Behavioral Neuroanatomy – Mesulam

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Introduction

Structural foundations of cog and beh domains take the form of partially overlapping large-scale networks organized around reciprocally interconnected cortical epicenters

  • Spatial Attention Network (Rt hemisphere)
  • Lang Network (Lt hemisphere)
  • Memory-Emotion Network (Limbic)
  • Executive Function-Comportment Network (Prefrontal)
  • Face-and-Object Identification Network (Ventral occipitotemporal)

 

Parts of the Cerebral Cortex

  • Human cortex contains approximately 20 billion neurons
  • Difficult to map
    • Brodmann’s map – microscopically identified variations
    • Today, folks use Brodmann’s in topographic way – problematic
    • Probably more accurate to say, for e.g., “middle temp gyrus”. Can be identified topographically and doesn’t need to be verified microscopically

 

5 major fxal subdomains of cerebral cortex

  1. Limbic
  • Corticoid (cortex-like) – simplified cytoarchitecture; certain basal forebrain structures
  • Allocortex – 2 formations: hippocampal complex and piriform cortex
    • Extensively interconnected with the hypothalamus
    • important in regulating the internal milieu and preserving self/species
    • specifically: mem, emtn, motiv, hormonal balance, and autonomic balance
  1. Paralimbic (Mesocortex)
  • Between allocortex and isocortex
  • Includes 5 formations: orbitofrontal, insula, temp pole, parahippocampal, cingulate
  • Divided into 2 grps: 1) Olfactocentric and 2) Hippocampocentric
  • Plays a critical role in channeling emotion to behaviorally relevant motor acts, mental content, and extrapersonal events
  1. Heteromodal Ass’n (Isocortex) – 6 layer homotypical architecture; High-order ass’n cortex
  • Includes: Prefrontal, post parietal, lateral temporal, and portions of parahippocampal
  • Most closely involved in perceptual elaboration and motor planning
  • 3 essential characteristics
    • neuronal responses not confined to single sens modality
    • sens inputs come from unimodal areas in mult modalities
    • lesions multimodal; never confined to tasks under control sing modality
  1. Unimodal Ass’n (Isocortex) – 6 layer homotypical architecture
  • Most closely involved in perceptual elaboration and motor planning
  • Upstream: only one synapse away from primary sensory area
    • Visual: BA18-19
    • Auditory: ?Sup temp gyrus (BA22); maybe BA21
    • Somatosens: unclear up/dwnstrm: sup parietal lobule(BA5, BA7); ? inf par
  • Downstream: 2 or more synapses from primary area
    • Visual: Fusiform, inf temp, middle temp
    • Auditory: ?anterior part of superior temp cortex (BA22)
  • 3 essential characteristics of unimodal
    • respond to stim in only single sensory modality
    • sens info comes from primary sens cortex
    • lesions yield deficits only in tasks guided by that modality
  • Primary Sensory-Motor (Idiotypic cortex)
    • Visual: Covers banks of calcarine fissure (BA17)
    • Auditory: Covers Heschl’s gyrus (BA41-42)
    • Somatosensory: Postcentral gyrus (BA3a, 3b, 1, 2)
    • Motor: Precentral gyrus (BA4 and probably BA6)
    • Vestibular: Posterior sylvian fissure (where temp lobe joins insula and parietal lobe)
    • Gustatory: BA43
    • Olfactory: At confluence of insular, orbitofrontal, and temporopolar areas
      • Vestibular, gustatory, and olfactory sensations – not same prominence as others in primate

 

Cortical Organization, Connectivity, and Transmodal Areas

  • 5 areas above have extramural (w/ other fxal zones) and intramural connections (w/ same zone)
  • Essential characteristic of primate brains – obligatory synaptic relays between stim and response
    • Allows for integrative processing (psych outcomes: cognition, consciousness, comportment)
    • This processing has two roles:
      • keeps motivationally-driven internal milieu from dominating
      • allows identical stimuli to trigger diff responses depending on context, experience, needs, consequences

 

Functions of Individual Cortical Zones: Primary Sensory/Motor Areas

  • Primary Visual “Striate” Cortex (BA17) – covers occipital pole and banks of calcarine fissure
    • 70% of retinal input is relayed to striate thru LGN
    • Entire visual field is mapped onto striate cortex with great spatial precision
    • Contralateral representation
    • Lesion of geniculostriate pathway – characteristic visual field deficits
  • Primary Auditory Cortex (BA41, 42) – located on Heschl’s gyrus
    • Inputs from MGN
    • Tonotopic organization in A1 so that low freq are represented more anteriorly
    • Does NOT display strict contralateral representation that visual and somatosensory display
    • MGN has projections both to A1 and aud ass’n areas; thus, complete cort deafness unlikely
    • Lesion to A1 (unilateral) – difficult to detect clinically
  • Primary Somatosensory – postcentral gyrus
    • Input primarily from ventroposterior lateral thalamic nucleus
    • Contralateral half of body surface is somatotopically mapped onto S1 in each hemisphere
    • Lesion to S1 – selective impairment in “cortical sensations” (e.g., 2-pt discrimination, touch localization, graphethesia, position sense, and stereognosis…..touch, pain, temp intact)
  • Primary Motor Cortex – precentral gyrus; closely parallels S1
    • Dominated by large pyramidal neurons
    • Lesions to M1 – poorly understood; may impair distal movements leave muscle tone and strength of proximal muscles intact??
    • Like S1, hand and foot in M1 have no callosal connectivity – rlted to handedness

 

Functions of Modality-Specific (Unimodal) Sensory Association Areas

  • Info processing enters first ‘associative’ area within modality specific (unimodal) ass’n area
  • Lesions give rise to 2 beh deficits:
    • Selective perceptual deficits that leave other fxs of that modality intact (e.g., achromatopsia)
    • Modality-specific agnosias (e.g., prosopagnosia, pure word deafness)

Visual Unimodal Ass’n – peristriate (BA18-19), parts of fusiform, inf temp, mid temp (BA37, 20, 21)

  • Each node continuously passing on info to others; connections are reciprocal; display relative rather than absolute specializations
  • Color (V4, maybe V8)
    • Posterior parts of lingual and fusiform sensitive to color
    • Lesions (unilateral) – contralateral loss of color perception (hemi-achromatopsia); if disrupt connections to lang cortex “color anomia”
  • Movement (V5, MST)
    • Middle temporal gyrus
    • Lesions (bilateral) – akinetopsia (can’t perceive visual motion)
  • Form and Complex Patterns (parts of fusiform, lingual, inf occipital gyri)
    • Elementary sens features above used by areas along ventral path for discrimination of form/complex patterns
  • Ventral Pathway: Faces, Objects, and Words
    • Fourth synaptic level – promote rapid identification of faces, objects, words
    • Face/object: Midportion of fusiform
      • Lesions – prosopagnosia, associative visual object agnosia
    • Word-form: fusiform, perhaps lateral occipitotemporal region; probably mediate a sort of processing where words are handled more like objects than symbols
      • Lesions – pure alexia
    • Dorsal Pathway: Spatial Orientation; Dorsal Occipitoparietal region (junction of BA19 and BA7)
      • Fourth synaptic level – encodes info in form of spatial vectors
      • Lesions: visuospatial disorientation syndromes (visual neglect; dressing apraxia; simultanagnosia; optic ataxia-deficit in reaching toward target; optic apraxia-oculomotor exploration deficits)
        • Balint’s Syndrome – optic ataxia, optic apraxia, simultanagnosia

Auditory Unimodal Association Areas (A1)

  • May also have ventral and dorsal organization
  • A1-pure tones and pitch; mid to anterior parts of sup temp gyrus-phonetic parameters
  • Lesions of unimodal aud ass’n cortex-auditory perceptual impairments (cortical deafness; pure word deafness; auditory agnosia for sounds; phonoagnosia-inability to recognize familiar voices)

Somatosensory Association Areas and Secondary Somatosensory cortex (BA1,2)

  • S2 area – participates in pain perception; lesions-loss of pain w/out loss of other somatosens
  • Somatosensory ass’n (BA5, 7 and ?anterior BA40&posterior insula)
    • Essential role in touch localization, manual exploration, coordination of reaching/grasping, and encoding of somatosensory memories
  • Lesions
    • Between SS cortex and parietal heteromodal-somatosensory integration deficits ass’d w/ dressing apraxia, neglect, and other aspects of spatial disorientation
    • Between SS and temoroparietal SS object recognition deficit-tactile agnosia
    • Between SS and lang network-pure agraphesthesia (analog of pure word deafness/pure alexia)
    • Between SS and premotor-modality specific tactile apraxia

 

Motor Association Areas

(Premotor-BA6, Supp Motor-mostlyBA6, Frontal Eye Fields-post parts of Broca’s and parts of BA8)

  • Motor ass’n areas anterior to M1 source of almost all cortical projections to M1
    • Stim will produce movements but higher threshold than M1-movement patterns much more intricate (e.g., bilateral)
    • Lesions: Reflect a disconnection between cognition and action; not impairment in strength or mobility; complex deficits of movement in absence of weakness, dystonia, dysmetria, or hyperreeflexia
  • Premotor
    • Receive input from # of unimodal/heteromodal areas so have access to info in all sens modalities
    • Respond to sens stimuli but usually according to movement that would follow
    • Intricate connections between post parietal and premotor areas
    • Lesions in premotor part Broca’s – dysarthria for speech but not singing
    • Lesions between BA6 and post lang network-ideomotor apraxia (inability to pantomime use of object upon command)
  • Supplementary
    • Role in coordinating multistep movement strategies ?maybe also in encoding procedural mem
    • Along w/ premotor-imp roles in motor planning and response selection; also initiation of motor; selection of motor responses
    • Lesions – may interfere with motor initiation but not other phases of movement
  • Lesion between pre/supp motor and Brocas->Transcortical motor aphasia; aphemia(nonaphasic, nondysarthric impairment of fluency)
  • Frontal Eye Fields
    • Lesions-impaired exploratory eye movements even when spot eye movements intact
  • Broca’s (premotor in BA44 and adjacent heteromodal cortex)
    • Critical role in translating neural word forms into articulatory sequence; in seq words/endings into utterances that have a meaning-appropriate syntactic structure

 

Temporal Heteromodal Cortex and Agnosias

(Recognition of Faces, Objects, Voices)

  • Heteromodal cortices in mid temp gyrus may link visual representation of faces with other assns (eg, name, voice)
    • Associative prosopagnosia – bilateral lesions in mid-to-ant parts of lingual and fusiform
      • When info from nonvisual modality is available, can recognize
    • Apperceptive prosopagnosia – deficit in spat integration of vis percept
      • Unable to determine if 2 faces alike
    • Pts with prosopagnosia can recog and name object classes (this is a face, car, etc), but not particular faces
    • Other associative agnosias – arise when unimodal areas specialized for perceptual encoding of objects damaged or when they fail to access the transmodal gateways allowing for integration
      • Associative Visual Object Agnosia – extends to level of categorical recognition; may represent lesion more upstream
      • Auditory agnosia – may reflect a disconnection of unimodal auditory areas from transmodal
        • eg, don’t associate ringing of telephone or ambulance siren
      • Phonoagnosia – (aud analog of prosopagnosia)-inability to recognize identity of familiar voices
      • Tactile agnosia – inability to recognize objects by palpation (Associative deficit)
    • Stereognosis – apperceptive deficit

 

Wernicke’s Areas as a Temporoparietal Transmodal Gateway for Language

  • Broca’s -BA44 and adjacent heteromodal prefrontal cortices
    • Synaptic/Articulatory pole
  • Wernicke’s-no accepted boundary (Post 1/3 of BA22, adjacent pars BA39-40, and ?mid temp gyrus)
    • Lexical/Semantic pole
    • Transmodal gateway coordinating reciprocal interactions btwn sensory reps of word forms and symbolic assns – give meaning to words
  • Aspects of lang network
    • Word forms – encoded within unimodal auditory and visual areas
    • Lexical labeling – component of object recognition (name is attribute like color, location, etc)
    • Word comp – object recognition task where perceptual features first lead to word is a word
    • Identification of individual word
    • Establishment of assns – define meaning using transmodal nodes in Wernicke’s
  • Lesions: Verbal associative agnosias
    • Pure alexia(word blindness) – disconnect between areas encode vis word form and vis input
      • Can arise when lesion of V1 in L and Splenium (region of CC conveys vis info across)
    • Pure word deafness – unimodal auditory cortex cut off frm aud input or can’t access transmodal
    • Pure agraphethesia-disconnect somatosensory ass’n from wernicke’s/lang network
    • Maybe from post parietal lesion

 

Functions and Syndromes of Posterior Parietal Heteromodal Cortex

Posterior Parietal Heteromodal area (BA37, 39, 40) – interactions related to praxis, language, visuomotor integration, generation of motor plans, and spatial attention

Lesions

  • Inf parietal lobule – Ideomotor apraxia – can’t use or understand pantomime of using object
  • Angular gyrus of Lang-dominant – anomia, alexia, acalculia, dysgraphia, finger identification, left-rt naming difficulties (last four – Gerstmann syndrome)
  • Heteromodal inferior par lobule in rt hemisphere – deficits in spat attn, visuospat integration, and drawing (Rt parietal syndrome) also, anosognosia, dressing apraxia, confusional states, route finding deficits, and disturbances in navigating body with respect to objects
  • Parietotemporal heteromodal – disturbances in mood and motivation

 

Prefrontal Heteromodal Cortex and Frontal Lobe Syndromes

  • Frontal lobes – represent 1/3 of cerebral hemispheres
  • Three functional sectors
    • Motor-Premotor – BA4, BA6, supp motor area, frontal eye fields (BA6), parts of Broca’s
      • Lesions: weakness, alteration of muscle tone, release of grasp reflexes, incontinence, akinesia, mutism, aprosody, apraxia, and some motor components of neglect and Broca’s aphasia
    • Paralimbic – ventral and medial part of frontal-part of ant cingulate (BA23, 32), parolfactory gyrus (gyrus rectus, BA25), and post orbitofrontal regions (BA11-13)
    • Heteromodal – BA9-10, ant. BA11-12, and BA45-47
  • Prefrontal cortex (generally refer to paralimbic and heteromodal areas)
    • Two fx axes:
      • Working memory-executive fx-attn (transmodal centers-prefrontal and post parietal)
      • Comportment (transmodal centers in prefrontal and orbitofrontal paralimbic cortex
    • Two general types of frontal lobe syndromes
      • Syndrome of frontal abulia – loss of creativity, initiative, and curiosity; apathy, emot blunting
        • Lesions to heteromodal cortex (ie, dorsolateral frontal area)
      • Syndrome of frontal disinhibition – loss of judgment, insight, and foresight
        • Lesions to paralimbic cortex (ie, orbitofrontal and medial frontal)
      • Neuropsychology of Frontal Lobe Disease
        • Attention
          • P300 response to novel stim critically dependent on prefrontal cortex
          • Frontal eye fields – part of network for exploring extrapers space and seeking motivationally relevant targets
        • Working Memory (volitional manipulation and on-line holding of info)
          • Unimodal ass’n cortex participate in working mem of own area of specialization
          • Lateral prefrontal cortex – supramodal role in orchestrating working mem in all domains
            • (like role of temp transmodal cortex in object recog; Wernicke’s in lang)
          • Two groups of processes: volitional manipulation and on-line holding
            • Volitional manipulation: Central Executive; prefrontal dorsolateral cortex
            • On-line maintenance: both prefrontal and post parietal cortex
          • Lesions to prefrontal or post par can disrupt working memory
          • Prefrontal- orient att’n focus toward internal mental processes; lesions – tilt emphasis away from internal mental processes toward stim-bound behavior
          • Posterior Parietal – orients toward extrapersonal space; lesions – tilt emphasis away from external sensory events and promote sens neglect
        • Metaphysiology of Prefrontal cortex
          • Even massive damage to prefrontal leaves all sens, perception, movement, and homeostasis fx intact
          • Prefrontal cortex has many interconnections with almost all other heteromodal, unimodal, paralimbic, and limbic areas; so, can activate, suppress, orchestrate networks of fxing
          • Important role in inhibiting impulses not appropriate to context
          • Neurons of prefrontal help to establish subjective reality; sensitive to behavioral relevance of stim – not surface properties
          • Cuz of working mem can simultaneously maintain mult external and internal phenomena
          • Orbitofrontal and other paralimbic components – transmodal nodes for binding thoughts, memories, and experiences with visceral and emot states
        • Frontal lobe versus Frontal network syndromes; tricky saying frontal lobe cuz of intricate connections; probably more accurate to refer to “Frontal network syndrome”
          • Manifestations of frontal lobe syndrome could result from:
            • Lesions in the head of the caudate or in mediodorsal thalamus
            • Multifocal white matter diseases
            • Metabolic encephalopathy
            • Multifocal partial lesions

 

Paralimbic (Mesocortical) Areas

  • Olfactocentric formations – temporal pole, insula, and post orbitofronal cortex
  • Hippocampocentric formations – parahippocampal “rhinal” cortices, retrosplenial area, cing gyrus, and subcallosal (paraolfactory) regions
  • Link cognition with visceral states and emotion; emphasize beh relevance over physical aspects
  • Critical to: 1)mem/learning; 2)channeling of emotion; 3)linkage of visceral state, immune responses, and endocrine balance to mental state; 4)perception *of pain, smell, and taste
  • Insula – abuts upon frontal and parietal opercula dorsally and supratemporal plane ventrally
    • Contains gustatory cortex, piriform olfactory cortex, and aud/vestibular areas
    • Also imp in mediating tactile learning and reaction to pain; may help link Wernicke/Broca’s
    • Lesions: pain asymbolia, tactile learning deficits
  • Orbitofrontal Cortex- designate entire ventral surface of frontal lobes
    • Critical role in integration of visceral and emotional states w/ cog and comportment
    • Posterior – behaviorally more “limbic”
    • Anterior – similar to dorsolateral
  • Temporal Pole – caps anterior tip of temporal lobe; jx with insula thru piriform cortex
    • Medially – olfactory-gustatory-visceral medially
    • Dorsally – auditory fx
    • Ventrally – visual
    • Laterally – multimodal integration
  • Cingulate complex and Medial Frontal Area – part of hippocampocentric grp
    • Includes: retrosplenial region; cing gyrus; and paraolfactory (essentially around CC)

 

Limbic Structures of the Septal Area, Nucleus Basalis, and Piriform Cortex

  • Behavioral specializations of these areas similar to paralimbic but more closely related to memory, drive, emotion
  • Septal Nuclei and the Nucleus Basalis of the Substantia Innominata
    • Basal Forebrain (medial septal nucleus, nuclei of Broca’s diagonal band, nucleus basalis of Meynert)
      • major cholinergic innervation of cortical surface; also contain GABA neurons
    • Piriform Cortex (primary olfactory cortex)
      • Inputs from the olfactory bulb; interconnected with the hypothalamus
      • Olfactory info does NOT have to be relayed thru thalamus
      • Unique importance of olfactory sensation to sexual, territorial, and feeding behaviors

 

The Amygdala, Emotion, and Affiliative Behaviors (Neuro of Value)

  • Extensive connections with the hypothalamus, hippocampus, and other limbic and paralimbic areas
  • Receives olfactory, gustatory and somatosensory, auditory, and visual info
  • Critical role of amygdala – channeling drive and emotion; acts as a transmodal gateway for linking sensory representations of reinforcers with each other and with the mental and autonomic correlates of emot and motiv valence
  • Lesions (hypoemotionality)
    • Hippocampal lesions – interfere with explicit recall of specific events but not with autonomic rxs
    • Amygdala lesions – leave explicit recall intact but abolish associated autonomic responses
    • Bilateral lesions of ant temp lobe, including amygdala – “Kluver-Bucy Syndrome”
      • Breakdown in the channeling of drive to appropriate visual target
        • indiscriminately initiate sexual activity
        • no longer show aggressive-aversive reaction to humans
        • mouth all objects – lose ability to distinguish edible from nonedible
      • Amygdala plays crucial role in modulating neural impact of sensory stimuli on each of 3 factors
        • Hedonistic value – amygdala is activated by aversive (not neutral) olfactory stim and fearful (not neutral) faces
        • Acquired assns – neutral stim don’t activate amygdala initially but do so after conditioned with fear
        • Motivational state – amygdala activated by pictures of food only when hungry
      • Dual role related to attn and memory
        • Attn – selective enhance processing resources allocated to events with emot value
        • Mem – mediate impact of emot valence on memory and also encode emot valence of stimuli
      • Participates in wide range of behaviors related to conspecific affiliative behaviors, social emotions, and their communication

 

The Hippocampus & Binding of Info into Explicit Memory (Neuro of Recollection)

  • Encoding of distance, color, movement, and form displays species-specific invariance
  • However, much of mental content, dependent on arbitrary assns – limbic/paralimbic (esp hippocampus and entorhinal cortex) imp in creating these assns
  • Hippocampo-entorhinal complex
    • Participates in regulation of emotion, but principle beh affiliation is memory and learning
    • Recalling *stable* knowledge – use transmodal areas outside of limbic/paralimbic
    • Recalling *new* info (obviously imp in sustaining knowledge) – use transmodal gateways within the limbic system (primarily hippo-entorhinal complex)
    • Lesions: dissociation btwn explicit learning of new experience and implicit-procedural learning
    • Suggests limbic system plays role in memory and learning by acting as neural gateway for encoding and retrieval NOT site where memories (engrams) are stored
    • Role – orchestrates the coherent storage and reactivation of this distributed info
  • Amnesic states
    • Severe only occur when hippocampo-entorhinal (and diencephalic connections) damaged
  • also see some memory impairments after lesions to the orbitofrontal, cingulate, or retrosplenial cortex
  • Why learning dependent on limbic structures – CNS needs to be protected from learning too rapidly and indiscriminately; initial screening is attnal systems; limbic system second line of defense
    • particularly prone to LTP; one of few areas that continue to display axonal sprouting
  • Implicit vs. Explicit Memory
    • Implicit memory – info remains sequestered, w/in unimodal and heteromodal ass’n areas
    • Explicit memory – info incorporated into a context thru the binding fx of the limbic nodes
  • Role of prefrontal cortex in memory
    • reconstruction of context and temporal order
    • on-line manipulation of encoding and retrieval (working memory)
    • associative search of internal data stores
    • contextual constraints to keep retrieved memory plausible

 

Limbic System

  • Components
    • Hypothalamus
    • Limbic components of cortex (allocortical and corticoid)
    • Paralimbic cortical belt
    • Limbic striatum, pallidum, ventral tegmental area, and the habenula
    • Limbic and paralimbic thalamic nuclei
  • Papez circuit (crucial in memory/learning; connections very, very strong)
    • Hippocampus-mammilary body-anterior thalamic nuclei-cingulate gyrus-presubiculum-entorhinal cortex-hippocampus
  • Components have greater capacity for synaptic plasticity; highly suited to encoding of new info but also highly vulnerable to pathological processes such as kindling and epilepsy
  • Behavioral specializations
    • Binding of distributed info related to recent events – supports memory
    • Channeling of emotion and drive (hunger, libido)
    • Linking of mental activity with autonomic, hormonal, and immunologic states
    • Coordination of affiliative behaviors related to social cohesion
    • Perception of smell, taste, and pain
  • Generally two spheres of influence
    • Amygdaloid – Olfactocentric paralimbic areas (emotion, motivation, affiliative behs, and autonomic-hormonal-immunological fxs)
    • Hippocampal – Papez components (learning and memory fxs)
  • Lesions
    • Limbic lesions: almost always give rise to multimodal impairment
    • Lesions interrupting connections btwn unimodal and limbic system give rise to modality specific disconnection syndromes like asymbolia for pain, visual hypoemotionality, visual amnesia, tactile lrning deficits
  • Limbic system most likely site of dysfx for many psychiatric diseases

 

Basal Ganglia and Cerebellum

  • Basal ganglia: critical role in automatic execution of learned motor plans
  • Cerebellum: regulates rate, range, and force of movement
  • Striatum (caudate, putamen, nucleus accumbens, olfactory tubercle)
    • Neural inputs from sub nigra and cereb cortex; does NOT send many projections back to cortex
    • Output of striatum predominantly to the globus pallidus then….striato-pallido-thalmo-cortico-striatal loop
    • Neostriatum – caudate and putamen
      • May play a critical role in acquisition and retention of procedural knowledge
      • Motor deficits – more linked with putamen
      • Cognitive deficits – more linked with caudate
    • Limbic striatum – nucleus accumbens and olfactory tubercle
      • Involved in neuropatholgy of Parkinson’s, Alzheimer’s, maybe Huntington’s
    • Behavioral specialization depends on where connected
      • g., caudate lesions can develop abulic form of frontal network syndrome
      • Mental state impairments with features of frontal lobe syndrome emerge in almost all basal ganglia diseases
    • Globus Pallidus
      • Crucial role in motor control; probably role in nonmotor fxs as well
      • Lesions – severe rigidity and bradykinesia; deficits in motivation, judgment, and insight (frontal syndrome like)
    • Cerebellum
      • Input from ipsilateral side of body and is interconnected with contralateral hemisphere
      • Lesions: give rise to ipsilesional motor symptoms
      • Intricate connections
        • Thru diaschisis frontal infarctions can cause acute contralateral cerebellar hypometabolism
      • Nonmotor affiliations – unlikely plays major role in explicit mem, lang, or spatial fxing
        • May globally influence state of info processing in all domains (like ascending cholinergic and noradrenergic pathways)
        • Lesions can impair perf on attention (digits), verb fluency, and reasoning (“frontal”)

 

Thalamus

  • Almost all nuclei have extensive reciprocal connections with cerebral cortex
  • Most nuclei have preferred cortical targets and each cortical area has principal source of thalamic input
  • Nuclei of primary sensory and motor areas
    • Sensory – ventroposterior lateral nucleus, ventromedial nucleus, ventroposterior inferior nucleus
    • LGN: relay nucleus for visual modality
    • MGN: relay for auditory modality
    • Motor – ventrolateral nucleus, ventroposterior lateral nucleus
  • Nuclei of modality-specific (unimodal) ass’n cortex
    • MGN – projections to A1 AND auditory ass’n cortex
    • Ventrolateral nucleus – motor association cortex; principal nuclei thru which basal ganglia and cerebellum influence of cerebral cortex
  • Transmodal nuclei of heteromodal, paralimbic, and limbic cortex
    • Medial dorsal nucleus – prefrontal heteromodal cortex
    • Medial pulvinar and lateral posterior nucleus – inf. Parietal lobule heteromodal cortices
    • Nuclei of “Anterior tubercle” – anterior nucleus and laterodorsal nucleus – connections to posterior cingulate cortex, retrosplenial area, Entorhinal cortex and hippocampal cortex
  • Reticular and intralaminar nuclei
    • Strong assns with reticular activating system

 

Channel Functions and State Functions

  • Many axonal pathways that interconnect one cortical area to another are organized in the form of point-to-point channels where sites of origin and termination are of approximately equal size
    • Language, spatial orientation, memory and emotion each subserved by large-scale networks which contain multiple point-to-point channels. Encode perceptual, motor, visceral, and affective components
  • In addition, each cortical area receives widespread modulatory connections which innervate entire cerebral cortex
    • Employ small amines and GABA as transmitters
    • Determine overall STATE of info processing rather than content of info that is being transmitted along the point-to-point channels
    • Most accessible to therapeutic manipulation – major targets of therapeutic efforts
    • Play very imp role in coordinating beh states related to arousal, attn, mood, and motivation
    • Small grp of neurons can induce rapid modulations in state of info processing
    • ABSENCE of reciprocal projections from cortex
    • Shift info processing based on demands of limbic system and internal milieu
    • Can alter tone, coloring, and interpretation of experience rather than content
    • Many psychiatric disturbances linked to these pathways
    • pathological biases in interpretation of events and experiences
  • Cholinergic and GABA from basal forebrain to cerebral cortex
    • Particularly responsive to novel and motivationally relevant sensory events
    • Enhance immediate neural impact and LT memory of motivationally relevant events
    • Acetycholine may also play role in working memory
  • Histaminergic from hypothalamus to cerebral cortex
  • Serotonergic from raphe nuclei to cerebral cortex
    • May modulate sensory gating of beh relevant cues
    • Also influence state of hunger and aggresivity
  • Noradrenergic projections from locus ceruleus to cerebral cortex
    • More responsive to motivational relevance (meaning) than to sensorial properties
    • Modulates novelty-seeking behaviors, focusing of attn, and resistance to distraction
  • Dopaminergic from substantia nigra and ventral tegmental area to cerebral cortex
    • Responsive to motivationally relevant stim; encode discrepancies between prediction and occurrence of reward
    • Important role in mediating processes related to substance addiction
    • Also working memory
  • Cholinergic from reticular formation to thalamus

 

Hemispheric Specialization and Asymmetry

  • Asymmetry of structure and fx NOT unique to humans
  • Purpose is unknown but may reflect biological advantage of concentrating the controlling components of network within single hemisphere in order to minimize transcallosal conduction delays
  • Left hemisphere specializations: Praxis and Language
    • 90% of population is said to be right-handed
    • L hemisphere more specialized for skilled movements (praxis)
      • Apraxias more commonly seen after damage to L hemisphere
      • Right motor cortex displays activation only when complex finger movements are performed by the contralat L hand whereas L hemisphere active during movement of either hand
      • Left hemisphere thus controls movements in both sides of body; R hemi controls contralat
    • 90% of right-handers and 60% of left-handers develop aphasia after damage to left hemisphere
    • Acalculia also more common after damage to left hemisphere
  • Right hemisphere specializations: Complex non-linguistic perceptual skills (including face identification)
    • R hemi better for melody and pitch; depth perception, spatial localization, identify geo shapes
      • R specialization esp apparent with COMPLEX tasks. (e.g., naïve listeners R hem superiority for melodies, tone sequences). Musically exp greater L hem specialization for these but particularly complex musical material still show R hemisphere activation
    • R hemi also better for faces
      • Altho, severe prosopagnosia seen after bilateral lesions (both hemispheres imp)
    • R lesions – impairment of complex visuospatial tasks
    • Memory processes also show hemispheric asymmetry
  • Right hemisphere specialization for Spatial Attention
    • R specialized for distributing attn for extrapersonal space; more tightly preserved than even L hem lang fx
    • Right hem – shift attn to both sides of space
    • Left hem – shift attn almost exclusively to contralateral hemispace
  • Right hemisphere specialization for Emotion and Affect
    • R hem may normally introduce a negative emot bias whereas left hem may introduce more pos bias
    • R hem more imp for coordinating nearly all aspects of affect and mood
      • Both expressing (prosody, fac expression gesture) and understanding
    • R hem specialization for experience of emotions as well
    • Modulation of mood and affect coordinated by limbic and nonlimbic components
      • Limbic – fund role in generating emotions, linking to visceral rxs, and channeling to targets
        • fx asymmetry probably much less pronounced at this level
      • Nonlimbic – integrating, interpreting, and communicating emotions
    • Right hemisphere specialization for paralinguistic aspects of communication
      • Left hem (Linguistic components) phoneme production, word choice, syntax, and grammar
      • Right hem – prosody, etc.; modulation of verbal output; pitch

 

Distributed Large-Scale Networks and their Cortical Epicenters

  • Structural foundations of cog and beh domains take the form of partially overlapping large-scale networks organized around reciprocally interconnected cortical epicenters
  • Enables parallel processing and contains multiple nodes where seamless transitions btwn parallel and serial processing can occur
  • Although various areas of networks are more specialized for certain behaviors, also play role in behaviors from other areas
  • Rt hemisphere dominant spatial attention network
    • Epicenter: Dorsal post parietal cortex, frontal eye fields, & cingulate gyrus
    • Parietal – specialization for perceptual representation of beh relevant locations
    • Frontal eye flds – choose and sequence exploratory and orienting movements
    • Cingulate – distribution of effort and motivation
    • Damage – deficits in spatial attn and exploration (neglect, simultanagnosia, Balint’s syndrome
  • Lt hemisphere dominant lang network
    • Epicenter: Wernicke’s and Broca’s
    • Broca’s – articulatory, syntactic, and grammatical aspects
    • Wernicke’s – lexical and semantic aspects
    • Damage – aphasia, alexia, agraphia
  • Limbic Memory-Emotion network
    • Epicenter: Hippocampo-entorhinal regions & amygdaloid complex
    • Hippocampal-entorhinal – memory and learning
    • Amygdala – drive, emotion, and visceral tone
    • Damage – deficits in memory, affiliative behs, and autonomic regulation
  • Prefrontal executive function-comportment network
    • Epicenter: Lateral prefrontal, orbitofrontal, and posterior parietal cortex
    • Prefrontal and orbitofrontal – coordination of comportment
    • Prefrontal and posterior parietal – working memory and related executive fxs
    • Damage to orbitofrontal/medial frontal – deficits in comportment
    • Damage to dorsolateral prefrontal – deficits of executive fx and working memory
  • Ventral occipitotemporal Face-and-object identification network
    • Epicenter: middle temporal gyrus and temporal pole
    • Damage – recognition deficits (object agnosia, prosopagnosia) usually bilateral lesions

fusiform gyrus common site of lesions probably cuz only area where vascular supply makes bilateral damage likely