Environmental Engineering Reference
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two visual stream hypothesis may be overly rigid. Recent investigation has shown that for
simple eye movements and pointing tasks, color information can be used to guide
movement (White, Kerzel, & Gegenfurtner, 2006). Pisella, Arzi, and Rossetti (1998)
studied the ability of humans to utilize color information to quickly update their
movements in a perturbation paradigm. While movement reorganization was possible
utilizing only color information, the results showed a distinct slowing of movement
reorganization. Brenner and Smeets (2004) also studied a similar paradigm, finding that
color could in fact be utilized rather quickly for task reorganization; however, they still
showed a minor slowing compared with movement reorganization based on luminance
information. Luminance contrast, while also important in perception, may have more
direct implications for motor output. Motion sensitivity is dependent on contrast
sensitivity and motion sensitivity is a hallmark of the neuronal structure of the dorsal
stream (Born & Bradley, 2005). Therefore luminance contrast may be an important source
of visual sensory feedback for motor output.
Properties of visual feedback are used both in the planning and online control of movement.
The specific role of luminance contrast for such processes has not been clearly identified,
and previous study of this topic is sparse. Recently Braun et al. (2008) investigated whether
initiation of eye movements differed when tracking two types of targets, one with
luminance contrast compared to the background and one isoluminant with the background
(i.e. defined by color only). They showed a strong and significant effect of target contrast on
speed of eye movement initiation, with tracking of isoluminant targets delayed by 50 ms.
They also showed lower eye accelerations to these no-contrast targets. For upper extremity
control, studies have shown mixed results. White, Kerzel, and Gegenfurtner (2006) showed
that there was no difference in accuracy or response latency when comparing simple rapid
aiming movements to targets of high luminance contrast versus isoluminant targets. In a
more complex task, Kleinholdermann et al. (2009) looked at the influence of the target
object's luminance contrast as subjects performed reach to grasp movements within a
desktop augmented (physical object with graphical overlay) environment. Participants were
not provided with a head-coupled stereoscopic view, nor were they provided any visual
representation of the hand. They were given a view of the environment that included only a
virtual image overlaying the actual target disk. The independent variables controlled by the
experimenters were the visual properties of chromatic and luminance contrast between the
target object and the environment background. The results of this study showed only a
minimal effect of luminance contrast on the formation of grasp aperture. They concluded
that isoluminant targets were as suitable for the motor planning of grasp as targets defined
by a luminance contrast or a luminance plus chromatic contrast. However, because current
theories of motor control rest on the premise that object location can be precisely identified
in relation to limb location (Wolpert, Miall, & Kawato, 1998) we contend that the lack of
visual feedback about the limb likely resulted in a ceiling effect for a number of performance
measures used by Kleinholdermann et al. Given that neuronal tuning properties make the
visual system particularly sensitive to change, it is logical that some property involving a
change in visual stimulus may be especially useful in this quick, precise identification of
object and limb spatial location. Luminance contrast is such a property. Future
experiments should expand upon the work of Kleinholdermann et al. by examining the
role of luminance contrast of both the target object and the effector limb for upper
extremity performance. Further, the Kleinholdermann et al. paper focused predominantly
