1) Dendritic delays play an important role in direction selectivity in retinal ganglion cells. Proximal synapses produce faster, sharper EPSP peaks, while distal synapses produce slower, broader peaks. 2) Rall (1964) modeled dendritic trees as isopotential compartments and showed that distal excitatory inputs produce slower rising, broader EPSP peaks compared to proximal inputs. 3) Direction selectivity requires temporal asymmetries like dendritic delays to produce higher EPSP peaks for the preferred direction versus the null direction. Several studies show direction selectivity involves temporal asymmetries in excitation and inhibition as well as nonlinear threshold mechanisms.

1. Delays in Passive Dendrites
1
•In earlier studies, dendrites modeled as passive cables,
deliver inputs to the soma
•Cable theory for dendrites (Rall, 1959) predicts
asymmetric voltage attenuation and summation effects
•Delay resulting between the synaptic input and output
spikes, depends on location of synapse
•Dendritic delays play role in direction selectivity
(Retinal ganglion cells)

2. Proximal Distal
Spatial sensitivity due to delays
2
• Dendritic tree reduced to
several isopotential
compartments
• Excitatory input delivered to
two adjacent compartments
• EPSP at the soma depends on
distal versus proximal synaptic
inputs
– Distal: EPSP rises slowly to a
broader and smaller peak
– Proximal: EPSP peak earlier and
sharper
Rall, 1964

3. Discrimination of spatiotemporal sequences
Direction Selectivity (DS) – fires more
spikes when stimulus is moved in the
preferred direction (DCBA); less or no
response in the null direction (ABCD)
3
Input sequence ABCD – earliest transient
with lower peak
Input sequence DCBA – delayed transient
with higher peak
Null
Pref
Rall studied passive DS – requires long
dendrites to produce significant
filtering effects
Basal dendrites not long enough

4. Direction Selectivity
Barlow and Levick, 1965
Null direction – inhibition (~C, ~B) prevents
activity from A and B
Preferred direction – inhibition arrives too
late (~C’, ~B’)
Several studies have shown that neural
mechanisms for DS involve:
1) Some form of temporal asymmetry in the
circuit – axonal or dendritic delays, asymmetry
between excitation and inhibition
2) A nonlinearity which detects or rejects
temporal coincidence – threshold mechanisms
of somatic or dendritic spikes, coincidence
detectors
4
Branco, 2014