Distinct modes of top-down cognitive processing in the ventral visual cortex.

Top-down cognitive control leads to changes in the sensory processing of the brain. In visual perception such changes can take place in the ventral visual cortex altering the functional asymmetry in forward and backward connections. Here we used fixation-related evoked responses of EEG measurement and dynamic causal modeling to examine hierarchical forward-backward asymmetry, while twenty-six healthy adults performed cognitive tasks that require different types of top-down cognitive control (memorizing or searching visual objects embedded in a natural scene image). The generative model revealed an enhanced asymmetry toward forward connections during memorizing, whereas enhanced backward connections were found during searching. This task-dependent modulation of forward and backward connections suggests two distinct modes of top-down cognitive processing in cortical networks. The alteration in forward-backward asymmetry might underlie the functional role in the cognitive control of visual information processing.

Task-dependent functional organizations of the visual ventral stream.

The visual hierarchy of the ventral stream has been widely studied. However, it remains unclear how the hierarchical system organizes its functional coupling during top-down cognitive process. The present fMRI study investigated task-dependent functional connectivity along the ventral stream, while twenty-eight participants performed object recognition tasks that required different types of visual processing: i) searching or ii) memorizing visual objects embedded in natural scene images or iii) free viewing of the same images. Utilizing a seed-based approach that explicitly compared task-specific BOLD time-series, we identified task-dependent functional connectivity of the visual ventral stream, demonstrating different correlation structures. Searching for a target object manifested both correlated and anti-correlated structures, separating the visual areas V1 and V4 from the posterior part of the inferior temporal cortex (PIT). In contrast, the ventral stream structure remained correlated during memorizing objects, but increased the correlation between the right V4 and PIT. On the other hand, V1 and V4 showed task-dependent activation, whereas PIT was deactivated. These results highlight the context-dependent nature of the visual ventral stream and shed light on how the visual hierarchy is selectively organized to bias object recognition toward features of interest.

Direct Electrophysiological Evidence for Prefrontal Control of Hippocampal Processing during Voluntary Forgetting.

Forgetting does not necessarily reflect failure to encode information but can, to some extent, also be voluntarily controlled. Previous studies have suggested that voluntary forgetting relies on active inhibition of encoding processes in the hippocampus by the dorsolateral prefrontal cortex (DLPFC) [1-4]. During attentional and sensorimotor processing, enhanced DLPFC theta power alongside increased alpha/beta oscillations are a neural signature of an inhibitory top-down mechanism, with theta oscillations reflecting prefrontal control and alpha/beta oscillations occurring in areas targeted by inhibition [5-12]. Here, we used intracranial EEG recordings in presurgical epilepsy patients implanted in DLPFC (n = 13) and hippocampus (n = 15) during an item-method directed forgetting paradigm. We found that voluntary forgetting is associated with increased neural oscillations in the low theta band (3-5 Hz) in DLPFC and in a broad theta/alpha/beta (6-18 Hz) frequency range in hippocampus. Combining time-lagged correlation analysis, phase synchronization, and Granger causality in 6 patients with electrodes in both DLPFC and hippocampus, we obtained converging evidence for a top-down control of hippocampal activity by the DLPFC. Together, our results provide strong support for a model in which voluntary forgetting relies on enhanced inhibition of the hippocampus by the DLPFC.

Human Hippocampal Dynamics during Response Conflict.

Besides its relevance for declarative memory functions, hippocampal activation has been observed during disambiguation of uncertainty and conflict. Uncertainty and conflict may arise on various levels. On the perceptual level, the hippocampus has been associated with signaling of contextual deviance and disambiguation of similar items (i.e., pattern separation). Furthermore, conflicts can occur on the response level. Animal experiments showed a role of the hippocampus for inhibition of prevailing response tendencies and suppression of automatic stimulus-response mappings, potentially related to increased theta oscillations (3-8 Hz). In humans, a recent fMRI study demonstrated hippocampal involvement in approach-avoidance conflicts. However, the more general significance of hippocampal activity for dealing with response conflicts also on a cognitive level is still unknown. Here, we investigated the role of the hippocampus for response conflict in the Stroop task by combining intracranial electroencephalography (iEEG) recordings from the hippocampus of epilepsy patients with region of interest-based fMRI in healthy participants. Both methods revealed converging evidence that the hippocampus is recruited in a regionally specific manner during response conflict. Moreover, our iEEG data show that this activation depends on theta oscillations and is relevant for successful response conflict resolution.