Civil Engineering Reference
In-Depth Information
FIGURE 23.6 Occlusion.
The neurons in both the magno- and parvocellular layers are innervated by axons descending
from the cortex, which support the top-down modulation of perception through factors such as
expectations and the perceived importance of a signal. Also, the retinal ganglion cells project to other
brain regions, such as brainstem cells that control pupil diameter as a function of light intensity, the
superior colliculus (mediates the organization of eye movements to keep objects in focus), and
the hypothalamus (involved in organizing circadian rhythm based on normal cycles of light and dark).
From the LGN, projections lead to the primary visual cortex where objects in the left visual field are
represented in the right hemisphere and vice versa. At this level, only simple visual sensations are avail-
able, which maintain a topographical representation of the pattern of retinal activity. This representation
is abandoned once the signals are sent to higher-order visual processing areas in the occipital, parietal,
and temporal lobes of the brain (Figure 23.7).
One major difference between neurons in the primary and higher-order visual areas is the size of
their receptive fields. Receptive fields of neurons in the primary cortex serve foveal vision and cover
less than 1
of visual angle. In contrast, receptive fields of neurons serving peripheral vision are a
few degrees of visual angle across. The overall visual field extends 180
8
8
horizontally and 130
8
vertically.
But why do we see what we see?
“Whilst part of what we perceive comes through our senses from the object before us, another part
(and it may be the larger part) always comes out of our head.”
(William James, 1890)
The previous brief overview of the morphology and physiology of the visual system and of selected
perceptual phenomena does not answer two very important questions about visual perception:
(a) “What will the user look at?” and (b) “What will things look like to the user?”. The first question
is concerned with the selection of visual objects for further processing, which, as mentioned earlier, is
affected both by top-down influences (such as operator expectations) and bottom-up factors (such
as the salience of objects). It has been suggested that an initial organization of the visual field that
supports figure-ground perception occurs at a preattentive level. In other words, the entire visual
field is processed automatically with the goal to detect basic features of objects such as colors, contrast,
or size (Treisman, 1986) and determine which objects should undergo further processing (Broadbent,
1958; Neisser, 1976).
Interface design can capitalize on this bottom-up process for the purpose of attention guidance
through the presentation of highly salient or conspicuous objects, that is, objects that are large,
bright, colorful, or flashing (Wickens, 1984) and thus are likely to capture attention.
It is important to note that these tendencies are easily overridden by top-down influences on the
selection of visual objects for in-depth processing. For example, people tend to start scanning in
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