The ability to maintain a stable attractor state among place cells and head direction cells is critical to the functioning of the model, while damage in the remaining feed-forward model components manifests in gradual degradation in the ability to represent the locations of objects and boundaries see accompanying Video 3. For example, if certain parts of the parietal window suffer from neuron loss, the reconstruction in imagery is impaired only at the locations in peri-personal space encoded by the missing neurons indeed, this can model representational neglect; Byrne et al.
The place cell population was more robust to silencing than the head-direction population containing only neurons , simply because greater numbers of neurons were simulated, giving greater redundancy. As long as a stable attractor state is present, the model can still encode and recall meaningful representations, giving highly correlated perceived and recalled patterns Figure 8B. The agent moves in a familiar environment and encounters a novel object. Upon navigating past the object, the agent initiates recall, reinstating patterns of neural activity similar to the patterns present during the original object encounter.
Correlations between patterns at encoding and recall remain similar to the noise-free case, see Figure 8C. Videos 3 and 4 show an instance from the neuron-loss and firing rate noise simulations respectively. However, a lesion to the head direction system head direction cells are found along Papez' circuit precludes the agent from laying down new memories, because the transformation circuit cannot drive the medial temporal lobe.
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A lesion to the head direction system head direction cells are found along Papez' circuit has been implemented similar to Simulation 1. The agent is supplied with the connection weights learned in Simulation 1. That is, the agent has acquired a memory before the lesion. However, even though cueing with the object re-activates the correct medial temporal representations, due to the lesion no reinstatement in the parietal window cannot occur, leading to retrograde amnesia for hippocampus-dependent memories in the model agent.
Note, it is hypothesized that a cognitive agent only has conscious access to the egocentric parietal representation, as suggested by hemispatial representational negelct Bisiach and Luzzatti, see main text. In the model, hippocampal place cells bind all scene elements together. The locations of these scene elements relative to the agent are encoded in the firing of boundary vector cells BVCs and object vector cells OVCs. We simulate one of these experiments Mumby et al. In Simulations 1.
We define a mismatch signal as the difference in firing of object vector cells during encoding versus recall modelled as imagery, at the encoding location , and assume that the relative amounts of exploration would be proportional to the mismatch signal. With an intact hippocampus Figure 9 ; Video 7 , the moved object generates a significant novelty signal, due to the mismatch between recalled top-down OVC firing and perceptual bottom-up OVC firing at the encoding location.
That detection of a change in position requires the hippocampus is consistent with place cells binding the relative location of an object via object vector cells to perirhinal neurons signalling the identity of an object.
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A Two objects are encoded from a given location left. After encoding, object one is moved further North. When the agent returns to the encoding location, the perceived position of object one differs from that at encoding blue line, middle panel. When the agent initiates recall right the perceived location of object 1 green filled circle and its imagined location end point of blue line differ.
Blue circle in panel D indicates the previously perceived position of object 1. Inset bar graphs show the concurrent activity of perirhinal cells PRo. E The mismatch in OVC firing results in near zero correlation between encoding and recall patterns for object 1 black bar , while object 2 white bar exhibits a strong correlation, so that object one would be preferentially explored. Note, the correlation for object two is less than 1 because of the residual OVC activity of the other object secondary peaks in both panels in D, driven by learned PC-to-OVC connections.
G-H An incidental match between learned and recalled OVC patterns can occur for either object at specific locations red arrow heads in second panel in G , but otherwise mismatch is signaled for both objects equally and neither object receives preferential exploration. The agent is faced with two objects and encodes them sequentially into memory.
Following some behavior one of the two objects is moved. Note, in real experiments the animal is removed for this manipulation. In simulation, this is unnecessary. Once the agent has returned to location of encoding, it is faced with the manipulated object array. The agent then initiates recall for objects one and two in sequence.
The patterns of OVC re-activation can be compared to the corresponding patterns during perception population vectors correlated, see main text. For the moved object, the comparison signals a change near zero correlation. That object would hence be preferentially explored by the agent, and the next movement target for the agent is set accordingly see main text.
Hippocampal lesions are implemented by setting the firing rates of hippocampal neurons to zero. A hippocampal lesion Figure 9 ; Video 8 precludes the generation of a meaningful novelty signal because the agent is incapable of generating a coherent point of view for recollection, and the appropriate BVC configuration cannot be activated by the now missing hippocampal input. Connections between object vector cells and perirhinal neurons see Figure 3D can still form during encoding in the lesioned agent. Thus some OVC activity is present during recall due to these connections.
However, this activity is not location specific. It only tells the agent that it has seen this object at a given distance and direction, but not where in the environment it was seen. Hence, the mismatch signal is equal for both objects, and exploration time would be split roughly evenly between them.
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However, if the agent happens to be at the same distance and direction from the objects as at encoding, then perceptual OVC activity will match the recalled OVC activity Figure 9G,H , which might correspond to the ability of focal hippocampal amnesics to detect the familiarity of an arrangement of objects if tested from the same viewpoint as encoding King et al. It shows a reproduction of the object novelty paradigm of Mumby et al.
The agent is faced with two objects and encodes an association between relative object location signaled by OVCs and object identity signaled by perirhinal neurons - see Video 7 for encoding phase. Due to the hippocampal lesion, these associations cannot be bound to place cells. Once one of the two objects is moved compare to Simulation 1. Recall is initiated at two distinct locations to highlight the following effect of the lesion: Since associations between OVCs and perirhinal neurons are not bound to a specific environmental location a comparison of OVC patterns between perception and recall signals mismatch everywhere for both objects except for the two special locations at which imagery is engaged in the video.
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At each of those locations, the neural pattern due to the learned association happens to coincide with the pattern during perception for one of the two objects. Hence no object can be singled out for enhanced exploration. Match and Mismatch is signaled equally for both objects see main text. Rats also show preferential exploration of a familiar object that was previously experienced in a different environment, compared with one previously experienced in the same environment, and this preference is also abolished by hippocampal lesions Mumby et al.
Bostock et al. Initiating recall of object A, belonging to context 1, in context 2, would re-activate the PC ensemble belonging to context 1, creating an imagined scene from context one which would mismatch the activity of PCs representing context two during perception. A hippocampal lesion precludes such a mismatch signal by removing PCs. Finally, it has been argued that object recognition irrespective of context is spared after hippocampal but not perirhinal lesions Aggleton and Brown, ; Winters et al.
Similarly, if a scene element a boundary or an object has been removed after encoding, probing the memorized MTL representation can reveal trace activity reflecting the previously encoded and now absent boundary or object. In rodents, it has been proposed that encoding and retrieval are gated by the theta rhythm Hasselmo et al. If rodent theta determines the flow of information encoding vs retrieval then it may be viewed as a periodic comparison between memorized and perceived representations, without deliberate recall of a specific item in its context that is, without changing the point of view.
In Simulations 2. There is no cue to recall anything specific, regular sensory inputs are continuously engaged, and we periodically switch between bottom-up and top-down modes at roughly theta frequency to allow for an on-going comparison between perception and recall.
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In Simulation 2. However, when the agent explores the environment, the barrier is absent Figure 10A. Due to the modulation of top-down connectivity, the memory of the barrier in form of BVC activity periodically bleeds into the parietal representation during perception Figures 10B1 , 2 and 3 and Video 9. The resultant dynamics carry useful information. First, letting the memory representation bleed into the perceptual one allows an agent, in principle, to localize and attend to a region of space in the egocentric frame of reference as indicated by parietal window activity where a change has occurred.
A mismatch between the perceived low bottom-up gain and partially reconstructed high bottom-up gain representations, can signal novelty compare to Simulations 1.
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Moores et al. A An environment containing a small barrier red outline has been encoded in the connection weights in the MTL, but the barrier has been removed before the agent explores the environment again. C1 High gain for top-down connections yields BVC firing rate maps with trace fields due to the missing boundary. Left: BVC firing rate map.
Right: An illustration of the BVC receptive field teardrop shape attached to the agent at a fixed allocentric direction and distance with the agent shown at two locations where the cell in the left panel fires maximally. C2 Same as C1 for a cell whose receptive field is tuned to a different allocentric direction.
Every time the agent traverses the location from which the object was encoded large red circle in D1 , learned PC-to-OVC connections periodically reactivate the associated OVC. D3 The same PCs also re-activate the associated perirhinal identity cell PRo , yielding a spatial trace firing field for a nominally non-spatial perirhinal cell red circle. However, a previously present boundary has been removed. The agent is supplied with a periodic akin to rodent theta modulation of the top-down connection weights please see main text.
The periodic modulation of these connections allows for a probing of the memorized spatial context without engaging in full recall and reveals the memory of the environment to be incongruent with the perceived environment. Time integrated neural activity from this simulation yields firing rate maps which show traces of the removed boundary see Figure 10 in the manuscript.
Note, the video is cut after 1 min to reduce filesize. The full simulation covers approximately s of real time.