Auditory Working Memory

Auditory working memory (AWM) is the process of keeping representations of auditory objects in mind for short duration when the sounds are not in the environment. This is different from phonological WM as these sounds cannot be assigned a semantic label.

Recent fMRI study on AWM in humans showed a network of activation in auditory cortex, hippocampus, and inferior frontal gyrus. They proposed a system for AWM where sound specific representations in auditory cortex are kept active by projections from hippocampus and inferior frontal cortex.

Unlike fMRI, in MEG direct neuronal activation is measured as tiny (femto Tesla) rapidly changing magnetic fields that are proportional to post synaptic dendritic currents in those pyramidal neurons that are tangentially oriented to the scalp and fire synchronously with other such neurons that are parallel to them within a cortical column. This non-invasive neurophysiological method has excellent milli-second temporal resolution of neuronal activity and good spatial resolution (better than EEG).

My MEG project aims to understand the spectro-temporal dynamics underlying this AWM system for maintaining pitch of a simple tone. What mechanisms underlie neural activity during pitch retention? What is the role of cerebellum in vocal rehearsal? 

In my experiment where pitch of one of two tones is required to be retained for 12 s, the neuro-magnetic response showed excitation throughout the maintenance phase when compared to silent pre-stimulus baseline which is consistent with existing literature. However, when compared to a control condition that required no memorisation, the response during maintenance was not sustained over the entire duration but instead decayed to control after initial excitation. Source localisation of the evoked response during maintenance against silent pre-stimulus baseline showed activity in the auditory cortex similar to that seen for encoding phase. Similarly, source localisation of the induced response during the first second of maintenance against pre-stimulus baseline showed suppressed alpha oscillations in the auditory cortex, enhanced theta oscillations in the medial prefrontal, enhanced beta in cerebellum, suppressed beta in left supramarginal gyrus. 

In my experiment I contrasted working memory for pitch of a tone against working memory for spacing of a visual sinusoidal grating. Source localisation of induced activity during first second of maintenance of auditory information against silent pre-stimulus baseline showed medial prefrontal theta enhancement, cerebellar beta enhancement, and auditory cortex alpha suppression, left supramarginal gyrus theta and beta suppression. Further, I found beta phase coupling between cerebellum and left inferior frontal gyrus (Broca's area) which was correlated with subject performance. So I propose that, representations of pitch are kept active in the auditory cortex through vocal rehearsal by Broca's area with predictive control by Cerebellum in tandem with Posterior Parietal Cortex and consolidated by the medial prefrontal cortex.