Functional Topography of Corticothalamic Feedback Enhances Thalamic Spatial Response Tuning in the Somatosensory Whisker/Barrel System

Abstract

Corticothalamic (CT) projections are approximately 10 times more numerous than thalamocortical projections, yet their function in sensory processing is poorly understood. In particular, the functional significance of the topographic precision of CT feedback is unknown. We addressed these issues in the rodent somatosensory whisker/barrel system by deflecting individual whiskers and pharmacologically enhancing activity in layer VI of single whisker-related cortical columns. Enhancement of corticothalamic activity in a cortical column facilitated whisker-evoked responses in topographically aligned thalamic barreloid neurons, while activation of an adjacent column weakly suppressed activity at the same thalamic site. Both effects were more pronounced when stimulating the preferred, or principal, whisker than for adjacent whiskers. Thus, facilitation by homologous CT feedback sharpens thalamic receptive field focus, while suppression by nonhomologous feedback diminishes it. Our findings demonstrate that somatosensory cortex can selectively regulate thalamic spatial response tuning by engaging topographically specific excitatory and inhibitory mechanisms in the thalamus.

Cortical control of adaptation and sensory relay mode in the thalamus

“…However, the spatially diffuse acetylcholine action of brainstem inputs which activates slow muscarinic receptors in the VPM may represent the overall level of vigilance and modulate the thalamus globally on slow time scales. In contrast, spatially and temporally precise changes in firing mode and adaptation likely are under L6 control because of the topographical precision of cortico-thalamic axons (40,41). These results somewhat contrast with a study by Olsen et al (23) in the visual thalamus in which tonic mode shifts by L6 were observed but did not reach significance level (SI Discussion).…”

Looking back: corticothalamic feedback and early visual processing

“…Similar mechanisms have been suggested to operate in both the somatosensory [57][58][59][60] and the auditory [5] systems.…”

Local Connections of Excitatory Neurons to Corticothalamic Neurons in the Rat Barrel Cortex

“…A recent study has further reported that activation of the barrel column enhances the sensory responses of relay neurons in the corresponding thalamic compartment, or barreloid (Temereanca and Simons, 2004). L6 CTN activation in a barrel column thus has an excitatory/ facilitatory influence on the activity of thalamic relay neurons, which transfer peripheral information to the cerebral cortex.…”

“…In this reciprocal circuit, L6 corticothalamic projection neurons (CTNs) are considered to enhance and tune thalamic responses to peripheral stimuli (Yuan et al, 1986;Yan and Suga, 1996;Przybyszewski et al, 2000;Alitto and Usrey, 2003;Temereanca and Simons, 2004;Thomson, 2010). Local connections between cortical layers are well developed (Briggs and Callaway, 2001;Mercer et al, 2005;Zarrinpar and Callaway, 2006;West et al, 2006;Lefort et al, 2009;Llano and Sherman, 2009;Lam and Sherman, 2010;Hooks et al, 2011), and in addition to external inputs, local translaminar inputs are involved in shaping CTN activity.…”