MIT Encyclopedia of the Cognitive Sciences - Cryptome

lxiv Neurosciences
clinical intervention, are all among the most important topics for the future of cognitive
neuroscience.
Dynamic Control of Sensitivity in the Mature Brain
Mature sensory systems have limited information processing capacities. An exciting
area of research in recent years has been that addressing the conditions under which
processing capacity is dynamically reallocated, resulting in fluctuations in sensitivity
to sensory stimuli. The characteristics of sensitivity changes are many and varied, but
all serve to optimize acquisition of information in a world in which environmental features
and behavioral goals are constantly in flux. The form of these changes may be
broad in scope or highly stimulus-specific and task-dependent. Changes may be
nearly instantaneous, or they may come about gradually through exposure to specific
environmental features. Finally, sensitivity changes differ greatly in the degree to
which they are influenced by stored information about the environment and the degree
to which they are under voluntary control.
Studies of the visual system reveal at least three types of sensitivity changes represented
by the phenomena of (1) contrast gain control, (2) attention, and (3) perceptual
learning. All can be viewed as recalibration of incoming signals to
compensate for changes in the environment, the fidelity of signal detection (such as
that associated with normal aging or trauma to the sensory periphery), and behavioral
goals.
Generally speaking, neuronal gain control is the process by which the sensitivity of
a neuron (or neural system) to its inputs is dynamically controlled. In that sense, all of
the forms of adult plasticity discussed below are examples of gain control, although
they have different dynamics and serve different functions.
Contrast Gain Control A well-studied example of gain control is the invariance of
perceptual sensitivity to the features of the visual world over an enormous range of
lighting conditions. Evidence indicates that the limited dynamic range of responsivity
of individual neurons in visual cortex is adjusted in an illumination-dependent manner
(Shapley and Victor 1979), the consequence of which is a neuronal invariance that can
account for the sensory invariance. It has been suggested that this scaling of neuronal
sensitivity as a function of lighting conditions may be achieved by response “normalization,”
in which the output of a cortical neuron is effectively divided by the pooled
activity of a large number of other cells of the same type (Carandini, Heeger, and
Movshon 1997).
Attention Visual ATTENTION is, by definition, a rapidly occurring change in visual
sensitivity that is selective for a specific location in space or specific stimulus features.
The stimulus and mnemonic factors that influence attentional allocation have been
studied for over a century (James 1890), and the underlying brain structures and
events are beginning to be understood (Desimone and Duncan 1995). Much of our
understanding comes from analysis of ATTENTION IN THE HUMAN BRAIN—particularly
the effects of cortical lesions, which can selectively interfere with attentional
allocation (VISUAL NEGLECT), and through electrical and magnetic recording (ERP,
MEG) and imaging studies—POSITRON EMISSION TOMOGRAPHY (PET) and functional
MAGNETIC RESONANCE IMAGING (fMRI). In addition, studies of ATTENTION IN THE
ANIMAL BRAIN have revealed that attentional shifts are correlated with changes in the
sensitivity of single neurons to sensory stimuli (Moran and Desimone 1985; Bushnell,
Goldberg, and Robinson 1981; see also AUDITORY ATTENTION). Although attentional
phenomena differ from contrast gain control in that they can be influenced by feedback
WORKING MEMORY as well as feedforward (sensory) signals, attentional effects
can also be characterized as an expansion of the dynamic range of sensitivity, but in a
manner that is selective for the attended stimuli.
Perceptual Learning Both contrast gain control and visual attention are rapidly
occurring and short-lived sensitivity changes. Other experiments have targeted neuronal
events that parallel visual sensitivity changes occurring over a longer time scale,