Attention as Gatekeeper

Part Of: Attention sequence
Followup To: An Introduction to the Attentional Spotlight
Content Summary: 600 words, 6 min read

Global Workspace Theory

The weakest noticeable sound is defined at 0 decibels. Imagine putting somebody into a scanner, and having them listen to two sounds:

  1. A trumpet playing at -5 dB
  2. A trumpet playing at 5 dB

The acoustic difference between the two waveforms are not very different. How similar are the patterns of brain activation?

Attention- GWT

Here we see that subliminal auditory stimuli only activate early perceptual areas. Consciousness brings with it a huge increase in neural activation! Why should this be?

Global Workspace Theory (GWT) posits that consciousness is involved in two mental operations:

  • Binding: perceptual features, distributed across the brain, are bound together into discrete objects
  • Broadcasting: these object networks are broadcast to the rest of cortex, allowing consumer systems to use & modify them.

Attention- GWT Architecture
Three properties of consciousness have long baffled philosophers:

  • Consciousness is small: we can only retain a few (less than 7) objects in our head at one time.
  • Consciousness is serial: we can’t read two books at the same time.
  • Consciousness is flexible: unlike state of the art AI software, human reasoning can effortlessly enter new domains.  

GWT explains these facts. Consciousness is…

  • … small because it is hard to keep global object networks distinct from one another.
  • … serial because it is a singleton: massively parallel modules engage the same centralized resource.
  • … flexible because any consumer system can augment the processing of any perceptual object.

The Role of Attention

Attention is a gatekeeper. Our perceptual systems process myriad sensory events, these must bid for entry into the Global Workspace. The brain contains circuitry that implements this selective process, choosing which perceptual objects to bind & broadcast.  

Attention- Gatekeeper Role

Let’s see if we can use this metaphor to make sense of the sprawling literature on attention.

Consolidating Taxonomies

There are three taxonomies of attention that you’ll find in the literature:

  1. Covert vs overt attention. As discussed in Attentional Spotlight, we can differentiate attending to objects in the periphery, versus saccading to attended targets.
  2. Bottom-up vs top-down attention.  Distinguishes unplanned attention (e.g., to loud noises) vs goal-based attention (e.g., “count the number of times the soccer ball is passed”).
  3. Feature vs spatial attention. Distinguishes attending to a feature (“look for all red things”) vs an object (“look for a red triangle”)

In an influential paper, Peterson & Posner (1990) present three attentional networks: functionally independent brain systems which do attention. These are:

  1. Alerting. This network is tightly linked to wakefulness. Startling events induces strong alerting, lounging on a couch less so.
  2. Orienting. These two networks (one dorsal, the other located more ventral) orients the organism to process incoming stimuli.
  3. Executive. This network supports complex task execution, and goal-oriented attention.

Peterson & Posner’s framework allows us to simplify the conceptual landscape:

Attention- Taxonomy Reduction

The Orienting network produces Bottom-Up (“externally-driven”) attention. Its dorsal arm contains mechanisms for covert and overt orienting.

The Executive network produces Top-Down (“internally-generated”) attention. Feature and Object attention are both a form of search template, and as such are constructed here.

An Attentional Organ

In my next post, I’m going to argue that the Dorsal Orienting network is the attentional gateway, full stop. It alone performs selection: a single gateway through which percepts pass into conscious awareness.

On this model, the arousal, ventral orienting, and executive networks play auxiliary roles, modulating our brain’s attentional gateway.

Attention- Architecture Overview

Until next time.

References

  • Peterson & Posner (1990). The attention system of the human brain

Baars: The Conscious Access Hypothesis, Origins and Recent Evidence

Article Details

Article: The conscious access hypothesis: origins and recent evidence
Author: Bernard J Baars
Published: 01/2014
Citations: 581 (note: as of 03/2014)
Link: Here (note: not a permalink).

Context

In 1988, Bernard Baars authored A Cognitive Theory of Consciousness, which presented his Global Workspace Theory (GWT) of consciousness. In short, he argues that consciousness is caused by global inter-brain sharing of information. This theory does not concern itself much with the construction of phenomenology, and thus does not qualify as a solution to the Hard Problem of Consciousness (which is well explained here).

Methodology

Scientific efforts to understand consciousness evoked vigorous philosophical objections. These were essentially the classic mind-body problems: how does private experience relate to the physical world? … Difficult conceptual questions are routine when the sciences turn to new topics. The traditional scientific response is simply to gather relevant evidence and develop careful theory. Ultimately, philosophical controversies either fade, or they compel changes in science if they have empirical consequences.

I like this quote. While it doesn’t encapsulate my sentiments on the role of philosophy, its call for empirical analysis was long overdue.

You may find yourself asking: how can neuroscience examine consciousness, if consciousness is private to the individual? Baars advocates using an operational definition of conscious awareness: consciousness is the ability to produce a reliable report. An example: suppose I flash a number (0-9) on your monitor, and then ask its value. Say I present the number three for 200 milliseconds. If I ask you what you saw, you would be able to report your conscious experience. But, say I present the same number for 2 milliseconds. If I then ask you what you saw, you would not be able to report the correct value better than a ten-sided die. By this means, I have acquired a variable that represents whether a task is associated with consciousness.

How can we causally distinguish between the effect of consciousness and, say, the effect of low IQ on a given task? Well, most neuroscientific inquiries into consciousness employ a technique Baars refers to as contrastive analysis. This technique involves comparing processes that induce conscious awareness only occasionally. Let’s suppose that, in the above example, 200ms corresponded to 98% correct reports, whereas 2ms corresponded to 3% of subjects being aware of the change consciously. I would then be tempted to “turn the display-time knob” so any one person has a 50% chance of perceiving the number, and then analyzing the differences between the two groups. To see an example of contrastive analysis beyond the above toy model, Baars cites Dehaene et al [1] as an exemplar.

A Philosophical Aside

It is, first, important to distinguish between operational definitions such as the above, and operationalism, which is a more extreme call to operationalize all scientific concepts. While the latter movement is today widely regarded as unhelpful, that doesn’t seem to problematize the desire to operationalize some definitions, such as consciousness or volition.

Let me sketch a problem that will be familiar to any philosophers. The question of philosophical zombie was memorably treated by Descartes: is it possible for a human being behave exactly as one who is conscious, reporting conscious experiences to anyone who may ask, but entirely devoid of an inner life? This metaphysical question has not been satisfactorily resolved. However, let us reframe this question in nomological terms: is consciousness causally linked to the human nervous system? If we provisionally accept the operational definition of consciousness above, we are in position to answer this question with data.

Evidences

The data seems to say yes. Consciousness hugely contributes to the functioning of our nervous system. In this paper, Baars sketches seven lines of evidence that have accumulated since his theory’s inception (1988).

  1. Conscious perception involves more than sensory analysis; it enables access to widespread brain sources, whereas unconscious input processing is limited to sensory regions.
  2. Consciousness enables comprehension of novel information, such as new combinations of words.
  3. Working memory depends on conscious elements, including conscious perception, inner speech, and visual imagery, each mobilizing widespread functions.
  4. Conscious information enables many types of learning, using a variety of different brain mechanisms.
  5. Voluntary control is enabled by conscious goals and perception of results.
  6. Selective attention enables access to conscious contents, and vice versa.
  7. Consciousness enables access to ‘self’: executive interpretation in the brain.

A wealth of data bolsters the above theses; I would point the interested reader to the article.

Baars goes on to claim that his GWT explains the above seven evidences. If GWT is to be overturned, its replacement must do even better.

Mechanisms of Brain Access

So, we see evidence of conscious activity being correlated with full-brain activation. But what mechanisms might produce full-brain activation? Baars identifies several research traditions exploring different (potentially complementary) answers to the question:

  • Dehaene and Changeux have focused on frontal cortex [1]
  • Edelman and Tononi on complexity in re-entrant thalamocortical dynamics [2]
  • Singer and colleagues on gamma synchrony [3]
  • Flohr on NMDA synapses [4]
  • Llinas on a thalamic hub [5]
  • Newman and Baars on thalamocortical distribution from sensory cortex [6]

Thoughts

Baars notes in his article that efforts to integrate research on attention and consciousness are long overdue. I would go a step further. His theory of consciousness also ought to be integrated with:

  • dual-process theory (theoreticians have already correlated System 2 with conscious awareness)
  • working memory (Alan Baddeley is already struggling to integrate his Central Executive with conscious awareness)

References

1. Dehaene, S. et al (2001) Cerebral mechanisms of word masking and unconscious repetition priming.
2. Tononi, G. and Edelmen, G.M. (1998) Consciousness and complexity.
3. Engel, A.K and Singer, W (2001) Temporal binding and the neural correlates of sensory awareness.
4. Flohr, H et al (1998) The role of the NMDA synapse in general anesthesia.
5. Llinas, R et al (1998) The neuronal basis for consciousness.
6. Newman, J and Baars, B.F. (1993) A neural attentional model for access to consciousness: a global workspace perspective.