Angular Tuning and Velocity Sensitivity in Different Neuron Classes Within Layer 4 of Rat Barrel Cortex

Abstract

Local circuitry within layer IV whisker-related barrels is preferentially sensitive to thalamic population firing synchrony, and neurons respond most vigorously to stimuli, such as high-velocity whisker deflections, that evoke it. Field potential recordings suggest that thalamic barreloid neurons having similar angular preferences fire synchronously. To examine whether angular tuning of cortical neurons might also be affected by thalamic firing synchrony, we characterized responses of layer IV units to whisker deflections that varied in angular direction and velocity. Barrel regular-spike units (RSUs) became more tuned for deflection angle with slower whisker movements. Deflection amplitude had no affect. Barrel fast-spike units (FSUs) were poorly tuned for deflection angle, and their responses remained constant with different deflection velocity. The dependence of angular tuning on deflection velocity among barrel RSUs appears to reflect the same underlying response dynamics that determine their velocity sensitivity and receptive field focus. Unexpectedly, septal RSUs and FSUs are largely similar to their barrel counterparts despite available evidence suggesting that they receive different afferent inputs and are embedded within different local circuits.

Spatiotemporal Characteristics of Neuronal Sensory Integration in the Barrel Cortex of the Rat

“…In fact, the level of the evoked response we have observed (0.7 a.p./stim) is well within the range of responses that have been reported recently both in urethane-anesthetized animals (Celikel et al 2004; 0.66 a.p./stim for layer IV cells and 0.39 a.p./ stim for layer II-III cells) and in fentanyl-sedated animals (see Fig. 3A in Lee and Simons 2004). Thus a very low level of activity such as that observed in Shimegi et al (1999Shimegi et al ( , 2000 might be necessary for the emergence of supralinear summation.…”

Stimulus-Dependent Changes in Spike Threshold Enhance Feature Selectivity in Rat Barrel Cortex Neurons

“…The degree of synchronization of thalamocortical input has been shown previously to play a key role for the encoding of velocity/acceleration in layer 4 of barrel cortex (Pinto et al, 2000(Pinto et al, , 2003, which results in an increase in dV m /dt of the synaptic response (Wilent and Contreras, 2004). A similar role for thalamocortical input synchronization in the encoding of direction has been suggested by the increase in the amplitude of local field potentials recorded in thalamic barreloids to preferred versus nonpreferred directions (Temereanca and Simons, 2003) and by the dependence of angular tuning on deflection velocity (Lee and Simons, 2004). We cannot determine whether decreases in input synchronization underlie the decrease in dV m /dt associated with nonpreferred directions.…”

Late Emergence of the Vibrissa Direction Selectivity Map in the Rat Barrel Cortex

“…In layer 2/3, inhibitory connections were assumed to have a shorter length scale than excitatory ones. L4 neurons were assumed to be directionally selective (Bruno et al, 2003;Lee and Simons, 2004;Andermann and Moore, 2006). After implementing uniform random connectivity between L4 and L2/3, the simulated integrate-and-fire L2/3 neurons thus received a collection of plastic and directionally selective feedforward inputs.…”

“…Neurons in SIbf (primary somatosensory cortex, barrel field) are known to have their activity modulated by numerous parameters. In addition to their tuning to the principal vibrissa identity (the barrel map), some neurons preferentially respond to specific directions of vibrissa deflections (Bruno et al, 2003;Lee and Simons, 2004), to high-frequency vibrations (Andermann et al, 2004), or to the apparent global motion of objects through the vibrissa pad (Jacob et al, 2008). However, no superposition of functional maps has been reproducibly observed in SIbf.…”