1.5 Cognitive Neuroscience Methods, Lateralization Testing, and Neuroplasticity

Cognitive Neuroscience Methods, Lateralization Testing, and Neuroplasticity

The GRE Psychology Test expects you to know not only which brain regions support a function but also how researchers know. Many questions describe a study and ask what a method can or cannot conclude, so the inferential logic matters as much as the anatomy.

1. The Lesion Method and the Logic of Dissociation

The oldest approach in neuropsychology studies what is lost after brain damage. Paul Broca's 1861 patient "Tan" and Carl Wernicke's 1874 cases linked language deficits to specific left-hemisphere sites, and the study of patient H.M. linked declarative memory to the medial temporal lobes.

• Single dissociation: A patient with lesion A is impaired on task X but performs normally on task Y. This suggests X depends on A, but the result could also arise if task X is simply harder or more demanding than task Y.
• Double dissociation: Patient 1 (lesion A) fails X but passes Y, while Patient 2 (lesion B) fails Y but passes X. The term was popularized by Hans-Lukas Teuber (1955). A double dissociation rules out a simple difficulty explanation and is the strongest neuropsychological evidence that two functions rely on separable systems. Classic examples include Broca's versus Wernicke's aphasia and patient K.F. (impaired short-term memory, intact long-term learning) versus H.M. (the reverse).

Limitations: Natural lesions are rarely confined to one structure, damage often reflects the territory of a blood vessel rather than a functional unit, and the brain reorganizes after injury, so the deficit observed months later may not reveal the original function. Experimental lesions in animals (electrolytic, aspiration, or neurotoxic lesions that spare fibers of passage) offer more control.

2. Electrophysiological Methods

High-Yield ERP Components

• P300 (P3): A positive wave peaking about 300 ms or later after a rare, task-relevant stimulus in an oddball paradigm. Its amplitude grows with stimulus improbability and attention, and its latency lengthens with harder stimulus evaluation.
• N400: A negative wave about 400 ms after a word that is semantically anomalous in context ("He spread the warm bread with socks"), first described by Marta Kutas and Steven Hillyard (1980).
• N170: A negative deflection about 170 ms after faces, larger for faces than for other objects.
• Mismatch negativity (MMN): An automatic response to a deviant sound in a repetitive sequence, present even when the listener is not attending.
• Error-related negativity (ERN): Generated in the anterior cingulate within about 100 ms of an error (see Section 6.4).

3. Hemodynamic and Metabolic Imaging

• PET (positron emission tomography): A radioactive tracer is injected. Fluorodeoxyglucose (FDG) tracks glucose metabolism, labeled water tracks blood flow, and receptor ligands can map dopamine or serotonin receptor density. PET is invasive (radiation) and slow, with images integrated over tens of seconds to minutes.
• fMRI (functional magnetic resonance imaging): Measures the BOLD (blood-oxygen-level-dependent) signal described by Seiji Ogawa (1990). Active neurons trigger an oversupply of oxygenated blood, and because deoxygenated hemoglobin is paramagnetic, the local signal changes. The hemodynamic response rises over several seconds and peaks roughly 5 to 6 seconds after neural activity, so fMRI has millimeter spatial resolution but temporal resolution of seconds.
• Structural MRI and DTI: Structural MRI images anatomy (gray matter volume, cortical thickness). Diffusion tensor imaging (DTI) tracks the direction of water diffusion along axons to reconstruct white-matter tracts such as the arcuate fasciculus or corpus callosum.

The Subtraction Method and Its Pitfalls

Most activation studies compare brain activity in an experimental task with a control task that differs in one component. The logic descends from Franciscus Donders' (1868) mental chronometry: if task B equals task A plus one extra process, then B minus A isolates that process. The assumption of pure insertion (adding a process does not change the others) is often violated.

Reverse inference is a common error: reasoning that because a region (say, the amygdala) was active, a particular mental state (fear) must have occurred. Russell Poldrack (2006) showed that this inference is weak whenever a region is active across many different tasks.

Spatial resolution ->      fine (mm)                           coarse (cm)
                    |  single-unit   fMRI   PET     MEG    EEG/ERP
Temporal resolution |  ms            s      s-min   ms     ms
Causal?             |  no            no     no      no     no

4. Methods That Test Causation

• Transcranial magnetic stimulation (TMS): A coil on the scalp produces a brief magnetic pulse that induces current in the cortex beneath it (Anthony Barker, 1985). Single pulses over motor cortex produce muscle twitches; repetitive TMS over a region can produce a temporary "virtual lesion," letting researchers test whether that region is necessary for a task in healthy people. Therapeutic rTMS is discussed in Section 13.3.
• Transcranial direct current stimulation (tDCS): Weak constant current shifts cortical excitability up (anodal) or down (cathodal). Effects are small and variable.
• Optogenetics (animals): Neurons are genetically engineered to express light-sensitive ion channels such as channelrhodopsin, so light pulses switch specific cell types on or off with millisecond precision (Boyden, Deisseroth, and colleagues, 2005).
• Pharmacological manipulation: Agonists, antagonists, and neurotoxins (for example, 6-hydroxydopamine to destroy dopamine neurons) test the role of specific transmitter systems.

5. Testing Hemispheric Lateralization

• Wada test (intracarotid sodium amobarbital procedure): Developed by Juhn Wada (1949). A barbiturate injected into one carotid artery briefly anesthetizes one hemisphere; if speech stops, that hemisphere is language-dominant. It was traditionally used before epilepsy surgery and is increasingly replaced by fMRI language mapping.
• Dichotic listening: Different words are played to the two ears simultaneously. Most right-handers show a right-ear advantage for verbal material because the stronger contralateral auditory pathway projects to the left hemisphere.
• Divided visual field (tachistoscopic) presentation: Brief stimuli flashed to one visual field reach the opposite hemisphere first, revealing faster or more accurate processing when the receiving hemisphere is specialized for the material (see the split-brain studies in Section 1.2).

6. Neuroplasticity: Experience Rewires the Brain

Donald Hebb (1949) proposed that when one neuron repeatedly helps fire another, the connection between them strengthens, an idea later summarized as "cells that fire together wire together" and supported by long-term potentiation (Section 1.3).

Adult neurogenesis: New neurons are generated throughout life in rodent dentate gyrus and olfactory bulb. Peter Eriksson and colleagues (1998) reported new hippocampal neurons in adult humans, but whether this continues at meaningful levels in older adults remains debated (two influential 2018 studies, by Sorrells and colleagues and by Boldrini and colleagues, reached opposite conclusions).