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

2. 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

3. 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

4. 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

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

6. 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

7. 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

8. 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

9. Please vote for this topic!
Cortico-
thalamic
topic