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Figure 3.4 Accommodating infinity and accommodating a short distance (simplified optical diagram)
As in accommodation, the convergence of eyes is done subconsciously, except if
the person wants to squint. The movement of eyes in the orbital globes to change
the convergence point and/or to follow a moving object can be very quick, with a
maximum speed of approximately 600 degrees per second.
Since accommodation and convergence of eyes is related to the depth of the object
being observed, there is a relation between the two. This correspondence is not innate
in humans, but acquired experimentally and subconsciously by infants. This natural
relation can become artificially incorrect when we look at stereoscopic images on
a screen: A virtual object placed behind the screen makes you converge the optical
axes on it while the eyes adapt to the screen. This modification of the convergence-
accommodation relation is likely to cause visual difficulties and strain to the user. We
will study this problem in the chapter “Stereoscopic restitution of vision''.
3.2.1.4 The retina
The retina is the place where the light energy is transformed into discrete electrochem-
ical signals. This signal comes out of the eye via the optic nerve. After having described
its general structure, we will describe each of the three retinal levels: photoreceptors,
bipolar cells and ganglion cells (Figure 3.5).
General organisation
The retina has several layers of neurons that cover the bottom of the eye. These layers
form a carpet of cells connected horizontally within the same layer and vertically from
one layer to other creating a strong pyramid-shaped neuronal architecture. In fact, if
the number of photoreceptors is in the region of 100 millions, the number of ganglion
cells whose axons constitute the optic nerve is approximately a million. Between the
photoreceptors and the ganglion cells, there are horizontal cells, bipolar cells and
amacrine cells. Paradoxically, the first layer of cells exposed to light is not that of
photoreceptors, but a layer of ganglion cells, except at the fovea. All axons of the
ganglion cells come out of the retina in the same region that has no photoreceptors,
called the blind spot or the optic disk. The photoreceptors are connected to each other
and also to the bipolar cells through horizontal cells, which form the outer fibre layer.
The bipolar cells create vertical links by connecting the photoreceptors to the cells of
the next layer.
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