Transformation from temporal to rate coding in a somatosensory thalamocortical pathway

Abstract

The anatomical connections from the whiskers to the rodent somatosensory (barrel) cortex form two parallel (lemniscal and paralemniscal) pathways. It is unclear whether the paralemniscal pathway is directly involved in tactile processing, because paralemniscal neuronal responses show poor spatial resolution, labile latencies and strong dependence on cortical feedback. Here we show that the paralemniscal system can transform temporally encoded vibrissal information into a rate code. We recorded the representations of the frequency of whisker movement along the two pathways in anaesthetized rats. In response to varying stimulus frequencies, the lemniscal neurons exhibited amplitude modulations and constant latencies. In contrast, paralemniscal neurons in both thalamus and cortex coded the input frequency as changes in latency. Because the onset latencies increased and the offset latencies remained constant, the latency increments were translated into a rate code: increasing onset latencies led to lower spike counts. A thalamocortical loop that includes cortical oscillations and thalamic gating can account for these results. Thus, variable latencies and effective cortical feedback in the paralemniscal system can serve the processing of temporal sensory cues, such as those that encode object location during whisking. In contrast, fixed time locking in the lemniscal system is crucial for reliable spatial processing.

Input-specific effects of acetylcholine on sensory and intracortical evoked responses in the “barrel cortex” in vivo

“…Repetitive stimulation of individual whiskers is characterized by a decrease in responsiveness in the neocortex. S1 neurons are generally ooly responsive to low frequency whiske~stimulation and often respond exclusively to the tirst stimulus presented (Ahissar, 2000;Simons, 1978;Swadlow, 1989). Field potential responses evoked in SI by whisker stimulation depress at frequencies above 5 Hz (Sheth et al, 1998).…”

“…In fact, bath VPM and Pom neurons depress when stimulated at frequencies above 5 Hz (Ahissar, 2000;Diamond et al, 1992). Depression ofVPM neurons to repetitive whisker stimulation will have a great impact on cortical sensory suppression because they carry the input producing the primary response in Si.…”

“…• dependent depression found in Pom neurons (Ahissar, 2000;Diamond et al, 1992) could also contribute to cortical sensory suppression, but only for long latency cortical responses because Pom neurons discharge much later than the primary cortical response.…”

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

“…Such POm TCAs also terminate in the dysgranular cortex that separates major body parts within the somatosensory cortex. This pathway—the so-called paralemniscal pathway—conveys signals related to vibrissal movement (Ahissar et al, 2000; Yu et al, 2006). Thus, in rats the two thalamic streams form largely non-overlapping, complementary patterns of terminations in and outside of layer IV, thereby establishing semi-independent circuits for tactile information processing (Koralek et al, 1988; Chmielowska et al, 1989; Lu and Lin, 1993; Kim and Ebner, 1999; Ahissar et al, 2001; Bureau et al, 2006).…”

Current status of functional MRI on small animals: application to physiology, pathophysiology, and cognition

“…Thus, Yang and colleagues (67) successfully performed functional MRI of the barrel cortex at stimulation frequencies of 8 Hz. Moreover, it has been shown that different frequencies may activate different parts of the same somatosensory circuit (95) and reveal the mutual interaction between different parts of the circuit.…”