Neuropsychology Option Week 5, 2006 How and why is the human brain different? The human species is psychologically and behaviourally unique. At the level

Neuropsychology Option Week 5, 2006

How and why is the human brain different?

• The human species is psychologically and behaviourally unique.

• At the level of behavioural ecology, humans are a special species simply by not being confined to a single ecological niche (Bingham, 1999)

• Beyond that there is a widespread consensus that the human species has unusual psychological capacities, even though there are wide differences in how these should be described or defined.


• Assuming unique psychological capacities of some kind, the neuropsychological question is then how these depend on special characteristics of the human brain (see e.g. Deacon, 1997a&b).

• Broadly speaking, the main answer to this question is “we don’t know”

• but there is a range of attempted answers which are worth exploring.

• The why? and how? questions are in fact difficult to separate, but the standard answer to why? would be because human evolution has made the human brain different (Bradshaw, 1997; Moll et al., 2005).


This standard evolutionary answer does not give us very much to go on, but “evolutionary psychology” of various kinds is becoming more popular, and there is a fair degree of consensus that present and future genetics will be able to add a great deal of detail to our knowledge of human brain evolution.

(See, e.g. Ramus, 2006 and Sikela, 2006 on page 7 of the handout)


Millions bottom


Millions top

“Lucy”


Fossilized footprints, discovered by Mary Leakey in Laetoli, Tanzania.

They are dated at 3.5 Mbp, and only the “Lucy” species is known from that time, but the imprints look very like modern human imprints.

Mary leakey footprints


Family Tree


Alemseged, Z., et al. (2006). A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, 443(7109), 296-301


Alemseged, Z., et al. (2006). A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, 443(7109), 296-301

Dikika is only 4km from where ‘Lucy’ was found (Australopithecus afarensis )

The Dikika specimen, from 3.3m yrs ago was about 3yrs old and probably female.

The legs were human-like for bi-pedal walking, but the arms and hands ape-like. The hyoid bone (for the larynx) was also ape-like


Napier (1980)

[not on handout

UCL Library]


Napier, ape hands


Napier – Screwtop


Brain Size

• This gives one simple answer to the how? question: the human brain is outstandingly big, given our body size.

• However it is clearly not the whole answer, since relating brain-size to body weight is complicated (Deacon, 1997b) and the human gain in behavioural terms is more extensive than would be predicted on size alone.

• Genes which may be responsible for primate and human brain expansion are currently being investigated (Ponting & Jackson, 2005; Evans et al., 2006; Pollard et al, 2006; Sikela, 2006; Tang, 2006)


Brain Size

• To what extent are human capacities what we would expect from a 1,500 primate, or 1,500 mammalian (cf dolphins, elephants) brain?

• Or are there special ingredients or “magic bullets” (Elston et al., 2006; Allman et al., 2005) which produce uniquely human capacities?

• Or are both the above correct?


Number of neurons in the nervous system

1,000,000,000,000350,000,000,000100,000,000,000

500,000,000300,000,000

50,000,000850,000250,000

20,000381302

• Homo sapiens (maybe 1014)• Chimpanzee• Rhesus monkey• Mouse• Octopus• Stickleback• Honey bee• Fruitfly• Sea slug• Thread worm male• Thread worm




In Striedter, G. F. (2005). Principles of brain evolution. and

Striedter, G. F. (2006). Precis of Principles of Brain Evolution. Behavioral and Brain Sciences, 29(1), 1-+. [not on paper handout, see intranet]

Striedter brain szie





Ponting and Jackson, 2005• …….recent advances from the cloning of two human genes

promise to make inroads in the area .. of brain size evolution.

• Microcephalin (MCPH1) and Abnormal spindle-like microcephaly associated (ASPM) are genes mutated in primary microcephaly, in which, the brain is of a size comparable with that of early hominids.

• It has been proposed that these genes evolved adaptively with increasing primate brain size. Subsequent studies have lent weight to this hypothesis by showing that both genes have undergone positive selection during great ape evolution.

• the evolutionary patterns of all four presently known primary microcephaly genes are consistent with the hypothesis that genes regulating brain size during development might also play a role in brain evolution in primates and especially humans (Evans, 2006; Tang 2006)


Lateralization

• One additional factor is lateralization. • Although there are some who suspect that

human brain lateralization is a development of primate asymmetries (Corballis, 2003), the data on population handedness suggests a very sharp distinction between the degree of handedness observed in other primate populations

• (which is zero according to the meta-analysis by Papademetriou et al., 2005)


Lateralization figureLateralization - old


• the human figure is 90% right handedness• which does not correlate completely with

language lateralization • but they may be related• And the link between physical lateralization

and human specializations may be strengthened

• By changing techniques (Buchel et al., 2004; Hutsler, 2003; Sun & Watson, 2006; Sun et al., 2006; Luders et al., 2006)

• and theoretical models (Monaghan & Shillcock, 2004)

Lateralization - new


They found that the left arcuate fasculus was larger in right handers


Luders, E., et al. (2006). Hemispheric asymmetries in cortical thickness. Cerebral Cortex, 16(8), 1232-1238.

• Used MRI on 60 healthy adults. • cortex in the left hemisphere was

generally thicker. • the precentral gyrus, middle frontal, anterior

temporal and superior parietal lobes were significantly thicker on the left

• but the inferior posterior temporal lobe and inferior frontal lobe were thicker on the right.

• Asymmetry profiles were similar in both sexes


• they compared gene expression levels in the perisylvian regions of human left-right cortex at fetal weeks 12, 14, and 19

• identified dozens of genes• “identified a subset of genes with human

asymmetry humans and altered expression levels between chimps and humans.”

• “Our results identify candidate genes involved in the evolution of human cerebral cortical asymmetry.”

Sun, T., et al. (2006). Genomic and evolutionary analyses of asymmetrically expressed genes in human fetal left and right cerebral cortex. Cerebral Cortex, 16, I18-I25.


Sun, T., & Walsh, C. A. (2006). Molecular approaches to brain asymmetry and handedness. Nature Reviews Neuroscience, 7(8), 655-662


Sun, T., & Walsh, C. A. (2006). Molecular approaches to brain asymmetry and handedness.


Hutsler, 2003

7 autopsies: 50-97 yrs of age


Monaghan, P., & Shillcock, R. (2004). Hemispheric asymmetries in cognitive modeling: Connectionist modeling of unilateral visual neglect. Psychological Review, 111(2), 283-308

They claimed that a hemispheric distinction between coarse- coding in the RH and fine- coding in the LH exists at the neuronal level as different sized receptive fields.

Simulations with connectionist models with these properties were successful in modeling various tests of unilateral visual neglect


nottebohm



Brain re-organization: expansion of the frontal lobes

• Apart from lateralization, other kinds of re-organization with-in the brain might have been possible within the time-scale of human evolution.

• The most popular hypothesis for many decades has been that the human frontal lobes, presumed to be the main site for planning and self-control, have either generally expanded or undergone some more detailed change (Deacon, 1997a&b; Schoenemann et al., 2005).


• However, careful MRI scanning of different great ape species and comparison with human scans has led to the conclusion that there has been no disproportionate expansion of the human frontal lobes


semendeferi1


The Semendeferi et al., (2002) table

Semendeferi table


• Schoenemann et al.(2005) recently suggested that prefrontal white matter is disproportionately larger in humans than in other primates

• but Sherwood et al. (2005) countered that a) the boundary between prefrontal and other cortex is not well defined; and b) that in any case, although the data showed humans having more white matter than the average primate, they did not show a difference between humans and great apes.

Brain re-organization: expansion of the frontal lobes

The sherwood 2005


Preuss (2004) suggests that what has happened is that, while the primary motor and sensory areas in the human brain are roughly the same size as those in apes,

Brain re-organization: expansion of the frontal lobes?

secondary areas (“association cortex”) has greatly expanded in all the lobes of the human brain.


Others have suggested that certain neuronal features of primate and human frontal lobes hold the key to understanding human intelligence:

pyramidal cells in prefrontal cortex (Elston et al., 2006)

or certain spindle cells in anterior cingulate and fronto-insular cortex (Allman et al. 2005; see back of handout, and p. 7 of handout for abstracts).


Fig. 4. Neurolucida tracings of pyramidal (left) and von Economo (right) neurons from Fronto-Insular (a) and Anterior Cingulate Cortex (b). Notice the vertical symmetry and relative sparseness of the VEN dendritic tree. [back of handout; Allman et al., 2005]


[Back of handout: Allman et al., 2005]


Allman, J. M., Watson, K. K., Tetreault, N. A., & Hakeem, A. Y. (2005). Intuition and autism: a possible role for Von Economo neurons. Trends in Cognitive Sciences, 9(8), 367-373. [p 7 of handout]

Von Economo neurons (VENs) are a recently evolved cell type which may be involved in the fast intuitive assessment of complex situations. As such, they could be part of the circuitry supporting human social networks.

We propose that the VENs relay an output of fronto-insular and anterior cingulate cortex to the parts of frontal and temporal cortex associated with theory-of-mind, where fast intuitions are melded with slower, deliberative judgments.

The VENs emerge mainly after birth and increase in number until age 4 yrs. We propose that in autism spectrum disorders the VENs fail to develop normally, and that this failure might be partially responsible for the associated social disabilities that result from faulty intuition.


Elston, G. N., et al. (2006). Specializations of the granular prefrontal cortex of primates: Implications for cognitive processing. Anatomical Record Part a-Discoveries in Molecular Cellular and Evolutionary Biology, 288A(1), 26-35. [Abstract on p. 7 of handout]

….we demonstrate that the basic neuronal building block of the cerebral cortex, the pyramidal cell, is characterized by marked differences in structure among primate species…….

…..pyramidal cells in the granular prefrontal cortex of humans had a disproportionately high number of spines…

The biological underpinnings of human intelligence remain enigmatic.

…. the highly branched, spinous neurons in the human granular prefrontal cortex (gPFC) may be a key component of human intelligence.



Kaas, J. H. (2005). From mice to men: the evolution of the large, complex human brain. Journal of Biosciences, 30(2), 155-165. [not on handout]


Elston et al 2006, abstract on p. 7 of handout. Note the absence of comparisons either with apes or with association cortex in other lobes.


Granular prefrontal cortex is represented by stipple. bar 2 cm for human and 1 cm for other species.(another paper from the Elston Lab)

However, since even the galago, a prosimian, has got some prefrontal cortex, it is another case where it looks as though the human brain is what would be expected in a typical primate brain expanded to 1500 ccs


The galago or bushbaby, a noctural cat-sized tree dweller, has brain organisation which is recognizable in terms of the larger rhesus monkey version.


An elephant brain = 3,886.7 cc

Hakeem, et al. (2005)

C is a bottle nose dolphin

Above is the elephant hippocampus, in pink


Plotnik, J. M., et al. (2006). Self-recognition in an Asian elephant. PNAS, online, October 30, 2006.

Movie1, movie2 movie3


Plotnik, J. M., et al. (2006). Self-recognition in an Asian elephant. PNAS, online, October 30, 2006.

Movie1, movie2 movie3

‘we report a successful MSR (mirror self-recognition) elephant study and report striking parallels in the progression of responses to mirrors among apes, dolphins, and elephants. These parallels suggest convergent cognitive evolution most likely related to complex sociality and cooperation


Marino, L., Sherwood, C. C., et al. (2004). Neuroanatomy of the killer whale (Orcinus orca) from magnetic resonance images. Anatomical Record Part a-Discoveries in

Molecular Cellular and Evolutionary Biology, 281A(2), 1256-1263. (not on handout)

A brain larger than ours, killer whale brain =~5,000 cc


• There still remains the difficulty that superficially the human brain is more similar to that of a chimpanzee than we might expect by comparing the behaviours of the two species. Thomas Huxley, a champion of Darwin’s put it this way in a supplement to Darwin’s book The Descent of Man —


End of lecture 1

• Start of lecture 2

Documents

Neuropsychology Option Week 5, 2006 How and why is the human brain different? The human species is psychologically and behaviourally unique. At the level