Chapter IV
VISUAL PROCESSING OF IN THREE-DIMENSIONAL ATTENTIONAL SYSTEM
In this chapter we will combine studies on the processing in the brain of the right versus left hemifield, the upper versus lower visual field processing, and the far versus near vision. Studies of the differences in the working of the right brain by Gazzinga, and many others. The provoking studies of Previc on the differences of the processing of the upper versus lower visual fields and in far and near space. I will quote the interesting remark of Mennemeier, Wertman and Heilman:
"Our finding compliment other studies that demonstrate neglect may occur in multiple spatial dimensions and provide evidence for a three-dimensional attentional system in humans".
The three dimensional attentional system is orientated dominantly for:
Figure 1. The processing of the three-dimensional space of the world around us
1. Evolutionary necessity of a duplication neuronal systems on each side of the body
As multicellular organisms discovered the advantage of moving through space presumably at first along a substrate, they inevitably encountered the problem of attractions and threats that lay to either side of the direction of motion. Turning this became desirable and laterality came into being, with receptors to detect what lay to the right or left and muscular contractility and disinhibition to provide appropriate steering. As a consequence duplicate neuronal systems arose on each side of the body.
The brain is split up in to two halves.
What is the nature of the exchange between the hemispheres? By what process is the decision achieved as to which hemisphere will lead at any given moment i.e. that is in charge. Especially the visual system is split in half. The amalgamation of the two visual inputs is not processed in the cortical area, but in the adjacent cortex which specialize by having rich and callosal interconnections (200 million nerves). They are exceedingly slow: 25 msec to transmit a one way message across the human callosum sometimes up to 100 msec. Repeated back and forth exchange is not possible with these slow transmissions. Therefore each hemisphere must achieve most of the multistage synaptic arguments; as a consequence each has to develop its own modus operandi.
2. Differences between left and right hemisphere
Figure 2. A design made by Jan Ebeltjes of his intuitive feeling that in our society the emotional right brain is destroyed by the overdominant left brain. The discrepancy between emotion and ratio.
a. Dominance of the hemispheres
Dominance of the right hemisphere:
Perception and identification of environmental and nonverbal sounds
The analyses of geometric and visual space
Stereognosis
The maintenance of the body image
The production of REM sleep during dreaming
Perception of aspects of musical stimuli
Good in perception of illusory contours.
.
Dominance of left hemisphere:
Expressive and receptive linguistic functioning grammar, syntax
Reading, writing, speaking
Verbal comprehension and verbal memory
Problem solving
Smart about its search for strategies
Perception of meaningful actions
Looking for explanations.
Difference between two hemispheres is their style of working:
The left hemisphere has sparser short-range connections, the right hemisphere has richly and widely connections.
The left hemisphere is dominant for learned movements. Patients with left hemisphere damage are unable to use everyday objects in normal way. The brain areas that are processing responses for these movements are dorsal lateral frontal and parietal cortices, striatum, thalamus and white matter fasciles (Rushworth,et.al 1997).
Difference between two hemispheres is their style of working (Van Kleeck, 1989).
b. Specific characteristics of the hemispheres in attention
The right hemisphere has attention for both visual hemifields, whereas the left hemisphere is only interested in the right visual hemifield. Right hemisphere is modulating left and right visual field. Left hemisphere more exclusively right side. Some parietal regions and their corresponding thalamic nuclei are larger in the right side of the brain (Mangun et.al., 1994).
The left hemisphere is more specialized for local attention and the right hemisphere for more global processing.
Left hemisphere is more interested in space close to the body especially when the objects are nearing us whereas the right hemisphere is more directed towards space farther away and object moving away from us. The brain has a special mechanism that objects moving away from us are apparently moving slower away from us so that we are able to watch them more carefully.
Figure 3. Left hemisphere is only interested in right hemifield whereas the right hemisphere is interested towards both hemifields (After, Weintraub, 1989).
This has been proven by experiments. When we look at a stick of 2 meters long which is hold by a person in a horizontal position in a direction away from us. When we look to this stick when it is left from us we activate the right hemisphere we will have the tendency to see the end of the stick nears to us going away from us, whereas the far end toward us. When we look at the stick when it is positioned right from us it is we have the opposite experience we want to the closest end toward us and the far end away. This means that looking to the left the stick appears shorter whereas to the right larger. If we stick our arms in from us stretch our arms in front of us the left arm will appear to be shorter than the right. The attention for left we want it closer to us and attention to the right of us will be more attend to the space away from us. Right-hand people turn their head to the right to solve verbal problem (left hemisphere) is more activated, when solving numeric and spatial problem we are turning are head to the left (right hemisphere).
Figure 4. Radial lines looks shorter at our left side than on the right side (After, Heilman et.al., 1995).
c. Naming of objects
Left hemisphere acts as an interpreter. The right hemisphere has not such a system. Michael Gazzinga (1992) has demonstrated this well with the following experiment.
The person two pictures one exclusively to the left hemisphere a chicken claw and to the right hemisphere a snow scene. He was then asked to choose from a number of pictures, which were associated with the two pictures shown either to the right and the left hemisphere. He chooses the shovel by his left hand and the chicken with his right hand. His explanation was "The chicken claw goes with the chicken and you need a shovel to clean out the chicken shed". The left brain interpreted the response in consistency with the knowledge of the relation chicken and claw and he did not include the information from the right about right hemifield snow scene. The left brain is acting as an interpreter, the right brain do not have such a system.
Figure 5. The talking brain of a split brain patient cannot interpret the emotional sensations of the right brain (Gazzaniga, 1998. Design of John W.Karapelou)
Another experiment shows the emotional role of the right hemisphere. A patient was shown only to his right hemisphere a film showing one person throwing another into the fire. She could not interpret actually what she saw but felt kind of scared and jumpy. She did not like the room or maybe she got nervous of her doctor. She whispered to some one else in the room I know I like Dr Gazzinga, but right now I’m scared of him for some reason. The emotional content of the stimulus has crossed over from right to the left hemisphere. The left hemisphere was unaware of the context that produced the emotion, but anyhow the left brain want to give an interpretation of the emotional feelings. Even if the right and left hemisphere of the cortex are cut emotions can be give from one hemisphere to the another by the connections between the subcortical structures.
The symbolic content of a stimulus is more processed in left hemisphere also neglect in reading written text. Left cerebral lesions want to hyper compensates the defect whereas right no showing this tendency.
d. Facial expression
Neural pathways that control voluntary and spontaneous facial expressions are different.
Spontaneous facial expressions are generates by both hemispheres, whereas voluntary expression only by the left hemisphere. Its sends it messages not only to enervate the lower facial muscles but also sends the commands via the corpus callosum to the right brain. Different neural pathways manage spontaneous facial expression. Each hemisphere sends signals straight down to parts of the brain that are responsible for enervating the facial muscles. A patient with a lesion in right part of the brain can not receive the input of the signals for generating a voluntary facial expression. The left part of the face controlled by the right hemisphere is unable to move the lower left part. But is capable of smiling spontaneously since those pathways are unaffected by the right brain damage. A Parkinson patient has a damage to the partway for a spontaneous smile but those for the voluntary expressions do work. They lose their masked face appearance when told to smile. (Gazzaniga, 1992)
Figure 6. Neural pathways that control voluntary (A) and spontaneous facial expressions( B).
C. A callosum sectioned patient (split brain) attempts to make a voluntary command. As the left hemisphere is processing this expression no command can be given via the corpus callosum to the right brain. Therefore the patient can not make a voluntary facial expression. (after Gazzaniga and Smylie, 1992)
e. Recognition of faces
Modular organization of processing of visual information concerns with the head within the upper bank of the superior temporal sulcus. There are cells selectively responsive to a particular view of the head and body. They are grouped in patches and separated by cell responsive to other stimuli.
There is an extensive connection between the dorsal and ventral systems. (Harries and Perret, 1991)
A patient could recognize faces from photos, cartoons etc when the faces were upright but as soon when the photo or the face was inverted he failed to recognize it. Face recognition depends on two different systems, one for recognizing the whole and one for the parts. When the face stimulus does not satisfy the domain- specific conditions. There are three interrelated components dedicated to face recognition;
1. A region in the occipital-temporal sulcus that is sensitive to facial features particularly the eyes
2. A region in the right lateral fusiform gyrus that is sensitive to configurational (second-order relational) features
3. Anterior region on the border of the fusiform and parahippocampal gyri on the right that is sensitive to semantic/physionomic aspects of facial identity and the on the left that is sensitive to names. The proximity of these various modules to one another explains why damage to this region typically leads to deficits in all aspects of higher order visual patterns. The patient described has a deficit in a special region (Mosvovitch et.al., 1997)
Perception of faces is associated with neural activity in the inferior part of the occipitotemporal cortex. Region activated selectively during gender categorization was found posteriorly in the inferior part of the occipital cortex. Regions during face identification task were found more anteriorly in the inferior part of the temporal lobe. Matching of unknown faces was associated with the activation of the occipital and temporal part of the fusiform gyrus. A small region on the left inferior temporal gyrus and other regions outside the inferior part of the temporal and occipital lobes were involved in maintaining a face representation in working memory (Clarke et.al., 1997)
Right hemispheric mechanisms are specialized for upright faces. A left hemispheric advantage is shown in face processing local elements of a face. (Hilger and Koenig, 1991)
The face cells are so typical designed for the recognition of faces that they are not responding to other stimuli like textures, brushes gratings, bars and edges of various colors and models of complex objects and emotional response like snakes spider and food. Neither single component of the faces sufficient nor doe the elimination of only one specific component causes an alimentation of the neuronal response. Removing altering the picture of a face or reducing the contrast to a very lower level reduces but does no eliminate the response. Line drawings produce weak response
Expression and identification of a face appear to be coded by separate populations of cells. For expression tend to be located within the superior temporal sulcus, whereas the cell sensitive identification to be located on the inferior temporal gyrus.
Figure 7 Some cells fire when seeing faces en profile and others with frontal faces (Desimone, 1991).
Why should face cells be distributed across cortical regions with such diverse properties? Face cells are contributed to several functionally distinct neural circuits. Why should faces be treated differently from other class of objects? For us the recognition of faces is of great importance not only to recognize of the specific individuals of the troop but especially for the social communication. It has taken the place for the olfaction. There is a decrease in the size of the olfactory bulb with the higher primates and an increase in visual inputs to the amygdala.
f. Language
Left cerebral specialization for language implies that the right visual hemifield superiority for word recognition is found even in languages that read from right to left. The left domination for language is not always the case.
Figure 8. The Japanese right/left hemisphere compared with non-Japanese left/right hemisphereRecordings of the amplitude of electrical signals generated from the brain while the person listen to sounds. A. Japanese listens to the vowel (left hemisphere). B. Non-Japanese listens to the vowel (right hemisphere). C. Japanese listens to the sound of a cricket D. Non-Japanese listens to the sound of a cricket. (After Tsunoda, 1989)
The Japanese have a very special habit for employing vowels. A sentence can be expressed only by vowels. The left hemisphere receives speech sound including emotional voices but is also receives sound with a similar structure like noise of waves wind and rain and all noise of nature. The left hemisphere picks up non-harmonic sounds while the right hemisphere dominates for harmonic sounds. The Japanese when they hear occidental musical instruments the right hemisphere is dominant whereas the left hemisphere with the traditional Japanese musical instruments. For most non-Japanese the sounds perceived by the left hemisphere are actually limited to syllables containing consonants, while vowels that express an emotion are perceived by the right hemisphere. Japanese, who employ many vowels is the emotion more expressed by the left hemisphere they use many vowels in their language. It is the environment not the race. It is determined by the exposure of language. It remains unchanged after the age of nine. A Japanese child raise as an American until the age of nine will perceive her emotions as a non Japanese predominantly with the right hemisphere.(Tsunoda, 1989)
g. Arithmetic problems
Dehaene et.al.. (1999) discovered that exact arithmetic recruits networks involved in word-association processes. (Left hemisphere). In contrast, approximate arithmetic shows language independence, relies on a sense of numerical magnitudes, and recruits bilateral areas of the parietal lobes involved in visuo-spatial processing (Dorsal pathway). Mathematical intuition may emerge from the interplay of these brain systems.
h. Eye movements
Eye movements are asymmetrically organized in the two hemispheres. Eye movement's reaction times were longer in patients with right hemispheric damage than left. Inability to extract information from left side of stimuli during eye fixation.
Right-handed persons turn their head to the right in solving verbal problems (left hemisphere) and to the left for spatial problems (Right hemisphere)
j. Movements of the hands
Right handers display right hand superiority in tasks such as rapid finger tapping and finger sequencing. This superiority has been suggests reflecting the ability on the left hemisphere to specify the value and timing of muscular forces that are required to move the limb from the home position to the target. There is left side (right hemisphere) superiority for determining spatial orientation and tactile space perception, a more holistic approach. The right hemisphere plays more a role in the movement preparation and the allocation of attention in space, whereas the left hemisphere more involved is the movement execution. These findings emphasize the different roles of the two cerebral hemispheres in goal-directed aiming rather then a high degree of cooperation between them (Hodges, 1997).
(Dolphins can sleep with one hemisphere while the other is awake. They retain locomotor activity during sleep Mukhametov, 1987).
3. Upper and lower visual field: a world of difference
"
It is clear, however that researchers studying left-right visual field differences in perception processing will need to address their relation to similar upper and lower visual field differences"(Christman, 1993)."Despite the phletora of research on visual fields effects, very few researchers have paid much attention to possible top-bottom differences (Bryden an Underwood, 1990)".
At the back of our brain there is the calcarine sulcus separating these two visual brain areas. As we have already seen the upper and lower visual field are separated in the extrastriate areas in V2, V3 and V4. A good demonstration of the separation in upper /lower versus left/right is shown by damage to the upper lip of the calcarine sulcus. It results in blindness of in the lower contralateral field of view a phenomenon known as quadrantanopia.
Horton and Hoyt (1991) describe two patients (one woman of 39 years and a man of 40 years) with defects of vision of the lower left visual field. This means they had a damage to the upper part of the right occipital lobe field. They both had no lesions in the area of the horizontal meridian (dividing lower and upper visual fields). Areas V2 and V3 are each divided along the horizontal meridian into separate halve flanking the striate cortex. The upper and lower quadrants in extrastriate cortex are physically isolated on opposite side of the cortex.
Figure 9. Quadrantic visual field defect in the lower part of part of the left visual field. (After Horton and Hoyt,1991)
I found two figures in the literature demonstrating the separation of the upper and lower visual fields in the striate and extrastriate cortex (V2, V3, and V4). One of Gattass et.al. 1988. They mapped the horizontal meridian extrastriate cortex. The representation of the central vision is magnified relative to that of the periphery. The size of the receptive fields increases with increasing eccentricity.
Figure 10. Separation of the upper and visual field representations in the striate and extrastriate cortex. Open circles indicate the horizontal meridian (After Gattess et.al.. 1988).
Figure 11. Representations of the upper and lower field's in the striate and extrastriate cortex of the macaque monkey. Open circles; Lower visual field, Dots upper visual fields. These representations are clearly separated from each other (After Colby and Duhamel 1991).
It is Previc who published in 1990 a provoking article in which he claims a functional specialization role for the upper and lower visual fields. His relation of the functions of these visual fields with near and far vision may have confused the discussion around his revolutionary ideas. It is in context that I discuss in a later chapter the functional differences between far and near vision.
It is the merit of Previc to draw attention to the neglect of the dichotomy between upper and lower visual fields. The emphasis between the left and right hemisphere has too much dominated the scientific literature.
In the literature the division in left and right half of the brain is well documented and well described and analyzed. This is not the case with the division of the lower and upper parts processing the upper and lower part of the visual field. I will cite some abstracts of articles which I found indicating a fundamental difference between the processing of the upper and lower visual fields.
Link 1. Differences in processing lower- and upper visual field.
It is important to compare these facts of the upper and lower retina with the division in left-right halves of the brain. The consequence of these data implies that there is a processing of the visual filed in four parts, which each quadrant has its specific tasks.
Previc gives a list of differences between the upper and lower visual fields. The most important are:
We don’t realize that the image of the world is projected reversed on the retina. Bottom of our lower visual field to the upper part of the retina on the upper part. Experiments have shown that wearing goggles that turns everything up side down show that after some time everything is back to normal. After a few days it is possible to ride a bicycle and after a few weeks even skiing. After removing the goggles the world is unstable again but recovery is now taken less time within an hour or so. You can see for yourself what happens with your movements when you looking in a mirror held obliquely against your forehead. You will see all upside down. It will be extremely difficult to poor tea into a cup!
Linden et.al. (1999) showed clearly that wearing prism- and mirror-inverting spectacles during 10 days showed a rapid visuomotor adaptation. But it did not lead to the return of upright vision, nor was the retinopy of early visual areas observed. It is a reinterpretation rather than a really physically inversions of the orientation of the perceive image. This study is very important because it is an illustration how the brain is capable to adapt to the changing situations, without changing his hard wire in the primary visual cortex. Sugita (1996) studied the left-right reversing in monkeys. Cells in V1 had no preference for orientation or direction of motion, which suggest deterioration of function rather than compensation. It is the higher brain areas who are responsible for the compensatory processes, especially in the parietal cortex.
V
ision has two tasks, first to create a perception of the external world in order to recognize objects and their interrelation. The second task is to direct action towards the recognizing objects. We are transforming the visual information into motor actions.In 1982 Ungerleiter and Miskin identified two distinct streams of processing in the macaque monkey brain: a ventral stream from the primary cortex to the inferior temporal lobe and a dorsal stream projecting from the visual cortex to the posterior parietal stream. The ventral steam plays a role in object identification (what) and the dorsal stream is involved in spatial vision, in order to localize an object in space (where).
In 1992 Goodale and Milner came up with another approach, especially for the dorsal stream. This stream provides critical information about the object parameters and is able to guide goal directed actions towards the object. After having recognize a nice fruit hanging on a tree (ventral stream) the dorsal stream takes care for reaching out and grasping the fruit with the hand and to put it into the mouth.
Figure 12: The dorsal and ventral pathways "Where" versus "What" ( After Ungerleiter and Miskin,1982).
They tested two patients RV and DF with brain damages that spare one of the two systems and not the other. The two patients were a mirror image of each other. RV had bilateral lesions of the occipitoparietal region (dorsal stream) whereas as DF had damage to the ventrolateral region of the occipital cortex (an area to be part of the ventral stream).
RV was asked to pick up a series of objects with an appropriate grip. Despite that the fact that the patient could recognize these objects from each other, she often failed to place her fingers on the appropriate grasp points when she attempted to pick up the objects.
Actually her spatial vision was intact, but she could not transform the visual space perception into action of grasping the object.
DF is a 35-year-old woman suffered from brain damage by carbon monoxide intoxication. She had damage ventrally in the lateral occipital region and the parasagittial occipito pariental region. DF showed poor perception of shape. When asked to reach out and pick up the objets she could correctly place these objects in a slot.
Also she could pick objects with different orientations on the table surface with her eyes closed. But she could not use the visual information for perceptual and cognitive purposes. DF was asked to indicate with index finger and thumb of her right hand the width of different objects without picking them up. The aperture between finger and thumb was not related to the width of the target objects.
Both in her verbal and her manual response was there no evidence that she was sensitive to difference in the dimensions of the stimuli. The visual processing underlying 'conscious' perpetual judgements must operate separately from the underlying the 'automatic' visuomotor guidance of skilled actions of the hand and limb.
These two different reactions lead Goodale and Milner to the following conclusion:
"The visual projection system to the human parietal cortex provide action-relevant information about the structural characteristics and orientation and not juts about the position. On the other hand projections to the temporal lobe may furnish our visual perceptual experience, and is these that we postulate to be severely damaged in DF".
Figure 13 Patient DF performed badly by indicating the width of objects by perceptual matching, but accurate by grasping the objects. The aperture of their fingers was correct (After Goodale and Milner, 1992)
Goodale and Humphrey published in 1998 an article in which the asked them selves. What is vision?
Their answer is:
"One function of vision is the creation of an internal model or percept of the external world- a model that can be used to recognition of objects and understanding their relations. There is another function of vision, which is concerned not with object recognition, but with object-directed action. In this case vision guides our actions to the world by transforming visual inputs into appropriate motor outputs. We will suggest that separate but interacting, visual systems have evolved for the perception of objets on the one hand and the control of actions directed to those objects on the other".
Specific characteristics of the dorsal system:
The dorsal system has more myelinization and therefore a quicker guidance, whereas the ventral regions a greater topographical precision more callosal representation.
The dorsal system is far away from emotional centers in the brain. They will be disturbed by memories. It is intimately connected not only with the primate forebrain but also with those brainstem structures, like the superior colliculus and various pontine nuclei, that play a critical role in the programming and control of movement. The dorsal structure is phylogenetically an ancient network. (Goodale and Humphrey 1998). The dorsal system works in real time with short memory. Once the movement is made the visuomotor coordinates used to program and guide the movement are lost.
Specific characteristics of the ventral system
The temporal lobe (ventral system) is considerably enlarged by humans. It is intimately connected with the medial temporal lobe and prefrontal cortex. It is involved in long term memory.
"If perceptual representations were to attempt to real metrics of all objects in the visual array the computational load would be astronomical. Only the relative position orientation and size and motion of the objects is of concern for perception."
Gregory (1997) has argued that higher level illusions including geometric illusion deceive the perceptual system (ventral stream), because the system makes false assumptions about the structure of the world based on stored knowledge. This is necessary to have constant image for a moving object. The dorsal system however is not deceived by these illusions and is guiding our hand and eyes towards the object where it really is.
We may perceive an object bigger than it really is, but we open our finger thumb grip correctly when reaching for it.
The distinctions between the ventral and dorsal pathways have an enormous application to our behavior in space. As main topic of I am going to explain is the fact that:
"We see objects distorted or wrong, but we grasp these objects correctly
!"A nice example how the ventral system is fooled is by looking at the famous Ebbinghaus illusion.
Two target circles of equal size, each surrounded by a circular array of either smaller or larger circles. The target circle surrounded by the larger circles appears smaller. But when the target circle surrounded by the larger circles is made a fraction larger than this circle seems to have the same size as the target circle surrounded by the smaller circles.
The dorsal system is however not fooled by this illusion. People were asked to pick up the disks. It appeared that the grip (the aperture of thumb and index finger) were determined by the real size of the disk.
Another example of the dissociation between action and perception is given by the illusion where a vertical line bisects a horizontal line. Visual perception of these lines gives the illusion that the vertical line is longer. But by attempting to reach out for the lines we are not fooled by the illusion. The same has been shown with other illusions like the Muller-Lyer illusion
Figure 14. Grasping the objects the mind is not fooled by the Ebbinghaus illusion (Goodale and Humphery 1998)
Perceiving is not the same as doing. We don't see what we do or we do what we are not seeing.
When we are sitting in a car next to the driver we see the world differently than the driver, because the driver must relay on his dorsal system He must compute the retinal size of the object for steering the wheel correctly. We have two different views of the same world, one for perceiving and the other for action.
It is important to realize that once the goal has been selected for action the two systems process the incoming visual information simultaneously. They are 'talking' to each other and are working together for the final motor output.
The receptive fields of the inferior temporal cortex have very little cells for the far peripheral vision. Most cells include the fovea. The posterior cortical cortex has a large representation of the peripheral visual fields.
I will give you some ideas of other scientists concerning the two pathways.
Bridgeman already in 1979 realizes that some information that is available to a motor oriented visual system is unavailable to the cognitive visual system. Held in 1968 has suggests that a separation of vision into two modes is necessary to explain the lability of visually guided behavior while perception of shape remains relatively stable. This seems in accordance with the conclusion of Abrams and Landgraf (1990) that the motor system appears to have access to spatial information that is unavailable to the perceptual system.
Bridgeman (1999) asked himself; "Why should we have two visual brain instead of just one". Why should we have a perceptual and a visuo-motor system? The dorsal system apparently can do the work when we are moving around in the world. A frog has only one motor system. The motor system acts in real time; here and now. The ventral system is able to live in the past and the future. If one asks people what they see, they report the contents of the ventral system.
Lesions in the temporal lobe lead to lasting deficits in learning of new visual discriminations and the recollection of previously learned memories (Otto et. al. 1992). The invariance property in the whole visual field mainly comes from the cooperation between two regions that are a priori known to extract two different types of information. One that has limited invariant capacities for object recognition in the center of the visual field and the other that can extract object locations in the periphery and drive eye movements to reset the pattern in the central region.
Turnball (1999):
"Incredible however, it seems that I sees the world in different ways, depending on which task I am engaged in - depending upon whether I am recognizing my mug or reaching for it".
": death follows for the primate whose representation of a branch may be beautiful but geometrically inaccurate".
"Mel Goodale said once to Turnball: The ventral stream is Devil's playground".
"God mighty well wish to exert his influence on the world through the dorsal stream, where his manipulations would go unnoticed by consciousness".
Mc.George does not agree with Milner and Goodale (1995) that the consciousness is confined to the ventral system. There is good evidence that in absence of awareness structural encoding of objects is possible up to a high level of encoding. Awareness is not a property of one system. Consciousness implies something that exists over and above the modular process.
The posterior parietal cortex consists of a mosaic of areas each with its own role. Different areas for reaching and grasping objects. Grasping implies transformation of a real thing into moment, reaching implies the transformation of an abstract construct into movement (Rizzolatti, 1997).
To look at or reach what we see spatial information must be transformed into a motor plan. The parietal cortex is such a area were planning for the action is proceeded. Neurons in area 7a and the lateral intraparietal area fire before and during visually guided saccades. Other neurons in area 7a and 5 are active before and during reaching visually guided arm movements. The action need not to be executed it is the intention to act that activate the parietal cortex. (Snyder, 1997)
Deubel et.al. (1999) comes up with a slightly different model of the dorsal and ventral systems. Their Visual Attention Model (VAM) suggests that a common selection mechanism exists for dorsal and ventral processing. This mechanism is suggested to select one object at a time in "early" stages of the visual system, resulting in an increased activation of the visual representations of this object in higher-level ventral and dorsal visual areas. This increased activation allows the selective perceptual analysis of the selected object to the level of recognition, and the selective computation of its spatial parameters such that saccading, reaching, and grasping movements are prepared. So, VAM suggests a strict one-object-at-a-time rule: Whenever a goal-directed action towards an object is prepared, only this movement target can be perceptually processed in higher-level ventral areas. On the other hand, whenever visual attention focuses on an item for the purpose of object recognition, no other objects can be selected for goal-directed actions.
During the preparation phase objects other than the movement target are temporarily excluded from ventral high level analysis.
Grady et.al. 1993. Old subjects has more activation of the occipito-temoral cortex during spatial task and more activation of superior parietal cortex during object task than did younger subjects, but less functional separation between dorsal and ventral. Age related reduction in processing efficiency less accurate in similarities and differences in faces performing slower also mental rotation.
Jeannerod et.al. (1995). The grasping action is a complicated thing. The fingers begin to shape during the transport of the hand. The grip aperture is bigger than the moment the fingers grip the object. Grasping movements are coded more globally in the interior parietal lobule and whereas they are more segmented in the area F5.
Processing of visual information is task dependent.
Brenner and Smeets (1996). Information of the objects' position and orientation guides the hand to the object and while information on the object's shape and size determines how the fingers move relative to the thumb to grasps it. The persons were right with the aperture of the thumb and finger only not about the weight to lift up because they were fooled by illusion. More force is applied to lift up larger objects. But visual information about the positions at which the object is grasped determines the aperture. Position and size are analyzed separately. Size is an intrinsic property of the object whereas the position is in relation to the observer. Different aspects of an action lifting versus grasping are controlled independently.
Jeannerod (1994) proposed a distinction between two streams one for pragmatic and the other for semantic representations. The first refers to rapid transformation of sensory input into motor commands and the last to the use of cognitive cues for generation of action. Decety (1999) proposes a third way. When perception has an explicit goal there is segregation in the two pathways dorsal and ventral. However when the perception has no explicit goal both visual pathways are implicated.
It shows again how important the choice of a task is the way visual signals are processed.
Perception of shape is independent of the size and position and also the cue that it defines it. The same shape can be recognized whether defined by a difference in luminance, by motion, or by texture. Neurons in the inferior temporal lobe did not vary with the size and position of a shape and also did not vary with the visual cue used to define the shape. It concerns of cues that are processed in ventral and dorsal visual pathways, this indicates a convergence of information of these two pathways to the inferior temporal lobe (Sáry et.al. 1993).
In his TINS lecture Sakata et.al.(1997) stressed the point that the parietal association cortex plays a crucial role in depth precession and visually guided hand movements. The dorsal area has two subsystems for motion vision and for coding position and 3 D features.
4. Difference in processing objects close by and far away
"The projection of the visual fields to opposite hemispheres is certainly consistent with the basic left-right division of the visual world. It has also proposed that a weaker subcortical dorsal-ventral brain axis mirrors the up-down split of the visual world, but the related near-far axis has evidently superseded the primordial vertical one in determining the structure of the primate higher visual cortical pathways" (Previc, 1990).
Selection pressures led to refinement visual guidance of manual behavior in peripersonal environment and a visual attention system for guiding hand to a fixated object. When the hand cannot directly be viewed a smooth pursuit tracking system monitoring the hand to bring fruit to mouth. Coming form the woods to the flatland vision have to cope more with the extrapersonal space. Later we shall see that the attention to near space is especially directed to the lower visual field close to the body, where as the right hemisphere toward space farther away and especially directed to the upper visual field. As we have seen that the visual field is processed in four separate areas of our brain, the space of our surrounding world is focussed for far space in the upper left visual field and the nearby space by the lower right visual field. Studies of brain impaired persons with lesions of the dorsolateral parietal occipital areas induce inattention of the lower vertical and near peripersonal space and lesions of the ventral temporal occipital induce inattention of upper vertical and far radial space. It will be of great interest to know in how far we can find examples in our daily life that we can underline this fact
Reading writing and working with tools occur in spatial positions that are close to the body (peripersonal space), whereas facial and emotional feature recognition occur more in spatial positions that are away from the body (extrapersonal space).
Previc postulates the close relation of the near and far vision respectively with the lower and upper visual field.
The increase functional specialization in the lower and upper visual fields in primates was promoted by advances in near (peripersonal) and visuomotor manipulatory skills and far (extrapersonal) visual capabilities. It has lead to functional specialization like the dorsal and ventral cortical divisions.
He discusses the important visual improvement of primates for the segregation of near and far visual space. He mentions four advances of particular importance to far (extrapersonal) vision:
For the near (peripersonal) vision there were two developments of importance
Figure 15 shows a close relationship between the lower visual field and reaching behavior. Reaching for objects and transporting them toward the mouth (Previc, 1991)
Human upper retina has more rods and cones per mm2 than its lower counterpart. In the lower parts of our visual field we need rapid processing because in this area it is more likely than in the upper part of the visual field that both predators and prey occur (Rapcsack et.al., 1988).
For Previc the distinction between far and near vision is the main topic of his thesis. The split in far and near vision led to important transformation in the primate visual system. The functional specialization must correspond with the three dimensional structure of visual space and the powerful attentional mechanisms associated with it. The near far dichotomy has an ecological basis. The near and far distinction shaped into the dorsal and ventral system. The dorsal system more for movement and depth mostly in peripersonal space and the ventral system more for color and object recognition. Local perception is better confined to central and peripheral perception for near vision.
The pursuit system is an instrument of near vision and a lower visual field superiority, whereas voluntary saccades have a closer link with the upper visual field in extrapersonal space.
The near system represents space for purposes of manipulating or avoiding objects with the body, whereas the far system represents space for purposes of acquiring and analyzing objects with the eyes grasping
A triad of ocular responses, accommodation, convergence and papillary constriction produces the near vision. Designed to focus of nearby objects. It is a phylogenetically recent phenomenon. Other specialized perceptual capabilities are bringing objects hand accurately guided from the visual periphery to the fixated object even if it cannot be viewed. Also global perception the reaching hand despite reduced contrast caused by its rapid motion as well as the diplodia and defocus resulting from the more distal fixation.
Visual experience exerts a profound influence over the formation of higher cortical visual maps and specialization's and even alter subcortical and retinal levels. Not only what is seen but also on what is attended.
Greater ganglion densities in the upper retina (lower visual field and near vision) may be subject of experience. This asymmetry has not been observed in children. Left-right vestibular asymmetry may be responsible for a further subdivision of near versus far visual perception into the left and right hemisphere. The existence of left vestibular dominance in most humans leads to a greater involvement of the right parietal lobe in vestibular processing.
The emergence of far vision gave rise to widespread transformations in the primate brain. The ventral location of far vision can be influenced by non-visual influences. There is a close relation between the ventral system and the primary auditory cortex (closely related to the extrapersonal space and limbic system (emotional associations).
Martin et.al. (1995) shows object knowledge are stored close to the regions that mediate perception of those attributes generation of color words activate in a region of the ventral temporal lobe just anterior to the area involved in the perception of color generation of action words in region middle temporal gyrus close to just anterior the area involved in perception of motion.
The primate lateral geniculate nucleus has under gone evolutionary changes. The different layers are more segregated. The parvo system (color) has increased considerably. Many features depend on visual experience.
There has also been a large increase of the temporal lobe. Face sensitive neurons in the temporal have not been reported in any species other than the rhesus monkey. The role of face expression for the higher primates is playing a great role in social communication.
The specialization if the inferior frontal lobe for object recognition and memory parallels that of the inferior temporal lobe, whereas adjacent dorsal region may duplicate many specialization of the parietal area. Eye movements elicited from the superior and inferior field may be related to the lower and upper visual field respectively.
Weintraub and Mesulam (1989) gave four components of a network for the mediation of attention in extrapersonal space.