A movement-sensitive area in auditory cortex.
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Auditory Cortex↗
Biomedical subjects
M G Woldorff
Publications and source records attributed to M G Woldorff.
Hemodynamic and electrophysiological study of the role of the anterior cingulate in target-related processing and selection for action.
A number of experiments requiring attention or other complex cognitive functions have found substantial activation in the anterior cingulate cortex (ACC). Some of these studies have suggested that this area may be involved in "selection for action," such as for selecting to respond to a target stimulus. Here, positron emission tomography (PET) and event-related potentials (ERPs) were used to study the effects of target probability during a demanding visual spatial attention task, in which the target percentage was either low (2%, 1 per approximately 26 sec) or high (16%, 1 per approximately 3.5 sec). As expected, ERPs to detected targets evoked large, bilaterally distributed P300 waves. The PET showed strong activation of the ACC, particularly dorsally, during all the attend conditions relative to passive. However, these PET activations did not significantly differ between the few-target and many-target conditions, showing only a small trend to be larger in the many-target case. Such results indicate that the bulk of the ACC activation does not reflect selection for action per se, while also suggesting that the ACC is not a likely source of the P300 effect. The current data, however, do not argue against the ACC serving a role in maintaining a vigilant or anticipatory state in which one may need to select for action, or in continually or repeatedly (i.e., for each stimulus) needing to resolve whether to select to act or to not act.
Adolescent↗
Lateralized auditory spatial perception and the contralaterality of cortical processing as studied with functional magnetic resonance imaging and magnetoencephalography.
Functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were used to study the relationships between lateralized auditory perception in humans and the contralaterality of processing in auditory cortex. Subjects listened to rapidly presented streams of short FM-sweep tone bursts to detect infrequent, slightly deviant tone bursts. The stimulus streams consisted of either monaural stimuli to one ear or the other or binaural stimuli with brief interaural onset delays. The onset delay gives the binaural sounds a lateralized auditory perception and is thought to be a key component of how our brains localize sounds in space. For the monaural stimuli, fMRI revealed a clear contralaterality in auditory cortex, with a contralaterality index (contralateral activity divided by the sum of contralateral and ipsilateral activity) of 67%. In contrast, the fMRI activations from the laterally perceived binaural stimuli indicated little or no contralaterality (index of 51%). The MEG recordings from the same subjects performing the same task converged qualitatively with the fMRI data, confirming a clear monaural contralaterality, with no contralaterality for the laterally perceived binaurals. However, the MEG monaural contralaterality (55%) was less than the fMRI and decreased across the several hundred millisecond poststimulus time period, going from 57% in the M50 latency range (20-70 ms) to 53% in the M200 range (170-250 ms). These data sets provide both quantification of the degree of contralaterality in the auditory pathways and insight into the locus and mechanism of the lateralized perception of spatially lateralized sounds.
Acoustic Stimulation↗
Randomized event-related experimental designs allow for extremely rapid presentation rates using functional MRI.
Previous studies have shown that hemodynamic response overlap severely limits the maximum presentation rate with event-related functional MRI (fMRI) using fixed intertrial experimental designs. Here we demonstrate that the use of randomized experimental designs can largely overcome this limitation, thereby allowing for event-related fMRI experiments with extremely rapid presentation rates. In the first experiment, fMRI time courses were simulated using a fixed intertrial interval design with intervals of 16, 3, and 1 s, and using a randomized design having the same mean intertrial intervals. We found that using fixed intertrial interval designs the transient information decreased with decreasing intertrial intervals, whereas using randomized designs the transient information increased with decreasing mean intertrial intervals. In a second experiment, fMRI data were collected from two subjects using a randomized paradigm with visual hemifield stimuli presented randomly every 500 ms. Robust event-related activation maps and hemodynamic response estimates were obtained. These results demonstrate the feasibility of performing event-related fMRI experiments with rapid, randomized paradigms identical to those used in electrophysiological and behavioral studies, thereby expanding the applicability of event-related fMRI to a whole new range of cognitive neurosciences questions and paradigms.
Cerebrovascular Circulation↗
Auditory attention in the congenitally blind: where, when and what gets reorganized?
Functional reorganization of auditory attention was studied in 12 congenitally blind subjects and 12 controls using high-density event-related potentials during a highly focused dichotic listening task. Reaction times for the attend-ear intensity-deviant targets were markedly faster for the blind. Brain activity associated with sustained attention (N1 effect, Nd), and with the automatic detection of deviants in an unattended channel (MMN), did not exhibit reorganization. In contrast, marked plasticity changes were reflected in late auditory attentional processing (attend-ear targets), in the form of a prolonged negativity (200-450 ms post-stimulus) that was absent in the sighted subjects. The plasticity changes in the blind had a time course indicating progressive recruitment of parietal and then occipital regions, providing new evidence for cross-modal sensory reorganization in the blind.
Adult↗
Intersubject variability of functional areas in the human visual cortex.
Intersubject variability of striate and extrastriate areas was mapped by conjoined use of positron emission tomography (PET) and magnetic resonance imaging (MRI). We used two dynamic bowtie-shaped random-dot patterns centered symmetrically around the vertical- and horizontal-meridian, respectively, presented during sequential PET scans in 11 subjects. Control condition was simple fixation on a central dot in absence of a surrounding random dot pattern. V1, V2, VP, V3, V3a, V4, V5, and "wordform" areas were identified. After spatial normalization to Talairach atlas space, mean locations and standard deviations about these mean locations for x-, y-, and z-axes were calculated for each area in both hemispheres and compared for differences. The mean standard deviation for all axes across all areas tested was found to be small (4.9 mm). No significant differences were found in the mean standard deviations for the x-, y-, and z-axes in the left hemisphere vs. their counterparts in the right hemisphere. However, when mean standard deviations in both hemispheres were polled together by axis, the mean standard deviation for the y-axis (5.3 mm) was found to be significantly different from the mean standard deviation for the x-axis (4.3 mm). Furthermore, in the left hemisphere, the mean standard deviation for the z-axis (5.7 mm) was significantly greater than the mean standard deviation for the x-axis (3.9 mm). The values reported in this study for mean location and standard deviation of visual areas can be used to establish confidence intervals for distinguishing normal variations from pathology and consequent brain reorganization.
Adolescent↗
Magnetoencephalographic recordings demonstrate attentional modulation of mismatch-related neural activity in human auditory cortex.
It is widely agreed that the negative brain potential elicited at 150-200 ms by a deviant, less intense sound in a repetitive series can be modulated by attention. To investigate whether this modulation represents a genuine attention effect on the mismatch negativity (MMN) arising from auditory cortex or attention-related activity from another brain region, we recorded both the MMN and the mismatch magnetic field (MMF) elicited by such deviants in a dichotic listening task. Deviant tones in the attended ear elicited a sizable MMF that was well modeled as a dipolar source in auditory cortex. Both the MMN and MMF to unattended-ear deviants were highly attenuated. These findings support the view that the MMN/MMF elicited in auditory cortex by intensity deviants, and thus the underlying feature-analysis and mismatch-detection processes, are not strongly automatic but rather can be gated or suppressed if attention is strongly focused elsewhere.
Acoustic Stimulation↗
Improving the temporal resolution of functional MR imaging using keyhole techniques.
Using a keyhole technique, it is shown that the data acquisition rate of gradient-echo imaging for functional MRI (fMRI) studies can be increased substantially. The resulting enhancement of the temporal resolution of fMRIs was accomplished without modifying the hardware of a conventional MRI system. High spatial resolution fMRI images were first collected with conventional full k-space acquisition and image reconstruction. Using the same data set, simulation reconstruction using the keyhole principle and zero-padding were performed for comparison with the full k-space reconstruction. No significant changes were found for fMRI images generated from the keyhole technique with a data sharing profile of 50% of the k-space. As k-space data sharing profiles increased to 75 and 87.5%, the keyhole fMRI images began to show only modest changes in activation intensity and area compared with the standard images. In contrast, zero-padding fMRI images produced a significant disparity both in activation intensity and area relative to the truly high-resolution fMRI images. The keyhole technique's ability to retain the intensity and area of fMRI information, while substantially reducing acquisition time, makes it a promising method for fMRI studies.
Adult↗
Selective listening at fast stimulus rates: so much to hear, so little time.
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Acoustic Stimulation↗
Integrating human brain maps.
Perception, action, cognition, and emotion can now be mapped in the brain by a growing family of techniques. Positron emission tomography, functional magnetic resonance imaging, event-related electrical potentials, event-related magnetic fields, and other non-invasive imaging techniques are rapidly evolving and providing an increasingly rich literature on the functional organization of the human brain. Although no two techniques map identical physiological processes or physical parameters, replications of functionally specific maps by different techniques indicate sufficient common ground for multimodality integration. The process of integration is multi-tiered. Recent advances in integration range from simple image fusion, to model-based synthetic analyses, to collective databases for neural-system modeling. Spatially, temporally, physiologically, and cognitively accurate computational models of the neural systems of human behavior are the ultimate objective of functional brain mapping. This objective will be reached only through integrating the diversity of modern brain-mapping methods.
Brain Mapping↗
Effects of spatial cuing on luminance detectability: psychophysical and electrophysiological evidence for early selection.
Three experiments were conducted to determine whether attention-related changes in luminance detectability reflect a modulation of early sensory processing. Experiments 1 and 2 used peripheral cues to direct attention and found substantial effects of cue validity on target detectability; these effects were consistent with a sensory-level locus of selection but not with certain memory- or decision-level mechanisms. In Experiment 3, event-related brain potentials were recorded in a similar paradigm using central cues, and attention was found to produce changes in sensory-evoked brain activity beginning within the 1st 100 ms of stimulus processing. These changes included both an enhancement of sensory responses to attended stimuli and a suppression of sensory responses to unattended stimuli; the enhancement and suppression effects were isolated to different neural responses, indicating that they may arise from independent attentional mechanisms.
Adolescent↗
Modulation of early sensory processing in human auditory cortex during auditory selective attention.
Neuromagnetic fields were recorded from human subjects as they listened selectively to sequences of rapidly presented tones in one ear while ignoring tones of a different pitch in the opposite ear. Tones in the attended ear evoked larger magnetic brain responses than did unattended tones in the latency ranges 20-50 msec and 80-130 msec poststimulus. Source localization techniques in conjunction with magnetic resonance imaging placed the neural generators of these early attention-sensitive brain responses in auditory cortex on the supratemporal plane. These data demonstrate that focused auditory attention in humans can selectively modulate sensory processing in auditory cortex beginning as early as 20 msec poststimulus, thereby providing strong evidence for an "early selection" mechanism of auditory attention that can regulate auditory input at or before the initial stages of cortical analysis.
Acoustic Stimulation↗
Distortion of ERP averages due to overlap from temporally adjacent ERPs: analysis and correction.
In studies of event-related potentials (ERPs), short interstimulus intervals (ISIs) are often employed to investigate certain neural or psychological phenomena. At short ISIs, however, the ERP responses to successive stimuli may overlap, thereby distorting the ERP averages. This paper describes a signal processing approach for analyzing the distortion of ERP averages due to such overlap. In general, the distortion is modeled in terms of mathematical convolutions of the ERP waveform elicited by each type of adjacent stimulus with the corresponding distribution in time of those stimuli relative to the averaging epoch. Using this framework, a number of implications of ERP overlap for experimental design and interpretation are examined, with special emphasis given to selective attention paradigms. It is shown that the possibility of confound due to ERP overlap is widespread in short-ISI experiments, and even the widely used procedure of stimulus randomization does not necessarily control for differential distortion of the ERPs to attended versus unattended stimuli. Problems due to ERP overlap can be particularly serious in short-ISI studies that examine how ERPs (and associated perceptual processes) are influenced by the nature of the preceding stimulus (i.e., stimulus sequence effects). A set of algorithms is presented for estimating and removing the residual distortion due to response overlap from recorded ERP averages. The use of these algorithms, collectively termed the Adjacent Response (Adjar) Technique, can alleviate many of the overlap-related problems that arise when short ISIs are used, thereby enhancing the power of the ERP technique.
Algorithms↗
Modulation of early auditory processing during selective listening to rapidly presented tones.
Two dichotic listening experiments were performed in which stimulus and task conditions were optimized for the early selection of inputs. Subjects listened selectively to sequences of rapidly presented tone pips in one ear while ignoring tone pips of a different pitch in the opposite ear. In both experiments, an enhanced positivity between 20 and 50 msec (the 'P20-50') was observed over central and frontal sites in the ERPs to the attended-channel tone pips. At longer latencies, the effects of attention appeared to include an amplitude modulation of several exogenous ERPs, including subcomponents of the central N1 (60-150 msec) and P2 (170-230 msec) waves and the temporal T complex (80-150 msec). In contrast, the attention effect prefrontally consisted of a broad negativity that appeared to be largely endogenous. A signal processing technique (Adjar) was employed to correct for distortion of mutually overlapping ERPs elicited by successive stimuli presented at short interstimulus intervals (ISIs). It was confirmed that the P20-50 attention effect was not the result of differential overlap from previous ERPs. In addition, this technique allowed an analysis to be made of the effects of the preceding stimulus type and ISI on the attention-sensitive ERPs, which provided further support for the view that highly focused selective attention can directly modulate exogenous components of the auditory ERP. Moreover, these sequence-dependent ERP modulations were paralleled by variations in target discrimination performance. Taken together, these results provide strong support for the early selection hypothesis that attention can serve to selectively bias or gate stimulus processing before full perceptual analysis has occurred.
Acoustic Stimulation↗
The effects of channel-selective attention on the mismatch negativity wave elicited by deviant tones.
The mismatch negativity (MMN) is an event-related brain potential elicited by infrequent, physically deviant sounds in a sequence of repetitive auditory stimuli. Two dichotic listening experiments that were designed to optimize the selective focusing of attention provided a strong test of Näätänen's proposal that the MMN is unaffected by attention and reflects the operation of a strongly automatic mismatch detection system. In Experiment 1, tones were presented at intervals of 120-320 ms, and the deviant tones (intensity decrements) in both the attended and unattended ears elicited negative waves consistent in waveshape, latency, and distribution with previously described MMNs. In contrast to previous reports, however, the MMN elicited by the unattended-channel deviant was markedly reduced (peak amplitude of less than 1 microV) relative to the corresponding negative wave elicited by the attended-channel deviants (3-4 microV), as well as relative to previously reported MMNs (3-6 microV) elicited by comparable deviations in stimulus intensity. In Experiment 2, which employed interstimulus intervals of 65-205 ms, the unattended-channel MMN elicited by the deviant fainter tones was barely discernible, whereas the corresponding attended-channel negativity was again about 3-4 microV. These findings call into question the assertion that the auditory mismatch detection process and the associated MMN wave are wholly independent of attentional influence. Rather, these data provide evidence that the processing of stimuli in unattended channels can be attenuated or gated at an early sensory level under conditions of highly focused auditory selective attention.
Acoustic Stimulation↗
Activation of human auditory cortex in retrieval experiments: an fMRI study.
In a previous functional magnetic resonance (fMRI) study, a subdivision of the human auditory cortex into four distinct territories was achieved. One territory (T1a) exhibited functional specialization in terms of a foreground-background decomposition task involving matching-to-sample monitoring on tone sequences. The present study more specifically determined whether memory-guided analysis of tone sequences is part of the T1a specialization. During the encoding periods, an arbitrary and unfamiliar four-tone-sequence (melody) played by one instrument was presented. The melody-instrument-combination was different in each period. During subsequent retrieval periods, learned and additional combinations were presented, and the tasks were either to detect the target melodies (experiment I) or the target instruments (experiment II). T1a showed larger activation during the melody retrieval. The results generally suggest that (1) activation of T1a during retrieval is determined less by the sound material than by the executed task, and (2) more specifically, that memory-guided sequential analysis in T1a is dominant over recognition of characteristic complex sounds.
Acoustic Stimulation↗