Cognitive Neuroscience Exam 2

3 types of movements

Reflexes, stereotyped movements, movements of self expression

Chain of command for movement

Cortical motor control centers → brainstem → central pattern generators → motor interneurons → motor neurons → skeletal muscle

Stroke causes difficulties in what kind of movement

Intentional movement; affects cortical motor control centers

Polio causes difficulties in what kind of movement

intentional, stereotyped, and reflexes; affects motor neurons

Central pattern generators

groups of neurons in the spinal cord and brainstem that can produce entire sequence of actions without cortical commands or internal feedback (walking, chewing, coughing, etc.); do not require sensory feedback but can be modified by feedback

Motor control in humans vs. cats after spinal cord resection

Cats only have gait, so they can walk normally after a resection of the spinal cord using central pattern generators; humans have gait and posture so they rely on cortical motor control centers to help with walking

Corticospinal Pathway

Starts at primary motor cortex and passes through cerebral hemisphere, midbrain, and medulla; contralateral projection: tract crosses entirely from one side of the brain to the opposite side of the body in the medulla; controls movements of the extremities

Primary Motor Cortex

sends signals to motor neurons to make muscles move; electrical stimulation results in movement; controls individual movements or sequences of movements that require the activity of multiple muscle groups; neurons fire 5-100 msec before the onset of movement

Lesions in Primary Motor Cortex

hemiplegia/hemiparesis (paralysis): loss/weakness of voluntary movement in contralateral side of the body; primitive reflexive mechanisms take over

Topographic Correspondence

Surface area devoted to controlling the movements of each body part proportional to precision of movements that can be executed by that part; plasticity

Secondary Motor Cortex

Planning and control of movements, internally and externally guided; premotor cortex and supplemental motor area

Lesions of Secondary Motor cortex

result in apraxia; problems with motor planning, patients have normal muscle strength and tone, can perform simple movements (opening/closing fist) but can’t link gestures to meaningful actions (e.g. waving hand or saluting)

Premotor Cortex

associated with external sensory-guided actions; neurons represent a “vocabulary” of actions (tearing, grasping) based on affordances; prepares movement based on external contingencies (hit brakes on red light, grab cookie on plate); neurons often have mirror properties (sensory and motor component)

Mirror Neurons

active during both action production and action perception; independent of how the action is inferred; basis for comprehending others’ actions rooted in own experience

Supplementary Motor Area

converting intention into self-initiated actions; initiation of internally generated as opposed to stimulus driven components; implementing and coordinating complex sequences retrieved from memory that follow a precise timing (e.g., playing a familiar tune on a musical instrument)

Alien Hand syndrome

Lesion of secondary motor area can leave contralateral hand at mercy of external stimuli that operate through the PMC; people experience a conflict between their declared will and action of one of their hands; complex movements of one hand are apparently directed towards a goal and are smoothly executed yet are unintended; patients are aware of their bizarre behavior but cannot inhibit it

Prefrontal cortex in motor control

involved in selection and maintenance of goals and responses; coordination of cognition generally (both external actions and internal thoughts); damage does not impair physical movement but actions become inappropriate or disorganized

Frith et al.

Move any finger vs. move that finger; PFC greater activation in free choice task; important for response selection and willed action

Subcortex - Direct Pathway

initiates movement; excitatory

Subcortex - Indirect pathway

inhibits movement; inhibitory

Cerebellum

Motor coordination, fine-tuning of movements; motor learning (no forgetting); modulates IPSILATERAL muscles

Cerebellum feedback loop

sensory signals → cerebellum → corrective feedback to motor areas of cerebral cortex

Damage to cerebellum

Ataxia: uncoordinated muscle movement; involving speech is called dysarthria; finger to nose test: movement can be performed but path is staggered, jerky, and overshoots target; difficulties in sensory-motor learning after damage to lateral portions of the cerebellum, difficulties in learning to adjust an action such as throwing a dart at a target after visual input is modified by having to wear prisms

Damage of Basal Ganglia

Parkinson’s Disease: reduces function of direct pathway, death of dopaminergic neurons in the substantia nigra, indirect pathway becomes overactive; akinesia (the inability to initiate spontaneous movement), bradykinesia (slowness of movement) tremors (rhythmic, oscillating movements)

Therapy for Parkinson’s Disease

L-Dopa: to produce more dopamine; deep brain stimulation: pacemaker like device connected to the electrodes and implanted subcutaneously, high frequency stimulation of subthalamic nuclei which project in an inhibitory fashion to pallidum

Huntington’s disease; basal ganglia damage

Reduces function of indirect pathway; hyperkinesia: involuntary, undesired movements; chorea: uncontrollable, jerky movements such as twitching and abrupt jerking of the body; athetosis: involuntary writhing contractions and twisting of the body into abnormal postures; Causes: inherited gene defect, autosomal dominant so person only needs one copy

Nature of sound

changes in air pressure: amplitude and frequency; pitch and loudness: psychological properties, frequency and amplitude: physical properties; pure tones are sinusoid but more complex sounds are sums of sinusoids

Outer ear

Pinna, external auditory canal, tympanic membrane (eardrum)

Middle ear

hollow region containing the ossicles

Ossicles

middle ear bones: malleus (hammer) Incus (anvil) stapes (stirrup)

Hydraulic amplification

surface area of eardrum is larger than stapes which amplifies

Cochlea

fluid filled, snail shaped structure containing the organ of Corti

Organ of Corti

sensory organ for the auditory system; basilar membrane and hair cells

Basilar membrane

varies in width and stiffness at different points along its length; high frequency sounds localize near the base of the cochlea while low frequency sounds localize near the apex; conversion of sound waves into action potentials happens in hair cells

Ear to Brain

Hair cells in cochlea convert sound energy to neural impulses and send them along to the primary auditory cortex; thalamus projects to primary auditory cortex; the primary auditory cortex is located in the superior portion of the temporal lobe (both hemispheres); core area is surrounded by secondary auditory cortex

Language

system of communication using sounds or symbols that enables us to express our feelings, thoughts, ideas, and experiences; all cultures have a language; languages are “unique but the same”: different words, sounds, and rules but they all have nouns, verbs, negatives, questions, past/present tense

Morphemes

smallest unit of language that has definable meaning or grammatical function; every word comprises of one or more morphemes; prefixes and suffixes

Phonemes

shortest segment of speech (sound) that if changed, changes the meaning of the word; different languages have different numbers of phonemes; not the same as letters; “rich” has 3 phonemes and 4 letters; top down process

Phonemic restoration effect

sounds missing from a speech signal appear to be heard; “there was time to *ave” (shave, wave, save, rave) depending on context

McGurk Illusion

conflicting acoustic and visual cues can lead to blended perception of sound, or visual signal overrides sound; superior temporal sulcus: auditory-visual integration of speech stimuli; TMS to STS reduces illusion

Primary auditory cortex: understanding speech

not speech specific; areas surrounding primary auditory cortex react specifically to speech (left: speech comprehension, production; Right: melodic aspects, prosody)

Segmentation problem

perceiving individual words in continuous flow is difficult; words are often not segmented from one another by pauses; acoustic variability: did you=dijoo

Segmentation problem: how do we map continuous stream of acoustic signals to discrete words

1. knowing the meaning of words

2. context of conversation

3. understanding of sounds and syntactic

   1. statistical learning: learning which sounds are more likely too follow one another

Saffran et al.

2 minute training; babies listened longer to words they had not been trained on; head turn preference technique

ERP

violations in semantics and syntax affect different ERP components; different brain structures support semantic and syntactic processing; different time courses for processing semantic and syntactic information (semantic happens first)

Semantic ERP analysis

N400; is word appropriate for wider context

Syntactic ERP analysis

P600; grammatical errors

Aphasia

a deficit in language comprehension or production, commonly due to a hemisphere stroke or insult; 40% of stroke patients

broca’s aphasia

mostly language production; anomia: difficulty finding words; speech: telegraphic, very effortful; lack function words (a, some, the , in, etc.); articulation problems (muscles of the tongues); agrammatism: problems with correct syntactic assignment; patients are aware and often frustrated

Wernicke’s Aphasia

brain: left posterior region of superior temporal gyrus; difficulty understanding spoken and written language; fluent speech with normal prosody; output nonsensical but patients unaware

Conduction aphasia

white matter tract connecting wernicke’s and broca’s area; problems with speech repetition; intact auditory comprehension; aware of disorder and can hear own speech errors but cannot correct them

Pure word deafness (auditory verbal agnosia)

inability to understand language, repeat words, and write from dictation; speech appears too fast or distorted; spontaneous speaking, reading, and writing are preserved; able to recognize non-verbal sounds

Reading

difficult because it’s culturally engineered skill; requires transforming visual code into auditory code; speech comprehension developed without explicit training but reading requires instruction

Neural substrates of written word processing

identification of orthographic units: occipitotemporal regions in left hemisphere (visual word form area); pre-semantic processing, responds to left and right visual field inputs, not sensitive to case change

Visual word form area

responds to written words more than consonant strings, irrespective of visual field; specific type of object recognition—letters and whole words; specialization develops after years of training; fMRI adaptation evidence shows that the VWFA is tuned to whole words, not just orthographically allowable letter strings

Dual-Route model of reading

Phonics reading: grapheme-phoneme conversion, slow, bit by bit; Whole word reading: visual semantic route, fast, affected by word frequency

Acquired dyslexias

surface dyslexia: damage to lexical-semantic route and reliance on grapheme-phoneme conversion; able to read non-words, regularization errors (yacht read as “yatched”)

Dual route model

visual analysis → letter recognition → visual lexicon → semantic memory → phonological lexicon → speech production

Phonological dyslexia

damage to grapheme-phoneme conversion; opposite problem to surface dyslexics; intact ability to read familiar words, but impairment of the ability to read pronounceable non-words

Acquired deep dyslexia

damage to grapheme-phoneme conversion and semantic memory; able to read real words better than non-words but makes semantic errors in reading (e.g. cat → dog)

two functions of eyes

Optical: focusing light on the retina; sensory: transduction of light projected on the retina and projection of neural signal to higher processing areas of the brain (transduction= conversion of light energy into neural code/signal)

How do we focus?

near triad: converging eyes, rounding lens, pupillary constriction

Photoreceptors

specialized neurons: rods and cones; contain photopigments: transmembrane protein molecules that are light sensitive; when exposed to light, photopigments become unstable and fall apart; alters membrane potential; photoreceptors convert light into electrochemical signal

Rods

~120 million; periphery of retina; photopigment: rhodopsin; high sensitivity: reacts to low levels of light, good at detecting light - b/w vision, most useful at night

Cones

~6 million; center of the retina (fovea); photopigment: photopsin; lower sensitivity: requires more intense levels of light sensitivity, active during daytime vision, color vision

Ganglion cells: receptive fields

specific region of visual space to which a particular cell responds; light from a particular location in space falls on a particular part of the retina, and stimulates specific subgroups of rods or cones, to which specific ganglion cells respond; brain knows where light has struck by knowing which ganglion cells are excited; the receptive fields of retinal ganglion cells have a center-surround structure; can help enhance contrast

Coding of visual information in retina

the center-surround organizations of the RGCs receptive fields helps to enhance contrast and detect edges. This is an efficient way to encode the interesting parts of a visual scene

Geniculo striate pathway

information from the left visual field hits the right part of the retina and travels to the right hemisphere of the visual cortex, vice versa

Primary Visual Cortex

first destination within the cortex is the primary visual cortex in the occipital; projections from the LGN to the primary visual cortex maintain their spatial organization; the mapping is so precise that we can specify a cortical magnification factor, which describes the millimeters of cortical surface that are devoted to one degree of angle in the visual world; cortical magnification value is dramatically higher for regions of the fovea compared to the periphery

Retinotopic organization

the spatial arrangement of light on the retina is retained in the response properties of V1 neurons (except inverted); damage to parts of area V1 results in blindness for the corresponding region of space (e.g. hemianopia, scotoma)

Cells in V1

simple, complex, and hyper complex; their receptive fields are responsive to bars of light oriented in particular ways

simple cells

respond to bars of different orientations; derive their response by combining the responses of several LGN center-surround cells; respond to different orientations

complex cells

also respond best to certain line orientations, but no on-off region; preference for moving lines

hyper-complex

end-stopped cells, prefer lines of certain lengths

Hubel & Wiesel

investigated how visual information is processed in cat’s LGN; found that the LGN, like ganglion cells, maintain center-surround receptive fields that respond best to spots of light rather than lines

Motion after-effect

adaptation in visual neurons that respond selectively to moving contours in the image, in the absence of image motion, cells tuned to different directions produce roughly equal responses; exposure to a particular direction of motion alters the balance of activity in favor of cells tuned to the opposite direction, causing the after-effect

V1

within the chunk of cortex representing each spatial location, there is a systematic organization of cells that respond to all the various line orientations

Binocular integration in V1

since the eyes are about 6cm apart from each other, they provide slightly different images of the same object (binocular disparity); the greater the disparity, the closer the objects must be; binocular neurons in occipital lobe are tuned to specific disparity between retinal inputs

Zeki et al. PET study

colored images compared to grayscale: V4 active and region is specialized for color; moving b/w square patterns compared to static patterns;V5 and V4 active, region specializedV for visual movement

V4

cells in V4 continue to respond to the same surface color if the light source is changed, whereas cells in V1 do not; patients with damage to area V4 see the world in black and white— they are called achromatopsic

V5

patients with bilateral damage to this region see the world in a series of still snapshots (akinetopsic); can detect movement in other senses like hearing

Ventral stream

travels to temporal lobe (what pathway)

Dorsal stream

terminates in the parietal lobe and processes spatial locations (where)

Patient DF (CO poisoning)

cortical atrophy with concentrated bilateral lesions in the ventral stream; visual agnosia; dissociation between cision-for-perception and vision-for-action

Blindsight

geniculostriate route may be specialized for conscious vision but other routes act subconsciously; damage to V1 leads to a clinical diagnosis of blindness (no conscious access to vision); someone with blindsight can get through an obstacle course just fine but cannot see or identify obstacles

Ventral visual processing stream

occipitotemporal and temporal regions that are devoted to processing visual stimuli; cells in posterior regions fire to relatively simple stimuli, but cells further along the ventral stream fire to more complex and specific stimuli

Receptive Fields

become larger as we move further along the ventral stream

Ventral stream - 2 trends as we move from posterior to anterior

cells respond to more complex and specific stimuli, receptive fields become larger

Pros and cons of large receptive fields

Pros: allows whole objects to be detected, object can be identified regardless of its size or location in space, inclusion of central - color/high acuity aids in object perception

Cons: some information about item’s position in space is lost

Failures in object recognition

agnosia: inability to process sensory information

visual agnosia: inability to recognize objects in visual modality that cannot be explained by other causes

apperceptive agnosia

difficulty forming a percept

sensory information is processed in a rudimentary way

data cannot be bound together to allow for perception of a meaningful whole

lesions in occipital regions

associative agnosia

patient can perceive object, but cannot assign meaning; is unable to access conceptual knowledge from visual inputs; doesn’t know what the object is used for, where it might be found, etc.

rarely exists in pure form

bilateral occipitotemporal lesions

visual agnosia or memory loss A patient presents with an inability to recognize many visual objects • You are tasked to determine whether the patient has memory loss or visual agnosia • How would you go about this

Test other senses, if they can identify objects on touch or sound then it is not memory loss, but visual agnosia

JBR (herpes encephalitis)

90% correct identification rate for scissors, clocks, chairs but 6% correct identification rate for tiger, pig, blue jay

visual knowledge of many inanimate objects is supplemented by kinesthetic codes that are developed through interactions with these objects

kinesthetic codes don’t exist for most living things

prosopagnosia

face blindness

how does the ventral stream achieve perceptual invariance

invariance to translation and changes in size are built up by MAX-pooling over units tuned to the same feature

complexity is built up by combining units tuned to different features

receptive field size increases

how does the brain take 2D information from the retina and create a 3D representation of an object such that it can be recognized from any viewpoint

viewpoint dependent theories: systematic integration or interpolation across a set of viewer-centered representations, in which stored representations of objects are specific to certain viewpoints of those objects

viewpoint-independent theories: perceptual system extracts structural information about the components of an object and their relationships to build 3D representations

Viewpoint dependent object recognition

problem: requires multiple representations of same object in memory

solution: find the best match

support: easier to recognize object from typical viewpoint

viewpoint-independent recognition

visual stimuli are broken down into their principal components (features)

from this the brain constructs a viewpoint-independent abstract 3D representation

Lateral Occipital Complex

recognition at the perceptual structural level; retinotopic representations are transformed into relatively abstract shape representations that support recognition across variation in size, precise form, and location (cue invariant, shape driven, no semantic information