The squirrel monkey entorhinal cortex: Architecture and medial frontal afferents

The cytoarchitecture of the periallocortex was studied in cresyl-violet-stained frontal and sagittal sections in six adult squirrel monkeys (Saimiri sciureus). The entorhinal area, located between the sulcus semiannularis and sulcus rhinalis in the rostral parahippocampal gyrus, has been divided into a caudal-medial Area 28a and rostral-lateral Area 28b. Of the six paleocortical laminae, Layer II is the most distinctive, for in 28a it consists of a deeply-stained, dense, continuous lamina and in 28b is interrupted into cell islands. Layer IV, lamina dissecans, is thick and irregular in 28b, thin and of uniform thickness in rostral 28a, and disappears in caudal 28a in transition to the isocortex of the more caudal parahippocampal gyrus. Further observations in Fink-Heimer silver material of fiber degeneration resulting from medial prefrontal ablations in these monkeys suggest that Areas 9 and 10 project to superficial layers (Layer II) of 28a whereas the orbital probably projects to the deeper layers (Layer V) of 28b. The topography and laminar specificity of prefrontal-entorhinal connections may have important functional consequences in terms of hippocampal input.     

Long-latency somatosensory evoked potentials

Theoretically, long-latency somatosensory evoked potentials (SEPs) provide information on the function of somatosensory associative cortical structures. Their potential role in clinical studies and research has been hampered by the lack of standardized methodology in the use of these SEPs. Other factors, such as drugs, simultaneous stimuli, and state of consciousness, also have far-reaching influences on the various parameters of long-latency SEPs. The knowledge of the origin of most SEP components is at best fragmentary; studies on clinical-electrophysiological correlations seem to be hopeful in this respect. As yet, clinical applications of long-latency SEPs are limited; for future research, studies of disturbances of SEPs are most promising, mainly with regard to diseases of the gray matter, the influence of drugs on the cerebral function, and psychopathology.     

Cytoarchitecture of the frontal lobe of the squirrel monkey

Six Saimiri sciurea brains were studied. Coronal, horizontal, and sagittal sections were stained with cresyl violet. The frontal lobe map shows, anterior to the cental sulcus, a triangular area 4, agranularis giganto-pyramidalis (FA). Anterior to area 4 is area 6, frontalis agranularis (FB), which differs from area 4 by the absence of Betz cells. Rostral to area 6 is area 8, frontalis intermedia (FC), a narrow, thick band of cortex, with a faint layer IV. Anterior to area 8 the cortex becomes thinner and more granular, its cells are smaller and this region is subdivided into area 9, frontalis granularis (FD), and area 10, fronto-polaris (FE). Area 44 (FCBm) is on the inferior frontal gyrus, this cortex is granular limb of the arcuate sulcus is thin and granular, contains very large pyramidal cells in layers V and III: area 8a (FDT). The orbital cortex posteriorly shows an evident loss of layer IV; area 11 (FF). Area 50 is a narrow band of koniocortex in the inner wall of the fronto-parietal operculum. On the gyrus rectus the cortex develops allocortical characteristics with a heavy layer V, area 12 (FG). The characteristics of limbic cortex (mesocortex) are present on the cingulate gyrus, area 24 (LA). Area 25 is found in the subcallosal region.     

Long-range focal collateralization of axons arising from corticocortical cells in monkey sensory-motor cortex

Small extracellular injections of HRP were placed into a stratum of corticocortical axons situated immediately deep to area 3b of the monkey somatic sensory cortex. This stratum had previously been demonstrated to contain corticocortical fibers linking the cytoarchitectonic fields of the somatic sensory cortex to one another and certain of them to the motor cortex. This method resulted in extremely successful filling of pyramidal cells, their axons, collateral axon branches, and terminations in areas 3b, 1, and 2 posterior to the injection and in areas 3a and 4 anterior to it. The major finding was that cells with somata situated in any one of these fields and with principal axons traversing the injection site have long collaterals, primarily in layers III and V, which can extend throughout their own cytoarchitectonic field and into one or more other fields. In these fields they give off focused, columnlike concentrations of terminal boutons, which can be separated from one another by 800 micron or more. The anterogradely labeled, primary corticocortical fibers, traced forwards into areas 3a and 4, have virtually identical focal terminations. These findings indicate that interareal connectivity in the sensory-motor cortex can be effected by the axon branches of single cells rather than by separate groups of cells, and this may form a basis for the convergence of place and modality information on single cells in the sensorimotor cortex, a convergence that is not seen in the thalamic input to this cortex.     

The ipsilateral corticocortical connections of area 7 with the frontal lobe in the monkey.

The corticocortical connections between area 7 and the frontal lobe have been studied in the monkey. Injections of HRP were made into area 7 of the parietal lobe or into area 46 in the walls of the principal sulcus. The two subdivisions of area 7, 7a or PG and 7b or PF, are connected with different parts of the frontal lobe, and each subdivision is connected with two distinct areas. Area 7b, PF, is connected in a well organized and somatotopic manner with the lower premotor area and with the lower part of area 46, below the fundus of the principal sulcus. Area 7a, PG, is connected with area 8a and with the upper part of area 46, above the fundus of the principal sulcus; it is suggested that the lower part of area 8a and the posterior part of area 46 are related to the central visual field, while the medial part of area 8a and the anterior part of area 46 are related to the periphery of the visual field. The corticocortical connections between area 7 and the frontal lobe are reciprocal and those passing from area 7 to the frontal lobe are 'feed-forward' and those to area 7 are 'feed-back'.     

Brain-stem auditory evoked potentials in squirrel monkey (Saimiri sciureus)

To more fully characterize brain-stem auditory evoked potentials (BAEPs) in non-human primates, BAEPs were recorded from chronically implanted epidural electrodes in 10 squirrel monkeys (Saimiri sciureus). The effects of stimulus intensity, repetition rate, and anesthesia (ketamine 20 mg/kg i.m.) on peak latencies and inter-peak intervals were evaluated. Monkey wave forms consisted of approximately 7 peaks (I-VII), each exhibiting similar latencies across sessions, with later peaks exhibiting greater variability. In some subjects, additional peaks (IIa, IIIa) and slow potentials were recorded. The slow potentials provided a substratum for peaks IV through VII. As with human, monkey peaks exhibited systematic changes in latency with changes in stimulus intensity or repetition rate. These shifts included significant decreases in latency with increasing intensity for peaks I-IV and increases in latency with increases in repetition rate for peaks III, V, and VI. Inter-peak intervals were similar to those observed in human. Furthermore, ketamine anesthesia significantly delayed the latencies of most peaks (except I, V, and VII). Some differences between monkey and human BAEPs were evident in the relative amplitude of specific peaks. For example, peak V is typically most prominent in human, while this was true for peak III in monkey. The similarities between unanesthetized monkey and human inter-peak intervals suggest that the times required for impulses to reach particular brain-stem areas are conserved across primate species that vary in brain size. This supports the hypothesis that comparably numbered BAEP peaks in monkey and human index homologous processes. The data also suggest that the differences between animal and human BAEPs commonly reported may result from the use of anesthetics. In summary, unanesthetized monkey BAEPs resemble human BAEPs in morphology, number of peaks, polarity, latency variability, inter-peak intervals, slow potentials superimposed on the high-frequency peaks, and variations in morphology, amplitude, and resolution of peaks as a function of recording site. Thus, unanesthetized monkey BAEPs may be an excellent model for investigating the neural substrates of human BAEP or for determining species differences in acoustic processing among primates.     

Posterior parietal cortex in rhesus monkey: II. Evidence for segregated corticocortical networks linking sensory and limbic areas with the frontal lobe

We have examined the circuitry connecting the posterior parietal cortex with the frontal lobe of rhesus monkeys. HRP-WGA and tritiated amino acids were injected into subdivisions 7m, 7a, 7b, and 7ip of the posterior parietal cortex, and anterograde and retrograde label was recorded within the frontal motor and association cortices. Our main finding is that each subdivision of parietal cortex is connected with a unique set of frontal areas. Thus, area 7m, on the medial parietal surface, is interconnected with the dorsal premotor cortex and the supplementary motor area, including the supplementary eye field. Within the prefrontal cortex, area 7m's connections are with the rostral sector of the frontal eye field (FEF), the dorsal bank of the principal sulcus, and the anterior bank of the inferior arcuate sulcus (Walker's area 45). In contrast, area 7a, on the posterior parietal convexity, is not linked with premotor regions but is heavily interconnected with the rostral FEF in the anterior bank of the superior arcuate sulcus, the dorsolateral prefrontal convexity, the rostral orbitofrontal cortex, area 45, and the fundus and adjacent cortex of the dorsal and ventral banks of the principal sulcus. Area 7b, in the anterior part of the posterior parietal lobule, is interconnected with still a different set of frontal areas, which include the ventral premotor cortex and supplementary motor area, area 45, and the external part of the ventral bank of the principal sulcus. The prominent connections of area 7ip, in the posterior bank of the intraparietal sulcus, are with the supplementary eye field and restricted portions of the ventral premotor cortex, with a wide area of the FEF that includes both its rostral and caudal sectors, and with area 45. All frontoparietal connections are reciprocal, and although they are most prominent within a hemisphere, notable interhemispheric connections are also present. These findings provide a basis for a parcellation of the classically considered association cortex of the frontal lobe, particularly the cortex of the principal sulcus, into sectors defined by their specific connections with the posterior parietal subdivisions. Moreover, the present findings, together with those of a companion study (Cavada and Goldman-Rakic: J. Comp. Neurol. this issue) have allowed us to establish multiple linkages between frontal areas and specific limbic and sensory cortices through the posterior parietal cortex. The networks thus defined may form part of the neural substrate of parallel distributed processing in the cerebral cortex.     

The corticocortical projections of the physiologically defined eye field in the rat medial frontal cortex

This study investigated in the rat the corticocortical projections of the frontal eye field (FEF), which is located within the medial frontal cortex. The experiments were carried out on Wistar rats. Seven animals received a single iontophoretic injection of Phaseolus vulgaris leucoagglutinin in an FEF site within the medial frontal cortex where intracortical microstimulation elicited eye movements. In these cases, anterogradely labeled fibers and terminal-like elements were found in both hemispheres. The densest labeling was seen in the injected hemisphere, where labeled fibers prevailed in the visual cortex and their laminar distribution differed between the primary and secondary visual cortices. Dense labeled fibers were also seen in the frontal and retrosplenial cortex, whereas a columnar arrangement of terminal-like elements was detected in a restricted part of area 1 of the somatosensory cortex. Contralaterally to the injection site, labeled fibers were distributed mainly in the homotopic region. In two animals, the tracer was injected in a site at the FEF border whose stimulation evoked eye and whisker movements. In these animals, a different distribution of labeling was observed with respect to the other rats in which the tracer was deposited within the FEF, and anterograde labeling was observed in areas 1 and 2 of the parietal cortex of both hemispheres; in addition, no labeling was observed in these cases in the primary visual cortex. These findings suggest that cortical sites confined within the rat FEF are implicated in the control of orienting and exploring behaviors in addition to the control of eye movement.     

Photically evoked unit activity in the tectum opticum of the squirrel monkey

Visually evoked unit responses of the tectum were studied in the squirrel monkey. The results showed that the superior colliculus had a definite functional laminar organization. Units responding to diffuse light were found in the upper part of the stratum griseum superficiale, units having antagonistically segregated receptive fields in the middle portion, and units sensitive to moving objects in the lower portion as well as in the stratum opticum. Retinotopic projection was also observed. The pretectal region contained mainly “tonic-on” and “inhibitory” units. No specific localization was observed in the distribution of these units.     

Input-output organization of jaw movement-related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and K?lliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.     

Long-latency motor evoked potentials in congenital hemiplegia.

OBJECTIVE: To investigate long-latency motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation in congenital hemiplegia (CH) and to seek for correlation with paretic hand movement deficits. METHODS: MEPs were recorded from the first dorsal interosseous of both hands in 12 CH patients and 12 age-matched controls; dexterity and upper limb function were quantitatively assessed in both groups. RESULTS: In CH patients, long-latency MEPs, occurring much later than the commonly reported MEPs, were frequently observed in the paretic and non-paretic hands. Four distinct groups of long-latency MEPs were found, each cluster being identified by its mean latency, namely 35, 85, 160 and 225 ms. The residual dexterity of the paretic hand was correlated with the presence of contralateral MEPs with a 20 and 225 ms latency and was negatively correlated with ipsilateral MEPs, irrespective of their latency. In controls, only few MEPs with a latency of 225 ms were found in 4 out of 12 subjects. CONCLUSIONS: The pattern of MEPs found in CH patients differs dramatically from that reported in adult stroke patients, suggesting that long-latency MEPs are a rather distinctive consequence of early corticospinal lesions. The hypothesis that a given cluster of long-latency MEPs is mediated by a particular pathway appears very unlikely. Rather, we suggest that an exacerbation of cortical and/or spinal excitability is at the origin of these long-latency MEPs.