Download ipsilateral

Perception is constructive
Perception is context dependent
Perception involves interpretation
Perception groups into meaningful units
Optic Nerve
•
•
•
•
•
Carries ~1.25 million RGCs from retina.
Is myelinated (unlike retinal nerve cells)
About 2 inches (5 cm) to optic chiasm
Fibers cross at the chiasm
After the chiasm the structure is called the
optic tract.
Ipsilateral and contralateral
• Varies across species.
• Frontal eyes have crossing; lateral eyes
do not.
• In humans, ~50% of fibers cross the
medial plane.
The first bifurcation:
• In some fish and amphibians, the majority of
LGN cells project to the superior colliculus.
• In mammals and reptiles, the majority of LGN
cells project to the cortex.
• In humans, ~80 of LGN cells project to the
cortex; ~20% of LGN cells project to the superior
colliculus.
The Superior Colliculus
• Cells in the superior colliculus are position
sensitive, but have ill-defined ON and OFF
regions.
• Thus, probably not concerned with what is
present, just with where something is.
The Superior Colliculus
• Activity initiates eye movements.
• Activity guides eye movements.
The Superior Colliculus
• Cells are multi-modal or multisensory.
– They respond to apparently co-occurring
sound and light.
– They respond more vigorously when both
stimuli are present than when just one is
present.
The Superior Colliculus
• Thus, the superior colliculus appears to
function to bring objects into fixation,
rather than to analyze objects.
Structure of the LGN
• Six layers bent in the middle.
• Layers 1-2 are magnocellular; layers 3-6
are parvocellular.
• Layers 1, 4, & 6 are contralateral; layers 2,
3, & 5 are ipsilateral.
Layers 1, 4, and 6 respond to information from the contralateral
eye, whereas layers 2, 3, and 5 respond to information from the
ipsilateral eye.
Red-colored dye crystals (Dye-I) were inserted into
the parvocellular layers and yellow/green-colored
crystals into the magnocellular layers (Photo
enhanced.)
P & M ganglion Cells (cf. p. 90)
Characteristic
Size
Conduction
% ganglion cells
Spatial resolution
Temporal resolution
Contrast sensitivity
P cells
small
slow
80%
high
low
low
M cells
large
fast
20%
low
high
good
P & M LGN cells
Characteristic
Color opponent
Spatial resolution
Temporal Resolution
P cells
yes
high
low
M cells
no
low
high
Color opponency in the LGN
• Most parvocellular cells display color
opponency.
• Some centers are excited by one color, but
inhibited by others.
• For example, red/green and blue/yellow.
Function of the LGN
• LGN appears to preserve retinal information
about the presence of edges.
• LGN receives information from both the retina
and the reticular activating system. Thus, the
LGN could be modulated by the overall level of
arousal.
• K cells may be involved in suppressing cortical
information processing during saccades.
• LGN also receives input from the cortex.