Neurobiological Background - Hierarchical Neural Networks for Image Interpretation - page 25

Information flows also laterally through the retina. Photoreceptors are connected
to each other by horizontal cells in the outer plexiform layer. The horizontal cells
mediate an antagonistic response of the center cell when the surround is exposed
to light. Amacrine cells are interneurons that interact in the inner plexiform layer.
Several types of these cells exist that differ greatly in size and shape of their dendritic
trees. Most of them are inhibitory. Amacrine cells serve to integrate and modulate
the visual signal. They also bring the temporal domain into play in the message
presented to a ganglion cell.
The result of the vertical and horizontal interaction is a visual signal which
has been spatiotemporally compressed and that is represented by different types of
center-surround features. Automatic gain control and predictive coding are achieved.
While all the communication within the retina is analog, ganglion cells convert the
signal into all-or-nothing events, the action potentials or spikes, that travel fast and
reliably the long way through the optic nerve.
Another area for which the layered structure has been investigated in depth is the
primary visual cortex, V1. As all cortical areas do, the 2mm thick V1 has six layers
that have specific functions, as shown in Figure 2.9. The main target for input from
the LGN is layer 4, which is further subdivided into four sublayers. While the axons
from M cells terminate principally in layer 4C α , the P cells send their output to
layer 4C β . Interlaminar LGN cells terminate in the blobs present in layers 2 and 3.
Not shown in the figure is feedback input from higher cortical areas that terminates
in layers 1 and 2.
Two major types of neurons are present in the cortex. Pyramidal cells are large
and project to distant regions of the cortex and to other brain structures. They are
always excitatory and represent the output of the computation carried out in their
cortex patch. Pyramidal cells from layers 2, 3, and 4B of V1 project to higher corti-
cal areas. Outputs from layers 5 and 6 lead to the LGN and other subcortical areas.
Stellate cells are smaller than pyramidal cells. They are either excitatory (80%)
or inhibitory (20%) and serve as local interneurons. Stellate cells facilitate the in-
(a) (b) (c)
Fig. 2.9. Cortical layers in V1: (a) inputs from LGN terminate in different layers; (b) resident
cells of various type; (c) recurrent information flow (adapted from [117]).