Historically, the locus coeruleus-norepinephrine (LC-NE) system has been implicated in arousal, but recent findings suggest that this system plays a more complex and specific role in the control of behavior than investigators previously thought. We review neurophysiological and modeling studies in monkey that support a new theory of LC-NE function. LC neurons exhibit two modes of activity, phasic and tonic. Phasic LC activation is driven by the outcome of task-related decision processes and is proposed to facilitate ensuing behaviors and to help optimize task performance (exploitation). When utility in the task wanes, LC neurons exhibit a tonic activity mode, associated with disengagement from the current task and a search for alternative behaviors (exploration). Monkey LC receives prominent, direct inputs from the anterior cingulate (ACC) and orbitofrontal cortices (OFC), both of which are thought to monitor task-related utility. We propose that these frontal areas produce the above patterns of LC activity to optimize utility on both short and long timescales.

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An Integrative Theory of Locus Coeruleus-Norepinephrine Function: Adaptive Gain and Optimal Performance.

Introduction to synaptic circuits, Gordon M.Shepherd and Christof Koch membrane properties and neurotransmitter actions, David A.McCormick peripheral ganglia, Paul R.Adams and Christof Koch spinal cord - ventral horn, Robert E.Burke olfactory bulb, Gordon M.Shepherd, and Charles A.Greer retina, Peter Sterling cerebellum, Rodolfo R.Llinas and Kerry D.Walton thalamus, S.Murray Sherman and Christof Koch basal ganglia, Charles J.Wilson olfactory cortex, Lewis B.Haberly hippocampus, Thomas H.Brown and Anthony M.Zador neocortex, Rodney J.Douglas and Kevan A.C.Martin Gordon M.Shepherd. Appendix: Dendretic electrotonus and synaptic integration.

The synaptic organization of the brain

Anatomical and physiological observations in monkeys indicate that the primate visual system consists of several separate and independent subdivisions that analyze different aspects of the same retinal image: cells in cortical visual areas 1 and 2 and higher visual areas are segregated into three interdigitating subdivisions that differ in their selectivity for color, stereopsis, movement, and orientation. The pathways selective for form and color seem to be derived mainly from the parvocellular geniculate subdivisions, the depth- and movement-selective components from the magnocellular. At lower levels, in the retina and in the geniculate, cells in these two subdivisions differ in their color selectivity, contrast sensitivity, temporal properties, and spatial resolution. These major differences in the properties of cells at lower levels in each of the subdivisions led to the prediction that different visual functions, such as color, depth, movement, and form perception, should exhibit corresponding differences. Human perceptual experiments are remarkably consistent with these predictions. Moreover, perceptual experiments can be designed to ask which subdivisions of the system are responsible for particular visual abilities, such as figure/ground discrimination or perception of depth from perspective or relative movement--functions that might be difficult to deduce from single-cell response properties.

https://science.sciencemag.org/content/sci/240/4853/740.full.pdf

Segregation of form, color, movement, and depth: anatomy, physiology, and perception

Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.

What we can do and what we cannot do with fMRI

Vision is critical for the functional and structural maturation of connections in the mammalian visual system. Visual experience, however, is a subset of a more general requirement for neural activity in transforming immature circuits into the organized connections that subserve adult brain function. Early in development, internally generated spontaneous activity sculpts circuits on the basis of the brain's "best guess" at the initial configuration of connections necessary for function and survival. With maturation of the sense organs, the developing brain relies less on spontaneous activity and increasingly on sensory experience. The sequential combination of spontaneously generated and experience-dependent neural activity endows the brain with an ongoing ability to accommodate to dynamically changing inputs during development and throughout life.

Synaptic Activity and the Construction of Cortical Circuits

We have tested the accuracy of Gordon Holmes' retinotopic map of human striate cortex by correlating magnetic resonance scans with homonymous field defects in patients with clearly defined occipital lobe lesions. Our findings indicate that Holmes underestimated the cortical magnification of central vision. In a revised map of the human striate cortex, we expand the area subserving central vision and reduce the area devoted to peripheral vision. These changes bring the map of human striate cortex into agreement with data reported for closely related nonhuman primate species.

The representation of the visual field in human striate cortex : a revision of the classic Holmes map

This year, the field of neuroscience celebrates the 50th anniversary of Mountcastle's discovery of the cortical column In this review, we summarize half a century of research and come to the disappointing realization that the column may have no function Originally, it was described as a discrete structure, spanning the layers of the somatosensory cortex, which contains cells responsive to only a single modality, such as deep joint receptors or cutaneous receptors Subsequently, examples of columns have been uncovered in numerous cortical areas, expanding the original concept to embrace a variety of different structures and principles A ‘column’ now refers to cells in any vertical cluster that share the same tuning for any given receptive field attribute In striate cortex, for example, cells with the same eye preference are grouped into ocular dominance columns Unaccountably, ocular dominance columns are present in some species, but not others In principle, it should be possible to determine their function by searching for species differences in visual performance that correlate with their presence or absence Unfortunately, this approach has been to no avail; no visual faculty has emerged that appears to require ocular dominance columns Moreover, recent evidence has shown that the expression of ocular dominance columns can be highly variable among members of the same species, or even in different portions of the visual cortex in the same individual These observations deal a fatal blow to the idea that ocular dominance columns serve a purpose More broadly, the term ‘column’ also denotes the periodic termination of anatomical projections within or between cortical areas In many instances, periodic projections have a consistent relationship with some architectural feature, such as the cytochrome oxidase patches in V1 or the stripes in V2 These tissue compartments appear to divide cells with different receptive field properties into distinct processing streams However, it is unclear what advantage, if any, is conveyed by this form of columnar segregation Although the column is an attractive concept, it has failed as a unifying principle for understanding cortical function Unravelling the organization of the cerebral cortex will require a painstaking description of the circuits, projections and response properties peculiar to cells in each of its various areas

The cortical column: a structure without a function

Thalamic nuclei are thought to funnel sensory information to the brain's primary cortical areas, which in turn transmit signals afresh to higher cortical areas. Here we describe a direct projection in the macaque monkey from the lateral geniculate nucleus (LGN) to the motion-selective middle temporal area (MTor V5), a cortical area not previously considered 'primary'. The constituent neurons are mostly koniocellular, send virtually no collateral axons to primary visual cortex (V1) and equal about 10% of the V1 population innervating MT. This pathway could explain the persistence of motion sensitivity in subjects following injury to V1, suggesting more generally that residual perception after damage in a primary area may arise from sparse thalamic input to 'secondary' cortical areas.

Bypassing V1: a direct geniculate input to area MT.

In two preliminary studies, normal macaque1 and squirrel monkey2,3 striate cortex cut parallel to the surface and stained for cytochrome oxidase (a mitochondrial enzyme) showed a striking pattern of regularly spaced patches. This was surprising, since until then no physiological or anatomical studies had suggested such a patchy organization. In the present study in the macaque we found that the patches were arranged in rows spaced about 350 µm apart. When one eye was injected with tritiated proline the rows of patches in layers II and III lay in register with the ocular-dominance bands seen autoradiographically in layer IVc. Removing one eye caused the patches in every other row to shrink and blanch. The rows of patches are therefore centred on the ocular dominance columns. Regions labelled by 2-deoxyglucose autoradiography after stimulating one eye with black-and-white stripes in all orientations consisted of rows of patches that lay in register with the cytochrome oxidase patches in every other row. On stimulating monkeys with stripes of a single orientation, the deoxyglucose-labelled regions formed a lattice that included the cytochrome oxidase patches but was more extensive. Thus either the deoxyglucose is not labelling the orientation columns at all, or the orientation columns coalesce in the areas marked by the cytochrome oxidase stain.

Regular patchy distribution of cytochrome oxidase staining in primary visual cortex of macaque monkey

Primary and secondary visual cortex (V1 and V2) form the foundation of the cortical visual system. V1 transforms information received from the lateral geniculate nucleus (LGN) and distributes it to separate domains in V2 for transmission to higher visual areas. During the past 20 years, schemes for the functional organization of V1 and V2 have been based on a tripartite framework developed by Livingstone & Hubel (1988). Since then, new anatomical data have accumulated concerning V1's input, its internal circuitry, and its output to V2. These new data, along with physiological and imaging studies, now make it likely that the visual attributes of color, form, and motion are not neatly segregated by V1 into different stripe compartments in V2. Instead, there are just two main streams, originating from cytochrome oxidase patches and interpatches, that project to V2. Each stream is composed of a mixture of magno, parvo, and konio geniculate signals. Further studies are required to elucidate how the patches and...

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Jonathan C. Horton

Papers

The representation of the visual field in human striate cortex : a revision of the classic Holmes map

The cortical column: a structure without a function

Bypassing V1: a direct geniculate input to area MT.

Regular patchy distribution of cytochrome oxidase staining in primary visual cortex of macaque monkey

THE CIRCUITRY OF V1 AND V2: Integration of Color, Form, and Motion