We will follow the flow of information about the visual world,
picking up at the photoreceptors and finishing in the cortex. We will learn
about the structure and function of the parts of the brain dedicated to
vision. At several points along the way, we will stop to see how these
details fit into more general, "big picture" themes in neuroscience and
Remember that the retina is a part of the brain, "reaching
out" to see the world. It is the only part of the brain that can be viewed
A quick overview of the route: the outputs of the retina
then pass through the optic nerve, cross and split at the optic chiasm,
through the optic tract to the LGN. From there, they pass to V1, or primary
visual cortex, and then on to "higher-level" brain areas. Get oriented
by looking at [the visual
pathways from below], and [the
visual pathways from the side].
1. Photoreceptor Signals
When a photoreceptor "captures" a photon, the protein part
of the photopigment ("opsin") can change shape ("become activated", "be
This change in the shape of the photopigment triggers a biochemical
cascade (we'll skip the details). This results in the hyperpolarization
of the receptor (i.e., its charge decreases relative to the outside).
In the dark (at rest), the photoreceptors are quite active,
constantly releasing neurotransmitter. After absorption of a photon, the
resulting hyperpolarization decreases the amount of neurotransmitter
released. This means that light actually turns receptors off.
These hyperpolarizations are graded responses. Gradual
increases in light intensity have gradual effects on neurotransmitter release.
The rods and cones are connected to horizontal and bipolar
cells. These cells then connect to the retinal ganglion cells. Refer to
detailed and more
2. Retinal Ganglion Cells
The retinal ganglion cells represent the output of the
retina. They exhibit several important properties that are characteristic
of many visual neurons. Begin by reviewing the basic
anatomy of a neuron: cell body ("soma"), dendrites (which receive inputs
from other cells), and the axon (which sends outputs to other cells).
Interlude. The Representation of the Visual Field
in the Brain.
As the pattern of light reflected off objects in the world
enters the eye, it is flipped upside-down (recall
from last lecture).
This upside-down pattern of light is the retinal image.
The spatial structure of the retinal image is preserved as
neurons from the retina connect to the LGN, and is still preserved further
along in the cortex.
Although you have two eyes, and slightly different images
in each eye, the brain does not keep the information from the two retinas
separate. Instead, it splits the world into a left and right visual fields.
After leaving the retina, the outputs of each eye are split.
The nasal (toward the nose) half of each eye's visual field crosses
from one side to the other at the optic chiasm. The temporal half
(towards the temple) remains on the same side as its eye-of-origin. This
splitting and crossing re-organizes the retinal outputs so that the left
hemisphere processes information from the right visual field, and
the right hemisphere processes information from the left visual field.
Check out these two figures which diagram this tricky wiring pattern: [visual
maps 1] [visual maps
3. Parallel Pathways and the LGN
Retinal ganglion cells actually come in 2 sorts: M (magnocellular,
or parasol) and P (parvocellular, or midget).
P cells also exhibit color-opponent responses: their firing
is also dependent on the wavelength of light in their receptive field.
M cells do not exhibit color-opponency.
M cells make transient responses: they fire action potentials
when a stimulus is introduced, but quickly fade if the stimulus does not
change. P cells, meanwhile, give sustained responses to stimuli in their
The division of M and P pathways becomes anatomically evident
in the lateral geniculate nucleus (LGN), a part of the thalamus that acts
as a relay station between the retina and the cortex.
While the receptive fields of LGN neurons look a lot like
retinal ganglion cells, the LGN does re-organize these circuits into distinct
There are 6 layers in the LGN. Bottom 2 layers are M cells,
upper 4 are P cells. Layers alternate eye of origin.
Parallel pathways generally exhibit these 4 main characteristics:
Physiologically/functionally distinct. For example,
the M cells conduct neural signals faster, while P cells represent more
constant stimulus presence. A simple hypothesis is that M cells contribute
to fast/transient processing (visual motion perception, eye movements)
while P cells contribute more to recognition (object recognition, face
Anatomically distinct. The dendritic trees of P cells
are always smaller than the M cells (remember that they're also called
"midget" cells). Note that dendritic trees of both types of cell get larger
as you move from fovea to periphery.
Complete coverage (or nearly complete). Both M and
P cells cover the entire retina.
Recombine. The M and P cells are separated in the
LGN (different layers) but recombine in visual cortex (although some separation
4. Gross Anatomy of the Cortex
Before we discuss visual cortex in detail, let's
stop and get oriented in the brain as a whole.
As you know, there are left and right hemispheres of the
brain. They are connected by a tract of fibers known as the corpus callosum.
In each hemisphere, there are 4 "lobes": frontal, temporal,
Visual information is processed primarily in the occipital
lobes, but parallel pathways extend into the temporal and parietal lobes
as information-processing becomes increasingly specialized.
Interlude. Spatial Organization in the Brain
The spatial organization of the brain often provides hints
about what the brain does to transform sensory input to useful information
for the guidance of action and thought. Spatial organization can be seen
at many different levels:
Functional specialization: different types of information
are processed in different parts of the brain (with varying degrees of
Columnar architecture: within a brain area, neurons with
similar (or complimentary) sensitivities lie close together, often in "columns"
Topography/Retinotopy: a "map" of the visual world (or, a
map of the retina) is preserved in many visual brain areas. E.g., adjacent
points of the visual world/retinal image are mapped onto (or processed
by) adjacent neurons. Just as there is a "retinal image", there is a "neural
image" in each visual area. People who study the visual system often use
the existence of multiple retinotopic maps to localize different brain
5. Primary Visual Cortex: V1
V1 has a topographic/retinotopic map of the visual world
(see above). This means that there is a "neural image" that retains the
spatial layout of the pattern of light that falls on the retina. This map
has several interesting characteristics:
Remember that there are 2 V1s in each person (left and right
hemispheres). Each V1 has a representation of the opposite half of the
visual field (e.g., left V1 has a map of the right visual field, and vice
versa). Note that each V1 does not simply receive input from the
opposite eye. The outputs of each retina are split (left half/right
half) and then run through the LGN to the appropriate V1.This diagram of
the visual fields is helpful.
Just as othe image of the world is inverted when projected
onto the retina, the retinotopic V1 map is upside down. As discussed earlier,
the right hemisphere's V1 has a topographic map of the left visual field,
and vice versa.
Cortical magnification: more cortical space is dedicated
to the fovea than the periphery (remember the higher density of photoreceptors
in the fovea, hence clearer vision).
There are 3 main types of cells in primary visual cortex.
Simple: receptive fields often have a long, narrow
bar of light (ON) and flanking (OFF) parts. Other types are the opposite
(responding to dark bars) or simply respond to a light/dark edge. [simple
cell receptive field types]
Complex: bars of light must be oriented correctly,
but can appear anywhere in the receptive field. Moving the bar through
the field produces a sustained response. Complex cells often show direction-selectivity:
they fire more when the bar moves in one direction, and are suppressed
by motion in the opposite direction. [complex
End-stopped (formerly Hypercomplex): Many simple and
complex cells exhibit length summation: if an appropriate bar is
placed in the visual field, they fire action potentials; if the bar is
made longer, they fire more, up to the extent of the full receptive field.
However, end-stopped cells increase their responses with increases in bar
length up to a limit that is smaller than the receptive field. [hypercomplex
Architecture of V1
Ocular dominance columns: as you move parallel to the surface
of V1, there are alternating columns of cells that are driven predominantly
by inputs to a single eye. These alternations between left and right eye
are the ocular dominance columns.[ocular
Orientation columns: as you move perpendicular to the surface,
the preferred orientation of the cells changes gradually from horizontal
to vertical and back again.
View a schematic
of the ocular dominance and orientation columns together.
Interlude. Defining and Separating Different Brain
Brain areas can be differentiated according to 4 main
Remember 'FACT' as a mnemonic.
Function: physiology. Neurons in different parts of the brain
are responsive to different aspects of the stimulus (= do different things).
Architecture: microanatomy can differ widely across brain
areas. For example, V1 is also referred to as "striate cortex" because
it has a series of stripes that run parallel to the surface; these stripes
end abruptly at the end of V1.
Connections: different areas feed forward and also receive
backward-reaching connections from distinct areas.
Topography: e.g., retinotopy. Each distinct visual area has
its own retinotopic map.
6. Secondary Visual Areas
There are approximately
30 visual areas after V1. The functional specialization hypothesis
drives much of the research about these areas. Some areas seem specialized
for processing a certain aspect of visual information. E.g., MT - motion,
V4 - color (?).
Cortical areas dedicated to vision are densely interconnected,
and can seem quite confusing
at first glance.
However, a more general organization is evident in a pair
of parallel pathways.
What pathway. Temporal lobe; recognition of objects.
Where pathway. Parietal lobe; motion, spatial orientation,