From spikes to liquid computing

From spikes to liquid computing

Research Update TRENDS in Cognitive Sciences Vol.5 No.9 September 2001 371 Meeting Report From spikes to liquid computing Henry Markram The Fifth ...

21KB Sizes 11 Downloads 76 Views

Research Update

TRENDS in Cognitive Sciences Vol.5 No.9 September 2001


Meeting Report

From spikes to liquid computing Henry Markram The Fifth International Conference on Cognitive and Neural Systems was held at the Dept of Cognitive and Neural Systems, Boston University, Boston, MA, USA, 31 May – 2 June 2001.

Around 300 participants gathered from 31 countries in Boston for the Fifth International Conference on Cognitive and Neural Systems meeting. A broad range of issues was addressed at this meeting, all with the common goal of trying to understand how the nervous system organizes itself and adapts to a rapidly changing world. Most studies were focused on the meeting’s two main themes: (1) ‘How does the brain control behavior?’, and (2) ‘How can technology emulate biological intelligence?’ One of the goals of the meeting was for the sessions to move smoothly between psychological and neurobiological data, and between biological models, mathematical studies and technological applications. There was also a smooth movement of topics between the more microscopic/biophysical studies and the system-level links between brain and behavior, as well as from perception through to action. This multi-dimensional space of topics was aimed at achieving a type of local coherence among clusters of talks, whilst also offering enough variety on each day to satisfy different intellectual appetites, and to provide a sense of variety and of new discovery. All this was, of course, only partly controllable through the 24 invited talks and tutorials. Beyond that, another 125 presentations (oral and poster) were contributed, and organized according to these underlying themes with some degree of success. At the physiological level, a series of new views were presented. The importance of precision in spike timing during learning received attention from invited speakers as well as oral and poster presenters. Larry Abbott’s (Brandeis University, Waltham, MA, USA) main claim was that this new angle of precision in the relative timing of input and output spikes automatically balances input spike rates with output spike rates. Leon Cooper

(Brown University, Providence, RI, USA) presented evidence to suggest that the width of the spike propagating back into dendrites largely determines the Ca2+ level near synapses, and when the Bienenstock–Cooper–Munro (BCM) model’s learning law is adapted to include weight changes as a function of internal Ca2+ levels, then the precise relative timing principle also yields the bidirectional modifications predicted by the BCM. Kepecs and Tegner (Brandeis University, Waltham, MA, USA) showed that the sliding threshold naturally emerges from this learning principle as synapses compete to fire the postsynaptic cell. No one, however, addressed the loopholes of this learning algorithm, such as non-linearities during trains of spikes, convergence and stabilities of single synaptic values (as opposed to the distribution), the need for noisy input to hold a distribution of synaptic values and maintain competition, and other more tricky issues. Exploring mechanisms of visual processing, Raph Freeman (University of California Berkeley, Berkeley, MA, USA) illustrated suppression across receptive fields and showed that neurons generally interact to suppress other neurons outside their receptive fields, but that in order for suppression to be effective they must be activated by their preferred stimulus – presumably indicative of a collaborative competition intrinsic to the intercolumnar circuitry. Nancy Kopell (Boston University, Boston, CA, USA) beautifully demonstrated the inevitable result of interacting neural assemblies: a mosaic of rhythms. The age-old question of their functional significance unfortunately still stands nobly. Wolf Singer (Max Planck Institute for Brain Research, Frankfurt, Germany) highlighted the fundamental importance of solving the superposition problem, where the massive combinatorial possibilities for different features of objects necessitates that each neuron be involved in multiple perceptions. The natural synchrony that dynamically emerges due to relatedness and compositionality rules, (presumably

embedded in the circuitry) is proposed as the Gestalt binding mechanism that allows objects to stand out and form unified perceptions while allowing neurons to simultaneously multiplex their tasks. Temporal correlations were also shown to be a key factor in meaningful information transfer from the thalamus to the auditory cortex by Christoph Schreiner (University of California San Francisco, San Francisco, CA, USA). ‘...the key challenge for biological systems is to organize functions dynamically...’

The speed limit for signal transmission from one population of neurons to another was addressed in the presentation by Wulfram Gerstner (EPFL, Lausanne, Switzerland), in which he concluded the non-intuitive result that it is actually the amplitude and type of noise in the membrane of each cell that sets the limit, dynamically. How information is processed during spike trains, rather than lumping a spike train into a single stimulus was approached in a novel way by Wolfgang Maass (University of Graz, Austria) and Henry Markram (Weizmann Institute of Science, Rehevot, Israel). They presented the new notion of ‘liquid computing’ which solves the generally intractable problem of extracting salient information from a dynamical system supported by recurrent circuits. He showed how ‘liquid recurrent circuits’ absorb spike trains into a highdimensional spatial map that can be used for real-time universal computation. Moving to hardcore perception, Nikos Logothetis (Max-Planck Institute for Biological Cybernetics, Tuebingen, Germany) presented a marathon account of how binocular rivalry can be generated by virtually any two stimuli depending on contrast, how the site of suppression in rivalry is in the striatum and how different circuitries mediate the suppression and perception. At an equally rapid pace, Steve Grossberg (Boston University, Boston, MA, USA), with a backdrop of the Mind–Body problem, stressed that the key challenge for

1364-6613/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1364-6613(00)01731-9


Research Update

biological systems is to organize functions dynamically to adapt to a rapidly changing world. He pointed out the inevitable uncertainty at each stage of information processing and that multiple stages of intra- and inter-stream interactions in parallel and hierarchical circuits (rather than independent modules) resolve the uncertainty and reveal a profound re-occurring ‘principle of complementarity’ – streams, properties and functions all come with their complementary pairs that must collaborate to form unitary perceptions. ‘Complementary computing’ was shown to explain the circuit mechanisms behind a host of psychophysical data. David Heeger (Stanford University, Stanford, CA, USA) pointed out that attention ‘coming from somewhere’ does not evoke a perception, but rather primes neurons for relevant inputs that will evoke the perception. One wonders, however, what mechanistic marvel will allow one to attend to different parts of an imagined scene or move around in a daydream! Maggie Shiffrar

TRENDS in Cognitive Sciences Vol.5 No.9 September 2001

(Rutgers University, Newark, NJ, USA) brilliantly showed how the laws of physics and reality conventions influence visual perception and that visual perception really can’t stand on its own ground because perception of moving objects is intimately associated with the acceptable physical limitations of their movements. Even the motor cortex responds when one watches a movie of a person in motion – but only if those movements are within the realm of the experience of the watcher! From the angle of brain circuits, Peter Strick (University of Pittsburgh, Pittsburgh, PA, USA) presented impressive data showing how the usual concept of the basal ganglia and cerebellum projecting largely to motor-related cortical areas is incorrect, and that in fact, nearly 80% of the inputs are topographically projected onto other regions of the neocortex and subcortical nuclei. One wonders, however, when we will begin to move to higher-order mapping topographies and complementary rules for convergent topographies. Yet another twist in the cerebellar story was

presented by Richard Ivry (University of California Berkeley, Berkeley, CA, USA), who showed that the cerebellum is crucial in perception of discrete events in time and not in the continuous perception of time. Daniel Bullock (Boston University, Boston, MA, USA) was probably the only speaker who satisfied biologists, anatomists and modelers alike by employing all the circuits of the frontal cortex and basal ganglia humanly possible in a model for action selection and reinforcement learning. Many other issues were raised in oral presentations and posters, with a spectrum of models that extended up to industrial applications. The meeting space facilitated intense discussions and friendly encounters between old and new friends alike. It was a welcoming and refreshing meeting that went far beyond the usual concepts of neural coding. Henry Markram Dept of Neurobiology, Weizmann Institute of Science, Rehevot 76100, Israel. e-mail: [email protected]

Rational approaches to rationality Luis A. Pérez-Miranda The Seventh International Colloquium on Cognitive Science was held at DonostiaSan Sebastian, Spain, 9–12 May 2001. The conference was organised by the Institute for Logic, Cognition, Language and Information (ILCLI), and the Dept of Logic and Philosophy of Science of the University of the Basque Country (UPV-EHU), Donostia-San Sebastian, Spain.

In this report, I will comment on some of the most important and interesting aspects of the Seventh International Colloquium on Cognitive Science (ICCS-01). As a significant proportion of the contributed papers presented at this year’s colloquium revealed, the main goal in contemporary cognitive science is not primarily to build a thinking machine, but to increase our understanding of different cognitive processes, such as perception, learning, reasoning, planning, decisionmaking, communication (literal and non-literal) and so forth. The diversity and use of different methods was also manifest in this colloquium, in sharp contrast with

the early days of artificial intelligence when it was thought that the symbolicrepresentation methodology was enough to do the job. As Gärdenfors has recently pointed out, ‘the current trend [in cognitive science] is to work with several forms of representations and data’ (Ref. 1, p. 14). This change of outlook has led us to a kind of methodological pluralism. Psychological experiments, observations and descriptions of authentic cognitive processes in practical action (‘situated cognition’) or simulation of human behaviour by means of programmes and robots are now common methods in cognitive science. And although the main research goal is more or less clear for the cognitive science community, the diversity of theories and methods is becoming so large that replacing the name cognitive science with cognitive sciences (or ‘sciences of cognition’) is more than justified. Even if situated at another level of analysis, several contributions to this conference showed us that we cannot easily overlook the ‘intrinsic’ philosophical

difficulties involved in the studies of human mind. Specifically, the ‘frame problem’, the problems of ‘mental causation’, the meaning problems posed by propositional attitudes ascriptions, the impact of theory building (in cognitive science) on epistemology, and the problem of ‘reductionism’ have all been under debate. So, in addition to presenting explanatory models of cognitive abilities, it is fair to say that this conference was also a first-order forum for the examination and discussion of questions that give rise to fundamental philosophical issues for the cognitive sciences. Leaving aside the somewhat expected discussion of new methodologies, two other issues attracted my attention at this meeting. Firstly, there was discussion of the fact that our knowledge and understanding of the cognitive mechanisms regulating human emotions, and in particular of those mechanisms related to music, remains remarkably limited. The way emotions arise from listening to music and the connection of emotion to music are still very much a

1364-6613/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1364-6613(00)01732-0