2013
2011
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Early in the physiological study of the motor cortex, one experimental question began to dominate the research. How are points in cortex connected to muscles? The question fosters a simplistic, feed-forward view of motor cortex in which its intrinsic processing is ignored and its function is assumed to be defined almost entirely by the cables that run down to the spinal cord, relay onto motor neurons, and thus cause muscle contraction. This perspective still pervades almost all modern thinking about the motor cortex. As a result, a more realistic view of motor cortex as a control network has been hindered. The study by Capaday et al. (2011), examining the lateral interactions among neurons in motor cortex, represents an important step beyond the limited muscle-map conception and toward a better understanding of the processing network within the cortex itself.
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2010
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Electrical stimulation of the motor cortex of monkeys elicits complex movements that combine many muscles and joints and that resemble fragments of the animal’s normal movement repertoire. Hand-to-mouth movements, reaching movements, defensive movements, and other ethologically relevant actions can be evoked. Different movements are evoked from different locations in motor cortex. The movement repertoire of the monkey appears to be mapped on the cortical sheet in a manner that preserves local continuity. Simple map schemes that have been proposed in the past, such as a map of the body, or a segregation of the motor cortex into separate areas that process different aspects of movement, explain only some aspects of motor cortex organization. More of the subtlety and complexity of the motor cortex topography can be explained by the principle of a highly dimensional movement repertoire that is flattened onto the cortical surface.
2009
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2008
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2007
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2006
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2005
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We used electrical microstimulation to study the organization of motor cortex in awake monkeys. Stimulation on a behaviorally relevant time scale (0.5-1 s) evoked coordinated, complex postures that involved many joints. Postures that involved the arm were arranged across cortex to form a map of hand positions around the body. This map encompassed both primary motor and lateral premotor cortex. Primary motor cortex appeared to represent the central part of the workspace, where monkeys most often manipulate objects with their fingers. These findings suggest that primary motor and lateral premotor cortex might not be arranged in a hierarchy, but instead might operate in parallel, serving different behavioral functions in different parts of the workspace. This hypothesis is also consistent with some of the previous data from motor and premotor cortex.
2004
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If a hornet flies toward your face, you might duck, squint, and lift your hand to block it. If the insect touches your hand, you might withdraw your hand, even pulling it behind your back. These defensive movements have a reflexive quality. They are fast and can occur without conscious planning or thought. They are similar in all people (see Figure 1). However, although they seem reflexive, defensive movements are also highly sophisticated. They can be elicited by touch, sight or sound. They involve coordination between different body parts, such as the arm and head. They are spatially specific: the body parts that move and the direction of movement are appropriate for the location of the threat. The movements can be stronger or weaker depending on external context or the internal state of the person. For example, someone whose “nerves are on edge” may give an exaggerated alerting response to an unexpected stimulus. What sensory-motor pathways in the brain coordinate this rich and complex behavior? We suggest that a special set of interconnected areas in the monkey brain monitors the location and movement of objects near the body and controls startle, flinch and defensive responses. This hypothesized “defensive” system, shown in Figure 2, includes the ventral intraparietal area (VIP), parietal area 7b, the polysensory zone (PZ) in the precentral gyrus, and the putamen. These brain areas are monosynaptically interconnected (Cavada and Goldman-Rakic 1989a,b; Cavada and Goldman-Rakic 1991; Kunzle 1978; Matelli et al. 1986; Mesulam et al. 1977; Parthasarathy et al. 1992; Weber and Yin 1984; Luppino et al., 1999). Of the four areas, PZ is closest to the motor output, sending direct projections to the spinal cord (Dum and Strick 1991). Electrical stimulation of PZ evokes defensive movements, such as withdrawal of the hand, squinting and turning of the head, ducking, or lifting the hand as if to defend the side of the head (Graziano, Taylor and Moore 2002).
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2003
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2002
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In a News and Views piece (“Stimulating research on Motor Cortex” 2002, 5:714), Strick comments on our recent finding that microstimulation of motor cortex evokes complex, coordinated behavior (1). A major concern that he raises is that, “one might ask whether electrical stimulation of the cortex is capable of revealing its function.” We agree that one should always ask such questions about all experimental methods. However, a large body of recent work, conspicuously not cited in Strick’s piece, successfully probes cortical function using electrical stimulation. For example, Newsome and colleagues (2) stimulated monkey visual area MT and influenced the monkey’s perceptual decisions about the direction of motion of visual stimuli. Romo and colleagues (3) stimulated primary somatosensory cortex and influenced the monkey’s perceptual decisions about tactile stimuli. Shadlen and colleagues (4) stimulated the frontal eye fields and influenced the monkey’s target selection. Many researchers have used electrical stimulation to study functional maps of eye and head movement (5-7). We took the well-established protocol of stimulating on a behaviorally relevant time scale and applied it to motor cortex. The stimulation durations that we used are within the range of these previous studies, and the current intensities are within the range used in the oculomotor studies. As in previous studies, we evoked meaningful behaviors.
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To reach for the computer mouse, sit upright in a chair or hold a journal in order to read it, indeed, to do most of the actions that we commonly perform, we rely on a representation of the spatial configuration of the body. How and where in the brain is the body represented and what are the psychological properties of this body schema? In this article we review first the neurophysiology and then the psychology of the body representation. One finding that has emerged from both approaches is that the body representation is not merely a registration of proprioceptive inputs about joint angle. Instead, the brain contains a sophisticated model of the body that is continually updated on the basis of multimodal input including vision, somesthesis and motor feedback. Neurophysiological studies in the monkey brain show that parietal area 5 is a critical node for processing the body’s configuration. Neurons in area 5 combine signals from different modalities in order to represent limb position and movement. Psychological studies show that the body schema is used to cross-reference between different senses, as a basis for spatial cognition and for movement planning.
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2001
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Damage to restricted parts of the brain can cause spatial confusion and even eliminate awareness of large parts of space around the body. The precise brain areas responsible for spatial awareness, however, are still in debate.
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The primate cerebral cortex has traditionally been divided into separate territories for vision, touch, audition and movement. These functions are known to overlap in many parts of cortex, but until recently the regions of overlap were not well studied. In this issue of Neuron, Bremmer et al. report a major advance in understanding at least one set of areas in the human brain in which the senses are integrated. This finding joins a growing set of work in monkeys and humans on the integration of the senses with each other and with the control of movement.
2000
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In 1870, Fritsch and Hitzig first studied primary motor cortex in the monkey brain using electrical stimulation, and in 1881, Hermann Munk used lesion methods to localize the primary visual cortex in the occipital lobe (cited in Gross 1998). Only now, more than one hundred years later, has neuroscience begun to identify the neuronal pathways that connect these two areas. We are finally beginning to understand the routes through which vision is transformed into action.
1999
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1998
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1997
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We find it effortless to reach toward or avoid nearby objects. However, the spatial and visuo-motor computations must be quite complicated, especially since our eyes, head, limbs, body, and the objects themselves may be continually changing positions. How does the brain construct a representation of the visual space surrounding the body, and how does this representation guide movement?