Understanding Perception Through Neural “Codes”

Walter J Freeman

doi:10.1109/tbme.2010.2095854

Abstract

A major challenge for cognitive scientists is to deduce and explain the neural mechanisms of the rapid transposition between stimulus energy and recalled memory-between the specific (sensation) and the generic (perception)-in both material and mental aspects. Researchers are attempting three explanations in terms of neural codes. The microscopic code: cellular neurobiologists correlate stimulus properties with the rates and frequencies of trains of action potentials induced by stimuli and carried by topologically organized axons. The mesoscopic code: cognitive scientists formulate symbolic codes in trains of action potentials from feature-detector neurons of phonemes, lines, odorants, vibrations, faces, etc., that object-detector neurons bind into representations of stimuli. The macroscopic code: neurodynamicists extract neural correlates of stimuli and associated behaviors in spatial patterns of oscillatory fields of dendritic activity, which self-organize and evolve on trajectories through high-dimensional brain state space. This multivariate code is expressed in landscapes of chaotic attractors. Unlike other scientific codes, such as DNA and the periodic table, these neural codes have no alphabet or syntax. They are epistemological metaphors that experimentalists need to measure neural activity and engineers need to model brain functions. My aim is to describe the main properties of the macroscopic code and the grand challenge it poses: how do very large patterns of textured synchronized oscillations form in cortex so quickly?

Highlights

THE challenges I see for neuroengineers are to define, measure, and explain extreme correlation lengths in brain activity, and to do this with existing tools for acquiring and modeling brain data
When and where does a population form an active state that executes a cognitive step? What kind of shape does the active state have? How large is it? What bounds it? How long does it last? How many neurons participate? Each step may be regarded as a cinematic frame that is combined with others of its kind as the substrate for perception and action
Unequivocal classification with respect to some measure of intentional behavior is essential for validation of the premise that the AM patterns provide the content transmitted by the macroscopic code

Summary

INTRODUCTION

THE challenges I see for neuroengineers are to define, measure, and explain extreme correlation lengths in brain activity, and to do this with existing tools for acquiring and modeling brain data. Specialized neural networks in the intrusive layers preprocess sensory information from the thalamus (microscopic) and transform it to mesoscopic patterns of action potentials. Simulations of population dynamics with nonlinear differential equations [3], [6] show that the negative feedback to the excitatory neurons from inhibitory interneurons (o) generates gamma oscillations (30-80 Hz) (Fig. 2, A), which are regeneratively amplified (B) by small increases in the strengths of Hebbian synapses on association and strongly damped by small decreases on habituation (C). They do this when cortical output is transmitted through a divergent-convergent pathway (Fig. 1, left) that by spatial integration enhances endogenous signals that everywhere have the same carrier frequency. The integration attenuates unsynchronized activity by smoothing [2], most notably the stimulus-bound cortical activity that is driven by exogenous sensory input, as distinct from interactively bound activity

THE ELEMENT OF THE MACROSCOPIC CODE

PROPERTIES OF THE MACROSCOPIC CODE

INTERPRETATION OF CORTICAL DYNAMICS