Reorganization of retinotopic cortical maps in adult mammals after lesions of the retina.

The organization of the visual cortex has been considered to be highly stable in adult mammals. However, 5 degrees to 10 degrees lesions of the retina in the contralateral eye markedly altered the systematic representations of the retina in primary and secondary visual cortex when matched inputs from the ipsilateral eye were also removed. Cortical neurons that normally have receptive fields in the lesioned region of the retina acquired new receptive fields in portions of the retina surrounding the lesions. The capacity for such changes may be important for normal adjustments of sensory systems to environmental contingencies and for recoveries from brain damage.

The evolution of visual cortex: where is V2?

A comparative analysis of the area of the cortex that is adjacent to the primary visual area (V1), indicates that the lateral extrastriate cortex of primitive mammals was likely to contain only a single visuotopically organized field, the second visual area (V2). Few, if any, other visual areas existed. The opposing hypothesis, that primitive mammals had a 'string' of small visual areas in the cortex lateral to V1 (as in some rodents), is not supported by studies of the organization of extrastriate cortex in other mammals, nor by the variability in this organization among extant rodents. A critical re-analysis of published evidence on the presence of multiple areas adjacent to V1 in some rodents has led to alternative interpretations of the organization of the areas in these regions.

Early experiences can alter the size of cortical fields in prairie voles (Microtus ochrogaster).

The neocortex of the prairie vole is composed of three well-defined sensory areas and one motor area: primary somatosensory, visual, auditory areas and the primary motor area respectively. The boundaries of these cortical areas are identifiable very early in development, and have been thought to resist alteration by all but the most extreme physical or genetic manipulations. Here we assessed the extent to which the boundaries of sensory/motor cortical areas can be altered by exposing young prairie voles (Microtus ochrogaster) to a chronic stimulus, high or low levels of parental contact, or an acute stimulus, a single dose of saline, oxytocin (OT), or oxytocin antagonist on the day of birth. When animals reached adulthood, their brains were removed, the cortex was flattened, cut parallel to the pial surface, and stained for myelin to identify the architectonic boundaries of sensory and motor areas. We measured the overall proportion of cortex that was myelinated, as well as the proportion of cortex devoted to the sensory and motor areas. Both the chronic and acute manipulations were linked to significant alterations in areal boundaries of cortical fields, but the areas affected differed with different conditions. Thus, differences in parental care and early exposure to OT can both change cortical organization, but their effects are not identical. Furthermore, the effects of both manipulations were sexually dimorphic, with a greater number of statistically significant differences in females than in males. These results indicate that early environmental experience, both through exposure to exogenous neuropeptides and parental contact, can alter the size of cortical fields.

The dorsomedial visual area of owl monkeys: connections, myeloarchitecture, and homologies in other primates.

Cortical connections of the dorsomedial visual area (DM) of owl monkeys were revealed with injections of the bidirectional tracer, wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP), or the retrograde fluorescent tracer, diamidino yellow. Microelectrode recordings in two cases identified DM as a systematic representation of the visual hemifield in a densely myelinated rectangle of cortex just rostral to the dorsomedial portion of the second visual area (V-II, or area 18). Cortex was flattened and cut parallel to the surface in all cases so that the myeloarchitectonic borders of DM and other areas such as the primary visual area (V-I or area 17), V-II or area 18, and the middle temporal visual area (MT) could be readily determined, and the surface view patterns of connections could be directly appreciated. The ipsilateral pattern of connections of DM were dense and visuotopically congruent with area 17, area 18, and MT, and moderate to dense connections were with the medial visual area (M), the rostral division of the dorsolateral visual area, the dorsointermediate area, the ventral posterior area, the caudal division of inferotemporal cortex (ITc), the ventral posterior parietal area, and visuomotor cortex of the frontal lobe. The connections of DM were concentrated in the cytochrome oxidase (CO)-dense blobs of area 17, the CO-dense bands of area 18, and the CO-dense regions of MT. Callosal connections of DM were with matched locations in DM in the opposite hemisphere, and with VPP. The ipsilateral connections of DM with area 17 were confirmed by injecting WGA-HRP into area 17 in one owl monkey. In addition to labelled cells and terminals in area 18 and MT, bidirectionally transported tracer was also apparent in DM. Evidence for the existence of DM in other primates was obtained by injecting area 17 and examining the areal patterns of connections and myeloarchitecture in three species of Old World monkeys, two additional species of New World monkeys, and prosimian galagos. In all of these primates, one of three major targets of area 17 was a densely myelinated zone of cortex just rostral to dorsomedial area 18, in the location of DM in owl monkeys. Thus, it seems likely that DM is a visual area common to all primates.

Thalamic connections of three representations of the body surface in somatosensory cortex of gray squirrels.

The anatomical tracer, wheat germ agglutinin, was used to determine the connections of electrophysiologically identified locations in three architectonically distinct representations of the body surface in the somatosensory cortex of gray squirrels. Injections in the first somatosensory area, S-I, revealed reciprocal connections with the ventroposterior nucleus (VP), a portion of the thalamus just dorsomedial to VP, the posterior medial nucleus, Pom, and sometimes the ventroposterior inferior nucleus (VPI). As expected, injections in the representation of the face in S-I resulted in label in ventroposterior medial (VPM), the medial subnucleus of VP, whereas injections in the representation of the body labeled ventroposterior lateral (VPL), the lateral subnucleus of VP. Furthermore, there was evidence from connections that the caudal face and head are represented dorsolaterally in VPM, and the forelimb is represented centrally and medially in VPL. The results also support the conclusion that a representation paralleling that in VP exists in Pom, so that the ventrolateral part of Pom represents the face and the dorsomedial part of Pom is devoted to the body. Because connections with VPI were not consistently revealed, the possibility exists that only some parts or functional modules of S-I are interconnected with VPI. Two separate small representations of the body surface adjoin the caudoventral border of S-I. Both resemble the second somatosensory area, S-II, enough to be identified as S-II in the absence of evidence for the other. We term the more dorsal of the two fields S-II because it was previously defined as S-II in squirrels (Nelson et al., '79), and because it more closely resembles the S-II identified in most other mammals. We refer to the other field as the parietal ventral area, PV (Krubitzer et al, '86). Injections in S-II revealed reciprocal connections with VP, Pom, and a thalamic region lateral and caudal to Pom and dorsal to VP, the posterior lateral nucleus, Pol. Whereas major interconnections between S-II and VPI have been reported for cats, raccoons, and monkeys, no such interconnections were found for S-II in squirrels. The parietal ventral area, PV, was found to have prominent reciprocal interconnections with VP, VPI, and the internal (magnocellular) division of the medial geniculate complex (MGi). The pattern of connections conforms to the established somatotopic organization of VP and suggests a crude parallel somatotopic organization in VPI. Less prominent interconnections were with Pol.(ABSTRACT TRUNCATED AT 400 WORDS)

The somatosensory thalamus of monkeys: cortical connections and a redefinition of nuclei in marmosets.

Thalamic connections of three subdivisions of somatosensory cortex in marmosets were determined by placing wheatgerm agglutinin conjugated with horseradish peroxidase and fluorescent dyes as tracers into electrophysiologically identified sites in S-I (area 3b), S-II, and the parietal ventral area, PV. The relation of the resulting patterns of transported label to the cytoarchitecture and cytochrome oxidase architecture of the thalamus lead to three major conclusions. 1) The region traditionally described as the ventroposterior nucleus (VP) is a composite of VP proper and parts of the ventroposterior inferior nucleus (VPi). Much of the VP region consists of groups of densely stained, closely packed neurons that project to S-I. VPi includes a ventral oval of pale, less densely packed neurons and finger-like protrusions that extend into VP proper and separate clusters of VP neurons related to different body parts. Neurons in both parts of VPi project to S-II rather than S-I. Connection patterns indicate that the proper and the embedded parts of VPi combine to form a body representation paralleling that in VP. 2) VPi also provides the major thalamic input into PV. 3) In architecture, location, and cortical connections, the region traditionally described as the anterior pulvinar (AP) of monkeys resembles the medial posterior nucleus, Pom, of other mammals and we propose that all or most of AP is homologous to Pom. AP caps VP dorsomedially, has neurons that are moderately dense in Nissl staining, and reacts moderately in CO preparations. AP neurons project to S-I, S-II, and PV in somatotopic patterns.

Five topographically organized fields in the somatosensory cortex of the flying fox: microelectrode maps, myeloarchitecture, and cortical modules.

Five somatosensory fields were defined in the grey-headed flying fox by using microelectrode mapping procedures. These fields are: the primary somatosensory area, SI or area 3b; a field caudal to area 3b, area 1/2; the second somatosensory area, SII; the parietal ventral area, PV; and the ventral somatosensory area, VS. A large number of closely spaced electrode penetrations recording multiunit activity revealed that each of these fields had a complete somatotopic representation. Microelectrode maps of somatosensory fields were related to architecture in cortex that had been flattened, cut parallel to the cortical surface, and stained for myelin. Receptive field size and some neural properties of individual fields were directly compared. Area 3b was the largest field identified and its topography was similar to that described in many other mammals. Neurons in 3b were highly responsive to cutaneous stimulation of peripheral body parts and had relatively small receptive fields. The myeloarchitecture revealed patches of dense myelination surrounded by thin zones of lightly myelinated cortex. Microelectrode recordings showed that myelin-dense and sparse zones in 3b were related to neurons that responded consistently or habituated to repetitive stimulation respectively. In cortex caudal to 3b, and protruding into 3b, a complete representation of the body surface adjacent to much of the caudal boundary of 3b was defined. Neurons in this area habituated rapidly to repetitive stimulation. We termed this caudal field area 1/2 because it had properties of both area 1 and area 2 of primates. In cortex caudolateral to 3b and lateral to area 1/2 (cortex traditionally defined as SII) we describe three separate representations of the body surface coextensive with distinct myeloarchitectonic appearances. The second somatosensory area, SII, shared a congruent border with 3b at the representation of the nose. In SII, the overall orientation of the body representation was erect. The lips were represented rostrolaterally, the digits were represented laterally, and the toes were caudolateral to the digits. The trunk was represented caudally and the head was represented medially. A second complete representation, PV, had an inverted body representation with respect to SII and bordered SII at the representation of the distal limbs. The proximal body parts were represented rostrolaterally in PV. Finally, caudal to both SII and PV, an additional representation, VS, shared a congruent border with the distal hindlimb representation of both SII and PV. VS had a crude topography, and receptive fields of neurons in VS were relatively large. Many neurons in VS responded to both somatosensory and auditory stimulation.

Connections of somatosensory cortex in megachiropteran bats: the evolution of cortical fields in mammals.

The cortical connections of the primary somatosensory area (SI or 3b), a caudal somatosensory field (area 1/2), the second somatosensory area (SII), the parietal ventral area (PV), the ventral somatosensory area (VS), and the lateral parietal area (LP) were investigated in grey headed flying foxes by injecting anatomical tracers into electrophysiologically identified locations in these fields. The receptive fields for clusters of neurons were mapped with sufficient density for injection sites to be related to the boundaries of fields, and to representations of specific body parts within the fields. In all cases, cortex was flattened and sectioned parallel to the cortical surface. Sections were stained for myelin and architectonic features of cortex were related to physiological mapping and connection patterns. We found patterns of topographic and nontopographic connections between 3b and adjacent anterior parietal fields 3a and 1/2, and fields caudolateral to 3b (SII and PV). Area 1/2 had both topographic and nontopographic connections with 3b, PP, and SII. Connections of SII and PV with areas 3b, 3a, and 1/2 were roughly topographic, although there was clear evidence for nontopographic connections between these fields. SII was most densely connected with area 1/2, while PV was most densely connected with 3b. SII had additional connections with fields in lateral parietal cortex and with subdivisions of motor cortex. Other connections of PV were with subdivisions of motor cortex and pyriform cortex. Laminar differences in connection patterns of SII and PV with surrounding cortex were also observed. Injections in the ventral somatosensory area revealed connections with SII, PV, area 1/2, auditory cortex, entorhinal cortex, and pyriform cortex. Finally, the lateral parietal field had very dense connections with posterior parietal cortex, caudal temporal cortex, and with subdivisions of motor cortex. Our results indicate that the 3b region is not homogeneous, but is composed of myelin dense and light regions, associated with 3b proper and invaginations of area 1/2, respectively. Connections of myelin dense 3b were different from invaginating portions of myelin light area 1/2. Our findings that 3b is densely interconnected with PV and moderately to lightly interconnected with SII supports the notion that SII and PV have been confused across mammals and across studies. Our connectional evidence provides further support for our hypothesis that area 1/2 is partially incorporated in 3b and has led to theories of the evolution of cortical fields in mammals.

Microelectrode maps, myeloarchitecture, and cortical connections of three somatotopically organized representations of the body surface in the parietal cortex of squirrels.

Microelectrode mapping methods and anatomical procedures were combined in the same animals to reveal the cortical connections of three architectonically distinct representations of the body surface in the somatosensory cortex of grey squirrels. In individual experiments, microelectrode multiunit recordings were used to determine the somatotopic organization of regions of the cortex and to identify sites for injections of the anatomical tracer, wheat germ agglutinin conjugated to horseradish peroxidase. After the brains were perfused, the cortex was separated from the brainstem, flattened, and cut parallel to the flattened surface to facilitate comparisons of areal connection patterns, physiological data, and architectonic subdivisions. Recordings in the primary (S-I) and secondary (S-II) somatosensory fields confirmed earlier descriptions of the somatotopic organization of these fields (Sur et al.: J. Comp. Neurol. 179:425-450, '78; Nelson et al.: J. Comp. Neurol. 184:473-490, '79). In addition, recordings in the cortex caudal to S-I and ventral to S-II revealed a third representation of the body surface in parietal cortex, the parietal ventral area (PV). Neurons in PV were responsive to light tactile stimulation of skin and hairs. Multiple unit receptive fields of neurons in PV were larger than those for neurons in S-I but similar in size to those for neurons in S-II. PV represented the contralateral body surface in a somatotopic manner that can be roughly characterized as an inverted "homunculus" with the limbs directed medially, the trunk located ventrally, and the face congruent with the representations of the upper lip and nose in S-I. Neurons in some electrode penetrations in PV were also responsive to auditory clicks. Microlesions placed at physiologically determined borders allowed all three somatic representations to be related to myeloarchitectonically defined fields. S-I was architectonically distinct as a densely myelinated region. Within S-I, a lightly myelinated oval of the cortex between the representation of the hand and face, the "unresponsive zone" (Sur et al.: J. Comp. Neurol. 179:425-450, '78), was an easily recognized landmark. S-II and PV corresponded to less densely myelinated fields. Other subdivisions such as motor cortex, primary auditory cortex, and visual areas 17 and 18 were distinguished. Connections were revealed by placing injections within S-I, S-II, or PV.(ABSTRACT TRUNCATED AT 400 WORDS)

Frontal eye field as defined by intracortical microstimulation in squirrel monkeys, owl monkeys, and macaque monkeys: I. Subcortical connections.

Intracortical microstimulation was used to define the borders of the frontal eye fields in squirrel, owl, and macaque monkeys. The borders were marked with electrolytic lesions, and horseradish peroxidase conjugated to wheat germ agglutinin was injected within the field. Following tetramethyl benzidine histochemistry, afferent and efferent connections of the frontal eye field with subcortical structures were studied. Most connections were ipsilateral and were similar in all primates studied. These include reciprocal connections with the following nuclei: medial dorsal (lateral parts), ventral anterior (especially with pars magnocellularis), central lateral, paracentral, ventral lateral, parafascicular, medial pulvinar, limitans, and suprageniculate. The frontal eye field also projects to the ipsilateral pretectal nuclei, subthalamic nucleus, nucleus of the posterior commissure, superior colliculus (especially layer four), zona incerta, rostral interstitial nucleus of the medial longitudinal fasciculus, nucleus Darkschewitsch, dorsomedial parvocellular red nucleus, interstitial nucleus of Cajal, basilar pontine nuclei, and bilaterally to the paramedian pontine reticular formation and the nucleus reticularis tegmenti pontis. Many of these structures also receive input from deeper layers of the superior colliculus and are known to participate in visuomotor function. These results reveal connections that account for the parallel influence of the superior colliculus and the frontal eye field on visuomotor function; suggest that there has been little evolutionary change in subcortical connections, and therefore function, of the frontal eye fields since the time that these lines of primates diverged; and support the conclusion that the frontal eye fields are homologous in New and Old World monkeys.

Frontal eye field as defined by intracortical microstimulation in squirrel monkeys, owl monkeys, and macaque monkeys. II. Cortical connections.

Physiological (intracortical microstimulation) and anatomical (transport of horseradish peroxidase conjugated to wheat germ agglutinin as shown by tetramethyl benzidine) approaches were combined in the same animals to reveal the locations, extents, and cortical connections of the frontal eye fields (FEF) in squirrel, owl, and macaque monkeys. In some of the same owl and macaque monkeys, intracortical microstimulation was also used to evoke eye movements from dorsomedial frontal cortex (the supplementary motor area). In addition, in all of the owl and squirrel monkeys, intracortical microstimulation was also used to evoke body movements from the premotor and motor cortex situated between the central dimple and the FEF. These microstimulation data were directly compared to the distribution of anterogradely and retrogradely transported label resulting from injections of tracer into the FEF in each monkey. Since the injection sites were limited to the physiologically defined FEF, the demonstrated connections were solely those of the FEF. To aid in the interpretation of areal patterns of connections, the relatively smooth cortex of owl and squirrel monkeys was unfolded, flattened, and cut parallel to the flattened surface. Cortex of macaque monkeys, which has numerous deep sulci, was cut coronally. Reciprocal connections with the ipsilateral frontal lobe were similar in all three species: dorsomedial cortex (supplementary motor area), cortex just rostral (periprincipal prefrontal cortex) to the FEF, and cortex just caudal (premotor cortex) to the FEF. In squirrel and owl monkeys, extensive reciprocal connections were made with cortex throughout the caudal half of the lateral fissure and, to a much lesser extent, cortex around the superior temporal sulcus. In macaque monkeys, only sparse connections were present with cortex of the lateral fissure, but extensive and dense connections were made with cortex throughout the caudal one-third to one-half of the superior temporal sulcus. In addition, very dense reciprocal connections were made with the cortex of the lateral, or inferior, bank of the intraparietal sulcus. Contralateral reciprocal connections in all three species were virtually limited to regions that correspond in location to the FEF and the supplementary motor area. The results of this study reveal connections between the physiologically defined frontal eye field and cortical regions known to participate in higher order visual processing, short-term memory, multimodal, visuomotor, and skeletomotor functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Cortical connections of areas 17 (V-I) and 18 (V-II) of squirrels.

Connections of visual cortex in squirrels were investigated by placing WGA-HRP injections, and in some cases fluorescent dyes, into area 17 (V-I) or area 18 (V-II). Results were related to architectonic fields determined in brain sections cut parallel to the surface of manually flattened cortex and to limited microelectrode mapping data. Injections in area 17 provided evidence for 1) a patchy pattern of horizontal intrinsic connections extending 1-2 mm from the injection site; 2) uneven, widely distributed connections with area 18 (V-II) and adjoining occipital-temporal (OT) cortex; and 3) callosal connections of large portions of area 17 with the 17/18 border zone. While restricted locations in area 17 had uneven interconnections over several mm of area 18, more rostral locations in area 17 related to more rostral locations in area 18, demonstrating a topographic tendency. Injections in area 18 revealed 1) zones of discontinuous connections with area 17 that followed a topographic pattern, 2) patches of intrinsic connections that spread over distances of up to 6-8 mm from the injection site; 3) two zones of uneven connections with OT cortex suggesting the locations of at least two visual areas, OTr and OTc; 4) connections with limbic cortex rostromedial to areas 17 and 18; 5) sparse connections with regions of temporal cortex lateral to OT; and 6) uneven callosal connections with area 18 and OT cortex. The widespread and unevenly distributed intrinsic callosal interconnection patterns of areas 17 and 18 contrast with the restricted excitatory receptive fields of neurons and the retinotopic patterns of representation in these fields. Although physiological evidence is presently lacking, the patchy connections suggest that areas 17 and 18 in squirrels are modularly organized.

Cortical connections of electrophysiologically and architectonically defined subdivisions of auditory cortex in squirrels.

Multiunit recordings with microelectrodes were used to identify and delimit subdivision of auditory cortex in squirrels. In the same animals, cortical connections of subdivisions of auditory cortex were determined by placing injections of the tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) into electrophysiologically defined locations. The electrophysiological results and patterns of connections were later related to myeloarchitectonic distinctions in brain sections cut parallel to the surface of the artificially flattened cortex. As previously described (Merzenich et al.: J. Comp. Neurol. 166:387-402, '76), a primary auditory field, A-I, was characterized by (1) neurons narrowly tuned to tone frequency; (2) a tonotopic map with high frequencies, which represented caudal to low frequencies; and (3) dense myelination. A-I was reciprocally connected with a rostral field, R, a parietal ventral somatosensory representation, PV, cortex ventral to A-I, and other nearby regions of cortex of the same hemisphere. Callosal connections of A-I were with A-I, R, and two or more other regions of temporal cortex. The less densely myelinated rostral field, R, also had neurons that were frequency tuned, but the neurons were often less securely driven. R appeared to have a tonotopic organization that roughly mirrored that of A-I. Ipsilateral connections of R included A-I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A-I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A-I, PV, and other locations in temporal cortex. Cortex in caudal PV, ventral to A-I, and ventral to R was responsive to auditory stimuli, but responses to pure tones were weak and inconsistent, and habituation to a repeated stimulus was rapid. The cortex responsive to auditory stimuli included some but not all of the cortex connected with A-I and R. The results lead to the conclusion that auditory cortex of squirrels contains at least two tonotopically organized fields, possibly as many as five or more auditory fields, and at least two auditory-somatosensory fields.

Connections of primary auditory cortex in the New World monkey, Saguinus.

Connections of primary auditory cortex (A-I) were investigated in the tamarin (Saguinus fuscicollis), a New World monkey. In each case, A-I was defined by multiunit recordings, and best frequencies were determined for neurons at different recording sites. Microlesions were placed to mark recording sites for correlation with cortical architecture. Following mapping, separate injections of up to three different tracers (HRP-WGA and fluorescent dyes) were placed into the representations of different frequencies within A-I. The results support several conclusions: (1) high to low frequencies are represented in a dorsocaudal to ventrorostral sequence in A-I, (2) intrinsic connections in A-I are more pronounced along isofrequency contours, (3) the pattern of connections between A-I and adjoining cortex suggests that this surrounding auditory cortex contains at least two tonotopically organized fields and possibly one or more additional auditory fields, (4) callosal connections of A-I are largely between parts of A-I matched for frequency representation, (5) thalamic connections of A-I include topographic connections with the ventral division of the medial geniculate complex (MGv) and more diffuse connections with the medial (MGm) and dorsal (MGd) divisions of the medial geniculate complex and the suprageniculate nucleus (Sg), and (6) A-I projects bilaterally to the dorsal cortex of the inferior colliculus.

Retinotopic organization of the primary visual cortex of flying foxes (Pteropus poliocephalus and Pteropus scapulatus).

The representation of the visual field in the occipital cortex was studied by multiunit recordings in seven flying foxes (Pteropus spp.), anesthetized with thiopentone/N2O and immobilized with pancuronium bromide. On the basis of its visuotopic organization and architecture, the primary visual area (V1) was distinguished from neighboring areas. Area V1 occupies the dorsal surface of the occipital pole, as well as most of the tentorial surface of the cortex, the posterior third of the mesial surface of the brain, and the upper bank of the posterior portion of the splenial sulcus. In each hemisphere, it contains a precise, visuotopically organized representation of the entire extent of the contralateral visual hemifield. The representation of the vertical meridian, together with 8-15 degrees of ipsilateral hemifield, forms the anterior border of V1 with other visually responsive areas. The representation of the horizontal meridian runs anterolateral to posteromedial, dividing V1 so that the lower visual quadrant is represented medially, and the upper quadrant laterally. The total surface area of V1 is about 140 mm2 for P. poliocephalus, and 110 mm2 for P. scapulatus. The representation of the central visual field is greatly magnified relative to that of the periphery. The cortical magnification factor decreases with increasing eccentricity, following a negative power function. Conversely, receptive field sizes increase markedly with increasing eccentricity, and therefore the point-image size is approximately constant throughout V1. The emphasis in the representation of the area centralis in V1 is much larger than that expected on the basis of ganglion cell counts in flat-mounted retinas. Thus, a larger degree of convergence occurs at the peripheral representations in the retino-geniculo-cortical pathway, in comparison with the central representations. The marked emphasis in the representation of central vision, the wide extent of the binocular field of vision, and the relatively large surface area of V1 reflect the importance of vision in megachiropterans.

How does evolution build a complex brain?

To understand how complex brains evolve one can examine a variety of the products of the evolutionary process and then infer the mechanisms that generate the differences observed. We address this issue using a number of techniques. We combine neurophysiological recording techniques with neuroanatomical tracing techniques and histochemical methods in an effort to accurately determine the functional subdivisions of the neocortex in a variety of mammals. By using these techniques we can determine common features of neocortical organization, or common cortical areas, which are considered homologous. We can observe modifications to patterns of cortical organization, or to cortical fields specifically, that are independently evolved and generally related to morphological and behavioural specializations. Comparative studies have led us to consider the development of the neocortex and the specific changes in developmental mechanisms that might account for the observed changes in extant adults. Both comparative studies and developmental studies allow us to formulate hypotheses regarding how the neocortex is constructed in the life of an individual, and in a lineage over time.

Cortical integration of parallel pathways in the visual system of primates.

Injections of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) were placed in the middle temporal visual area (MT) of squirrel monkeys to reveal the distributions of interconnections with functionally distinct modules in areas 17 and 18. In agreement with previous reports, brain sections cut parallel to the surface of manually flattened cortex and reacted for cytochrome oxidase (CO) revealed CO dense blobs in area 17 and alternating CO dense thick and thin bands separated by CO light interbands in area 18. Alternate sections stained for myelin indicated that the CO light interblobs and interbands are more densely myelinated than the CO dense blobs and bands. Our major finding is that projections from MT to areas 17 and 18 are both to modules projecting to MT and modules projecting to other targets. In area 17, the cells in the middle layers projecting to injection sites in MT typically were distributed in several short merging and diverging rows, suggesting the convergence of projections from several matched orientation columns in area 17 to the restricted injection site in MT. Backward projections from MT to more superficial layers in area 17 were distributed more evenly across cortex and over a wider area of cortex. These terminations were dense throughout the interblob cortex which includes all orientation columns and neurons projecting to area 18, but were light over the blobs. As previously reported, neurons in area 18 projecting to MT were located in one set of the CO dense bands. However, these bands appeared to be thin rather than thick.(ABSTRACT TRUNCATED AT 250 WORDS)

Photic preference of the short-tailed opossum (Monodelphis domestica).

The gray short-tailed opossum (Monodelphis domestica) is a nocturnal South American marsupial that has been gaining popularity as a laboratory animal. However, compared to traditional laboratory animals like rats, very little is known about its behavior, either in the wild or in a laboratory setting. Here we investigated the photic preference of the short-tailed opossum. Opossums were placed in a circular testing arena and allowed to move freely between dark (0 lux) and light (∼1.4, 40, or 400 lux) sides of the arena. In each of these conditions opossums spent significantly more time in the dark than in the illuminated side and a greater proportion of time in the dark than would be expected by chance. In the high-contrast (∼400 lux) illumination condition, the mean bout length (i.e., duration of one trip on the light or dark side) was significantly longer on the dark side than on the light side. When we examined the number of bouts greater than 30 and 60s in duration, we found a significant difference between the light and dark sides in all light contrast conditions. These data indicate that the short-tailed opossum prefers the dark to the light, and can also detect very slight differences in light intensity. We conclude that although rats and opossums share many similar characteristics, including ecological niche, their divergent evolutionary heritage results in vastly different behavioral capabilities. Only by observing the behavioral capabilities and preferences of opossums will we be able to manipulate the experimental environment to best elicit and elucidate their behavior and alterations in behavior that can arise from experimental manipulations.

Area 17 lesions deactivate area MT in owl monkeys.

The middle temporal visual area, MT, is one of three major targets of the primary visual cortex, area 17, in primates. We assessed the contribution of area 17 connections to the responsiveness of area MT neurons to visual stimuli by first mapping the representation of the visual hemifield in MT of anesthetized owl monkeys with microelectrodes, ablating an electrophysiologically mapped part of area 17, and then immediately remapping MT. Before the lesions, neurons at recording sites throughout MT responded vigorously to moving slits of light and other visual stimuli. In addition, the relationship of receptive fields to recording sites revealed a systematic representation of the contralateral visual hemifield in MT, as reported previously for owl monkeys and other primates. The immediate effect of removing part of the retinotopic map in area 17 by gentle aspiration was to selectively deactivate the corresponding part of the visuotopic map in MT. Lesions of dorsomedial area 17 representing central and paracentral vision of the lower visual quadrant deactivated neurons in caudomedial MT formerly having receptive fields in the central and paracentral lower visual quadrant. Most neurons at recording sites throughout other parts of MT had normal levels of responsiveness to visual stimuli, and receptive-field locations that closely matched those before the lesion. However, neurons at a few sites along the margin of the deactivated zone of cortex had receptive fields that were slightly displaced from the region of vision affected by the lesion into other parts of the visual field, suggesting some degree of plasticity in the visual hemifield representation in MT.(ABSTRACT TRUNCATED AT 250 WORDS)

Responsiveness and somatotopic organization of anterior parietal field 3b and adjoining cortex in newborn and infant monkeys.

Microelectrodes were used to record from somatosensory areas 3b and adjoining areas 3a and 1 in newborn monkeys. At birth, area 3b was responsive to cutaneous stimuli and had an adult-like somatotopic organization in marmosets and one squirrel monkey, while areas 1 and 3a had only limited responsiveness. In newborn macaque monkeys, cortex was unresponsive to cutaneous stimuli; however, by 1 month, areas 3b and 1 appeared to be adult-like in responsiveness and somatotopic organization.