Red Circle & Green Square or Green Circle and Red Square?

The Binding Problem

Binding likely takes place across small and large anatomical scales.

Static vs. Dynamic BindingStatic binding => conjunctive representation; collections of visual

elements and properties map onto individual units of the representation, one unit per conjunction

Hummel (2003)

Distributive Encoding of Conjunctions: no need to have all possible conjunctions separately

represented.

Problem: Ambiguity solved at one level, but still present at another => Still a need for Grandmother cells?

Potential Solution 1:

Potential Solution 2: Dynamic Binding: separate properties represented by separate units, conjunctions occur via some changing tag external to the units themselves.

Hummel (2003)

Neural synchrony: things that fire together go together.

Engels et al. (1992)

Trujillo et al. (2002)

McNaughton, 2001

Fries et al., 2001

Rate Coding: information encoding via firing rates

Temporal Coding: information encoding via timing relationships

Static vs. Dynamic Binding often equated with….

Vs.

Problems with the Synchrony Hypothesis

1) Observed synchrony may be an epiphenomenon of neural firings.

Neurons can fire in synchrony by random chance (Shadlen and Movshon, 1999).

Synchrony might simply reflect the bidirectional connectivity of the cortex (O’Reilly and Munakata, 2000; Lamme,

2001).

Counter-evidence against epiphenomenal view:

impaired odor detection in bees with a reduction in synchrony of olfactory cells while preserving high firing rates (Laurent et al., 1997).

2) Further processing of bound information requires novel and unknown dynamic mechanisms.

McNaughton, 2001

Maybe temporal Coding might provide a dynamic mechanism for processing? More research is needed…..

One problem: timing codes highly sensitive to noise (O’Reilly and Munakata, 2000; Mazurek and Shadlen, 2000).

Attention and Binding

Sometime binding fails…..

“illusory conjunctions”: human subjects presented with an may perceive incorrect feature conjunctions when attention is divided over different objects and features. (Triesman, 1999; Wolfe and Cave, 1999).

Attention may be the “tag” that binds features together.

Human electrophysiological evidence:

Greater fronto-central gamma-range spectral power to attended than unattended auditory stimuli.

Tiitinen, Sinkkonen, Reinikainen, Alho, Lavikainen, and Naatanen (1993)

Neural Synchrony and Attention

McNaughton,2001

Primate electrophysiological evidence:

Synchronization of pre-synaptic neuron inputs upon post-synaptic cells is increased with increased attention.

(Fries, Reynolds, Rorie, and Desimone, 2001). This synchronization may boost temporal and spatial summation

of EPSPs and IPSPs, and thus increase the chance to depolarize post-synaptic cell.

Conjecture:

Neurons within the loci of attention may selectively gate signals depending upon whether input cells are

synchronized.

IMHO: Binding by convergence (Singer, 1999) binding occurs via convergence of signals among neuron in feedforward, feedback, and lateral connections.

Information is irreversibly (i.e. nonlinearly) convolved (“blended” or transformed) at each synaptic step, i.e. nonlinear transformation.

Attention findings: Synchrony facilitates communication, => high synchrony can be a marker of good

communication

Synchrony might also contribute to synaptic plasticity :Dynamic interplay between LTD and LTP could work to create new connections in response to a stimulus that are reset to near initial conditions when the stimulus is removed (e.g. Loebel and Tsyodyks, 2002).

Ngezahayo, Schachner, and Artola (2000)

Speculation: such dynamic LTD/LTP interplay may resemble phenomena of “metaplasticity”: changes in the ability to induce subsequent LTD/LTP.