Perception Lecture. Monday August 24, 18.00 - 19.00.

Wolf Singer, "Distributed processing and temporal codes in cortical networks"

The cerebral cortex presents itself as a distributed dynamical system with the characteristics of a small world network. The neuronal correlates of cognitive and executive processes often appear to consist of the coordinated activity of large assemblies of widely distributed neurons. These features require mechanisms for the selective routing of signals across densely interconnected networks, the flexible and context dependent binding of neuronal groups into functionally coherent assemblies and the task and attention dependent integration of subsystems. In order to implement these mechanisms, it is proposed that neuronal responses should convey two orthogonal messages in parallel. They should indicate i) the presence of the feature to which they are tuned and ii) with which other neurons (specific target cells or members of a coherent assembly) they are communicating. The first message is encoded in the discharge frequency of the neurons (rate code) and it is proposed that the second message is contained in the precise timing relationships between individual spikes of distributed neurons (temporal code). It is further proposed that these precise timing relations are established either by the timing of external events (stimulus locking) or by internal timing mechanisms. The latter are assumed to consist of an oscillatory modulation of neuronal responses in different frequency bands that cover a broad frequency range from < 2 Hz (delta) to > 40 Hz (gamma) and ripples. These oscillations limit the communication of cells to short temporal windows whereby the duration of these windows decreases with oscillation frequency. Thus, by varying the phase relationship between oscillating groups, networks of functionally cooperating neurons can be flexibly configurated within hard wired networks. Moreover, by synchronizing the spikes emitted by neuronal populations, the saliency of their responses can be enhanced due to the coincidence sensitivity of receiving neurons in very much the same way as can be achieved by increasing the discharge rate. Experimental evidence will be reviewed in support of the coexistence of rate and temporal codes. Evidence will also be provided that disturbances of temporal coding mechanisms are likely to be one of the pathophysiological mechanisms in schizophrenia.

Rank Lecture. Tuesday August 25, 18.00 - 19.00.

Patrick Cavanagh, "The position sense"

How is position coded in the visual system? Many of the visual areas in the brain are organized as retinotopic maps where adjacent neurons respond to adjacent locations on the retina. An object’s position, at least for the fronto-parallel (XY) plane, is therefore naturally carried by its retinotopic coding throughout the visual areas. Clearly, this retinotopic information must be corrected for eye, head, and body movements (to reference locations in the world), but recent physiology suggests this is feasible. In comparison, position in depth must be constructed from multiple cues and lacks an explicit dimension for representation. Errors in depth perception are common, including depth reversals, whereas there are few such dramatic errors in XY location. Nevertheless, there are some – saccade-induced compression and mislocalization, and motion induced shifts, for example – where the perceptual and retinotopic locations of a stimulus disagree and these force us to reject retinotopy as the underlying code for position. Our studies suggest that position is constructed on one map and that target locations on this map attempt to follow and, where possible, predict target locations but occasionally lag or miscalculate in the attempt. These errors in construction underlie several perceptual mislocalizations and reveal possible mechanisms used to follow and predict target locations.

Human and non human primates can recover the third dimension of shape, depth structure, from four cues: shading, texture, motion and disparity. We begin to understand how this is achieved by the primate visual system. Three cues, texture, motion and disparity are treated in relatively similar ways. In macaque three parietal regions, CIP, anterior LIP and AIP, house neurons selective for first or second order gradients of texture, speed, or disparity. In humans four parietal regions are involved, the anterior two of which might be homologous of the two anterior IPS regions in monkey. The main species difference is a weaker sensitivity for the motion cue in macaque compared to human. In the macaque ventral visual pathway fewer regions are involved: MT/V5 and FST for motion, TEs for disparity and TEO for texture. In humans similarly only few regions, located in or below the hMT/V5+ complex are involved. These ventral regions also house gradients selective neurons. The shading cue appears to be processed only in the ventral pathway: in TEO and posterior TE in the monkey and in a corresponding region in caudal ITG in humans. The neuronal mechanism is still unclear and might include more than just the extraction of luminance gradients. All regions involved in processing depth structure also process 2D shape, and hence can provide a complete description of 3D shape.

Siemens Lecture. Thursday August 27, 18.00 - 19.00.

Nikos Logothetis, "In vivo connectivity: MRI, paramagnetic tracers and electrical stimulation"

Neuroanatomical cortico-cortical and cortico-subcortical connections have been examined mainly by means of degeneration methods and anterograde and retrograde tracer techniques. Although such studies have demonstrated the value of the information gained from the investigation of the topographic connections between different brain areas, they do require fixed, processed tissue for data analysis and therefore cannot be applied to animals participating in longitudinal studies. Capacities such as plasticity and learning are indeed best studied with non-destructive techniques that can be applied repeatedly and, ideally, combined with neuroimaging or electrophysiology studies.

The recent development of MR-visible tracers that can be infused into a specific brain region and are transported anterogradely transsynaptically is one such technique. Simultaneous electrical stimulation (ES) and fMRI (esfMRI) is another. In fact, esfMRI offers a unique opportunity not only to study connectivity, but also to visualize networks underlying electrostimulation-induced behaviors, to map the neuromodulatory systems, or to develop electrotherapy and neural prosthetic devices. In my talk I’ll present new data on MR-visible tracers and esfMRI that show the capacity of these methods for the study of connectivity, of cortical microcircuits, and of cortical network reorganization induced by long term potentiation of synapses in subcortical structures, e.g. in hippocampus.