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Sequential involvements of the perirhinal cortex and hippocampus in the recall of item-location associative memory in macaques [1]

['Cen Yang', 'School Of Psychological', 'Cognitive Sciences', 'Peking University', 'Beijing', 'Center For Life Sciences', 'Yuji Naya', 'Pku-Idg Mcgovern Institute For Brain Research', 'Beijing Key Laboratory Of Behavior', 'Mental Health']

Date: 2023-06

The standard consolidation theory suggests that the hippocampus (HPC) is critically involved in acquiring new memory, while storage and recall gradually become independent of it. Converging studies have shown separate involvements of the perirhinal cortex (PRC) and parahippocampal cortex (PHC) in item and spatial processes, whereas HPC relates the item to a spatial context. These 2 strands of literature raise the following question; which brain region is involved in the recall process of item-location associative memory? To solve this question, this study applied an item-location associative (ILA) paradigm in a single-unit study of nonhuman primates. We trained 2 macaques to associate 4 visual item pairs with 4 locations on a background map in an allocentric manner before the recording sessions. In each trial, 1 visual item and the map image at a tilt (−90° to 90°) were sequentially presented as the item-cue and the context-cue, respectively. The macaques chose the item-cue location relative to the context-cue by positioning their gaze. Neurons in the PRC, PHC, and HPC, but not area TE, exhibited item-cue responses which signaled retrieval of item-location associative memory. This retrieval signal first appeared in the PRC, followed by the HPC and PHC. We examined whether neural representations of the retrieved locations were related to the external space that the macaques viewed. A positive representation similarity was found in the HPC and PHC, but not in the PRC, thus suggesting a contribution of the HPC to relate the retrieved location from the PRC with a first-person perspective of the subjects and provide the self-referenced retrieved location to the PHC. These results imply distinct but complementary contributions of the PRC and HPC to recall of item-location associative memory that can be used across multiple spatial contexts.

To address this question, we used the item-location association (ILA) paradigm in macaque electrophysiology [ 19 ]. In this study, before recording sessions, we first repeatedly trained monkeys to associate visual items with locations on a background map image (Figs 1B and S1 and S2 ). Because the map image was presented at a tilt with a random orientation, the monkeys were encouraged to store a common relationship between the items and corresponding locations relative to the map image across the past trials rather than to store a unique spatiotemporal context in each trial. Thus, the long-term association memory tested by the ILA paradigm could be classified as “semantic-like” rather than “episodic-like,” even though it is controversial whether nonhuman primates have semantic memory and/or episodic memory [ 20 , 21 ]. In each trial, one of the learned visual items was presented as an item-cue, and then the randomly tilted map image was presented as a context-cue. The monkeys made a saccade to the location assigned to the item-cue relative to the context-cue ( Fig 1C ). We recorded spike firings of single neurons during the monkeys performing the ILA task. We examined neuronal responses to the item-cues in the absence of the map image (i.e., before the context-cue presentation), which might remind the monkeys of locations associated with the item-cues. To evaluate mnemonic components carried by neural responses to the item-cue, we used 2 types of measures. In the first measure, we tested the presence/absence of ILA effect by examining neurons’ responses to pairs of the item-cues (“co-locating items”) assigned to the same locations (“co-locations”) on the map image. In the second measure, we evaluated the neural representations of the retrieved locations during the item-cue period by comparing with those of the target locations where the monkeys fixated during the choice-fixation period. The first measure revealed an earlier appearance of the item-location associative effect in the PRC before HPC. The second measure indicated that neural representations of retrieved item-cue locations were related with those of external space where the monkeys viewed, only in the HPC but not in the PRC. These results suggest that the PRC and the HPC are involved in remembering the semantic-like item-location associative memory in distinct manners. In addition to the PRC and HPC, we recorded signals from area TE (TE) of the ventral pathway and the PHC as control brain regions.

Concerning the responsible brain regions for the 2 subcategories of declarative memory, previous neuropsychological studies have indicated the critical involvement of the hippocampus (HPC) in encoding both new episodic memory and semantic memory, whereas HPC lesions impair recollection of episodic memory but spare retrieval of semantic memory [ 7 – 11 ]. In contrast, the anterior temporal lobe, including the perirhinal cortex (PRC), is involved in conceptual processing, which depends on factual knowledge stored as semantic memory. The functional dissociation between the PRC and the HPC is also suggested by a series of dual-process models in an item recognition paradigm [ 12 – 14 ]; the PRC contributes to recognition by providing item familiarity, whereas the HPC contributes to recognition by recollecting the spatiotemporal context in which the item was presented. These models are consistent with anatomical evidence suggesting separate information processing along 2 distinct pathways from neocortical areas to the HPC via the PRC and the parahippocampal cortex (PHC) and their relational processing in the HPC [ 15 – 18 ]. The PRC receives visual input from the ventral pathway including area TE, while the PHC receives visual inputs from the dorsal pathway in addition to the posterior part of the ventral pathway. The distinct mnemonic functions of the PRC and the HPC in these 2 lines of preceding literature (“semantic memory vs. episodic memory” and “familiarity/item vs. recollection/relation”) raise the question of which brain area is involved in the retrieval of semantic memory, particularly when it contains spatial components.

(a) (Left) Through repetitive experiences (e.g., reading a book in a study room, etc.), ILAs (e.g., Statue of Liberty ↔ northeast in the US) can be stored in the allocentric frame. The stored memory, which does not accompany a unique spatiotemporal context from a single episode (e.g., watching TV in a living room), could be categorized into semantic memory. (Right) The recall process of the item-location associative memory may differ between the presence (i) and absence (ii) of a map on which the item’s location can be shown. (i) When the map is present, the item can be located on it no matter how it is tilted (0° to 360°), suggesting the semantic recall in the allocentric frame. (ii) In the absence of the map, the item can be located in an assumed spatial context (e.g., top for the north), providing mental imagery of the retrieved location from the first-person perspective. The outline map image of the US was made based on an image from public domain, “Pixabay” ( https://pixabay.com/illustrations/us-map-outline-us-map-america-1674031/ ). (b) ILA pattern. Two items, 1 from set A (e.g., I-A) and the other from set B (e.g., I-B), were assigned to each location (e.g., co-location I) on the map image. Scale bars for both item and map stimuli, with a 5° visual angle. The background map image was made based on an image (EMU 13) from public domain, “USGS” ( https://www.usgs.gov/media/images/emu-13 ). (c) Schematic diagram of the ILA task. An item-cue and a context-cue were sequentially presented in each trial. The monkeys’ gaze was fixated on the center until the end of the delay-2, then saccade to the target location (indicated by the red arrowhead and red dashed circle) according to the 2 cues. A successful trial was rewarded with juice paired with feedback showing the associated location of the item-cue on the context-cue. Relative sizes of the stimuli were magnified for display purposes. ILA, item-location associative.

Semantic memory, the memory of factual knowledge, is a subcategory of declarative memory [ 1 , 2 ]. Semantic memory is acquired through repetitive experiences that share common facts in various spatiotemporal contexts. In each experience, this common fact would eventually be stored as semantic memory separately from the unique spatiotemporal context. This notion contrasts with another subcategory of declarative memory (i.e., episodic memory), which allows us to reexperience a particular past event [ 1 ]. Therefore, semantic memory is essentially formed in an allocentric manner, while episodic memory accompanies the unique spatiotemporal contexts to reconstruct a particular personal event in the “self-referenced” manner or the “first-person perspective” [ 1 , 3 – 6 ]. However, when we recall semantic knowledge, particularly that containing spatial components, we may bring it to mind from a first-person perspective. Assume a scenario where someone asks where the Statue of Liberty is in the United States; when a map is presented, even if at a tilt, the Statue of Liberty can be easily located on it ( Fig 1A ), which would suggest an allocentric representation of semantic memory. In the absence of the map, you may get mental imagery of the retrieved location to represent it from a first-person perspective (e.g., rightward) by assuming a particular spatial context (e.g., top for the north).

(a) Correlation matrices between population vectors for co-locations of item-cues and those for target locations in each MTL area. The population vectors consist of activity from the recorded neurons in each area (PRC, n = 319; HPC, n = 456; PHC, n = 232). TR, top-right; BR, bottom-right; BL, bottom-left; TL, top-left. (b) Representational similarity between co-locations and target locations was estimated as a mean value of correlation coefficients for 4 combinations of co-locations and target locations corresponding to each of −90°, 0°, and 90° map image conditions. Error bar, standard deviation. Asterisk indicates results of two-tailed permutation test: P < 0.0001***. Source data are available in S1 Data . HPC, hippocampus; MTL, medial temporal lobe; PHC, parahippocampal cortex; PRC, perirhinal cortex.

We examined this tendency in the population by conducting representational similarity analysis (RSA) using the responses from all the recorded neurons in each area (n = 319, PRC; n = 456, HPC; n = 232, PHC) because only a few neurons showed task-related activity in both task periods in the PRC (n = 8) and the PHC (n = 6). We calculated a correlation coefficient between the N-dimensional population vector for the response to each co-location during the item-cue period and that of the response to each target location during the choice-fixation period in each area (see Materials and methods and S9 Fig ). “N” was the number of the recorded neurons in each area. According to 3 orientations (−90°, 0°, or +90°) of the map image, co-locations were combined with particular target locations (e.g., “co-location I” and “top-left” in −90° orientation) to form a total of 4 combinations in each orientation. The average of the correlations across the 4 combinations showed a significantly positive value in the HPC (r = 0.14 and P < 0.0001, two-tailed permutation test) and the PHC (r = 0.18, P < 0.0001) only when assuming the co-locations on a map image with 0° orientation ( Fig 6 ). These results suggest that the co-location of the item-cue was represented relative to the 0° map image before the presentation of the context-cue in the HPC and the PHC. The selective conjunction of the co-locations and the 0° map image might be due to the training history in which the monkeys had learned the ILA under the context-cue with 0° orientation during the initial training (“default condition”) [ 19 ], although we do not exclude a possibility that the default condition might be just due to the symmetric property in orientation of the context-cue between −90° and 90°. In contrast to the HPC and the PHC, a positive correlation was not found for any orientation of the map image in the PRC (P = 0.73, 0.77, 0.06 for −90°, 0°, and 90°, respectively). The nonsignificant result of the RSA in the PRC could neither be explained by the sample size nor the strengths of task-related signals because they were even larger in the PRC than in the PHC (n = 319 in PRC versus 232 in PHC, sample size; r = 0.63 versus 0.63, co-location index; 26.3% × 6.0% versus 9.5% × 10.8%, item-cue × target location selective neurons). Together, the comparison of the response patterns between the item-cue and choice-fixation periods suggests that the HPC and the PHC represent the co-locations of the item-cue stimuli in relation to a particular spatial context. Conversely, the PRC might represent co-locations irrespective of their relationship with any particular spatial context.

(a) Lines, SDFs for each item-cue during the item-cue period (left) or SDF for each target location during the choice-fixation period (right). I-IV, co-location I-IV. Brown bar, presentation of the item-cue. TR, top-right; BR, bottom-right; BL, bottom-left; TL, top-left. Gray bar, fixation on the target location. (b) Mean responses during the item-cue period for each co-location (top) and during the choice-fixation period for each target location (bottom). Error bar, standard error. Significant selectivity for the co-locations (P < 0.0001***, F (3, 42) = 56.6, one-way ANOVA) and for the target locations (P < 0.0001***, F (3, 42) = 129.3, one-way ANOVA). (c) Combinations of item- and context-cues resulted in the bottom-left (BL) target location. Mean responses during the choice-fixation period for combinations resulted in the BL target location. Error bar, standard error. Responses were not significantly different among the combinations that resulted in the BL target location (P = 0.9942, F (2, 8) = 0.0058, one-way ANOVA). The background map image was made based on an image (EMU 13) from public domain, “USGS” ( https://www.usgs.gov/media/images/emu-13 ). Source data are available in S1 Data . HPC, hippocampus; SDF, spike density function.

Next, we compared the preferred locations of neurons during the item-cue period to those during the choice-fixation period. Fig 5 displays the responses of one HPC neuron that showed selective activity during both task periods. This neuron signaled co-location III during the item-cue period, whereas it signaled the bottom-left target location during the choice-fixation period ( Fig 5A and 5B ). The bottom-left was assigned as a target location in trials with combinations of “co-location II & 90°,” “co-location III & 0°,” and “co-location IV & −90°” ( Fig 5C ). The response patterns to the target locations during the choice-fixation period and those to the co-locations during the item-cue period were positively correlated when the co-locations were assumed to be positioned relative to the 0° context-cue (r = 0.99), but not to the −90° context-cue (r = −0.27) nor the 90° context-cue (r = −0.27) ( S8 Fig ). These results suggest that the HPC neuron represented the co-location of the item-cue stimulus relative to a particular context-cue during the item-cue period, although the context-cue was not yet presented.

To compare the responses between the 2 task periods in each MTL area, we first examined whether neurons exhibiting item-cue selective responses during the item-cue period (i.e., item-cue selective neurons) also showed target-selective responses during the choice-fixation period. We found that the item-cue selective neurons had a significant tendency to show target-selective responses in the HPC (17.6%, P = 0.0170, χ 2 = 5.7, d.f. = 1, χ-square test) and PHC (27.3%, P = 0.0087, χ 2 = 6.88, d.f. = 1), but not in the PRC (9.5%, P = 0.1075, χ 2 = 2.59, d.f. = 1) ( S6B Fig ). We confirmed this area difference by considering the strengths (R 2 values) of item and target selectivity for all recorded neurons ( S7 Fig ). These results suggest that the neural representations during the item-cue period may be explained by activities for target locations during the choice-fixation period in the HPC and the PHC but not in the PRC.

The item-cues elicited item-cue selective activity according to their co-locations, which were determined relative to the map image. However, it is yet to be addressed whether co-location-related activity signals the retrieved location in relation to a particular spatial context under real space. To answer this question, we compared the responses during the item-cue period with those during the choice-fixation period in which the monkeys fixated on a target location ( Fig 1C ). Before the comparison, we examined the representation of task-related information during this choice-fixation period by applying a three-way (item-cue, context-cue, target) ANOVA (P < 0.01) for each neuron in the MTL. A substantial number of neurons exhibited differential responses among the target locations (target-selective) in all the MTL areas (6.0% in PRC, 12.1% in HPC, and 10.8% in PHC) ( S6A Fig , S1 Table ), whereas only a negligible number (approximately 1.5%) of neurons showed item-cue or context-cue selective responses across the 3 subareas ( S6A Fig and S1 Table ). These results indicate that the MTL neurons represented the target location where the monkeys presently gazed during the choice-fixation period rather than the information from the preceding task events (i.e., item-cue and context-cue).

We next examined which area in the MTL first exhibited the association signal by comparing the time courses of the population-averaged co-location indices among the 3 MTL areas. To this end, we selected neurons with high co-location indices (r > 0.8), founding that the population-averaged co-location index increased earlier in the PRC (n = 30) than in the HPC (n = 83, P = 0.0382, two-tailed permutation test) ( Fig 4C ). The increase in the population-averaged co-location index in the HPC was followed by that in the PHC (n = 8), although the delay was not statistically significant (P = 0.1048, two-tailed permutation test). These results hold for neurons that were selected at different criteria (r > 0.7 or significant co-location effect, S4 Fig ). The time course of the co-location index was also examined for each neuron with high co-location indices (r > 0.8) (see Materials and methods and S5 Fig ). The half-peak times of the co-location indices for individual neurons were shorter in the PRC than in the HPC (P = 0.0395, KS = 0.29, Kolmogorov–Smirnov test) and PHC (P = 0.0201, KS = 0.57) ( Fig 4D ). Considering the dense fiber projections from the TE to the PRC [ 24 , 25 ], the perceptual information signaling the item-cue in TE may elicit a reinstatement of the item-location associative memory in the PRC, which is consistent with previous physiological studies examining other types of association memory paradigms [ 26 – 33 ].

(a) Distributions of co-location indices for item-cue selective neurons in each area. Red, TE (n = 47); blue, PRC (n = 84); purple, HPC (n = 136); green, PHC (n = 22). Co-location indices in all MTL areas were significantly positive (P < 0.0001, Y = 0, for PRC, P < 0.0001, Y = 0, for HPC, P = 0.0009, Y = 0, for PHC, two-sided Wilcoxon signed-rank test) and greater than those in TE (P = 0.0043, KS = 0.31 for PRC; P < 0.0001, KS = 0.59 for HPC; P = 0.0056, KS = 0.43 for PHC; Kolmogorov–Smirnov test). (b) Cumulative frequency histograms of co-location indices for item-cue selective neurons in 2 monkeys. For both monkeys, the HPC showed the largest co-location index (median = 0.92 and 0.84 for monkeys B and C, respectively), followed by the other 2 MTL areas (median = 0.86 and 0.35 in the PRC, 0.78 and 0.59 in the PHC for monkeys B and C, respectively). The co-location index was close to the lowest in the TE for both monkeys B (0.17) and C (−0.04). (c) Time course of population-averaged co-location indices. Lines and shading, mean and SEM across the item-cue selective neurons with high co-location indices (r > 0.8) in MTL areas. The population-averaged co-location index increased earlier in the PRC than in the HPC (P = 0.0382*, two-tailed permutation test). Arrow, half-peak time. (d) Cumulative frequency histograms of half-peak time of co-location indices for individual neurons. Half-peak times were calculated for each item-cue selective neurons with high co-location indices (r > 0.8) in MTL areas. The half-peak times were shorter in the PRC than in the HPC (P = 0.0395*, KS = 0.29, Kolmogorov–Smirnov test) and PHC (P = 0.0201*, KS = 0.57). Source data are available in S1 Data . HPC, hippocampus; MTL, medial temporal lobe; PHC, parahippocampal cortex; PRC, perirhinal cortex; SEM, standard error of the mean.

We evaluated the association effect in each area by calculating the co-location index for each item-cue selective neuron. In addition to the HPC, which showed a high co-location index (median, r = 0.89, P < 0.0001, two-sided Wilcoxon signed-rank test) in our previous study [ 19 ], both the PRC and the PHC showed significantly positive co-location indices (r = 0.63, P < 0.0001 in PRC; r = 0.63, P = 0.0009 in PHC) ( Fig 4A and 4B ). In contrast to the striking association effect in the medial temporal lobe (MTL), TE did not show a significantly positive co-location index (r = 0.16, P = 0.0754), which is consistent with previous lesion studies indicating preferential involvement of TE in the perceptual processing of visual objects relative to the MTL (PRC) [ 23 ].

(a) Solid lines and dashed lines indicate SDFs in trials with item-cues from the stimulus sets A and B, respectively. Dark and light gray shading, 90% confidence interval of 10,000 bootstraps (see Materials and methods ) for the stimulus sets A and B, respectively. Black and gray dots, raster plots for the stimulus sets A and B, respectively. Brown bar, presentation of the item-cue. The background map image was made based on an image (EMU 13) from public domain, “USGS” ( https://www.usgs.gov/media/images/emu-13 ). (b) Mean discharge rates of the neuron during the item-cue period for each item. r, correlation coefficient. Asterisk indicates the result of a two-tailed permutation test: P = 0.0024***. Source data are available in S1 Data . PRC, perirhinal cortex; SDF, spike density function.

Fig 3A shows an example of item-cue selective neurons from the PRC (P = 0.0002, F (7,78) = 4.78) (see S3 Fig for example neurons from TE, HPC, and PHC). This neuron exhibited the largest responses to 2 item-cues that were assigned to co-location II as well as the second-largest responses to 2 item-cues that were assigned to co-location III, indicating that similar response patterns were observed for the item-cues with the same co-locations (“co-locating items”). We evaluated the correlated responses to the 4 pairs of co-locating items by calculating the Pearson correlation coefficient between responses to item-cues “I-A”–“IV-A” and those to item-cues “I-B”–“IV-B” as the “co-location index.” If a neuron showed a pattern of stimulus selectivity that was independent of the items’ co-locations, the expected value of the neuron’s co-location index was “zero.” The co-location index of the example neuron shown in Fig 3 was significantly positive (r = 0.98, P = 0.0024, two-tailed permutation test), suggesting an effect of the item-location associative memory on its response pattern to the item-cues.

We analyzed the activities of single neurons during the item-cue period, including the presentation of the item-cue (0.3 s) and following delay-1 (0.7 s) periods, to examine their involvement in item-location associative memory. We first selected neurons that exhibited differential item-cue responses among the 8 visual items (item-cue selective) at a threshold of P < 0.01 (one-way ANOVA). We found a large proportion of the item-cue selective neurons in the PRC (26.3%), TE (28.0%), as well as in the HPC (29.8%) ( Fig 2B and S1 Table ). In contrast, the proportion of the item-cue selective neurons was significantly smaller in the PHC (9.5%) than in the other 3 areas (P < 0.001 for each area, χ-square test). This result was consistent with previous studies indicating preferential processing of item information in the ventral path-PRC stream [ 15 – 17 , 22 ].

(a) Recording regions. MRI images corresponding to the coronal planes anterior 4, 10, and 17 mm from the interaural line of monkey C (right hemisphere). The recording regions are the PRC, HPC, and PHC of the MTL and TE. A reference electrode implanted in the center of the chamber was observed as a vertical line of shadow in the coronal plane at A10. amts, anterior middle temporal sulcus. rs, rhinal sulcus. ots, occipital temporal sulcus. D, dorsal. V, ventral. L, lateral. M, medial. (b) Percentage of item-cue selective neurons out of recorded neurons in each area. Parentheses, number of recorded neurons in each area. Dashed line, 1.0% chance level. The number of item-cue selective neurons was significantly larger than the chance level (P < 0.0001 for each area, one-tailed binomial test). Asterisk indicates results of a χ-square test: PHC and TE, P < 0.0001***, χ 2 = 23.3, d.f. = 1; PHC and PRC, P < 0.0001***, χ 2 = 24.5, d.f. = 1; PHC and HPC, P < 0.0001***, χ 2 = 36.0, d.f. = 1. Source data are available in S1 Data . HPC, hippocampus; MTL, medial temporal lobe; PHC, parahippocampal cortex; PRC, perirhinal cortex.

Two rhesus macaques were trained to perform the ILA task. In the ILA task, 4 visual item pairs (“co-locating items”) were assigned to 4 different locations (“co-locations”) on an image of a map [ 19 ]. The same 8 visual items and 1 map image were used during all the recording sessions ( Fig 1B ). In each trial, a randomly chosen visual item was presented as an item-cue. Then, the map image was presented at a tilt with a randomly chosen orientation (−90° to 90°, 0.1° step) as a context-cue ( Fig 1C ). By moving their gaze positions, the monkeys reported a target location (e.g., Top-left in the trial n, Fig 1C ), which corresponded to a location assigned to the item-cue (e.g., co-location I) on the tilted map image of the context-cue (e.g., −90°). While the monkeys performed the ILA task, we recorded single-unit activity from a total of 1,175 neurons from the 4 brain areas ( Fig 2A and S1 Table ). During the recording session, the orientation of the context-cue was pseudorandomly chosen from among 5 orientations (−90°, −45°, 0°, 45°, and 90°). The task was correctly performed by both monkeys (chance level = 25%) at rates of 80.5% ± 8.4% (mean ± standard deviation; Monkey B, n = 435 recording sessions) and 95.1% ± 5.8% (Monkey C, n = 416 recording sessions). We used only correct trials for the following analyses. The HPC data was partly reported in a previous study [ 19 ].

Discussion

Using the same ILA task, we previously presented the retrieved co-location information represented in the HPC [19]. However, it remains unclear whether the co-location information was retrieved in the HPC or whether the HPC received a retrieval signal from other upstream brain areas. To address this problem, the present study examined the parahippocampal cortical areas, including the PRC and PHC, which provide signal inputs to the HPC. In addition to the HPC, item-cue selective neurons in the PRC and the PHC showed significantly correlated responses to pairs of co-locating items that the monkeys had learned to associate with the same co-locations in advance (Fig 4A and 4B). Among these 3 areas, correlated responses first appeared in the PRC (Fig 4C and 4D). In contrast to the HPC and parahippocampal cortical areas, correlated responses were not found in TE, which reportedly provides the PRC with perceptual information of visual objects [25,26]. These results suggest that the PRC was first involved in the memory retrieval process before the HPC. We also examined the representation property of the co-location information in each MTL area and found that the retrieved co-location information was represented in relation to a particular spatial context (i.e., 0° of the map image) in the HPC and the PHC but not in the PRC (Fig 6). These results suggest involvements of both PRC and HPC in remembering item-location associative memory that was learned as common information across trials with different context-cues, choice responses, and reward outcomes (Fig 7).

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TIFF original image Download: Fig 7. Two-stage recall model of semantic memory. Schematic diagram of neuronal signals in the MTL during the item-cue period in the ILA task. Perceptual signal of the item-cue transmits from the neocortex to the PRC, in which the item information would be transformed to the mnemonically linked location information, which is represented in an allocentric manner. The retrieval signal would transmit (via the ERC) to the HPC, in which the allocentric location would be combined with the default context to construct the self-reference representation of the retrieved location. The self-referenced location signal would spread from the HPC (via ERC) to PHC. HPC, hippocampus; ILA, item-location associative; MTL, medial temporal lobe; PHC, parahippocampal cortex; PRC, perirhinal cortex. https://doi.org/10.1371/journal.pbio.3002145.g007

In this study, the effect of item-location associative memory was first assessed by correlated responses to pairs of co-locating items. Similar to the present results, an increase in correlated responses to pairs of items from TE to the PRC was also found in previous studies using the pair-association paradigm [27,34,35], which examined a direct association between items [36,37]. Using the ILA task, this study indicated that the PRC also served as an indirect linkage between items via their co-locations on the map image. In contrast to the significant involvements of the PRC in both memory paradigms, a small but significant correlation effect in TE was found only in the pair-association paradigm [27,38]. In the ILA paradigm, the lack of an association effect in TE might be explained by the unwanted recall of co-locating items from item-cues because the retrieved location information signaled forward from the PRC to HPC. Conversely, the PRC provided TE with a feedback signal to substantiate the item–item association effect in the pair-association paradigm [26,38]. Secondly, to examine the effect of item-location associative memory, we compared activities during the item-cue period with those during the choice-fixation period. This study showed that neurons in the MTL areas exhibited selective responses to target locations where the monkeys positioned their gaze for choice. The target-selective responses during the choice-fixation period could be explained by locations relative to “multiple spatial reference frames” [39], including head-center, external landmarks (e.g., computer screen), and the to-be-retained context-cue in mind, which could be represented in either the head-center or external landmark frame. This ambiguity makes it difficult to distinguish the target-selective activity in the self-referenced frame from that in the allocentric frame. However, the gaze-position-dependent activity during the choice-fixation period still seems to reflect the first-person perspective because it depended on what the subjects perceived at that time.

Our previous study using the same ILA task reported that the HPC showed target-selective activity during the context-cue period, during which the animal maintained fixation at the center of the display [19]. However, substantial number of neurons in the HPC exhibited the item-cue (co-location) and the context-cue information in addition to the target information. The intermingled task-related information during the context-cue period contrasts with the exclusive representation of the target-selective information during the choice-fixation period in the present study. This tendency was confirmed in the other MTL areas (S10 Fig). Taken together, the activity during the choice-fixation period would be more appropriate than those during the context-cue period as a reference to examine the “first-person perspective” effect on the activity during the item-cue period, particularly when we conduct population-coding data analysis such as the RSA, which includes all recorded neurons in each area. Therefore, to examine the first-person perspective effect on the retrieved location information during the item-cue period, the comparison with the choice-fixation period seems to be the most appropriate in this study, although it may not yet be a direct measure of the first-person perspective effect. A future study should assess the effect by developing a new behavioral paradigm, in which subjects report their mental imagery during a memory task, and directly compare neuronal activity with the subjective memory content that they directly report at that time [40].

The item-location associative memory isolated from a particular spatial context (e.g., the map image with 0° orientation) in the PRC is consistent with previous literature suggesting its involvement in semantic memory [25,26,41–44] as well as the allocentric representations of semantic memory [4,5]. The isolation from the particular context may also explain the previous studies suggesting a selective involvement of the PRC in familiarity rather than in recollection [13,45]. The reactivation of item-location associative memory in the PRC may induce familiarity [46], but the reactivated mnemonic information in the PRC may not provide a first-person perspective that could be reexperienced in mind with a particular spatial context [4,6,47]. Therefore, the location information in the PRC may not be directly coupled with mental imagery as recollection. The poor access of the reactivated item-location associative memory to mental imagery may also explain previous studies suggesting the exclusive processing of object-related information in the PRC [12,14], although recent physiological studies, including ours, have reported space-related information represented by PRC neurons [22,25,48–50]. Here, we hypothesize that the PRC might be involved in retrieving semantic memory even when it includes the space-related information, although its reinstated spatial contents may not be directly experienced in the mind because of the allocentric representation of semantic memory [4].

In contrast to the PRC, the retrieved location was represented in relation to a particular spatial context (0° orientation of the map image) in the HPC and the PHC. An accompaniment of a particular spatial context is consistent with previous studies suggesting the involvement of the HPC in semantic recall with a vivid mental image [51–53], which presumably involves concrete representations from the first-person perspective. Of the 2 areas, the HPC showed a much larger retrieval signal than the PHC, which was estimated by the proportions of the item-cue selective neurons (29.8% in the HPC versus 9.5% in the PHC) and their co-location indices (median r-value, 0.89 versus 0.63) (Figs 2B and 4A and 4B). Moreover, the time courses of the co-location index elevated earlier in the HPC than in the PHC (half-peak time, 225 ms versus 284 ms) (Fig 4C and 4d), which may imply a transmission of the retrieved information from the HPC to the PHC. Together, it may be reasonable to deduce a critical involvement of the HPC rather than the PHC in accompanying the semantic-like item-location associative memory with a particular spatial context to represent the memory component in the first-person perspective. A future study should address whether the PHC just serves as a relay center between the HPC and the dorsal pathway [18,54] or processes the retrieval signal from the HPC. Because we could not find a qualitative difference in the retrieval signal between the PHC and HPC, it might be important to devise a new research paradigm to reveal the role of PHC.

Representations of the retrieved location in the default context appeared inconsistent with the definition of semantic memory, which should be formed in an allocentric manner [1,4,55], because the retrieved location was related to only 1 particular spatial context. However, after the context-cue presentation, the retrieved location was fitted to a spatial context given by the context-cue in the HPC [19]. In short, the retrieved location can be represented in the self-referenced frame among multiple spatial contexts, including the default context. However, the fitness of the self-referenced location at any spatial context amounts to indicate the allocentric location of the item-cue on the background map [56]. Another possibility might be that retrieval during the item-cue period reflected an episode that the animals experienced in a particular single trial during the training. This episodic-like memory might match the presence of the particular spatial context. However, it seems unlikely for the animals to recall the episode of a particular single trail, because the animals had experienced trials with the same item-cue and context-cue many times before. In addition, if the animals should have recalled the episodic-like memory, the item-cue would remind a subject of subsequent task events in a to-be-retrieved trial, such as the appearance of a context-cue, disappearance of a fixation dot (small white square), appearance of choice-fixation dots (four small white squares), choice response, and presence/absence of reward delivery in addition to the co-location information that was not shown at least before the choice response. Moreover, the recall of episodic memory might also reinstate contexts including information of preceding and succeeding trials. Together, episodic-like memory would provide the animals with plenty of unnecessary information to solve each trial of the ILA task. Therefore, it is reasonable to consider that the item-location associative memory investigated in this study could be classified as a semantic-like memory rather than an episodic-like memory, although some neural mechanisms might be shared between the 2 types of memory to represent the retrieved memory content in its default context.

One remaining question is how the retrieved location was accompanied by a particular spatial context in the absence of the map image. Using the same ILA task, we previously showed a constructive process in which the HPC neurons fitted the retrieved location to the spatial context given by the context-cue [19]. Similarly, we hypothesize that before the context-cue presentation, HPC neurons combined item-location associative memory and default context to represent item-location from the first-person perspective (Fig 7). Considering the anatomical hierarchy of the PRC linking the ventral pathway and the HPC for object-related information [18,25,57], and the earlier rise of the co-location index in the PRC than in the HPC after the item-cue presentation, it seems reasonable to interpret that the HPC neurons received the allocentric item-location from the PRC via the ERC and fit it to the default context in the absence of the map image (Fig 7). Based on the present investigation of semantic-like memory in nonhuman primates, we propose a two-stage recall process for semantic memory. In this model, the PRC and HPC would contribute to the recall of semantic memory in distinct but complementary manners; the PRC might serve for semantic recall in the allocentric frame, while the HPC might serve for constructing the first-person perspective [56].

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[1] Url: https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002145

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