194 II. BIOLOGICAL  FOuNDATIONS
properties of different tissue types. These 
can be either high-  resolution images of brain 
structure or lower resolution images of brain 
function. Functional imaging with fMRI is 
possible because blood flow and oxygen 
use increase with neural activity, so that 
the blood-  oxygen- level dependent (BOLD) 
signal indicates which regions of the brain 
are active at a given time. One limitation of 
fMRI is that it assesses relative rather than 
absolute levels of neural activation; activa -
tion during one experimental condition 
must be contrasted with activation during 
another to get a meaningful signal.
In addition to activation, fMRI can also 
be used to assess functional connectivity — 
patterns of temporal synchrony between dif -
ferent parts of the brain. Brain regions that 
show similar temporal patterns of activation 
and deactivation during some portion of a 
scan are deemed to be functionally connect -
ed. This analysis does not require a contrast; 
hence, it is often applied to data collected 
during periods of rest in the scanner. Pat -
terns of spontaneous functional connectivity 
during rest closely resemble networks that 
are activated by specific tasks (Laird et al., 
2011; Smith et al., 2009), a discovery that 
has enabled mapping of the major networks 
of the brain (Choi, Yeo, & Buckner, 2012; 
Yeo et al., 2011).
MRI is so widely used in part because it is 
noninvasive. This is in contrast to neuroim -
aging techniques such as positron emission 
tomography (PET) or single-  photon emission 
computed tomography (SPECT) that require 
injection of radioactive material before scan -
ning. The risk inherent in these techniques 
has meant they are less used in personality 
neuroscience, but they have the advantage of 
allowing measurement of receptors or trans -
porters of particular neurotransmitters. All 
of these neuroimaging techniques are valu -
able for their impressive spatial resolution.
2. Electrophysiological techniques . Elec -
troencephelography (EEG) measures the 
brain’s electrical activity through the skull 
using electrodes on the scalp. It can assess 
neural activity by the millisecond, which is 
much higher temporal resolution than what 
is currently available from fMRI or other 
neuroimaging techniques, but it suffers from 
greatly reduced spatial resolution. Other electrophysiological techniques, such as 
electrocardiography and assessment of elec -
trodermal activity, use peripheral nervous 
system activity to draw inferences about 
neural processes related to emotion and mo -
tivation.
3. Assays of endogenous psychoactive 
substances . Measurements of substances 
such as hormones or neurotransmitter me -
tabolites in blood, saliva, urine, or spinal 
fluid can be used to implicate specific neuro -
biological systems in personality.
4. Psychopharmacological manipulation . 
Psychoactive chemicals can be used as drugs 
to probe neurotransmitter systems involved 
in personality traits. Effects of administer -
ing a drug (vs. a placebo) can be assessed 
through effects on behavior or on biologi -
cal parameters measured through one of the 
three categories of assessment already de -
scribed. If the effects of the pharmacological 
manipulation are moderated by a trait, this 
suggests that variation in the trait is related 
to variation in the neural system targeted by 
the drug.
5. Molecular genetics . The study of varia -
tion in DNA can be considered neuroscience 
when it focuses on genes expressed in the 
brain. Molecular genetics has been in a rapid 
state of flux as a field. Much early work on 
personality relied on candidate gene stud -
ies, targeting variation in specific genes that 
seemed likely to be linked to personality. 
Unfortunately, results have been largely in -
consistent, and most candidate gene studies 
have failed to replicate. Such failures do not 
indicate a lack of genetic influences on per -
sonality, as all personality traits are substan -
tially heritable (Johnson & Krueger, 2004; 
Loehlin, McCrae, Costa, & John, 1998; 
Riemann, Angelitner, & Strelau, 1997). 
Rather, they indicate that complex traits 
are massively polygenic, influenced by many 
thousands of variations in the genome, most 
of which have only miniscule effects on a 
given trait (Munafò & Flint, 2011). Because 
of these challenges, we largely avoid citing 
candidate gene studies.
These diverse methods allow research on 
the neural correlates of personality traits. 
An important caveat is that such research 