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Steady-state visual evoked potentials (SSVEPs) were recorded from the scalp of human subjects who were cued to attend to a rapid sequence of alphanumeric characters presented to one visual half-field while ignoring a concurrent sequence of characters in the opposite half-field. These two-character sequences were each superimposed upon a small square background that was flickered at a rate of 8.6 Hz in one half-field and 12 Hz in the other half-field. The amplitude of the frequency-coded SSVEP elicited by either of the task-irrelevant flickering backgrounds was significantly enlarged when attention was focused upon the character sequence at the same location. This amplitude enhancement with attention was most prominent over occipital-temporal scalp areas of the right cerebral hemisphere regardless of the visual field of stimulation. These findings indicate that the SSVEP reflects an enhancement of neural responses to all stimuli that fall within the "spotlight" of spatial attention, whether or not the stimuli are task-relevant. Recordings of the SSVEP provide a new approach for studying the neural mechanisms and functional properties of selective attention to multi-element visual displays. The brain mechanisms of visual selective attention have been studied extensively in humans by means of noninvasive recordings of transient visual evoked potentials (VEPs) that reveal spatio-temporal patterns of neural activity in cortical sensory areas (1, 2). In the case of visual-spatial attention, stimuli presented to an attended location in the visual fields elicit enhanced VEP amplitudes in extrastriate visual cortex during the interval 80-200 ms poststimulus (1,(3)(4)(5). This early enhancement of evoked cortical activity supports the hypothesis that attention to location produces an amplification of sensory information arising from stimuli within the central focus or "spotlight" of spatial attention (6).Although a good deal has been learned about the brain's sensory systems through recordings of transient evoked potentials to individual stimuli, this approach has intrinsic limitations for investigating processes of visual attention. One major constraint is that transient VEPs are elicited optimally by stimuli having abrupt onsets that are presented at fairly long intervals (typically 0.3-1.5 s). When stimuli having these properties are used in attention experiments, however, it becomes difficult to maintain a state of focused attention upon relevant stimuli and to ignore irrelevant stimuli (7).An alternative approach that may circumvent this limitation is to record the steady-state visual evoked potential (SSVEP) in response to a visual stimulus that is repeated at a steady rate of 8-10 Hz or more (e.g., a flickering light). The Only a few previous studies have examined changes in the SSVEP as a function of cognitive variables (9-12). In a vigilance task, Silberstein et al. (11) found that the SSVEP to an irrelevant 13-Hz flickering background was reduced in amplitude over parieto-occipital scalp areas during a...

Event-related brain potentials (ERPs) elicited by multidimensional auditory stimuli were recorded from the scalp in a selective-attention task. Subjects listened to tone pips varying orthogonally between two levels each of pitch, location, and duration and responded to longer duration target stimuli having specific values of pitch and location. The discriminability of the pitch and location attributes was varied between groups. By examining the ERPs to tones that shared pitch and/or locational cues with the designated target, we inferred interrelationships among the processing of these attributes. In all groups, stimuli that failed to match the target tone in an easily discriminable cue elicited only transitory ERP signs of selective processing. Tones sharing the "easy" but not the "hard" cue with the target elicited ERPs that indicated more extensive processing, but not as extensive as stimuli sharing both cues. In addition, reaction times and ERP latencies to the designated targets were not influenced by variations in the discriminability of pitch and location. This pattern of results is consistent with parallel, self-terminating models and holistic models of processing and contradicts models specifying either serial or exhaustive parallel processing of these dimensions. Both the parallel, self-terminating models and the holistic models provide a generalized mechanism for hierarchical stimulus selections that achieve an economy of processing, an underlying goal of classic, multiple-stage theories of selective attention.

A finite element network model has been developed to predict the macroscopic elastic shear modulus and the area expansion modulus of the red blood cell (RBC) membrane skeleton on the basis of its microstructure. The topological organization of connections between spectrin molecules is represented by the edges of a random Delaunay triangulation, and the elasticity of an individual spectrin molecule is represented by the spring constant, K, for a linear spring element. The model network is subjected to deformations by prescribing nodal displacements on the boundary. The positions of internal nodes are computed by the finite element program. The average response of the network is used to compute the shear modulus (mu) and area expansion modulus (kappa) for the corresponding effective continuum. For networks with a moderate degree of randomness, this model predicts mu/K = 0.45 and kappa/K = 0.90 in small deformations. These results are consistent with previous computational models and experimental estimates of the ratio mu/kappa. This model also predicts that the elastic moduli vary by 20% or more in networks with varying degrees of randomness. In large deformations, mu increases as a cubic function of the extension ratio lambda 1, with mu/K = 0.62 when lambda 1 = 1.5.

The properties of linguistic attention were examined by recording event-related brain potentials (ERPs) to probe stimuli mixed with dichotically presented prose passages. Subjects either shadowed (repeated phrase by phrase) or selectively listened to one passage while ERPs were recorded from electrodes overlying midline sites, left-hemisphere speech areas, and corresponding areas of the right hemisphere. Mixed with each voice (a male voice in one ear, a female voice in the other) were four probe stimuli: digitized speech sounds (but or /a/ as in father) produced by the same speaker and tone bursts at the mean fundamental and second formant frequencies of that voice. The ERPs elicited by the speech probes in the attended ear showed an enhanced negativity, with an onset at 50 ms-100 ms and lasting up to 800 ms-1,000 ms, whereas the ERPs to the second formant probes showed an enhanced positivity in the 200 ms-300 ms latency range. These effects were comparable for shadowing and selective listening conditions and remained stable over the course of the experiment. The attention-related negativity to the consonant-vowel-consonant probe (but) was most prominent over the left hemisphere; other probes produced no significant asymmetries. The results indicate that stimulus selection during linguistic attention is specifically tuned to speech sounds rather than simply to constituent pure-tone frequencies or ear of entry. Furthermore, it appears that both attentional set and stimulus characteristics can influence the hemispheric utilization of stimuli.

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.

Event‐related potentials (ERPs) were recorded from human subjects in response to short trains of rapidly presented tone pips of two frequencies. Subjects listened selectively to tones of one frequency as directed by a visual cue prior to each train and responded to occasional longer‐duration target tones of that frequency. The early negative difference (Nd) between ERPs elicited by the tones when attended versus unattended was minimal for the first stimulus in each train but developed rapidly, attaining stable values by about the third stimulus. The development of this early Nd was most rapid when two attended tones began a train. Long time‐base averages revealed the concomitant development of a standing negativity over the course of each train. In contrast, late positive ERPs distinguished attended from unattended target tones even when they occurred in the first position of a train, prior to the development of Nd. The latency of this late positivity together with the reaction time to the targets decreased substantially as the Nd developed. It was concluded that subjects can select stimuli at various stages of analysis, but employment of the early selection mechanism indexed by Nd enables an economy of processing and faster and more accurate task performance. This mode of early selection is not pre‐set in advance of a stimulus train, however, at least under the present conditions.

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