Neuroimaging for Cognitive
Research
Obtaining evidence from the
Brain
• Lesion studies (ling. aphaisiology)
• Neuroimaging (CT, PET, SPECT, fMRI, EEG,
MEG)
• Direct manipulation
– cell recordings (single and array)
– electrical stimulation
– neurochemical stimulation (barbiturates - Wada)
Imaging Approaches
Functional
Structural
MRI
X-ray
fMRI
SPECT
PET
CT/CAT
EEG
ECG
Electro-Encephalograghy
(EEG)
• Electrical current originating in the cortical areas
• Measured across scull and tissue - adjustments
for physical properties
-
+
current
EEG
• Strengths:
– Relatively easy to administer and cheap
– High temporal resolution (miliseconds)
• Weaknesses
– Hard to interpret (noise, artifacts)
– Low spatial resolution
EEG signal analysis
• Event Related Potentials (ERP)
• Electrical activity on an electrode or a group
of electrodes averaged over many trials
• Positive and negative peaks at different
points in time from the stimulus presentation
• 0-150 ms - perception
• 150-350 ms - phonological/syntactic
• 350-600 ms - conceptual/semantic
ERP temporal resolution
From: brainvat.wordpress.com
ERP spatial resolution
ERP caveats
• Signals from multiple sources (general
body function unrelated to cognition)
– Multiple presentations of the stimuli
• Uncertainty of the signal source
– Multiple “dipoles” may be responsible for
the strength of signal at a given location
– Source can be verified with other imaging
methods (e.g., PET)
ERP components
• Three dimensional representation
– Direction: Negative vs. Positive deflection
– Latency: time from stimulus onset
– Gross location: frontal, temporal, occipital,
etc.
• P1, N1, P2, N2, P3, N400, P600
ERP components
P1
N1
P2
N2
P3
N400
P600
P1/N1
• P1
– 50ms – auditory, 100ms – visual
– General attention/arousal
• N1
– Selective attention to stimulus
characteristics
– Stimulus discrimination
P2/N2
• P2 – obligatory cortical potential
– Low individual variability and high reproducibility
– Stimulus classification
– Sensitive to pitch and loudness (auditory)
• N2
– Stimulus discrimination
– Deviation of stimulus from expectation
P3
• Stimulus classification and response
preparation
• Varies with stimulus complexity
• Possibly associated with memory and
attention
N400
• Sensitive to language (not music) specific anomalies
• Semantic but not syntactic processing
• May reflect the degree of anticipation/preactivation
From Kutas & Hillyard 1980
P600
• Memory and language
– Old-new response (greater for old
information)
– Syntactic Positive Shift (Kutas and Hilliard,
1983)
• Syntactic processing load due to parsing failure
• Elicited with syntactic and morphosyntactic
violations (agreement, phrase structure,
subcategorization, syntactic ambiguity)
Magneto-Encephalography
• Similar to EEG in some respects
• Detects very weak magnetic fields resulting from electrical
activity
–
–
–
–
Earth - 1010
Urban noise - 1010
Epileptic spike - 1,000
Sensory evoked response - 100
• Tens of thousands of neurons firing in the same direction
• Detected with Superconducting Quantum Interface Device
(SQUID)
• Orthogonal to EEG
• Dipole source model
-
+
current
MEG
• Strengths
– High temporal resolution
• Weaknesses
– Sensitivity to magnetic interference
– Hard to administer
– Hard to interpret (noise, artifacts)
MEG and synchronous
cognitive networks
Use Case: Study of Silent
Meaning
• Pylkkanen and McElree (JCN, 2007)
• Semantic Compositionality
– Strict/compositional version – semantics
are always expressed in syntax
– Alternative version – some semantic
interpretations are non-compositional –
independent of syntax
Compositional vs. Noncompositional Meaning
• The author began the article
– Activity (writing) is implied
– “Coerced complement”
• The author wrote the article
– Activity is explicit
• The author astonished the article
– Semantically anomalous
Sources of Neural Response
Results
• Anterior Medial Field response (350500ms) sensitive to complement
coercion
• M350 component in the left temporal
area is sensitive to semantic anomaly
• Consistent with ERP findings for N400
component