Abstract
Sensory processing is associated with gamma frequency oscillations (30–80 Hz) in sensory cortices. This raises the question whether gamma oscillations can be directly involved in the representation of time-varying stimuli, including stimuli whose time scale is longer than a gamma cycle. We are interested in the ability of the system to reliably distinguish different stimuli while being robust to stimulus variations such as uniform time-warp. We address this issue with a dynamical model of spiking neurons and study the response to an asymmetric sawtooth input current over a range of shape parameters. These parameters describe how fast the input current rises and falls in time. Our network consists of inhibitory and excitatory populations that are sufficient for generating oscillations in the gamma range. The oscillations period is about one-third of the stimulus duration. Embedded in this network is a subpopulation of excitatory cells that respond to the sawtooth stimulus and a subpopulation of cells that respond to an onset cue. The intrinsic gamma oscillations generate a temporally sparse code for the external stimuli. In this code, an excitatory cell may fire a single spike during a gamma cycle, depending on its tuning properties and on the temporal structure of the specific input; the identity of the stimulus is coded by the list of excitatory cells that fire during each cycle. We quantify the properties of this representation in a series of simulations and show that the sparseness of the code makes it robust to uniform warping of the time scale. We find that resetting of the oscillation phase at stimulus onset is important for a reliable representation of the stimulus and that there is a tradeoff between the resolution of the neural representation of the stimulus and robustness to time-warp.
Highlights
General background In recent years, there has been a growing interest in understanding how temporal information of sensory stimuli is encoded by sensory corticies
This raises the question whether the gamma oscillations can be directly involved in the representation of timevarying stimuli, including stimuli whose time scale is larger than that of a gamma cycle
Such a model was suggested by Hopfield [5], and later was studied in the contex of diphone discrimination [21]. In this model subthreshold oscillatory input acts to coordinate the firing of cells so that a downstream neuron can read out a population code based on synchrony of firing. The implementation of this idea had a memory of about 200 ms, in a way that varied along a given stream of speech; the time scale of the memory depended on a dynamically changing ‘‘Lyapunov exponent’’; the more negative this quantity, the shorter the memory and the more stable the representation
Summary
General background In recent years, there has been a growing interest in understanding how temporal information of sensory stimuli is encoded by sensory corticies (see, e.g., [1,2,3,4,5,6,7,8]). Sensory processing has been shown to be associated with the appearance of gamma oscillations in various sensory corticies (see, e.g., [16,17,18,19,20]) This raises the question whether the gamma oscillations can be directly involved in the representation of timevarying stimuli, including stimuli whose time scale is larger than that of a gamma cycle. Such a model was suggested by Hopfield [5], and later was studied in the contex of diphone discrimination [21]. To represent a signal having a natural time scale of more than one gamma period, we use multiple periods explicitly in the representation
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