Hemispheric Asymmetry in Emotional Face Processing: An Event-Related fMRI Study

Abstract

The lateralization of emotional processing has been debated for decades. We used event-related fMRI to examine whether the right hemisphere dominates emotion recognition in facial expressions. Eighteen participants viewed briefly-presented happy, sad, and neutral faces during 3T scanning. Right amygdala and inferior temporal regions showed selective responses to emotional versus neutral faces, supporting right-hemisphere primacy.

Introduction

The question of hemispheric specialization in emotional processing has persisted since the classical work of Sackheim and colleagues in the 1970s. While behavioural and lesion studies have suggested right-hemisphere dominance for emotion recognition, neuroimaging evidence has been mixed. The advent of event-related fMRI offers unprecedented temporal and spatial resolution for characterizing the neural basis of emotional face processing, allowing researchers to isolate the precise brain regions and timing associated with emotion detection.

Previous work has predominantly used static, sustained paradigms that may not reflect the rapid, automatic nature of emotional perception in natural settings. By employing brief stimulus presentations (150 ms) and matched timing across emotional categories, the present study provides a more ecologically valid test of hemispheric contributions to emotion recognition. We hypothesized that structures within the right hemisphere, particularly the amygdala and temporal cortex, would show selective amplification for emotional faces.

Method

Participants

Eighteen healthy, right-handed undergraduate volunteers (mean age 21.3 years, SD 1.8; 10 female) participated in the study. All participants had normal or corrected-to-normal vision, no history of neurological or psychiatric illness, and were screened for MRI contraindications. Participants provided informed consent in accordance with institutional review board guidelines.

Procedure

Stimuli consisted of 96 high-contrast, grayscale photographs of emotional expressions from the NimStim database (48 happy, 24 sad, 24 neutral). Each face was presented for 150 ms during a 3-second inter-stimulus interval, permitting sustained hemodynamic response measurement. The task required participants to indicate whether each face was "happy" or "sad" (neutral faces were classified as "neither"). Responses were collected via button press using an MRI-compatible response device. Two functional runs of 192 trials (4 min 44 sec per run) were acquired. A high-resolution T1-weighted anatomical image was also acquired for registration.

Functional images were acquired using a Siemens 3T Magnetom scanner with a gradient-echo echo-planar sequence (TR 2000 ms, TE 30 ms, 32 axial slices, 3.5 mm isotropic voxels). Data were preprocessed using SPM2: realignment, slice-timing correction, spatial normalization to MNI space, and Gaussian smoothing (8 mm FWHM). Event-related activation maps were generated using a canonical hemodynamic response function with temporal and dispersion derivatives.

Results

Behavioural accuracy was high across conditions (mean 89.3%, SD 6.1%), with no significant differences between emotional and neutral faces (F(1,17) = 1.22, p = 0.28). Reaction times showed expected main effects of emotion (F(2,34) = 12.45, p < 0.001), with faster responses to happy faces (M = 487 ms, SD = 102) than sad (M = 531 ms, SD = 118) or neutral (M = 515 ms, SD = 109) faces.

Neuroimaging results revealed robust bilateral amygdala activation for emotional versus neutral faces (right: MNI 22, -8, -14; t(17) = 4.32, p < 0.001; left: MNI -20, -6, -16; t(17) = 3.18, p = 0.006). The right inferior temporal gyrus (MNI 54, -48, -12; t(17) = 5.01, p < 0.001) showed significantly greater activation for emotional faces compared with neutral, with a rightward asymmetry (right > left contrast at p < 0.05). No significant differences emerged between happy and sad expressions in these core regions.

Discussion

Our findings provide converging evidence for right-hemisphere specialization in the detection of emotional facial expressions, at least when emotions are processed automatically and rapidly. The selective activation of right amygdala and temporal regions aligns with clinical observations from hemispheric lesion studies and supports contemporary theories of emotion lateralization. The lack of distinction between happy and sad faces in core affective regions suggests that emotional valence detection (emotional versus neutral) may rely on partially distinct neural circuits from emotional categorization.

These results have implications for understanding affective disorders characterized by emotion recognition deficits, including depression and social anxiety. Future work should examine whether medication or cognitive interventions normalizing right-hemisphere function might alleviate emotion processing difficulties. Additionally, longitudinal designs comparing clinical samples to healthy controls would clarify whether hemispheric asymmetries predispose to or result from affective pathology.

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