Role of Spike Timing in the Forelimb Somatosensory Cortex of the Rat

Abstract

The aim of this study was to test the hypothesis that the significance of spike timing in somatosensory processing is not a specific feature of the whisker cortex but a more general characteristic of the primary somatosensory cortex. We recorded ensembles of neurons using microwire arrays implanted in the deep layers of the forelimb region of the rat primary somatosensory cortex in response to step stimuli delivered to the cutaneous surface of the contralateral body. We used a recently developed peristimulus time histogram (PSTH)-based classification method to investigate the temporal precision of the code by evaluating how changing the bin size (from 40 to 1 msec) would affect the ability of the ensemble responses to discriminate stimulus location on a single-trial basis. The information related to the discrimination was redundantly distributed within the ensembles, and the ability to discriminate stimulus location increased when decreasing the bin size, reaching a maximum at 4 msec. In our experiment, at 4 msec bin size the first spike per neuron after the stimulus conveyed almost as much information as the entire responses, so the temporal precision of the code was preserved in the first spikes. Subsequent spikes were less frequent but conveyed more information per spike. Finally, not only the trials correctly classified but also the trials incorrectly classified conveyed information about stimulus location with a similar temporal precision. We conclude that the role of spike timing in cortical somatosensory processing is not an exclusive feature of the highly specialized rat trigeminal system, but a more general property of the rat primary somatosensory cortex.

Tradeoffs and Constraints on Neural Representation in Networks of Cortical Neurons

“…In this study, using generic networks of cortical neurons as a model system, we follow the path of a stimulus-reconstruction approach to compare the representational efficacy of four types of popular schemes, two rate-base and two time-based: population-counthistogram (Schwartz, 1993;Hupé et al, 2001;Fiorillo et al, 2003), spike-count (Arabzadeh et al, 2006;Foffani et al, 2009;Jacobs et al, 2009), time-to-first-spike (Petersen et al, 2001;Foffani et al, 2004;Johasson and Birznieks, 2004;Gollisch and Meister, 2008;Gollisch and Meister, 2008), and rank-order Van Rullen and Thorpe, 2001;VanRullen et al, 2005;Shahaf et al, 2008). Notwithstanding limitations associated with the stimulus-reconstruction approach in relation to brain function, it served us well in the present context as a mean for estimating the total information content, embedded in a given response feature, about an input.…”

“…(2) Spike-count: The timing of individual spikes is omitted, and only the number of spikes evoked by each (identified) neuron in a predefined time interval is considered (Arabzadeh et al, 2006;Foffani et al, 2009;Jacobs et al, 2009). (3) Time-to-first-spike: The precise time delay from the stimulus to the first spike evoked by each (identified) neuron is considered (Petersen et al, 2001;Foffani et al, 2004;Johasson and Birznieks, 2004;Gollisch and Meister, 2008;Gollisch and Meister, 2008). (4 Of course, other types of representation schemes do exist (e.g., correlation-based, or general spike patterns); however, the above four types seem to cover a wide enough range of response features.…”

Redundancy and Synergy of Neuronal Ensembles in Motor Cortex

“…Even if such a classifier exists, there is no way to verify that rules used by a statistical classifier have any relationship to the true mutual information or rules used by the animal. For these reasons, and according to the data processing inequality (Cover and Thomas, 1991;Schneidman et al, 2003), we and others (Averbeck et al, 2003;Foffani et al, 2004) can only estimate a lower bound of mutual information between neural data and behavioral outcomes. Nevertheless, we believe that the statistical pattern recognition approach is a practical solution to the problem of interpreting high-dimensional neuronal ensemble data.…”

Developmental synaptic plasticity at the thalamocortical input to barrel cortex: Mechanisms and roles

“…Thus development or induction of LTP results in action potentials evoked in layer IV neurons by TC stimulation that exhibit the high timing precision. Such timing precision is thought to be necessary for information processing in mature somatosensory cortex (Celikel et al, 2004;Foffani et al, 2004;Ghazanfar et al, 2000;Panzeri et al, 2001). If such precisely timed action potentials are desirable, the role of the slow kainate receptor-mediated EPSP in neonatal animals is not immediately apparent.…”