Biomedical Engineering Reference
In-Depth Information
information sources as “sensory” inasmuch as they are not considered to carry affer-
ent information from the peripheral to the central nervous system. Nonetheless, these
information sources form a critical component of active engagement with one's envi-
ronment and may have a strong influence on the nature and quality of spatial infor-
mation that is available for acquiring environmental knowledge. It is worth noting
that these sources of information are also tightly constrained and dictated by one's
goals. Our brief discussion of these information sources draws heavily on the con-
ceptual distinctions made by Chrastil and Warren [
18
] who used these concepts in
their recent review of the extensive and complicated literature on active and passive
contributions to spatial knowledge.
Efference copy [
47
] is a simultaneous record of the motor commands from the
central nervous system to the musculature that enables organisms to account for the
difference between external stimulation and the stimulation that arises as a conse-
quence of their own actions. For example, as we discussed above, when one turns
one's head clockwise, the visual system has access to laminar optic flow in a coun-
terclockwise direction. Logically, such a pattern of optic flow could signal a counter-
clockwise rotation of the visible environment around a stationary viewpoint, rather
than a rotation of a viewpoint in a stationary environment. However, people are typ-
ically able to distinguish these possibilities by accounting for the fact that motor
commands actively produced a set of expected visual consequences (but see [
43
]for
situations when people cannot). This knowledge of one's motor commands consti-
tutes efference copy. We consider the efferent copy of motor commands to contain
information about both the implicit or explicit intentions used to move in and interact
with the environment, as well as information about the strength of these intentions.
In this way, efference copy can be used to generate a set of expectations about the
consequences of one's actions (see [
12
]). Discrepancies between the expected and
the perceived consequences of a set of motor commands can indicate a need for
perceptual recalibration or can be used as an indicator of the precision of one's
intentions.
Other efferent sources of information have origins in “higher-level” cognition
and include factors such as constraints about how and where to allocate attention,
decisions about how and where to navigate, and the ability to transform spatial infor-
mation. Chrastil and Warren [
18
] point out that these internal sources of information
are often confounded or conflated in studies that examine the relative effects of active
versus passive navigation on the acquisition of spatial knowledge. However, because
virtual environments (VEs) enable yoked playback of others' interactive experiences,
computer simulations have recently provided researchers a helpful methodological
tool for teasing some of these influences apart. For example, Christou and Bülthoff
([
19
], Experiment 3) asked participants to explore a model of a complex attic space
on a desktop virtual reality system and to familiarize themselves with the environ-
ment and its contents. One group of participants navigated through the environment
actively, by manipulating a trackball that controlled the position and orientation of
their simulated viewpoint. A second group of participants had access to the same
visual information as the first, but passively viewed playbacks of the explorations
made by matched participants in the other condition. Differences between these
