Biomedical Engineering Reference
In-Depth Information
Light direction
Ganglionic cells
Fovea
Bipolar cells
Photoreceptor
Pigmented epithelium
Cones
Rods
FIgUre 10.7
(See color insert.)
Histological characteristic of the retina.
so-called accommodative strabismus (Griffin 1982). In minor, physiological hypermetropy
this movement will not be manifest, but it will remain latent, because fusion, a central
analysis process, contrasts it, activating the lateral recti. The simple apparently static fixa-
tion requires a fair coordination of various couples of agonist and antagonist muscles; stri-
ated, voluntary muscles; and of the accommodative system muscles, smooth, involuntary
muscles (Traccis and Zambardieri 1996).
The image forming on the retina is made up of a central component, in a retinal sense,
foveal, and a peripheral component. This one can be divided between the information
in the right perceptual hemispace and the left perceptual hemispace. As an effect of the
dioptric convergence of light, rays place themselves on the left and on the right hemiretina,
respectively (Figure 10.8).
The visual information travels along the ganglion axons and reunites on the blind spot
to form the optic nerve. The nerve, coming out from the bulb, is myelinated, and it pro-
ceeds via the orbit to the foramen; the nerve then converges and crosses in the optic chiasm
(Figure 10.9).
This is a rather critical structure because here is where the information from both eyes'
inherent homologous perceptual hemispace is first integrated. After this crossing, any
information from what is in the left visual space, and has reached the left or the right
eye, travels in the same bundle of fibers of the right optic tract, and vice versa on the
other side.
The fibers in the optic tract reach the lateral geniculate nucleus (LGN), a thalamic
nucleus (Hubel 1995). At this level the pathway of the peripheral fibers differentiates from
the one of the foveal fibers. Two different pathways can be individuated, one for WHERE
the object is and one for WHAT the object is. The fibers, which are already differentiated
because of the retinal level, in this tract are also spatially segregated. From the LGN, the
fibers, called optic radiation, reach the occipital cortex. In this area called 17 or V1, all
processing that allows the extraction of the elementary characteristic of the images takes
place. We thus find a columnar organization, separated for each eye, as well as neurons
that can detect light only if surrounded by dark (on center) or vice versa (off center), or
lines of a certain length or angular orientation, moving toward a direction or its opposite,
