Transformation from temporal to rate coding in a somatosensory thalamocortical pathway

• Published in: Nature (London) Vol. 406; no. 6793; pp. 302 - 306
• Main Authors: Ahissar, Ehud, Sosnik, Ronen, Haidarliu, Sebastian
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 20-07-2000; Nature Publishing Group
• Subjects: Afferent Pathways; Animals; Biological and medical sciences; Brain; Brain Stem - physiology; Fundamental and applied biological sciences. Psychology; lemniscus; Male; Nervous system; Rats; Rats, Wistar; Rodents; Senses; Somatosensory Cortex - physiology; Somesthesis and somesthetic pathways (proprioception, exteroception, nociception); interoception; electrolocation. Sensory receptors; Thalamus - physiology; Vertebrates: nervous system and sense organs; vibrissa; Vibrissae - physiology; whiskers; Cerebral cortex; Rat; Rodentia; Central nervous system; Electrophysiology; Somatosensory cortex; Vertebrata; Somatotopy; Mammalia; Somesthetic pathway; Thalamus; Mechanical stimulus; Vibrissa; Brain (vertebrata)
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/35018568

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.