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Learn How the Brain works, how we take input to brain. Understand what exactly is attention and its bottle necks and get to know what is Top Down Processing and Bottom Up Processing.
Colour in object perception and memory – Part 1 Snehlata Jaswal HUL 211 OBJECT PERCEPTION AND MEMORY Colour as a feature Colour is one of the features of an object. Color can only exist when three components are present: a viewer, an object, and light. Although pure white light is perceived as colorless, it actually contains all colors in the visible spectrum. HUL 211 OBJECT PERCEPTION AND MEMORY Coloured objects When white light hits an object, it selectively blocks some colors and reflects others; only the reflected colors contribute to the viewer's perception of color. Objects may be classified as luminous and illuminated. Luminous objects are themselves the source of light – such as the sun, the stars, an incandescent bulb, etc. the amount of light energy they give off is known as radiance. Most objects, however only reflect light, and are therefore said to be illuminated. It follows that the amount of incident light falling on an object or surface is called illuminance. Further, the intensity of light reflected from an illuminated surface is called luminance. E.g. the amount of light emanating from the sun or the bulb in the room is radiance, the amount of light falling on this page is illuminance, and the amount of light reflected from the page is luminance. All these contribute to the psychological experience of brightness. Changes in the brightness of adjacent areas in the visual field is an important factor in the formation of contours and is also the basis of perception of form and image formation. HUL 211 OBJECT PERCEPTION AND MEMORY Receiving information about Colour The human eye senses this spectrum using a combination of rod and cone cells for vision. Rod cells are better for low-light vision, but can only sense the intensity of light, whereas while cone cells can also discern color, they function best in bright light. Three types of cone cells exist in your eye, each being more sensitive to either short (S), medium (M), or long (L) wavelength light. The set of signals possible at all three cone cells describes the range of colors we can see with our eyes – the visible spectrum from ~400 nm to 700 nm. HUL 211 OBJECT PERCEPTION AND MEMORY Transduction of radiant energy In the initial stage of vision, radiant energy must be transduced, or transformed, into a neural form. Physical energy acts on light-sensitive tissue to produce impulses that convey sensory information. The kind of tissue that is responsive to radiant energy is found even in the simplest organisms. Some organisms, such as the single-celled amoeba, possess no specialized light receptors; rather, the entire body is light sensitive. However, most animals have a region on their body that is maximally sensitive to light. Nevertheless, mere responding to light is quite different from actually forming a visual image. Indeed, many of the light-sensitive structures of lower forms of life act primarily to concentrate light on a light-sensitive pigment. That is, they serve as light-gathering rather than image-forming organs. It is in advanced stages of evolution that an image-forming eye developed. The human visual system is the eye, attuned to detecting light or radiant energy. cause it to focus an image on the retina. HUL 211 OBJECT PERCEPTION AND MEMORY The human eye The vertebrate eye is built on a single basic plan: from fish to mammal, all vertebrate eyes possess a photosensitive layer called a retina and a lens whose optical properties cause it to focus an image on the retina HUL 211 OBJECT PERCEPTION AND MEMORY Sclera and Cornea The eyeball, lying in a protective socket of the skull, is a globular structure just under 1 inch (about 20 mm) in diameter. The outer covering of the eyeball is a tough white, opaque coat about 1 mm in thickness called the sclera (seen as the white of the eye). The sclera at the front of the eye becomes the translucent membrane called the cornea. Light rays entering the cornea are refracted, or bent, by its surface. In fact, most of the refraction of light rays reaching the eye is performed by the cornea. If the cornea is misshapen, producing astigmatism, the person experiences a blurring of some of the incoming light. The cornea has no blood vessels because they would block the incoming light. This is why corneas can be easily transplanted with little fear of tissue rejection. Since there are no blood vessels in the cornea to provide nutrition, the cornea receives nutrition HUL 211 OBJECT PERCEPTION AND MEMORY Aqueous humor and choroid Since there are no blood vessels in the cornea to provide nutrition, the cornea receives nutrition from the aqueous humor, a watery liquid that resembles the cerebrospinal fluid. The aqueous humor fills the anterior chamber immediately in the back of the cornea. Though it is being continuously recycled, the canal through which it leaves the anterior chamber can become blocked, especially in the elderly, resulting in a buildup of pressure called glaucoma. A second layer of the eyeball, the choroid, is attached to the sclera. The choroid is about 0.2 mm thick. It consists largely of blood vessels and provides a major source of nutrition for the eye. In addition, the choroid is a heavily pigmented, dark structure; this enables the absorption of most extraneous light entering the eye, thereby reducing reflections within the eyeball that might blur the image. However, some nocturnal animals possess a retinal layer, called the tapetum, that reflects back some of the light entering the eye. It is the reflection of light from the retina that accounts for the eerie glow, or “eyeshine”, that appears occasionally from the eyes of many nocturnal animals. HUL 211 OBJECT PERCEPTION AND MEMORY Iris and pupil In front of the eye the choroid is modified to form the iris. The iris lies between the cornea and the lens. It is a disk-like, colored membrane, consisting of two smooth muscles. One function of the iris is to regulate the amount of light entering the eye. The iris surrounds the pupil, which is actually just a hole in the middle of iris through which the light passes. When lighting conditions are poor, the iris opens (or dilates) to increase the size of the pupil. By contrast, in bright light the iris closes, constricting or reducing, the size of the pupil. In the human young adult, the pupil is round and its size ranges from about 2 to 9 mm in diameter. This enables more than a 20-fold variation in area and in the amount of light admitted into the eye. Generally, the pupil’s reaction to the overall conditions of lightning is reflexive. Shining a bright light in the eye produces Whytt’s reflex (a reflex identified by the physiologist Robert Whytt in 1751): an immediate constriction of the pupil in response to bright light. the inability to demonstrate Whytt’s reflex may indicate neural injury to the CNS. HUL 211 OBJECT PERCEPTION AND MEMORY Lens The lens is a transparent spherical body located directly behind the pupil. It is slightly yellow in color and is composed of an outer layer (the lens capsule) that contains fibers arranged like the layers of an onion (Koretz and Handelman, 1988). The cornea bends light rays as they enter the eye. The lens completes the task of bringing light waves into focus on the photoreceptors that line the rear of the eye. Because the lens can change its shape, it can focus light rays from both nearby and faraway objects through a process called accommodation. The ciliary muscle surrounds the lens and is attached to it by means of tiny fibers called the Zonules of Zinn. HUL 211 OBJECT PERCEPTION AND MEMORY Accommodation When we are looking at a distant object (20 feet away or ore), the ciliary muscle relaxes, which causes the muscle to expand and pull on the zonules. In this state (unaccommodated), the lens is pulled out to its flattest shape, so it will bend the incoming light the least. When you are looking at an object that is nearer, the ciliary muscle contracts, which allows the lens to return to its natural shape. In this state (accommodated), the lens will bend light the most. As we grow older, however, the lens loses its ability to accommodate. This condition, called presbyopia, is due to several factors. The lens actually continues to grow throughout our lifetime, so it is thicker in older people. To correct for this deficit, people have to wear reading glasses, or bifocals if their vision needed correction prior to the onset of presbyopia. It is crucial that the lens be clear so that the light can pass through it. A cataract is a cloudy lens, which can happen as a result of injury or disease. Alternatively, an intraocular lens, a substitute lens, can be inserted through the pupil after the defective lens has been removed. HUL 211 OBJECT PERCEPTION AND MEMORY Retina and vitreous humor The various structures - from the cornea through the lens - serve to bring images into focus on the eye’s photoreceptors. The retina is a layer of light receptors, or photoreceptors and nerve cells at the rear of the eye. The transduction of light energy takes place in retina. The photoreceptors absorb light rays, and transform them into information that can be transmitted by the neurons, or nerve cells. Between the lens and the retina is the posterior chamber that contains the vitreous humor, a thick jelly like substance, the pressure from which helps to maintain the shape of the eyeball, despite external pressures. HUL 211 OBJECT PERCEPTION AND MEMORY Fovea, rods, and cones The photoreceptors are most densely packed within the fovea, thus it is the portion of the retina that produces clearest vision. Humans have a single fovea located in the center of the retina. Many mammals lack the fovea, and some animals (horses and birds) have two foveas in each eye. In horses this is a marvellous evolutionary adaptation, as it enables the horse to see clearly the ground at its feet and the space directly ahead of it at the same time. A bundle of neurons, the optic nerve, carries information away from the retina at the optic disc. The optic disc has no photoreceptors, so you cannot see anything that falls on this part of the retina. The optic disc therefore creates a blind spot. Cones and rods are the two kinds of photoreceptors in the retina that transduce the light information into neural information. HUL 211 OBJECT PERCEPTION AND MEMORY Cones vs. Rods Characteristic Cones Rods Kind of vision Colour Black and white Shape Fat and pointed Thin and blunt Number 7 million 125 million Sensitivity Poor Excellent Distribution Throughout retina, Periphery of retina, not concentrated in fovea in fovea Dark adaption Rapid, but high Slow, but low threshold threshold Light required for best Well lit Dimly lit functioning Number of receptors for Few Many one ganglion cell Disc shedding Evening Morning Photo pigment Three types Rhodopsin Acuity Excellent HUL 211 OBJECT PERCEPTION AND MEMORY Poor Beyond rods and cones The information from the cones and rods is transmitted through the bipolar cells to the next level in the chain, the ganglion cells. Ganglion cells take the information from the bipolar cells and bring it toward the brain. Recent research has revealed three structurally different ganglion cells: X cells are the most common, they respond in a steady, sustained fashion during stimulation, so they are important for picking up precise details about the visual stimulus. They are perhaps important to see the pattern in non-moving stimuli. Y cells are probably involved in the perception of movement and stereoscopic depth, but are insensitive to color because they cannot differentiate among the cone inputs they receive. Both X and Y cells possess a central receptive field, with antagonistic surrounds, and hence work best in the presence of contrast in the stimulus. W cells respond to more homogeneous, evenly distributed stimulation. The most slowly conducting of the three types of ganglion cells, they seem to respond best to moving stimuli. Less is known about them, probably because they appear to show a wider range of attributes. HUL 211 OBJECT PERCEPTION AND MEMORY Between the fovea and ganglion cells HUL 211 OBJECT PERCEPTION AND MEMORY Bipolar, Horizontal and Amacrine cells The chain of interconnections from the photoreceptors to the bipolar cells to the ganglion cells is a vertical chain. However, the information also travels horizontally across the retina through horizontal cells and amacrine cells. Horizontal cells allow communication among photoreceptors. They also interconnect bipolar cells. Furthermore, horizontal cells communicate with one another. Amacrine cells allow communication between bipolar cells and between ganglion cells. They are also interconnected by means of other amacrine cells. In fact, with widely branching dendrites, they are perhaps the most common cells in the retina. HUL 211 OBJECT PERCEPTION AND MEMORY From the eye to the brain The optic nerve is a bundle of ganglion cells leaving the eye. Most of the ganglion cells in the optic nerve terminate in the lateral geniculate nuclei. However, before they do, half of the ganglion cells of either eye cross over to the other side of the brain at the optic chiasm. The ganglion cells from the left half of the left eyes do not cross over at the optic chiasm, but continue back to the left lateral geniculate nucleus. Likewise, the ganglion cells from the right half of the right eye remain on the right side of the brain. However, the ganglion cells from the right half of the left eye cross over to the right side at the optic chiasm. The ganglion cells from the left half of the right eye cross over to the left side of the brain. As a result, information from the left visual field of both eyes is carried to the right visual cortex, and information from the right visual field of both eyes is carried to the left visual cortex. HUL 211 OBJECT PERCEPTION AND MEMORY From the eye to the brain HUL 211 OBJECT PERCEPTION AND MEMORY Optic nerve and optic tract The bundle of ganglion cells is called the optic tract after it crosses the optic chiasm. Groupings of ganglion cells are first called the optic nerve. Then after passing through the optic chiasm, the cells are regrouped and called the optic tract. No synapses are found in the optic chiasm, so it serves no function in terms of processing the visual stimuli. In the absence of a synapse, the information from the retina remains intact and untransformed. So the change in name reflects the fact that the optic tract contains information from half of each eye, but that information is kept separate until later in the visual system. Optic chiasm is merely the location where the portions of each optic nerve cross over to the other side of the brain. The first synapse of the ganglion cells in the optic nerve is either in the superior colliculus or the lateral geniculate nucleus. So the first opportunity for further processing of visual information occurs in these two locations. HUL 211 OBJECT PERCEPTION AND MEMORY Superior colliculus Some of the optic tract fibers go to the superior colliculus. The Superior colliculus is a relatively primitive part of the midbrain that in humans is important for eye movements as well as other perceptual functions. Two superior colliculi are found in the visual system, one for each optic tract. In humans, some of the Y and all of the W ganglion cells go to the superior colliculus. Remember that the Y cells are sensitive to movement rather than details about shape. The superior colliculus receives information from the auditory system as well as information from the skin senses (e.g. touch). This non-visual information is arranged to coincide spatially with the organization of the visual input. Thus the superior colliculus probably works toward integration of information from various senses. HUL 211 OBJECT PERCEPTION AND MEMORY Lateral Geniculate Nucleus Most of the optic tract fibers go to the Lateral Geniculate Nucleus. Lateral means “on the side”. Geniculate means “bent like a knee”. Nucleus means “little nut”. So a lateral geniculate nucleus looks like a little nut that is bent like a knee, located on each side of the brain. The lateral geniculate nucleus (LGN) is part of the thalamus where 80% of the ganglion cells that began in the retina finally stop, transferring their information to new neurons. (Most of the remaining 20% stop at the superior colliculus). Because the ganglion cells entering the LGN have passed over the optic chiams, input to the LGN comes from both eyes. However, this input from the two eyes is kept separate in the LGN in a layered, or laminated, fashion. In fact, the LGN contains six layers, three from each eye. HUL 211 OBJECT PERCEPTION AND MEMORY Lateral Geniculate Nucleus The visual pathway appears to have three parallel information systems, two of which are present in the LGN (Hubel,1988; Livingstone, 1987). The parvocellular system (small cell) receives input from the X cells. The magnocellular system (large cell) receives input from the Y cells. The former is involved in the detailed and colour processing of non-moving stimuli, whereas, the latter is involved in movement and depth perception. Tremendous differences are found in LGN from species to species. In human beings, input to the LGN comes not only from the retina, but also from other parts of the brain. So LGN is not simply a relay station designed to keep the ganglion cells to a manageable length, but a place where information is processed. The inputs from the other areas of the brain must be important to LGN functioning, but researchers are currently exploring the effects of these inputs. HUL 211 OBJECT PERCEPTION AND MEMORY Visual cortex The visual cortex is the part of the cerebral cortex, or outer part of the brain, concerned with vision. The visual cortex is in the rear part of the brain (occipital lobe). The rear portion of the visual cortex has been labeled Areas 17,18 and 19. Neurons from the lateral geniculate nuclei terminate in Area 17 of the visual cortex, which is also referred to as the primary visual cortex or the striate cortex. Striate means “striped”; a microscopic investigation of this area of the cortex reveals pale stripes. The clear majority of research on cortical processing in vision has concentrated on the primary visual cortex (Area 17). Two other regions at the back of the brain are also concerned with vision. These are Area 18 and Area 19, often called the secondary visual cortex (as opposed to the primary visual cortex) or the extra striate cortex (where extra- means “beyond” as in “extraterrestrial”). The extra striate cortex appears to be involved in complex aspects of visual processing. HUL 211 OBJECT PERCEPTION AND MEMORY Visual cortex There is an overrepresentation of information from the fovea in the cortex. This cortical magnification helps to construct an accurate three-dimensional representation of the object. Hubel and Wiesel, who shared the Nobel Prize with Robert Sperry in 1981, isolated three kinds of neurons in the visual cortex: simple, complex, and hypercomplex, which respond to increasingly selective stimuli. This is in consonance with the principle in visual processing that cells become more selective at higher levels. Photoreceptors respond when the light reaches them, ganglion cells and LGN cells respond only if the center of the receptive field contrasts with the surrounding area, simple cells respond to lines, complex cells require moving lines, hypercomplex cells require moving lines of particular dimensions. HUL 211 OBJECT PERCEPTION AND MEMORY Three stages in cortical processing Zeki and Marini (1998) Used fMRI as participants viewed objects in their natural and unnatural colours and their achromatic counter parts to chart the stages of processing of colour in the brain. Concluded that there are three stages in cortical colour processing: 1. Based on V1 and maybe V2 and mainly concerned with registering the presence, intensity, and differences of different wavelengths. 2. Based on V4 and concerned with automatic color constance operations without regard to memory, judgement, and learning 3. Based on inferior temporal and frontal cortex, concerned with ‘object colour’ involving memory and past experience HUL 211 OBJECT PERCEPTION AND MEMORY Thank you HUL 211 OBJECT PERCEPTION AND MEMORY
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