Sweigart, S. C. | bioRxiv Science

bioRxiv2022

Adding meaning to memories: How parietal cortex combines semantic content with episodic experience

Neuroimaging studies of human memory have consistently found that univariate responses in parietal cortex track episodic experience with stimuli (whether stimuli are old or new). More recently, pattern-based fMRI studies have shown that parietal cortex also carries information about the semantic content of remembered experiences. However, it is not well understood how memory-based and content-based signals are integrated within parietal cortex. Here, we used voxel-wise encoding models and a recognition memory task to predict the fMRI activity patterns evoked by complex natural scene images based on (a) the episodic history and (b) the semantic content of each image. Models were generated and compared across distinct subregions of parietal cortex and for occipitotemporal cortex. We show that parietal and occipitotemporal regions each encode memory and content information, but they differ in how they combine this information. Among parietal subregions, angular gyrus was characterized by robust and overlapping effects of memory and content. Moreover, subject-specific semantic tuning functions revealed that successful recognition shifted the amplitude of tuning functions in angular gyrus but did not change the selectivity of tuning. In other words, effects of memory and content were additive in angular gyrus. This pattern of data contrasted with occipitotemporal cortex where memory and content effects were interactive: memory effects were preferentially expressed by voxels tuned to the content of a remembered image. Collectively, these findings provide unique insight into how parietal cortex combines information about episodic memory and semantic content.

neuroscience↗

bioRxiv2018

Parietal representations of stimulus features are amplified during memory retrieval and flexibly aligned with top-down goals

In studies of human episodic memory, the phenomenon of reactivation has traditionally been observed in regions of occipitotemporal cortex (OTC) involved in sensory experience. However, reactivation also occurs in lateral parietal cortex (LPC), and recent evidence indicates that reactivation of stimulus-specific information may be stronger in LPC than in OTC. These observations raise a number of questions about the nature of memory representations in LPC and their relation to representations in OTC. Here, we report two fMRI experiments that quantify stimulus feature information (color and object category) within LPC and OTC, separately during perception and memory retrieval, in male and female human subjects. Across both experiments, we show a clear dissociation between OTC and LPC: while feature information in OTC is relatively stronger during perception than memory, feature information in LPC is relatively stronger during memory than perception. Thus, while OTC and LPC represent common stimulus features, they preferentially represent this information during different stages. We show that this transformation of feature information across regions co-occurs with stimulus-level reinstatement within LPC and high-level OTC. In Experiment 2, we consider whether feature information in LPC during memory retrieval is flexibly and dynamically shaped by top-down goals. Indeed, we find that dorsal LPC preferentially represents retrieved feature information that addresses current goals. In contrast, ventral LPC represents retrieved features independent of current goals. Collectively, these findings provide insight into the nature and significance of mnemonic representations in LPC and constitute an important bridge between putative mnemonic and control functions of parietal cortex.

neuroscience↗