Past events

I will discuss different models that implement distinct limit cases of the Braitenberg view of the cortex as a two-level associative network, with A (long-range) and B (local) systems of connections. In one limit case, local networks are assumed structureless, and they can be collapsed onto single Potts variables in order to analyse global cortical dynamics, and the effect of macroscopic correlations. In another limit case, local nets have internal metric connectivity, which can be exploited to code continuous parameters topographically, a "where" representation. This models allow to analyse a local version of the what/where dilemma, a conflict to which evolution has proposed multiple solutions, all, frankly, unsatisfactory...I will discuss different models that implement distinct limit cases of the Braitenberg view of the cortex as a two-level associative network, with A (long-range) and B (local) systems of connections. In one limit case, local networks are assumed structureless, and they can be collapsed onto single Potts variables in order to analyse global cortical dynamics, and the effect of macroscopic correlations. In another limit case, local nets have internal metric connectivity, which can be exploited to code continuous parameters topographically, a "where" representation. This models allow to analyse a local version of the what/where dilemma, a conflict to which evolution has proposed multiple solutions, all, frankly, unsatisfactory...

The biological visual system is extremely complex; the coordination between the neurological system, the ocular muscles, and the photoreceptors of the retina make it possible for the visual system to produce a continues 3D representation of the real world which provides the ability to distinguish between objects in space, track them, and estimate their relative distances and velocities. For such complex abilities, the retinal image should be persistent enough for the brain to evaluate it, but ephemeral enough to permit a high sampling rate and in order to overcome physical limitations on constant exposure of the photoreceptors. In order to provide accurate depth information it is also required that there is a synchronization between the movement of both eyes. These requirementS are addressed by a complex neuromuscular system that produces multitimescale and synchronization behaviors that are not yet fully understood. We investigated the roles of these different time scale behaviors, especially how they are expressed in the different spatial directions (vertical versus horizontal). In addition, in primates with frontally placed eyes, the synchronization properties of fixational eye movements is related to binocular coordination in order to provide stereopsis, and thus this was also investigated. Results show different scaling behavior between horizontal and vertical movements. When the small ballistic movements, i.e., microsaccades, are removed, the scaling behavior in both axis become similar. Our findings suggest that microsaccades enhance the persistence at short time scales mostly in the horizontal component and much less in the vertical component. We here applied also the phase synchronization decay method to study the synchronization between six combinations of binocular fixational eye movement components. We found that only the right and left horizontal are synchronized with each other and the right and left vertical. Furthermore, the vertical components are significantly more synchronized than the horizontal components. These differences may be due to the need for continuously moving the eyes in the horizontal plane, in order to match the stereoscopic image for different viewing distances.

In this talk I will describe a recent controversy that has arisen regarding the intrinsic properties of Purkinje cells and explain the importance of this controversy to our understanding of Cerebellar function. In brief, it has been shown that Purkinje cell membrane potential is bistable, but there remains significant disagreement about whether this bistability has a functional role. In our lab, we addressed the controversy by recording from Purkinje cells in an awake animal and testing to see whether bistability that had been observed in vitro and in anaesthetized animals could also be seen in a behaving animal. Our findings will not settle the controversy, nor settle the question of the Cerebellum's functional role, but they will significantly shift the terms of the debate. We found that all of the predictions we tested confirmed the potential for a functional role for Purkinje cell bistability. This will force a serious re-evaluation of our understanding of Cerebellar circuitry.

Understanding the functional neuro-anatomy of face processing in the human brain is a long-standing goal of Cognitive Neuroscience. Up to the early 90’s, the most important source of knowledge was from lesion studies, i.e. making correlations between the localization of lesions in groups of brain-damaged patients and their face recognition impairments. The influence of the cognitive approach in Neuropsychology, with an emphasis on single-case functional investigations, as well as the advent of neuroimaging studies in the healthy brain, have considerably reduced the importance of lesion studies in clarifying the neuro-anatomical aspects of face processing. In this talk, my goal will be to illustrate how neuroimaging investigations of single-cases of acquired prosopagnosic patients can still greatly increase our knowledge in this field. Neuroimaging studies of the normal brain have shown that the middle fusiform gyrus (‘FFA’) and the inferior occipital gyrus (‘OFA’) are activated by both detection and identification of faces. Among other observations, our studies of the patient PS, a case of prosopagnosia with normal object recognition, show that the right ‘FFA’ can be recruited to detect faces independently of the ‘OFA’ of the same hemisphere (Rossion et al., 2003). However, fMRI-adaptation investigations suggest that both areas are necessary to perform individual discrimination of faces (Schiltz et al., 2006). Recent observations also show that the the same brain area, here the right ‘FFA’, may be impaired at individual face discrimination while performing normal individual object discrimination. This suggests that clusters of neurons coding specifically for different categories in this area (Grill-Spector et al., 2006) can be functionnally independent. Finally, when structurally intact, non-face preferring areas such as the ventral part of the lateral occipital complex (vLOC) may subtend residual individual discrimination of faces following prosopagnosia. Altogether, these studies show that faces are processed through multiple pathways in the human brain, with a subset of these areas responding preferentially to faces being critical for efficient face recognition.

An alternative model to the Declarative-Memory & Cognitive Map theories of the function of the hippocampus is suggested. the new model may explain the deficits described in the famous case of H.M., who displayed total anterograde amnesia following a surgery in which a bilateral dissection of the whole medial-temporal lobe (MTL) was perfromed (Scoville and Milner, 1957) . According to the model, the main functions of the MTL are: (1) to act as an integrator (2) to detect novelty. The integrator function is used, for example, for generation of the place-cell and grid-cell system. Normally, the MTL is integrating an episode until it detects a novel situation. Once the MTL detects such a novel situation, it sends the executive brain (perhaps the basal ganglia and/or prefrontal cortex) a message that it is time to play a novel behavioral game. In the case of H.M., where the MTL is missing, the executive brain never gets the message that an episode is novel, and thus continues to play "old games". In principle, at least, if this model is correct, H.M. could be cured from his memory problem, if the executive brain would have received the missing novelty signals artificially.