Now we're starting to get into some of my research! Cool.
Let's say that I'm at a home listening to iTunes when I get a phone call from my wife. I'm a child of grunge, raised by a 70s father, who was into 90s rave scene, so depending on my mood I'm either listening to Alice in Chains, Credence, or Orbital. In any case, the music is way too loud and I can't hear my wife to save my skin.
In order to improve my chances of hearing her I have one of two options. I can either:
- Turn down the noise (pause iTunes), or;
- Increase the signal/gain (turn up the volume on my phone).
Of course, I could also do both, but that might be overkill.
This analogy applies to human attention as well. For whatever evolutionary reason, we humans have evolved with highly sensitive sensory organs with a very limited attentional bandwidth.
Just to give you an idea of how sensitive (see here for references):
- We can detect as few as 2 photons entering the retina. Two. As in, one-plus-one. Under ideal conditions, a young, healthy person can see a candle flame from 30 miles away. That's like being able to see a candle in Times Square from Stamford, Connecticut. Or seeing a candle in Candlestick Park from Napa Valley.
- The limits to our threshold of hearing may actually be just above Brownian motion. That means that we can almost hear the random thermal movements of atoms.
- We can also smell as few as 30 molecules of certain substances.
We appear to have a "processing bottleneck" however that prohibits us from simultaneously cognitively processing all that information.
To deal with this high sensitivity combined with low processing capability, our brains "filter out" a lot of the incoming information either directly sensory level or somewhere father along the neural network before getting into conscious cognition.
For example, most people think that when light enters our eyes that makes neurons activate. While true a little farther down the line, it turns out that the photoreceptors of our eyes are actually always firing action potentials (the main signals the brain uses to communicate) in the dark. When a photon enters the retina at the back of the eye, it actually causes the photoreceptor neuron to stop firing an action potential!
From a signal fidelity standpoint this makes a lot of sense even though it costs a lot in terms of energy to keeps these neurons "always on" in the dark.
Imagine again my iTunes scenario from above. Sure, if I turn up the volume (increase the signal) I will be able to hear my wife better, but there's still the music noise in the background. But if I turn down the noise by pausing iTunes, it becomes a lot easier to hear my wife even if I'm at the lowest volume setting on my phone.
In other words: it's easier for your "signal" to be the total shutting-off of noise than trying to amplify the signal above the noise.
The main theoretical mechanisms for how attention operates in the human brain to enhance a sensory stimulus is by shutting down/inhibiting competing stimuli and modulating the gain (making more sensitive) the sensory stimulus itself.
The brain appears to do both things. While we're not totally sure how
Over on my answer to Neuroanatomy: What are the primary functions of the dorsolateral prefrontal cortex? I talked about some of my research with people who have lesions to their frontal lobes.
It turns out that if I'm asking a person to pay attention to a certain part of their visual world and ignore another part, activity in the part of their visual cortex that represents the attended space is enhanced while activity in the part of their visual cortex that represents the ignored space is not (or is even suppressed).
Furthermore, patients with lesions to their prefrontal cortex (important for guiding attention, etc.) show altered activity all the way back in their visual cortex such that they don't show this attention-guided enhancement of activity. This is illustrated in the figure below from my 2010 PNAS study  that shows that patients with prefrontal lesions have high "alpha" activity (an index of suppressed functioning) in the visual cortex on the same side of the brain as their lesion.
There's even some evidence that when you need to pay attention to a specific auditory frequency–for example at the frequency at which your baby cries–the neurons in your ear that represent that frequency might actually physically move so that they respond more easily to that frequency .
Humans are incredible.