Current Trends of fMRI in Vision Science: A Review Current Trends of fMRI in Vision Science: A Review

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Introduction
Studying brain functional activities is an area that is experiencing rapid interest in the field of neuroimaging.Functional magnetic resonance imaging (fMRI) has provided vision science researchers a powerful and noninvasive tool to understand eye function and correlate it with brain activities.In this chapter, we focus on the physiological aspects followed by a literature review.More specifically, to motivate and appreciate the complexity of the visual system, we will begin with a description of specific stages the visual pathway, beginning from the distal stimulus and ending in the visual cortex.More importantly, the development of ascending visual pathway will be discussed in order to help in understanding various disorders associated with it such as monochromacy, albinism, amblyopia (refractive, strabismic).In doing so we will divide the first half into two main sections, the visual pathway and the development of the ascending pathway.The first of these sections will be mostly an anatomy review and the latter will discuss the development of this anatomy with specific examples of disorders as a result of abnormal development.We will then discuss fMRI studies with focus on vision science applications.The remaining sections of this chapter will be highlighting the work done on mainly oculomotor function, some perception and visual dysfunction with fMRI and investigate the differences and similarities in their findings.We will then conclude with a discussion on how this relates to neurologists, neuroscientists, ophthalmologists and other specialists.

Background
To motivate the discussion we begin by asking, what is the problem in visual perception?This will be answered briefly.In visual perception, we have both a distal and a proximal stimulus.The distal stimulus is what the subject is looking at, usually at a distance.In the case of vision, it determines the pattern of light arriving at the cornea.The proximal stimulus hits the sense organs directly.In the case of vision, it is the pattern of light arriving at the retina, for instance as a result of looking at the distal stimulus.There are several features that distinguish the distal and proximal stimuli.The distal stimulus is 3-dimensional, independent of point of view, upright, and has no lens blur or filter.An example of the latter two is that when we look at a person their head is on top and their feet are on the bottom and the physical person does not get blurred.The proximal on the other hand is 2-dimensional, depends on point of view, inverted, blurred and filtered by the lens.So the main problem in visual perception becomes clearer; that is to retrieve information about the distal stimulus with only the proximal stimulus to work with.This is important because it affects the perceptual representation which is the endpoint of the perceptual process.Perceptual representation is the state of the visually-guided motor behavior (keeps us from bumping into things), visual pattern recognition, visual understanding, and memory.Basically, as the subject sees an object (distal stimulus), the input falls on the retina (proximal stimulus) and an output of the distal stimulus is perceived via perceptual representations.Note, that this is not the same as the distal stimulus, because there are two kinds of perception, veridical and illusory.There are many examples of visual illusions, in which the perceptual representation suggests an incorrect distal stimulus.That is, the apparent distal stimulus differs from the veridical distal stimulus.With this concept, we can now refine the problem in visual perception, as trying to understand how the visual system creates a perceptual representation of the distal stimulus with only the proximal stimulus as an input.Why is this a problem?Because the relationship of distal to proximal is not one to one, that is a distal stimulus can be seen as many proximal stimuli and proximal stimuli can be many distal stimuli.This leads to the inverse problem of trying to recover a visual representation from the input, even when many representations are consistent with the proximal stimulus.Thus, this is a motivation to begin discussing the visual pathway and understand the retinal (proximal) input to the brain.

Visual pathway
The visual pathway consists of many stages.We will focus on the ganglion cells, lateral geniculate nucleus (LGN), and the primary visual cortex (V1).The ascending visual pathway begins when light hits the back of the retina and stimulates the photoreceptors (rods and cones).These photoreceptors transform radiant energy into electrical activity, which is transmitted to retinal bipolar cells and then into retinal ganglion cells.The retina has several layers and sub-layers with corresponding cells, such as ganglion, amacrine, bipolar and horizontal.Each of these cells play a role in the visual system and have their own receptive fields.Again, in this chapter we choose to focus and discuss the ganglion cells.

Ganglion cells
There are two major classes of ganglion cells.The smaller midget, or parvo, cells comprise about 80 percent of these cells and the larger parasol, or magno, cells about 10 percent (Lennie et al., 1990).As with other cells in the retina, these ganglion cells have their own receptive fields known as center surround with either on-center (off-surround) or off-center (on-surround).There are several differences between these two types of cells.Parvo cells are dominant in the fovea as opposed to the magno cells, which are dominant in the periphery.The parvo cells are also characterized as having a sustained response while the magno have a transient response (Purpura et al., 1990;Schiller & Malpeli, 1978).At any given eccentricity, parvo cells have a higher spatial resolution, lower contrast sensitivity, slower conduction velocity, and a more sustained response than do magno cells (Shapley et al., 1981).The parvo cells have low contrast sensitivity and detect color and form, while the magno have high contrast sensitivity and detect motion.Parvo cells rarely respond well to luminance contrasts below 10%, whereas magno cells often respond to stimuli with contrasts as low as 2% (Purpura et al., 1988;Sclar et al., 1990;Shapley et al., 1981).In addition to these two, there are other types of ganglion axons that exist; the more common of these are the konio cells which are small bistratified cells (Kaas et al., 1978).They are common in the parafovea, have low contrast sensitivity, and detect color.The major difference between the konio cells and the other two is that the konio have a uniform receptive field and thus have no spatial opponency.To many investigators the term konio has become synonymous with the blue-yellow pathway, just as parvo is now equated, too simplistically, with the red-green pathway (Sincich & Horton, 2005).But this is not always the case because, konio cells constitute a heterogeneous population of cells, some lacking blue-yellow color opponency (Hendry & Reid, 2000).The axons of all these ganglion cells exit the eye, forming the optic nerve and synapse in the midbrain.Since the diameter of the optic nerve and the number of the ganglion cell axons it contains are limited by the structure of the skull, not all the information that falls upon the retina is transmitted to the brain proper (Schwartz, 2004).Although there are more than 100 million photoreceptors within the retina, there are only 1 million ganglion cells, revealing an extensive degree of neural convergence (Curcio & Allen, 1990;Osterberg, 1935).At the optic chiasm, ganglion cell fibers from the nasal retina of each eye cross over to join the temporal fibers of the fellow eye to form the optic tract (Schwartz, 2004).The long axons of the retinal ganglion cells leave the eye, form the second cranial nerve (the optic nerve), and synapse in the dorsal lateral geniculate nucleus (dLGN), a midbrain structure (Schwartz, 2004).We will now discuss the LGN.

Lateral geniculate nucleus (LGN)
The primary target of the optic tract is the dorsal lateral geniculate nucleus (dLGN), a thalamic nucleus.In higher vertebrates, such as carnivores and primates, axons from the two eyes converge onto their primary target, the dorsal lateral geniculate nucleus (dLGN), but occupy distinct regions (the eye-specific layers) within this target (Guillery, 1970;Kaas et al., 1972;Linden et al., 1981).In primates (Rakic, 1976;1977), the axonal terminals of ganglion cells of the two eyes initially share common territories within the dLGN, but through a process that eliminates inappropriately placed branches, projections from the two eyes become restricted to their appropriate layer.Most, but not all, retinal ganglion cells synapse in the six-layered structure.Layers 2, 3, and 5 receive input from the ipsilateral eye, whereas layers 1, 4, and 6 receive input from the contralateral eye, Fig. 1.The dorsal four layers, which are constituted of comparatively small neurons called parvo, or P-cells, are the parvocellular layers (layers 3,4,5,6).Larger neurons, commonly called magno or M-cells, comprise the two ventral magnocellular layers (layers 1,2).Axons from midget ganglion cells synapse on P-cells in the dLGN to form the parvo pathway, while axons from the parasol cells synapse on dLGN M-cells to form the magno pathway.The layers between the parvocellular and magnocellular layers contain very small neurons (konio cells).Studies have shown that konio cells provide the only direct geniculate input to layers 1-3 (Hendry & Yoshioka, 1994).The subcortical projection from the retina to cerebral cortex is strongly dominated by the two pathways (M and P pathways) the magnocellular and parvocellular subdivisions of the lateral geniculate nucleus (Shapley & Perry, 1986).The parvo layers receive input from color-opponent midget ganglion cells, whereas the magno layers are supplied by broadband parasol ganglion cells (Perry et al., 1984).Parvo pathway neurons show color opponency of either the red/green or blue/yellow type, which means that they respond to color change regardless of the relative luminance of the colors (Derrington & Lennie, 1984).The blue-yellow ganglion cells project to the konio layers just ventral to the third and fourth parvocellular layers (Calkins & Hendry, 1996).Layers 5 and 6 have on-center receptive fields, and layers 3 and 4 have off-center receptive fields.Layers 1 and 2 have both on-and off-center receptive fields.These projections from the retina to the LGN then lead to the visual cortex.Fig. 1.Retinal ganglion cell projections to the lateral geniculate nucleus (LGN) of the thalamus.Note that layers 1,4, and 6 of the LGN receive visual information from the contralateral retina, whereas layers 2,3, and 5 receive visual information from the ipsilateral retina.

Primary visual cortex (V1)
The cells of dLGN send most of their axons to the cerebral cortex, specifically, the primary visual cortex (V1) along with the visual field representation in the retina and primary cortex.Inputs to V1, which are stratified by magno, parvo, and konio, become thoroughly intermingled by passage through the elaborate circuitry of V1 (Sincich & Horton, 2005).There are about 8 or 9 layers in V1.Layer 4 consists of three sublayers, 4A, 4B, and 4C.Layer 4C also is subdivided into 4Cα, and 4Cβ.The projections from the LGN go specifically to layer 4C and the information flows up and down from there (Merigan & Maunsell, 1993).The projections from parvocellular layers terminate primarily in layers 4A and 4Cβ, whereas those from magnocellular geniculate terminate in layer 4Cα (Fitzpatrick et al., 1985).Layer 4B receives direct input from 4Cα (M pathway), but not 4Cβ (P pathway) (Lund & Boothe, 1975;Lund et al., 1979).Layer 4Cβ projects to the blobs and interblobs (Horton & Hubel, 1981;Humphrey & Hendrickson, 1980).The blobs also receive major inputs from the M pathway by way of layers 4B and 4Cα (Blasdel et al., 1985;Fitzpatrick et al., 1985;Lachica et al., 1992;Lund, 1988).Fig. 2 gives the details of these connections.More recently, Yazar et al. (2004) have found that some geniculate fibers terminate in both layers 4Cβ and 4A, implying either a direct parvo input to 4A or a konio input to 4Cβ.In layer 3B the cells in blobs and interblobs receive input from parvo (4Cβ), magno (4Cα), konio (4A), or mixed (4B) layers, in a range of relative synaptic strengths (Sawatari & Callaway, 2000).Cells in both 4Cα and 4Cβ project to layers 5 and 6 (Callaway & Wiser, 1996;Lund & Boothe, 1975).Feedback from layer 6 to the LGN is segregated only partially with respect to magno  and parvo, thus mixing the geniculate channels (Fitzpatrick et al., 1994).There are two main types of cells in V1, stellate and pyramidal.The stellate cells are small interneurons found in layers 2-6 and the pyramidal cells are large relay neurons found in layers 2, 3, 5, and 6.The stellate cells are simple cells because of their receptive fields.The pyramidal cells are complex cells.The simple cells' receptive fields are of a certain size, are oriented in a certain way, and are sensitive to phase.They increase their rate of firing when stimulated in some places, and reduce it when stimulated in other places.The simple cells respond to a single spot of light and are additive and linear.The complex cells do not respond to a single spot of light, rather they respond to edges and bars, and are not sensitive to spatial phase.Many of the complex cells respond best to stimuli that move in one direction.So, if the stimulus is stationary, in the opposite direction, or a spot of light then the complex cells' receptive field will have no response.The complex cells are non-additive and are non-linear.Both the simple and complex cells respond to most proximal stimuli.All together, these cortical cells are tuned for spatial frequency, position, and orientation.This distinction is important in designing visual stimuli for fMRI studies to understand normal and abnormal visual function.

Development of the ascending pathway
We now describe how the visual pathway develops and the effects of abnormal development.During development anatomical projection patterns are restructured and functional reorganization takes place (Campbell & Shatz, 1992;Hubel & Wiesel, 1977;Shatz & Kirkwood, 1984;Wiesel, 1982).There are at least two ways by which neurons can be wired up accurately: connections may be specified from the outset, or synapse formation may initially follow an approximate wiring diagram, with precision achieved by the elimination of inappropriate inputs and the stabilization and growth of appropriate connections (Goodman & Shatz, 1993;Purves & Lichtman, 1985).The ganglion cells, LGN, and V1 are all wired up in a "retinotopic" fashion; meaning that the order of points on the retina (proximal stimulus) are preserved.In this mapping, the points that are further away from each other on the retina will be further away on the brain.It is easy to see that the proximal image is retinotopically related to the distal stimulus, simply because of the optics of the eye.However the retinotopic mapping from the retina to the LGN and from the LGN to V1 is harder to appreciate.Studies of patients with localized cortical damage showed that the receptive fields of neurons within area V1 are retinotopically organized (Holmes, 1918;1944;Horton & Hoyt, 1991).As a matter of fact, the development of the retinotopic map is a general process for the central nervous system.Cell bodies are born early in embryogenisis; axons and dendrites come later.The nerve growth is then guided mechanically, probably by glial cells, to their overall destination.The patterns of activity of the neurons themselves determine the exact position of the synapses that are formed.Ganglion cells travel up the concentration gradient to the LGN.Target cells send guiding chemical messages, giving crude directions to the cells' overall destination by their concentration gradient.These chemical signposts act like beacons that attract the cells to project to approximately the correct part of the target tissue.At the same time the chemical signposts repel growth cones from the wrong axons.These guidance molecules also govern the decussation at the optic chiasm by signaling the retinal ganglion cells to either cross or not to to cross.The activity of adjacent retinal ganglion cells is correlated (Galli & Maffei, 1988), and "waves" of activity sweep across the retina during early life (Meister et al., 1991).Although the waves could potentially underlie the refinement of many retinal projection patterns, activity may not be required for establishing the M and P pathways of the primate retina that develop prenatally, and which show no apparent gross structural refinement with ensuing development (Meissirel et al., 1997).The immature and light-insensitive retina spontaneously generates a pattern of rhythmic bursting activity during the period when the connectivity patterns of retinal ganglion cells are shaped (Wong, 1999.).After the cells find a region, the wave then enforces precise ordering at the target.Thus the retinotopic map is finalized via the wave.Prenatal refinement of the retinotopic projections is achieved by these spontaneous waves of activation that propagate across the retina.Here ganglion cells are linked together by means of electrical synapses in a rough network and charge fluctuates randomly.The random response of one cell starts a wave of activity and the cells that fire together will eventually wire together.These spontaneous waves cause neighboring retinal regions to fire at about the same time.In fact, the correlation between the responses of cells is directly related to their separation on the retina (Wong, 1999.).So, the first principle of refinement is that cells that are neighbors tend to respond together.The second principle of refinement is that cells that fire together wire together.If there are two cells, 1 and 2, that are close to each other on the retina then when they fire together they will form neighboring synapses at the LGN.But cell 3, which is far from the first two on the retina will fire separately and thus synapse at the LGN separately.This is how the LGN is retinotopically wired up at birth along with V1 and other retinotopic cortical areas.Hence, the waves in the prenatal retina setup the relation between retina and brain.As for the postnatal retina, responses to stimuli set up the relation between the proximal stimulus and the brain.The postnatal wave may help guide the formation of synapses and determine which erroneous synapses are cut out for the normal mapping.When they arrive at their destinations, each process synapses over a relatively large area.Since target cells have lots of cells synapsing onto them, there are a lot more synapses present in V1 at 6 months and 1 year than in an adult.The process of the synapse starts as each axon from different cell bodies tries to take over a large piece of visual cortex and inevitably overlap occurs.At these regions of overlap a competition occurs, and the cell with the most or strongest synapse claims that region and the other synapses pull back.This synaptic elimination is a key element in the refinement of connectivity in both the central and peripheral nervous systems (Cowan et al., 1984;Goodman & Shatz, 1993;Lichtman et al., 1999;Nguyen & Lichtman, 1996;Purves & Lichtman, 1985).This produces a retinotopic map that has less overlap than before, and has many fewer synapses.If there is a vacant area then other nearby cells synapse onto it without meeting any competition and in turn increase their synaptic field.This process of being able to change as a result of experience is called plasticity, and is required for normal development.It determines how the visual system is wired up during normal development.The synaptic development occurs at different time scales across the brain.For V1 the development ends from about 8 to 16 years and culling happens at about 1-2 years.If there is any difficulty or blur in one eye or an eye turn while these synapses are being formed and refined, the subject will develop a visual disorder.This leads us into the next section.

Disorders of the ascending pathway
We will now discuss several visual disorders associated with the ascending pathway before reviewing fMRI research in vision science.The disorders are: rod monochromacy, albinism, refractive amblyopia, and strabismic amblyopia.

Rod monochromat
Rod monochromat, also known as complete achromatopsia, is an autosomal recessive disease.The rod vision is normal but cone vision is completely absent, meaning there is no fovea.In a normal subject, the fovea is what projects to V1, so what happens to the foveal representation in this case?In a rod monochromat, the visual acuity is about 20/200 and the spectral sensitivity is that of rhodopsin, meaning there are big losses in the red compared to normal.As a result of not having cones, there is no color vision and the patient has photophobia and severe visual impairment due to glare.The fovea is grossly abnormal with no reflex and may have a few cones which may contain rhodopsin.As a result of this abnormality, pendular nystagmus forms.With respect to the ascending pathway, the vacant space of the part of V1 that normally receives signals from the fovea is occupied during the synaptic development stage by synapses originating in the parafoveal retina of the achromat.

Albinism
Albinism is characterized by a systematic misrouting of the connections between the retina and the visual cortex.The ascending projection in an albino is almost entirely crossed.Note the normal projection that is crossed is about 55%.This miswiring can produce nystagmus and strabismus.The clinical features of albinism include hypopigmentation of the fundus, and iris.There are variable degrees of pigmentation of the iris, hair, skin.Tyrosinase negative albino (oculocutaneous) individuals may be completely white with a visual acuity range from 20/60 -20/400, but is usually worst than 20/200.Tyrosinase positive albino may look hypopigmented or even essentially normal with visual acuity range from 20/60 -20/400, but is usually better than 20/200.More clinical features related to the eye include a very light fundus because there is no melanin in the retinal pigment epithelium (RPE).There is little differentiation of the fovea from the surrounding retina.Albinos also have high myopia or high hyperopia.In the albino system there is more than 90% decussation at the optic chiasm.This means that the guidance molecules during development failed to stop the neurons from going the opposite direction.For a better understanding of the ascending pathway abnormalities in albinos we will do a comparison with normals.If a distal stimulus is presented on the right hand side of a normal subject then the expected pathway from the right eye nasal retina would cross the optic chiasm and end up in the contralateral visual cortex (left visual cortex).For the same stimulus on an albino subject, the resulting signal would be the same as the normal.If the distal stimulus is changed to the left hand side for the normal, and looking at the right eye temporal retina, then the signal would not cross the optic chiasm and would end up in ipsilateral visual cortex (right visual cortex).The same repeated for the albino reveals the opposite since the majority of the neurons cross the optic chiasm and end up in the contralateral visual cortex again.The primary lesion in albinism is a genetically determined lack of melanin or melanosomes as mentioned earlier.As a side point, melanin is very important for many aspects of neurological development.For instance, the neural crests pigment and its location on the embryo is determined by melanin.Melanin is also involved in production of dopamine and serotonin and many other neurotransmitters related to neuroendocrine function.
Current Trends of fMRI in Vision Science: A Review www.intechopen.com

Refractive amblyopia
Refractive/deprivation amblyopia is a result of the receptive fields not being used early in life.Thus, the culling at about 1 year postnatal removes their synaptic connections because lack of function.Specifically, the proximal stimulus is blurred during the critical period, meaning the high spatial frequencies are reduced or eliminated from the visual image, causing high spatial frequency tuned channels to either never develop, or be lost.In this case, the low spatial frequencies pass unattenuated, so the low spatial frequency tuned channels develop normally.The effect of this blur in refractive amblyopia is the direct loss of contrast sensitivity at high spatial frequencies, which is equivalent to a loss of visual resolution acuity.As for the remaining spatial frequency channels, they stay relatively normal because they are stimulated normally during the critical period.This illustrates the principle that the receptive fields must be used if they are to be maintained.If the proximal stimuli do not stimulate the receptive fields effectively, the cells tend to stop responding to the intended stimulus even if it is presented occasionally.The cell may begin to respond to other stimuli, and therefore develop a new receptive field.The input from the other eye is likely to grab the synapse area because of competition.As a result there is anisometropia, an unequal refractive error in the two eyes.Thus, the eye with the larger refractive error continues to experience chronic blur.Dominance of the good eye becomes exaggerated during development, because of competition between incoming signals.Most cells in the primary visual cortex come to have predominant input from the good eye.If one eye is handicapped during the competition, it tends to lose its synaptic connections.Thus, the development of ocular dominance columns in amblyopia is distorted, and depends on the age at which deprivation begins.The most dangerous periods of refractive amblyopia are in the first 6 months.

Strabismic amblyopia
The cells in the ascending pathway are labeled lines.Labels relate to position on the retina and therefore position in the proximal stimulus.Labels also relate to spatial frequency and orientation.Labeled lines are important because the brain only knows what the ascending pathway tells it.If the labels are abnormal, vision is also abnormal.In strabismic amblyopia, the lines are mislabeled, which leads to distorted vision.In normal retinotopic organization, labels relate position in the distal stimulus to position upon the retina.Strabismic amblyopia is thought to be due to disordered (scrambled) retinotopic mapping between the LGN and V1 of the signals from one eye; therefore, leading to abnormal visual experience.The waves that happen after birth are not normal because the eye is not always pointing in the right direction.Recall that cells fire together after birth because of the wave of activity produced by the usual retinal stimulus.This postnatal wave may help guide the formation of synapses and determines which erroneous synapses are cut out for the normal mapping.This eye turn in early childhood produces an abnormal wave.The connection between the retina and the LGN remains normal because it is wired up prenatally, but the connection between the LGN and V1 is not.When cortical cells fire together abnormally they wire together abnormally.Clinical consequences of this disorder at the primary visual cortex are impaired visual recognition, crowding (nearby stimulus information obscures attended item), poor vernier acuity, poor stereo acuity, poor grating orientation identification acuity, and often near normal grating resolution acuity.The high spatial frequency gratings do not look like uniform gray, so they can be detected, but they are badly distorted, so the amblyope cannot discriminate between vertical and horizontal.

fMRI and oculomotor function
FMRI studies of the oculomotor function have been mostly limited to normal subjects and have concentrated on voluntary pursuit, saccadic eye movements and optokinetic nystagmus (OKN).Table 1 summarizes the details of these studies, imaging parameters and visual Current Trends of fMRI in Vision Science: A Review www.intechopen.comstimuli.Tanabe et al. (2002) have noted that fMRI studies of oculomotor function have employed few subjects and the reliability of mapping-out brain sites involved in oculomotor control have not been established.This statement was made almost 10 years ago and a lot has been accomplished since then.Overall, there appears to be two parallel cortical oculomotor systems for pursuit and saccadic eye movements.Both pursuit and saccadic eye movements appear to activate the same cortical areas including the frontal eye fields (FEF, precentral cortex), supplementary eye fields (SEF, superior frontal cortex), parietal eye fields (PEF, intraparietal cortex), precuneus, and MT/V5.However, pursuit or saccadic eye movements may selectively activate subregions of these cortical areas.Petit & Haxby (1999) found that the pursuit related activation areas were usually smaller than and consistently inferior to and/or posterior to the saccadic related activation areas.Dieterich et al. (2000) have shown that small field horizontal OKN as well as voluntary saccadic eye movements activate areas of both cerebellar hemispheres including the superior semilunar lobule, simple lobule, quadrangular lobule and inferior semilunar lobule.In addition, activation was found in the middle cerebellar peduncle, dentate nucleus, culmen (medially), and uvula of the cerebellar nuclei.Fixation during OKN suppressed activation in the uvula and culmen.Dieterich et al. (1998) also found OKN to activate subcortical areas including the caudate nucleus, putamen, globus pallidus and paramedium thalamus.Fixation increased activity in the FEF and anterior cingulate gyrus.(Dieterich et al., 2000) used a rotating drum that contained "colored figures" to stimulate OKN amplitude that ranged from 2 − 13 o visual angle, suggesting a mixture of voluntary and involuntary OKN or only voluntary OKN.Most recently it has been shown that voluntary OKN generates more cortical activation than does involuntary OKN (Kashou et al., 2006;2010;Konen et al., 2005).Specifically, Kashou et al. (2010) showed that activation sites for OKN studies are dependent on subject instruction which influence the type of OKN generated.Bense et al. (2006a) found that there was no direction dependent activation in cortical eye fields, but there was asymmetry in the paramedian visual cortex areas.Also they found stronger activation in the hemisphere contralateral to slow OKN phase (pursuit).Bense et al. (2006b) found cerebellar activation was localized in the oculomotor vermis.In a comparison of gratings versus dots to stimulate an optokinetic response, the gratings evoked more activation in FEF, PEF, MT/V5 and the cerebellar area VI (Schraa-Tam et al., 2008).Saccades in humans have been found to activate the precentral sulcus in FEF and in the precuneus along the intraparietal sulcus (IPS), extending in both superior and inferior parietal lobules (Luna et al., 1998).Saccades are traditionally divided into "reflexive" and "voluntary" saccade.Mort et al. (2003), demonstrated that voluntary saccades produced greater activation within FEF and the saccade related area of IPS.In an oculomotor study on oscillatory, predictable and unpredictable saccade, Konen et al. (2004) showed that predictable saccades with the shortest saccadic latency led to the most pronounced cerebral activity both in terms of cortical areas involved and signal intensity.The activation of FEF has also been found to be correlated with saccade reaction time (Connolly et al., 2005).Saccades are also distinguished as either pro or anti if they are made toward or away a stimulus respectively.Cornelissen et al. (2002) found similar BOLD activation in FEF during both pro-and antisaccades.It was suggested in a study looking at functional interactions between pro-and antisaccades that the presupplementary motor area (pre-SMA) coordinates with the FEF to maintain a controlled, preparatory set for task appropriate oculomotor execution (Miller et al., 2005).Saccade frequency and amplitude was varied (Kimmig et al., 2001) and high correlation between frequency and BOLD signal was found along with higher BOLD activation in antisaccades over prosaccades.Merriam et al. (2001) found that comparison of visually guided saccades with fixation revealed activation in all three cortical eye fields: SEF, FEF, and PEF.In addition, the cerebellar vermis (declive and folium) and the bilateral cerebellar hemispheres (superior semilunar lobule) were associated with visually guided saccades (Hayakawa et al., 2002).In differentiating saccade inhibition from generation, the right supramarginal gyrus was responsible for inhibition and the right lateral FEF and bilateral intraparietal sulcus were responsible for antisaccade generation (Ettinger et al., 2008).Unlike pro-and anti-, corrective saccades may also occur, specifically during saccades, pursuit and fixation.This eye movement activated the anterior inferior cingulate, bilateral middle and inferior frontal gyri, bilateral insula and cerebellar areas (Haller et al., 2008).FEF activation during smooth pursuit performance was found to be smaller than during saccades (Petit et al., 1997).The performance of pursuit eye movements induced activations in the cortical eye fields also activated during the execution of visually guided saccadic eye movements, namely in the precentral cortex [FEF], the medial superior frontal cortex [SEF], the intraparietal cortex [PEF], and the precuneus, and at the junction of occipital and temporal cortex (MT/MST) cortex (Petit & Haxby, 1999).Rosano et al. (2002) localized the saccade-related area to the upper portion of the anterior wall of the precentral sulcus and the pursuit-related area to a deeper region along the anterior wall, extending in some subjects to the fundus or deep posterior wall.It was suggested that the lateral occipitotemporal cortex has extraretinal signals during pursuit (Barton et al., 1996).Significant activation in V1 and V2 in both hemispheres as well as additional bilateral activation in the lateral extent of Brodmann's area 19 and 37 (BA 19/37) was evident during smooth pursuit (Freitag et al., 1998).Pursuit performance, relative to visual fixation, elicited activation in three areas known to contribute to eye movements in humans and in nonhuman primates: the frontal eye field, supplementary eye field, and intraparietal sulcus.It also activated three medial regions 13 Current Trends of fMRI in Vision Science: A Review www.intechopen.comnot previously identified in human neuroimaging studies of pursuit: the precuneus and the anterior and posterior cingulate cortices.All six areas were also activated during saccades (Berman et al., 1999).Tanabe et al. (2002) found activation consistently in dorsal cortical eye fields and cerebellum.Many studies are still being pursued on normal eye movements with hopes of mapping out or isolating specific anatomical areas responsible with the goal of future diagnostic and therapeutic interventions.
Before moving on to visual dysfunction we want to briefly mention a few visual perception studies.Goodyear & Menon (2001) were the first to demonstrate reproducible high resolution (0.55 mm x 0.55 mm) capabilities of fMRI in humans when using short duration (<6 sec) visual stimuli.Mullen et al. (2010) studied how the responses of the visual pathway to temporal frequency are modified as signals are transfered between the LGN and V1 to the dorsal and ventral streams (V2, V3, VP, V3A, VA, and MT).They concluded that the dorsal and ventral pathways develop characteristic differences in temporal processing that affect chromatic and achromatic stimuli.Differentiation between the magnocellular and parvocellular visual pathways has been recently demonstrated (Liu et al., 2006).Conner et al. (2004) compared retinotopic maps of children with adults in hopes that the study would be useful reference for studies of children with visual disorder, such as amblyopia.Retinotopic mapping is of importance in understanding visual field; a step by step study on this process has been summarized (Warnking et al., 2002).Studying the effects of age showed that the volume and degree of fMRI activation decreased with increasing age, particularly over the age of 40 years (Lewis et al., 2003;2004).

fMRI and visual dysfunction
fMRI studies have been undertaken in normal subjects and in patients with amblyopia, commonly known as lazy-eye (Algaze et al., 2002;2005;Goodyear et al., 2000;Leguire et al., 2004a;b;2011a;Lewis et al., 2003;2004;Rogers, 2003).Goodyear et al. (2000) showed that there were always fewer activated fMRI voxels during amblyopic stimulation than during normal eye stimulation.Algaze et al. (2002) also showed that the volume and level of occipital visual cortical activation was less from the amblyopic eye compared to the dominant eye of amblyopes or to normal eyes.Rogers (2003) and Algaze et al. (2005) have shown that L-dopa, a drug used in the treatment of Parkinson's disease, caused a reduction in volume of activation of occipital visual cortex while it improved visual acuity -a counterintuitive finding.(Yang et al., 2003) showed that the volume ratio between the amblyopic and sound eye stimulation significantly increased after L-dopa treatment.More recently, the amblyopic eye showed marked reduction in activation in the fusiform gyrus, with normal activation in the collateral sulcus (Lerner et al., 2006).Responses to grating stimuli showed reduced responses in higher areas on the central visual pathway (Muckli et al., 2006).
In albinism, there is an abnormal chiasmic projection system which favors the contralateral hemisphere (Schmitz et al., 2004).For example, in oculocutaneous albinism and in ocular albinism, monocular stimulation yields a greater fMRI reponse in the contralateral hemisphere than the ipsilateral hemisphere because of misrouting of the eye's afferents favoring the contralateral hemisphere.After using standard fMRI statistical analysis tools, the number of voxels activated in each hemisphere were counted for each subject.A crossing ratio was then computed by subtracting the voxels activated contralaterally from the ipsilateral ones and dividing by the total number activated.The mean of these ratios for left and right eyes were then calculated for correlations.

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Functional Magnetic Resonance Imaging -Advanced Neuroimaging Applications www.intechopen.com Reduced signal and greater asymmetry in the visual cortex has been shown in optic neuritis (ON) patients, compared with controls (Langkilde et al., 2002).They also showed that the volume of visual cortical activation was significantly correlated to the result of the contrast sensitivity test.They used an asymmetry index I a to calculate the relative difference between size of activated area in the left and right hemisphere, in a similar fashion to the above study.This was done by simply counting the number of voxels in each hemisphere and taking the absolute value of the difference and dividing by the total number of voxels in both hemispheres.A value of I a = 1 meant 100% asymmetry while a value of I a = 0 meant no asymmetry.Toosy et al. (2002) showed that visual cortex activation is reduced during photic stimulation, whilst extra-occipital areas are extensively activated with a peak blood oxygen level dependent response during the OFF phase of the stimulus paradigm.More recently they suggested a genuine adaptive role for cortical reorganization within extrastriate visual areas early after optic neuritis (Toosy et al., 2005).Reduced activation was seen in V1 during stimulation of the affected eye, compared to the normal eye (Levin et al., 2006).Parents of children with autism or Asperger Syndrome (AS) showed atypical brain function during both visual search and emotion recognition (Baron-Cohen et al., 2006).Hadjikhani et al. (2004a) found that retinotopic maps of individuals with autism were similar to normal subjects, indicating that low level visual processing is normal.A case study by Sunness et al. (2004) illustrated that retinotopic mapping can be performed successfully in patients with central scotomas from macular disease.An increase in the activation of the prefrontal cortex and intraparietal sucli and decrease in the visual cortex was reported in patients with macular degeneration (Little et al., 2008).The ability to look at anatomical reorganization of the visual cortex was demonstrated in a case of callosal agenesis and colpocephaly (Bittar et al., 2000), and in alteration by vascular lesions (Schlosser et al., 1997).Analyzing oculomotor recovery from ischemic lesions in frontal and parietal eye fields using visually triggered saccades has been recently implemented (Nyffeler et al., 2011).
In an eye blink inhibition study, patients with Tourette syndrome showed higher activation in the middle frontal gyrus, dorsal anterior cingulate and temporal cortices compared to controls (Mazzone et al., 2010).Most recently the declive of the cerebellum has been shown to be associated with INS (Leguire et al., 2011b).Similarly the cerbellar vermis, also has been found to be active in patients with bipolar disorder while performing SPEM (Martin et al., 2011).fMRI activation during downward smooth pursuit was less in both flocculi of the cerebellum for patients with DBN than controls (Kalla et al., 2006).Reduced activation in the paraflocular lobule and in the ponto-medullary brainstem of the patients was also seen (Hüfner et al., 2007).Saccadic oscillations in patients with opsoclonus may be a result of disinhibition of the cerebellar fastigial nuclei (Helmchen et al., 2003a;b).Monitoring vision therapy using fMRI for patients with CI revealed increase in activity in the frontal areas, cerebellum and brainstem (Alvarez et al., 2010).Understanding SPEM is also of interest in schizophrenia where greater activity in both posterior hippocampi and the right fusiform gyrus have been reported (Tregellas et al., 2004).The same investigators also found that nicotine was associated with greater activity in the anterior and posterior cingulate gyri, precuneus and area MT/MST and less activity in the hippocampus and parietal eye fields in patients with schizophrenia (Tregellas et al., 2005).Data from Hadjikhani et al. (2001) suggested that an electrophysiological event such as cortical spreading depression (CSD) generates migraine aura in the visual cortex.This was determined using a standard t statistic computing the difference between activation amplitude during off period preceding aura.The time courses for independent voxels were then extracted from specific visual areas.A reference baseline (mean) and standard deviation was computed on the first 6 cycles and the pixels that exhibited a higher mean plus standard deviation and a standard deviation less than the reference standard deviation for at least 2 cycles were considered as activated.The visual cortex of patients with idiopathic Parkinsons disease with and without visual hallucinations were examined by Holroyd & Wooten (2006).They found that patients with visual hallucinations had increased activation in the visual association cortex and deficits in the primary visual cortex.Again these are samples of the fMRI studies published in literature.

Discussion
In this chapter we aimed to discuss the basics of visual development and then review fMRI vision science research.To recap, there are three main principles in visual development: labeled lines, cells firing together wire together, and synaptic competition.In summary, sensory cells send the same kind of signal, regardless of how, or how strongly, they are stimulates (labeled lines).The relations between the retina and the LGN, and between the LGN and the cortex, are crudely wired up at birth, by prenatal "visual" experience of the wave.That wire up is refined and related to the proximal stimulus by genuine postnatal visual experience and synaptic competition.This refinement includes creation of new synapses and culling of old ones.Abnormalities early in life can cause disorders in the visual pathway.Rod monochromats do not have the normal photoreceptor connections from the retina and thus the rods take over the synaptic fields where the fovea usually falls in V1.Albinos seem to have a dysfunction in the chemical signposts that separate the nasal and temporal retina projections.In refractive amblyopia, there is a blur in the proximal stimuli of one eye and high frequency cells are not fully developed in V1 because they are cut out during the refinement process.Strabismic amblyopes suffer from an eye turn early on that causes an abnormal wave which leads to miswiring between the LGN and V1.The use of functional MRI has proved to be a successful imaging modality in understanding the visual development process and for basic research in vision science of controls and patients.Currently, neuroscientists, neurologists, ophthalmologists and others are using this imaging modality extensively to study vision science related problems.Further development of these studies will allow noninvasive diagnostic, pre-, and post-surgical techniques with the aim of improving the clinical sensitivity and specificity for visual cortex diagnosis.

Limitations of fMRI interpretation
The key to interpreting fMRI data is to understand the problem being studied.In this chapter some applications from vision science were discussed to show the extensiveness of the field.
The more one knows about vision science in general the better they will be able to make an informed interpretation of the fMRI activation.However it is essential to have this knowledge before designing an fMRI study.It is also necessary to have firm knowledge of the MR technology and physics in order to appreciate the complexities and intricacies of the process.This in turn would help minimize errors and confounds in the results.The main limitations of interpretation lies in the knowledge of the user.Unfortunately, some believe that this is a pushbutton technology and whatever comes out is perfect.On the contrary, a good understanding of the field, in this case vision science, the technology, and the art of designing an fMRI experiment, will allow for respect and caution when interpreting and analyzing the

Future developments in fMRI
The advancement in technology will have the biggest influence on the future developments in fMRI.Most of the studies presented here were on 1.5 or 3 Tesla systems but ultra high field (UHF) 7 and 8 Tesla systems are now regulary being used for human research.The limiting factor for UHF MRI are the head coils, however continuous effort is being made for optimization and improvement.In the next few years 1.5 and even 3 Tesla systems will seem old in the field of research as the new UHF magnets have superior resolution (down to the μm).This will enhance the visualization of cortical areas and allow the parcellation of smaller anatomical regions such as the LGN and allow the functional localization of subregions that otherwise would be bulked into one region in the current scanners.Clinical imaging developments in the short term are focusing on enhancing the 3 Tesla technology by transition from 8 channel head coils to 32 channels so there will be a delay before the UHF systems make their way into hospitals.

Fig. 2 .
Fig. 2. Block diagram of ganglion cell mapping from retina through LGN, V1, and other cortical areas.

Table 1 .
Specifications of fMRI studies performed on normal eye movements.

Table 2 .
Table 2 lists a few pathologies related to vision investigated using fMRI.Pathologies investigated using fMRI.