CCN talk March 14, 2018

Michael Hasselmo

Director for Systems Neuroscience, Department of Psychological and Brain Sciences, Boston University

Neural coding of space and time in entorhinal cortex

Time: 4:00-5:00

Place: Moore Hall, room 202

Abstract

Episodic memory is defined as memory for events occurring at a specific place and time. Neurophysiological data from brain regions in behaving rodents demonstrate neuronal response properties that may contribute to coding of place and time for episodic memory. Data shows the coding of spatial location by place cells in hippocampus (O'Keefe, 1976) and grid cells in entorhinal cortex (Hafting et al., 2005), both by firing rate and by spiking phase relative to network theta rhythm oscillations (O'Keefe and Recce, 1993; Hafting et al., 2008; Climer et al., 2016). Neurons in medial entorhinal cortex also code running speed in terms of firing rate and theta rhythmicity (Kropff et al., 2015; Hinman et al., 2016), and many models of grid cells use path integration of running speed plus direction to compute location. Head direction cells have been shown in structures including postsubiculum (Taube et al., 1990; Taube, 2007) and inactivation of head direction cell input impairs grid cell coding (Winter et al., 2015). Experimental data shows that inactivation of input from the medial septum impairs the responses of neurons coding space (Brandon et al., 2011), and this is associated with a loss of the coding of running speed by theta rhymicity (Hinman et al., 2016). Entorhinal grid cells and hippocampal place cells also code temporal intervals as time cells (Kraus et al., 2013; 2015) and inactivation of medial septum also impairs coding of time (Wang et al., 2014). These effects on the coding of space and time may involve modulation of cellular neural mechanisms of resonance and rebound spiking (Giocomo et al., 2007; Shay et al., 2015). Earlier work showed that coding of location could also involve coding of distance to environmental barriers by allocentric boundary cells (O'Keefe and Burgess, 1996; Byrne, Becker, Burgess, 2007), and recent work in our laboratory demonstrates coding of barriers in egocentric coordinates by egocentric boundary cells which could interact with head direction cells to code allocentric boundaries (Hinman et al., 2017). Computational models demonstrate how processing of optic flow or visual feature angles could also underlie coding of location by grid cells (Raudies and Hasselmo, 2015) suggesting how sensory input could contribute to coding of location as well as how location could be associated with sensory input in episodic memory.