Perception and Attention - Computational Explorations in Cognitive Neuroscience - page 225

8.2.2 The LGN of the Thalamus Another dynamic aspect of thalamic processing has

to do with attention . Indeed, one of the biggest clues
about thalamic function comes from the large number
of backprojections from the visual cortex back into the
thalamus. According to some estimates, these backpro-
jections outnumber the forward-going projections by an
order of magnitude (Sherman & Koch, 1986)! These
backprojections are generally thought to play a role in
controlling the attentional processing performed by the
thalamus, where some aspects or regions of the visual
scene are dynamically focused on to the exclusion of
others (e.g., LaBerge, 1990; Crick, 1984). Because the
thalamus has all of the visual input concentrated in a
relatively compact structure, it may be uniquely suited
for implementing the kind of competitive activation dy-
namics across the entire visual scene that result in at-
tentional effects. A similar argument can be made re-
garding attentional competition across modalities, be-
cause all sensory modalities except olfaction go through
the thalamus, and could potentially compete with each
other there. Further, the thalamus is thought to be im-
portant for modulating levels of arousal (e.g., sleep ver-
sus waking) by controlling cortical sensory input.
Although we will explore models of visual attention
in this chapter, these models are based on cortically-
mediated attentional processing, not thalamic attention.
However, similar principles are likely to apply. Given
that we also are not focusing on motion processing, this
means that our models do not really capture the contri-
butions of the LGN. We will however, take advantage
of the laminar (layered) organization of the LGN to or-
ganize the on- and off-center inputs to our models.

The thalamus is generally regarded as the “relay sta-
tion” of the brain, because its many different nuclei re-
lay sensory signals up to the cortex (and other places).
The visual nucleus of the thalamus is called the lateral
geniculate nucleus or LGN , and it relays information
from the retina to the visual cortex. The LGN also orga-
nizes the retinal signals somewhat, using a six-layered
structure (though there is recent evidence for six more
layers). This organization is based in part on the eye of
origin, with 3 layers for one eye and 3 for the other, and
the spatial layout of the visual inputs that retains the to-
pography of the retina (i.e., it is a retinotopic mapping,
preserved in many cortical areas as well). The on- and
off-center cells may also be organized into different tha-
lamic layers.
Another aspect of LGN organization involves two
different categories of neurons, called magnocellular
(M, magno meaning big) and parvocellular (P, parvo
meaning small). Generally speaking, the M cells have
broader receptive fields (i.e., poorer acuity), poor wave-
length selectivity, better motion selectivity, and better
contrast sensitivity (i.e., low light or small brightness
differences) compared to the P cells, which are high res-
olution (small receptive fields), and have better wave-
length sensitivity. Thus, it is tempting to think of P
cells as uniquely contributing to form processing, and
M cells to motion processing. This is not correct, how-
ever, as both types participate in form processing, al-
though to varying degrees.
Increasingly, people are finding that the thalamus is
responsible for a number of important forms of process-
ing beyond simply organizing and relaying information
from the retina. However, the fact remains that the basic
on- and off-center coding of information from the retina
is passed on relatively intact to the visual cortex. Thus,
instead of elaborating the complexity of the structural
information being conveyed by the visual signal (i.e.,
what it reflects about the structure of the visual scene),
the thalamus appears to contribute more to the dynamic
aspects of this information. For example, aspects of the
temporal dynamics of the magnocellular cells, that are
important for motion processing, appear to be computed
in the LGN itself.

8.2.3 Primary Visual Cortex: V1

The next major processing area in the visual stream
is the primary visual cortex , also known as area V1 ,
which is at the very back of the occipital lobe in pri-
mates. 1 Area V1 builds on the thalamic input by pro-
ducing a richer set of representations that provide the
basis for subsequent cortical processing. We will focus
on those representations that capture more complex and
useful aspects of visua l form.
1 Visual information is also subsequently processed in various sub-
cortical brain areas as well.