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circle: it is the circle which passes through the fixation point and the nodal point of
each eye. Determination of the empiric horopter depends on different criteria as per
the definition selected (Tyler, 1991). The points situated ahead of the horopter are
said to be converging or in crossed disparity, whereas those behind the horopter are
diverging or in direct disparity.
If the retinal images are near and have slight horizontal disparities, the binocular
stimuli are perceived in a single image due to fusion of points that are within the
Panum's area (area in grey in the previous figure). This is known as binocular vision.
The objects are perceived to be in depth in front of or behind the fixation point by
binocular vision. However, binocular vision and appearance of depth are damaged
by vertical disparity (Tyler, 1991). Beyond Panum's area, the stimuli of each eye are
perceived separately. They are split, known as diplopia.
3.2.2.4 Neurophysiological mechanisms of the perception of depth
Depth can be estimated after mapping. This problem is far from being trivial because
disparity is an ambiguous cue. In fact, the same amount of disparity can be associated to
different distances depending on the convergence of vision. Therefore, to estimate the
actual depth, the information of disparity must be graded as per the convergence. This
process is called constancy of depth. Activity of cortical cells processing the disparity
should thus be extremely specialised and modulated by the fixation distance. Trotter
observes that whenwe modify the real distance of depth, the activity of neurons changes
remarkably. Neurons always prefer the same retinal disparity, for example “nearer''
or “farther'' from the fixation point. However, their level of activity depends to a
great extent on the distance from the object. Trotter shows that coherent perception
of the three-dimensional space is the result of a combination of messages issued from
the retina and the extra-retinal information regarding the position of eyes, made in a
group of neurons in the primary visual cortex. The most probable information is the
proprioceptive signal from the extrinsic eye muscles (Trotter, 1995).
To conclude this section on visual perception of depth, we can say the following
regarding the phenomenon of “3D'' vision: From a real 3D object, an observer receives
two 2D images on his two retinas. His visual system deduces a 3D perception from
these images. In an on-screen artificial 3D vision, we display two 2D images calculated
on the basis of a computer-generated 3D model of the object. The retinas receive two
2D images which are perceived by the visual system as 3D!
3.2.3 Psychophysical characteristics of vision
In this paragraph we will specify some characteristics useful to understand a visual
interface. These characteristics correspond to the efforts currently undertaken to
improve visual interfaces:
Improvement in the image definition;
Increase in the vision field;
Stereoscopic vision and
Immersion of eyes
Let's start this discussion with sensitivity of the human eye, i.e. its capacity to react to
very low levels of stimulation.
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