Geoffrey Brookshire

Electrically stimulating prefrontal cortex at retrieval improves recollection accuracy

Neuroimaging and brain damage studies suggest that dorsolateral prefrontal cortex (dlPFC) is involved in the cognitive control of episodic recollection. If dlPFC is causally involved in retrieval, then transcranial direct current stimulation (tDCS) of this brain region should increase recollection accuracy, especially when recollection is difficult and requires cognitive control. Here, we report the first brain stimulation experiment to directly test this hypothesis. We administered tDCS to dlPFC immediately after studying to-be-learned material but just prior to recollection testing, thereby targeting retrieval processes. We found that stimulation of dlPFC significantly increased recollection accuracy, relative to a no-stimulation sham condition and also relative to active stimulation of a comparison region in left parietal cortex. There was no significant difference in the size of this increase between hemispheres. Moreover, these dlPFC stimulation effects were behaviorally selective, increasing accuracy only when participants needed to recollect difficult information. Electrically stimulating dlPFC allowed people to more accurately recollect specific details of their experiences, demonstrating a causal role of dlPFC in the retrieval of episodic memories.

Visual cortex entrains to sign language

Significance Language comprehension is thought to rely on a combination of specialized and general-purpose neural mechanisms. When people listen to speech, low-frequency oscillations in cerebral cortex (<8 Hz) become entrained to quasirhythmic fluctuations in volume. Entrainment of auditory cortex to speech rhythms has been well documented, but its functional significance has remained unclear. By showing similar entrainment of visual cortex to sign language, we establish that this phase-locking is not due to specific properties of auditory cortex or of oral speech perception. Rather, low-frequency entrainment is a generalized cortical strategy for boosting perceptual sensitivity to informational peaks in time-varying signals. Despite immense variability across languages, people can learn to understand any human language, spoken or signed. What neural mechanisms allow people to comprehend language across sensory modalities? When people listen to speech, electrophysiological oscillations in auditory cortex entrain to slow (<8 Hz) fluctuations in the acoustic envelope. Entrainment to the speech envelope may reflect mechanisms specialized for auditory perception. Alternatively, flexible entrainment may be a general-purpose cortical mechanism that optimizes sensitivity to rhythmic information regardless of modality. Here, we test these proposals by examining cortical coherence to visual information in sign language. First, we develop a metric to quantify visual change over time. We find quasiperiodic fluctuations in sign language, characterized by lower frequencies than fluctuations in speech. Next, we test for entrainment of neural oscillations to visual change in sign language, using electroencephalography (EEG) in fluent speakers of American Sign Language (ASL) as they watch videos in ASL. We find significant cortical entrainment to visual oscillations in sign language <5 Hz, peaking at ∼1 Hz. Coherence to sign is strongest over occipital and parietal cortex, in contrast to speech, where coherence is strongest over the auditory cortex. Nonsigners also show coherence to sign language, but entrainment at frontal sites is reduced relative to fluent signers. These results demonstrate that flexible cortical entrainment to language does not depend on neural processes that are specific to auditory speech perception. Low-frequency oscillatory entrainment may reflect a general cortical mechanism that maximizes sensitivity to informational peaks in time-varying signals.