#31 - Cortex Function

Question 1

Primary Visual Cortex

(V1 / striate cortex)

Answer 1

• Occipital lobe
• Located medially on the banks of the calcarine fissure extending around the occipital pole
• Left visual field is represented in right visual cortex; vice versa
• Lower visual field is represented in dorsal bank; vice versa
• Central (macular) vision is located at the occipital pole
• Visual fields are represented as a retinotopic map
• Damage = CONTRA scotomas, quadrantopsia, or hemianopsia

Question 2

Secondary visual (extrastriate) cortex

Answer 2

• Occipital lobe
• Secondary visual areas form the extrastriate cortex surrounding V1, analyze specific attributes of visual stimuli
• V2: strongly interconnected with V1, responds to complex patterns
• V3: contains dorsal (V3a) and ventral (VP) divisions
• Dorsal V3a neurons receive input form V1 and V2, respond to large scale motion
• Ventral VP receives input mostly from V2, involved in object recognition
• Thus, two visual streams become separated at V3:
  
  • Dorsal stream:
    
    • Object and relative body motion
    • Placing of objects in external space (where?)
    • Control of visually-guided eye movements
  • Ventral stream
    
    • Pattern and object recognition (what?)

Question 3

Area V4

Answer 3

• Occipital Lobe
• Visual association area: ventral stream and temporal lobe
• Area V4 is concerned in the assemblage of object representations
• Output directed to posterior inferior temporal lobe, “IT”
  
  • Left IT: processing feature information like color, texture, shape, symbols associated with language (e.g., writing)
    
    • Damage: alexia with agraphia
  • Right IT: holistic, non-verbal perception, facial recognition
    
    • Damage: prosopagnosia
• General lesion to ventral stream: visual form agnosia = Inability to recognize objects, discriminate between simple geometric shapes and orientation

Question 4

Area V5

Answer 4

• Occipital lobe
• Visual Association Areas - Dorsal Stream & Parietal lobe
• Area V5 is located in the posterior end of the middle temporal gyrus
• Sends info to the posterior parietal cortex, including parietal eye fields
  
  • PEF: Generate saccadic eye movements evoked by novel visual/auditory stimuli
• Involved in motion perception and assemblage of objects in visual space
• Lesion to dorsal stream (posterior parietal cortex) = Balint’s Syndrome
  
  • Simultanagnosia
  • Ocular apraxia: absence of purposeful eye movement
  • Optic ataxia: lack of coordination btwn visual and motor

Question 5

Primary Auditory Cortex

Answer 5

• Temporal lobe
• Located in transverse temporal gyri (Heschl’s gyri)
• Is tonotopically organized
• Lesions: may lead to loss of awareness of sound, but reflexes involving sound remain intact
• Receives binaural input with contralateral dominance
• Receives thalamic input from medial geniculate nuclei
  
  • Inferior colliculus > MGN > sublenticular projection from thalamus to roof of Heschl’s gyrus

Question 6

Auditory “belt” cortex

Answer 6

• Temporal lobe
• Ring of associatoin cortex surrounding the primary auditory cortex
• LEFT: involved in decoding temporal aspects of sounds
  
  • Ex: sequence, duration, intervals
• RIGHT: involved in decoding spectral aspects of sounds
  
  • Ex: frequency/pitch, harmonics
• Plays a general role in “auditory imagery”; fills gaps in speech (L) and in melodies (R)
• Hierarchical nature of auditory processing:
  
  • Primary auditory cortex responds to simple stimuli (tones) and downstream regions are selectively responsive to more complex stimuli (band noise, speech) with neural codes progressively more abstract the farther away from Heschl’s gyri you get

Question 7

Wernicke’s Area

Answer 7

• Temporal lobe; left posterior superior temporal gyrus
• Decodes verbal info
• Activated by phonemic content (perceptually distinct units of sound in language) and gaps between speech sounds
• Damage = sensory aphasia
  
  • Inability to understand language in written or spoken form
  • Speech is fluent, effortless, melodic, but jumbled and unintelligible, normal grammar, syntax, rate, and intonation but they can’t express themselves meaningfully using language.
• Similar area on the right/dominant side appears to be involved in prosody (various rhythms, intonations in normal speech)
  
  • Lesions to R = sensory aprosodia, inability to detect prosodic elements (humor, sarcasm) in speech

Question 8

Auditory Association What, Where Streams

Answer 8

• Ventral “what” stream: directed at temporal cortex
  
  • Plays a general role in auditory object recognition, including perception of vocalizations and speech
• Dorsal “where” stream: directed at parietal cortex
  
  • Pertains to significance of the location, motion of the sound source

Question 9

Dorsal Stream

Answer 9

Where

Parietal cortex

Question 10

Ventral Stream

Answer 10

What

Temporal cortex

Question 11

Lateral, Inferior Temporal Lobe

Answer 11

• Areas key to interpretation and categorization of “what”
• Left
  
  • Caudal parts are important for retrieval of the general names (common nouns) of objects (dogs, carrots)
  • Anterior parts (temporal pole) are important for retrieval of proper nouns that denote unique entities (individuals, places)
  • Lesions = anomia (naming defect) with severity increasing with lesions closer to the temporal pole
• Right
  
  • Recognition of objects and retrieval on non-verbal info pertaining to these objects

Question 12

Medial/Mesial Temporal Lobe

Answer 12

• Includes parahippocampal gyrus, hippocampus, and amygdala
• Critical for memory formation, emotion, and sexual behavior

Question 13

Primary Somatosensory Cortex

Answer 13

• Parietal Lobe
• Rostral-caudal in postcentral gyrus, has a somatotopic map (sensory homunculus) of contra body
• Granular layer of cortex
• Brodmann’s areas:
  
  • Area 3: primary somatosensory cortex
    
    • Receives most of output of VPL and VPM
  • Area 3a: proprioception
  • Area 3b: pressure and vibration sense
  • Area 1: texture processing
  • Area 2: estimating size and shape
• Postcentral gyrus lesions:
  
  • Loss of 2-point discrimination
  • Agraphesthesia: inability to recognize written letters or numbers traced on skin
  • Astereognosis: inability to identify an object by active touch of the hands without other sensory input

Question 14

Somatosensory Association Areas

Answer 14

• Parietal lobe
• Integrates complex stimuli in the external and internal environment
• All have a crude sensory homunculus
• Ventrolateral zone: parietal operculum, which has 4 association areas
  
  • Lesion leads to tactile agnosia = patients cannot recognize or name objects by touch
• Dorsomedial zone: supplementary somatosensory area, has 1 association area
  
  • Lesion (especially on R) leads to constructional apraxia = difficulty with simple drawing, assembling blocks

Question 15

Inferior Parietal Lobule Components

Answer 15

• Supramarginal gyrus
• Angular gyrus
• Right, Left IPL

Question 16

Right Inferior Parietal Lobule

Answer 16

• Parietal Lobe
• Critical role in self-perception (holistic) and the position of the body in external space
• Critical role in visuospatial tasks (understanding complex patterns) and using spatial knowledge to perform complex tasks
• Damage to right IPL leads to hemispatial neglect = left side of body and world is ignored
  
  • Common from R-sided MCA strokes
  • Visual neglect of the left visual field is prominent even though visual pathways are intact
  • Right neglect doesn’t really occur because R representation is bilateral while L is only on right hemisphere

Question 17

Left Inferior Parietal Lobule

Answer 17

• Parietal lobe
• Supramarginal gyrus: language perception and processing
  
  • Lesions produce sensory (Wernicke’s-like) aphasia and impairments in verbal working memory
• Angular gyrus: transfers visual info to Wernicke’s area to make meaning out of visually perceived words
• Lesions of BOTH left supramarginal and angular gyri lead to:
  
  • Agraphia: loss of ability to write
  • Acalculia: loss of ability to understand math
  • Right-left disorientation
  • Finger agnosia: inability to distinguish own fingers and fingers of others
  • ALL OF THESE SIGNS TOGETHER = Gerstmann’s Syndrome

Question 18

Medial Parietal Lobe (precuneus)

Answer 18

• Involved in mental imagery, concerning self, memory tasks, and visuospatial imagery
• Shows highest degree of metabolic activity during waking
• Probably plays a role in self-consciousness and resting consciousness (hub in default mode network)
• DMN: large-scale brain network known to have highly correlated activity patterns
  
  • Participates in:
    
    • Neurological basis of “Self”
    • Engaged when thinking about others
    • Engaged when remembering past and imagining future

Question 19

vmPFC

Answer 19

Valence, emotional charge of experiences

Question 20

dlPFC

Answer 20

concerned with how to interact with stim via decision making and working memory

Question 21

Frontal Eye Fields

Answer 21

• Anterior to premotor cortex
• Production of non-tracking, voluntary eye movement
• Saccades reposition eyes when you voluntarily move them to select a new target in your visual field
• FEF sends projections to the superior colliculus, which controls pattern generators in the pontine reticular formation for eye position
• Destructive lesion (typical of certain MCA infarctions) to FEF causes deviation of the eyes to the ipsilateral side during the acute phase
  
  • FEF on each side controls voluntary conjugate gaze to the contralateral visual field
  • So when FEF is damaged, signaling from FEF on the intact side is unopposed and both eyes deviate toward the side of the lesion