Developmental neurobiology

In this dissertat ion, the early visual system is used to explore the role of efficiency in
the general organization of the nervous system. Efficient representation theory predicts
that neurons dynamically change their responses to changes in the environment in order to
maintain their efficiency. To directly test the predication of this theory, a computational
model and a neurophysiological experiment are used. Using a computational model, we investigate the sparseness of the response of filters at
each stage of the model of the visual pathway. We find that the temporal bandpass filter
and the rectification in each stage improves the efficiency of the response representation.
Moreover, we find that ON/nonlagged responses carry more information than OFF/ lagged
responses in signals with low signal-to-noise ratios. In the neurophysiological experiment, the response of LGN cells is measured and compared
to their input from the retina in awake cats during free-viewing of natural time-varying
images using quasi-intracellular recording technique. We find that the neural responses in
the retina and the LGN are efficient. However, the LGN response is more efficient, sparser and less correlated than the retina's response, and it carries less information about eye
movements than the retina's. As a result the LGN represents the visual world with fewer
spikes. The LGN response changes with the variation of visual input. The temporal correlation
of the visual input changes with saccade timing. Accordingly, the temporal receptive field of
the LGN also changes in order to maintain the decorrelation of the LGN response regardless
of the saccade. The retina-thalamic transmission changes during and after a saccade in order to transmit
useful information to the visual cortex and decreases during a saccade in order to eliminate
the variation of the visual input during a saccade. However, the transmission increases after
a saccade to facilitate the transmission of new information due to the new gaze direction in
the visual environment. The temporal receptive field of the LGN, derived from the efficacy of the thalamic
transmission, is causal and bimodal. Such a receptive field decorrelates the visual input
and improves the sparseness of the LGN response representation.

This study examined the hypothesis that cognitive immaturity may serve an adaptive purpose for children at a time in ontogeny when they are not capable of ensuring their own survival. Participants were presented pairs of scenarios of 3- and 9-year-old children expressing either immature or mature cognition. Participants were asked to select the child (immature vs. mature) which best reflected each of 11 different psychological traits that were ultimately grouped into 3 trait dimensions: cute, deceptive, and smart. Participants received one of 6 pairs of scenarios reflecting examples of either intuitive cognition or nonintuitive cognition. Participants selected the immature child as being more cute and less deceptive than the mature child for the intuitive vignettes, but not for the nonintuitive vignettes. This pattern suggests that some forms of immature cognition do indeed bias adults to feel more favorably toward the children who express them and may foster positive parent-child relationship.

Perception and behavior are mediated by a widely distributed network of brain areas. Our main concern is, how do the components of the network interact in order to give us a variety of complex coordinated behavior? We first define the nodes of the network, termed functional units, as a strongly coupled ensemble of non-identical neurons and demonstrate that the dynamics of such an ensemble may be approximated by a low dimensional set of equations. The dynamics is studied in two different contexts, sensorimotor coordination and multisensory integration. First, we treat movement coupled to the environment as a driven functional unit. Our central hypothesis is that this coupling must be minimally parametric. We demonstrate the experimental validity of this hypothesis and propose a theoretical model that explains the results of our experiment. A second example of the dynamics of functional units is evident in the domain of multisensory integration. We employ a novel rhythmic multisensory paradigm designed to capture the temporal features of multisensory integration parametrically. The relevant parameters of our experiment are the inter-onset interval between pairs of rhythmically presented stimuli and the frequency of presentation. We partition the two dimensional parameter space using subjects perception of the stimulus sequence. The general features of the partitioning are modality independent suggesting that these features depend on the coupling between the unisensory subsystems. We develop a model with coupled functional units and suggest a candidate coupling scheme. In subsequent chapters we probe the neural correlates of multisensory integration using fMRI and EEG. The results of our fMRI experiment demonstrate that multisensory integration is mediated by a network consisting of primary sensory areas, inferior parietal lobule, prefrontal areas and the posterior midbrain. Different percepts lead to the recruitment of different areas and their disengagement for other percepts. In analyzing the EEG data, we first develop a mathematical framework that allows us to differentiate between sources activated for both unisensory and multisensory stimulation from those sources activated only for multisensory stimulation. Using this methodology we show that the influences of multisensory processing may be seen at an early (40--60 ms) stage of sensory processing.

Finding novel compounds that affect neuronal or muscular function is of great interest, as they can serve as potential pharmacological agents for a variety of neurological disorders. For instance, conopeptides have been developed into powerful drugs like the painkiller PrialtTM. Most conopeptides, however, have yet to be characterized, revealing the need for a rapid and straightforward screening method. We have designed a novel bioassay, which allows for unbiased screening of biological activity of compounds in vivo against numerous molecular targets on a wide variety of neurons and muscles in a rapid and straightforward manner. For this, we paired nanoinjection of compounds with electrophysiological recordings from the Giant Fiber System of Drosophila melanogaster, which mediates the escape response of the fly.