Comparisons of sequentially presented vibrotactile frequencies have been extensively studied using electrophysiological recordings in nonhuman primates. Although neural signatures for working memory aspects of such tasks were recently also identified in human oscillatory EEG activity, homologue correlates of the comparison process are yet unknown. Here, we recorded EEG activity while participants decided which of two sequentially presented vibrotactile stimuli had a higher frequency. Because choices in this type of task are known to be systematically biased by the time-order effect, we applied Bayesian modeling to account for individual choice behavior. Using model-based EEG analysis, we found that upper beta band amplitude (∼20–30 Hz) was modulated by participants' choices. The modulation emerged ∼750 msec before a behavioral response was given and was source-localized to premotor areas.Importantly, the choice-dependent modulation of beta band amplitude was invariant to different motor response mappings and reflected the categorical outcome of the subjective comparison between the two frequencies. Consistently, this pattern was evident for both correct and incorrect trials, indicating that the beta band amplitude mirrors the internal representation of the comparison outcome. Our data complement previous findings in nonhuman primates and corroborate that the beta band activity in premotor areas reflects the categorical outcome of a sensory comparison prior to translation into an effector-specific motor command.

Do ongoing brain states determine conscious perception of an upcoming stimulus? Using the high temporal resolution of EEG, we investigated the relationship between prestimulus neuronal oscillations and the perceptibility of two competing somatosensory stimuli embedded in a backward masking paradigm. We identified two prestimulus EEG signatures predictive for a suprathreshold yet weak target stimulus to become perceptually resistant against masking by a stronger distractor stimulus: (i) over left frontal cortex a desynchronization of the regional beta rhythm (∼20 Hz) 500 msec prior to a perceived target, and (ii) a subsequent additional attenuation of both mu (∼10 Hz) and beta “idling” rhythms over those pericentral sensorimotor cortices which are going to process the upcoming target stimulus. Furthermore, across subjects the probability for target perception strongly correlates with the individual absolute level of pre-target amplitudes in these frequency bands and locations. These signatures significantly differed from the EEG characteristics preceding detected and undetected single stimuli. We suggest that the early activation of left frontal areas involved in top–down attentional control is critical for preventing backward masking and leads the preparation of primary sensory cortices: The ensuing prestimulus suppression of sensory idling rhythms warrants an intensified poststimulus processing, and thus, effectively promotes conscious perception of suprathreshold target stimuli embedded into an ecologically relevant condition featuring competing environmental stimuli.

We used concurrent TMS–fMRI to test directly for hemispheric differences in causal influences of the right or left fronto-parietal cortex on activity (BOLD signal) in the human occipital cortex. Clinical data and some behavioral TMS studies have been taken to suggest right-hemisphere specialization for top–down modulation of vision in humans, based on deficits such as spatial neglect or extinction in lesioned patients, or findings that TMS to right (vs. left) fronto-parietal structures can elicit stronger effects on visual performance. But prior to the recent advent of concurrent TMS and neuroimaging, it was not possible to directly examine the causal impact of one (stimulated) brain region upon others in humans. Here we stimulated the frontal or intraparietal cortex in the left or right hemisphere with TMS, inside an MR scanner, while measuring with fMRI any resulting BOLD signal changes in visual areas V1–V4 and V5/MT+. For both frontal and parietal stimulation, we found clear differences between effects of right- versus left-hemisphere TMS on activity in the visual cortex, with all differences significant in direct statistical comparisons. Frontal TMS over either hemisphere elicited similar BOLD decreases for central visual field representations in V1–V4, but only right frontal TMS led to BOLD increases for peripheral field representations in these regions. Hemispheric differences for effects of parietal TMS were even more marked: Right parietal TMS led to strong BOLD changes in V1–V4 and V5/MT+, but left parietal TMS did not. These data directly confirm that the human frontal and parietal cortex show right-hemisphere specialization for causal influences on the visual cortex.