While the sense of hearing is clearly the dominant channel for speech perception, humans are surprisingly good at reading the lips of one’s conversation partners, a phenomenon referred to as speech reading. This ability has been demonstrated already in early psychophysics studies to significantly enhance speech perception, especially when speech is to be perceived under noisy conditions. There is a fairly good amount of neuroimaging literature on the underlying neural mechanisms. In these studies, visual speech stimuli (i.e., articulatory gestures) have been reported to modulate auditory cortical processing, with some evidence pointing to speech motor system first being activated by visual speech and then influencing auditory-cortical processing via an efference copy.
In their recent study, Chu et al. (2013) studied the neural basis of speech reading by presenting 19 healthy volunteers with silent videoclips of a person articulating vowels during event-related functional magnetic resonance imaging (fMRI). Speech reading activated a wide range of occipital, temporal, and prefrontal cortical areas. The authors used structural equation modeling to estimate information flow during speech reading between the activated areas. The results suggested that there is parallel information flow from extrastriate areas to anterior prefrontal areas and, further, feedback information flow from the anterior prefrontal areas to posterior-superior temporal lobe auditory areas. These effective connectivity estimates thus support the model wherein speech reading influences auditory-cortical areas via prefrontal speech motor areas, possibly in the form of an efference copy that might facilitate speech perception.
Reference: Chu Y-H, Lin F-H, Chou Y-J, Tsai K W-K, Kuo W-J, Jaaskelainen IP. Effective cerebral connectivity during silent speech reading revealed by functional magnetic resonance imaging. PLoS ONE (2013) 8: e80265. http://dx.doi.org/10.1371/journal.pone.0080265
In the scientific quest to unravel the neural basis of many perceptual and cognitive functions, animal models are very important in complementing the findings obtained in non-invasive human neuroimaging studies. Furthermore, even though there are many species-specific aspects to cognition (e.g., human language), for those perceptual-cognitive functions that do generalize across species, animal models often offer the only possibility to test decisively between alternative hypotheses. Further, development of animal research methods is advancing at astounding speed. Two-photon calcium imaging is a relatively new method that allows simultaneous recording from large (~hundreds) populations of neurons, however, the method has been limited to recording from limited number of cortical layers at a time, and it has not been possible to record the neural populations over extended periods of time, which would be very useful in studies of, for example, the neural basis of various types of learning.
With the method recently published by Andermann et al. (2013) it is now possible to record extensive populations of neurons simultaneously from all six cortical layers over extended periods of time, even for months. The authors surgically implanted glass microprisms in somatosensory and visual cortical areas of mice, which then allowed chronic two-photon imaging of hundreds of neurons and from all layers simultaneously, in awake animals. The authors point out that their novel methodology, when combined with advances in genetic, pharmacological, and optogenetic methods (using which individual neurons in a population can be selectively suppressed and excited), can considerably expand the highly exciting capabilities offered by two-photon imaging in animal-model studies of the neural basis of perceptual and cognitive functions.
Reference: Andermann ML, Gilfoy NB, Goldey GJ, Sachdev RNS, Wölfel M, McGormick DA, Reid RC, Levene MJ. Chronic cellular imaging of entire cortical columns in awake mice using microprisms. Neuron (2013) e-publication ahead of print. http://dx.doi.org/10.1016/j.neuron.2013.07.052
Visual-cortex GABA concentrations predict incidence of cognitive failures in daily life in healthy volunteers
Since the amount of information one receives in daily life by far exceeds the limited capacity of one’s processing resources, selecting relevant information and suppressing irrelevant information is a vital ability. The link between this cognitive ability, termed selective attention, and cognitive failures in daily life (e.g., failing to notice things, getting distracted) is well established. On the other hand, gamma-aminobutyric acid (GABA), the most common inhibitory neurotransmitter in the human brain, has been observed to contribute to visual cortex selectivity to stimuli, a function that is an integral part of selective attention. What has not been investigated before, however, is whether inter-individual variability in the amount of visual-cortical GABA is linked with the frequency of cognitive failures in daily life.
In their recent study, Sandberg et al. (2013) had 36 healthy participants fill out a cognitive failures questionnaire, where participants were asked to self-rate frequency with which they experience common cognitive failures in perception, memory, and motor function. They then underwent 3T whole-head structural magnetic resonance imaging and focal magnetic resonance spectroscopy measurement of GABA concentration was obtained with two voxels placed in 1) calcarine sulcus in occipital cortex and 2) in the anterior part of the superior parietal lobule. It was observed that GABA concentrations in the visual cortex correlated with the incidence of self-reported cognitive failures. In contrast, the authors failed to see any correlation between GABA concentrations in the parietal voxel and cognitive failures. The authors however observed that gray matter volume in left superior parietal lobule and occipital GABA concentration independently predicted cognitive failures.
These exciting results first of all demonstrate nicely that it is possible to predict inter-individual variability in cognitive failures that take place in daily life with inter-individual differences in local brain neurochemical properties. The results further add an important piece of evidence pointing to the role of GABA in cognitive processing by suggesting that visual-cortical GABA concentrations impact selective attention under ecologically valid conditions as estimated by the questionnaire items. Third, these results suggest that the role of GABA in modulating selective attention is specific to the sensory cortical areas, whereas gray matter volume in parietal cortex additionally contributes to frequency of cognitive failures in daily life.
Reference: Sandberg K, Blicher JU, Dong MY, Rees G, Near J, Kanai R. Occipital GABA correlates with cognitive failures in daily life. Neuroimage (2013) e-publication ahead of print. http://dx.doi.org/10.1016/j.neuroimage.2013.10.059