Incremental grouping verses phase binding

When an image appears to the visual system, rapid feedforward processing (within about 120ms) leads to an activity patterns called base-groupings that are distributed across many cortical areas. Base-groupings are coded by single neurons tuned to multiple features. The question arises as to how more complex structures are bound together from a combination of base groupings from many cortical areas. For example, a line contour may require base-groupings that code for many smaller line segments that make up  part of the contour to be bound together as a whole. The combined pattern may not be catered for by an explicitly wired base-grouping. Opinions are polarized as to what methods the brain uses to bind disparate activations together. I will here outline two contenders.

Some suggest that groups of neurons across the cortex synchronise their firing patterns binding disparate parts of brain activation together (e.g. a population encoding for red and a population encoding for circle synchronize to encode for a red circle). This is said to explain rhythmic oscillations in the brain, particularly in the gamma band (30-40 hertz). Others believe this to be a mere epiphenomenon. In the 2006 Annual Review of Neuroscience Roelfsema points out that some recent studies on monkeys observed no direct relationship between synchrony and perceptual grouping, and in some instances, grouping is even associated with a reduction in synchrony.

Incremental grouping proposed my Roelfsema et al are said to make use of horizontal and feedback connections to enhances the responses of neurons coding features that are bound in perception. ‘By the time that the base representation has been computed, neurons that respond to features of the same object are linked by the interaction skeleton. This also holds for neurons that respond to widely separated image elements, although these are only indirectly connected through a chain of cells responsive to interspersed image elements. A rate enhancement has to spread through the interaction skeleton in order to make these additional groupings explicit.’

However, this does not blow synchronization theory out of the water. Firing rate labelling proposed by Roelfsema begs for an explanation of how the rate encodes the label bacause a simply higher rate of firing does not seem to say enough unless the rate itself shares a code between populations or unless we are to believe in some kind of threshold beyond which binding occurs. In addition, Pascal Fries explains how oscillatory rhythmic excitability fluctuations produce temporal windows for communication due to relaxation time needed between firing and when the a neuron is ready to again receive signals. ‘Only coherently oscillating neuronal groups can interact effectively, because their communication windows for input and for output are open at the same times’. A detail which adds an extra level of depth for the synchronization camp.

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