PERSONALITY IS DEFINED as ingrained patterns of thought, feeling, and behavior characterizing an individual's unique lifestyle and model of adaptation resulting from constitutional factors, development, and social experience. It has conventionally been conceptualized as consisting of several factors or dimensions. Representative examples of hypothesis-driven and empirically based major personality models are the Temperament and Character Inventory and the NEO Personality Inventory–Revised (NEO-PI-R), respectively.1,2
The brain substrates associated with the five dimensions of the NEO-PI-R have been studied in healthy subjects. The neuroticism score of the NEO-PI-R was negatively correlated with the ratio of the brain volume to the remainder of the intracranial volume on magnetic resonance imaging (MRI) and with insula metabolism on positron emission tomography (PET).3,4 The extraversion score was positively correlated with orbitofrontal cortex metabolism on PET and with increased cerebral blood flow in the anterior cingulate gyrus, temporal lobes, and posterior thalamus on single-photon emission computed tomography (SPECT).4,5 Significant correlations have also been observed in patients with psychiatric disorders: the depression score (one of the subscales of neuroticism) was correlated with MRI abnormalities in early-onset Alzheimer's disease,6 the extraversion score was correlated with gray matter volume loss in the right posterior fusiform gyrus in patients with chronic schizophrenia,7 and the agreeableness score was positively correlated with right orbitofrontal lobe volume and negatively correlated with left orbitofrontal lobe volume in frontotemporal dementia patients.8
Near-infrared spectroscopy (NIRS) is a recently available functional brain imaging technique in which cerebral blood volume (CBV) changes are monitored by measuring increases in oxygenated hemoglobin concentration ([oxy-Hb]) and decreases in deoxygenated hemoglobin concentration ([deoxy-Hb]). NIRS is particularly suitable for personality studies because it enables measurement in a natural setting, compared with other functional brain imaging techniques such as PET and functional MRI.9–11 For example, NIRS enables examination of subjects in a sitting position, with their eyes open, and while speaking using a small apparatus by their bedside, with a fine time resolution.12 Due to its technical advantages, NIRS addresses one of the main problems of functional neuroimaging personality studies: lack of a natural setting during neuroimaging.
Here, we examined differential relationships between personality traits assessed using the NEO-PI-R and frontal and temporal lobe activation during a verbal fluency task (VFT) using multichannel NIRS in healthy male subjects. Activation changes in brain functions were able to be monitored in a natural setting. In addition to the usual task-activated condition, frontal lobe and temporal lobe activation was also examined under two motivationally modified versions of the task-activated condition: a monetary reward condition and a goal-oriented condition. These conditions were used because human behaviors are, in general, motivated by reward and social interaction, and because task-induced brain activation is considered to be modified by the motivational state of the subject at the time of examination and hence to be differentially related to personality depending on the type of motivation.13
We hypothesized that the personality dimensions of the NEO-PI-R would be correlated with brain activation, and that these relationships would vary between the monetary reward, goal-oriented, and control conditions.14,15 All of the dimensions of the NEO-PI-R could be correlated with brain activation, because motivation is assumed to consist of various aspects of brain function. For example, personality dimensions reflecting interpersonal relationships such as extraversion and agreeableness could be positively correlated with brain function, especially under the goal-oriented condition, because subjects with high extraversion or agreeableness are assumed to be more motivated in a natural setting with an examiner. Personality dimensions reflecting interpersonal relationships and emotional features such as openness and neuroticism could be positively and negatively correlated with brain function, respectively, particularly under the monetary reward and the goal-oriented conditions, because motivation is considered to be related to openness and to be decreased during high-tension situations such as an examination. In addition, personality dimensions reflecting an effort-related nature such as conscientiousness could be positively correlated with brain function, because motivation is closely correlated with effort.