Selective attention effects on somatosensory evoked potentials

Short and long latency somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded over the contralateral hemisphere. Simultaneously, signals evoked by the same stimulus were monitored at Erb's point. Recordings were made during three conditions which were presented in a different random order to each of the subjects tested. During the control condition the subjects were instructed to attend to and count the number of electrical pulses delivered to the median nerve of their right hand. During the two task conditions, in addition to the pulses, the subjects received stimulation on the dorsal surface of one of their hands. This consisted in drawing circles lightly for the duration of SEP recording using a cotton swab (Q-tip). During the trial, the Q-tip was momentarily withdrawn 15 to 20 times and the subject's task was to ignore the pulses, attend to this cutaneous stimulation and count the number of times the cutaneous stimulation was interrupted. SEPs to the pulse were significantly greater in amplitude when cutaneous stimulation was delivered to the same hand as the pulse (the right hand) than when it was delivered to the left hand. This effect was confined to the long-latency SEPs and did not appear in either the Erb's point response or the short latency SEPs. These data indicate that selective attention to peripheral stimulation is a relatively late process mediated by cortical mechanisms and argue against the notion of early suppression of irrelevant stimulus channels in subcortical centers or in the periphery.(ABSTRACT TRUNCATED AT 250 WORDS)     

The interactions of multisensory integration with endogenous and exogenous attention

Stimuli from multiple sensory organs can be integrated into a coherent representation through multiple phases of multisensory processing; this phenomenon is called multisensory integration. Multisensory integration can interact with attention. Here, we propose a framework in which attention modulates multisensory processing in both endogenous (goal-driven) and exogenous (stimulus-driven) ways. Moreover, multisensory integration exerts not only bottom-up but also top-down control over attention. Specifically, we propose the following: (1) endogenous attentional selectivity acts on multiple levels of multisensory processing to determine the extent to which simultaneous stimuli from different modalities can be integrated; (2) integrated multisensory events exert top-down control on attentional capture via multisensory search templates that are stored in the brain; (3) integrated multisensory events can capture attention efficiently, even in quite complex circumstances, due to their increased salience compared to unimodal events and can thus improve search accuracy; and (4) within a multisensory object, endogenous attention can spread from one modality to another in an exogenous manner.     

Neurocomputational approaches to modelling multisensory integration in the brain: A review

The Brain’s ability to integrate information from different modalities (multisensory integration) is fundamental for accurate sensory experience and efficient interaction with the environment: it enhances detection of external stimuli, disambiguates conflict situations, speeds up responsiveness, facilitates processes of memory retrieval and object recognition. Multisensory integration operates at several brain levels: in subcortical structures (especially the Superior Colliculus), in higher-level associative cortices (e.g., posterior parietal regions), and even in early cortical areas (such as primary cortices) traditionally considered to be purely unisensory. Because of complex non-linear mechanisms of brain integrative phenomena, a key tool for their understanding is represented by neurocomputational models. This review examines different modelling principles and architectures, distinguishing the models on the basis of their aims: (i) Bayesian models based on probabilities and realizing optimal estimator of external cues; (ii) biologically inspired models of multisensory integration in the Superior Colliculus and in the Cortex, both at level of single neuron and network of neurons, with emphasis on physiological mechanisms and architectural schemes; among the latter, some models exhibit synaptic plasticity and reproduce development of integrative capabilities via Hebbian-learning rules or self-organizing maps; (iii) models of semantic memory that implement object meaning as a fusion between sensory–motor features (embodied cognition). This overview paves the way to future challenges, such as reconciling neurophysiological and Bayesian models into a unifying theory, and stimulates upcoming research in both theoretical and applicative domains.     

Selective attention to human voice enhances brain activity bilaterally in the superior temporal sulcus

We demonstrated that neonatal isolation (1-h pup isolation; postnatal days 2-9) impairs context-induced fear conditioning in adult male rats and tends to enhance this effect and foot shock sensitivity in females. In this study, we examine the effects of brief (i.e., handling; 15 min) and prolonged (3 h) maternal separations (postnatal days 1-21) on fear conditioning and foot shock sensitivity in adult male and female rats. Identical training and test conditions from our prior study were employed so comparisons of the three early life stressors could be made. Context- and cue-elicited freezing and ultrasonic vocalizations (USVs; 22 kHz) were measured after 10 tone-shock training trials in Experiment 1. In Experiment 2, foot shock responses (flinch, jump, sonic vocalizations) to escalating shock levels were assessed. Brief maternal separation impaired context- and cue-conditioned fear in rats of both sexes as assessed by USVs. Prolonged maternal separation only impaired context fear in female rats. There were no effects on foot shock sensitivity. Results of this and other studies suggest that early life stress impairs fear conditioning in adult rats whereas stress experienced in adulthood has the opposite effect. These opposing effects may reflect developmental differences on stress-induced alterations on hippocampal regulation of the hypothalamic-pituitary-adrenal axis.     

Neurons and behavior: the same rules of multisensory integration apply

Combinations of different sensory cues (e.g. auditory and visual) that are coincident in space enhance the responses of multisensory superior colliculus neurons, while the responses of these same neurons are depressed if the stimuli are separated in space. Using a behavioral paradigm modeled after that used in physiological studies, the present experiments demonstrate that the rules governing multisensory integration at the level of the single neuron also predict the responses to these stimuli in the intact behaving animal.     

Combining steady-state visual evoked potentials and f MRI to localize brain activity during selective attention

Brain activity was studied with fMRI and steady-state visual evoked potentials (SSVEPs) in separate sessions as subjects attended to letter sequences in either the right or left visual field. The two letter sequences were superimposed on small square backgrounds that flickered at 8.6 and 12.0 Hz, respectively, and elicited SSVEPs at the flicker frequencies. The amplitude of the frequency-coded SSVEP elicited by either of the flickering backgrounds was enlarged when attention was focused upon the letter sequence at the same location. Source analysis of the SSVEP waveforms localized the attentional modulation to ventral and lateral extrastriate visual cortex, which corresponded to zones of activation observed with fMRI. Hum. Brain Mapping 5:287–292, 1997. © 1997 Wiley-Liss, Inc.     

Superadditivity in multisensory integration: putting the computation in context

Single-neuron studies have highlighted dramatic enhancements in neural activity consequent to multisensory integration. Most notable are 'superadditive' enhancements in which the multisensory response exceeds the sum of those evoked by the modality-specific stimulus components individually. Although all multisensory enhancements may have perceptual/behavioral consequences, superadditivity, which suggests a nonlinear combination of modality-specific influences, seems to have had a disproportionate influence within the multisensory literature. This influence has been reinforced by the increasing application of noninvasive techniques such as functional imaging and event-related potential recording, which depend on response nonlinearities to demonstrate underlying multisensory processes. In promoting the idea that many multisensory behaviors may not rely on superadditivity, we consider more recent single-neuron studies that place its incidence in context.     

Selective attention differentially affects brainstem auditory evoked potentials of electrodermal responders and nonresponders

Brainstem auditory evoked potentials (BAEPs) were recorded in adult male chronic schizophrenics on maintenance neuroleptic therapy, drug-free alcoholics, and normal subjects. Subjects were subdivided into electrodermal responders (Rs) and nonresponders (NRs). Attention was directed toward auditory or visual stimuli. Independent of diagnosis, latencies of BAEP wave V were longer in NRs when visual rather than auditory stimuli were attended to, while there was no task effect for Rs. This finding is interpreted as a sign of excessive selective filtering of auditory stimuli in NRs. In the alcoholic NRs, wave I-V conduction times were longer than those in any other subgroup, possibly indicating retarded neural transmission.     

The effects of divided attention on encoding- and retrieval-related brain activity: A PET study of younger and older adults

Divided attention (DA) disrupts episodic encoding, but has little effect on episodic retrieval. Furthermore, normal aging is associated with episodic memory impairments, and when young adults are made to encode information under DA conditions, their memory performance is reduced and resembles that of old adults working under full attention (FA) conditions. Together, these results suggest a common neurocognitive mechanism by which aging and DA during encoding disrupt memory performance. In the current study, we used PET to investigate younger and older adults' brain activity during encoding and retrieval under FA and DA conditions. In FA conditions, the old adults showed reduced activity in prefrontal regions that younger adults activated preferentially during encoding or retrieval, as well as increased activity in prefrontal regions young adults did not activate. These results indicate that prefrontal functional specificity of episodic memory is reduced by aging. During encoding, DA reduced memory performance, and reduced brain activity in left-prefrontal and medial-temporal lobe regions for both age groups, indicating that DA during encoding interferes with encoding processes that lead to better memory performance. During retrieval, memory performance and retrieval-related brain activity were relatively immune to DA for both age groups, suggesting that DA during retrieval does not interfere with the brain systems necessary for successful retrieval. Finally, left inferior prefrontal activity was reduced similarly by aging and by DA during encoding, suggesting that the behavioral correspondence between these effects is the result of a reduced ability to engage in elaborate encoding operations.     

Dissociation of perception and action in audiovisual multisensory integration

The ‘temporal rule’ of multisensory integration (MI) proposes that unisensory stimuli, and the neuronal responses they evoke, must fall within a window of integration. Ecological validity demands that MI should occur only for physically simultaneous events (which may give rise to non‐simultaneous neural activations), and spurious neural response simultaneities unrelated to environmental multisensory occurrences must somehow be rejected. Two experiments investigated the requirements of simultaneity for facilitative MI. Experiment 1 employed an reaction time (RT)/race model paradigm to measure audiovisual (AV) MI as a function of AV stimulus‐onset asynchrony (SOA) under fully dark adapted conditions for visual stimuli that were either rod‐ or cone‐isolating. Auditory stimulus intensity was constant. Despite a 155‐ms delay in mean RT to the scotopic vs. photopic stimulus, facilitative AV MI in both conditions occurred exclusively at an AV SOA of 0 ms. Thus, facilitative MI demands both physical and physiological simultaneity. Experiment 2 investigated the accuracy of simultaneity and temporal order judgements under the same stimulus conditions. Judgements of AV stimulus simultaneity or temporal order were significantly influenced by stimulus intensity, indicating different simultaneity requirements for these tasks. The possibility was considered that there are mechanisms by which the nervous system may take account of variations in response latency arising from changes in stimulus intensity in order to selectively integrate only those physiological simultaneities that arise from physical simultaneities. It was proposed that separate subsystems for AV MI exist that pertain to action and perception.     

Selective attention, channel capacity, and the average evoked response in human subjects.

Traditional studies of selective attention and the Average Evoked Response (AER) have employed naive subjects who performed non-verbal tasks designed to capture attention. The performance of highly trained subjects on a shadowing task, which is verbal in nature, reveals striking effects on the auditory AER which differ from those findings obtained with naive subjects. AER attenuation was found to be related to the level of the subject's sophistication in the performance of the task. Channel capacity is viewed as dynamic rather than fixed. Theoretical implications are discussed.     

Differential effects of object-based attention on evoked potentials to fearful and disgusted faces

Event-related potentials (ERPs) were used to investigate the role of attention on the processing of facial expressions of fear and disgust. Stimuli consisted of overlapping pictures of a face and a house. Participants had to monitor repetitions of faces or houses, in separate blocks of trials, so that object-based attention was manipulated while spatial attention was kept constant. Faces varied in expression and could be either fearful or neutral (in the fear condition) or disgusted or neutral (in the disgust condition). When attending to faces, participants were required to signal repetitions of the same person, with the facial expressions being completely irrelevant to the task. Different effects of selective attention and different patterns of brain activity were observed for faces with fear and disgust expressions. Results indicated that the perception of fear from faces is gated by selective attention at early latencies, whereas a sustained positivity for fearful faces compared to neutral faces emerged around 160 ms at central–parietal sites, independent of selective attention. In the case of disgust, ERP differences began only around 160 ms after stimulus onset, and only after 480 ms was the perception of disgust modulated by attention allocation. Results are interpreted in terms of different neural mechanisms for the perception of fear and disgust and related to the functional significance of these two emotions for the survival of the organism.     

Formal models and quantitative measures of multisensory integration: a selective overview

Multisensory integration (MI) is defined as the neural process by which unisensory signals are combined to form a new product that is significantly different from the responses evoked by the modality‐specific component stimuli. In recent years, MI research has seen exponential growth in the number of empirical and theoretical studies. This study presents a selective overview of formal modeling approaches to MI. Emphasis is on models and measures for behavioral paradigms, such as localization, judgment of temporal order or simultaneity, and reaction times, but some concepts for the modeling of single‐cell spike rates are treated as well. We identify a number of essential concepts underlying most model classes, such as Bayesian causal inference, probability summation, coactivation, and time window of integration. Quantitative indexes for measuring and comparing the strength of MI across different paradigms are also discussed. Whereas progress over the last years is remarkable, we point out some strengths and weaknesses of the modeling approaches and discuss some obstacles toward a unified theory of MI.