Only a few dozen sequential processing stages are allowed in acts such as visual recognition of a scene in a fraction of a second.

This is an Important Constraint on brain algorithms. Who cares if a neuron dies? There is almost no distinction between data and algorithm nearly. Synaptic strengths determine memories but algorithm is system dynamics 
cf in LISP has the data/program distinction has being relaxed.

Various specialised areas or cortices;  visual, motor, somatosensory and associational. 

Biological Background

The neo-cortex is the most recent and highly 

developed level of nerve tissue of about 0.2m2 in area and 2-3 mm in thickness, strongly 

convoluted to save space, forming the exterior of both brain hemispheres.

There are about 100,000 closely interconnected nerve cells per square millimeter.

Structure of Processing Units ; Dendritic Tree, Cell Body and Axon cf input, processing and output functions

Dendritic tree 400 micro meters in radius around neuron and sums the output signals of the surrounding neurons in the form of an electrical potential which is sends to the cell body (soma) of the neuron. (Tom Doris suggests an alternative “resonate and fire” paradigm”)
If potential > threshold then a short electrical spike is produced, conducted along the axon which can be a fraction ofa millimeter to several meters. The axon also branches out and can conduct to several thousand target neurons. The contacts are either on the dendritic tree or else on the cell body. The pulse causes release of neuro-transmitter which in turns leads to a change in potential in the dendritic tree or cell body of the target neuron. This may have an excitatory or inhibitory effect on the neuron.