John M. Allman

The anterior cingulate cortex. The evolution of an interface between emotion and cognition.

Abstract: We propose that the anterior cingulate cortex is a specialization of neocortex rather than a more primitive stage of cortical evolution. Functions central to intelligent behavior, that is, emotional self‐control, focused problem solving, error recognition, and adaptive response to changing conditions, are juxtaposed with the emotions in this structure. Evidence of an important role for the anterior cingulate cortex in these functions has accumulated through single‐neuron recording, electrical stimulation, EEG, PET, fMRI, and lesion studies. The anterior cingulate cortex contains a class of spindle‐shaped neurons that are found only in humans and the great apes, and thus are a recent evolutionary specialization probably related to these functions. The spindle cells appear to be widely connected with diverse parts of the brain and may have a role in the coordination that would be essential in developing the capacity to focus on difficult problems. Furthermore, they emerge postnatally and their survival may be enhanced or reduced by environmental conditions of enrichment or stress, thus potentially influencing adult competence or dysfunction in emotional self‐control and problem‐solving capacity.

A real-time neural system for color constancy

A neural network approach to the problem of color constancy is presented. Various algorithms based on Lands retinex theory are discussed with respect to neurobiological parallels, computational efficiency, and suitability for VLSI implementation. The efficiency of one algorithm is improved by the application of resistive grids and is tested in computer simulations; the simulations make clear the strengths and weaknesses of the algorithm. A novel extension to the algorithm is developed to address its weaknesses. An electronic system that is based on the original algorithm and that operates at video rates was built using subthreshold analog CMOS VLSI resistive grids. The system displays color constancy abilities and qualitatively mimics aspects of human color perception.

Book Review: Two Phylogenetic Specializations in the Human Brain

In this study, two anatomical specializations of the brain in apes and humans are considered. One of these is a whole cortical area located in the frontal polar cortex (Brodmann’s area 10), and the other is a morphologically distinctive cell type, the spindle neuron of the anterior cingulate cortex. The authors suggest that the spindle cells may relay to other parts of the brain—especially to area 10, the outcome of processing within the anterior cingulate cortex. This relay conveys the motivation to act. It particularly concerns the recognition of having committed an error that leads to the initiation of adaptive responses to these adverse events so as to reduce error commission. This capacity is related to the development of self-control as an individual matures and gains social insight. Although the anterior cingulate deals with the individual’s immediate response to changing conditions, area 10 is involved in the retrieval of memories from the individual’s past experience and the capacity to plan adaptive responses. The authors suggest that these neurobehavioral specializations are crucial aspects of intelligence as defined as the capacity to make adaptive responses to changing conditions. The authors further hypothesize that these specializations facilitated the evolution of the unique capacity for the intergenerational transfer of the food and information characteristic of human extended families.

The von Economo neurons in the frontoinsular and anterior cingulate cortex

The von Economo neurons (VENs) are large bipolar neurons located in the frontoinsular cortex (FI) and limbic anterior (LA) area in great apes and humans but not in other primates. Our stereological counts of VENs in FI and LA show them to be more numerous in humans than in apes. In humans, small numbers of VENs appear the 36th week postconception, with numbers increasing during the first 8 months after birth. There are significantly more VENs in the right hemisphere in postnatal brains; this may be related to asymmetries in the autonomic nervous system. VENs are also present in elephants and whales and may be a specialization related to very large brain size. The large size and simple dendritic structure of these projection neurons suggest that they rapidly send basic information from FI and LA to other parts of the brain, while slower neighboring pyramids send more detailed information. Selective destruction of VENs in early stages of frontotemporal dementia (FTD) implies that they are involved in empathy, social awareness, and self‐control, consistent with evidence from functional imaging.

The Effect of Gaze Angle and Fixation Distance on the Responses of Neurons in V1, V2, and V4

What we see depends on where we look. This paper characterizes the modulatory effects of point of regard in three-dimensional space on responsiveness of visual cortical neurons in areas V1, V2, and V4. Such modulatory effects are both common, affecting 85% of cells, and strong, frequently producing changes of mean firing rate by a factor of 10. The prevalence of neurons in area V4 showing a preference for near distances may be indicative of the involvement of this area in close scrutiny during object recognition. We propose that eye-position signals can be exploited by visual cortex as classical conditioning stimuli, enabling the perceptual learning of systematic relationships between point of regard and the structure of the visual environment.

Magnification in striate cortex and retinal ganglion cell layer of owl monkey: a quantitative comparison

Magnification, the relative size of the neural representation of a portion of the visual field, decreases more rapidly with increasing visual field eccentricity in striate cortex than in the retinal ganglion cell layer of the owl monkey (Aotus trivirgatus); the proportion of the cells in striate cortex devoted to central vision is much larger than the comparable proportion of retinal ganglion cells. Magnification in striate cortex is a power function of magnification in the retinal ganglion cell layer. A formula for convergence (ganglion cells to cortical neurons) follows from this relationship.

The Scaling of White Matter to Gray Matter in Cerebellum and Neocortex

It is known that the white matter of neocortex increases disproportionately with brain size. However, relatively few measurements have been made of white matter/gray matter scaling in the cerebellum. We present data on the volumes of white and gray matter in both structures, taken from 45 species of mammals. We find a scaling exponent of 1.13 for cerebellum and 1.28 for neocortex. The 95% confidence intervals for our estimates of these two exponents do not overlap. This difference likely reflects differences in the connectivity and/or micro-structure of white matter in the two regions.

Evolution of Specialized Pyramidal Neurons in Primate Visual and Motor Cortex

The neocortex of primates contains several distinct neuron subtypes. Among these, Betz cells of primary motor cortex and Meynert cells of primary visual cortex are of particular interest for their potential role in specialized sensorimotor adaptations of primates. Betz cells are involved in setting muscle tone prior to fine motor output and Meynert cells participate in the processing of visual motion. We measured the soma volumes of Betz cells, Meynert cells, and adjacent infragranular pyramidal neurons in 23 species of primate and two species of non-primate mammal (Tupaia glis and Pteropus poliocephalus) using unbiased stereological techniques to examine their allometric scaling relationships and socioecological correlations. Results show that Betz somata become proportionally larger with increases in body weight, brain weight, and encephalization whereas Meynert somata remain a constant proportion larger than other visual pyramidal cells. Phylogenetic variance in the volumetric scaling of these neuronal subtypes might be related to species-specific adaptations. Enlargement of Meynert cells in terrestrial anthropoids living in open habitats, for example, might serve as an anatomical substrate for predator detection. Modification of the connectional and physiological properties of these neurons could constitute an important evolutionary mode for species-specific adaptation.

Brains, maturation times, and parenting<

Finch and Sapolsky propose that the slow development of human infants and their consequent long period of dependency on their parents have favored the evolution of genes that retard brain senescence, specifically recently evolved variants of the apolipoprotein E gene. We examine here the probable reasons why human maturation is so slow, and the influence of this slow development on parental dependence and patterns of survival. Large brains are expensive in terms of energy, anatomic complexity, and the time required to reach particular stages of postnatal maturation. We hypothesize that the maturational time costs arise from the fact that the brain is unique among the organs of the body in requiring a great deal of interaction with the environment (learning experience) to achieve adult competence, and thus that the brain serves as a rate-limiting factor governing the maturation of the entire body. Although the brain achieves its adult size at an earlier age than the other organs of the body, it does not become structurally and functionally mature until some point after sexual maturity [30]. The classical studies of developmental myelination by Flechsig [14,15] indicate that the brain matures slowly in stepwise hierarchies proceeding, for example, from the thalamus to the primary cortical sensory areas to the higher cortical areas of the temporal, parietal, and frontal lobes. Quartz and Sejnowski [33] have proposed that the brain builds sequentially from one level to the next on the basis of experience, and thus larger brains may require more time to mature, in part because they have more levels. We have examined the time costs associated with enlarged brains by analyzing the relationships between average brain size and the average times required to reach various stages of postnatal maturation, such as the eruption of various classes of teeth and reproductive maturity, in different primate species. Because both brain and developmental timing variables are related to body mass, we have first extracted the statistical effect of mass for each variable and then compared the residual values related to brain

Magnetic resonance microscopy of iron in the basal forebrain cholinergic structures of the aged mouse lemur

Increased non-heme iron levels in the brain of Alzheimers disease (AD) patients are higher than the levels observed in age matched normal subjects. Iron level in structures that are highly relevant for AD, such as the basal forebrain, can be detected post mortem with histochemistry. Because of the small size of these structures, in vivo MR detection is very difficult at conventional field magnets (1.5 and 4 T). In this study, we observed iron deposits with histochemistry and MR microscopy at 11.7 T in the brain of the mouse lemur, a strepsirhine primate which is the only known animal model of aging presenting both senile plaques and neurofibrillary degeneration. We also examined a related species, the dwarf lemur. Iron distribution in aged animals (8 to 15 years old) agrees with previous findings in humans. In addition, the high iron levels of the globus pallidus is paralleled by a comparable contrast in basal forebrain cholinergic structures. Because of the enhancement of iron-dependent contrast with increasing field strength, microscopic magnetic resonance imaging of the mouse lemur appears to be an ideal model system for studying in vivo iron changes in the basal forebrain in relation to aging and neurodegeneration.