Lecture 6 + Chapter 13
Vision and Agnosia
-~50% of cortex devoted to visual perception
-Stimulus in the visual system is light (electromagnetic energy)
-In retina there are
1. Photorecepter ; there are 2 types
-Rods sensitive to dim light, are used mainly for night vision
-cones transducer bright light and are used for daytime vision. There are 3 types
2.Bipolar cells use action pot. in ganglion cells.
3. Ganglion cells sends axons into the brain proper.
From the Eye to the CNS Pg 206-208.
-Signal processing of visual information is performed
-Photorecepter bipolar cels Ganglion cells
-Axons of the ganglion cells form a bundle = The optic nerve
-Visual information is transmitted to the CNS via optic nerve.
-optic nerve splits into two parts before entering brain
-temporal branch = ipsilateral
-nasal branch = contralateral
-point of crossover = optic chaism
-Once inside the brain, each optic nerve divides into pathways that differ with respect to where they
-Each eye is divided into two identical halves
-within each eye, one half receives stimulation from the left visual field and the other from the right
-Lateral or temporal branch stays on the same side
-Medial or nasal branch crosses over
-optic chiasm before entering the nerve, two optic nerves (one from each eye) meet and form this
Geniculo- Cortical pathway
- 90% of the visual information
-consists of axons that exit from the LGN(Lateral geniculate nuclueus) and ascends to the cortex
LGN is the primary relay center for visual information receives from the retina of the eye;
found in thalamus. LGN has 6 layers: layer 2,3,5 receive fibers from ipsilateral eye. Layer 1,4,6 from contralateral
-Almost all the fibers terminate in the primary visual area of the occipital lobe.
Geniculo-striate pathway pg 208-209
-Major visual pathway from the retina to the LGN to the striate cortex is the geniculostriate pathway
- 10% of visual information
-Consists of axons that exit from the LGN and go to subcorital structures ( pulvinar and superior
10% of optic tract constitutes more fibers that the entire auditory system
Big role in visual attention
Sometimes viewed as a more primitive visual system
-takes part in pattern recognition and conscious visual functions
-second main takes part in visual pathway is detecting and orienting to visual stimulation.
-So by the time infor reaches primary visual cortex
Visual information in the cortex has been processed by at least five distinct neurons:
Photoreceptors, bipolar cells, ganglion cells , LGN cells, and cortical cells
summary THUS FAR
-Primary visual cortex (90%)
-Pulvinar and Superior colliculus (10%)
-Superior collicus(10% - projects back to thalamus and then to cortex)- tectopulvinar pathway
tectopulvinar pathway : relays from the eye to the superior coliculus in the midbrain tectum and
reaches the visual areas in the temporal and parietal lobes through relays in the lateral posterior-
pulvinar complex of the thalamus.
Visual Cortex – Primary visual cortex
-Different names for primary visual cortex
-Brodmann’s area 17
-V1 (the striate cortex)- Primary vision area: receives the largest input from the LGN and project to all
other occipital regions. also the first processing level in the hierarchy
Retinal Topography and Cortical Blindness pg 354
-Damage to LGN causes blindness ( but rare)
-Damage of the primary visual cortex causes blindness ( common consequence of stroke)
-Hemianopia – loss of vision in either the left or right visual field (ex. [ |x]; x indicates loss of vision)
-Quadrantanopia- blindness in one quadrant of the visual field (ex. * | ‘+; ‘ indicates loss of vision)
-Damage to the optic tract, LGN or V1. Cortical Blindness and Consciousness
Area around the right calcarine fissure was removed
Reported not seeing anything in the left visual field
Able to point to light in the left visual field
Blindsight – Residual visual abilities within a field defect in the absence of acknowledged
Importance of subcortical visual pathways
Visual Association areas
- ~30 cortical visual areas with distinct functions
-V1 is initial projection region of LGN
- Do not synapse sequentially to V2, V3, V4, etc
-How do we define a visual area? Based on how areas respond to visual stimuli
Vision for action
-ex. When reaching for a particular object such as a cup, the fingers form a specific pattern that enables
a person to grasp the cup.
-Movement is guided by vision
-Vision for action must be sensitive to movement of the garget. Ex. Catching a ball requires specific
information about the location, trajectory speed and shape of the object.
-Vision for action is a function of the parietal visual areas in the dorsal stream
-the shaping of the hand corresponding to different object is due to dorsal stream.
Area MT (aka V5 Motion)
- Cells in area MT respond to movement but no colour
Visual pathways beyond the occipital lobe
-vision evolved first for motion , not for recognition
-The system of knowing what an object is includes the flow of visual information from area V1 to the
temporal lobe in the ventral stream.
- The system controlling the visual guidance of movements includes the flow of information from area
V1 to the parietal lobe in the dorsal stream
-those with dorsal stream damage can consciously report seeing objects but cannot reach accurately or
shape the hand appropriately when reaching.
Motion – area v5( MT)
Colour – Area V4. Visual cortex – functional dissocations
- Double dissociation
-MT is activated by motion ,but v4 is not.
-V4 is activated by colour, but MT is not
Disorder of Visual Pathways
-monocular blindness – loss of sight in the eye with destruction of the retina or optic nerve
-bitemporal hemianopita –occurs when a lesion of the medial region of the optic chiasm severs the
crossing fiber. leads to loss of vision of both temporal fields
Basically, you lose both the outer peripheral field of vision.
-nasal heminanopia – Lesion of the lateral chiasm results in a loss of vision of one nasal field.
Basically, you lose one of the inner field of vision. If its binasal then you its both.
-Homonymous hemianopia – occurs from a complete cut of the optic tract, l