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227
Temporal autocorrelation in univariate linear modelling of fMRI data
- pP Y C W P k nk N p Var(Yk ) (Yk ) 0 1 C CR 1 Var(Y ) P k nk N Var(Y k ) 0 1 C MI H(X;Y ) H(X) H(Y ) 1 0 C NMI H(X;Y ) H(X)+H(Y
, 2000
"... In functional magnetic resonance imaging statistical analysis there are problems with accounting for temporal autocorrelations when assessing change within voxels. Techniques to date have utilized temporal filtering strategies to either shape these autocorrelations or remove them. Shaping, or “color ..."
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Cited by 211 (10 self)
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In functional magnetic resonance imaging statistical analysis there are problems with accounting for temporal autocorrelations when assessing change within voxels. Techniques to date have utilized temporal filtering strategies to either shape these autocorrelations or remove them. Shaping, or “coloring, ” attempts to negate the effects of not accurately knowing the intrinsic autocorrelations by imposing known autocorrelation via temporal filtering. Removing the autocorrelation, or “prewhitening, ” gives the best linear unbiased estimator, assuming that the autocorrelation is accurately known. For single-event designs, the efficiency of the estimator is considerably higher for prewhitening compared with coloring. However, it has been suggested that sufficiently accurate estimates of the autocorrelation are currently not available to give prewhitening acceptable bias. To overcome this, we consider different ways to estimate the autocorrelation for use in prewhitening. After highpass filtering is performed, a Tukey taper (set to smooth the spectral density more than would normally be used in spectral density estimation) performs best. Importantly, estimation is further improved by using nonlinear spatial filtering to smooth the estimated autocorrelation, but only within tissue type. Using this approach when prewhitening reduced bias to close to zero at probability levels as low as 1 � 10 �5. © 2001 Academic Press Key Words: FMRI analysis; GLM; temporal filtering; temporal autocorrelation; spatial filtering; singleevent; autoregressive model; spectral density estimation; multitapering.
The link between social cognition and self-referential thought in the medial prefrontal cortex.
, 2005
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Separating processes within a trial in event-related functional MRI. I. The method. NeuroImage 13
, 2001
"... Many cognitive processes occur on time scales that can significantly affect the shape of the blood oxygenation level-dependent (BOLD) response in eventrelated functional MRI. This shape can be estimated from event related designs, even if these processes occur in a fixed temporal sequence (J. M. Oll ..."
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Cited by 113 (3 self)
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Many cognitive processes occur on time scales that can significantly affect the shape of the blood oxygenation level-dependent (BOLD) response in eventrelated functional MRI. This shape can be estimated from event related designs, even if these processes occur in a fixed temporal sequence (J. M. Ollinger, G. L. Shulman, and M. Corbetta. 2001. NeuroImage 13: 210–217). Several important considerations come into play when interpreting these time courses. First, in single subjects, correlations among neighboring time points give the noise a smooth appearance that can be confused with changes in the BOLD response. Second, the variance and degree of correlation among estimated time courses are strongly influenced by the timing of the experimental design. Simulations show that optimal results are obtained if the intertrial intervals are as short as possible, if they follow an exponential distribution with at least three distinct values, and if 40 % of the trials are partial trials. These results are not particularly sensitive to the fraction of partial trials, so accurate estimation of time courses can be obtained with lower percentages of partial trials (20–25%). Third, statistical maps can be formed from F statistics computed with the extra sum of square principle or by t statistics computed from the cross-correlation of the time courses with a model for the hemodynamic response. The latter method relies on an accurate model for the hemodynamic response. The most robust model among those tested was a single gamma function. Finally, the power spectrum of the measured BOLD signal in rapid event-related paradigms is similar to that of the noise. Nevertheless, highpass filtering is desirable if the appropriate model
Optimization of experimental design in fMRI: a general framework using a genetic algorithm
, 2003
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When keeping in mind supports later bringing to mind: neural markers of phonological rehearsal predict subsequent remembering
- J Cogn Neurosci
, 2001
"... Abstract & The ability to bring to mind a past experience depends on the cognitive and neural processes that are engaged during the experience and that support memory formation. A central and much debated question is whether the processes that underlie rote verbal rehearsal-that is, working mem ..."
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Cited by 79 (9 self)
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Abstract & The ability to bring to mind a past experience depends on the cognitive and neural processes that are engaged during the experience and that support memory formation. A central and much debated question is whether the processes that underlie rote verbal rehearsal-that is, working memory mechanisms that keep information in mind-impact memory formation and subsequent remembering. The present study used eventrelated functional magnetic resonance imaging (fMRI) to explore the relation between working memory maintenance operations and long-term memory. Specifically, we investigated whether the magnitude of activation in neural regions supporting the on-line maintenance of verbal codes is predictive of subsequent memory for words that were roterehearsed during learning. Furthermore, during rote rehearsal, the extent of neural activation in regions associated with semantic retrieval was assessed to determine the role that incidental semantic elaboration may play in subsequent memory for rote-rehearsed items. Results revealed that (a) the magnitude of activation in neural regions previously associated with phonological rehearsal (left prefrontal, bilateral parietal, supplementary motor, and cerebellar regions) was correlated with subsequent memory, and (b) while rote rehearsal did not-on average-elicit activation in an anterior left prefrontal region associated with semantic retrieval, activation in this region was greater for trials that were subsequently better remembered. Contrary to the prevalent view that rote rehearsal does not impact learning, these data suggest that phonological maintenance mechanisms, in addition to semantic elaboration, support the encoding of an experience such that it can be later remembered. &
fMRI responses to video and point light displays of moving humans and manipulatable objects
- J. Cogn. Neurosci
"... & We used fMRI to study the organization of brain responses to different types of complex visual motion. In a rapid event-related design, subjects viewed video clips of humans perform-ing different whole-body motions, video clips of manmade manipulable objects (tools) moving with their character ..."
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Cited by 69 (7 self)
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& We used fMRI to study the organization of brain responses to different types of complex visual motion. In a rapid event-related design, subjects viewed video clips of humans perform-ing different whole-body motions, video clips of manmade manipulable objects (tools) moving with their characteristic natural motion, point-light displays of human whole-body motion, and point-light displays of manipulable objects. The lateral temporal cortex showed strong responses to both moving videos and moving point-light displays, support-ing the hypothesis that the lateral temporal cortex is the cortical locus for processing complex visual motion. Within the lateral temporal cortex, we observed segregated responses to different types of motion. The superior temporal sulcus (STS) responded strongly to human videos and human point-light displays, while the middle temporal gyrus (MTG) and the inferior temporal sulcus responded strongly to tool videos and tool point-light displays. In the ventral temporal cortex, the lateral fusiform responded more to human videos than to any other stimulus category while the medial fusiform preferred tool videos. The relatively weak responses observed to point-light displays in the ventral temporal cortex suggests that form, color, and texture (present in video but not point-light displays) are the main contributors to ventral temporal activity. In contrast, in the lateral temporal cortex, the MTG responded as strongly to point-light displays as to videos, suggesting that motion is the key determinant of response in the MTG. Whereas the STS responded strongly to point-light displays, it showed an even larger response to video displays, suggesting that the STS integrates form, color, and motion information. &
An event-related fMRI investigation of implicit semantic priming
- J Cogn Neurosci
, 2003
"... & The neural basis underlying implicit semantic priming was investigated using event-related fMRI. Prime-target pairs were presented auditorily for lexical decision (LD) on the target stimulus, which was either semantically related or unrelated to the prime, or was a nonword. A tone task was als ..."
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Cited by 58 (0 self)
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& The neural basis underlying implicit semantic priming was investigated using event-related fMRI. Prime-target pairs were presented auditorily for lexical decision (LD) on the target stimulus, which was either semantically related or unrelated to the prime, or was a nonword. A tone task was also administered as a control. Behaviorally, all participants demonstrated semantic priming in the LD task. fMRI results showed that for all three conditions of the LD task, activation was seen in the superior temporal gyrus (STG), the middle temporal gyrus (MTG), and the inferior parietal lobe, with greater activation in the unrelated and nonword conditions than in the related condition. Direct comparisons of the related and unrelated conditions revealed foci in the left STG, left precentral gyrus, left and right MTGs, and right caudate, exhibiting significantly lower activation levels in the related condition. The reduced activity in the temporal lobe suggests that the perception of the prime word activates a lexical– semantic network that shares common elements with the target word, and, thus, the target can be recognized with enhanced neural efficiency. The frontal lobe reductions most likely reflect the increased efficiency in monitoring the activation of lexical representations in the temporal lobe, making a decision, and planning the appropriate motor response. &
Neural mechanisms of visual object priming: Evidence for perceptual and semantic distinctions in fusiform cortex.
- Neuroimage,
, 2003
"... Abstract Previous functional imaging studies have shown that facilitated processing of a visual object on repeated, relative to initial, presentation (i.e., repetition priming) is associated with reductions in neural activity in multiple regions, including fusiform/lateral occipital cortex. Moreove ..."
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Cited by 42 (7 self)
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Abstract Previous functional imaging studies have shown that facilitated processing of a visual object on repeated, relative to initial, presentation (i.e., repetition priming) is associated with reductions in neural activity in multiple regions, including fusiform/lateral occipital cortex. Moreover, activity reductions have been found, at diminished levels, when a different exemplar of an object is presented on repetition. In one previous study, the magnitude of diminished priming across exemplars was greater in the right relative to the left fusiform, suggesting greater exemplar specificity in the right. Another previous study, however, observed fusiform lateralization modulated by object viewpoint, but not object exemplar. The present fMRI study sought to determine whether the result of differential fusiform responses for perceptually different exemplars could be replicated. Furthermore, the role of the left fusiform cortex in object recognition was investigated via the inclusion of a lexical/semantic manipulation. Right fusiform cortex showed a significantly greater effect of exemplar change than left fusiform, replicating the previous result of exemplar-specific fusiform lateralization. Right fusiform and lateral occipital cortex were not differentially engaged by the lexical/semantic manipulation, suggesting that their role in visual object recognition is predominantly in the visual discrimination of specific objects. Activation in left fusiform cortex, but not left lateral occipital cortex, was modulated by both exemplar change and lexical/semantic manipulation, with further analysis suggesting a posterior-to-anterior progression between regions involved in processing visuoperceptual and lexical/semantic information about objects. The results are consistent with the view that the right fusiform plays a greater role in processing specific visual form information about objects, whereas the left fusiform is also involved in lexical/semantic processing.
Detection power, estimation efficiency, and predictability in event-related fMRI
- NEUROIMAGE 13: 759–773
, 2001
"... Experimental designs for event-related functional magnetic resonance imaging can be characterized by both their detection power, a measure of the ability to detect an activation, and their estimation efficiency, a measure of the ability to estimate the shape of the hemodynamic response. Randomized d ..."
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Cited by 37 (2 self)
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Experimental designs for event-related functional magnetic resonance imaging can be characterized by both their detection power, a measure of the ability to detect an activation, and their estimation efficiency, a measure of the ability to estimate the shape of the hemodynamic response. Randomized designs offer maximum estimation efficiency but poor detection power, while block designs offer good detection power at the cost of minimum estimation efficiency. Periodic single-trial designs are poor by both criteria. We present here a theoretical model of the relation between estimation efficiency and detection power and show that the observed trade-off between efficiency and power is fundamental. Using the model, we explore the properties of semirandom designs that offer intermediate trade-offs between efficiency and power. These designs can simultaneously achieve the estimation efficiency of randomized designs and the detection power of block designs at the cost of increasing the length of an experiment by less than a factor of 2. Experimental designs can also be characterized by their predictability, a measure of the ability to circumvent confounds such as habituation and anticipation. We examine the relation between detection power, estimation efficiency, and predictability and show that small increases in predictability can offer significant gains in detection power with only a minor decrease in estimation efficiency.
