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Summary Perceptual and Motor Development. Task 2. Visual Streams
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PAM Task 2 Visual DevelopmentLearning goals:
1. How do different shifts of eye gaze develop and what brain changes underlie this
development? link it to Schiller’s model
2. How is the development of oculomotor control different in ASD compared to TD
children?
Extra info (Bernstein en internet)
Visual pathways
Information reaches the brain via the optic nerve, a bundle of fibers made up out of the axons
of ganglion cells. After leaving the retina, about half the fibres of the optic nerve cross over to
the opposite of the brain at a structure called the optic chiasm. The optic chiasm is part of the
bottom surface of the brain; beyond the chiasm the fibres ascend to the brain itself. The axons
from most of the ganglion cells in the retina synapse in the thalamus, in a specific region
called the lateral geniculate nucleus (LGN). Neurons in the LGN then send the visual input to
the primary visual cortex, which lies in the occipital lobe, and also send from here to other
areas of the cortex. LGN is organised in multiple layers of neurons:
Layer 1, 2
Large cells, called magnocellular pathways
Input from Y-ganglion cells
Very rapid conduction
Colour blind system
Layer 3-6
Parvocellular
Input from X- ganglion cells
Colour vision
Moderate velocity.
Magnocellular cells: Rods; necessary for the perception of movement, depth, and small
differences in brightness
Parvocellular cells: Cones; long- and medium-wavelength ("red" and "green" cones);
necessary for the perception of color and form (fine details).
Neurons of different layers respond to particular aspects of visual stimuli: what and where
pathways. These sensations are then sent to the cortex.
What and where pathways refer to a proposed organization of the visual system. It describes
two information processing streams originating in the occipital cortex, dorsal (which goes
to parietal cortex) and ventral (which goes to temporal cortex).
The two strongest pathways linking the eye to the brain are those projecting to the dorsal part
of the LGN in the thalamus, and to the superior colliculus (or optic tectum, main visual
processor of the brain). The retina only accounts for a small percentage of LGN input in these
cases. 95% of input in the LGN comes from the visual cortex, superior colliculus, pretectum,
thalamic reticular nuclei, and local LGN interneurons. Regions in the brainstem that are not
involved in visual perception also project to the LGN.
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