The Perception of Contrast and Color

1. The Perception ofContrast and Color Ch. 6 (cont’d)

2. Outline • Contrast: The Perception of Edges • Brightness-Contrast Detectors in the Mammalian Visual System • Seeing Color

3. Contrast:The Perception of Edges

4. Contrast:The Perception of Edges • Edges are the most important stimuli in our visual world; they define the position and extent of things • Edge perception is the perception of a contrast between two adjacent areas of the visual field; this lecture will focus on how the visual system controls brightness contrast

5. Contrast:The Perception of Edges • The perception of edges is so important that there is a mechanism in the visual system that enhances our perception of brightness contrast; this is called contrast enhancement; thus what we see is even better than the physical reality

6. Contrast:The Perception of Edges • The Mach bands illusion is the result of contrast enhancement; in it, light areas that are near the border with a dark region appear lighter than they really are, and the area of the dark region that lies along this border looks darker than it really is

7. Lateral Inhibition:The Physiological Basis of Contrast Enhancement • The mechanisms of contrast enhancement were studied in the eye of the horseshoe crab because of its simplicity; it is a simple compound eye, with eye of its individual receptors (ommatidia) connected by a lateral neural network (the lateral plexus)

8. Lateral Inhibition:The Physiological Basis of Contrast Enhancement • When ommatidium is activated, it inhibits its neighbors via the lateral plexus; contrast enhancement occurs because receptors near an edge on the dimmer side receive more lateral inhibition than receptors further away from the edge…

9. Lateral Inhibition:The Physiological Basis of Contrast Enhancement • …while receptors near the edge on the brighter side receive less lateral inhibition than the receptors on the brighter further way from the edge

10. Brightness-Contrast Detectors in the Mammalian Visual System

11. Mapping Receptive Fields • In the late 1950’s Hubel and Wiesel developed a method that became the standard method for studying visual system neurons

12. Mapping Receptive Fields • Visual stimuli are presented on a screen in front of a subject (cat or monkey); the image is artificially focused on the retina by an adjustable lens in front of the eye

13. Mapping Receptive Fields • Once an extracellular electrode is positioned in the neural structure of interest so that it is recording the action potentials of only one neuron, the neuron’s receptive field is mapped; the receptive field of a neuron is the area of the visual field within which appropriate visual stimuli can influence the firing of that neuron

14. Mapping Receptive Fields • Once the receptive field is defined, the task is to discover what particular stimuli presented within the field are most effective in changing the cell’s firing

15. Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway • Most retinal ganglion cells, lateral geniculate nucleus neurons, and the neurons in lower layer IV of the striate cortex have similar receptive fields: they are smaller in the foveal area; they are circular; they are monocular; and they have both an excitatory and an inhibitory area separated by a circular boundary

16. Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway • Neurons in these regions have 2 patterns of responding: (1) on firing; or (2) inhibition followed by off firing

17. Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway • The most effective way to influence the firing of these neurons is to fully illuminate only the “on area” or the “off area” of its receptive field; if one light is shone in the “on area” and one is simultaneously shone in the “off area”, their effects cancel one another out by lateral inhibition; these neurons respond little to diffuse light

18. Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway • In effect, many neurons of the retina-geniculate-striate pathway respond to brightness contrast between the centers and peripheries of their visual fields

19. Simple Cortical Cells • Simple cortical cells in the striate cortex have receptive fields like those that were just described, except that the two areas of the receptive fields are divided by straight lines

20. Simple Cortical Cells • These cells respond best to bars or edges of light in a particular location in the receptive field and in a particular orientation (e.g., 45 degrees) • All simple cells are monocular

21. Complex Cortical Cells • Most of the cells in the striate cortex are complex cells; they are more numerous but are like simple cells in that they respond best to straight-line stimuli in a particular orientation; they are not responsive to diffuse light

22. Complex Cortical Cells • Complex cells are unlike simple cells in that the position of the stimulus within the receptive field does not matter; the cell responds to the appropriate stimulus no matter where it is in its large receptive field • Over half of the complex cells are binocular, and about half of those that are binocular display ocular dominance

23. Hubel and Wiesel’s Model of Striate-Cortex Organization • When you record from visual cortex using vertical electrode passes, you find: (1) cells with receptive fields in the same part of the receptive field, (2) simple and complex, cells that all prefer the same orientation, and (3) binocular complex cells that are all dominated by the same eye (if they display ocular dominance)

24. Hubel and Wiesel’s Model of Striate-Cortex Organization • When you record visual cortex using horizontal electrode passes you find: (1) receptive field location shifts slightly with each electrode advance, (2) orientation preferences shifts slightly with each electrode advance and (3) ocular dominance periodically shifts to the other eye with electrode advances

25. Hubel and Wiesel’s Model of Striate-Cortex Organization • Ocular dominance columns can be visualized by injecting a large does of radioactive amino acids in one eye, waiting several days, and then subjecting the cortex to audioradiography; ocular dominance columns are clearly visible in lower layer IV as alternating patches of radioactivity and non-radioactivity

26. Hubel and Wiesel’s Model of Striate-Cortex Organization • Columns of vertical-line-preferring neurons have been visualized by injecting radioactive 2-DG and then moving vertical stripes back and forth in front of the animal for 45 min.; the subjects were then immediately killed and their brains sectioned; columns of radioactivity were visible through all layers of striate cortex except lower layer IV

27. (In-class Video)

28. Seeing Color

29. Component Theory • Also called trichromatic theory • Component processing occurs at receptor level • One of three different photopigments coats each cone; each photopigment reacts optimally to a particular part of the spectrum of electromagnetic energy; the ratio of cones activated at a particular part of the color spectrum creates a summed stimulus and thus color differentiation

30. Opponent Process Theory • Opponent processing occurs at all levels of the visual system beyond the receptors • Three classes of cells: one that becomes more active to red and less active to green; one that becomes more active to blue and less to yellow; and one that is more active to bright and less active to dark areas

31. Opponent Process Theory • With regards to color vision, red-green and blue-yellow opponent cells fire in response to input from cones; when red cells are “on” one cannot see green and when yellow cells are “on” one cannot see blue

32. Color Constancy • This is the tendency for an object to be perceived as the same color despite major changes in wavelengths of light that it reflects • Perception of color constancy allows objects to be distinguished in a memorable way

33. Color Constancy • Retinex theory of color vision follows the premise that the color of an object is determined by its reflectance and the visual system calculates the reflectance of surfaces by comparing the ability of a surface to absorb light in the three bandwidths corresponding to the three classes of cones

34. Color Constancy • Dual-opponent cells provide the means to analyze contrast between wavelengths reflected by adjacent areas of their receptive fields

35. Cortical Mechanisms of Vision and Audition Ch. 7

36. Outline • General Concepts (hierarchy, sensation and perception) • Cortical Mechanisms of Vision • The Auditory System

37. The Traditional Hierarchical Sensory-System Model • Traditionally, sensory-system organization has been viewed according to the following model: input flows from receptors to thalamus, to primary sensory cortex, to secondary sensory cortex, and finally to associated cortex

38. The Traditional Hierarchical Sensory-System Model • Primary sensory cortex is cortex that receives direct input from the thalamic sensory relay nuclei • Secondary sensory cortex is cortex that receives input primarily from the primary cortex • Associated cortex is cortex that receives input from more than one sensory system

39. The Traditional Hierarchical Sensory-System Model • The major feature of this model is its serial and hierarchical organization (any system with components that can be assigned ranks) • Sensory systems are thought to be hierarchical in two ways:

40. The Traditional Hierarchical Sensory-System Model • Sensory info is thought to flow through brain structures in order of their increasing neuroanatomical complexity • Sensation is thought to be less complex than perception

41. The Traditional Hierarchical Sensory-System Model • The neuroanatomical hierarchy is thought to be related to the functional hierarchy; perception is often assumed to be a function of cortical structures

42. Sensation and Perception • Sensation refers to the simple process of detecting the presence of a stimulus • Perception refers to the complex process of integrating, recognizing, and interpreting complex patterns of sensations

43. The Current Model of Sensory-System Organization • It is now clear that sensory systems are characterized by a parallel, functionally segregated, hierarchical organization

44. The Current Model of Sensory-System Organization • Parallel: sensory systems are organized so that information flows between different structures simultaneously along multiple pathways • Functionally segregated: sensory systems are organized so that different parts of the various structures specialize in different kinds of analysis • Hierarchical: as noted, information flows through brain structures in order of their increasing neuroanatomical and functional complexity

46. The Current Model of Sensory-System Organization • The Critical Question: If different types of information are processed in different specialized zones that are found in different structures connected by multiple pathways, how are complex stimuli perceived as an integrated whole? This is known as the binding problem

47. Cortical Mechanisms of Vision • The occipital cortex and some of the parietal and temporal cortex is visual cortex • Primary visual cortex is in the occipital lobe; secondary visual cortex is in the prestriate cortex (surrounding primary) and inferotemporal cortex; and most of the visual association cortex is posterior parietal cortex

48. Scotomas and Blindsight • Individuals with damage to primary visual cortex have scotomas or areas of blindness in corresponding areas of the visual field • Amazingly, when forced to guess, some brain-damaged patients can respond to stimuli in their scotomas (e.g., can grab a moving object or guess the direction of its movement) all the while claiming to see nothing; this is called blindsight

49. Scotomas and Blindsight • Blindsight is thought to be mediated by visual pathways that are not part of the retina-geniculate-striate system; one hypothesis is that the r-g-s system mediates pattern and color perception, whereas a system involving the superior colliculus and the pulvinar nucleus of the thalamus mediates the detection and localization of objects in space

50. Scotomas and Blindsight • This phenomenon emphasizes that parallel models (multiple-path) models rather than serial models (single-path) are needed to explain many perceptual phenomena