T1 Accuracy Research Articles

No Evidence for Early Modulation of Evoked Responses in Primary Visual Cortex to Irrelevant Probe Stimuli Presented during the Attentional Blink

BackgroundDuring rapid serial visual presentation (RSVP), observers often miss the second of two targets if it appears within 500 ms of the first. This phenomenon, called the attentional blink (AB), is widely held to reflect a bottleneck in the processing of rapidly sequential stimuli that arises after initial sensory registration is complete (i.e., at a relatively late, post-perceptual stage of processing). Contrary to this view, recent fMRI studies have found that activity in the primary visual area (V1), which represents the earliest cortical stage of visual processing, is attenuated during the AB. Here we asked whether such changes in V1 activity during the AB arise in the initial feedforward sweep of stimulus input, or instead reflect the influence of feedback signals from higher cortical areas.Methodology/Principal FindingsEEG signals were recorded while participants monitored a sequential stream of distractor letters for two target digits (T1 and T2). Neural responses associated with an irrelevant probe stimulus presented simultaneously with T2 were measured using an ERP marker – the C1 component – that reflects initial perceptual processing of visual information in V1. As expected, T2 accuracy was compromised when the inter-target interval was brief, reflecting an AB deficit. Critically, however, the magnitude of the early C1 component evoked by the probe was not reduced during the AB.Conclusions/SignificanceOur finding that early sensory processing of irrelevant probe stimuli is not suppressed during the AB is consistent with theoretical models that assume that the bottleneck underlying the AB arises at a post-perceptual stage of processing. This suggests that reduced neural activity in V1 during the AB is driven by re-entrant signals from extrastriate areas that regulate early cortical activity via feedback connections with V1.

The attentional blink in schizophrenia: Isolating the perception/attention interface

Previous work has demonstrated that several aspects of visual processing are impaired in schizophrenia, including early perceptual processes and later higher-order processes. However, it remains unclear whether the stage of processing where early perception and later higher-order processes interact is impaired in schizophrenia. The current research examined this interface in schizophrenia using the attentional blink (AB) paradigm. We administered two rapid serial visual processing (RSVP) tasks to 143 patients with schizophrenia or schizoaffective disorder and 80 healthy controls: 1) a single target detection task, to measure basic visual perception; and 2) a dual target detection task, to measure the AB effect. In the dual target task, the two target stimuli (T1 and T2) were presented at varying positions or “lags” within a rapid sequential stream of distractor stimuli. Participants verbally identified the target stimuli. Both groups showed the expected AB effect, with T2 accuracy being poorest 200–500 ms after presentation of T1. However, patients showed an exaggerated AB effect compared to the healthy controls, with significantly reduced detection of T2, even after correcting for performance on the single target task. The reduction in accuracy was steeper and more pronounced in the patients during the AB lags, and it extended to lags before and after the typical AB. This performance pattern on the AB task suggests that patients with schizophrenia exhibit both deficits in visual processing at the interface of perceptual and attentional processing and a general attentional deficit.

Is all sparing created equal? Comparing lag-1 sparing and extended sparing in temporal object perception.

When two targets (T1, T2) are presented amongst a rapid stream of distractors, T2 accuracy is impaired if the targets are separated by at least one distractor (attentional blink). However, this impairment largely disappears if the targets follow one another directly (lag-1 sparing), and, in fact, as many as four or five consecutive targets may be identified quite accurately under these conditions (extended sparing). Although all current models propose a common mechanism for both lag-1 and extended sparing, this hypothesis has yet to be tested. To this end, we examined the effect of various types of attentional switches, known to impact lag-1 sparing, on extended sparing in order to determine whether they would have a similar effect. Results suggested substantial parallels between the two types of sparing. We discuss these results in terms of a unified account of sparing in temporal object perception.

Attentional blink and repetition blindness.

When two masked, to-be-attended targets are presented within half a second of each other, report accuracy for the second target (T2) is impaired relative to when the two targets are presented farther apart in time or relative to when the first target (T1) can be ignored. This effect is known as the attentional blink (AB). An additional T2 accuracy deficit is observed if T1 and T2 are identical or highly similar on a task-relevant dimension. This effect is known as repetition blindness (RB). For both AB and RB, targets are typically imbedded in rapid serial visual presentation (RSVP) streams and the dual-task attention cost lasts approximately half a second. Given the high degree of superficial similarity, AB and RB are often considered to be related phenomena. Although research thus far has suggested that both phenomena reflect limits of the attentional system and how attention is allocated when needing to organize stimuli for entrance into awareness, these two phenomena are dissociable; RB is not simply an enhanced AB. Furthermore, investigations of AB and RB have taken quite different courses over the last two decades. The AB has been investigated extensively with a variety of experimental, behavioral, neurophysiological, and clinical approaches, and has become widely used as a paradigm of convenience with which to study other effects. In contrast, studies of RB have tended to manipulate the nature of the target information to understand the level of representation that supports RB. WIREs Cogni Sci 2011 2 336-344 DOI: 10.1002/wcs.129 For further resources related to this article, please visit the WIREs website.

T1 difficulty affects the AB: manipulating T1 word frequency and T1 orthographic neighbor frequency

Colored target words were presented with distractor nonwords in a rapid serial visual presentation (RSVP) task. In Experiment 1, the attentional blink (AB) effect on T2 accuracy was larger when T1 was a difficult (low-frequency) word than when it was a high-frequency word. In Experiment 2 the effect of T1 frequency on the AB was replicated in a between-participants design, and the frequency of T1's one-letter different neighbors (e.g., case, bare, for care) interacted with T1 frequency in its effects on T2 accuracy. Experiment 3 confirmed the effect of T1 frequency over 6T1-T2 lags. The effects of T1 characteristics were sensitively assessed in the AB and were more consistent with resource depletion theories than control-process accounts.

Theta Phase Synchrony and Conscious Target Perception: Impact of Intensive Mental Training

The information processing capacity of the human mind is limited, as is evidenced by the attentional blink-a deficit in identifying the second of two targets (T1 and T2) presented in close succession. This deficit is thought to result from an overinvestment of limited resources in T1 processing. We previously reported that intensive mental training in a style of meditation aimed at reducing elaborate object processing, reduced brain resource allocation to T1, and improved T2 accuracy [Slagter, H. A., Lutz, A., Greischar, L. L., Francis, A. D., Nieuwenhuis, S., Davis, J., et al. Mental training affects distribution of limited brain resources. PloS Biology, 5, e138, 2007]. Here we report EEG spectral analyses to examine the possibility that this reduction in elaborate T1 processing rendered the system more available to process new target information, as indexed by T2-locked phase variability. Intensive mental training was associated with decreased cross-trial variability in the phase of oscillatory theta activity after successfully detected T2s, in particular, for those individuals who showed the greatest reduction in brain resource allocation to T1. These data implicate theta phase locking in conscious target perception, and suggest that after mental training the cognitive system is more rapidly available to process new target information. Mental training was not associated with changes in the amplitude of T2-induced responses or oscillatory activity before task onset. In combination, these findings illustrate the usefulness of systematic mental training in the study of the human mind by revealing the neural mechanisms that enable the brain to successfully represent target information.

Distractor Inhibition Predicts Individual Differences in the Attentional Blink

BackgroundThe attentional blink (AB) refers to humans' impaired ability to detect the second of two targets (T2) in a rapid serial visual presentation (RSVP) stream of distractors if it appears within 200–600 ms of the first target (T1). Here we examined whether humans' ability to inhibit distractors in the RSVP stream is a key determinant of individual differences in T1 performance and AB magnitude.Methodology/Principal FindingsWe presented subjects with RSVP streams (93.3 ms/item) of letters containing white distractors, a red T1 and a green T2. Subjects' ability to suppress distractors was assessed by determining the extent to which their second target performance was primed by a preceding distractor that shared the same identity as T2. Individual subjects' magnitude of T2 priming from this distractor was found to be negatively correlated with their T1 accuracy and positively related to their AB magnitude. In particular, subjects with attenuated ABs showed negative priming (i.e., worse T2 performance when the priming distractor appeared in the RSVP stream compared to when it was absent), whereas those with large ABs displayed positive priming (i.e., better T2 performance when the priming distractor appeared in the RSVP stream compared to when it was absent). Thus, a subject's ability to suppress distractors, as assessed by T2 priming magnitude, predicted both their T1 performance and AB magnitude.Conclusions/SignificanceThese results confirm that distractor suppression plays a key role in RSVP target selection and support the hypothesis that the AB results, at least in part, from a failure of distractor inhibition.

Optimization and validation of a fully‐integrated pulse sequence for modified look‐locker inversion‐recovery (MOLLI) T1 mapping of the heart

To optimize and validate a fully-integrated version of modified Look-Locker inversion-recovery (MOLLI) for clinical single-breathhold cardiac T1 mapping. A MOLLI variant allowing direct access to all pulse sequence parameters was implemented on a 1.5T MR system. Varying four critical sequence parameters, MOLLI was performed in eight gadolinium-doped agarose gel phantoms at different simulated heart rates. T1 values were derived for each variant and compared to nominal T1 values. Based on the results, MOLLI was performed in midcavity short-axis views of 20 healthy volunteers pre- and post-Gd-DTPA. In phantoms, a readout flip angle of 35 degrees , minimum TI of 100 msec, TI increment of 80 msec, and use of three pausing heart cycles allowed for most accurate and least heart rate-dependent T1 measurements. Using this pulse sequence scheme in humans, T1 relaxation times in normal myocardium were comparable to data from previous studies, and showed narrow ranges both pre- and postcontrast without heart rate dependency. We present an optimized implementation of MOLLI for fast T1 mapping with high spatial resolution, which can be integrated into routine imaging protocols. T1 accuracy is superior to the original set of pulse sequence parameters and heart rate dependency is avoided.

High‐resolution T1 mapping of the brain at 3T with driven equilibrium single pulse observation of T1 with high‐speed incorporation of RF field inhomogeneities (DESPOT1‐HIFI)

To investigate an alternative approach to correct for flip angle inaccuracies in the driven equilibrium single pulse observation of T1 (DESPOT1) T1 mapping method. While DESPOT1 is a robust method for rapid whole-brain voxelwise mapping of the longitudinal relaxation time, the approach is inherently sensitive to inaccuracies in the transmitted flip angle, defined by the B1 field, which become more severe with increased field. Here we propose an extension of the DESPOT1 technique, involving the additional acquisition of an inversion-prepared SPGR image alongside the conventional multiangle DESPOT1 data. From these combined data both B1 and T1 may be determined with high accuracy and precision. The method is evaluated at 3T with phantom and in vivo imaging experiments, with derived T1 estimates compared with values calculated from multiple inversion time inversion recovery data. The method provides robust correction of flip angle variations, with less than 5% error compared with reference values for T1 between 300 msec and 2500 msec. The described approach, dubbed DESPOT1-HIFI, permits whole-brain T1 mapping at 3T, with 1 mm(3) isotropic voxels, in a clinically feasible time (approximately 10 minutes) with T1 accuracy greater than 5% and with high precision.

Phase-encoding strategies for optimal spatial resolution and T1 accuracy in 3D Look–Locker imaging

The Look-Locker (LL) imaging method provides an accurate and efficient approach for mapping the spin-lattice relaxation time, T(1). However, the same recovery of signal during LL image acquisition required to estimate T(1) also results in unwanted modulation of k-space. This is particularly problematic with 3D LL imaging as the number of phase-encoding steps during the recovery interval (e.g., 16) increases in an effort to reduce imaging times. This modulation of k-space has the effect of introducing a point spread function (PSF), which can lead to either image blurring (if the earlier tip angles are assigned to the centre of k-space) or edge enhancement (if the earlier tip angles are assigned to the edges of k-space), thus corrupting T(1) estimation, particularly for small objects. In this study, the PSF and its effect on the acquired images for four different interleaved phase-encode schemes (centric-in, centric-out, sequential and hybrid-sequential) are simulated for a range of T(1), tip angle and 3D LL acquisition parameters expected in practice. It is shown by simulation and confirmed experimentally in phantoms that a hybrid sequential phase-encoding scheme reduces image blurring while maintaining T(1) accuracy ( approximately 2%) and precision (2%) over a range of object sizes down to 2 pixels (2 mm).

Relationships between attentional blink magnitude, RSVP target accuracy, and performance on other cognitive tasks

When two masked, to-be-attended targets are presented within approximately half a second of each other, performance on the second target (T2) suffers, relative to when the targets are presented further apart in time or when the first target (T1) can be ignored. This pattern of results is known as the attentional blink (AB). Typically, participants differ with respect to the magnitude of their AB and their overall target accuracy. Despite investigations as to what participant characteristics may influence AB performance (e.g., age, brain damage, or mood state), there has been no focused examination of whether individual differences in cognitive performance measures predict the magnitude of the AB or overall rapid serial visual presentation(RSVP) target accuracy. Our university studentparticipants performed single-target and dual-target RSVP tasks, as well as a selection of cognitive tasks that did not use RSVP presentations, with color, letter, digit, and object stimuli. Overall performance on each of the RSVP targets (T1, T2, and single target) was predicted by speeded manual and vocal identification times to isolated stimuli and by performance with other RSVP targets. However, the magnitude of the AB was predicted only by T1 accuracy, not by any other performance measures. The results suggest that individual differences in AB magnitude do not result from differences in effective RSVP target encoding and are not well explained by varied information-processing abilities.

Attentional control and capture in the attentional blink paradigm: Evidence from human electrophysiology

We studied attentional control mechanisms using electrophysiological methods, focusing on the N2pc event-related potential (ERP), to track the moment-by-moment deployment of visual spatial attention. Two digits (T1 and T2, both red or both green, and masked, were embedded in a rapid serial visual presentation of letter distractors with an SOA of 200 ms or 800 ms. T1 was at fixation, whereas T2 was 3° to the left or right of fixation and presented with a concurrent equiluminant distractor digit in a different colour. T1 and T2 were reported in one block of trials, and only T2 in another block (order counterbalanced). Accuracy for T2 was lower at short SOA than at long SOA when both T1 and T2 were reported, suggesting an attentional blink (AB) effect. It was difficult to ignore T1 because T1 had the same colour as T2, producing a large deficit in T2 accuracy at short SOA in the control condition. The amplitude of the N2pc ERP component was attenuated in the short-SOA condition relative to the long-SOA condition, both in the experimental and the control conditions, suggesting that T1 involuntarily captured visual spatial attention and that while attention was deployed on T1, the processing of T2 was significantly impaired.

Modified Look‐Locker inversion recovery (MOLLI) for high‐resolution T1 mapping of the heart

A novel pulse sequence scheme is presented that allows the measurement and mapping of myocardial T1 in vivo on a 1.5 Tesla MR system within a single breath-hold. Two major modifications of conventional Look-Locker (LL) imaging are introduced: 1) selective data acquisition, and 2) merging of data from multiple LL experiments into one data set. Each modified LL inversion recovery (MOLLI) study consisted of three successive LL inversion recovery (IR) experiments with different inversion times. We acquired images in late diastole using a single-shot steady-state free-precession (SSFP) technique, combined with sensitivity encoding to achieve a data acquisition window of < 200 ms duration. We calculated T1 using signal intensities from regions of interest and pixel by pixel. T1 accuracy at different heart rates derived from simulated ECG signals was tested in phantoms. T1 estimates showed small systematic error for T1 values from 191 to 1196 ms. In vivo T1 mapping was performed in two healthy volunteers and in one patient with acute myocardial infarction before and after administration of Gd-DTPA. T1 values for myocardium and noncardiac structures were in good agreement with values available from the literature. The region of infarction was clearly visualized. MOLLI provides high-resolution T1 maps of human myocardium in native and post-contrast situations within a single breath-hold.

Separate and Shared Sources of Dual-Task Cost in Stimulus Identification and Response Selection

There is often strong interference if a second target stimulus (T2) is presented before processing of a prior target stimulus (T1) is complete. In the “Psychological Refractory Period” (PRP) paradigm, responses are speeded and interference manifests as increased response time for T2. In the “Attentional Blink” (AB) paradigm, stimuli are masked and responses unspeeded; interference manifests as reduced T2 accuracy. While different causes have usually been considered for PRP and AB phenomena, recent evidence has supported a unified account based on a single, shared restriction on concurrent processing. Here we show that a full assessment of separate and shared resource limitations requires direct comparison of hybrid PRP/AB trials with corresponding pure PRP and AB cases. Randomizing trial types in such a comparison also brings substantial benefit in addressing possible changes in task preparation or readiness. The data from two such experiments—combining speeded auditory (SA) and unspeeded visual (UV) task events—provide clear evidence for both separate and shared resource limitations. Often interference is strongest for T1 and T2 events of the same type, reflecting predominantly different limitations in SA and UV processing. With modest increases in demand, however, interference between different event types can also be made arbitrarily large, reflecting arbitrarily important shared limitations. For even such simple tasks as these, T1–T2 interference reflects a combination of relatively local and relatively global sources.

T1 measurements in cell cultures: a new tool for characterizing contrast agents at 1.5T.

The objective of this work was to assess the feasibility and accuracy of T1 and relaxivity measurements in cell cultures using 1.5T magnetic resonance imaging (MRI) with the long-term goal to develop a tool for evaluation of novel paramagnetic agents in a realistic macromolecular environment. This initial study was carried out using MCF-7 cells treated with independently determined concentrations of Gd-DTPA. Two cell culture systems were evaluated: cell pellets and single layers of cells grown on microporous inserts. High-resolution T1 measurements of cell cultures were acquired with two dimensional Inversion Recovery Fast Spin Echo (2D-IR-FSE), three dimensional Inversion Recovery Fast Spin Echo (3D-IR-FSE), and 3D-SPGR sequences. The T1 and relaxivity accuracy of these sequences was confirmed with aqueous Gd-DTPA samples of known concentration. Relaxivities of 1.71 +/- 0.15 [mM(-1)second(-1)] and 1.55 +/- 0.50 [mM(-1)second(-1)] were measured in the cell pellets and cell monolayers, respectively, and were different from the value of 4.3 [mM(-1)second(-1)] for Gd-DTPA in water. Both cell pellets and monolayers are suitable for initial assessment of novel MR contrast agents.

Quantitative Magnetic Resonance Imaging With The Mixed Turbo Spin-echo Pulse Sequence: A Validation Study

The quantitative accuracy of images generated with mixed turbo spin echo (mix-TSE) pulse sequence and model-conforming quantitative algorithms is tested experimentally with a multi-sample phantom. Quantitative T1 and T2 accuracy within a few percent is demonstrated for samples representing tissues with MR relaxation times spanning the full T1 and T2 biologic ranges, with the exception of T2 measurements of simple fluids with very long T2 relaxation times. To correct this problem, a simple approach based on the known proportionality T1 μT2 exhibited by simple fluids is proposed. The results show that the mix-TSE pulse sequence can be used to generate accurate and high quality quantitative image sets with large anatomic coverage and near isotropic high spatial resolution. In conclusion, the mix-TSE pulse sequence could become a valuable tool for clinical applications of quantitative MR imaging.

T2 accuracy on a whole-body imager.

MR oximetry requires a T2 measurement that is accurate within 5% in vivo. Simple methods are susceptible to signal loss and tend to underestimate T2. Current methods utilize RF pulses or RF cycling patterns that prevent signal loss at each data acquisition. However, using these methods with imperfect pulses, T2 tends to be overestimated due to temporary storage of the magnetization along the longitudinal axis where it decays more slowly with a time constant T1 > T2. To reduce the T1 dependence while preventing signal loss, we utilize simple 90x180y90x composite pulses and good RF cycling patterns. These trains are critical for T2 accuracy over typical ranges of RF and static field inhomogeneities and refocusing intervals. T1 signal decay during each 90x180y90x pulse must be accounted for to yield accuracy within 5% when the pulse-width is 10% or more of the refocusing interval. A simple correction scheme compensates for this T1-related error effectively.

Cardiac T1 calculations from MR spin-echo images.

Normally, cardiac triggering in MR spin-echo imaging restricts repetition times (TR) to integral values of the cardiac period (TC), and introduces irregularities in TR due to variations in TC. We have investigated how much these restrictions decrease the accuracy and precision of spin-lattice relaxation (T1) values of the myocardium calculated from two cardiac-triggered spin-echo images. By introducing additional excitation pulses, TR can effectively be reduced to a fractional value of TC and considerable improvement in T1 precision is possible. For TC = 800 ms, the improvement in T1 precision is 30% when two spin-echo images of TR = 1/2 X TC and TR = 2 X TC are used to calculate T1 instead of two images with TR = TC and TR = 2 X TC. The irregularities in TR decrease both T1 precision and accuracy. Irregularities of the order of 15% in a mean TR of 800 ms produce a fourfold decrease in precision. Since irregularities in TC easily exceed 15%, MRI data should be acquired when individual TR values are approximately within +/- 15% of the subject's mean TC.