[show abstract][hide abstract] ABSTRACT: The laminar pattern of axonal termination from prefrontal (caudal orbitofrontal, rostral orbitofrontal and lateral areas) to anterior temporal areas (entorhinal cortex, perirhinal cortex and area TE) and from temporal to prefrontal areas was investigated with the aid of anterograde tracers. Both regions are characterized by structural heterogeneity, and include agranular, dysgranular and granular cortical types, denoting, respectively, the absence, incipience and presence of granular layer 4. In addition, both the prefrontal and anterior temporal cortices are composed of areas that have related though specialized functions. The pattern of cortical axonal termination was associated with both the structural type of the cortex of origin and the structure of the destination cortex. Thus, efferent fibers from a single origin in either prefrontal or anterior temporal cortex terminated in different patterns depending on their target area. Conversely, axons terminated in different patterns in a single target area, prefrontal or anterior temporal, depending on their area of origin. Projections from agranular or dysgranular type cortices (e.g. medial temporal areas and caudal orbitofrontal areas) terminated mostly in the upper layers of granular cortices (e.g. area TE and lateral prefrontal areas), and projections from granular cortices terminated mostly in the deep layers of agranular or dys- granular cortices. A robust projection from dysgranular orbitofrontal areas terminated in the deep layers of the agranular entorhinal cortex. Projections from prefrontal areas to area TE terminated in the upper layers, and may facilitate focused attention on behaviorally relevant stimuli processed through reciprocal pathways between prefrontal and temporal cortices.
[show abstract][hide abstract] ABSTRACT: Medial prefrontal cortices in primates have been associated with emotion, memory, and complex cognitive processes. Here we investigated whether the pattern of cortical connections could indicate whether the medial prefrontal cortex constitutes a homogeneous region, or if it can be parceled into distinct sectors. Projections from medial temporal memory-related cortices subdivided medial cortices into different sectors, by targeting preferentially caudal medial areas (area 24, caudal 32 and 25), to a lesser extent rostral medial areas (rostral area 32, areas 14 and 10), and sparsely area 9. Area 9 was distinguished by its strong connections with premotor cortices. Projections from unimodal sensory cortices reached preferentially specific medial cortices, including a projection from visual cortices to area 32/24, from somatosensory cortices to area 9, and from olfactory cortices to area 14. Medial cortices were robustly interconnected, suggesting that local circuits are important in the neural processing in this region. Medial prefrontal cortices were unified by bidirectional connections with superior temporal cortices, including auditory areas. Auditory pathways may have a role in the specialization of medial prefrontal cortices in species-specific communication in non-human primates and language functions in humans.
The Journal of Comparative Neurology 09/1999; 410(3):343-67. · 3.66 Impact Factor
[show abstract][hide abstract] ABSTRACT: Prefrontal cortices have been implicated in autonomic function, but their role in this activity is not well understood. Orbital and medial prefrontal cortices receive input from cortical and subcortical structures associated with emotions. Thus, the prefrontal cortex may be an essential link for autonomic responses driven by emotions. Classic studies have demonstrated the existence of projections between prefrontal cortex and the hypothalamus, a central autonomic structure, but the topographic organization of these connections in the monkey has not been clearly established. We investigated the organization of bidirectional connections between these areas in the rhesus monkey by using tracer injections in orbital, medial, and lateral prefrontal areas. All prefrontal areas investigated received projections from the hypothalamus, originating mainly in the posterior hypothalamus. Differences in the topography of hypothalamic projection neurons were related to both the location and type of the target cortical area. Injections in lateral eulaminate prefrontal areas primarily labeled neurons in the posterior hypothalamus that were equally distributed in the lateral and medial hypothalamus. In contrast, injections in orbitofrontal and medial limbic cortices labeled neurons in the anterior and tuberal regions of the hypothalamus and in the posterior region. Projection neurons targeting orbital limbic cortices were more prevalent in the lateral part of the hypothalamus, whereas those targeting medial limbic cortices were more prevalent in the medial hypothalamus. In comparison to the ascending projections, descending projections from prefrontal cortex to the hypothalamus were highly specific, originating mostly from orbital and medial prefrontal cortices. The ascending and descending connections overlapped in the hypothalamus in areas that have autonomic functions. These results suggest that specific orbitofrontal and medial prefrontal areas exert a direct influence on the hypothalamus and may be important for the autonomic responses evoked by complex emotional situations.
The Journal of Comparative Neurology 09/1998; 398(3):393-419. · 3.66 Impact Factor
[show abstract][hide abstract] ABSTRACT: Cortical areas are linked through pathways which originate and terminate in specific layers. The factors underlying which layers are involved in specific connections are not well understood. Here we tested whether cortical structure can predict the pattern as well as the relative distribution of projection neurons and axonal terminals in cortical layers, studied with retrograde and anterograde tracers. We used the prefrontal cortices in the rhesus monkey as a model system because their laminar organization varies systematically, ranging from areas that have only three identifiable layers, to those that have six layers. We rated each prefrontal area based on the number and definition of its cortical layers (level 1, lowest; level 5, highest). The structural model accurately predicted the laminar pattern of connections in approximately 80% of the cases. Thus, projection neurons from a higher-level cortex originated mostly in the upper layers and their axons terminated predominantly in the deep layers (4-6) of a lower-level cortex. Conversely, most projection neurons from a lower-level area originated in the deep layers and their axons terminated predominantly in the upper layers (1-3) of a higher-level area. In addition, the structural model accurately predicted that the proportion of projection neurons or axonal terminals in the upper to the deep layers would vary as a function of the number of levels between the connected cortices. The power of this structural model lies in its potential to predict patterns of connections in the human cortex, where invasive procedures are precluded.