OPTO 5320 VISION SCIENCE I

Transcription

1 OPTO 5320 VISION SCIENCE I Monocular Sensory Processes of Vision: Color Vision Mechanisms of Color Processing

2 . Neural Mechanisms of Color Processing A. Parallel processing - M- & P- pathways B. Second stage mechanisms 1. Opponent & non-opponent mechanisms in LGN 2. Perceptual correlates of second stage processing C. Third stage color vision mechanisms D. Fourth stage color vision mechanisms

3 Color Vision Mechanisms Stage 2 Synaptic Interactions in the Retina Feed-forward and feed-back synapses of retinal neurons in the inner and outer plexiform layers provide connections for spatial interactions between cone types. Receptors (R) drive both bipolar (B) and horizontal (H) cells. Bipolar cells terminate on both ganglion (G) and amacrine (A) cells.

4 Receptive Fields of Neurons in the Visual Pathway Receptive field locations are defined by the area on the retina (or visual space) where stimuli affect the response of a neuron. Microelectrode A neurons response properties reflect the relationship between stimulus parameters and neural responses. In many cases, the spectral properties of stimuli are an important variable in determining the neural response amplitude.

5 Parallel Processing in the M- and P-Pathways. 1. Segregation of visual functions in the anatomically independent magnocellular (M-) and parvocellular (P-) pathways. 2. The larger P(alpha) ganglion cells (10%) comprise the M- path. Response properties: low spatial frequency high temporal frequency color non-opponent high contrast gain 3. The medium-sized P(beta) ganglion cells (80%) comprise the P-path. Response properties: high spatial frequency low temporal frequency color opponent lower contrast gain Livingstone M, Hubel D. Science 240: , 1988

6 Parallel Processing from Retina to the Lateral Geniculate Nucleus P (phasic) ganglion cells project to the magnocellular layers of the LGN. P (tonic) ganglion cells project to the parvocellular layers of the LGN.

7 The Lateral Geniculate Nucleus 1. The first common site in the afferent pathways of the two eyes, but the two monocular pathways remain segregated. 2. Laminae 2,3 & 5 receive input from the ipsilateral eye. Layers 1,4 & 6 from the contralateral eye. 3. Layers 1 & 2 are the magnocellular laminae and layers 3-6 are the parvocellular laminae. 4. Neurons from corresponding points of the retinas are aligned across layers along projection lines.

8 Receptive Field Organization of LGN Cells Neurons in the LGN have a concentric, center-surround organization with opposite or antagonistic responses in the center and surround. The receptive fields may be either excitatory (on-center) or inhibitory (off-center). For these cells there is no response with diffuse white stimuli because the center and surround responses cancel.

9 Color Response Properties of LGN Cells Responses of the LGN cells can be divided into two general categories. Spectrally opponent cells respond with an increase in firing rates to some parts of the spectrum and by a decrease in other parts. Spectrally non-opponent cells which respond to all wavelengths with either an increase or a decrease in firing rates. Comparison with psychophysical data indicates that the non-opponent and opponent cells are the mechanisms underlying the discrimination of hue, saturation, and brightness.

10 Responses of Color- Opponent LGN Cells An example from a parvocellular neuron shows the spectral responses that are excitatory responses over short wavelengths and inhibitory responses over long wavelengths. The responses define a +G-R opponent response. DeValois, Abramov & Jacobs, 1966

11 Responses of Color- Opponent LGN cells Mean spectral response curves for the R/G opponent neurons demonstrate that the +G-R and +R-G neurons are essentially mirror images. The null point of the response is at about 600 nm with peak excitatory or inhibitory responses at nm and at nm. The form of the spectral response is best explained by a subtractive, linear combination of the M- and L-cone outputs. DeValois, Abramov & Jacobs, 1966

12 Receptive Field Organization of R/G Opponent LGN Cells +L The spatial arrangement of receptive fields of opponent cells is a center-surround organization of antagonistic inputs from M- and L-cone types. -M Monochromatic stimuli produce the characteristic opponent responses, but white light is not an effective stimulus. -L +M

13 Responses of Color- Opponent LGN Cells As with the R/G opponent cells, the mean spectral response curves for the +Y-B and +B-Y neurons are essentially mirror images. The null point of the response is at about 480 nm with peak excitatory or inhibitory responses at nm and at The form of the spectral response is best explained by a subtractive, linear combination of the LM nonopponent neurons and S-cone outputs. DeValois, Abramov & Jacobs, 1966

14 Wavelength Discrimination Normal color vision: Very small differences in wavelength (1 2 nm) can be discriminated on the basis of hue. Finest discrimination at 490 nm and 590 nm. Poorer discrimination in the middle of the visible spectrum (530 nm) and at the ends of spectrum Wavelength (nm) Graham, CH, (1966)

15 R/G opponent cell Y/B opponent cell Neural Mechanisms of Color Vision - Hue Discrimination The relative change in neural responses of color opponent mechanisms, as a function of wavelength, underlies hue discrimination. The Y/B neurons are more sensitive to variation in wavelength over the short wavelengths The R/G neurons are more sensitive to variation to wavelength over the long wavelengths. Hue discrimination represents the lower envelope of the two channels.

16 Neural mechanisms of color vision - hue discrimination Hue discrimination nctions are based on the smallest wavelength difference between a test and comparison field required to discriminate between the hues in the two fields. Hue discrimination for subjects with normal trichromatic vision have minima at 490 and 590 nm, with maxima at 530 and at the ends of the spectrum.

17 Opponent and Non-Opponent Responses The retinal circuits for combining the outputs of the cones (first stage mechanisms) to obtain opponent and non-opponent responses (second stage mechanisms).

18 Brightness Discrimination (increment-threshold spectral sensitivity) For normal color vision, increment-threshold spectral sensitivity functions have three peaks in the visible spectrum (440, 520, & 620 nm). The three peaks are produced by neural (subtractive) combinations of the responses of the three cone photopigments, but they are not the photopigment sensitivity functions.

19 Percent color score Color Naming Color naming requires 4 responses classes (blue, green, yellow, red) with modifiers e.g., greenish blue. Certain wavelengths elicit essentially pure color names unique hues. Some combinations do not occur as a single color, i.e., red-green and blue-yellow form opponent pairs

20 Receptive Field Organization of Y/B Opponent LGN cells +L+M The scheme of the spatial organization of Y/B neurons is similar to the R/G opponent cells except for the difference in the specific cone inputs. -S -L-M +S

21 Non-Opponent Responses of LGN Neurons Responses from magnocellular neurons are either excitatory or inhibitory at all wavelengths. Most of the properties of non-opponent neurons are similar to the L-response type neurons in the retina. DeValois, Abramov & Jacobs, 1966

22 Neural Mechanisms of Color Vision - Brightness Discrimination Brightness discrimination is well described by the responses of non-opponent cells. The upper example shows the responses of excitatory nonopponent cells (triangles) compared to the monkey s photopic luminosity function by CFF (circles). The lower example shows the relative responses of magnocellular phasic cells (symbols) compared to a photopic luminosity function at an eccentricity of 10 arcdeg.

23 Brightness Discrimination (luminous efficiency) Ferry Porter law (CFF ~ log(i)). Critical flicker fusion is directly proportional to the logarithm of the light intensity Borders are least distinct when the boundary separates stimuli of equal luminance Maximum luminous efficiency for wavelengths near 555 nm. The V function is the basis of photometry. Flicker photometry or minimally distinct border methods produce smooth unimodal V functions, which represent the sum of the responses of the MW- and LW-cones.

24 Neural mechanisms of color vision - saturation discrimination Psychophysical determination of saturation discrimination (colorimetric purity) involves measurement of the amount of white light added to monochromatic light to obtain a difference in the perception of saturation. Saturation discrimination functions show that monochromatic stimuli at 570 nm appear less saturated than longer or shorter wavelengths.

25 Neural mechanisms of color vision - saturation discrimination A stimulus of any wavelength will generate activity of both nonopponent and opponent mechanisms. The perceived saturation of a stimulus is determined by the ratio of the opponent to non-opponent activity. The upper figure represents the spectral sensitivity of the nonopponent neurons (dashed line) compared to the combined sensitivities of R/G & Y/B cells. The lower figure shows the ratio of opponent to non-opponent sensitivity (filled circles). DeValois, Abramov & Jacobs, 1966

26 Saturation Discrimination (colorimetric purity) Wavelengths in the region of nm (yellows) appear less saturated than those nearer the ends of the visible spectrum (blue or red). Wavelength (nm) Graham, CH, (1966)

27 Receptive Field Organization of Non- Opponent LGN Cells +L+M Non-opponent cells receive input from cones pathways with either excitatory or inhibitory synapses to the center and pathways of the opposite sign to the surround -L-M Some non-opponent cells may lack the center-surround organization - the center mechanism extends over the whole receptive field. +L+M -L-M

28 Models of Neural Circuits for Color Vision S-cone M-cone L-cone Six distinct response types were found for color processing Y-B opponent output + + Non-opponent output - + +R-G opponent output Four types of color opponent neurons. 1) +R-G 2) +G-R 3) +Y-B 4) +B-Y Two types of nonopponent neurons. 1) Excitatory 2) Inhibitory

29 Perceptual Attributes of Color Color naming R/G and Y/B opponent mechanisms Hue discrimination Y/B opponent mechanism over short wavelengths and R/G opponent mechanisms over long wavelengths Photopic luminosity discrimination excitatory and inhibitory non-opponent mechanisms Saturation discrimination ratio of opponent to non-opponent responses at each wavelength (stage-three mechanisms) Increment threshold with white-light adaptation - Y/B opponent mechanism over short wavelengths and R/G opponent mechanisms over long wavelengths

30 Perceptual Correlates of Neural Mechanisms Second stage neural processing accounts for the major perceptual attributes of color - color naming, brightness discrimination, and hue discrimination. Saturation discrimination probably represents third stage mechanisms formed by the combination of opponent and non-opponent mechanism outputs at a cortical site. Perceptual correlates of the second stage opponent and non-opponent mechanisms should be also measurable.

31 Parallel Processing from Retina to the Striate Cortex Magnocellular neurons project to layer 4C of striate cortex (V1), then to layer 4B and on to prestriate area V2. Parvocellular neurons project to layer 4C of striate cortex, then blobs or interblobs of cortex and on to area V2.

32 Segregation on Cortical Areas for Specialized Visual Functions Stage 3 Mechanisms Ocular dominance (RE) - (LE) Inter-blobs Color blobs Orientation columns Schematic representation of the modular segregation of the brain. The largest modules are ocular dominance (OD) columns. Superimposed on the OD organization are the interblobs, blobs, and orientation columns. Neurons in the blobs have opponent wavelength specificity, consistent with color vision mechanisms of the LGN.

33 Color Modules in Striate and Prestriate Cortex The architecture of visual areas of the brain is for functional modules (ocular dominance columns, orientation columns, color blobs, interblobs). Neurons in the blobs are not orientation specific. V2 V1 About 1/2 of blob neurons are wavelength-selective, doubleopponent cells and the others are broadband (nonopponent). Blob neurons project to the thin stripes in area V2.

34 Double-Opponent Color Cells +M -L +L -M Third stage mechanisms: The major elaboration of color information processing in the visual cortex is the formation of double-opponent neurons. Opponent neurons of one type (e.g., +M-L) are combined with the opposite type (e.g., +L-M) +M-L to create neurons with color opponency in both +L-M the center and surround of the receptive field. Double-opponent receptive field Double-opponent neurons respond to chromatic contrast, but not luminance contrast.

35 red green Luminance contrast red green Chromatic contrast Luminance vs. Chromatic Contrast A stimulus with luminance contrast can be generated by superimposing red and green gratings, with a grating phase in which the luminance adds. A stimulus with chromatic contrast can be generated by superimposing red and green gratings, with a grating phase in which the luminance does not add.

36 Optimal Detection Mechanisms for Luminance vs. Chromatic Contrast Receptive field type +M+L -M-L +M-L +L-M The optimal detection mechanism for a grating with luminance contrast is a non-opponent neuron with a center-surround organization when the bar width is equal to the diameter of the receptive field center. Similarly, the optimal detection mechanism for a grating with chromatic contrast is a doubleopponent neuron with a centersurround organization when the bar width is equal to the diameter of the receptive field center.

37 Spatial Contrast Sensitivity for Iso-Luminant Chromatic Gratings Luminance gratings Iso-luminant chromatic gratings Comparisons of contrast sensitivity data for luminance and chromatic gratings exhibit difference in properties for gratings detected by hue vs brightness. The double-opponent mechanisms are high-pass filters with lower sensitivities to higher spatial frequencies, compared to luminance contrast functions. The properties of the chromatic contrast sensitivity function may depend on the specific colors chosen for the grating. Mullen, KT, J Physiol 359: , 1985

38 Processing of Color from V1 to V2 and V4 Neurons in the color rich blobs of V1 project to the thin stripes of V2 and then to the extrastriate area V4.

39 Perceptual visual pathways past striate (V1) and prestriate (V2) visual areas Connections between extrastriate areas combine stimulus properties. Stage 4 mechanisms. Area V4 is strongly involved with color vision information processing, with many cells that respond to the perceived color of a stimulus rather than its wavelength composition.

40 Visual area V4 of the human brain The brain slice illustrates the anatomical location of V4, in respect to the other major visual areas of the brain.

41 Color Agnosia. Zeki, 1993 One of the strongest lines of evidence for V4 as the brain center for color perception is the achromatopsia caused by lesions of V4. A patient, with normal color vision by clinical tests, was asked to draw from memory a banana, a tomato, a cantaloupe, and green leaves. The patient s spatial vision seems to have been preserved, but color perception is deeply affected.

42 Mechanisms of Color Processing Recap Parallel processing of visual information. Four stages of color vision processing. Neural networks to account for the perceptual attributes of color. A cartoon representation of the connections in the visual pathway from LGN to striate cortex to prestriate cortex to extrastriate cortex.