for analysis of visual
meanings
o Numbered visual area II (V2), visual area III (V3), etc.
o Importance of all areas is variation aspects of visual image progressively
dissected and analyzed
Primary visual cortex has 6 distinct layers
Geniculocarine fibers terminate mainly in layer IV (organized into subdivisions)
Rapidly conducted signals from Y retinal ganglion cells terminate in layer IVcα; relayed from there vertically both
outward toward cortical surface and inward toward deeper levels
Visual signals from medium-sized optic nerve fibers, derived from X
ganglion cells in retina, terminate in layers IVa and IVcβ; from there,
signals transmitted vertically both toward surface of cortex and to
deeper layers (these transmit accurate point-to-point type vision and
color vision
Visual cortex organized structurally in vertical columns of neuronal cells
o Same vertical columnar organization found throughout
cerebral cortex for other senses as well (also in motor and
analytic cortical regions)
o Each column represents functional unit
After optic signals terminate in layer IV, they are further processed as
they spread outward and inward along each vertical column unit
o Processing deciphers separate bits of visual info at successive
stations along pathway
o Signals that pass outward eventually transmit signals for short
distances laterally in cortex
o Signals that pass inward excite neurons that transmit signals
greater distances
Interspersed among columns of primary and some secondary visual areas are special column-like areas (color
blobs); receive lateral signals from adjacent visual columns and are activated specifically by color signals
Signals from each eye remain separated from each other when they arrive in layer IV of primary visual cortex;
signals from one eye enter columns of every other stripe, alternating signals from second eye
o Deciphers whether respective areas of 2 visual images from eyes in register with each other; deciphered
info used to adjust directional gaze of separate eyes so they will fuse with each other
o Info observed about degree of register of images from 2 eyes allows person to distinguish distance of
objects by stereopsis
After leaving primary visual cortex, info analyzed by
o Analysis of 3D position, gross form, and motion of objects – pathway that tells where every object is
during each instant and whether it is moving
 After leaving primary visual cortex, signals flow into posterior midtemporal area and upward to
occipitoparietal cortex
 At anterior border of parietal cortex, signals overlap with signals from posterior somatic
association areas that analyze 3D aspects of somatosensory signals
 Position-form-motion pathway; signals mainly transmitted from Y optic nerve fibers of retinal Y
ganglion cells (rapid signals, but only black-and-white)
o Analysis of visual detail and color – pass from primary visual cortex into secondary visual areas of
inferior, ventral, and medial regions of occipital and temporal cortex
 Separate portions of pathway specifically dissect out color
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Concerned with recognizing letters, reading, determining texture of surfaces, determining
detailed color of objects, and deciphering what object is and what it means
Neuronal Patterns of Stimulation During Analysis of Visual Image
 Areas of max excitation occur along sharp borders of visual pattern; visual signal in primary visual cortex
concerned mainly with contrasts in visual scene rather than with noncontrasting areas
o Equally stimulated adjacent retinal receptors mutually inhibit one another
o At any border in visual scene, mutual inhibition doesn’t occur, and intensity of stimulation of most
neurons proportional to gradient of contrast
 Visual cortex also detects direction or orientation of each line or border; results from linear organizations of
mutually inhibiting cells that excite second-order neurons when inhibition occurs all along line of cells where
there is contrast edge
o For each orientation of line, specific neuronal cells stimulated (simple cells; found mainly in layer IV of
primary visual cortex)
 As visual signal progresses farther away from layer IV, some neurons respond to lines oriented in same direction
but not position specific (complex cells; stimulated by both of set of parallel lines)
 Some neurons in outer layers of visual columns stimulated only by lines or borders of specific lengths, specific
angulated shapes, or images that have other characteristics
o As one goes farther into analytical pathway of visual cortex, progressively more characteristics of each
visual scene deciphered
 Color detected by means of color contrast; contrasting against white mainly responsible for color constancy
(when color of illuminating light changes, color of white changes with light, and appropriate computation in
brain allows red to be interpreted as red, even though illuminating light has changed color entering eyes)
o Mechanism depends on fact that contrasting colors (opponent colors) excite specific neuronal cells
o Initiated details of color contrast detected by simple cells; more complex contrasts detected by complex
and hypercomplex cells
 Removal of primary visual cortex causes loss of conscious vision (blindness); blind people can still at times react
subconsciously to changes in light intensity, movement in visual scene, or rarely some gross patterns of vision
o Vision subserved by neuronal pathways that pass from optic tracts mainly into superior colliculi and
other portions of older visual system
 Field of vision – visual area seen by eye at given instant
o Perimetry – diagnosing blindness in specific portions of retina by having subject look with one eye closed
and other eye looking toward central spot directly in front of eye; small dot of light or small object
moved back and forth in all areas of field of vision, and subject indicates when spot of light or object can
be seen and can’t
o Blind spot caused by lack of rods and cones in retina over optic disc 15o lateral to central point of vision
o Scotomata – blind spots found in portions of field of vision other than optic disc area; frequently caused
by damage to optic nerve resulting from glaucoma, allergic reactions, or toxic conditions (lead, tobacco)
o Retinitis pigmentosa – portions of retina degenerate; excessive melanin pigment deposits in
degenerated areas; usually causes blindness in peripheral field of vision first and gradually encroaches
on central areas
 Destruction of optic chiasm prevents crossing of impulses from nasal half of each retina to opposite optic tract;
nasal half of each retina blinded (temporal fields of vision blinded; bitemporal hemianopsia)
 Interruption of optic tract denervates corresponding half of each retina on same side as lesion; neither eye can
see objects to opposite side of head (homonymous hemianopsia)
Eye Movements and Their Control
 CN III, CN IV, and CN VI vaguely connected in brainstem nuclei by way of medial longitudinal fasciculus
 Strong signals also sent from body’s equilibrium control centers in brain stem into oculomotor system (from
vestibular nuclei by way of medial longitudinal fasciculus)
 Fixation movements controlled by voluntary fixation or involuntary fixation
o Voluntary fixation movements controlled by cortical field located bilaterally in premotor cortical regions
of frontal lobes; bilateral dysfunction or destruction makes difficult for person to unlock eyes from one
point of fixation and move them to another point; usually necessary to blink or put hand over eyes for
short time, allowing eyes to be moved
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Fixation mechanisms that causes eyes to
lock on object of attention once found
controlled by secondary visual areas in
occipital cortex, located mainly anterior to
primary visual cortex
o When fixation area destroyed bilaterally,
animal has difficulty keeping eyes directed
toward given fixation point or may become
totally unable to do so
Involuntary locking fixation results from negative
feedback mechanism that prevents object of
attention from leaving foveal portion of retina
Eyes normall have continuous tremor (caused by
successive contractions of motor units in ocular
muscles), slow drift of eyeballs in one direction, and
sudden flicking movements (controlled by
involuntary fixation mechanism)
o When spot of light fixed on foveal region of
retina, tremulous movements cause spot to move back and forth across cones, and drifting movements
cause spot to drift slowly across cornea
o Each time spot drifts as far as edge of fovea, sudden reflex reaction occurs producing flicking movement
that moves spot back to center of fovea
o Involuntary fixation capability mostly lost when superior colliculi destroyed
When visual scene moving continually, eyes fix on one highlight after another in visual field, jumping from one
to next quickly (saccades); movements are opticokinetic movements
o Saccades occur so rapidly that more than 90% of time dedicated to fixation, not saccades
o Brain suppresses visual image during saccades, so person not conscious of movements
During reading, person usually makes several saccadic movements of eyes for each line; visual scene not moving
past eyes, but eyes trained to move by several successive saccades across visual scene to extract important info
o Same saccades occur when person observes painting (saccades follow lines of painting)
Pursuit movement – eyes remaining fixed on moving object; highly developed cortical mechanism automatically
detects course of movement of object and rapidly develops similar course of movement for eyes
o Eyes begin to jump by saccades in approximately same pattern of movement as object; after few more
seconds, eyes develop progressively smoother movements and finally follow movement almost exactly
Even after visual cortex destroyed, sudden visual disturbance in lateral area of visual field often causes
immediate turning of eyes in that direction; doesn’t occur if superior colliculi have been destroyed
o Various points of retina represented topographically in superior colliculi in same way as in primary visual
cortex with less accuracy
o Principal direction of flash of light in peripheral retinal field mapped by colliculi, and secondary signals
transmitted to oculomotor nuclei to turn eyes
o Superior colliculi also have topographical maps of somatic sensations from body and acoustic signals
from ears
o Optic nerve fibers from eyes to colliculi (responsible for rapid turning movements) are branches from
rapidly conducting Y fibers with one branch going to visual cortex and other going to superior colliculi
o Superior colliculi and other regions of brainstem also strongly supplied with visual signals transmitted in
type W optic nerve fibers; function unclear
o Signals relayed from superior colliculi through medial longitudinal fasciculus to other levels of brainstem
to cause turning of whole head and body toward direction of disturbance
o Other types of non-visual disturbances (strong sounds or stroking side of body) cause similar turning of
eyes, head, and body, but only if superior colliculi intact
Visual images in eyes normally fuse with each other on corresponding points of retinas; visual cortex helps
o Interactions occur between cortical neurons to cause interference excitation in specific neurons when 2
visual images not in register; provides signal transmitted to oculomotor apparatus to cause convergence
or divergence or rotation of eyes so fusion can be reestablished; once corresponding points of retinas in
register, excitation of specific interference neurons in visual cortex disappears
 Even when eyes fused with each other, it is still impossible for all corresponding points in both visual images to
be exactly in register at same time; degree of nonregister provides neural mechanism for stereopsis (important
mechanism for judging distances of closer objects)
o Mechanism based on fact that some of fiber pathways from retinas to visual cortex stray 1-2o on each
side of central pathway; some optic pathways from both eyes exactly in register for objects 2 m away
and different set in register for objects 25 m away
o Distance determined by which set or sets of pathways excited by non-register or register
 Strabismus (squint or cross-eye) – lack of fusion of eyes in one or more visual coordinates (horizontal, vertical, or
rotational); combinations of directional strabismus can occur
o Caused by abnormal set of fusion mechanism of visual system; in young child’s early efforts to fixate
eyes on same object, one eye fixates satisfactorily while other fails to do so, or both fixate satisfactorily
but never simultaneously; patterns of conjugate movement of eyes become abnormally set in neuronal
control pathways, so eyes never fuse
o Visual acuity highly dependent on proper development of CNS synaptic connections from eyes
Autonomic Control of Accommodation and Pupillary Aperture
 PNS preganglionic fibers arise in Edinger-Westphal nucleus and pass in CN III to ciliary ganglion behind eye
o Preganglionic fibers synapse with postganglionic PNS neurons, which send fibers through ciliary nerves
o Nerves excite ciliary muscle and sphincter of iris
 SNS innervation originates in interomediolateral horn cells of T1; fibers then enter SNS chain and pass upward to
superior cervical ganglion, where they synapse with postganglionic neurons
o Postganglionic SNS fibers spread along surfaces of carotid artery and successively smaller arteries until
they reach eye
o SNS fibers innervate radial fibers of iris and several extraocular muscles of eye
 Accommodation results from contraction or relaxation of ciliary muscle; constriction causes increased refractive
power of lens; regulated by negative feedback mechanism that automatically adjusts refractive power of lens to
achieve highest degree of visual acuity
o When eyes suddenly change distance of fixation point, lens changes strength in proper direction to
achieve new state of focus
o Chromatic aberration (red light rays focus slightly posteriorly to blue light rays because lens bends blue
rays more than red rays); eyes able to detect which color in better focus and use that to accommodate
mechanism to make lens stronger or weaker
o Neural mechanisms for convergence cause simultaneous signal to strengthen lens of eye
o Because fovea lies in hollowed-out depression that is slightly deeper than remainder of retina, clarity of
focus in depth of fovea different from clarity of focus on edges
o Degree of accommodation of lens oscillates slightly all the time; visual image becomes clearer when
oscillation of lens strength changing in appropriate direction and becomes poorer when lens strength
changes in wrong direction
o Brain cortical areas that control accommodation closely parallel those that control fixation movements;
transmission of motor signals to ciliary muscle through pretectal area in brainstem, then through
Edinger-Westphal nucleus, then by PNS nerve fibers to eyes
 Stimulation of PNS excites pupillary sphincter muscle, decreasing pupillary aperture (miosis)
 Stimulation of SNS excites radial fibers of iris and causes mydriasis
 Neuronal pathway for pupillary light reflex – when light impinges on retina, few resulting impulses pass from
optic nerves to pretectal nuclei; from there, secondary impulses pass to Edinger-Westphal nucleus and back
through PNS fibers to constrict sphincter of iris; in darkness, reflex inhibited, resulting in dilation of pupil
o Light brightness on retina increases with square of pupillary diameter
 CNS diseases can damage nerve transmission of visual signals from retinas to Edinger-Westphal nucleus,
blocking pupillary reflexes; occurs in syphilis, alcoholism, encephalitis, etc.
o Block usually occurs in pretectal region of brain stem; can result from destruction of some small fibers in
optic nerves
o Final nerve fibers in pathway through pretectal area to Edinger-Westphal nucleus mostly inhibitory
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When inhibitory effect lost, nucleus becomes chronically active, causing pupils to remain mostly
constricted; pupils can constrict a little more if Edinger-Westphal nucleus stimulated through some
other pathway
 When eyes fixate on near object, signals that cause accommodation of lens and those that cause
convergence of eyes cause mild degree of pupillary constriction
 Pupil that fails to respond to light but does respond to accommodation (Argyll Robertson pupil)
important diagnostic sign of CNS disease such as syphilis
SNS nerves to eye occasionally interrupted (Horner’s syndrome), frequently in cervical SNS chain
o Blood vessels on corresponding side of face and head persistently dilated