ECOG Dynamics Word Processing Canolty 2007

May 21, 2009 10:32 PM

Ryan Canolty may be the 34 year old who was capped twice... once while in his mother's womb.

One neuroimaging method with

excellent combined spatial and temporal resolution is electrocorticography

(ECoG) recorded directly fromthe human cortex using subdural electrodes.

The ECoG technique has several advantages over EEG and MEG. The sub-

dural ECoG signal is an order of magnitude stronger in amplitude than

scalp recorded EEG and is not affected by the ocular and muscle artifacts

which contaminate scalp EEG. Furthermore, the source of the signal may

be more precisely estimated. Most importantly, the ECoG signal provides

access to high frequency electrical brain activity (60–200 Hz) not readily

seen in the scalp EEG. The high gamma (HG) band (80–200 Hz) has been

shown to be a strong index of sensory-, motor-, and task-related cortical

activation across multiple tasks including language processing (Crone et

al., 1998a,b; Crone et al., 2001a,b; Edwards et al., 2005). HG is largely

invisible to scalp EEG due to amplitude attenuation and spatial low-pass

filtering (Nunez and Srinivasan, 2006). HG amplitude can exist as high as

5–10V on the cortex and is likely at least an order of magnitude less on

the scalp.

The four patients (all females, age range 35–45 years) participating in

this study were candidates for surgical treatment for medically refractory

epilepsy. Each had undergone a craniotomy for chronic implantation of

a subdural electrode array and depth electrodes. The placement of the

electrodes was determined on clinical grounds and varied for each sub-

ject, but included coverage of an 8 cm×8 cm area centered over the

left frontotemporal region for each of the four subjects described here.

Implantation was followed by approximately 1 week of continuous mon-

itoring of the ECoG in order to more precisely localize (1) the seizure

focus for later resection, and (2) critical language and motor areas to be

avoided during resective surgery.

DISCUSSION

These ECoG results demonstrate an

orderly and automatic flow of word processing in the human temporal

lobe. In particular, the HG band identifies the cortical regions involved in

word processing with a greater spatial and temporal specificity than any

other frequency band tested. Word processing involves sequential activa-

tion of the post-STG,mid-STG, and STS and these results validate previous

spatial results regarding the cortical regions involved in word processing,

and, in turn, language comprehension. These neuroanatomical results

support lesion and neuroimaging studies which have shown word-related

activity to occur in the post-STG, mid-STG, and STS

However, these

results also reveal the temporal flow of information between these distinct

brain regions and support a component of serial processing in language.

This study complements and extends Binder and colleagues (2000) by

demonstrating that word processing first activates the post-STG, then the

mid-STG, and finally the STS.

the STS.

Hickok and Poeppel (2007) propose a hierarchical model of word pro-

cessing with parallel analysis of a word for its acoustic-spectral content

by auditory regions and for its phonetic content by the STS, and later for

its meaning-based content by regions in the posterior middle and inferior

portions of the temporal lobe. Importantly, they specify that both the left

and the right hemispheres are involved in speech processing (i.e., pre-

dominately the ventral stream). In this study, we were unable to thoroughly

address their argument for parallel processing, because all of our subjects

had electrode grids placed over their left hemisphere for clinical purposes.

In regards to information flow, however, we did observe a systematic flow

of word processing beginning with acoustic processing in the auditory

cortices and ending with meaning-based processing in the STS. Note that

Hickok and Poeppel argue that the STS is involved in phonetic processing.

Our paradigmdid not included phonemes so we cannot definitely conclude

that the STS is solely involved in the processing of words for meaning and

not involved in phonetic level analysis.We can, however, conclude that our

data supports their proposal of a hierarchically organized ventral stream,

which may not necessarily correspond with their functional subdivision of

the temporal cortex.

In accord with P

ECOG Dynamics Word Processing Canolty 2007

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