Ulla Martens

It is an integral function of the human brain to sample novel information from the environment and to update the internal representation of the external world. The formation of new memories is assumed to be orchestrated by neuronal oscillations, the rhythmic synchronization of neuronal activity within and across cell assemblies. Specifically, successful encoding of novel information is associated with increased theta oscillations (3-8Hz) and theta coupled gamma activity (40-120Hz), and a decrease in alpha oscillations (8-12Hz). However, given the correlative nature of neurophysiological recordings, the causal role of neuronal rhythms in human memory encoding is still unclear. Here, we experimentally enhance the formation of novel memories by a visual brain stimulation at an individually adjusted theta frequency, in contrast to the stimulation at an individual alpha frequency. Critically, the memory entrainment effect by the theta stimulation was not explained by theta power per se , but was driven by visually evoked theta-gamma coupling in wide spread cortical networks. These findings provide first evidence for a functional role of the theta rhythm and the theta-gamma neuronal code in human episodic memory. Yet more strikingly, the entrainment of mnemonic network mechanisms by a simplistic visual stimulation technique provides a proof of concept that internal rhythms align with visual pacemakers, which can entrain complex cognitive functions in the wake human brain.

The present study investigated implicit and explicit recognition processes of rapidly perceptually learned objects by means of steady-state visual evoked potentials (SSVEP). Participants were initially exposed to object pictures within an incidental learning task (living/non-living categorization). Subsequently, degraded versions of some of these learned pictures were presented together with degraded versions of unlearned pictures and participants had to judge, whether they recognized an object or not. During this test phase, stimuli were presented at 15 Hz eliciting an SSVEP at the same frequency. Source localizations of SSVEP effects revealed for implicit and explicit processes overlapping activations in orbito-frontal and temporal regions. Correlates of explicit object recognition were additionally found in the superior parietal lobe. These findings are discussed to reflect facilitation of object-specific processing areas within the temporal lobe by an orbito-frontal top-down signal as proposed by bi-directional accounts of object recognition.

Due to methodological problems in separating the brain responses to multiple stimuli, there is a profound lack of electrophysiological studies investigating object recognition in the context of natural scenes. We overcome this limitation by tagging coincidentally presented natural scenes and objects to different presentation frequencies. In due course, the scenes produced a different steady-state visual evoked potential (SSVEP) than the objects. We analyzed the effect of semantic (in)-coherency between natural scenes and a salient object on SSVEP amplitudes and topography. Additionally, the neuronal generators of the coherency effect were analyzed by means of Variable Resolution Electromagnetic Tomography (VARETA). Results showed: 1) An attentional focus on the scene in coherent and on the object in incoherent conditions. 2) Right inferior and middle frontal neuronal sources of the incoherency effect indicate a top-down attentional shift from the scene to the object. 3) Lateralization effects in the anterior temporal lobe indicate right hemispheric scene processing and left hemispheric object processing. Taken together, we state that the SSVEP is an ideal tool to investigate multiple stimuli displays with an excellent signal-to-noise ratio to investigate neuronal generators of higher cognitive functions. The attentional advantage of incoherent objects in a scene is likely the result of a mismatch between scene-based expectations (i.e. activated concepts) and activated semantic object representations.

In the present study, behavioral and electrophysiological markers of information processing—the lateralized readiness potential, the N170, and the P300—were recorded in order to assess the functional and temporal organization of facial identity and expression processing. A two-choice go/no-go task was used in which facial expression (happy vs. angry) determined response hand and response execution depended on facial familiarity (familiar vs. unfamiliar). The duration of facial identity and expression processing was manipulated in separate experiments. Together, the present findings in measures of overt and covert response activation indicate that facial identity is analyzed in parallel with, and typically somewhat faster than, facial expression. These data support a parallel model of face perception that assumes partial output from facial identity and expression processes to motor activation processes.

Classical theories assume that unconscious automatic processes are autonomous and independent of higher-level cognitive influences. In contrast, we propose that automatic processing depends on a specific configuration of the cognitive system by top-down control. In 2 experiments, we tested the influence of available attentional resources and previously activated task sets on masked semantic priming in a lexical decision task. In Experiment 1, before masked prime presentation, participants were engaged in an easy or hard primary task that differentially afforded attentional resources. Semantic priming was attenuated when the primary task was hard, that is, when only little attentional resources were available. In Experiment 2, a semantic or perceptual induction task differentially modulated subsequent masked semantic priming. Hence, unconscious automatic processing depends on the availability of attentional resources and is susceptible to top-down control.