A nerve is a cell whose body gets electrical input from other nerves by dendrites and projects electric pulses down an axon to other nerves (Figure 6.9). If nerves get electricity from dendrites and send it to projections, as trees send water from roots to leaves, the figure shows the nerve upside-down.
Dendrites into a nerve body are said to add up to fire a nerve based on an input threshold so in Figure 6.10, input from neurons B and D fire the nerve A, but B and C don’t as they don’t reach its threshold of four. Nerves selectively transmit electrical impulses to other nerves.
In embryos, nerves grow out from the brain to form the retina so light entering the eye touches the brain directly. If the retina was a photoelectric cell, it would pass on pixel data if say 1 is black and 9 is white. It works equally well if 9 is black and 1 is white, as long as the definition is absolute, but a designer would have to define that.
Brains had no designer so evolution took both options as it always does. One type of retinal cell responds to light above the background level and another type responds to light below that level.
In Figure 6.11, cell 1 responds to white and cell 2 to black. Instead of defining data absolutely, retinal cells respond relative to background light, then interact to excite or inhibit each other to amplify the borders later used to recognize object shapes.
Vision identifies an object by making one side figure and the other ground. In Figure 6.12, making black the figure just gives blobs but making it background lets you read “MAIL BOX”. The brain uses context to unravel visual data ambiguity, as one must choose the right context to see an object.
The human cortex is a nested hierarchy that processes data in six layers labelled I to VI, as lower units feed higher ones. The first step after the nerve is a hundred or so nerves about the thickness of a hair called a microcolumn:
“… current data on the microcolumn indicate that the neurons within the microcolumn receive common inputs, have common outputs, are interconnected, and may well constitute a fundamental computational unit of the cerebral cortex …” (Cruz, 2005)
About a hundred microcolumns then form a cortico-cortical column that sends axons to nerves nearby. They then form into a macrocolumn of about a million nerves, about 3mm wide, with cortical links. Macrocolumns then form about 32 Brodmann areas (Figure 6.13) of maybe a hundred million nerves for functions like language. Cortical processing then builds up from nerves as follows (Nunez, 2016) p91:
1. Microcolumns. A hundred or so nerves about .03mm wide.
2. Cortico-cortical columns. A thousand or so nerves about .3mm wide.
3. Macrocolumns. A million or so nerves about 3mm wide.
4. Brain areas. A hundred million or so nerves of various sizes.
Brain areas then form four lobes about 50mm wide separated by deep fissures (Figure 6.14). The occipital lobe handles visual data, the parietal lobe handles body image and space relations, the temporal lobe handles sound and memory associations and the frontal lobe handles plans and intentions. It can stop other parts doing socially improper acts, so a person with frontal lobe damage may know how to behave socially but can’t stop inappropriate acts like touching. These lobes form the hemispheres that together are the cortical brain.
The visual hierarchy starts when the eye detects a photon, which data is then subject to layer upon layer of processing to detect relevant features. For example, cells in layer IV are found to fire for different line angles (Figure 6.15) and others respond to other features.
Scientists estimate that each eye inputs about 8.75 Megabits a second and the brain in total receives over 20 Mbps, so as James said in 1892, our first impression was probably information overload:
“The baby, assailed by eyes, ears, nose, skin, and entrails at once, feels it all as one great blooming, buzzing confusion”
Computers handle information overload by compressing a video to a smaller information set that keeps the key features but is easier to handle. Visual processing does the same by reducing sense data to features that map reality to less information. When a baby’s brain can transform data from millions of optic nerves into a smaller set of objects, it can relate to the world better. Reducing sensory data to what is relevant is the brain helping us to survive.
Computer processing is mostly linear but brain hierarchies have bottom-up, lateral and top-down links. Sense data flows up and down the processing hierarchy as a two-way flow. Top-down paths predict, interrogate and check lower processing as higher processing “experts” check for consistency or errors (Dehaene, 2014) p139. While bottom-up paths process data as computers do, lateral paths establish context and top-down links review lower processing.
Is Figure 6.16 an old or young lady? If you see a young lady, can you see an old one or the reverse? To do this you must rerun your visual processing. The visual system makes a best guess but you can ask for a redo because nerves go down as well as up. The processing is “out of sight and out of mind” but it can be redone by top-down control. All perception is a hypothesis of an ambiguous world.
Such “subconscious” processing might be assumed to be primitive but the spinning ballerina illusion (Figure 6.17) suggests otherwise. Clicking the link shows a ballerina spinning but the rotation is ambiguous so you can see her spin clockwise or anti-clockwise. Try to see her spin the other way. If you can’t, pause the video and if you see an extended leg at the front, imagine it at the back, or vice-versa. Restart the video and if she spins the other way, you just reprogrammed some complex unconscious visual processing.
The optic nerve has about a million axons but the auditory nerve only has about 50,000, so its processing base is narrower than for vision.
In Figure 6.18, processing network resources applied to a narrow base give deep processing. There is a trade-off between base width and processing depth, so if hemispheres of equal processing capacity specialize, the narrow base of sound can be processed deeper than vision. The left hemisphere then handles language because its earlier sound specialization allows the deeper processing that language requires. Left and right hemisphere specializations are different processing hierarchy structures.