Image Processing Reference
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Fig. 1.6. On the
left
, the direction sensitivity of a cell in V1 is illustrated. On the
right
, the
sensitivity of simple cells to position, which comes on top of their spatial direction sensitivity,
is shown
are mapped to orthogonal lines in addition to the fact that the central retina is mapped
to a relatively large area in V1.
1.7 Spatial Direction, Velocity, and Frequency Preference
Neurons in V1 have radically different receptive field properties compared to the
center-surround response pattern of the LGN and the ganglion cells of the retina.
Apart from the input layer 4 and the blobs, the V1 neurons respond vigorously only
to edges or bars at a particular
spatial direction
, [114], as illustrated by Fig. 1.6. Each
cell has its own spatial direction that it prefers, and there are cells for (approximately)
each spatial direction. The receptive field patterns that excite the V1 cells consist in
lines and edges as has been illustrated in Fig. 1.8. Area V1 contains two types of
direction-sensitive cells,
simple cells
and
complex cells.
These cells are insensitive
to the color of light falling in their receptive fields.
Simple cells respond to bars or edges having a specific direction at a specific po-
sition in their receptive fields, Fig. 1.6. If the receptive field contains a bar or an edge
that has a different direction than the preferred direction, or the bar is not properly
positioned, the firing rate of a simple cell decreases down to the biological zero firing
rate, spontaneous and sporadic firing. Also, the response is maintained for the entire
duration of the stimulus. The density of simple cells decreases with increased ec-
centricity of the retinal positions they are mapped to. Their receptive fields increase
in size with increased eccentricity. This behavior is in good agreement with that of
