Faculty Bibliography

Pellionisz and Llinas proposed, years ago, a geometric interpretation towards understanding brain function. This interpretation assumes that the relation between the brain and the external world is determined by the ability of the central nervous system (CNS) to construct an internal model of the external world using an interactive geometrical relationship between sensory and motor expression. This approach opened new vistas not only in brain research but also in understanding the foundations of geometry itself. The approach named tensor network theory is sufficiently rich to allow specific computational modeling and addressed the issue of prediction, based on Taylor series expansion properties of the system, at the neuronal level, as a basic property of brain function. It was actually proposed that the evolutionary realm is the backbone for the development of an internal functional space that, while being purely representational, can interact successfully with the totally different world of the so-called 'external reality'. Now if the internal space or functional space is endowed with stochastic metric tensor properties, then there will be a dynamic correspondence between events in the external world and their specification in the internal space. We shall call this dynamic geometry since the minimal time resolution of the brain (10-15 ms), associated with 40 Hz oscillations of neurons and their network dynamics, is considered to be responsible for recognizing external events and generating the concept of simultaneity. The stochastic metric tensor in dynamic geometry can be written as five-dimensional space-time where the fifth dimension is a probability space as well as a metric space. This extra dimension is considered an imbedded degree of freedom. It is worth noticing that the above-mentioned 40 Hz oscillation is present both in awake and dream states where the central difference is the inability of phase resetting in the latter. This framework of dynamic geometry makes it possible to distinguish one individual from another. In this paper we shall investigate the role of dynamic geometry in brain function modeling and the neuronal basis of consciousness

Comments on an article by U. Awret (see record 2008-14313-001). The fascination of Velasquez's painting Las Meninas stems largely from the ambiguous relationship between the painting as a whole, viewed by a single perceiver, and the variety of different perceptual viewpoints it invites. This situation resonates strongly with a central puzzle in the study of consciousness: the apparent unity of perceptual experience despite multiple sense modalities. Understanding more of this latter might help to explain the way we respond to the painting. Given that sensory inputs generate but a fractured representation of universals, the issue of perceptual unity concerns the mechanisms that allow these different sensory components to be gathered into one global image. In recent years, this has been described as 'binding', to be implemented by temporal conjunction. Alternatively, since categorizations are generated by spatial mapping of the primary sensory cortex and its associated cortical structures, a more dynamic interaction based on temporal coherence may generate dissipative functional structures capable of a rapid a change as the perception they generate. Thus, a simultaneity mapping may be envisioned that takes advantage of the parallel and synchronous organization of the brain networks in order to generate perception.

OBJECTIVE: To determine the source localization(s) of the midlatency auditory magnetic response M50, the equivalent of the P50 potential, a sleep state-dependent waveform known to habituate to repetitive stimulation. METHODS: We used a paired stimulus paradigm at interstimulus intervals of 250, 500 and 1000 ms, and magnetoencephalographic (MEG) recordings were subjected to computational methods for current density reconstruction, blind source separation, time-frequency analysis, and data visualization to characterize evoked dynamics. RESULTS: Each subject showed localization of a source for primary auditory evoked responses in the region of the auditory cortex, usually at a 20-30 ms latency. However, responses at 40-70 ms latency that also decreased following the second stimulus of a pair were not localizable to the auditory cortex, rather showing multiple sources usually including the frontal lobes. CONCLUSIONS: The M50 response, which shows habituation to repetitive stimulation, was not localized to the auditory cortex, but showed multiple sources including frontal lobes. SIGNIFICANCE: These MEG results suggest that sources for the M50 response may represent non-auditory, perhaps arousal-related, diffuse projections to the cortex

Thalamocortical in vivo and in vitro function was studied in mice lacking P/Q-type calcium channels (Cav2.1), in which N-type calcium channels (Cav2.2) supported central synaptic transmission. Unexpectedly, in vitro patch recordings from thalamic neurons demonstrated no gamma-band subthreshold oscillation, and voltage-sensitive dye imaging demonstrated an absence of cortical gamma-band-dependent columnar activation involving cortical inhibitory interneuron activity. In vivo electroencephalogram recordings showed persistent absence status and a dramatic reduction of gamma-band activity. Pharmacological block of T-type calcium channels (Cav3), although not noticeably affecting normal control animals, left the knockout mice in a coma-like state. Hence, although N-type calcium channels can rescue P/Q-dependent synaptic transmission, P/Q calcium channels are essential in the generation of gamma-band activity and resultant cognitive function

The lattice-like structure of the cerebellar cortex and its anatomical organization in two perpendicular axes provided the foundations for many theories of cerebellar function. However, the functional organization does not always match the anatomical organization. Thus direct measurement of the functional organization is central to our understanding of cerebellar processing. Here we use voltage sensitive dye imaging in the isolated cerebellar preparation to characterize the spatio-temporal organization of the climbing and mossy fiber (MF) inputs to the cerebellar cortex. Spatial and temporal parameters were used to develop reliable criteria to distinguish climbing fiber (CF) responses from MF responses. CF activation excited postsynaptic neurons along a parasagittal cortical band. These responses were composed of slow ( approximately 25 ms), monophasic depolarizing signals. Neither the duration nor the spatial distribution of CF responses were affected by inhibition. Activation of MF generated responses that were organized in radial patches, and were composed of a fast ( approximately 5 ms) depolarizing phase followed by a prolonged ( approximately 100 ms) negative wave. Application of a GABA(A) blocker eliminated the hyperpolarizing phase and prolonged the depolarizing phase, but did not affect the spatial distribution of the response, thus suggesting that it is not the inhibitory system that is responsible for the inability of the MF input to generate beams of activity that propagate along the parallel fiber system

Transmitter release at chemical synapses is triggered by high calcium concentration microprofiles at the presynaptic cytosol. Such microprofiles, generated by the opening of voltage-dependent calcium channels at the presynaptic plasma membrane, have been defined as calcium concentration microdomains. Using total internal reflection fluorescent microscopy in conjunction with calcium and vesicular release indicator dyes, we have directly visualized the close apposition of calcium concentration microdomains and synaptic release sites at single synaptic terminals from the CNS from rat cerebellar mossy fiber and squid optic lobe. These findings demonstrate the close apposition of calcium entry and release sites and the dynamics of such site locations over time. Kinetic analysis shows that vesicles can be released via two distinct mechanisms: full-fusion and kiss-and-run. Calcium triggers vesicular motion toward the membrane, and the speed of such movement is calcium concentration-dependent. Moreover, the immediately available vesicular pool represents molecularly trapped vesicles that can be located at a larger distance from the plasma membrane than the field illuminated by total internal reflection fluorescent microscopy