Manuel Gomez-Ramirez

The strength of anticipatory spatial biasing predicts target discrimination at attended locations: a high-density EEG study

Cueing relevant spatial locations in advance of a visual target results in modulated processing of that target as a consequence of anticipatory attentional deployment, the neural signatures of which remain to be fully elucidated. A set of electrophysiological processes has been established as candidate markers of the invocation and maintenance of attentional bias in humans. These include spatially‐selective event‐related potential (ERP) components over the lateral parietal (around 200–300 ms post‐cue), frontal (300–500 ms) and ventral visual (> 500 ms) cortex, as well as oscillatory amplitude changes in the alpha band (8–14 Hz). Here, we interrogated the roles played by these anticipatory processes in attentional orienting by testing for links with subsequent behavioral performance. We found that both target discriminability (d’) and reaction times were significantly predicted on a trial‐by‐trial basis by lateralization of alpha‐band amplitude in the 500 ms preceding the target, with improved speed and accuracy resulting from a greater relative decrease in alpha over the contralateral visual cortex. Reaction time was also predicted by a late posterior contralateral positivity in the broad‐band ERP in the same time period, but this did not influence d’. In a further analysis we sought to identify the control signals involved in generating the anticipatory bias, by testing earlier broad‐band ERP amplitude for covariation with alpha lateralization. We found that stronger alpha biasing was associated with a greater bilateral frontal positivity at ∼390 ms but not with differential amplitude across hemispheres in any time period. Thus, during the establishment of an anticipatory spatial bias, while the expected target location is strongly encoded in lateralized activity in parietal and frontal areas, a distinct non‐spatial control process seems to regulate the strength of the bias.

Oscillatory Sensory Selection Mechanisms during Intersensory Attention to Rhythmic Auditory and Visual Inputs: A Human Electrocorticographic Investigation

Oscillatory entrainment mechanisms are invoked during attentional processing of rhythmically occurring stimuli, whereby their phase alignment regulates the excitability state of neurons coding for anticipated inputs. These mechanisms have been examined in the delta band (1–3 Hz), where entrainment frequency matches the stimulation rate. Here, we investigated entrainment for subdelta rhythmic stimulation, recording from intracranial electrodes over human auditory cortex during an intersensory audiovisual task. Audiovisual stimuli were presented at 0.67 Hz while participants detected targets within one sensory stream and ignored the other. It was found that entrainment operated at twice the stimulation rate (1.33 Hz), and this was reflected by higher amplitude values in the FFT spectrum, cyclic modulation of alpha-amplitude, and phase–amplitude coupling between delta phase and alpha power. In addition, we found that alpha-amplitude was relatively increased in auditory cortex coincident with to-be-ignored auditory stimuli during attention to vision. Thus, the data suggest that entrainment mechanisms operate within a delimited passband such that for subdelta task rhythms, oscillatory harmonics are invoked. The phase of these delta-entrained oscillations modulates alpha-band power. This may in turn increase or decrease responsiveness to relevant and irrelevant stimuli, respectively.

The deployment of intersensory selective attention: a high-density electrical mapping study of the effects of theanine.

Objective: Ingestion of the nonproteinic amino acid theanine (5-N-ethylglutamine) has been shown to increase oscillatory brain activity in the so-called alpha band (8-14 Hz) during resting electroencephalographic recordings in humans. Independently, alpha band activity has been shown to be a key component in selective attentional processes. Here, we set out to assess whether theanine would cause modulation of anticipatory alpha activity during selective attentional deployments to stimuli in different sensory modalities, a paradigm in which robust alpha attention effects have previously been established. Methods: Electrophysiological data from 168 scalp electrode channels were recorded while participants performed a standard intersensory attentional cuing task. Results: As in previous studies, significantly greater alpha band activity was measured over parieto-occipital scalp for attentional deployments to the auditory modality than to the visual modality. Theanine ingestion resulted in a substantial overall decrease in background alpha levels relative to placebo while subjects were actively performing this demanding attention task. Despite this decrease in background alpha activity, attention-related alpha effects were significantly greater for the theanine condition. Conclusion: This increase of attention-related anticipatory alpha over the right parieto-occipital scalp suggests that theanine may have a specific effect on the brains attention circuitry. We conclude that theanine has clear psychoactive properties, and that it represents a potentially interesting, naturally occurring compound for further study, as it relates to the brains attentional system.

The development of multisensory speech perception continues into the late childhood years.

Observing a speaker’s articulations substantially improves the intelligibility of spoken speech, especially under noisy listening conditions. This multisensory integration of speech inputs is crucial to effective communication. Appropriate development of this ability has major implications for children in classroom and social settings, and deficits in it have been linked to a number of neurodevelopmental disorders, especially autism. It is clear from structural imaging studies that there is a prolonged maturational course within regions of the perisylvian cortex that persists into late childhood, and these regions have been firmly established as being crucial to speech and language functions. Given this protracted maturational timeframe, we reasoned that multisensory speech processing might well show a similarly protracted developmental course. Previous work in adults has shown that audiovisual enhancement in word recognition is most apparent within a restricted range of signal‐to‐noise ratios (SNRs). Here, we investigated when these properties emerge during childhood by testing multisensory speech recognition abilities in typically developing children aged between 5 and 14 years, and comparing them with those of adults. By parametrically varying SNRs, we found that children benefited significantly less from observing visual articulations, displaying considerably less audiovisual enhancement. The findings suggest that improvement in the ability to recognize speech‐in‐noise and in audiovisual integration during speech perception continues quite late into the childhood years. The implication is that a considerable amount of multisensory learning remains to be achieved during the later schooling years, and that explicit efforts to accommodate this learning may well be warranted.

Tactile shape discrimination recruits human lateral occipital complex during early perceptual processing

Neuroimaging studies investigating somatosensory‐based object recognition in humans have revealed activity in the lateral occipital complex, a cluster of regions primarily associated with visual object recognition. To date, determining whether this activity occurs during or subsequent to recognition per se, has been difficult to assess due to the low temporal resolution of the hemodynamic response. To more finely measure the timing of somatosensory object recognition processes we employed high density EEG using a modified version of a paradigm previously applied to neuroimaging experiments. Simple geometric shapes were presented to the right index finger of 10 participants while the ongoing EEG was measured time locked to the stimulus. In the condition of primary interest participants discriminated the shape of the stimulus. In the alternate condition they judged stimulus duration. Using traditional event‐related potential analysis techniques we found significantly greater amplitudes in the evoked potentials of the shape discrimination condition between 140 and 160 ms, a timeframe in which LOC mediated perceptual processes are believed to occur during visual object recognition. Scalp voltage topography and source analysis procedures indicated the lateral occipital complex as the likely source behind this effect. This finding supports a multisensory role for the lateral occipital complex during object recognition. Hum Brain Mapp, 2010.

Preserved executive function in high-performing elderly is driven by large-scale recruitment of prefrontal cortical mechanisms

High‐density electrical mapping of event‐related potentials was used to investigate the neural processes that permit some elderly subjects to preserve high levels of executive functioning. Two possibilities pertain: (1) high‐performance in elderly subjects is underpinned by similar processing mechanisms to those seen in young adults; that is, these individuals display minimal functional decay across the lifespan, or (2) preserved function relies on successfully recruiting and amplifying control processes to compensate for normal sensory‐perceptual decline with age. Fifteen young and nineteen elderly participants, the latter split into groups of high and low performers, regularly alternated between a letter and a number categorization task, switching between tasks every third trial (AAA‐BBB‐AAA…). This allowed for interrogation of performance during switch, repeat, and preparatory pre‐switch trials. Robust effects of age were observed in both frontal and parietal components of the task‐switching network. Greatest differences originated over prefrontal regions, with elderly subjects generating amplified, earlier, and more differentiated patterns of activity. This prefrontal amplification was evident only in high‐performing (HP) elderly, and was strongest on pre‐switch trials when participants prepared for an upcoming task‐switch. Analysis of the early transient and late sustained activity using topographic analyses and source localization collectively supported a unique and elaborated pattern of activity across frontal and parietal scalp in HP‐elderly, wholly different to that seen in both young and low‐performing elderly. On this basis, we propose that preserved executive function in HP‐elderly is driven by large‐scale recruitment and enhancement of prefrontal cortical mechanisms. Hum Brain Mapp, 2009.

“What” and “Where” in Auditory Sensory Processing: A High-Density Electrical Mapping Study of Distinct Neural Processes Underlying Sound Object Recognition and Sound Localization

Functionally distinct dorsal and ventral auditory pathways for sound localization (WHERE) and sound object recognition (WHAT) have been described in non-human primates. A handful of studies have explored differential processing within these streams in humans, with highly inconsistent findings. Stimuli employed have included simple tones, noise bursts, and speech sounds, with simulated left–right spatial manipulations, and in some cases participants were not required to actively discriminate the stimuli. Our contention is that these paradigms were not well suited to dissociating processing within the two streams. Our aim here was to determine how early in processing we could find evidence for dissociable pathways using better titrated WHAT and WHERE task conditions. The use of more compelling tasks should allow us to amplify differential processing within the dorsal and ventral pathways. We employed high-density electrical mapping using a relatively large and environmentally realistic stimulus set (seven animal calls) delivered from seven free-field spatial locations; with stimulus configuration identical across the “WHERE” and “WHAT” tasks. Topographic analysis revealed distinct dorsal and ventral auditory processing networks during the WHERE and WHAT tasks with the earliest point of divergence seen during the N1 component of the auditory evoked response, beginning at approximately 100 ms. While this difference occurred during the N1 timeframe, it was not a simple modulation of N1 amplitude as it displayed a wholly different topographic distribution to that of the N1. Global dissimilarity measures using topographic modulation analysis confirmed that this difference between tasks was driven by a shift in the underlying generator configuration. Minimum-norm source reconstruction revealed distinct activations that corresponded well with activity within putative dorsal and ventral auditory structures.

Preliminary Evidence of Pre-Attentive Distinctions of Frequency-Modulated Tones that Convey Affect

Recognizing emotion is an evolutionary imperative. An early stage of auditory scene analysis involves the perceptual grouping of acoustic features, which can be based on both temporal coincidence and spectral features such as perceived pitch. Perceived pitch, or fundamental frequency (F0), is an especially salient cue for differentiating affective intent through speech intonation (prosody). We hypothesized that: (1) simple frequency-modulated tone abstractions, based on the parameters of actual prosodic stimuli, would be reliably classified as representing differing emotional categories; and (2) that such differences would yield significant mismatch negativities (MMNs) – an index of pre-attentive deviance detection within the auditory environment. We constructed a set of FM tones that approximated the F0 mean and variation of reliably recognized happy and neutral prosodic stimuli. These stimuli were presented to 13 subjects using a passive listening oddball paradigm. We additionally included stimuli with no frequency modulation (FM) and FM tones with identical carrier frequencies but differing modulation depths as control conditions. Following electrophysiological recording, subjects were asked to identify the sounds they heard as happy, sad, angry, or neutral. We observed that FM tones abstracted from happy and no-expression speech stimuli elicited MMNs. Post hoc behavioral testing revealed that subjects reliably identified the FM tones in a consistent manner. Finally, we also observed that FM tones and no-FM tones elicited equivalent MMNs. MMNs to FM tones that differentiate affect suggests that these abstractions may be sufficient to characterize prosodic distinctions, and that these distinctions can be represented in pre-attentive auditory sensory memory.

Temporal Correlation Mechanisms and Their Role in Feature Selection: A Single-Unit Study in Primate Somatosensory Cortex

How neurons pay attention Top-down selective attention mediates feature selection by reducing the noise correlations in neural populations and enhancing the synchronized activity across subpopulations that encode the relevant features of sensory stimuli.

Neural mechanisms of selective attention in the somatosensory system

Selective attention allows organisms to extract behaviorally relevant information while ignoring distracting stimuli that compete for the limited resources of their central nervous systems. Attention is highly flexible, and it can be harnessed to select information based on sensory modality, within-modality feature(s), spatial location, object identity, and/or temporal properties. In this review, we discuss the body of work devoted to understanding mechanisms of selective attention in the somatosensory system. In particular, we describe the effects of attention on tactile behavior and corresponding neural activity in somatosensory cortex. Our focus is on neural mechanisms that select tactile stimuli based on their location on the body (somatotopic-based attention) or their sensory feature (feature-based attention). We highlight parallels between selection mechanisms in touch and other sensory systems and discuss several putative neural coding schemes employed by cortical populations to signal the behavioral relevance of sensory inputs. Specifically, we contrast the advantages and disadvantages of using a gain vs. spike-spike correlation code for representing attended sensory stimuli. We favor a neural network model of tactile attention that is composed of frontal, parietal, and subcortical areas that controls somatosensory cells encoding the relevant stimulus features to enable preferential processing throughout the somatosensory hierarchy. Our review is based on data from noninvasive electrophysiological and imaging data in humans as well as single-unit recordings in nonhuman primates.