Visual, Tactile, and Multimodal Information Processing - Fundamentals and Assessment Tools for Occupational Ergonomics - page 498

TABLE 23.1 Comparison of Rods and Cones Properties and Functions
Rods Cones

High sensitivity to light Lower absolute sensitivity

Low acuity High acuity

Achromatic: one type of pigment Chromatic: three types of pigments

More numerous (100 to 120 million) Less numerous (6 to 7 million)

Highly convergent retinal pathways Less convergent retinal pathways

. Parallax or relative motion, that is, more distant objects show smaller movement across visual field
as the observer moves
. Occlusion, that is, if one object is in front of another (with respect to the viewer), it “over-writes”
the other object in the image (Figure 23.6)

It is important to note that depth perception is affected by illumination and texture. For example, objects
showing less texture tend to be perceived as being further away. This explained the difficulties that pilots
experience when they are flying at night and

or over an extended body of snow or calmwater. The lack of
surface texture creates the risk that pilots overestimate their altitude.
Visual perception beyond the retinal level is carried by the axons of the ganglion cells, which form the
optic nerve. The optic nerve leaves the eye through the retina in the area of the optic disk (better known
as the “blind spot” because of the absence of receptor cells, and thus the inability to perceive stimuli, in
this area). Part of each branch of the optic nerve crosses over in an area in front of the pituitary gland at
the optic chiasm and then reaches the lateral geniculate nucleus (LGN), a walnut-sized nuclear complex
in the thalamus.
The LGN comprises several layers, including two magnocellular layers and four parvocellular layers.
The magnocellular layers receive input from larger retinal ganglion cells. They are characterized by
high sensitivity, low spatial resolution, high temporal resolution, and little or no color selectivity.
These cells and layers are concerned primarily with the perception of motion. The parvocellular layers
are innervated by smaller ganglion cells and exhibit low sensitivity, high spatial resolution, low temporal
resolution, and color selectivity. Thus, their role is to transmit spatially detailed visual information such
as form and color sensations.

/

FIGURE 23.5 Linear perspective.