Adaptive behavior in daily living requires that individuals are able to learn in situations where feedback is unclear and potentially inconsistent. The present data confirm earlier findings that depressive illness compromises individuals' ability to use and respond to feedback. Specifically, individuals with MDD were more likely to reverse responding following misleading negative feedback. This effect appeared relatively specific to MDD, as similarly-depressed individuals with BD showed behavioral performance which did not differ significantly from that of controls, and also showed quantitatively-similar patterns of neural activity to the control group. An important strength of the present study is that all patients were unmedicated at the time of testing.
The neuroimaging findings indicate complementary mechanisms in prefrontal cortical and limbic circuitry for utilizing trial-by-trial feedback to optimize task performance. Flexible adaptation of behavior on the reversal task was accompanied by increased hemodynamic activity in the vlPFC and dmPFC. In addition, healthy subjects displayed deactivation, presumably signifying a reduction in neural transmission (
Drevets et al., 1995;
Drevets and Raichle, 1998), in an amygdala ROI, in response to negative feedback compared to correct responses. This reduced amygdala activity was directly predictive of their ability to suppress reversal in response to misleading feedback. Recent work has suggested that the lateral and medial PFC may implement top-down control over amygdala activity in situations where subjects are required to inhibit or re-appraise emotional responses to negatively-valenced stimuli (
Ochsner et al., 2002;
Pezawas et al., 2005;
Phelps and LeDoux, 2005). The amygdala forms part of a limbic circuit comprising the ventral PFC, ventral anterior cingulate and ventral striatum that has wide-ranging involvement in processing motivational salience and emotional learning and memory (
Phelps and LeDoux, 2005;
Phillips et al., 2003).
The response to task feedback contains both emotional and informational components. Psychological theories of depression have highlighted an increased sensitivity to negative emotional information (
Gotlib et al., 2004;
Murphy et al., 2003), thought to contribute to the instantiation of a negative cycle (
Teasdale, 1983). In addition, depressed individuals are unable to use the informational component conveyed in feedback to facilitate task performance (
Murphy et al., 2003). Translational models of depression have highlighted how affective responses may be regulated by top-down control processes (
Amat et al., 2005;
Robbins, 2005), which may confer resilience to affective illness in the healthy population. For example, it is known that a perceived lack of control over one's environment leads to future inaction, which has provided the influential cognitive model of depression termed “learned helplessness” (
Seligman, 1972).
Phillips et al. (2003) proposed a dorsal system involved in the regulation of affective states and consequent behavior in an effortful manner and a ventral system that modulates the automatic regulation of emotion. Decreases in dorsal PFC metabolism have been identified in depressed subjects with BD (
Ketter et al., 2001) and MDD (
Baxter et al., 1989). In contrast, increased activity has been identified in the ventral PFC during the depressed phase of MDD (
Drevets et al., 1992) and BD (
Ketter et al., 2001).
Goel and Dolan (2003) identified differential activation between dorsal/dorsolateral and ventromedial PFC networks during ‘hot’ and ‘cold’ reasoning, with increased dorsal/lateral PFC activity and reduced ventromedial PFC activity during ‘cold’ reasoning and a reciprocal pattern for ‘hot’ reasoning. This supports the argument that increased activity in dorsal and lateral areas in HC and BD subjects may be associated with attempts to exert cognitive control over an emotional response. It should be noted that activity in the dmPFC and vlPFC was particularly elevated in HC and BD subjects when they incorrectly reversed their responses following misleading negative feedback.
Drevets et al. (1998) proposed that increased activity in these areas may compensate for or modulate activation in limbic structures during emotional processing. Activity in dorsal PFC has been associated with suppression of sad responses (
Levesque et al., 2003). Increased dmPFC and vlPFC activity in the HC and BD groups may be central to the ability to maintain correct responding in the face of misleading negative feedback and in suppressing responses to the immediate emotional feedback.
Behavioral reversal on the PRL task activated a circuit comprising the vlPFC in the control group, consistent with earlier work using this task (
Cools et al., 2002) (see
Supplementary material). The vlPFC signal change was previously shown to be significantly greater on reversal switches than on other trials where negative feedback was received, but where subjects did not switch responding (
Cools et al., 2002). The vlPFC may play a role in behavioral inhibition, by modulating the transmission of motivational information from limbic areas to the motor system.
O'Doherty et al. (2003) identified an adjacent area of caudolateral OFC where activity increased during trials on which punishing feedback resulted in a response reversal on the next trial. In the present data, increased vlPFC activity in response to negative feedback was correlated with subjects' ability to inhibit inappropriate switches, in the BD group. This further supports the role for this region in behavioral inhibition.
Some limitations should be noted. The final group sizes were relatively small (
n=12–15 per group) after several subjects were excluded due to excessive movement, and these findings merit replication in larger samples. The between-group differences in reversal- and feedback-related brain activity were not statistically significant at the whole-brain level after voxel-wise correction for multiple comparisons, but were evident in
a priori regions of interest in the PFC (vlPFC, dmPFC) and amygdala. The PFC ROIs were defined as spheres of 10 mm radius around the peak voxels from an earlier experiment using the same task (
Cools et al., 2002), in order to capitalise on the high degree of replicability in the pattern of frontal activation associated with this task. Nonetheless, we accept that spherical ROIs do not capture the complex morphology of these frontal subregions. In order to test our predictions of amygdala dysregulation in mood disorders, we used anatomical ROIs based on the AAL template (
Tzourio-Mazoyer et al., 2002), given that the earlier work using the PRL task had not identified any changes in amygdala signal (
Cools et al., 2002;
Evers et al., 2005). It is likely that the earlier studies may have overlooked the amygdala signal change through the use of one-tailed statistical tests that would only detect regions where negative feedback was associated with signal
increases.
In conclusion, this novel study in unmedicated, depressed subjects with mood disorders clarifies the mechanism by which the normal processing of feedback is dysfunctional in MDD. The MDD subjects showed attenuated prefrontal cortical responses during reversal shifting and additionally failed to deactivate the amygdala in response to misleading feedback relative to positive feedback. This latter response was predictive of healthy subjects' capacity to ignore misleading feedback and may confer resilience in the face of uncontrollable or stressful situations. Notably, abnormalities in response to misleading negative feedback appeared specific to unipolar depressive illness, as they did not extend to BD subjects who scored similarly on depression severity ratings. These findings hold profound implications for pharmacological and psychological approaches to treatment in unipolar and bipolar depressions.