First-In-First-Out Item Replacement in a Model of Short-Term Memory Based on Persistent Spiking

Abstract

Persistent neuronal firing has been modeled in relation to observed brain rhythms, especially to theta oscillations recorded in behaving animals. Models of short-term memory that are based on such persistent firing properties of specific neurons can meet the requirements of spike-timing-dependent potentiation of synaptic strengths during the encoding of a temporal sequence of spike patterns. We show that such a spiking buffer can be simulated with integrate-and-fire neurons that include a leak current even when different numbers of spikes represent successive items. We propose a mechanism that successfully replaces items in the buffer in first-in-first-out (FIFO) order when the distribution of spike density in a theta cycle is asymmetric, as found in experimental data. We predict effects on the function and capacity of the buffer model caused by changes in modeled theta cycle duration, the timing of input to the buffer, the strength of recurrent inhibition, and the strength and timing of after-hyperpolarization and after-depolarization (ADP). Shifts of input timing or changes in ADP parameters can enable the reverse-order buffering of items, with FIFO replacement in a full buffer. As noise increases, the simulated buffer provides robust output that may underlie episodic encoding.

Gamma Oscillatory Firing Reveals Distinct Populations of Pyramidal Cells in the CA1 Region of the Hippocampus

“…Of the two pyramidal groups, TroPyr cells are able to span the whole theta cycle because their preferred theta firing phases did not change strongly during gamma oscillations and were distributed widely. This enables them to fire in temporal sequences in different gamma cycles and may encode movement paths during thetaphase precession, as previously suggested (Lisman, 2005;Koene and Hasselmo, 2007). On the linear track, we have been able to confirm that the theta-phase precession profile of these cells did not change during gamma oscillations.…”

“…Because we found that place-selective firing was maintained during gamma oscillations, it is possible that gamma-modulated temporal sequences are formed during theta-phase precession (Lisman, 2005;Koene and Hasselmo, 2007). To test directly whether this is the case, we recorded place cell activity during theta-phase precession and we examined whether the phase precession profile of cells was altered during gamma oscillations.…”

“…Gamma oscillations may organize temporal firing sequences within theta cycles (Lisman and Idiart, 1995;Lisman, 2005;Koene and Hasselmo, 2007). However, this type of temporalsequence code may not occur during gamma oscillations if the pyramidal cell firing does not span the whole theta cycle.…”

Modulation of Gamma and Alpha Activity during a Working Memory Task Engaging the Dorsal or Ventral Stream

“…It has been proposed that oscillatory brain activity plays an essential role for the neuronal dynamics required for sustaining working memory representations (Lisman and Idiart, 1995;Jensen, 2006;Koene and Hasselmo, 2007). Experimentally, oscillations in various frequency bands have been shown to be modulated differently during working memory operations in both humans and animals, but their exact functional role remains to be determined.…”

Mechanisms underlying memory enhancement in humans

“…Axmacher and colleagues reported that theta phase modulates gamma power during working memory retention and furthermore that the strength of the coupling correlates with working memory performance (Axmacher, Henseler, Jensen, Weinreich, Elger & Fell 2010). Several lines of studies suggests a model in which the number of gamma cycles within a theta cycle accounts for the working memory span (Jensen & Lisman, 1998;Koene & Hasselmo, 2007;Lisman & Idiart, 1995).…”