“Attractor Dynamics in the Hippocampal Representation of the Local Environment” and “Attractors in Memory”: The authors recorded spike activity from place cells in the hippocampus of rats as rats explored environments. They compared these spike activities to the rat’s relative location in its environment. Because of the remapping phenomenon, wherein some cells will change their activity when some aspect, e.g. the shape, of the environment changes, the authors posit that the rat’s place cells, which selectively fire consistently at particular regions on the environment, contain information about both the rat’s environment and the rat’s relative position within that environment. Remapping occurs at different rates; in particular, it is slower when a rat transitions between a very familiar environment and one with which it is not familiar. The authors designed an experiment such that the remapping process would be sped up by differentiating environments not just in shape, but in color and texture as well. The authors suggest that the place cell’s switching between two states (circle and square morph material), may evidence the use of attractors, which would influence the hippocampus to fire in multiple patterns, thereby facilitating a framework on which to encode new memories. This study is the first to show the cooperative behavior in hippocampal pyramidal cells. The authors additionally find that greater remapping is seen when proximal and distal cues were changed in tandem as compared to being changed individually while the other remained constant. In concluding, Willis et al. hypothesize that the ability to discriminate patterns (“pattern separation”) occurs in the dentate gyrus of the hippocampus, while the CA3 recurrent collaterals are responsible for autoassociative integration (which requires coordinated activity).
1. The news and views article discusses differences in activity patterns in the hippocampus being caused by varied environment shape, but that an in-between environment shape elicits a mix of responses between the original shapes (square and circle). How sensitive are these shape-dependent activity patterns? Would an almost-square elicit a perfect square pattern, or a distorted square pattern?
2. Are the differences in activity patterns from different environmental shapes based off of the viewer’s perception of those environmental shapes, or a lower-level processing system? In other words, if a rat stayed in an environment that was almost square, but not quite, would a low level processing system cause its activity patterns to react differently than a perfect square, or would this difference in reaction only occur if the rat could consciously tell that its environment was not a perfect square?
3. The authors do not specifically define what an attractor is. Is there a definition of an attractor that exists outside the context of this experiment?
4. At the end of their article, the authors suggest that “representations or charts could serve to reduce interference between environments by providing orthogonal representations for each [environment].” How would the provision of orthogonal representations accomplish this?
“Neuronal correlates of parametric working memory in the prefrontal cortex”: This study sought to answer the question of how a memory of one sensory pulse is held in mind to be compared to the experience of another sensory pulse. In order to investigate this question, the authors used single cell recordings in monkeys while monkeys were receiving these sensory pulses and carried out a task that asked them to discriminate between these two pulses. Four monkeys learned to perform the task, and their neural activity was recorded through seven microelectrodes in different regions of the prefrontal cortex. Sixty-five percent of the recorded neurons recorded by the authors were shown to regulate their activity (discharge rates) as a function of the frequency of the first stimulus pulse (“baseline function”), thereby indicating a working memory of the first stimulus pulse before the second pulse occurred. The authors further delineated between positive monotonic cells, which increased their firing frequency at higher frequencies, and negative monotonic cells, which increased their firing rate at lower frequencies. Neurons also had different firing frequencies during the interval between the first and second stimulus pulse. In particular, some of the neurons maintained their firing rates throughout the course of the interval, while others did not. In order to extract responses that encoded the memory of an original pulse from those that encode the anticipation of a second pulse, the authors recorded from a new monkey with a constant first pulse frequency, allowing them to circumvent information about the first pulse stimulus from the required motor response; in other words, since the first pulse was always the same, this signal would be eliminated in the data, leaving just the information encoding the required memory response. Despite this control, the authors still found similar results as in the first portion of their experiment, enforcing the hypothesis that these monotonic neurons were in fact encoding the memory representation for magnitude of the pulse. Overall, these findings serve the hypothesis that 1) ventral regions of the lateral PFC are utilized by the brain in working memory tasks that do not involve spatial memory and 2) that the PFC maintains memory information for non-visual memories (in this case, somatosensory memories).
1. How did the authors decide to test the prefrontal cortex as a region of interest to test this experiment in? Might there have been other regions of the brain, e.g. hippocampus, with populations of neurons carrying the same information?
2. Of their findings in this study, the authors pose the question: “Whether this multimodal capacity is subserved by intermingled but modality-specific neurons, or whether it is subserved by individually multimodal neurons, remains to be investigated in paradigms involving more than one sensory modality”. What might be a paradigm used to test this phenomenon in a multi-sensory modality?