Corticothalamic projections deliver enhanced-responses to medial geniculate body as a function of the temporal reliability of the stimulus.

KEY POINTS In response to a less temporally distinct repeating sound with low modulation depth, medial geniculate body (MGB) single-units show a switch from adaptation toward repetition-enhancement. Repetition enhancement was reversed by blockade of MGB inputs from the auditory cortex. Collectively, these data argue that diminished acoustic temporal cues such as weak modulation engage cortical processes to enhance coding of those cues in auditory thalamus. ABSTRACT Aging and challenging signal-in-noise conditions are known to engage the 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. A: The ascending sensory code from inferior colliculus (IC) shows adaptive responses to distinct repeating modulated stimuli which are reflected in unit responses at the level of medial geniculate body (MGB). B: MGB unit responses to weakly modulated/less distinct stimuli engage additional cortical resources as predictions changing counterproductive adaptive responses toward repetition-enhancement. C: Corticothalamic (CT)/top-down blockade during weakly modulated/less distinct stimuli blocks delivery of predictive information to MGB reversing repetition-enhancement back toward adaptive responses. This article is protected by copyright. All rights reserved.