An fMRI investigation of covertly and overtly produced mono- and multisyllabic words.

Shuster LI, Lemieux SK.

Brain and Language 93 (2005):20-31.

The Insula: what is it? An forgotten island

of cortex hiding behind the lateral fissure

Ill-defined functions may include Visceral sensory Vestibular Motor Supplementary motor

Speech movements

Background: Speech and the Insula

Left anterior insular damage linked to acquired apraxia of speech (AOS) (Dronkers, 1996)

AOS characterized as, "a disorder in the motor planning of articulatory movements”

Lesion-overlap method 25 chronic stroke patients

with AOS 19 patients non-AOS

aphasics Hence, “this area seems to

be specialized for the motor planning of speech“

AOS Non-AOS

Background: Speech and the Insula Insular damage-AOS correlation does

not appear to hold for acute-stage stroke patients (Hillis et al., 2004)

Insula currently thought to be involved in motor movements of speech E.g. coordination of muscle movements

rather than planning (Ackermann & Riecker, 2004)

Overview Goal:

Examine the role of the insula in overt and covert speech production

Methods: fMRI Blood Oxygenation Level Dependent (BOLD)

responses Overtness contrast Syllabic-length contrast

Logic: Overtness contrast should identify general overt

speech areas Syllable-length contrast should show specific areas

with graded activation in motor planning of utterances

Shuster and Lemieux (2005)

Behavioral task: Participants (n=10) auditorily

prompted to overtly or covertly “say” individual words 30 monosyllabic nouns 30 tetrasyllabic nouns Words frequency-matched à la

Thorndike and Lorge (1944)

(No additional behavioral tasks; no behavioral measures taken)

Shuster and Lemieux (2005): Methods

Repeat the word to yourself as quickly as you can, without moving any part of your mouth. Just hear

yourself saying the word inside your head. Be careful not to move any part of your mouth and try not to swallow during the response time

Thorndike and Lorge (1944) and Kucera and Francis (1967) frequency ratings for non-zero frequency one- and four- syllable words.

R2 = 0.6511

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But do either of these criticisms really matter here?

-Length effects may be difficult to interpret.

Criticisms of Materials Length likely confounded with

concreteness Thorndike-Lorge?

Criticisms of the tasks Image-naming or stem-completion

would have been a better task Phonological vs. articulatory rehearsal

Participants may have been ‘sleeping’ in covert trials A behavioral measure was needed

Covert-subtraction-based differences difficult to interpret

Shuster and Lemieux (2005): Methods

Design details: Event-related design

Allows isolation of speech/motion artifacts Requires long ISIs (1250 ms!)

Overt and covert response blocks of 12.5 minutes each All words presented in each block Randomized word orders Block order counterbalanced between subjects

Shuster and Lemieux (2005): Methods

Imaging details: 1.5T scanner Anatomical scans

.86 x .86 x 1.2mm voxels Functional scans

20 interleaved axial slices acquired 3.43 x 3.43 x 5 mm voxels No baseline

BOLD responses analyzed via AFNI Subtraction methods for overtness and length

Shuster and Lemieux (2005): Methods

An interesting analysis detail… “the stimulus time

series was convolved with a gamma variate function”

Translation: speech-motion artifacts removed by correlating observed data with an idealized ‘motion artifact’ function

Overview of results More overall activity in overt than

covert speech Included more activity in general

cognitive regions More overall activity in multisyllabic

than monosyllabic word production

Shuster and Lemieux (2005): Results

Shuster and Lemieux (2005):Results

Overtness effects

Covert > Overt

Overt > Covert

Mono- vs. Multisyllable effects

Mono- > Multisyllabic

Multi- > Monosyllabic

Shuster and Lemieux (2005):Results

Left Insula

More active in overt speech

No more active with longer words

Active in speech production but maybe not in sequencing speech movements

Shuster and Lemieux (2005): Discussion

Left inferior parietal lobule (and the left parietal cortex in general)

More active with longer (overt) words

Consistent with previous production and apraxia studies

Indicates involvement in word-length dependent processes (e.g. articulatory planning, sequencing, monitoring)

Shuster and Lemieux (2005): Discussion

Left Superior Temporal Sulcus (STS) More active in overt

speech No more active with

longer words Activity may reflect

perception of self-generated speech Confounded by

auditory stimulus presentation

Shuster and Lemieux (2005): Discussion

Conclusions: Covert and overt speech produced

qualitatively different response patterns, including more activation of general cognitive regions, suggesting that covert production may not be a good substitute for overt production

Shuster and Lemieux (2005)

Conclusions: The left insula is active in overt

speech production, but shows no utterance-length-dependent effects, suggesting a revised role in overt speech production

The left parietal cortex, however, does show more length-related activity

Shuster and Lemieux (2005)

Questions for discussion Any thoughts on the role of the insula in

speech? Monitoring in the left inferior parietal cortex? What might have been happening in the

“inner speech condition? How might this change our interpretation of overt

vs. covert results? Occipital activation?

Shuster and Lemieux (2005)

“Clearly, as Bennett and Netsell (1999) noted, further studies are required before the specific role of the left insula in speech production can be established”

--Shuster and Lemieux (2005)