Abstract
The sensorimotor cortex is responsible for the generation of movements and interest in the ability to use this area for decoding speech by brain–computer interfaces has increased recently. Speech decoding is challenging however, since the relationship between neural activity and motor actions is not completely understood. Non-linearity between neural activity and movement has been found for instance for simple finger movements. Despite equal motor output, neural activity amplitudes are affected by preceding movements and the time between movements. It is unknown if neural activity is also affected by preceding motor actions during speech. We addressed this issue, using electrocorticographic high frequency band (HFB; 75–135 Hz) power changes in the sensorimotor cortex during discrete vowel generation. Three subjects with temporarily implanted electrode grids produced the /i/ vowel at repetition rates of 1, 1.33 and 1.66 Hz. For every repetition, the HFB power amplitude was determined. During the first utterance, most electrodes showed a large HFB power peak, which decreased for subsequent utterances. This result could not be explained by differences in performance. With increasing duration between utterances, more electrodes showed an equal response to all repetitions, suggesting that the duration between vowel productions influences the effect of previous productions on sensorimotor cortex activity. Our findings correspond with previous studies for finger movements and bear relevance for the development of brain-computer interfaces that employ speech decoding based on brain signals, in that past utterances will need to be taken into account for these systems to work accurately.
Highlights
The execution of everyday voluntary body movements generally occurs without effort and is the result of the concerted action of different neural processes and brain areas
We focused mostly on the ventral parts of the sensorimotor cortex as this area has previously been shown to be responsible for the generation of speech movements (Penfield and Boldrey 1937; Crone et al 2001; Towle et al 2008; Pei et al 2011; Bouchard et al 2013) and has been the focus of BCI-studies for the classification of speech sounds and articulator movements (Bleichner et al 2015)
Since the difference for subject A is relatively small, and there is no significant difference for subject B, these results suggest that there was not a strong overall difference between vowel production duration for the three repetition rates for these two subjects
Summary
The execution of everyday voluntary body movements generally occurs without effort and is the result of the concerted action of different neural processes and brain areas. Imaging and patient studies have shown that repeating a movement in a discrete way (with short pauses between each movement) may involve different brain areas than performing the same movements in a continuous way (without short pauses between each movement; Kennerley et al 2002; Spencer et al 2003; Miall and Ivry 2004; Schaal et al 2004), even though the movements are almost identical. There is evidence for a non-linear relationship between movement-performance and neural activity in the sensorimotor cortex. During repeated finger movements, the amplitude of sensorimotor neural activity, as measured with fMRI and electrocorticography (ECoG), was shown to decline over repetitions, despite equal movement output (Hermes et al 2012b; Siero et al 2013; for a comparison between BOLD and ECoG see: Logothetis et al 2001; Hermes et al 2012a; Siero et al 2014)
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