Leabra Implementation Reference - Computational Explorations in Cognitive Neuroscience - page 447

C.4 Unit-Level Variables

LeabraUnit unit object:

Variable Description
spec Determines the spec that controls this unit (not in NetView).
pos Determines location of unit within layer (not in NetView).
ext flag Reflects type of external input to unit (not in NetView).
targ Target activity value (provided by external input from event).
ext External activation value when clamped (provided by external input from event).
act Activation value (what is sent to other units, y j ).
net Net input value ( ￿ j ) computed as normalized weights times activations — exci-
tation only (equation 2.16), inhibition is computed separately as g i ( gc.i ) either
by kWTA or directly by unit inhib.

The bias weight is a LeabraCon object hanging off of the unit — it is managed by
its own LeabraBiasSpec in the LeabraUnitSpec.

bias

act eq Rate-code equivalent activity value (time-averaged spikes when using discrete

spiking activation, or just a copy of act when already using rate code activation).

Average activation over long time intervals, as integrated by time constant in
adapt thr (see UnitSpec). Useful to see which units are dominating, and to
adapt their thresholds if that is enabled.

act avg

act m Minus phase activation value, set after settling in minus phase and used for learning.

act p Plus phase activation value, set after settling in plus phase and used for learning.

act dif Difference between plus and minus phase activations — equivalent to the error

contribution for unit ( Æ j ).

da Delta activation: change in activation from one cycle to the next, used for deter-

mining when to stop settling.

vcb Voltage-gated channel basis variables that integrate activation over time to deter-

mine if channels should be open or closed (channels are not active by default):

Hysteresis channel (excitatory) basis variable — typically hysteresis is triggered
after unit achieves brief sustained level of excitation as reflected in this basis vari-
able.

.hyst

Accommodation channel (inhibitory) basis variable — typically accommodation
(fatigue) is triggered after unit is active for a relatively long time period as reflected
in this more slowly-integrating basis variable.

.acc

Channel conductances for the different input channel types except excitatory input
(which is in net ).

gc

Leak channel conductance (a constant, not visible in NetView).

.l

Inhibition channel conductance, computed by kWTA or direct unit inhibition.

.i

Hysteresis (voltage-gated excitation) channel conductance.

.h

Accommodation (voltage-gated inhibition) channel conductance.

.a

I net Net current produced by all channels: what drives the changes in membrane potential.

v m The membrane potential, integrates over time weighted-average inputs across dif-

ferent channels, provides basis for activation output via thresholded, saturating
nonlinear function.