Brain evolution and PFC fractionation Flashcards
(41 cards)
Brain volume
Steadily increased relative to body weight in homo-lineage by a factor of approx. 2 from 2 million years ago.
(Lewin & Foley, 2004).
Human brain no bigger than it should be
Reached current size approx. 100,000 years ago.
Symbolic developments occurred 50k to 40k years ago.
Brain size is not everything.
- among humans, only small proportion of intelligence differences attributed to size. (Rushton & Ankey, 2009).
Paleoneuroanatomical evidence
Hominin cranial fossils preserve evidence of:
- overall brain size.
- cerebral asymmetry.
- cortical sulcul patterns - leaves impressions on the endocra surface.
Fossils suggest 3 major stages of hominin brain evolution
Stage 1 (3.5-2mya):
- brain reorganisation without substantial expansion - includes relative expansion of posterior parietal association cortex at the expense of the occipital cortex.
- may have been important for emergence of stone tool making by 2.6mya (Semaw et al, 2003).
- some suggest stage 1 involved in prefrontal lobe shape (Falk et al 2000).
Stage 2 (2-0.5mya):
- sudden increase in brain size associated with appearance of homohabilis.
- followed by gradual expansion related to body size increases in homo-erectus.
- first appearance of modern human like cerebral asymmetries in hobo habilis including enlargement of the Broca’s cap region in left LPFC (BA44).
Stage 3 (0.5-0.02mya): - past 15k years decreasing body size brought human mean brain size down a bit.
Overall (excluding Broca’s area cap enlarging) evidence of frontal lobe size and reorganisation is limited.
- fractionated function.
Evidence from comparative neuroanatomy: relative size of the frontal cortex?
Differences are large in size of FC between humans and primates.
- but FC in humans and great apes occupies a similar proportion of the cortex of the cerebral hemispheres (Semendeferi, Schenker & Damasio, 2002).
Evidence from comparative neuroanatomy: frontal vs. prefrontal
Not relative size of the frontal cortex but of the PFC - evidence of PFC reorganisation.
Prefrontal area argued to be fractionated (a substance is divided during phase transition into smaller quantities). in 1 of 3 ways:
- presence of granular layer 4 (Stellate and other smaller cells).
- projection area of the mediodorsal nucleus of the thalamus.
- motorically “silent” area when stimulated.
Evidence from comparative neuroanatomy: larger in humans
Human PFC especially enlarged compared to great apes - Passingham & Smaers (2014).
BA10 (Polar PFC) is larger in humans relative to the rest of the brain than in apes - Semendeferi et al (2001).
Cytoarchitectonics & granular cells of the PFC: granule cells
= interneuron.
- defined by its smallness.
Benefits of size:
- density and number of connections.
In the PFC: only primates have granular layer 4 in PFC.
- thickness of layer 4 increases as one foes from caudal to rostral along the medial and orbital surfaces of the frontal lobe.
- area can be agranular enough to warrant exclusion from special status even if some granule cells present.
Cytoarchitectonics & granular cells of the PFC: cerebral cortex layers
Cerebral cortex = outer layer of cerebrum/cortex.
- largest and most prominent part of the brain.
- cerebral cortex has 4 lobes.
Layers:
- molecular layer.
- external granular layer.
- external pyramidal layer.
- internal granular layer.
- internal pyramidal layer.
- multiform layer.
Cytoarchitectonics & granular cells of the PFC: phases of evolution of the PFC
Early mammals develop agranular areas of PFC: medial and orbital PFC/insular cortex.
- primates alone have granular cortex.
- rats only have agranular PFC.
Lateral and polar granular PFC last to appear during anthropoid evolution.
Granular PFC appeared in early primates as they adapted to life confined by trees - caudal PFC and areas of OPFC:
- function in the assessment of value or primary reward (e.g. food - Passingham & Wise, 2012).
Several new granular PFC areas appeared during anthropoid evolution - grew larger, foraged more (by reducing choices that increased risk of predation or wasted effort), became dependent on food and vulnerable to falls:
- dorsal PFC (BA9/46).
- ventral PFC (BA45/47).
- polar PFC (BA10).
Lateral and polar granular PFC - last to appear during anthropoid evolution.
Brain became wider and more rounded at the front during homonoid evolution.
Cytoarchitectonics & granular cells of the PFC: what did granular PFC add?
Evolved to implement new, faster, general-purpose mechanism - in response to adaptive pressures.
- supports older, reinforcement-learning mechanism.
Granular PFC generates goals appropriate to goals and needs.
Cytoarchitectonics & granular cells of the PFC: dorsal paracingulate cortex connects to dorsal PFC
Modern humans probably evolved after Broca’s expansion seen in homo habilis.
- when frontal lobe PFC reached modern state - before advanced tool use, abstract thought, language etc (approx. 70-40kya).
Elston et al (2006): granular cortex is 80% in humans and 55% in chimps.
Elston et al (2001): pyramidal cells in layer 3 = 70% more spinous in humans then monkeys.
Schenker et al (2005): human PFC has larger volume of short nerve fibre connections connecting parts of PFC.
Areas of human specialisation (at cell level of PFC):
- broca’s area (BA 44/45).
- lateral part of the polar PFC (BA 10).
- dorsal anterior cingulate/medial PFC (BA 32).
> BA 10 and BA 32 lack homologues in monkeys.
Cytoarchitectonics & granular cells of the PFC: connections
PFC = brains controller.
- has connections.
Synaptogenesis last longer in PFC =than other regions (Bianchi et al, 2013; Levitt, 2003).
Cytoarchitectonics & granular cells of the PFC: brain development, mirrors brain evolution
Cortical expansion during evolution matches expansion during development (Hill et al, 2010).
Ultimately, newer PFC regions (lateral and polar) are better learning devices (Passingham & Wise, 2012).
Leaves us more open to cultural influences than other species whose brain stops developing earlier and are more influenced by genes.
Cytoarchitectonics & granular cells of the PFC: abstraction increases with granularity
Processing hierarchy in brain (Badre, 2008):
- posterior
> anterior
> caudal
> rostral.
- processing moves from concrete to abstract.
Granular PFC at the apex of processing hierarchy (Passingham & Wise):
- allowing integration of all info necessary to generate goals from current context and events based on knowledge of current value.
Cytoarchitectonics & granular cells of the PFC: PFC summary
Brain got wider and more round at front.
- lateral and polar regions (populated by granular cells) - during hominoid evolution.
Regions of PFC enlarged compared to other apes:
- particularly BA10.
Evidence of frontal lobe changes during hominin evolution:
- particularly in Broca’s cap in lateral PFC which would have widened brain further.
Orbitiomedial regions of the PFC
Independent variation in size of FC areas across hominoids (Bonobos, chimps, orangutans and humans):
- dorsal (Schenker, Sedgouttes & Semendeferi, 2005).
- polar (Semendeferi, Armstrong, Schlicher, Siller & Van Hoesen, 2001).
- orbital (Semendeferi, Armstrong, Schlicher, Siller & Van Hoesen, 1998).
Variation correlates with behavioural differences between species:
- suggests degree of evolutionary independence between LPFC and VMPFC (Stout, 2010).
Orbitiomedial regions of the PFC: orang-utans
Unusually small orbital frontal cortex.
Solitary and simple organisation (Schenker et al, 2005; Semendeferi et al, 1998).
Orbitiomedial regions of the PFC: Phineas Gage
Impaled through frontal lobe.
- hardworking, energetic, clear thinking -> impatient, rude, angry etc.
Lateral PFC untouched.
Medial PFC wall destroyed and some of BA10 - suggests involvement in controlling anger and organisation.
Orbitiomedial regions of the PFC: orbital and VMPFC - reward
Shown human orbital PFC conveys info about expected rewards (London et al, 2000; Rolls et al, 2008).
Damage to orbital PFC - disturbance in learning and decision making tasks involving reward evaluation (Rolls, 2000; Bechara et al, 1994).
IOWA/Bechara gambling tasks:
- select from difference decks to receive rewards or penalties.
- some decks give good rewards but heavy penalties vs. moderate rewards for small penalties.
- orbitofrontal patients don’t avoid selecting heavy penalty piles.
Orbitiomedial regions of the PFC: early damage to prefrontal cortex
Impairment of social and moral behaviour related: Anderson et al (1999):
- normal upbringing, no family history of psychiatric disease, socially well adapted siblings.
- normal neurological profiles in both patients except for behavioural defects.
Orbitiomedial regions of the PFC: reward and OFC
VMPFC seems important in reward/punishment processing.
Warm, pleasant feelings associated with activity in brain - Rolls, Grabenhorst & Parris (2008).
Orbitiomedial regions of the PFC: financial rewards/losses
More lip up representations when receiving rewards but still some activity for punishment in human OFC. (O’Doherty, Kringelbach, Rolls, Hornak & Andrews, 2001).
Orbitiomedial regions of the PFC: OM regions summary
Involved in socialising, control of emotions, reward processing:
- eg. warm pleasant feelings.
- Phineas Gage.
Involved in simple decision making prevents risky and silly behaviour:
- eg. gambling tasks.
Little evidence for relative expansion at any stage during hominin evolution.
