Image Processing Reference
In-Depth Information
1
Neuronal Pathways of Vision
Humans and numerous animal species rely on their visual systems to plan or to
take actions in the world. Light photons reflected from objects form images that
are sensed and translated to multidimensional signals. These travel along the visual
pathways forward and backward, in parallel and serially, thanks to a fascinating chain
of chemical and electrical processes in the brain, in particular to, from, and within
the visual cortex. The visual signals do not just pass from one neuron or compart-
ment to the next, but they also undergo an incredible amount of signal processing
to finally support among others, planning, and decision-action mechanisms. So im-
portant is the visual sensory system, in humans, approximately 50% of the cerebral
cortex takes part in this intricate metamorphosis of the visual signals. Here we will
present the pathways of these signals along with a summary of the functional prop-
erties of the cells encountered on these. Although they are supported by the research
of reknowned scientists that include Nobel laurates, e.g., Santiago Ramon y Cajal
(1906), and David Hubel and Thorsten Wiesel (1983), much of the current neuro-
biological conclusions on human vision, including what follows, are extrapolations
based on lesions in human brains due to damages or surgical therapy, psychological
experiments, and experimental studies on animals, chiefly macaque monkeys, and
cats.
1.1 Optics and Visual Fields of the Eye
The eye is the outpost of the visual anatomy where the light is sensed and the 3D
spatio-temporal signal
, which is called image, is formed. The “spatial” part of the
name refers to the 2D part of the signal that, at a “frozen” time instant, falls as a
picture on light-sensitive retinal cells,
photoreceptors
. This picture is a spatial signal
because its coordinates are in length units, e.g., millimeters, representing the distance
between the sensing cells. As time passes, however, the amount of light that falls on
a point in the picture may change for a variety of reasons, e.g., the eye moves, the
object in sight moves, or simply the light changes. Consequently the sensed amount
of photons at every point of the picture results in a 3D signal.
