Corticothalamic feedback connections HST .722 - Brain Mechanisms of Hearing and Speech Topic Defense Michael Slama 11/01/07

Introduction Background Parts of the thalamus are involved in audition: Lateral Part of the Posterior Nucleus Reticular Nucleus Medial Geniculate Body MGB is an obligatory connection for the IC and the auditory cortex: Parallel ascending pathways Descending pathways from the cortex to the thalamus = corticothalamic feedback connections

Introduction More on corticothalamic connections: Outline: MGB receives as many connections from the IC than from the AC They originate in layers V and VI (retrograde labeling studies) They are reciprocal (target the same location from which they receive their input) Outline: Effects of cortical inactivation on thalamic processing Effects of cortical stimulation on thalamic processing Possible functional roles of these connections

Effect of cortical inactivation on thalamus activity (Villa et al, 1991) Methods: Anesthetized cats Auditory cortex is cooled down thanks to ice-cold water circulation (to inactivate the cortical synapses) Single unit recordings from various locations of the thalamus Results: Spontaneous activity decreased for most units, but did not change or increased for some Changes in tuning (bandwidth and best frequency) Changes in the temporal structure in response to noise Interesting to note: Changes depended on the recording location (not a simple modulation of activity) For most units, activity was back to normal after the inactivation was over

Effect of cortical inactivation on thalamus activity (Villa et al, 1991)

Effects of cortical stimulation on thalamic activity (He et al, 2002) Methods: Electrical stimulation of the cortex of guinea pigs Recordings from the MGB Electrical stimulation+delay+acoustic stimulus Results: Influence of the stimulation current is not monotonic (low=+, medium=++, large=-) Delay had an influence Tuning curves were affected Temporal structure changed

Possible functional roles Adaptive filtering (Villa et al, 1991): Thalamic activity is precisely shaped by the cortex Model=processing parameters are adjusted as a function of filter output Could be useful for source segregation Speech separation (Alain et al, 2005): fMRI study with 2 vowels presented simultaneously Significant extra activity in the left thalamus when comparing 1 and 2 correctly identified vowels Consistent with the adaptive filtering view

Why vote for this topic? Because: It is interesting Papers with single unit recording in the thalamus and fMRI suggest that the cortex finely shapes the thalamic activity, which could play a role in source segregation Many aspects can be discussed: What neural circuits? What are other functions for these connections? To what extent can this descending pathway compared to other cortical descending pathways? It relates well to other topics covered: Adaptive filter model has similarities with the DCN model we studied Left-right differences and speech processing Could be compared to the olivocochlear efferents topic

Please vote for this topic! Cortico- thalamic topic