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SUMMARYObjects and landmarks are crucial for guiding navigation and must be integrated into the cognitive map of space. Studies of object coding in the hippocampus have primarily focused on activity of single cells. Here we record simultaneously from large numbers of hippocampal CA1 neurons to determine how the presence of a salient object in the environment alters single-neuron and neural-population dynamics of the area. Only a small number of cells fired consistently at the object location, or at a fixed distance and direction from it; yet the majority of the cells showed some change in their spatial firing patterns when the object was introduced. At the neural population level, these changes were systematically organized according to the animal’s distance from the object. This organization was widely distributed across the cell sample, suggesting that some features of cognitive maps – including object representation – are best understood as emergent properties of neural populations.

The medial entorhinal cortex (MEC) hosts many of the brain’s circuit elements for spatial navigation and episodic memory, operations that require neural activity to be organized across long durations of experience1. While location is known to be encoded by a plethora of spatially tuned cell types in this brain region2–6, little is known about how the activity of entorhinal cells is tied together over time. Among the brain’s most powerful mechanisms for neural coordination are network oscillations, which dynamically synchronize neural activity across circuit elements7–10. In MEC, theta and gamma oscillations provide temporal structure to the neural population activity at subsecond time scales1,11–13. It remains an open question, however, whether similarly powerful coordination occurs in MEC at behavioural time scales, in the second-to-minute regime. Here we show that MEC activity can be organized into a minute-scale oscillation that entrains nearly the entire cell population, with periods ranging from 10 to 100 seconds. Throughout this ultraslow oscillation, neural activity progresses in periodic and stereotyped sequences. This activity was elicited while mice ran at free pace on a rotating wheel in darkness, with no change in its location or running direction and no scheduled rewards. The oscillation sometimes advanced uninterruptedly for tens of minutes, transcending epochs of locomotion and immobility. Similar oscillatory sequences were not observed in neighboring parasubiculum or in visual cortex. The ultraslow oscillation of activity sequences in MEC may have the potential to couple its neurons and circuits across extended time scales and to serve as a scaffold for processes that unfold at behavioural time scales, such as navigation and episodic memory formation.

Since its discovery by Hubel and Wiesel in 1959, orientation selectivity has been observed in every mammal for which the neuronal response selectivity of primary visual cortex (V1) has been examined. In some animals, like cats and monkeys, anatomically close V1 neurons have similar preferred orientations, giving rise to maps of orientation preferences. However, sharp selectivity is also observed in mice, squirrels and rats, whose V1 has no orientation map. This means that neurons with different preferred orientations are intermixed. This second scenario is called salt-and-pepper organization and leads to question the structural organization of the intracortical connections. With this study, we intend to analyze the differences between both scenarios focusing on selectivity properties, and to clarify the effect of plasticity on orientation selectivity. We study a computational model of layer 2/3 and a reduced one-dimensional model of orientation selective neurons, both in the balanced state. We also analyze a plasticity mechanism that involves spike-timing dependent plasticity (STDP) [1]. This rule implies that if the postsynaptic spike comes after the presynaptic spike, the connection becomes stronger. For the reverse order it becomes weaker. Inhibitory synapses are taken as non-plastic. The selectivity is quantified using the Orientation Selectivity Index (OSI) of the activity profile. According to the theory of balanced networks [2] applied to spatially structured networks [3], we find that for a given profile of the input, selectivity of the cortical activity is determined by the ratio between the first Fourier component of the connectivity and its mean value. As a consequence, for large connectivity salt-and-pepper structures are more selective than systems with orientation maps: our simulations indicate that the mean OSI for salt-and-pepper is 0.57, while it is 0.27 for orientation map. Moreover, for systems with orientation maps selectivity could be increased by taking connection probabilities that are broader. Regarding plasticity, we find that under certain conditions STDP can indeed improve selectivity but it works in a somehow unexpected way, by decreasing the modulated part of the intracortical connectivity with respect to the non-modulated part. We find this conclusion to be valid both for systems with salt-and-pepper organization and with orientation maps. This can be understood in terms of the relative change between the background connectivity and the functionally modulated part.

Orientation selectivity is ubiquitous in the primary visual cortex (V1) of mammals. In cats and monkeys, V1 displays spatially ordered maps of orientation preference. Instead, in mice, squirrels, and rats, orientation selective neurons in V1 are not spatially organized, giving rise to a seemingly random pattern usually referred to as a salt-and-pepper layout. The fact that such different organizations can sharpen orientation tuning leads to question the structural role of the intracortical connections; specifically the influence of plasticity and the generation of functional connectivity. In this work, we analyze the effect of plasticity processes on orientation selectivity for both scenarios. We study a computational model of layer 2/3 and a reduced one-dimensional model of orientation selective neurons, both in the balanced state. We analyze two plasticity mechanisms. The first one involves spike-timing dependent plasticity (STDP), while the second one considers the reconnection of the interactions according to the preferred orientations of the neurons. We find that under certain conditions STDP can indeed improve selectivity but it works in a somehow unexpected way, that is, effectively decreasing the modulated part of the intracortical connectivity as compared to the non-modulated part of it. For the reconnection mechanism we find that increasing functional connectivity leads, in fact, to a decrease in orientation selectivity if the network is in a stable balanced state. Both counterintuitive results are a consequence of the dynamics of the balanced state. We also find that selectivity can increase due to a reconnection process if the resulting connections give rise to an unstable balanced state. We compare these findings with recent experimental results. Fil: Gonzalo Cogno, Ximena Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Mato, German. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina