Lesion Studies

• Making sense of Lesion studies

Lesion Studies

• Why are there only certain kinds of deficits associated with lesions? Why not every possible deficit?

Lesion Studies

• Logic of Lesion Studies:– damaged area plays a role in accomplishing whatever task is deficient after the lesion

• Warning:– This isn’t the same as saying the lesioned area “does” the operation in question

– examples:

• normal behaviour may be altered to accommodate lesion

• lesion might cause “upstream problem” or general deficit

– e.g. attention problem “looks like” specific deficit if you only test one specific demanding task

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– First, compare lesion to healthy control

Per

form

ance

TaskA

Lesion X

HealthyThis difference indicates deficit

better

worse

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– But maybe this is a general deficit! - use 2nd task

TaskA

Lesion X

HealthyThis difference indicates deficit

Per

form

ance

better

worse

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– But maybe this is a general deficit! - use 2nd task

TaskA

Lesion X

Healthy

B

Per

form

ance

better

worse

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– But maybe this is a general deficit! - use 2nd task

TaskA

Lesion X

Healthy

B

indicates that deficit is selective

Per

form

ance

better

worse

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– This result is called a single dissociation

TaskA

Lesion X

Healthy

B

indicates that deficit is selective

Per

form

ance

better

worse

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– What if Task A is just harder than B? - add a 2nd group

TaskA

Lesion X

Healthy

B

Lesion YPer

form

ance

better

worse

Lesion Studies

• Designing Lesion Studies– “design tasks that diagnose the function of specific

operations”

– This result is a double dissociation – suggests functional independence of distinct regions or processes

TaskA

Lesion X

B

Lesion YLesion YPer

form

ance

better

worse

Comparing Imaging Techniques - Advantages

• fMRI

– ubiquitous

– high spatial resolution

– non-invasive/safe

• PET

– quiet

– labels variety of molecules

• EEG/ERP– inexpensive– high temporal resolution– non-invasive/safe

• MEG– high-temporal resolution– good but limited spatial

resolution– non-invasive/safe

• Unit Recording– very high spatial resolution– high temporal resolution

• Lesions– real-life

Comparing Imaging Techniques - Disadvantages

• fMRI

– loud

– expensive

– limited temporal resolution

• PET

– very expensive

– limited safety

– invasive

– limited temporal resolution• EEG/ERP

– limited spatial resolution– can be difficult to interpret

• MEG– limited spatial resolution– difficult to interpret

• Unit Recording– very invasive– can be hard to see “big picture”

• Lesions– very invasive

Temporal and Spatial Resolution are Traded Off

Cognitive Psychology

• High resolution instrumentation is of no use if you don’t understand the cognitive operations that you are trying to image

• Cognitive psychologists have worked to understand mental operations for over a century

• The enterprise of Cognitive Neuroscience is predicated on cognitive psychology

Cognitive Operations

• What does the brain actually do?