Mean signal and response time influences on multivoxel signals of contextual retrieval in the medial temporal lobe

Abstract

IntroductionThe medial temporal lobe supports integrating the “what,” “where,” and “when” of an experience into a unified memory. However, it remains unclear how representations of these contextual features are neurally encoded and distributed across medial temporal lobe subregions.MethodsThis study conducted functional magnetic resonance imaging of the medial temporal lobe, while participants retrieved pair, spatial, and temporal source memories. Multivoxel classifiers were trained to distinguish between retrieval conditions before and after correction for mean signal and response times, to more thoroughly characterize the multivoxel signal associated with memory context.ResultsActivity in perirhinal and parahippocampal cortex dissociated between memory for associated items and memory for their spatiotemporal context, and hippocampal activity was linked to memory for spatial context. However, perirhinal and hippocampal classifiers were, respectively, driven by effects of mean signal amplitude and task difficulty, whereas the parahippocampal classifier survived correction for these effects.ConclusionThese findings demonstrate dissociable coding mechanisms for episodic memory context across the medial temporal lobe, and further highlight a critical distinction between multivoxel representations driven by spatially distributed activity patterns, and those driven by the regional signal.

Parahippocampal Cortex Processes the Nonspatial Context of an Event

“…The largest degree of spatial processing occurred in studies that used scenes as stimuli (Kensinger and Schacter 2006;Tendolkar et al 2008;Duarte et al 2011) or asked participants to imagine scenes as part of the encoding task (Davachi et al 2003;Kahn et al 2004;Staresina et al 2011). Other aspects of source memory studies that might result in a contribution from spatial processing include manipulating screen position of encoded stimuli (Cansino et al 2002;Ross and Slotnick 2008;Hayama et al 2012;Yu et al 2012;Reas and Brewer 2015), using object images as stimuli (Cansino et al 2002;Peters et al 2007;Manelis et al 2013), and encoding tasks that require responses about the size of an object (Ranganath et al 2004;Hayes et al 2011). Although these manipulations do not precisely align with spatial functions attributed to PHc they have been cited as reason to doubt nonspatial hypotheses of PHc function (Epstein and Ward 2010).…”