Corticothalamic Projections Deliver Enhanced-Responses to Medial Geniculate Body as a Function of the Temporal Reliability of the Stimulus

Abstract

Aging and challenging signal-in-noise conditions are known to engage use of cortical resources to help maintain speech understanding. Extensive corticothalamic projections are thought to provide attentional, mnemonic and cognitive-related inputs in support of sensory inferior colliculus (IC) inputs to the medial geniculate body (MGB). Here we show that a decrease in modulation depth, a temporally less distinct periodic acoustic signal, leads to a jittered ascending temporal code, changing MGB unit responses from adapting responses to responses showing repetition-enhancement, posited to aid identification of important communication and environmental sounds. Young-adult male Fischer Brown Norway rats, injected with the inhibitory opsin archaerhodopsin T (ArchT) into the primary auditory cortex (A1), were subsequently studied using optetrodes to record single-units in MGB. Decreasing the modulation depth of acoustic stimuli significantly increased repetition-enhancement. Repetition-enhancement was blocked by optical inactivation of corticothalamic terminals in MGB. These data support a role for corticothalamic projections in repetition-enhancement, implying that predictive anticipation could be used to improve neural representation of weakly modulated sounds. Key pointsO_LIAging has been shown to increase temporal jitter in the ascending acoustic code prompting use of cognitive/attentional mechanisms to help better understand communication-like signals. C_LIO_LIAuditory thalamus receives extensive projections from cortex that are implicated in delivering higher-order cortical computations to enhance thalamic responses. C_LIO_LIThe present study modeled aging in young rats by using temporally less distinct stimuli shown to alter the pattern of MGB unit responses from response adaptation to repetition-enhancement. Enhanced responses to repeating less temporally distinct modulated stimuli were reversed when inputs from cortex to auditory thalamus were blocked. Collectively, these data argue that low salience temporal signals engage cortical processes to enhance coding of weakly modulated signals in auditory thalamus. C_LI