LANGUAGE PRODUCTION
4. SUMMARY
1.5. Advantages and Disadvantages of Interleaved Acquisition Techniques Acoustic gradient noise during task performance causes measurable BOLD-contrast
signal modulations in both cortical and subcortical structures (Bandettini et al., 1998). In conventional continuous data acquisition procedures, the gradient noise occurs equally in the control and task conditions. Assuming that the signal changes due to gradient noise are linearly additive with other sources of signal change (e.g. task-related changes), in principle it should be possible to subtract the noise effects. In this way categorical data analysis approaches based on image subtraction, such as Student’s t-test, can be used to identify regions that exhibit task-related activity. Although it is reasonable to assume that linear additivity for gradient noise might hold for many cortical regions, it is less likely that this assumption is reasonable for cortical areas known to be responsive to auditory stimuli (Talavage & Edmister, 2004). Non-linear effects and interference are particularly to be expected at frequencies near those generated by the gradients.
By employing a longer TR than customarily used in conventional continuous imaging, interleaved acquisition techniques have potential increased sensitivity to small signal changes, resulting from the improved contrast-to-noise that occurs with longer TR inter-vals as a result of more fully recovered longitudinal relaxation. However, the loss of sig-nal due to incomplete recovery of the longitudisig-nal magnetization with shorter TRs has been shown to be outweighed by the increased statistical power gained by the larger num-ber of samples collected in that situation (Constable & Spencer, 2001).
When using interleaved approaches, the proportion of time spent in performance of the target task is reduced, resulting in a situation in which the subject makes relatively fewer responses during the imaging session than during a continuous acquisition experiment utilizing the same overall run length. Therefore, interleaved techniques may be less sen-sitive because of the smaller number of samples collected in the same acquisition period.
However, the reduced power related to the smaller sample size may be balanced by the fact that, when using interleaved gradients, the subjects alternate more frequently between target and control tasks, with an attendant reduction in response habituation and therefore greater task-related signal modulation.
Also with interleaved acquisition techniques, susceptibility artifacts resulting from jaw and tongue motion can be greatly reduced. Experiments involving verbal responses are necessarily associated with orofacial movement that can induce significant susceptibility artifacts in medial temporal and orbitofrontal cortical regions. As these areas are involved in language production and understanding, it is obvious that tasks involving orofacial movement could be associated with signal drop-out or image distortion in the very regions that would be the principal objects of study. Speech is also likely to be associated
with periodic head motion, resulting from the mechanical coupling of the jaw and skull such that relatively small jaw movements can result in large rotational movements of the skull. Since even small amounts of uncorrected interscan motion can result in both false-positive and false-negative effects in statistical maps, even when using interleaved techniques it is advisable to take precautions to minimize head motion using comfortable restraints that allow unfettered jaw motion.
To illustrate the application of this method, Figure 4 shows an example of interleaved acquisition used to detect activity related to single word reading. An interleaved design was used to reduce the problem of susceptibility artifacts resulting from head and jaw movements. This is an important issue in understanding the brain mechanisms responsi-ble for language processing, because evidence from PET studies, that are uncontaminated by motion related susceptibility artifacts, indicates that covert and overt naming responses engage different neural processing systems (Bookheimer et al., 1995). Results from a single subject are shown for the comparison of reading words silently versus fix-ation (bottom) and reading words aloud versus fixfix-ation (top).
Figure 4. Interleaved and continuous acquisition compared.
HANDBOOK OF PSYCHOLINGUISTICS Continuous
Block vs. vs.
Reference Speech task event-related interleaved Comments
Abrahams et al. Verbal fluency, Block Interleaved
(2003) confrontation naming
Abrahams et al. Verbal fluency, Block Interleaved
(2004) confrontation naming
Aldenkamp et al. Naming task Block Interleaved
(2003)
Baciu, Rubin, Word fluency Block Continuous Soft articulation requested;
Decorps, and comparison of aloud and silent
Segebarth (1999) word fluency
Barch et al. Stroop task, verb Block Continuous Avoid scanning at throat and
(1999) generation, noun mouth, discard data acquired
reading during speech; comparison of
aloud and silent Stroop task and noun reading
Barch Brover, Sabb, Verb generation Event-related Continuous and Noll (2000)
Barrett, Pike, and Paus (2004) Reading before and Event-related Interleaved after mood induction
Birn, Bandettini, Cox, and Speech production Block and Continuous
Shaker (1999) event-related
FUNCTIONAL NEUROIMAGING OF SPEECH PRODUCTION139 and Peterson (2003)
Burton, Noll, Word repetition Block Continuous
and Small (2001)
de Zubicaray, Wilson, Picture-word task Event-related Interleaved McMahon, and Muthiah (2001)
de Zubicaray, McMahon, Picture-word task Event-related Interleaved Eastburn, and Wilson (2002)
Dietz, Jones, Gareau, Word reading Block Interleaved Comparison of aloud and silent
Zeffiro, and Eden (2005) word reading
Dietz, Jones, Word reading Block Interleaved and Comparison of aloud and silent
Twomey, Zeffro, continuous word reading
and Eden, (in press)
Eden et al. (2004) Word reading Block Interleaved
Frenck-Mestre, Word and sentence Block Continuous
Anton, Roth, Vaid, reading and Viallet (2005)
Grabowski et al. Object naming Event-related Continuous Adaptive pacing
(2006) algorithm
Haller, Radue, Sentence generation, Event-related Continuous Erb, Grodd, and word and sentence
Kircher (2005) reading
Hashimoto and Sentence reading Block Interleaved
Sakai (2003)
(Continued)
HANDBOOK OF PSYCHOLINGUISTICS Table 1
(Continued)
Continuous
Block vs. vs.
Reference Speech task event-related interleaved Comments
He et al. (2003) Chinese word and Block Continuous Comparison of aloud and silent
pinyin reading Chinese word and pinyin reading
Heim, Opitz, and Picture naming, Event-related Continuous
Friederici (2002) grammatical gender
Huang, Carr, and Cao Letter naming, animal Event-related Continuous Comparison of aloud and silent
(2002) name generation letter naming and animal
name generation Jung, Prasad, Qin and Name repetition Event-related Continuous Active noise cancellation Anderson (2005) (in new order)
Kan & Thompson- Picture naming Block Interleaved Comparison of aloud and
Schill (2004) silent picture naming
Kemeny et al. (2006) Sentence generation Block Continuous
Kemeny,Ye, Birn, and Object and action naming Event-related Continuous Braun (2005)
Kircher, Brammer, Levelt, Continuous speech Event-related Continuous Bartels, and McGuire (2004) production
Leger et al. Picture naming and Block Continuous
(2002) picture/word rhyming
FUNCTIONAL NEUROIMAGING OF SPEECH PRODUCTION141
McCarthy, Blamire, Word generation, Block Continuous Comparison of aloud and silent
Rothman, Gruetter, word repetition word generation
and Shulman (1993)
Naeser et al. (2004) Speech production Block Continuous Dynamic susceptibility
contrast fMRI technique
Nelles et al. (2003) Read and Event-related Continuous
generate words
Neumann et al. (2003) Sentence reading Event-related Continuous Neumann et al. (2005) Sentence reading Event-related Continuous
Owen, Borowsky, Word naming Block Continuous
and Sarty (2004)
Palmer et al. Word stem Event-related Continuous Comparison of aloud and silent
(2001) completion word stem completion
Peck et al. (2004) Word generation Event-related Continuous
Phelps, Hyder, Blamirc, Word repetition, antonym Block Continuous and Shulman (1997) generation, word generation
(Continued)
HANDBOOK OF PSYCHOLINGUISTICS (Continued)
Continuous
Block vs. vs.
Reference Speech task event-related interleaved Comments
Preibisch et al. (2003a) Sentence reading Event-related Continuous Preibisch et al. (2003b) Word and sentence reading Event-related Continuous
Riecker, Ackermann, Speech and melody Event-related Continuous Comparison of aloud and silent
Wildgruber, Dogil, and production speech and melody production
Grodd (2000)
Rosen, Ojemann, Oilinger, Word stem completion Event-related Continuous Comparison of aloud and silent
and Petersen (2000) word stem completion
Shuster and Word reading Event-related Continuous Comparison of aloud and silent
Lemieux (2005) word reading
Small et al. (1996) Word reading Block Continuous
Turkeltaub, Eden, Jones, Word reading Block Interleaved
and Zeffiro (2002)
Viswanath, Karmonik, King, Speech production Event-related Continuous Rosenfield, and Mawad (2003)
Yetkin et al. Word generation Block Continuous Comparison of aloud and silent
(1995) word generation
With the interleaved acquisition mechanism, jaw movement does not occur during data acquisition, resulting in reduced signal drop-out and geometric distortion effects in the derived statistical maps. The data show that reading aloud and reading silently make dif-ferent demands on the inferior frontal gyrus (IFG).
Because of their lengthy TR intervals, interleaved techniques have limited temporal resolution and are optimal for experimental designs requiring imaging of the entire brain at the relatively low sampling rates of six times per minute or less. Two to four seconds are required to image the entire brain utilizing echo-planar imaging. As the hemodynamic response to a brief movement takes ten seconds to complete, it is possible to image the entire cerebrum during the peak of the hemodynamic modulation.
Because of their relative insensitivity to acoustic and motion artifacts, the interleaved techniques produce activity maps that are comparable to or better than those derived using continuous acquisition, possibly because the reduced inter-scan head motion would result in less temporal image misregistration and therefore higher resulting levels of statistical significance. In addition, interleaved techniques employ a longer TR that can result in increased sensitivity to small signal changes due to the improved contrast-to-noise. In Table 1 we present a guide to some of the recent fMRI work using speech responses to study the neural mechanisms of cognitive processing. Both continuous and interleaved techniques are employed using both block and event-related timing arrange-ments.