Neural evidence for recognition of naturalistic videos in monkey hippocampus

Abstract

The role of the hippocampus in recognition memory has long been a source of debate. Tasks used to study recognition that typically require an explicit probe, where the participant must make a response to prove they remember, yield mixed results on hippocampal involvement. Here, we tasked monkeys to freely view naturalistic videos, and only tested their memory via looking times for two separate novel versus repeat video conditions on each trial. Notably, a large proportion (>30%) of hippocampal neurons differentiated these videos via changes in firing rates timelocked to the duration of their presentation on screen, and not during the delay period between them as would be expected for working memory. Many of these single neurons (>15%) contributed to both retrieval conditions, and differentiated novel from repeat videos across trials with trial-unique content, suggesting they detect familiarity. The majority of neurons contributing to the classifier showed an enhancement in firing rate on repeat compared with novel videos, a pattern which has not previously been shown in hippocampus. These results suggest the hippocampus contributes to recognition memory via familiarity during free-viewing.

Primacy of vision shapes behavioral strategies and neural substrates of spatial navigation in marmoset hippocampus

“…Macaque monkey hippocampal neurons have been putatively classified as interneuron/pyramidal cells based on firing rate alone 61 , 63 , where interneuron cells are typically of the fast-spiking type (average firing rates above 10 Hz). To increase the robustness of classification in our analysis and avoid potential species-specific biases that firing rate thresholding classification could introduce (since it has only been used in macaque monkeys), we implemented an unsupervised classification algorithm ( k -means, MATLAB function k- means) using BI and average firing rate as features.…”

Primacy of vision shapes behavioral strategies and neural substrates of spatial navigation in the hippocampus of the common marmoset

“…Excitatory pyramidal neurons are commonly regarded as the cells responsible for transmitting and processing spatial information in the hippocampus 1,50-52 , while inhibitory interneurons have been associated with the encoding of non-spatial variables like speed 53 , or with synchronizing activity in the hippocampal network 54 . To quantify spatial tuning to either place or view with conventional methods, we classified neurons based on their bursting properties and firing rate [55][56][57][58][59] into either putative interneurons or putative pyramidal cells (Fig. 3c), and we used exclusively putative pyramidal cells to assess variables related to spatial tuning.…”

“…Macaque monkey hippocampal neurons have been putatively classified as interneuron/pyramidal cells based on firing rate alone 56,58 , where interneuron cells are typically of the fast-spiking type (average firing rates above 10Hz). To increase the robustness of classification in our analysis and avoid potential species-specific biases that firing rate thresholding classification could introduce (since it has only been used in macaque monkeys), we implemented an unsupervised classification algorithm (k-means, MATLAB function kmeans) using burst index and average firing rate as features.…”

Primacy of vision shapes behavioral strategies and neural substrates of spatial navigation in the hippocampus of the common marmoset

“…Macaque monkey hippocampal neurons have been putatively classified as interneuron/pyramidal cell based on firing rate alone 56,58 , where interneuron cells are typically of the fast-spiking type (average firing rates above 10Hz). To increase the robustness of classification in our analysis and avoid potential species-specific biases that firing rate thresholding classification could introduce (since it has only been used in macaque monkeys), we implemented an unsupervised classification algorithm (k-means, MATLAB function kmeans) using burst index and average firing rate as features.…”