Almost everyone who has a limb amputated will experience a phantom limb--the vivid impression that the limb is not only still present, but in some cases, painful. There is now a wealth of empirical evidence demonstrating changes in cortical topography in primates following deafferentation or amputation, and this review will attempt to relate these in a systematic way to the clinical phenomenology of phantom limbs. With the advent of non-invasive imaging techniques such as MEG (magnetoencephalogram) and functional MRI, topographical reorganization can also be demonstrated in humans, so that it is now possible to track perceptual changes and changes in cortical topography in individual patients. We suggest, therefore, that these patients provide a valuable opportunity not only for exploring neural plasticity in the adult human brain but also for understanding the relationship between the activity of sensory neurons and conscious experience. We conclude with a theory of phantom limbs, some striking demonstrations of phantoms induced in normal subjects, and some remarks about the relevance of these phenomena to the question of how the brain constructs a 'body image.'

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The perception of phantom limbs. The D. O. Hebb lecture.

The species-specific organizational property of speech is a continual mouth open-close alternation, the two phases of which are subject to continual articulatory modulation. The cycle constitutes the syllable, and the open and closed phases are segments - vow- els and consonants, respectively. The fact that segmental serial ordering errors in normal adults obey syllable structure constraints sug- gests that syllabic "frames" and segmental "content" elements are separately controlled in the speech production process. The frames may derive from cycles of mandibular oscillation present in humans from babbling onset, which are responsible for the open-close al- ternation. These communication-related frames perhaps first evolved when the ingestion-related cyclicities of mandibular oscillation (as- sociated with mastication (chewing) sucking and licking) took on communicative significance as lipsmacks, tonguesmacks, and teeth chat- ters - displays that are prominent in many nonhuman primates. The new role of Broca's area and its surround in human vocal communication may have derived from its evolutionary history as the main cortical center for the control of ingestive processes. The frame and content components of speech may have subsequently evolved separate realizations within two general purpose primate mo- tor control systems: (1) a motivation-related medial "intrinsic" system, including anterior cingulate cortex and the supplementary motor area, for self-generated behavior, formerly responsible for ancestral vocalization control and now also responsible for frames, and (2) a lateral "extrinsic" system, including Broca's area and surround, and Wernicke's area, specialized for response to external input (and there- fore the emergent vocal learning capacity) and more responsible for content.

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The frame/content theory of evolution of speech production.

The thalamus has long been seen as responsible for relaying information on the way to the cerebral cortex, but it has not been until the last decade or so that the functional nature of this relay has attracted significant attention. Whereas earlier views tended to relegate thalamic function to a simple, machine-like relay process, recent research, reviewed in this article, demonstrates complicated circuitry and a rich array of membrane properties underlying the thalamic relay. It is now clear that the thalamic relay does not have merely a trivial function. Suggestions that the thalamic circuits and cell properties only come into play during certain phases of sleep to effectively disconnect the relay are correct as far as they go, but they are incomplete, because they fail to take into account interesting and variable properties of the relay that, we argue, occur during normal waking behavior. Although the specific function of the circuits and cellular properties of the thalamic relay for waking behavior is far from clear, we offer two related hypotheses based on recent experimental evidence. One is that the thalamus is not used just to relay peripheral information from, for example, visual, auditory, or cerebellar inputs, but that some thalamic nuclei are arranged instead to relay information from one cortical area to another. The second is that the thalamus is not a simple, passive relay of information to cortex but instead is involved in many dynamic processes that significantly alter the nature of the information relayed to cortex.

Functional organization of thalamocortical relays

Nervous system reorganization following injury.

The superior colliculus is a laminated midbrain structure that acts as one of the centers organizing gaze movements. This review will concentrate on sensory and motor inputs to the superior colliculus, on its internal circuitry, and on its connections with other brainstem gaze centers, as well as its extensive outputs to those structures with which it is reciprocally connected. This will be done in the context of its laminar arrangement. Specifically, the superficial layers receive direct retinal input, and are primarily visual sensory in nature. They project upon the visual thalamus and pretectum to influence visual perception. These visual layers also project upon the deeper layers, which are both multimodal, and premotor in nature. Thus, the deep layers receive input from both somatosensory and auditory sources, as well as from the basal ganglia and cerebellum. Sensory, association, and motor areas of cerebral cortex provide another major source of collicular input, particularly in more encephalized species. For example, visual sensory cortex terminates superficially, while the eye fields target the deeper layers. The deeper layers are themselves the source of a major projection by way of the predorsal bundle which contributes collicular target information to the brainstem structures containing gaze-related burst neurons, and the spinal cord and medullary reticular formation regions that produce head turning.

The mammalian superior colliculus: laminar structure and connections.

Microelectrode maps of somatosensory inputs were related to cortical architecture and patterns of cortical connections to provide evidence for five subdivisions of the somatosensory or sensorimotor cortex in North American opossums (Didelphis marsupialis). Microelectrode recordings revealed three systematic representations of the body surface. A large mediolaterally oriented representation was identified as the primary somatosensory area (S1) by its relative position, somatotopy, architecture, and connections. S1 represented the hindlimb, trunk, forelimb, and face in a mediolateral sequence. Two additional representations of cutaneous receptors were found caudolateral to S1, each with face representations adjacent to the border of lateral S1 and other body-part representations progressing more caudally toward the auditory cortex. We identified the more dorsal field as the second somatosensory area (S2) and the more ventral field as the parietal ventral area (PV). Tracers injected into S1 labeled neurons and terminals in architectonically distinct fields rostral and caudal to S1, the somatosensory caudal area (SC) and the somatosensory rostral area (SR). Movements could be evoked by microstimulation from sites scattered over S1, SR, and the frontal cortex, but thresholds were high and uncharacteristic of motor cortex. S2 and PV merged caudally with the cortex responsive to auditory stimuli, possibly A1, and neurons in some caudal recording sites in PV were activated by both auditory and cutaneous stimuli. Primary (V1) and secondary (V2) visual areas were also identified by microelectrode mapping, architecture, and connections. In addition, at least part of the cortex between V2 and the somatosensory cortex had visual connections. Thus, most of the dorsolateral cortex of opossums appears to be somatosensory, auditory, or visual.

Topography, architecture, and connections of somatosensory cortex in opossums: Evidence for five somatosensory areas

Transection of a sensory nerve in adults results in profound abnormalities in sensory perception, even if the severed nerve is surgically repaired to facilitate accurate nerve regeneration. In marked contrast, fewer perceptual errors follow nerve transection and surgical repair in children. The basis for this superior recovery in children was unknown. Here we show that there is little or no topographic order in the median nerve to the hand after median nerve section and surgical repair in immature macaque monkeys. Remarkably, however, in the same animals the representation of the reinnervated hand in primary somatosensory cortex area (area 3b) is quite orderly. This indicates that there are mechanisms in the developing brain that can create cortical topography, despite disordered sensory inputs. Presumably the superior recovery of perceptual abilities after peripheral nerve transection in children depends on this restoration of somatotopy in the central sensory maps.

Central reorganization of sensory pathways following peripheral nerve regeneration in fetal monkeys.

This study used biocytin and horseradish peroxidase (HRP) to examine the intrinsic connections of the cytochrome oxidase (CO) rich blob and CO poor nonblob zones within layer III of striate cortex in two primate species, nocturnal prosimian bush babies (Galago crassicaudatus) and diurnal simian squirrel monkeys (Saimiri sciureus). Our main objective was to determine whether separate classes of lateral geniculate nucleus (LGN) cells projected to separate superficial layer zones or layers in either species. There were three significant findings. First, we confirm that layer III consists of three sublayers, IIIA, IIIB, and IIIC in both species. Layer IIIA receives input from layers IIIB, IIIC, and V, with little or no input from LGN recipient layers IV and VI. Layer IIIB receives its input from nearly every cortical layer. Layer IIIC, receives input principally from layers IV alpha [which receives its input from magnocellular (M) LGN cells] and from layers V and VI. Taken together with other findings on the extrinsic connections of these layers, our data suggest that IIIA and IIIC provide output to separate hierarchies of visual areas and IIIB acts as a set of interneurons. Second, we find that, as in macaque monkeys, cells in both IV beta and IV alpha of bush babies and squirrel monkeys project to layer IIIB, converging within the blobs. These results suggest that information from all LGN cell classes [parvocellular (P), M, and the Koniocellular (K) or their equivalents] may be integrated within the blobs. Thus, blobs in all of these primates may perform a function that transcends visual niche differences. Third, our data show a species specific difference in the connections of the IIIB nonblobs; nonblobs receive indirect input via IV alpha from the LGN M pathway in bush babies but receive indirect input via IV beta from the LGN parvocellular (P) pathway in squirrel monkeys. These findings indicate that the role of nonblob zones within striate cortex differs from that of blob zones and takes into account visual niche differences.

Intrinsic connections of layer III of striate cortex in squirrel monkey and bush baby: correlations with patterns of cytochrome oxidase.

The connections of a wedge of densely myelinated cortex along the dorsomedial border of V2 were determined by injecting tracers into this region in macaque monkeys. According to previous descriptions, this cortex would constitute parts of dorsal V3 and V3A, or a dorsomedial visual area, DM, homologous to the DM described in prosimians and New World monkeys. Injections of wheat germ agglutinin conjugated to horseradish peroxidase or various fluorescent tracers demonstrated connections with architectonically defined V1, V2, and the middle temporal area, as well as regions of visual areas known as the ventral posterior parietal area, the rostral dorsolateral area or rostral V4, ventral posterior cortex and more rostral cortex in the ventral temporal lobe, and medial and dorsointermediate areas. Other sparser and less consistently revealed connections were with the medial superior temporal area, the area of the fundus of the superior temporal sulcus, and the caudal dorsolateral area. Distributions of labeled cells in V1 varied in relationship to the pattern of cytochrome oxidase blobs and interblobs in a manner suggesting a heterogeneous pattern of terminations from blob and interblob regions within DM. Major similarities in overall connections of the DM region in macaques with DM connections described in New World monkeys and prosimian galagos support the conclusion that the same visual area, DM, has been identified in all these primates.

Cortical connections of the dorsomedial visual area in old world macaque monkeys.

Slow lorises (Nycticebus coucang) are nocturnal prosimian (i.e. strepsirhine) primates, closely related to bushbabies (Galago spp.). We examined the organization

Pamela D. Beck

Papers

Topography, architecture, and connections of somatosensory cortex in opossums: Evidence for five somatosensory areas

Central reorganization of sensory pathways following peripheral nerve regeneration in fetal monkeys.

Intrinsic connections of layer III of striate cortex in squirrel monkey and bush baby: correlations with patterns of cytochrome oxidase.

Cortical connections of the dorsomedial visual area in old world macaque monkeys.

Areal, modular, and connectional organization of visual cortex in a prosimian primate, the slow loris (Nycticebus coucang).