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Transcript
The Visual System Periphery
Bear et al.
The peripheral specializations that\ transduce light are
extremely complex. Noteworthy are the focusing
mechanisms and the muscular system that controls
eye position. Focus is achieved by the cornea
(constant) and the lens (adaptive).
The visual field is that part of the environment that the
focusing mechanism can direct to the retina.
In front facing eyes (primates, carnivores) the 2 eyes
have overlapping visual fields and so can achieve
binocular vision.
The Retina 1 (General Background)
Bear et al.
The initial detection of light is via photoreceptors- specialized cells that can transduce a small
number of photons into an electrical signal. Light has to pass through the retinal neurons
before it reaches the photoreceptors.
The retinal circuitry leads from photoreceptors to bipolar cells to ganglion cells; ganglion cell
axons project from the retina to the brain via the optic nerve.
Two sets of interneurons, horizontal and amacrine cells, modify the transmission of visual
information from photoreceptors to ganglion cells.
It is important to remember that photoreceptors and bipolar cells do not spike- they produce
graded potentials; ganglion cells do produce classic action potentials.
The Retina: Photoreceptors 1
Bear et al.
There are 2 types of photoreceptors: rods (all wavelengths) and cones (3 subtypes: each
sensitive to a specific range of wavelengths, for color vision). Cones are present at highest
density in the foveal region- this is the region you use for focus. In the foveal region, ganglion
cells receive input from single cones (ultra-high acuity vision) while in peripheral retina there is
extensive convergence for greater sensitivity and less acuity.
The Retina: Photoreceptors 2
Tuning curves for the 3 types of cones. Note
that, although they peak at different “colors”,
there is extensive overlap of their tuning
curves. This is a population code: the central
visual system must compare the response of
the different cones in order to produce the
perception of color. Color vision is a very
difficult area with an immense literature.
Although it is immensely important for
primates, we will not cover it in this course.
Bear et al.
Photoreceptors are depolarized (active) in
the dark and release neurotransmitter.
When the rhodopsin in the outer segment
membrane disks absorb photons, they
cause the Na+ channels to close, thus
hyperpolarizing the rods.
The Retina: Photoreceptors 3
Rhodopsin constitutively binds retinal; in its inactive
form this prevents rhodopsin from binding to a Gprotein. When light hits retinal, it converts to an active
form that causes rhodopsin to bind the G-protein. The
G-protein in turn activates a phosphodiesterase that
then removes the cyclic GMP from a sodium channel.
The removal of the bound cGMP causes the Na+
channel to close and the photoreceptor
hyperpolarizes.
Bear et al.
The Retina: Circuitry and Function 1
Photoreceptors make synaptic contacts onto two types of
neurons: bipolar cells and horizontal cells. The horizontal
cells are GABAergic (inhibitory) interneurons. The
horizontal cells contact the bipolar cells at a site near to
where they receive input from the photoreceptor; this
complicated synaptic region is within invaginations of the
photoreceptors and the details are beyond the scope of this
course.
The key point is that there are two kinds of bipolar cells: On
center bipolar cells and Off center bipolar cells. The
functional distinction is that On center cells respond to light
with depolarization; Off center cells respond to darkness
with depolarization.
The cellular reason for this distinction is that these bipolar
cells have different receptors for the photoreceptor
neurotransmitter (likely glutamate).
Bear et al.
The Retina: Circuitry and Function 2
Bear et al.
Cone pedicles (b) have ribbon synapses; On bipolar (blue) and horizontal cell (yellow)
dendrites invaginate into the cone pedicle. Off bipolar dendrites (purple) remain on the
outside.
The Retina: Circuitry and Function 3
Kandel et al.
Rods are depolarized in the dark and release glutamate; this excites the Off-center bipolar
and inhibits the On-center bipolar. In the light, the rod hyperpolarizes and stops releasing
glutamate; the On-center bipolar now depolarizes (disinhibits) and the Off-center bipolar
hypeerpolarizes. The bipolar cells then contact ganglion cells selectively, producing On and
Off-center ganglion cells; ganglion cells spike.
The Retina: Ganglion Cells 1
Bear et al.
The center-surround antagonistic organization of ganglion cell RFs reduces the
spatial redundancy of the visual signals- regions of constant illumination do not
cause discharge and only information about changes in illumination are
transmitted to higher centers of the brain.
Of course other cells do signal the overall level of illumination- the ones that
project to hypothalamus and pretectum- but there are only a small number of
such cells.
The Retina: Ganglion Cells 2
Bear et al.
The major distinction among ganglion cells is On versus Off-center. Within each class there
are many additional varieties. One major distinction is between M (magnocellular) and ) P
(parvocellular) types. The M type is large and phasic- it responds well to movement over a
large receptive field. The P type is small and tonic- it responds well to stationary input over
a small receptive field. The projections from M and P types stay partly separate up to
cortex and basically define two key visual operations: sensitivity to motion (M type) and to
object identification (P type).
There are many additional types of ganglion cells senstive to a variety of spatio-temporal
patterns. We won’t go into these varieties in this course.
The M-type cells can be considered as reducing temporal redundancy by only
responding to changes in illumination over time.
Molecular Specification in the Retina
Wassle
Calbindin (red) and calretinin (green) are 2 calcium binding proteins. Other molecular
markers demonstrate an even more complicated chemical organization; this can be
correlated with morphological and functional properties. The main thing to get out of
this slide is the precision of neuronal connectivity since that is usually hard to see in
other brain area.
The Retina carries out more complicated computations than
Center-Surround
Berry, 1999
The upper panel shows the effect of a light flash in
the RF center of a ganglion cell. The cell fires with
a latency of ~100 ms- this delay is too long.
A moving object causes the same cell to begin
firing ~300 ms before the object reaches the RF
center. The cell anticipates (predicts) the
location of the moving object.
Schwartz, 2007
Ganglion cells anticipate (predict) the timing of a
light pulse. First their response to the
repeated,predictable pulses is to stop firing- they
cancel the expected signal to reduce redundancy.
When the signal stops, they then respond strongly at
exactly the time when the signal should have
appeared.
Projections of the Retina 1
The suprachiasmatic nucleus is a diencephalic (hypothalamus) structure.
The superior colliculus is a mesencephalic (midbrain) structure.
The lateral geniculate is a diencephalic (thalamus) structure.
The Superior Colliculus:
The Interface between Sense Input and Directed Movement
Fish, Frog
Mammals
Retina
Tectum
Retina
Pallium
Memory
Brainstem
Spinal Cord:
Eye/Body Movement
Oriented towards Visual Input
Superior
Colliculus
Cortex: Reward,
Memory, Intentions
Brainstem
Spinal Cord:
Eye/Head/Body Movement
Oriented towards Visual Input
Primates: Saccadic Eye Movements
The tectum receives retinal input and can direct the eyes (or head/body) towards the stimulus
(e.g. food). This is the orienting response. The tectum (at least frogs, rodents) can also
direct body away from a stimulus (predator).
Other senses also use tectal circuitry for orienting responses: audition, touch,
electroreception.
In mammals cortical input can prepare the superior colliculus for intended movements and for
movements that will produce rewards.
The Superior Colliculus 2
Torres, 2005
In a fish the tectum is responsible for orienting
movements towards visual stimuli.
Boehnke, 2008
In primates the superior colliculus is still responsible
for orienting but receives massive cortical input that
is the main determinant of where to direct the eyes.
Fecteau and Munoz (2006) have hypothesized that the superior colliculus generates a priority map that
determines what to orient towards.
The priority map is computed from a salience map (sensory input- how big, bright etc is the input)- and
a relevance map (cortex)- how important is the input. The relevance map must be learned.
In this way the phylogenetically ancient tectum is co-opted to serve the complex sensory systems of
mammals.
Projections of the Retina 2
Bear et al.
The partial decussation of the optic nerve resuls in the left visual field being represented
on the right side of the brain and vice versa. This holds for animals with binocular vision.
For animals with side-facing eyes, the retinal projections are entirely crossed.
Projections of the Retina 3
Bear et al.
The main targets of the retina are the superior colliculus (optic tectum) and the LGN.
The tectum is responsible for orientation to visual and auditory input (eye and head
movements). The LGN transmits retinal input to cortex for perception is most highly
evolved in primates. The view for many years was that the thalamus, inlcuding LGN,
was just a relay to the cortex and did not process its input in any significant way.
The Lateral Geniculate Nucleus 1
Retinal input to the LGN keeps the eyes separate
and keeps the P and M pathways separate. LGN
neurons have the same receptive fields as
retinal ganglion cells: On or Off centers +
surround. So is the LGN merely a relay nucleus
that does not process retinal input?
Emphatically no.
Bear et al.
The Lateral Geniculate Nucleus 2
Krahe and Gabbiani
Thalamic relay cells, including LGN, express a T-type Ca channel that is inactivated at rest.
When the cell is hyperpolarized (by an IPSP) the T channel inactivation is removed and the
next excitatory input activates a long lasting Ca2+ spike that triggers bursts of action
potentials. So LGN cells can produce isolated spikes or spike bursts. Certain types of
neurons have biophysical mechanisms that cause bursting, while others don’t. Is bursting a
special code?
The Lateral Geniculate Nucleus 3
Krahe and Gabbiani
The LGN is the main source of visual input to cortex. Yet it receives less than 20% of its
input from cortex. About 80% of LGN input is feedback from visual cortex and it is
topographically organized: an LGN projecting to a region of cortex receives feedback
from the same region. One function of the cortical feedback is to control LGN bursting. It
has been suggested that LGN bursts are a “wake up call” to cortex: bursts tell cortex
that some visual input is very important.
The Lateral Geniculate Nucleus 4
Lesica and Stanley
Modern studies of sensory processing typically use both natural stimuli and noise stimuli
designed to mimic some but not all of the characteristics of natural stimuli. Natural
stimuli are of course most relevant to the animal; noise stimulli are easy to work with
mathematically; so using both is often the most informative.
The Lateral Geniculate Nucleus 5
Lesica and Stanley
LGN bursts are triggered by objects; they presumably signal something different from
isolated spikes. LGN is not merely a relay station but shapes information flowing to
the cortex.
The Lateral Geniculate Nucleus 6
Feature-linked synchronization of LGN relay cells
caused by feedback from visual cortex.
Feedback
Present +
visual input:
2 LGN cells
show
correlated
firing.
The neural code in LGN presumably includes
correlated firing of relay neurons. This code
is controlled by feedback.
1.
The thalamus is not merely a feedback
center.
2. Correlated activity across a neural
population can be an important code.
3. Spike bursts can be an important code.
Visual input No
Feedback: No
correlations
are seen.
Sillito et al.