Generating words according to a given rule relies on retrieval-related search and postretrieval control processes. Using fMRI, we recently characterized neural patterns of word generation in response to episodic, semantic, and phonemic cues by comparing free recall of wordlists, category fluency, and letter fluency [Shapira-Lichter, I., Oren, N., Jacob, Y., Gruberger, M., & Hendler, T. Portraying the unique contribution of the default mode network to internally driven mnemonic processes. Proceedings of the National Academy of Sciences, U.S.A., 110, 4950–4955, 2013]. Distinct selectivity for each condition was evident, representing discrete aspects of word generation-related memory retrieval. For example, the precuneus, implicated in processing spatiotemporal information, emerged as a key contributor to the episodic condition, which uniquely requires this information. Gamma band is known to play a central role in memory, and increased gamma power has been observed before word generation. Yet, gamma modulation in response to task demands has not been investigated. To capture the task-specific modulation of gamma power, we analyzed the EEG data recorded simultaneously with the aforementioned fMRI, focusing on the activity locked to and immediately preceding word articulation. Transient increases in gamma power were identified in a parietal electrode immediately before episodic and semantic word generation, however, within a different time frame relative to articulation. Gamma increases were followed by an alpha-theta decrease in the episodic condition, a gamma decrease in the semantic condition. This pattern indicates a task-specific modulation of the gamma signal corresponding to the specific demands of each word generation task. The gamma power and fMRI signal from the precuneus were correlated during the episodic condition, implying the existence of a common cognitive construct uniquely required for this task, possibly the reactivation or processing of spatiotemporal information.

The ability to selectively perceive items in the environment may be modulated by the emotional content of those items. The neural mechanism that underlies the privileged processing of emotionally salient content is poorly understood. Here, using fMRI, we investigated this issue via a binocular rivalry procedure when face stimuli depicting fearful or neutral expressions competed for awareness with a house. Results revealed an interesting dissociation in the amygdala during rivalry condition: Whereas its dorsal component exhibited dominant activation to aware fearful faces, a ventral component was more active during the suppression of fearful faces. Moreover, during rivalry, the dorsal and ventral components of the amygdala were coupled with segregated cortical activations in the brainstem and medial PFC, respectively. In summary, this study points to a differential involvement of two clusters within the amygdala and their connected networks in naturally occurring perceptual biases of emotional content in faces.

Interpretation of emotional context is a pivotal aspect of understanding social situations. A critical component of this process is assessment of danger levels in the surrounding, which may have a direct effect on the organism's survival. The limbic system has been implicated in mediating this assessment. In situations of uncertainty, the evaluation process may also call for greater involvement of prefrontal cortex for decision-making and planning of an appropriate behavioral response. In the following study, morphed face images depicting emotional expressions were used to examine brain correlates of subjective uncertainty and perceptual ambiguity regarding danger. Fear and neutral expressions of 20 faces were morphed, and each of the face videos was divided into three sequences of equal length representing three levels of objective certainty regarding the expressions neutral, fear, and ambiguous. Sixteen subjects were scanned in a 1.5-T scanner while viewing 60 × 6-sec video sequences and were asked to report their subjective certainty regarding the level of danger surrounding the face on a four-level scale combining definite/maybe and danger/no-danger values. The individual responses were recorded and used as the basis for a “subjective protocol” versus an “objective protocol.” Significant activations of the amygdala, dorsomedial prefrontal cortex, and dorsolateral prefrontal cortex were observed under the subjective protocol of internally driven uncertainty, but not under objective stimuli-based ambiguity. We suggest that this brain network is involved in generating subjective assessment of social affective cues. This study provides further support to the “relevance detector” theory of the amygdala and implicates its importance to behavior relying heavily on subjective assessment of danger, such as in the security domain context.

Hemispheric lateralization of emotional processing has long been suggested, but its underlying neural mechanisms have not yet been defined. In this functional magnetic resonance imaging study, facial expressions were presented to 10 right-handed healthy adult females in an event-related visual half-field presentation paradigm. Differential activations to fearful versus neutral faces were observed in the amygdala, pulvinar, and superior colliculus only for faces presented in the left hemifield. Interestingly, the left hemifield advantage for fear processing was observed in both hemispheres. These results suggest a leftward bias in subcortical fear processing, consistent with the well-documented leftward bias of danger-associated behaviors in animals. The current finding highlights the importance of hemifield advantage in emotional lateralization, which might reflect the combination of hemispheric dominance and asymmetric interhemispheric information transfer.

Recent neuroimaging studies have described a differential activation pattern associated with specific object images (e.g., face-related and building-related activation) in human occipito-temporal cortex. However, it is as yet unclear to what extent this selectivity is due to differences in the statistics of local object features present in the different object categories, and to what extent it reflects holistic grouping processes operating across the entire object image. To resolve this question it is essential to use images in which identical sets of local features elicit the perception of different object categories. The classic Rubin vase-face illusion provides an excellent experimental set to test this question. In the illusion, the same local contours lead to the perception of different objects (vase or face). Here we employed a modified Rubin vase-face illusion to explore to what extent the activation in face-related regions is attributable to the presence of local face features, or is due to a more holistic grouping process that involves the entire face figure. Biasing cues (gratings and color) were used to control the perceptual state of the observer. We found enhanced activation in face-related regions during the “face profile” perceptual state compared to the “vase” perceptual state. Control images ruled out the involvement of the biasing cues in the effect. Thus, object-selective activation in human face-related regions entails global grouping processes that go beyond the local processing of stimulus features.