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.
My MEG project aims to understand the dynamics underlying this proposed system. What mechanisms underlie neural activity during retention? What is the role of hippocampus in AWM?
Here is a visual summary of this project
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 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 response during the first second of maintenance against control condition showed activity in the auditory cortex. So I conclude that, for the retention of a single tone in memory, representations of the acoustic stimulus are maintained by the activation of the auditory cortex at the start of the retention phase but it is not persistent throughout the delay.
In my second experiment I contrasted working memory for pitch of a tone against working memory for spacing of a visual sinusoidal grating. Source localisation of activity during maintenance of auditory information against visual information showed delta-theta suppression in primary auditory cortex (Heschl's Gyrus), theta and gamma suppression in left Inferior Frontal Gyrus (IFG) also known as Broca's area. Further source localisation of activity during 5th sec of maintenance against silent pre-stimulus baseline showed gamma suppression in Medial Temporal Lobe (MTL) or more specifically Hippocampus given my spatial priors from earlier fMRI study. So I conclude that, representations of auditory stimulus are kept active in sensory cortex through coupling of auditory cortex (gamma) with Broca's area and Hippocampus (delta to theta).