Dendritic Slow Dynamics Enables Localized Cortical Activity to Switch between Mobile and Immobile Modes with Noisy Background Input

Mounting lines of evidence suggest the significant computational ability of a single neuron empowered by active dendritic dynamics. This motivates us to study what functionality can be acquired by a network of such neurons. The present paper studies how such rich single-neuron dendritic dynamics affects the network dynamics, a question which has scarcely been specifically studied to date. We simulate neurons with active dendrites networked locally like cortical pyramidal neurons, and find that naturally arising localized activity – called a bump – can be in two distinct modes, mobile or immobile. The mode can be switched back and forth by transient input to the cortical network. Interestingly, this functionality arises only if each neuron is equipped with the observed slow dendritic dynamics and with in vivo-like noisy background input. If the bump activity is considered to indicate a point of attention in the sensory areas or to indicate a representation of memory in the storage areas of the cortex, this would imply that the flexible mode switching would be of great potential use for the brain as an information processing device. We derive these conclusions using a natural extension of the conventional field model, which is defined by combining two distinct fields, one representing the somatic population and the other representing the dendritic population. With this tool, we analyze the spatial distribution of the degree of after-spike adaptation and explain how we can understand the presence of the two distinct modes and switching between the modes. We also discuss the possible functional impact of this mode-switching ability

Persistence of figure-ground organization in the visual cortex

The question of whether the visual system uses a short-term memory store and the nature of that storage is a topic of ongoing research. Electrophysiological studies have found that neurons in visual cortex modify their activity within a fraction of a second in response to stimulus changes and do not seem to show evidence of memory. However, the long phases of stable perception of reversible figures suggest a memory that persists for seconds. Often the ambiguity in reversible figures is in the foreground-background structure, whether they be Necker cubes, rotating spheres of dots, or images like Rubin's vase-face figure that have multiple coherent interpretations which vary based on the assignment of foreground and background in the image elements. One of the ways in which the brain encodes foreground-background structure is by assigning contours to figures. Neurons in area V2 of macaques are selective to which side of a contour is figure and which side is ground. This selectivity has been termed border ownership tuning, and it very likely plays a role in the perception of figure and ground. Therefore, it seems possible that the perceptual persistence found in these ambiguous displays could come from persistence in the border ownership signal. This thesis tested this hypothesis by examining how the border ownership signal in single neurons decayed when the figure ground cues changed in different ways. Primarily, the goal was to see if the signal showed persistence when the figure ground cues that assigned border ownership were removed and the image became ambiguous regarding figure-ground. I found that the border ownership signal did persist, slowly decaying over more than one second after the stimulus became ambiguous for figure-ground. However, when the figure-ground cues reversed, reversing the border ownership assignment, the signal rapidly reversed in response to this change. The signal did not accumulate over multiple figure presentations, but instead each new figure reset the encoding mechanism. Thus the system is both able to preserve the information that has been removed and reset in response to new information that comes in. Further experiments were performed to study the characteristics of the persisting signal. I found that the persistence was robust, existing for border ownership signals generated by a variety of figure-ground cues. It also could survive the interruption of activity in the border ownership encoding neurons, ruling out an activity dependent loop in these neurons as a mechanism. I also show in preliminary results from one monkey that this signal may contribute to stable perception across saccades and object movements. The results of a third monkey who did not seem to show the same persistence as the other two are also discussed. This thesis shows that the visual system does have a memory store for figure-ground information. This should contribute to our understanding of how the border ownership mechanism works and how the visual system performs the daunting task of perception in an actively changing visual world

Sources of variability in numbers of live births in wild golden lion tamarins (Leontopithecus rosalia).

We examined the effects of several variables on the number of live births in multiparous females in a wild population of golden lion tamarins (Leontopithecus rosalia). Independent variables included the number of infants born to a female the previous breeding season, the number of infants weaned the previous breeding season, the female's age and body mass, the number of adult males and helpers in the group, and the inbreeding coefficient of the offspring. We also tested the hypothesis that trapping and chemical immobilization during pregnancy affected the number of live births. Female body mass was the only statistically significant predictor of the number of live-born infants in the current season when both first and second peaks were included. Characteristics that predicted higher numbers of infants in the first peak of a season were the number of infants born the previous season and the body mass of the female. The greater the number of infants born the previous season, the greater the number of infants born in the first peak of the current season. Factors positively correlated with the number of live births in the second peak within a season included the number of infants born the previous season, as well as the number of available helpers. Due to sample size constraints, the analysis of litters in the second peak did not include body mass of the female as a variable. Inbreeding and handling did not affect the number of live births. We found no evidence that current reproduction negatively impacts future reproduction in this species. We also found no evidence for an age-related reduction in fertility

Sources of variability in numbers of live births in wild golden lion tamarins (Leontopithecus rosalia).

We examined the effects of several variables on the number of live births in multiparous females in a wild population of golden lion tamarins (Leontopithecus rosalia). Independent variables included the number of infants born to a female the previous breeding season, the number of infants weaned the previous breeding season, the female's age and body mass, the number of adult males and helpers in the group, and the inbreeding coefficient of the offspring. We also tested the hypothesis that trapping and chemical immobilization during pregnancy affected the number of live births. Female body mass was the only statistically significant predictor of the number of live-born infants in the current season when both first and second peaks were included. Characteristics that predicted higher numbers of infants in the first peak of a season were the number of infants born the previous season and the body mass of the female. The greater the number of infants born the previous season, the greater the number of infants born in the first peak of the current season. Factors positively correlated with the number of live births in the second peak within a season included the number of infants born the previous season, as well as the number of available helpers. Due to sample size constraints, the analysis of litters in the second peak did not include body mass of the female as a variable. Inbreeding and handling did not affect the number of live births. We found no evidence that current reproduction negatively impacts future reproduction in this species. We also found no evidence for an age-related reduction in fertility