In recent years, many new cortical areas have been identified in the macaque monkey. The number of identified connections between areas has increased even more dramatically. We report here on (1) a summary of the layout of cortical areas associated with vision and with other modalities, (2) a computerized database for storing and representing large amounts of information on connectivity patterns, and (3) the application of these data to the analysis of hierarchical organization of the cerebral cortex. Our analysis concentrates on the visual system, which includes 25 neocortical areas that are predominantly or exclusively visual in function, plus an additional 7 areas that we regard as visual-association areas on the basis of their extensive visual inputs. A total of 305 connections among these 32 visual and visual-association areas have been reported. This represents 31% of the possible number of pathways if each area were connected with all others. The actual degree of connectivity is likely to be closer to 40%. The great majority of pathways involve reciprocal connections between areas. There are also extensive connections with cortical areas outside the visual system proper, including the somatosensory cortex, as well as neocortical, transitional, and archicortical regions in the temporal and frontal lobes. In the somatosensory/motor system, there are 62 identified pathways linking 13 cortical areas, suggesting an overall connectivity of about 40%. Based on the laminar patterns of connections between areas, we propose a hierarchy of visual areas and of somatosensory/motor areas that is more comprehensive than those suggested in other recent studies. The current version of the visual hierarchy includes 10 levels of cortical processing. Altogether, it contains 14 levels if one includes the retina and lateral geniculate nucleus at the bottom as well as the entorhinal cortex and hippocampus at the top. Within this hierarchy, there are multiple, intertwined processing streams, which, at a low level, are related to the compartmental organization of areas V1 and V2 and, at a high level, are related to the distinction between processing centers in the temporal and parietal lobes. However, there are some pathways and relationships (about 10% of the total) whose descriptions do not fit cleanly into this hierarchical scheme for one reason or another. In most instances, though, it is unclear whether these represent genuine exceptions to a strict hierarchy rather than inaccuracies or uncertainities in the reported assignment.

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Distributed Hierarchical Processing in the Primate Cerebral Cortex

The two basic phenomena that define the problem of visual attention can be illustrated in a simple example. Consider the arrays shown in each panel of Figure 1. In a typical experiment, before the arrays were presented, subjects would be asked to report letters appearing in one color (targets, here black letters), and to disregard letters in the other color (nontargets, here white letters). The array would then be briefly flashed, and the subjects, without any opportunity for eye movements, would give their report. The display mimics our. usual cluttered visual environment: It contains one or more objects that are relevant to current behavior, along with others that are irrelevant. The first basic phenomenon is limited capacity for processing information. At any given time, only a small amount of the information available on the retina can be processed and used in the control of behavior. Subjectively, giving attention to any one target leaves less available for others. In Figure 1, the probability of reporting the target letter N is much lower with two accompa­ nying targets (Figure la) than with none (Figure Ib). The second basic phenomenon is selectivity-the ability to filter out un­ wanted information. Subjectively, one is aware of attended stimuli and largely unaware of unattended ones. Correspondingly, accuracy in identifying an attended stimulus may be independent of the number of nontargets in a display (Figure la vs Ie) (see Bundesen 1990, Duncan 1980).

Neural Mechanisms of Selective Visual Attention

The Prefrontal Cortex, Fifth Edition, provides users with a thoroughly updated version of this comprehensive work that has historically served as the classic reference on this part of the brain. The book offers a unifying, interdisciplinary perspective that is lacking in other volumes written about the frontal lobes, and is, once again, written by the award-winning author who discovered "memory cells," the physiological substrate of working memory. The fifth edition constitutes a comprehensive update, including all the major advances made on the physiology and cognitive neuroscience of the region since publication in 2008. All chapters have been fully revised, and the overview of prefrontal functions now interprets experimental data within the theoretical framework of the new paradigm of cortical structure and dynamics (the Cognit Paradigm), addressing the accompanying social, economic, and cultural implications. * Provides a distinctly interdisciplinary view of the prefrontal cortex, covering all major methodologies, from comparative anatomy to modern imaging* Unique analysis and synthesis of a large body of basic and clinical data on the subject (more than 2000 references)* Written by an award-winning author who discovered "memory cells," the physiological substrate of working memory* Synthesizes evidence that the prefrontal cortex constitutes a complex pre-adaptive system* Incorporates emerging study of the role of the frontal lobes in social, economic, and cultural adaptation

The prefrontal cortex

Unilateral neglect reflects a disturbance in the spatial distribution of directed attention. A review of unilateral neglect syndromes in monkeys and humans suggests that four cerebral regions provide an integrated network for the modulation of directed attention within extrapersonal space. Each component region has a unique functional role that reflects its profile of anatomical connectivity, and each gives rise to a different clinical type of unilateral neglect when damaged. A posterior parietal component provides an internal sensory map and perhaps also a mechanism for modifying the extent of synaptic space devoted to specific portions of the external world; a limbic component in the cingulate gyrus regulates the spatial distribution of motivational valence; a frontal component coordinates the motor programs for exploration, scanning, reaching, and fixating; and a reticular component provides the underlying level of arousal and vigilance. This hypothetical network requires at least three complementary and interacting representations of extrapersonal space: a sensory representation in posterior parietal cortex, a schema for distributing exploratory movements in frontal cortex, and a motivational map in the cingulate cortex. Lesions in only one component of this network yield partial unilateral neglect syndromes, while those that encompass all the components result in profound deficits that transcend the mass effect of the larger lesion. This network approach to the localization of complex functions offers an alternative to more extreme approaches, some of which stress an exclusive concentration of function within individual centers in the brain and others which advocate a more uniform (equipotential or holistic) distribution. In human beings, unilateral neglect syndromes are more frequent and severe after lesions in the right hemisphere. Also, right hemisphere mechanisms appear more effective in the execution of attentional tasks. Furthermore, the attentional functions of the right hemisphere span both hemispaces, while the left hemisphere seems to contain the neural apparatus mostly for contralateral attention. This evidence indicates that the right hemisphere of dextrals has a functional specialization for the distribution of directed attention within extrapersonal space.

A cortical network for directed attention and unilateral neglect.

The sections in this article are:


1
Essence of Prefrontal Function: Regulation of Behavior by Representational Knowledge
11
Subdivisions of Prefrontal Cortex

12
Global Nature of Prefrontal Syndrome in Humans

13
Animal Model for Prefrontal Function in Humans

14
Delayed-Response Tests and Varying Interpretations of Their Functional Significance

15
Distractability and Perseveration: Secondary Consequences of Basic Defect in Representational Memory

16
Representational Memory in Wisconsin Card Sort and Other Diagnostic Tests of Prefrontal Function in Humans

17
Localization of Delayed-Response Function: Principal Sulcus

18
Circuit Basis of Visuospatial Functions




2
Accessing and “On-Line” Processing of Representations in Visuospatial Domain: Parietal-Prefrontal Connections
21
Visuospatial Representational Memory in Humans

22
Spatial-Mnemonic Nature of Delayed-Response Deficit: Domain-Specific Memory Loss

23
Topography of Representational Memory in Prefrontal Cortex

24
Electrophysiological Evidence of Spatial-Mnemonic Processes in Principal Sulcus

25
Parietal-Prefrontal Connectivity

26
Columnar and Laminar Framework for Feedforward and Feedback Mechanisms

27
Functional Significance of Parietal-Prefrontal Collaboration




3
Long-Term Memory and “Off-Line” Processing: Prefrontal-Limbic Connections
31
Role of Hippocampus in Spatial Memory

32
Multiple Connections Between Principal Sulcus and Hippocampal Formation

33
Quadripartite Neural Network: Parietal-Temporal-Cingulate-Prefrontal Circuit

34
Limbic Contribution to Spatial Memory




4
Response Initiation and Inhibition: Projections to Striatum, Tectum, Thalamus, and Premotor Cortex
41
Motor-Control Functions of Prefrontal Cortex

42
Cortical-Striatal Pathway and Related Feedback Loops

43
Cortical-Tectal Pathway

44
Thalamic-Cortical Systems

45
Prefrontal-Premotor Connections: Anterior Supplementary Motor Cortex Relays

46
Functional Speculations




5
Modulatory Mechanisms: Brain Stem Catecholamine Projections
51
Activation of Cognitive Machinery

52
Concentration and Synthesis of Catecholamines in Primate Cortex

53
Brain Stem Innervation of Prefrontal Cortex

54
Delayed-Response Deficits and Recovery Produced by Catecholamine Loss and Replacement in Prefrontal Cortex

55
Circuit Basis for Neuromodulation in Principal Sulcus




6
Multiple Subsystems of Prefrontal Cortex: Unity or Diversity of Function
61
Unity or Diversity of Prefrontal Function

62
Frontal Eye Fields

63
Inferior Convexity

64
Orbital Prefrontal Cortices

65
Problem of Integration




7
Diseases Affecting Prefrontal Cortex
71
Schizophrenia: Loss of Corticocortical Processing and Regulation of Behavior by Representational Knowledge

72
Wernicke-Korsakoff Syndrome: Loss of Thalamocortical and Brain Stem Modulatory Mechanisms

73
Huntington's Chorea and Parkinson's Disease: Loss of Prefrontal-Striatal Mechanisms and Initiation or Inhibition of Motor Response

74
Overview of Neurobiology of Disease




8
Summary

Circuitry of Primate Prefrontal Cortex and Regulation of Behavior by Representational Memory

The cortical projections of the inferior pulvinar and adjacent lateral pulvinar in the rhesus monkey (macaca mulatta): An autoradiographic study

Auditory--visual interaction in single cells in the cortex of the superior temporal sulcus and the orbital frontal cortex of the macaque monkey.

An autoradiographic study of the projections of the pretectum in the rhesus monkey (Macaca mulatta): evidence for sensorimotor links to the thalamus and oculomotor nuclei.

A comparison of the organization of the projections of the dorsal lateral geniculate nucleus, the inferior pulvinar and adjacent lateral pulvinar to primary visual cortex (area 17) in the macaque monkey.

Michael Rezak

Papers

The cortical projections of the inferior pulvinar and adjacent lateral pulvinar in the rhesus monkey (macaca mulatta): An autoradiographic study

Auditory--visual interaction in single cells in the cortex of the superior temporal sulcus and the orbital frontal cortex of the macaque monkey.

An autoradiographic study of the projections of the pretectum in the rhesus monkey (Macaca mulatta): evidence for sensorimotor links to the thalamus and oculomotor nuclei.

A comparison of the organization of the projections of the dorsal lateral geniculate nucleus, the inferior pulvinar and adjacent lateral pulvinar to primary visual cortex (area 17) in the macaque monkey.

Extrageniculate projections to layers VI and I of striate cortex (area 17) in the rhesus monkey (Macaca mulatta).