University Studies 15A:
Consciousness I
The Neurobiology of Vision
The Neurobiology of Vision provides an example for the
challenges of understanding consciousness.
The first challenge is simply to grasp the problem biology presents.
If we cannot see that visual awareness, whatever it is in the mind, is a
reconstruction based on layer upon layer of processing in the brain,
we will not see that there is a problem to be explained in the first
place.
We will not agree that the science matters in principle.
We will not agree that the details of the neurobiology are relevant as
a challenge to our experience of visual awareness.
We will not agree that our experience of visual awareness is relevant
as a challenge to getting the details of the neurobiology right.
If we can come to see the neurobiology as relevant to the problem of visual
experience, the next challenge is to synthesize the two levels of description.
1. Our account of visual experience must account for the neurobiology.
a. It must account for the constructed character of visual awareness
b. It must account for the particular details of the biological
mechanisms
2. The neuroscientific account must respect the experiential facts of
visual consciousness
a. It must provide an account adequate to explain (at least roughly)
the qualities of first person experience
b. That is, it will need to integrate the visual system into a “self,” into
semantic networks, memory, emotions, and saliency.
We start this week with the visual system itself, not with its integration into
other networks of meaning and memory.
We will set out the layers of mediation, the biological sub-systems upon
which visual experience is built.
I hope you will agree that the biology of vision does indeed present a
challenge to our understanding of visual consciousness.
We start with the eye.
We are not concerned with questions
of optics. We want to know how the
optical information is changed into
neuronal responses.
For this, we look at the retina.
The retina has three basic layers:
1. On the surface of the retina,
ganglion cells connect to the optic
nerve.
2. In the middle, bipolar cells
process responses of the rods and
cones and pass activation to the
ganglion for more aggregation.
3. Behind the other neurons are
the rods and cones that are the
photoreceptor cells.
Photoreceptors:
1. The rods are not tuned for color but are
sensitive, working well with low light
The rods are not tuned for color but are
sensitive, working well in low light
There are many more rods than cones
on the periphery of the retina, fewer
near the fovea (see below)
Photoreceptors:
2. The cones are tuned for three wavelengths of light, labeled “long,” “medium,”
and “short”
Horizontal Cells:
These are a source of lateral inhibition for
the rods and cones.
The lateral inhibition begins the process of
defining “center-surround” receptive fields.
That is, given the right connections, if all the
receptors surrounding a particular rod/cone
are firing, the horizontal cells activated by
those receptors may inhibit the central
receptor.
Or the other way around: the firing of the
central receptor may inhibit the firing of its
neighbors
Bipolar Cells:
There are rod bipolar cells and cone bipolar
cells. There are none with synaptic
connections to both.
Rod bipolar cells are connected to amacrine
cells rather directly to ganglion cells.
Bipolar cells come in two types: “On”
bipolar cells (which spike most quickly when
photoreceptors indicate the presence of
light) and “Off” bipolar cells (that spike most
in the dark).
Amacrine Cells:
Seventy percent of the neurons connecting
to the ganglion cell are amacrine cells.
As noted, rod bipolar cells are connected to
amacrine cells rather directly to ganglion
cells.
Amacrine cells seem to be involved in lateral
inhibition among the bipolar cells
However, amacrine cells remain something of
a mystery after decades of work.
Ganglion Cells:
Ganglion cells gather data from a collection
of bipolar cells that in turn have gathered
data from a collection of rods or cones..
As a result, ganglion cells have a receptive
field.
They are either off-center on-surround or
on-center off-surround.
Baars and Gage show the response pattern for an “on-center off-surround” ganglion cell.
The cell spikes at a moderate rate for weak on-center light (as determined by the
photoreceptor data transmitted via the bipolar cells) and spikes strongly for strong oncenter light.
The cell spikes slowly when there is some light in the “surround” and slowest for full
light in the “surround”
As Baars and Gage explain, the resulting spiking patterns turn the ganglion
cells into neurons that respond most strongly to edges where information
changes.
Hence the retina performs a first level of data reduction that reorganizes the
visual field:
The axons of the ganglion cells form the optic nerve and transmit data that
emphasizes edges.
However, we still are not through with the design of the retina.
At one place in the retina, the ganglion and bipolar cells have been pushed to
the side to allow a concentration of cone cells to have particularly precise
information about the light coming into the eye.
This is the fovea:
We can tract the center of visual attention by tracking the shifting of the fovea
as it focuses on different part of a visual scene.
This is foveation:
The optic nerve then splits the visual data into right and left streams and
carries the data to the contralateral (“other-side”) lateral geniculate nucleus
(LGN) in the thalamus
As Baars and Gage note, the receptive fields of
the neurons in the LGN are basically like those of
the ganglion cells in the retina.
That is,
“On-center off-surround” and
“Off-center on-surround”
However, it is important to stress that 10% of the
connections to the LGN come from the retina
and 90% from V1 and other “up-stream” layers.
The LGN must be performing types of signal
enhancement based on previously extracted
patterns at higher levels.
We finally get to V1, the Primary Visual Cortex (this is a macaque brain)
As we have discussed before, the simple cells of V1 extract line-segment
information from the receptive-field activations sent from the LGN:
Note that the “data” transmitted at each stage is in terms of spiking rates.
However, beyond the “simple cells,” other cell assemblies in V1 extract other
types of patterns and combinations of patterns.
Some cell assemblies detect direction of motion (changes in light over a
“retinal map” in V1).
Some cell assemblies detect motion of line segments with particular angles.
Since data is coming in from both eyes, some cell assemblies detect
differences between the data provided by the two eyes (binocular
disparities).
Some cell assemblies add in color information based on comparing the
spiking rate of L, M, and S cones.
In all these systems, remember that the network of neurons work by
difference, by dividing the incoming data into mutually differentiated,
mutually inhibiting groupings.
Onward and Upward: Beyond V1
Baars and Gage provide us with an excellent summary image:
This is the so-called “What” Pathway, a.k.a. the Ventral Pathway
The story starts to get a bit complicated as we head to higher areas.
The dorsal stream, the
“Where” pathway next
goes to the “middle
temporal area’” (MT).
For the moment,
however, follow the
red arrows for the
ventral pathway and
remember that the
“Where” pathway is
the dorsal stream.
We’ll explore what happens next to turn visual data into visual consciousness
after the mid-term.