Adaptive integration of self-motion and goals in posterior parietal cortex.

Rats readily switch between foraging and more complex navigational behaviors such as pursuit of other rats or prey. These tasks require vastly different tracking of multiple behaviorally significant variables including self-motion state. To explore whether navigational context modulates self-motion tracking, we examined self-motion tuning in posterior parietal cortex neurons during foraging versus visual target pursuit. Animals performing the pursuit task demonstrate predictive processing of target trajectories by anticipating and intercepting them. Relative to foraging, pursuit yields multiplicative gain modulation of self-motion tuning and enhances self-motion state decoding. Self-motion sensitivity in parietal cortex neurons is, on average, history dependent regardless of behavioral context, but the temporal window of self-motion integration extends during target pursuit. Finally, many self-motion-sensitive neurons conjunctively track the visual target position relative to the animal. Thus, posterior parietal cortex functions to integrate the location of navigationally relevant target stimuli into an ongoing representation of past, present, and future locomotor trajectories.

Cortical and hippocampal dynamics under logical fragmentation of environmental space.

Navigation is often constrained to pathways, and a recurring problem concerns whether to turn left or right when approaching an intersection. We examined this problem during T-maze performance in which the maze location in the recording environment varied over five-trial blocks and analyzed the associated positional firing patterns of hippocampal CA1 and posterior parietal cortex neurons. An arbitrary partitioning of the environmental space determined the left versus right turning rule for T-maze behavior. Under these conditions, rats learned the logical fragmentation of allocentric space into left turn and right turn sub-regions. Paradoxically, under these conditions, the spatial tuning of both posterior parietal cortex and hippocampal CA1 neurons followed the frame of reference given by the T-maze, as opposed to the location in the environment. Moreover, first trials within each block were associated with distinct firing rate changes for both posterior parietal cortex and hippocampal CA1 neurons. These data support a model where spatial tuning by hippocampus and cortex can interact to guide choice behavior in complex, path-based environments where a correct turn choice varies across environmental locations, and as a function of recent experience.

Locomotor action sequences impact the scale of representation in hippocampus and posterior parietal cortex.

The hippocampus and posterior parietal cortex are implicated in both episodic memory and encoding of position in an environment. In the present study, we examine the impact of locomotor behaviors associated with movement in both the horizontal and vertical dimensions on population activity patterns in these two brain structures. We utilized a five-looped, squared spiral track containing stair segments, ramp segments, and flat segments. In addition to encoding locations along the full route, posterior parietal cortex population activity demonstrates strong pattern recurrence for similar action types at different locations in the environment. Additionally, posterior parietal and hippocampal neurons exhibit parallel modulation in the scale of representation that follows behavioral dynamics required for track traversal. These findings build on prior work examining spatial mapping in the vertical dimension and provide a better understanding of how a series of actions and visited locations can be coordinated in the generation of episodic memory.

Age-related changes in place learning for adjacent and separate locations.

The effect of spatial interference on place learning was examined in young and old rats. Rats were trained on a radial 8-arm maze to discriminate between a reward arm and a nonreward arm that either were adjacent to each other (high spatial interference) or separated by a distance of 2 arm positions (low spatial interference). Each rat was tested until reaching a criterion of 9 correct choices out of 10 trials across 2 consecutive days. The data revealed that old rats committed significantly more errors than young rats when the arms were adjacent and spatial interference was high. However, no group differences were detected when the arms were separated and spatial interference was low. Group differences also were not detected in the number of trials required to reach the learning criterion in either condition. The results indicate that age-related brain changes result in increased errors during place learning, particularly when spatial interference is high, suggesting that spatial pattern separation might be less efficient in aged animals.

Age-related changes in detection of spatial novelty.

Age-related changes in novelty detection for object-place associations was assessed in 6-mo and 25-mo-old Fisher 344/Brown Norway (F344/BN) rats. Old rats showed significant deficits compared to young rats in detecting spatial displacement of objects. The data suggest that object-place novelty detection is impaired in aged F344/BN rats using a rapidly acquired, exploratory-based task. The results may have important implications for the selection of efficient memory paradigms for future aging studies.

Age-related differences in the anticipation of future rewards.

The present study examined the anticipation of future reward in 7-mo- and 26-mo-old Fischer 344/Brown Norway rats. Young and old rats were divided randomly and assigned into one of two conditions. In the Contrast Condition, subjects were given a water solution containing 2% sucrose for 3 min followed immediately by a water solution containing 32% sucrose for 3 min. In the No-Contrast Condition, subjects were given a water solution containing 2% sucrose for 3 min followed immediately by a water solution containing 2% sucrose for 3 min. Across 10 days of testing in the Contrast Condition, young rats showed significantly less intake of the less preferred 2% sucrose solution, whereas old rats showed increased intake of the 2% sucrose solution. Young rats showed a significant increase in intake of the preferred 32% sucrose solution compared to aged rats across the 10-day testing period with the exception of days 8-10 where intake did not differ between groups. In the No-Contrast Condition, there were no significant differences between young and old rats, with both groups consuming significantly more of the first 2% solution than the second 2% solution. Therefore, these data suggest that age-related changes may impair the ability to anticipate future rewards.