  The mechanisms underlying how activity in the visual pathway may give rise
through neural plasticity to many of the features observed experimentally in
the early stages of visual processing was provided by Linkser in a seminal,
three-paper series. Owing to the complexity of multi-layer models, an implicit
assumption in Linsker's and subsequent papers has been that propagation delay
is homogeneous and plays little functional role in neural behaviour. We relax
this assumption to examine the impact of distance-dependent axonal propagation
delay on neural learning. We show that propagation delay induces low-pass
filtering by dispersing the arrival times of spikes from presynaptic neurons,
providing a natural correlation cancellation mechanism for distal connections.
The cut-off frequency decreases as the radial propagation delay within a layer
increases relative to propagation delay between the layers, introducing an
upper limit on temporal resolution. Given that the PSP also acts as a low-pass
filter, we show that the effective time constant of each should enable the
processing of similar scales of temporal information. This result has
implications for the visual system, in which receptive field size and, thus,
radial propagation delay, increases with eccentricity. Furthermore, the network
response is frequency dependent since higher frequencies require increased
input amplitude to compensate for attenuation. This concords with
frequency-dependent contrast sensitivity in the visual system, which changes
with eccentricity and receptive field size. We further show that the proportion
of inhibition relative to excitation is larger where radial propagation delay
is long relative to inter-laminar propagation delay. We show that the addition
of propagation delay reduces the range in the cell's on-center size, providing
stability to variations in homeostatic parameters.
