Functional magnetic resonance imaging (fMRI) of the human brain was used to study whether the amygdala is activated in response to emotional stimuli, even in the absence of explicit knowledge that such stimuli were presented. Pictures of human faces bearing fearful or happy expressions were presented to 10 normal, healthy subjects by using a backward masking procedure that resulted in 8 of 10 subjects reporting that they had not seen these facial expressions. The backward masking procedure consisted of 33 msec presentations of fearful or happy facial expressions, their offset coincident with the onset of 167 msec presentations of neutral facial expressions. Although subjects reported seeing only neutral faces, blood oxygen level-dependent (BOLD) fMRI signal in the amygdala was significantly higher during viewing of masked fearful faces than during the viewing of masked happy faces. This difference was composed of significant signal increases in the amygdala to masked fearful faces as well as significant signal decreases to masked happy faces, consistent with the notion that the level of amygdala activation is affected differentially by the emotional valence of external stimuli. In addition, these facial expressions activated the sublenticular substantia innominata (SI), where signal increases were observed to both fearful and happy faces--suggesting a spatial dissociation of territories that respond to emotional valence versus salience or arousal value. This study, using fMRI in conjunction with masked stimulus presentations, represents an initial step toward determining the role of the amygdala in nonconscious processing.

Dorsal anterior cingulate cortex (dACC) is a brain region that subserves cognition and motor control, but the mechanisms of these functions remain unknown. Human neuroimaging and monkey electrophysiology studies have provided valuable insights, but it has been difficult to link the two literatures. Based on monkey single-unit recordings, we hypothesized that human dACC is comprised of a mixture of functionally distinct cells that variously anticipate and detect targets, indicate novelty, influence motor responses, encode reward values, and signal errors. As an initial test of this conceptualization, the current event-related functional MRI study used a reward-based decision-making task to isolate responses from a subpopulation of dACC cells sensitive to reward reduction. As predicted, seven of eight subjects showed significant (P < 10 ؊4 ) dACC activation when contrasting reduced reward (REDrew) trials to fixation (FIX). Confirmatory group analyses then corroborated the predicted ordinal relationships of functional MRI activation expected during each trial type (REDrew > SWITCH > CONrew > FIX). The data support a role for dACC in reward-based decision making, and by linking the human and monkey literatures, provide initial support for the existence of heterogeneity within dACC. These findings should be of interest to those studying reward, cognition, emotion, motivation, and motor control. ʈ A nterior cingulate cortex (ACC) lies on the medial surfaces of the brain's frontal lobes and encompasses subdivisions that play key roles in cognitive, motor, and emotional processing (1). Dorsal ACC (dACC) in humans includes cortex on the dorsal and ventral banks of the cingulate sulcus, and overlaps the territory occupied by the rostral cingulate motor area (CMAr) in monkeys (2, 3), which has projections directly to the spinal cord (4) and motor and limbic cortices (5). Convergent data (6, 7) has established that dACC specifically subserves cognition (8) and motor control (9), but the mechanisms by which this region operates have not been elucidated. Based primarily on work in humans, different functions have been ascribed to this area, including attention-for-action͞target selection (10, 11), motivational valence assignment (12), motor response selection (13-15), error detection͞performance monitoring (16, 17), competition monitoring (18), anticipation (19), working memory (20), novelty detection (21), and reward assessment (22), but no single unifying model explains the diverse results from neuroimaging and electrophysiological studies (1). In addition to the intrinsic importance of providing new information about normal cognition and motor control, determining how dACC works is essential because abnormalities of different ACC subdivisions have been implicated in the pathophysiology of many neuropsychiatric disorders (23).Single-unit recording studies have confirmed heterogeneity in monkey CMAr. Niki and Watanabe (24) identified timing (stimulus anticipation) units, and others sensitive to targets, motor responses, rewa...