Download Visual System Part 1 – Visual Perception

The visual system III
Tectum and thalamus
The primary visual pathway
Superior colliculus
& pretectum
From www.perret-optic.ch
The primary visual pathway
1 000 000 retinal
ganglion cells
40 000 000 LGN
relay cells
The Lateral Geniculate Nucleus (LGN)
P
P
P
C
P
I
M
C
I
M
I
C
JUST A RELAY?
Inputs to the LGN
• 30 % from local GABAergic interneurons
• 30 % from the brain stem
• 30 % from layer 6 of the visual cortex
• 10 % from retinal ganglion cells
The LGN – Circuitry
Thalamic
Reticular
Nucleus
Reticular
formation
Sherman (2007)
However…
Receptive fields in the LGN are identical to those in the retina.
What on earth does the thalamus
do?
The LGN – Circuitry
Drivers and modulators
Thalamic
Reticular
Nucleus
Reticular
formation
Sherman (2007)
Ionotropic connections – synchrony and
selectivity
• Increased synchrony (Sillito et al., 1994), and
thus the efficiency of the thalamus at evoking
responses in V1
• Increased center-surround selectivity
(Murphy & Sillito, 1984)
Sillito et al. (1994)
Metabotropic connections –
the T-current
The T-current produces a long-lasting
depolarization, causing the thalamic neuron to
fire a burst of spikes
The T-current is inactivated when the neuron is
depolarized ( > -55 mV), then the neuron fires
tonically.
The inactivation of the T-current is reversed at ~
-60 mV, inducing the neuron to fire in bursts.
Metabotropic connections can cause such a
slow hyperpolarization inputs
Thus, modulatory feedback from higher brain
areas can control the activity mode in which the
LGN processes retinal input
Llinas & Steriade (2003)
Bursting during sleep – shutting the system
down
Bursting during perception – alerting the
system?
More faithful stimulus
coding
Higher signal-tonoise ratio
- better identification!
-better detection!
Sherman (2001)
What the LGN does
• It selects and amplifies retinal inputs:
– Sparser firing through tonic inhibition (Hubel & Wiesel, 1961)
– Strong synchrony: Because retinal inputs diverge onto LGN
neurons, up to 30% of spikes are fired in synchronous events.
These are significantly more likely to drive V1.
– Selectivity for synchronous and burst events:
Spikes arriving within 10 ms of one another are 5 times more
likely to elicit an output, and even 12 times more likely to elicit
a synchronous output.
• Thus, the retinal code is decompressed and
distributed
Usrey et al. (2000)
What the LGN does
• It works as a gatekeeper for the visual
cortex
– By modulating strength of synchrony, cortex
can control efficiency of thalamic input
– By modulating burst mode, it can control the
responsiveness to the outer world into nonresponsive, alert/expectant, and continuous
processing
Usrey et al. (2000)
Summary III –
The thalamus…
• continues the segregation in magno- and parvocellular
pathway throughout the six layers of the LGN
• is not a passive relay
• filters, decompresses and restructures the signals from
the retina into a more distributed, temporally precise code
• favours strong synchronous inputs, and often produces
highly synchronized discharges
•can be modulated retinotopically by feedback from the
cortex and the brain stem to enter tonic, bursting and
oscillatory processing modes
• may thus play a crucial role for attentional selection etc.