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In-Depth Information
2.1 Cortical Processing of Stimuli
The optic nerves, transmitting sensory information from each eye, proceed poste-
rior and medially to unite at the optic chiasm. There, fibers from the nasal halves
of each retina cross over to the opposite hemisphere. Fibers from the temporal
halves project to the hemisphere on the same side. The result is that signals from
the same regions of the visual field are projecting to the same hemisphere; thus,
the left half of the visual field projects to the right half of retina, which in turn
sends neural signals to the right hemisphere.
There is a point-to-point relation between retina, lateral geniculate nucleus, and the
primary visual cortex. Impulses from the upper and lower halves of the visual field are
located in different parts of the optic radiation, and consequently also project into dif-
ferent areas of the primary visual cortex V1. This is called retinotopic projection, as the
distribution of stimulation on the retina is preserved in the cortex.
The primary visual cortex V1 seems to separate the pattern of light falling on
the retina into discrete features. Apparently, these are retinal location, orientation,
movement, wavelength, and the difference between the two eyes. In subsequent
cortical processing these features are further differentiated. Therefore, the primary
visual cortex has the task of sorting visual information and distributing it to other,
more specialized cortical areas.
Two visual streams have been identified by Ungerleider and Mishkin in 1982
that originate from the primary visual cortex: the dorsal or parietal stream, and the
ventral or temporal stream. Apparently, the first correlates more to location, depth
and movement, whereas the latter is more connected to color, spatial detail and
form [6, 7]. Goldstein, on the basis of the experiments performed by Ungerleider
and Mishkin, suggests that perceiving the location of an object is attributed to the
dorsal stream, whereas the ventral stream determines the object's identity - the
where
and
what
dimensions of vision [8].
Although the basic mechanisms of sensory information transmission are well
understood, a detailed understanding of how visual input is processed and inter-
preted is still missing. Especially the transition from neuronal reaction to percep-
tion, i.e. the process of attaching a meaning to the stimulus, remains unexplained.
Based on the findings introduced above, it seems that for the perception of visual
quality, the ventral stream is of higher importance than the dorsal stream. How
this affects visual perception remains unclear.
2.2 Perception and Attention
From experience we know that perceived visual quality is highly context- and
task-dependent. This is related to the way we generally perceive stimuli: Neisser's
model of the Perceptual Cycle describes perception as a setup of schemata,
perceptual exploration and stimulus environment [9]. These elements influence
each other in a continuously updated circular process (see Fig. 1). Thus, Neisser's
model describes how the perception of the environment is influenced by back-
ground knowledge which in turn is updated by the perceived stimuli.
