Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 36

Fig. 3.3 Ferroelectric
(hysteric) characteristic (not
drawn to an accurate scale)

Fig. 3.4 Delay and short-term
memory symbols

neurotransmitters also affect delay. Delay is denoted by a simple rectangle
(Fig. 3.4 ) with an input and an output and with the word Delay. Signals in dendrites
could theoretically go either way, left-to-right or right-to-left. In a neuron, signals
tend to flow from a synaptic input to a bouton output, in which case an arrowhead is
sometimes used to indicate the direction of signaling. If relevant or critical, the
amount of delay is specified to be a value represented by Δ .

Short-Term Memory Neurons

Neurons often emit perhaps ten pulses per burst lasting a total of a few milliseconds.
In contrast, STM neurons are modeled as producing bursts extending up to several
seconds. This is because a long neural burst may represent a STM for a given
attribute, such as shape or color. A selective collection of such features, of course,
builds an entire image.
STM neurons are assigned the oval symbol in Fig. 3.4 . The duration of a STM
burst is assumed variable,

, from milliseconds to seconds. This neuron is triggered
at T and produces a pulse burst at Q; the direction of the signal is usually understood
from the context in which it is used. An STM neuron is generally triggered via a
synapse, as opposed to a conductive contact, because STM dendrites are unique;
they support longer pulses and so must have lower membrane loss. Note that both
inputs and outputs are bursts of pulses; a short trigger to the input T and usually a
longer burst from the output Q.
In addition to representing memory attributes, STM neurons serve to produce
timing signals in a neural system. The duration

ψ

is fixed at some biologically
useful value that does not change easily, since it depends on dendritic parameters.

ψ