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chap. 3. The Photoreceptor Mosaic

The S-Cone Mosaic

DHE 76101

Spring semester

Nam Kyung Hyun

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Three Types of Cone Photoreceptors

Three classes of cones

Each with a different photopigment

Different spectral sensitivity

Maximum Response

S-Cone: 440nm

M-Cone: 530nm

L-Cone: 560nm

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Spatial Sampling Distribution

Behavioral Measurements

Punctate Sensitivity of the Blue Sensitive Mechanism. 1981. Williams et. al.

Concept

Poor acuity of S-cones Sparsely represented in retina

Sensitivity of the S-cone is significantly higher in the short-wavelength part

of spectrum

From spatial pattern of visual sensitivity, the distribution of S-cones are ableto be inferred

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Behavioral Measurement

Punctate Sensitivity of the Blue Sensitive Mechanism.

(1981. Williams et. al.)

METHOD

Apparatus

S: Tungsten SourceL1: Maxwellian Lens (images filament in AP)

L2: Maxwellian Lens (images Light source)

L3: Maxwellian Lens (background channel)

SH: Shutter

W: neutral density Wedge (controls flash intensity)

F: filter (determines wavelength)

PA: Aperture (defines test spot)

MS: Micrometer Stage (provides crosshair for adjustment)

AP: Artificial Pupil

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Behavioral Measurement

Method

3 observers

Presenting blue test target on a steady yellow background

Steady yellow background suppresses sensitivity of the M, L-cones

Thresholds were measured at121 foveal locations, spaced 5 arc apart.

Subjects visually fixated on a small mark and press a button

Stimulus presentation

Indicate whether or not the stimulus was visible

Map the sensitivity

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Behavioral Measurement

Result

The S-cones are very few in the fovea center so causing a so-called

S-cone blind spot

Typical separation between the inferred S-cones is about 8 12 minutesof visual angle.

5 7 cones per degree of visual angle

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Biological Measurement

Marc and Sperling (1977)

Used a stain that is taken up by cones when they are active

Stimulate short wavelength receptors

Typical separation between the stained cone was about 6 min of arc.

DeMonasterio et al. (1985)

Applied Procion-yellow dye in the Macaque retina

Cones are absent from central fovea

1 degree from the central fovea: peak density

S-cones spaced widely compared to the other cones

Limitation

1) animal experiment (baboon) species related difference

2) The arguments identifying the special cones as S-cones are ratherindirect

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Dyed with Procion-black

Dyed with Lucifer-yellow

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Biological Measurement

Curcio et al. (1991)

Used biological marker directly react to the S-cones in the humanretina

The measurements agree well quantitatively with Williamspsychophysical measurements

Average spacing between the S-cones is 10min. Of visual angle.

Anatomical observation: Wider inner segment

inserted further into subretinal space than neighboring cones

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Why are the S-cones widely spaced?

Spatial factor: Axial chromatic aberration of the lens blurs the short-wavelength component of the image.

Functional demand: If the boundary is blurred , light intensitychanges more slowly. S-cones will generally be coding slowertemporal variations

Flaws of this assumption

Other important factors in the design of visual pathway

Ex) rod vision: acuity is not a dominant factor

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