3.1. Image ratings
A 2 (Image Type: pleasant, neutral) × 2 (Group: MDD, control) MANOVA was performed for both valence and arousal ratings. For valence ratings, there was a significant main effect of image type, F(1,19) = 70.79, p < .00001, with pleasant images rated as more positive than neutral images. There was no significant main effect of group, F(1,19) = 0.77, p = 0.64, and the Image Type × Group interaction was not significant, F(1, 19) = 0.17, p = 0.68. For arousal ratings, no significant main effects or interactions were observed, p’s > 0.20.
3.2. Reaction time
A 2 (Reward Type: image, money) × 2 (Group: MDD, control) mixed-model MANOVA was performed to model button press reaction times in the scanner. There was a main effect of reward type, F(1, 20) = 12.53, p = 0.002, such that reaction times were significantly faster to money than image trials. There was no significant main effect of group or group × reward type interaction, F(1, 20) = 0.20, p = 0.66 and F(1, 20) = 0.33, p = 0.57, respectively. A second set of MANOVAs was performed for each reward type, comparing trials for which success was or was not possible (i.e., control trials). For both image and money trials, there was a significant main effect of success possibility, F(1, 20) = 31.40, p = 0.00002 and F(1, 20) = .22.75, p = 0.0001, respectively, such that participants responded more quickly when cued that success was possible. There were no significant main effects of group or success × group interactions, p’s > 0.60.
3.3. Imaging data
Anticipation and outcome phases were analyzed separately using a whole-brain analytic approach. Within- and between-group analyses were performed for both monetary and pleasant image rewards, examining potential win versus non-potential win contrasts during the anticipatory phase and for wins versus non-win contrasts during the outcome phase. In addition, Group × Reward type interaction analyses were performed. Between-groups and interaction results were masked by whole-sample (i.e., both groups combined) activation maps of Reward (images + money, images, or money) > Non-Reward contrasts for the appropriate task phase, therefore limiting significant results to regions activated by the task. See Supplemental Figure 1
for regions included in these masks. Within-group analyses were conducted to determine if the task elicited activation in typical reward-related regions for both MDD and control groups, consistent with previous literature (e.g., Knutson et al., 2008
). A more conservative z
score threshold of 3.5 was used for within-subjects analyses. Tables of within-groups results for both monetary and pleasant image rewards are available from the authors upon request. Activation localizations were based on Harvard-Oxford cortical and subcortical structural probabilistic atlases, with Brodmann area identification via Talairach Daemon, as implemented in FSLView v3.0.
3.4. Within-Groups Results for Monetary Rewards
With regards to anticipation of monetary win versus non-win trials in the control group, results were largely consistent with previous studies (e.g., Knutson et al., 2008
). Activation was observed in bilateral thalamus, extending to brainstem and through the striatum including caudate and nucleus accumbens to the subcallosal cortex. Activation was also observed in dorsal anterior cingulate and paracingulate gyrus, extending to supplementary motor cortex, as well as in left orbitofrontal cortex. In addition to the thalamo-striatal and frontal activations, clusters were observed in bilateral cerebellum, occipital fusiform, lateral occipital cortex, supramarginal gyrus, insula, and right precuneus and superior parietal lobule. MDD participants showed activation in bilateral thalamus, pallidum, caudate, accumbens and putamen; bilateral insula, with activation extending to orbitofrontal cortex on the left; anterior cingulate; and supplementary motor cortex; as well as bilateral cerebellum, right central opercular cortex, and left lateral occipital cortex.
With regards to monetary win versus non-win outcomes, win outcomes were associated with activation in right insula and lingual gyrus in both control and MDD groups. In addition, left cerebellum and occipital fusiform were activated in the control group.
3.5. Within-Groups Results for Pleasant Image Rewards
With regards to anticipation of pleasant image win versus non-win trials in the control group, anticipation of a positive image was associated with activation in right caudate and bilateral supplementary motor cortex. Winning outcomes were associated with activation in right intercalcarine cortex. Pleasant image win versus non-win contrasts were not associated with activation in any regions in the MDD group for either anticipation or outcome phases.
3.6. Within-Groups Comparisons between Reward Types
Within-group analyses were conducted to compare activations between reward types. Activation tables are available from the authors by request; see Supplemental Figure 2
. Within the control group, regions that showed greater activation to anticipation of monetary than pleasant image rewards included right subcallosal cortex, bilateral anterior paracingulate gyrus, left anterior cingulate, bilateral insula, bilateral hippocampus, left central opercular cortex, parahippocampal gyrus extending to lingual gyrus, bilateral thalamus extending to caudate, right precuneus, several bilateral regions of cerebellum, and regions of occipital cortex. Among the MDD group, greater activation to monetary than pleasant image rewards was observed in bilateral anterior cingulate and right paracingulate, right insula, bilateral parahippocampal gyrus, left thalamus, right putamen, left pallidum, right supramarginal gyrus, and several bilateral regions of cerebellum and occipital cortex. Neither the control group nor the MDD group demonstrated regions of greater activation to pleasant image than monetary rewards.
In the outcome phase, the control group showed greater activation to pleasant image than monetary rewards in right precentral gyrus, superior parietal lobe, and lateral occipital cortex. No regions showed greater activation to monetary rewards. In the MDD group, greater activation was observed for monetary than pleasant image rewards in right precuneus, right lateral occipital cortex, bilateral lingual gyrus, and right cerebellum. No regions showed greater activation to pleasant image than monetary rewards in the MDD group.
3.7. Between-Groups Comparisons for Monetary Rewards
Comparisons between the MDD and control groups were performed separately for anticipation of win versus non-win monetary rewards and for actual win versus non-win monetary outcomes. For anticipation, MDD participants showed greater activation than controls in a medial region of frontal pole. Control participants showed greater activation than the MDD group in anterior cingulate, right orbitofrontal cortex, right lateral frontal pole, left frontal operculum, left hippocampus, left planum polare, bilateral temporal pole, and regions of occipital cortex. See . For outcome, no regions showed greater activation for MDD than control participants. Control participants showed greater activation than the MDD group in right temporal pole (see ).
Table 2 Clusters showing significant group differences and Group × Reward Type interactions for anticipation of a) money > pleasant image, b) money, and c) pleasant image rewards. All voxels within the cluster are z > 2.58, with a minimum (more ...)
Table 3 Clusters showing significant group differences and Group × Reward Type interactions for outcome of a) money > pleasant image, b) money, and c) pleasant image rewards. All voxels within the cluster are z > 2.58, with a minimum of (more ...)
3.8. Between-Groups Comparisons for Pleasant Image Rewards
Comparisons between the MDD and control groups were performed separately for anticipation of win versus non-win pleasant image rewards and for win versus non-win pleasant image outcomes. For both anticipation and outcome, no regions showed greater activation for MDD than control participants. For anticipation, control participants showed greater activation than the MDD group in clusters in the anterior cingulate cortex and paracingulate gyrus, right middle frontal gyrus, right pallidum, right parahippocampal gyrus, bilateral precentral gyrus, precuneus, and areas of brainstem (see ). For outcome, control participants showed greater activation than the MDD group in clusters in bilateral frontal pole, anterior cingulate cortex, right caudate and putamen, right precentral gyrus, bilateral precuneous, right posterior cingulate gyrus, right intracalcarine cortex, left lateral occipital cortex, and bilateral occipital fusiform gyrus (see ).
3.9. Anticipatory Responses to Monetary versus Pleasant Image Rewards
One striatal region, the right putamen, showed a significant Group × Reward Type interaction, with MDD participants showing a stronger Money > Pleasant Image contrast than controls during win anticipation. Regions that showed greater Money > Pleasant Image activation for control than MDD participants included clusters within right hippocampus, thalamus, and occipital pole (see and ).
Key brain areas showing significant Group × Reward Type effects during reward anticipation and outcome. Starred bars on graphs differ at p < .05 in follow-up t tests.
3.10. Monetary versus Pleasant Image Reward Outcomes
Regions that showed a significant Group × Reward Type interaction, with MDD participants showing a stronger Money > Pleasant Image contrast than controls during win versus non-win outcomes, included clusters within the right precentral gyrus, insula, intracalcarine cortex, several occipital regions, and right cerebellum. No regions showed greater Money > Pleasant Image contrasts for control than MDD participants (see and )2