Salience as Unit of Bidding
Recall our Attention as Gatekeeper metaphor:
Our perceptual systems process myriad sensory events, these must bid for entry into the capacity-limited Global Workspace.
Attention, then, is a kind of auction. The unit of bidding is salience. Let me explain.
Imagine a landscape with money on the ground. This is rather unexpected: not many experiences of natural scenes include such images. Salience and consciousness are related: the bag of money is one of the first things to enter awareness.
The salience map hypothesis is that the brain constructs a topographic map to compute salience distributions.
The salience map hypothesis is meant literally: if you were standing over an exposed brain, and could transduce electrical activity into light, you would physically see the salience map tattooed onto the cortical surface.
Notice how this salience map contains a peak of activity at the location of the money. The money stimulus has evoked the strongest bid.
The Computation of Salience
Your visual system receives information from the retina into what is called a primary visual area, V1. From there, information is carried along several diverging cortical streams. Think: carrier pigeons dispatched to the four corners of the globe.
One pathway, the ventral stream (the soft underbelly of the brain) is responsible for extracting features (e.g., color, shape, texture) from retinal imagery. Features are used in object recognition: if an unknown object has the shape shape as your pre-existing Trumpet memory, then you will identify it as a trumpet!
Another thing that features do, however, is generate salience bids. Psychophysics has revealed a wide swathe of visual properties that induce salience. Here are some examples:
- Motion: objects moving quickly or erratically
- Contrast: significantly brighter or darker than background
- Novelty: violates contextual expectations; occur with low-probability.
We should expect salience to be grounded in biological fitness: that information with high survival value would select for high salience. This is in fact the case. The above salience-triggers are precisely the sorts of things we would expect e.g., predators to produce.
Premotor Theory of Attention
The following circuitry are associated with eye movement:
Call this the saccade circuit. The foveal spotlight moves via the following mechanism:
Signals from FEF & LIP travel to iSC, which engages the (tremendously complicated) oBN network responsible for generating eye movement.
However, in the 1990s, researchers began to notice that the saccade circuit is also involved in attention! Three streams of evidence have since confirmed this suspicion:
- Human imaging studies (e.g., Corbetta et al 1998) discovered eye movements and visuospatial attention both activate identical regions within the saccade circuit.
- Primate electrophysiology studies (e.g., Moore and Armstrong 2003) showed that microstimulation of FEF enhanced visual responses in V4 neurons that represented the same spatial location.
- Human TMS studies (e.g., Ruff et al 2006) blasted FEF with a magnetic pulse, and observed attention-like effects within early visual cortex (e.g., V1). This suggests
Taken together, these data motivate the Premotor Theory of Attention, which holds that visuospatial attention constitutes preparation for a saccade event.
Given the weight of evidence supporting it, the premotor theory is now the consensus view among neuroscientists. Of course, the theory is only a starting point. Much contemporary attention research elaborates on this basic mechanism.
Via Premotor Theory, we have successfully discovered a selection mechanism within the cortex. This adds some meat to our attention as gatekeeper metaphor:
A Dual Map Hypothesis of Spatial Attention
So far, we have seen three trends in the literature:
- Neuroeconomics argues that saccadic choice is implemented via Winner-Take-All (WTA) on utility maps.
- Salience maps are increasingly viewed as indispensable to exogenous attention, and suspected to reside in posterior parietal cortex.
- Premotor Theory suggests that saccadic choice and attentional choice utilize the same circuit.
Let me throw my hat into the ring, and present a novel hypothesis to weld these themes together.
- Conjecture 1: FEF contains a saccade utility map, and LIP contains a salience map.
- Conjecture 2: Corticocortical pathways between FEF and LIP synchronize these maps (“high saliency is high saccade utility”)
- Conjecture 3: WTAs in FEF induce saccades. They represent decisions to relocate the foveal spotlight.
- Conjecture 4: WTAs in LIP represent decisions related to the attentional spotlight. They initiate bind & broadcast operations necessary to admit an object into the Global Workspace, and send optimization signals downstream, modifying processing as far as V1.
Call this the Dual Map Hypothesis.
Recall that “large” simulations of FEF induces both saccades & attentional signals, whereas “moderate” stimulations only affects attention. Is this incompatible with my Dual Map Hypothesis?
No. This result is, in fact, predicted by our theory, given the following conditions:
- The electrical impulse travels across the FEF-LIP bridge, affecting both topographic maps
- The resolution threshold of FEF is considerably higher than that of LIP.
- This isn’t terribly difficult to suppose. Saccadic decisions are more metabolically and temporally expensive, after all.
Until next time.