FREQUENCY OF SEEING EXPERIMENTS (Hecht, Shlaer and Pirenne, 1942) - - PowerPoint PPT Presentation

100 80 60 40 20 % seen 10 100 photons at cornea = 12 = 2 = 7 Θ Θ Θ FREQUENCY OF SEEING EXPERIMENTS (Hecht, Shlaer and Pirenne, 1942) CONCLUSION: Θ = 5-7 photons absorbed spread over 500 rods PROBLEM: No way to account for false positives (noise) fixation point target target covers ~500 rods

Signal and noise distributions and false positives (on board)

IMPLICATIONS OF BEHAVIORAL SENSITIVITY

cone bipolar cell cone rod rod bipolar cell AII amacrine cell ganglion cell

• phototransduction
• single photons reliably transduced
• synaptic transmission
• reliable transmission of single

photon responses

• neural coding
• absorption of a single photon alters
• ptic nerve activity

But the task of the retina is in fact more challenging ...

• At visual threshold photons < 0.1% of

the rods contribute signals while all rods generate noise

• Under these conditions averaging is a

disaster - instead requires separation of signal from noise

• General problem in nervous system

keep discard

CONVERGENCE AND SPARSE SIGNALING IN MAMMALIAN RETINA

What is optimal readout of array of detectors when small fraction active?

Baylor et al., 1984 van Rossum and Smith, 1998 Field and Rieke, 2002

keep discard

Baylor et al., 1984 van Rossum and Smith, 1998 Field and Rieke, 2002

CONVERGENCE AND SPARSE SIGNALING IN MAMMALIAN RETINA

• At visual threshold photons < 0.1% of

the rods contribute signals while all rods generate noise

• Under these conditions averaging is a

disaster - instead requires separation of signal from noise

• General problem in nervous system
• Sizeable behavioral consequences for

getting this right!

RODS GENERATE PHOTON-LIKE NOISE EVENTS AND CONTINUOUS NOISE

60 40 20 time (sec) 1 pA

1 pA 0.2 sec ave photon-like noise event ave single photon response

continuous noise photon-like noise event Baylor et al., 1984 Greg Field

• Mouse rod-rod bipolar signal transfer is

nonlinear.

• dependence of response on flash

strength

• discreteness of dim flash response
• Nonlinear signal transfer eliminates or

severely attenuates majority of rod’s single photon responses.

• Rejection of noise more than compensates

loss of signal - thus rod bipolars provide near-optimal readout of rod signals near visual threshold.

keep discard

cone bipolar cell cone rod rod bipolar cell AII amacrine cell ganglion cell

THE ROD BIPOLAR PATHWAY

DIM FLASH RESPONSES OF RODS GROW LINEARLY WITH FLASH STRENGTH

• 15
• 10
• 5

pA 0.6 0.4 0.2 0.0 sec log(R/Rmax) log(Rh*)

• 2
• 1

1 rods rod bipolar

why log-log?

Field and Rieke, 2002

RESPONSES OF ROD BIPOLARS BUT NOT RODS GROW SUPRALINEARLY WITH FLASH STRENGTH

• 2
• 1

log(R/Rmax) 1

• 1

log(Rh*/rod)

• 150
• 100
• 50

pA 0.6 0.4 0.2 0.0 time (sec)

• 15
• 10
• 5

pA 0.6 0.4 0.2 0.0 sec log(R/Rmax) log(Rh*)

• 2
• 1

1 rods rod bipolar

Field and Rieke, 2002

Does rod-rod bipolar signal transfer separate rod signal and noise?

14 12 10 8 6 time (sec) 1 pA photocurrent flash times keep throw away

MOUSE ROD SINGLE PHOTON RESPONSES ARE PARTIALLY OBSCURED BY NOISE 60 40 20 # responses 4 2 amplitude

0.6 Rh*

0.6 0.4 0.2 0.0 sec 2 pA rods rod bipolar

Field and Rieke, 2002

ROD BIPOLARS GENERATE DISCRETE RESPONSES TO DIM FLASHES 0.6 0.4 0.2 0.0 sec 20 pA 8 6 4 2 # responses 4 2 amplitude

0.6 Rh*/rod

60 40 20 # responses 4 2 amplitude

0.6 Rh*

0.6 0.4 0.2 0.0 sec 2 pA rods rod bipolar

Field and Rieke, 2002

• Mouse rod-rod bipolar signal transfer is

nonlinear.

• Nonlinear signal transfer eliminates or

severely attenuates majority of rod’s single photon responses.

• Rejection of noise more than compensates

loss of signal - thus rod bipolars provide near-optimal readout of rod signals near visual threshold.

keep discard

MODEL FOR ROD-ROD BIPOLAR SIGNAL TRANSFER

gain amplitude probability amplitude flash strength # responses amplitude

nonlinearity

Σ

predict nonlinearity and discreteness of rod bipolar response sum rods

Signal and noise distributions and discrimination (on board)

• Mouse rod-rod bipolar signal transfer is

nonlinear.

• Nonlinear signal transfer eliminates or

severely attenuates majority of rod’s single photon responses.

• Rejection of noise more than compensates

loss of signal - thus rod bipolars provide near-optimal readout of rod signals near visual threshold.

keep discard

DISTRIBUTION OF ROD RESPONSES AT VISUAL THRESHOLD 10-8 10-6 10-4 10-2 2 1

• 1

amplitude 1.0 0.5 0.0 nonlinearity gain 10-8 10-6 10-4 10-2 probability 2 1

• 1

amplitude 1.0 0.5 0.0

signal noise nonlinearity

Rod experiments (~1 Rh*) Visual threshold (0.0001 Rh*)

Field and Rieke, 2002