Thalamus--Physiology

The thalamus has been traditionally viewed as a structural relay to specific cortical
areas behaviorally associated with sensory or motor functions, and thalamic nuclei that
function in this manner are referred to as 'relay nuclei·. However. the parts of the
thalamus interconnecting limbic association cortices (functionally involved in memory.
reward, emotion. and decision making) comprise the midline and intralaminar nuclei. The
midline thalamus has not been examined fully at the anatomical, physiological. or
behavioral level, and may serve as an important relay between cortical and subcortical
structures and the limbic system. The work incorporated into this dissertation included
five axonal tract tracing projects that were conducted in the rat. to explore and test the
hypothesis that the midline thalamus serves as an important interface between limbic
structures including the amygdala. nucleus accumbens. medial prefrontal cortex and
hippocampal formation.
An important finding was the demonstration of a closed anatomical loop between the
hippocampal formation, the ventral medial prefrontal cortex and the ventral midline
thalamus: CA 1/subiculum > PLIIL > RE > CA 1/subiculum. Another finding was that 1) the hippocampal formation innervates the entire medial prefrontal cortex; and 2) the
hippocampal formation projects more heavily to ventral as compared to dorsal cortices in
the mPFC. The paraventricular, parataenial, rhomboid and reuniens nuclei of the midline
thalamus were shown to distribute to limbic structures important for cognitive
processing: the amygdala, nucleus accumbens, hippocampal formation, parahippocampal
cortex, and the prefrontal cortex. Present results demonstrate that the ventral midline
nuclei (reuniens and rhomboid) extensively innervate limbic cortical structures (the
medial prefrontal cortex and hippocampal formation) whereas dorsal midline nuclei
(paraventricular and parataenial) distribute more heavily to subcortical limbic structures
(the amygdala and the nucleus accumbens). These midline nuclei may, therefore, relay
information between these limbic areas. This connectivity suggests that the midline
nuclei could further be subdivided from the intralaminar and relay groups. The midline
thalamic nuclei would, therefore, comprise the limbic thalamus.

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.