SENSATION AND PERCEPTION
3.1 Vision
3.1.1 Visual System
The sensory receptors in the eye are sensitive to energy within a limited range of the electromagnetic spectrum. One way of characterizing such energy is as continuous waves of different wavelengths. The visible spectrum ranges from wavelengths of approximately 370 nm (billionths of a meter) to 730 nm. Any energy outside this range, such as ultraviolet rays, will not be detected because they have no effect on the receptors.
Light can also be characterized in terms of small units of energy calledphotons. Describing light in terms of wavelength is important for some aspects of perception, such as color vision, whereas for others it is more useful to treat it in terms of photons. As with any system in which light energy is used to create a representation of the physical world, the light must be focused and a clear image created. In the case of the eye, the image is focused on the photoreceptors located on the retina, which lines the back wall of the eye.
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Sclerotic coat
Lens
Vitreous humor
Choroid coat Retina
Optic nerve
Blind spot Foveal pit Lid
Pupil
Cornea
Iris Lid
To nose Aqueous humor
Line of sight
Figure 5 Structure of the human eye. (From Schiffman, 1996.)
Focusing System Light enters the eye (see Figure 5) through the cornea, a transparent layer that acts as a lens of fixed optical power and provides the majority of the focusing. The remainder of the focusing is accomplished by the crystalline lens, whose power varies automatically as a function of the distance from the observer of the object that is being fixated. Beyond a distance of approximately 6 m, the far point, the lens is relatively flat;
for distances closer than the far point, muscles attached to the lens cause it to become progressively more spherical the closer the fixated object is to the observer, thus increasing its refractive power. The reason why this process, calledaccommodation, is needed is that without an increase in optical power for close objects their images would be focused at a point beyond the retina and the retinal image would be out of focus. Accommodation is effective for distances as close as 20 cm (the near point), but the extent of accommodation, and the speed at which it occurs, decreases with increasing age, with the near point receding to approximately 100 cm by age 60. This decrease in accommodative capability, called presbyopia, can be corrected with reading glasses. Other imperfections of the lens system—myopia, where the focal point is in front of the receptors;hyperopia, where the focal point is behind the receptors; andastigmatism, where certain orientations are out of focus while others are not—also typically are treated with glasses.
Between the cornea and the lens, the light passes through the pupil, an opening in the center of the iris that can vary in size from 2 to 8 mm. The pupil size is large when the light level is low, to maximize the amount of light that gets into the eye, and small when the light level is high, to minimize the imperfections in imaging that arise when light passes through the extreme periphery of the lens system. One additional consequence of these changes in image quality as a function of pupil size is that thedepth of field, or the distance in front of or behind a fixated object at which
the images of other objects will be in focus also, will be greatest when the pupil size is 2 mm and decrease as pupil size increases, at least up to intermediate diameters (Marcos et al., 1999). In other words, under conditions of low illumination, accommodation must be more precise and work that requires high acuity, such as reading, can be fatiguing (Randle, 1988). When required to accommodate to near stimuli, adults show accommodative pupil restrictions that increase the depth of field. This tendency for restriction in pupil size is much weaker for children (Gisl´en et al., 2008; the chil- dren in their study were 9–10 years of age), most likely due to the superior accommodative range of their lenses.
If the eyes fixate on an object at a distance of approx- imately 6 m or farther, the lines of sight are parallel. As the object is moved progressively closer, the eyes turn inward and the lines of sight converge. Thus, the degree of vergence of the eyes varies systematically as a function of the distance of the object being fixated. The near point for vergence is approximately 5 cm, and if an object closer than that is fixated, the images at the two eyes will not be fused and a double image will be seen.
The natural resting states for accommodation and vergence, calleddark focus anddark vergence, respec- tively, are intermediate to the near and far points (Leibowitz and Owens, 1975; Andre, 2003; Jaschinski et al., 2007). One view for which there is considerable support is that dark focus and vergence provide zero ref- erence points about which accommodative and vergence effort varies (Ebenholtz, 1992). A practical implication of this is that less eye fatigue will occur if a person working at a visual display screen for long periods of time is positioned at a distance that corresponds approx- imately to the dark focus and vergence points. As with most other human characteristics of concern in human factors and ergonomics, considerable individual differ- ences in dark focus and vergence exist. People with far dark-vergence postures tend to position themselves
farther away from the display screen than will those with closer postures (Heuer et al., 1989), and they also show more visual fatigue when required to perform close visual work (Jaschinski-Kruza, 1991).
Retina If the focusing system is working properly, the image will be focused on the retina, which lines the back wall of the eye. Objects in the left visual field will be imaged on the right hemi-retina and objects in the right visual field on the left hemi-retina; objects above the point of fixation will be imaged on the lower half of the retina, and vice versa for objects below fixation. The retina contains the photoreceptors that transduce the light energy into a neural signal;
their spatial arrangement limits our ability to perceive spatial pattern (see Figure 6). There also are two layers of neurons, and their associated blood vessels, that process the retinal image before information about it is sent along the optic nerve to the brain. These neural layers are in the light path between the lens and the photoreceptors and thus degrade to some extent the clarity of the image at the receptors.
There are two major types of photoreceptors,rods and cones, with three subtypes of cones. All photore- ceptors contain light-sensitive photopigments in their outer segments that operate in basically the same man- ner. Photons of light are absorbed by the photopigment when they strike it, starting a reaction that leads to the generation of a neural signal. As light is absorbed, the photopigment becomes insensitive and is said to be bleached. It must go through a process of regenera- tion before it is functional again. Because the rod and cone photopigments differ in their absolute sensitivities to light energy, as well as in their differential sensitiv- ities to light across the visual spectrum, the rods and cones have different roles in perception.
Rods are involved primarily in vision under very low levels of illumination, what is calledscotopic vision. All rods contain the same photopigment, rhodopsin, which is highly sensitive to light. Its spectral sensitivity func- tion shows it to be maximally sensitive to light around 500 nm and to a lesser degree to other wavelengths. One consequence of there being only one rod photopigment is that we cannot perceive color under scotopic condi- tions. The reason for this is easy to understand. The rods will respond relatively more to stimulation of 500 nm than they will to 560-nm stimulation of equal inten- sity. However, if the intensity of the 560-nm stimulus is increased, a point would be reached at which the rods responded equally to the two stimuli. In other words, with one photopigment, there is no basis for distinguish- ing among the wavelength differences associated with color differences from intensity differences.
Cones are responsible for vision in daylight, or what is known as photopic vision. Cone photopigments are less sensitive to light than rhodopsin, and hence cones are operative at levels of illumination at which the rod photopigment has been effectively fully bleached. Also, because there are three types of cones, each containing a different photopigment, cones provide color vision.
As explained previously, there must be more than one photopigment type if differences in the wavelength of
stimulation are to be distinguished from differences in intensity. The spectral sensitivity functions for each of the three cone photopigments span broad ranges of the visual spectrum, but their peak sensitivities are located at different wavelengths. The peak sensitivities are approximately 440 nm for the short-wavelength (“blue”) cones, 540 nm for the middle-wavelength (“green”) cones, and 565 nm for the long-wavelength (“red”) cones.
Monochromatic light of a particular wavelength will produce a pattern of activity for the three cone types that is unique from the patterns produced by other wavelengths, allowing each to be distinguished perceptually.
The retina contains two landmarks that are important for visual perception. The first of these is the optic disk, which is located on the nasal side of the retina. This is the region where the optic nerve, composed of the nerve fibers from the neurons in the retina, exits the eye. The significant point is that there are no photoreceptors in this region, which is why it is sometimes called theblind spot. We do not normally notice the blind spot because (1) the blind spot for one of the eyes corresponds to part of the normal visual field for the other eye and (2) with monocular viewing, the perceptual system fills it in with fabricated images based on visual att- ributes from nearby regions of the visual field (Araragi et al., 2009). How this filling in occurs has been the subject of considerable investigation, with evidence from physiological studies and computational modeling suggesting that the filling in is induced by neurons in the primary visual cortex through slow conductive paths of horizontal connections in the primary visual cortex and fast feed-forward/feedback paths by way of the visual association cortex (Matsumoto and Komatsu, 2005; Satoh and Usui, 2008). If the image of an object falls only partly on the blind spot, the filling in from the surrounding region will cause the object to appear complete. However, if the image of an object falls entirely within the blind spot, this filling in will cause the object to not be perceived.
The second landmark is thefovea, which is a small indentation about the size of a pinhead on which the image of an object at the point of fixation will fall. The fovea is the region of the retina in which visual acuity is highest. Its physical appearance is due primarily to the fact that the neural layers are pulled away, thus allowing the light a straight path to the receptors. Moreover, the fovea contains only cones, which are densely packed in this region.
As shown in Figure 6, the photoreceptors synapse with bipolar cells, which in turn synapse with ganglion cells; the latter cells are the output neurons of the retina, with their axons making up the optic nerve. In addition, horizontal cells and amacrine cells provide interconnections across the retina. The number of ganglion cells is much less than the number of photo- receptors, so considerable convergence of the activity of individual receptors occurs. The neural signals generated by the rods and cones are maintained in distinct pathways until reaching the ganglion cells (Kolb, 1994).
In the fovea, each cone has input into more than one ganglion cell. However, convergence is the rule outside the fovea, being an increasing function of distance from
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Horizontal cell
Amacrine Bipolar cell
cell
Ganglion cell
Light
Rods Cones
Figure 6 Neural structures and interconnections of vertebrate retina. (From Dowling and Boycott, 1966.)
the fovea. Overall, the average convergence is 120:1 for rods as compared to 6:1 for cones. The degree of convergence has two opposing perceptual consequences.
Where there is little or no convergence, as in the neurons carrying signals from the fovea, the pattern
of stimulation at the retina is maintained effectively complete, thus maximizing spatial detail. When there is considerable convergence, as for the rods, the activity of many photoreceptors in the region is pooled together, optimizing sensitivity to light at the cost of detail. Thus,
the wiring of the photoreceptors is consistent with the fact that the rods operate when light energy is at a premium but the cones operate when it is not.
The ganglion cells show several interesting proper- ties pertinent to perception. When single-cell recording techniques are used to measure their receptive fields (i.e., the regions on the retina that when stimulated pro- duce a response in the cell), these fields are found to have a circular, center-surround relation for most cells.
If light presented in a circular, center region causes an increase in the firing rate of the neuron, light presented in a surrounding ring region will cause a decrease in the firing rate, or vice versa. What this means is that the ganglion cells are tuned to respond primarily to discon- tinuities in the light pattern within their receptive fields.
If the light energy across the entire receptive field is increased, there will be little if any effect on the firing rate. In short, the information extracted and signaled by these neurons is based principally on contrast, which is important for perceiving objects in the visual scene, and not on absolute intensity, which will vary as a func- tion of the amount of illumination. Not surprisingly, the average receptive field size is larger for ganglion cells receiving their input from rods than for those receiv- ing it solely from cones and increases with increasing distance from the fovea.
Although most ganglion cells have the center- surround receptive field organization, two pathways can be distinguished on the basis of other properties. The ganglion cells in theparvocellular pathway have small cell bodies and relatively dense dendritic fields. Many of these ganglion cells, called midget cells, receive their input from the fovea. They have relatively small receptive fields, show a sustained response as long as stimulation is present in the receptive field, and have a relatively slow speed of transmission. The ganglion cells in the magnocellular pathway have larger cell bodies and sparse dendritic trees. They have their receptive fields at locations across the retina, have relatively large receptive fields, show a transient response to stimulation that dissipates if the stimulus remains on, have a fast speed of transmission, and are sensitive to motion. Because of these unique characteristics and the fact that these channels are kept separated later in the visual pathways, it has been thought that they contribute distinct information to perception. The parvocellular pathway is presumed to be responsible for pattern perception and the magnocellular pathway for high-temporal-frequency information, such as in motion perception and perception of flicker. The view that different aspects of the sensory stimulus are analyzed in specialized neural pathways has received considerable support in recent years.
Visual Pathways The optic nerve from each eye splits at what is called theoptic chiasma(see Figure 7).
The fibers conveying information from the nasal halves of the retinas cross over and go to the opposite sides of the brain, whereas the fibers conveying information from the temporal halves do not cross over. Functionally, the significance of this is that for both eyes input from the right visual field is sent to the left half of the brain and
input from the left visual field is sent to the right half.
A relatively small subset of the fibers (approximately 10%) splits off from the main tract and the fibers go to structures in the brain stem, the tectum, and then the pulvinar nucleus of the thalamus. Thistectopulvinar pathway is involved in localization of objects and the control of eye movements.
Approximately 90% of the fibers continue on the pri- marygeniculostriate pathway, where the first synapse is at the lateral geniculate nucleus (LGN). The distinction between the parvocellular and magnocellular pathways is maintained here. The LGN is composed of six lay- ers, four parvocellular and two magnocellular, each of which receives input from only a single eye. Hence, at this level the input from the two eyes has yet to be com- bined. Each layer is laid out in a retinotopic map that provides a spatial representation of the retina. In other respects, the receptive field structure of the LGN neu- rons is similar to that of the ganglion cells. The LGN also receives input from the visual cortex, both directly and indirectly, by way of a thalamic structure called the reticular nucleus that surrounds the LGN (Briggs and Usrey, 2011). This feedback likely modulates the activ- ity in the LGN, allowing the communication between the visual cortex and LGN to be bidirectional.
From the LGN, the fibers go to the primary visual cortex, which is located in the posterior cortex. This region is also called the striate cortex (because of its stripes),area 17, orarea V1. The visual cortex consists of six layers. The fibers from the LGN have their synapses in the fourth layer from the outside, with the parvocellular neurons sending their input to one layer (4Cβ) and the magnocellular neurons to another (4Cα), and they also have collateral projections to different parts of layer 6. The neurons in these layers then send their output to other layers. In layer 4 the neurons have circular-surround receptive fields, but in other layers, they have more complex patterns of sensitivity. Also, whereas layer 4 neurons receive input from one or the other eye, in other layers most neurons respond to some extent to stimulation at either eye.
A distinction can be made between simple cells and complex cells (e.g., Hubel and Wiesel, 1977). The responses of simple cells to shapes can be determined from their responses to small spots of light (e.g., if the receptive field for the neuron is plotted using spots of light, the neuron will be most sensitive to a stimulus shape that corresponds with that receptive field), whereas those for complex cells cannot be. Simple cells have center-surround receptive fields, but they are more linear than circular; this means that they are orientation selective and will respond optimally to bars in an orientation that corresponds with that of the receptive field. Complex cells have similar linear receptive fields and so are also orientation selective, but they are movement sensitive as well. These cells respond optimally not only when the bar is at the appropriate orientation but also when it is moving. Some cells, which receive input from the magnocellular pathway, are also directionally sensitive: they respond optimally to movement in a particular direction. Certain cells, calledhypercomplex cells, are sensitive to the length of