A map of visual space in the primate entorhinal cortex

Place-modulated activity among neurons in the hippocampal formation presents a means to organize contextual information in the service of memory formation and recall. One particular spatial representation, that of grid cells, has been observed in the entorhinal cortex (EC) of rats and bats, but has yet to be described in single units in primates. Here we examined spatial representations in the EC of head-fixed monkeys performing a free-viewing visual memory task. Individual neurons were identified in the primate EC that emitted action potentials when the monkey fixated multiple discrete locations in the visual field in each of many sequentially presented complex images. These firing fields possessed spatial periodicity similar to a triangular tiling with a corresponding well-defined hexagonal structure in the spatial autocorrelation. Further, these neurons showed theta-band oscillatory activity and changing spatial scale as a function of distance from the rhinal sulcus, which is consistent with previous findings in rodents. These spatial representations may provide a framework to anchor the encoding of stimulus content in a complex visual scene. Together, our results provide a direct demonstration of grid cells in the primate and suggest that EC neurons encode space during visual exploration, even without locomotion.

Electrical synapses coordinate activity in the suprachiasmatic nucleus

In the suprachiasmatic nucleus (SCN), the master circadian pacemaker, neurons show circadian variations in firing frequency. There is also considerable synchrony of spiking across SCN neurons on a scale of milliseconds, but the mechanisms are poorly understood. Using paired whole-cell recordings, we have found that many neurons in the rat SCN communicate via electrical synapses. Spontaneous spiking was often synchronized in pairs of electrically coupled neurons, and the degree of this synchrony could be predicted from the magnitude of coupling. In wild-type mice, as in rats, the SCN contained electrical synapses, but electrical synapses were absent in connexin36-knockout mice. The knockout mice also showed dampened circadian activity rhythms and a delayed onset of activity during transition to constant darkness. We suggest that electrical synapses in the SCN help to synchronize its spiking activity, and that such synchrony is necessary for normal circadian behavior.

Perirhinal and postrhinal contributions to remote memory for context

The perirhinal (PER) and postrhinal (POR) cortices, two components of the medial temporal lobe memory system, are reciprocally connected with the hippocampus both directly and via the entorhinal cortex. Damage to PER or POR before or shortly after training on a contextual fear conditioning task causes deficits in the subsequent expression of contextual fear, implicating these regions in the acquisition or expression of contextual memory. Here, we examined the contribution of PER and POR to the processing of remotely learned contextual information. Male Long-Evans rats were trained in an unsignaled contextual fear conditioning paradigm. After training, rats received bilateral neurotoxic lesions to PER or POR or sham control surgeries at three different training-to-lesion intervals: 1, 28, or 100 d after training. Two weeks after surgery, lesioned and control rats were returned to the training context to assess contextual fear as measured by freezing. Rats with PER or POR damage froze significantly less in the training context than control rats but were not different from each other. The severity of the deficit did not differ across training-to-lesion intervals for any group. This pattern of deficits differs from that of posttraining hippocampal lesions, for which longer training-to-lesion intervals produce significantly more fear-conditioned contextual freezing than shorter training-to-lesion intervals. In the absence of such a retrograde gradient in the present study, our interpretation is that PER and POR have an ongoing role in the storage or retrieval of representations for context. Alternatively, these regions may be involved in a more extended consolidation process that becomes apparent beyond 100 d after learning.

Abrupt maturation of a spike-synchronizing mechanism in neocortex

Synchronous activity is common in the neocortex, although its significance, mechanisms, and development are poorly understood. Previous work showed that networks of electrically coupled inhibitory interneurons called low-threshold spiking (LTS) cells can fire synchronously when stimulated by metabotropic glutamate receptors. Here we found that the coordinated inhibition emerging from an activated LTS network could induce correlated spiking patterns among neighboring excitatory cells. Synchronous activity among LTS cells was absent at postnatal day 12 (P12) but appeared abruptly over the next few days. The rapid development of the LTS-synchronizing system coincided with the maturation of the inhibitory outputs and intrinsic membrane properties of the neurons. In contrast, the incidence and magnitude of electrical synapses remained constant between P8 and P15. The developmental transformation of LTS interneurons into a synchronous, oscillatory network overlaps with the onset of active somatosensory exploration, suggesting a potential role for this synchronizing system in sensory processing.

Recognition Memory in Marmoset and Macaque Monkeys: A Comparison of Active Vision

The core functional organization of the primate brain is remarkably conserved across the order, but behavioral differences evident between species likely reflect derived modifications in the underlying neural processes. Here, we performed the first study to directly compare visual recognition memory in two primate species-rhesus macaques and marmoset monkeys-on the same visual preferential looking task as a first step toward identifying similarities and differences in this cognitive process across the primate phylogeny. Preferences in looking behavior on the task were broadly similar between the species, with greater looking times for novel images compared with repeated images as well as a similarly strong preference for faces compared with other categories. Unexpectedly, we found large behavioral differences among the two species in looking behavior independent of image familiarity. Marmosets exhibited longer looking times, with greater variability compared with macaques, regardless of image content or familiarity. Perhaps most strikingly, marmosets shifted their gaze across the images more quickly, suggesting a different behavioral strategy when viewing images. Although such differences limit the comparison of recognition memory across these closely related species, they point to interesting differences in the mechanisms underlying active vision that have significant implications for future neurobiological investigations with these two nonhuman primate species. Elucidating whether these patterns are reflective of species or broader phylogenetic differences (e.g., between New World and Old World monkeys) necessitates a broader sample of primate taxa from across the Order.

Oscillatory correlates of memory in non-human primates

The ability to navigate through our environment, explore with our senses, track the passage of time, and integrate these various components to form the experiences which make up our lives is shared among humans and animals. The use of animal models to study memory, coupled with electrophysiological techniques that permit the direct measurement of neural activity as memories are formed and retrieved, has provided a wealth of knowledge about these mechanisms. Here, we discuss current knowledge regarding the specific role of neural oscillations in memory, with particular emphasis on findings derived from non-human primates. Some of these findings provide evidence for the existence in the primate brain of mechanisms previously identified only in rodents and other lower mammals, while other findings suggest parallels between memory-related activity and processes observed in other cognitive modalities, including attention and sensory perception. Taken together, these results provide insight into how network activity may be organized to promote memory formation, and suggest that key aspects of this activity are similar across species, providing important information about the organization of human memory.

Comparing rapid rule-learning strategies in humans and monkeys

Inter-species comparisons are key to deriving an understanding of the behavioral and neural correlates of human cognition from animal models. We perform a detailed comparison of macaque monkey and human strategies on an analogue of the Wisconsin Card Sort Test, a widely studied and applied multi-attribute measure of cognitive function, wherein performance requires the inference of a changing rule given ambiguous feedback. We found that well-trained monkeys rapidly infer rules but are three times slower than humans. Model fits to their choices revealed hidden states akin to feature-based attention in both species, and decision processes that resembled a Win-stay lose-shift strategy with key differences. Monkeys and humans test multiple rule hypotheses over a series of rule-search trials and perform inference-like computations to exclude candidates. An attention-set based learning stage categorization revealed that perseveration, random exploration and poor sensitivity to negative feedback explain the under-performance in monkeys.

A Comparison of Rapid Rule-Learning Strategies in Humans and Monkeys

Interspecies comparisons are key to deriving an understanding of the behavioral and neural correlates of human cognition from animal models. We perform a detailed comparison of the strategies of female macaque monkeys to male and female humans on a variant of the Wisconsin Card Sorting Test (WCST), a widely studied and applied task that provides a multiattribute measure of cognitive function and depends on the frontal lobe. WCST performance requires the inference of a rule change given ambiguous feedback. We found that well-trained monkeys infer new rules three times more slowly than minimally instructed humans. Input-dependent hidden Markov model-generalized linear models were fit to their choices, revealing hidden states akin to feature-based attention in both species. Decision processes resembled a win-stay, lose-shift strategy with interspecies similarities as well as key differences. Monkeys and humans both test multiple rule hypotheses over a series of rule-search trials and perform inference-like computations to exclude candidate choice options. We quantitatively show that perseveration, random exploration, and poor sensitivity to negative feedback account for the slower task-switching performance in monkeys.