F3 - Visual Stream - Central Computation of Visual Stimuli PDF

PHYSIO F3 - Visual stream - Central computation of visual stimuli Ganglion cell axons exit the retina through a circular region in its nasal part called the optic disk (or optic papilla), where they bundle together to form the optic nerve. Axons in the optic nerve run a straight course to the optic...

PHYSIO F3 - Visual stream - Central computation of visual stimuli Ganglion cell axons exit the retina through a circular region in its nasal part called the optic disk (or optic papilla), where they bundle together to form the optic nerve. Axons in the optic nerve run a straight course to the optic chiasm at the base of the diencephalon. In humans, about 60% of these fibers cross in the chiasm; the other 40% continue toward thalamus and midbrain targets on the same side. Once past the optic chiasm, the ganglion cell axons on each side form the optic tract. Thus, the optic tract, unlike the optic nerve, contains fibers from both eyes. The partial crossing (decussation) of ganglion cell axons at the optic chiasm allows information from corresponding points on the two retinas to be processed by approximately the same cortical site in each hemisphere, an important feature considered in the next section. The ganglion cell axons in the optic tract reach a number of structures in the diencephalon and midbrain. The major target in the diencephalon is the dorsolateral geniculate nucleus of the thalamus. Neurons in the lateral geniculate nucleus, like their counterparts in the thalamic relays of other sensory systems, send their axons to the cerebralcortex via the internal capsule. These axons pass through a portion of the internalcapsule called the optic radiation and terminate in the primary visual cortex(V1), or striate cortex (also referred to as Brodmann’s area 17), which lies largely along and within the calcarine fissure in the occipital lobe. The retinogenic-ulostriate pathway, or primary visual pathway, conveys information that is essential for most of what is thought of as seeing; damage anywhere along this route results in serious visual impairment. A second major target of ganglion cell axons is a collection of neurons that lie between the thalamus and the midbrain in a region known as the pretectum. Although small in size compared with the lateral geniculate nucleus, the pretectum is particularly important as the coordinating center for the pupillary light reflex (i.e., the reduction in the diameter of the pupil that occurs when sufficient light falls on the retina). The initial component of the pupillary light reflex pathway is a bilateral projection from the retina to the pretectum. Pretectal neurons, in turn, project to the Edinger–Westphal nucleus, a small group of nerve cells that lies close to the nucleus of the oculomotor nerve (cranial nerve III) in the midbrain. The Edinger–Westphal nucleus contains the preganglionic parasympathetic neurons that send their axons via the oculomotor nerve to terminate on neurons in the ciliary ganglion. Neurons in the ciliary ganglion innervate the constrictor muscle in the iris, which decreases the diameter of the pupil when activated. Shining light in the eye leads to an increase in the activity of pretectal neurons, which stimulates the Edinger–Westphal neurons and the ciliary ganglion neu- rons they innervate, thus constricting the pupil. One is the suprachiasmatic nucleus of the hypothalamus, a small group of neurons at the base of the diencephalon. The retinohypothalamic pathway is the route by which variation in light levels influences a spectrum of visceral functions that are entrained to the day–night cycle. Another target is the superior colliculus, a prominent structure visible on the dorsal surface of the midbrain. The superior colliculus coordinates head and eye movements to visual (as well as other) targets. 1.​ Visuotopical arrangement With both eyes open, the two foveas normally align on a single target in visual space, causing the visual fields of both eyes to overlap extensively. This binocular field consists of two symmetrical visual hemifields (left and right). -​ The left binocular hemifield includes the nasal visual field of the right eye and the temporal visual field of the left eye. -​ the right hemifield includes the temporal visual field of the right eye and the nasal visual field of the left eye. The temporal visual fields are more extensive than the nasal visual fields, reflecting the sizes of the nasal and temporal retinas, respectively. As a result, vision in the periphery of the field of view is strictly monocular, mediated by the most medial portion of the nasal retina. The optic tract contains the axons of ganglion cells that originate in both eyes and represent the contralateral field of view. Optic tract axons terminate in an orderly fashion within their target structures, thus generating well-ordered maps of the contralateral hemifield. For the primary visual pathway, the map of the contralateral hemifield established in the lateral geniculate nucleus is maintained in the projections of the lateral geniculate nucleus to the striate cortex. -​ The fovea is represented in the posterior part of the striate cortex, whereas the more peripheral regions of the retina are represented in progressively more anterior parts of the striate cortex. -​ The upper visual field is mapped below the calcarine sulcus, and the lower visual field is mapped above it. As in the somatosensory system, the amount of cortical area devoted to each unit of area of the sensory surface is not uniform, but reflects the density of receptors and sensory axons that supply the peripheral region. Like the representation of the hand region in the somatosensory cortex, the representation of the macula is therefore disproportionately large, occupying most of the caudal pole of the occipital lobe (this discrepancy is called “cortical magnification”). From the lateral geniculate nucleus, the fibers that run in the optic radiations below the parietal lobe will drive information from the inferior visual field reaching the primary visual cortex above the calcarine fissure while the fibers that pass below the temporal lobe forming Meyer’s loops will drive information deriving from the superior visual field and will reach the primary visual cortex below the calcarine fissure. Some of the optic radiation axons run out into the temporal lobe on their route to the striate cortex, a branch called Meyer’s loop. Meyer’s loop carries information from the superior portion of the contralateral visual field. More medial parts of the optic radiation, which pass under the cortex of the parietal lobe, carry information from the inferior portion of the contralateral visual field. Damage to parts of the temporal lobe with involvement of Meyer’s loop can thus result in a superior homonymous quadrantanopsia. Lesions in precise area of the tract will lead to different types of impairment: -​ a lesion close to the Meyer’s loop leads to the superior homonymous quadrantanopsia a loss of information coming from the superior quadrant -​ a lesion more medial to the optic tract and below to the parietal lobe it would lead to the inferior homonymous quadrantanopsia; -​ lesion E in the picture will instead lead to the loss of all the fibers directed to the visual cortex and will manifest as the homonymous hemianopsia, therefore the loss of half of the visual field in both eyes. o​ A characteristic of the homonymous hemianopsia is the macula sparing: even if there was a damage at the level of the cortical visual system, the foveal vision is not lost. The reason of this phenomenon is not clear yet.  2.​ Functional organization of the striate visual cortex Neurons in LGN and retina The properties of neurons in the lateral geniculate nucleus and at level of the retina are very similar. They are -​ organized in a center-surround receptive field organization, -​ very sensitive to luminance increase and decrease (they are well activated by spot of light) -​ monocular, they receive information from only one eye. Neurons in cortical areas Neurons in the cortical areas, so after the LGN, are mostly unaffected to spot of light, they are binocular as they merge information from both eyes and they are sensitive to light-dark bar or edges presented in specific orientation. Much of the current understanding of the functional organization of visual cortex had its origin in the pioneering studies of David Hubel and Torsten Wiesel at Harvard Medical School. The method they used was primarily microelectrode recordings in anesthetized animals that reported the responses of individual neurons in the lateral geniculate nucleus and the cortex to various patterns of retinal stimulation. The responses of neurons in the lateral geniculate nucleus were found to be remarkably similar to those in the retina, with a center–surround receptive field organization and selectivity for luminance increases or decreases. However, the small spots of light that were so effective at stimulating neurons in the retina and lateral geniculate nucleus were largely ineffective in visual cortex. Instead, most cortical neurons in cats and monkeys responded vigorously to light–dark bars or edges, and only if the bars were presented at a particular range of orientations within the cell’s receptive field. The responses of cortical neurons are thus tuned to the orientation of edges, much as cone photore-ceptors are tuned to the wavelength of light; the peak in the tuning curve (the orientation to which a cell is most responsive) is referred to as the neuron’s preferred orientation. By sampling the responses of a large number of single cells, Hubel and Wiesel demonstrated that all edge orientations were roughly equally represented in visual cortex. As a result, a given orientation in a visual cene appears to be “encoded” in the activity of a distinct population of orientation-selective neurons. a.​ Cortical neurons in V1 The neurons are called orientation selectivity neurons since they are tuned for specific stimuli orientation. All bar orientations are equally represented in the visual cortex, and they are sensitive for the -​ Orientation of the bar -​ Length of the bar -​ Direction of the edge moved across the receptive field: stimulus can be stronger when the bar moves to the left than to the right. These cells are classified into 2 subgroups: ​ Simple cells: composed by spatially on-off response zone. They respond when a bar enters or leaves the off region. ​ Complex cells: they exhibit a mixed on-off responses throughout their receptive field as they integrate information arriving from simple cells in serial processing. ​ Hypercomplex cells are also present. b.​ Orientation selectivity in V1 It has been hypothesized that orientation selectivity at the level of V1 is present throughout its layers, except for layer IV where the information from the lateral geniculate nucleus arrives and which presents center-surround receptive fields. In layer IV in fact neurons have unoriented receptive field. The projections of simple cells of IV onto neurons of layer IIIB, causing the alignment of the circular center-surrounding receptive fields of many afferent neurons creates a receptive field with a specific orientation. 3.​ Binocularity and depth perception Unlike neurons at early stages in the primary visual pathway, most neutrons in the striated cortex are binocular. The LGN is divided into 6 layers and each of them receives input from monocular neurons: layer 1,4,6 receive info from the contralateral eye, while layer 2,3,5 receive info from the ipsilateral eye. (Image on the left) At the striate cortex inputs from both eyes arrive at the layer IV and from there the information of the two eyes get integrated one with the other so the receptive field from monocular becomes binocular. Mixing the information of the eyes represents the physiological base of stereoptosis: the sense of depth. Because the two eyes look at the world from slightly different angles, object that lie in front or behind the plane of fixation project to non-corresponding points on the two retinas. The disparity of the two eye views of object nearer or furtherer than the fixation point is perceived as depth. Diplopia or double vision is the perception of a single object as two objects (the experiment of the pen) since the image falls on non-corresponding portions of the two retinas. In the primary visual cortex, there are specific neurons that are maximally activated when the stimulus falls in an non-corresponding part of the retina. -​ far cells discharge to disparities beyond the plane of fixation. -​ near cells discharge to disparities in front of the place of fixation. 4.​ Orientation of the striate cortex The striated cortex is organized in columns of specialized neurons, similarly to the somatosensory system, meaning that in one column there are neurons that exhibit similar orientation preferences (besides level IV that, as stated before does not have neurons sensitive to orientation). As shown in the image on the left. An electrode oriented horizontally will instead show different selectivity orientation (Image on the right). The columnar organization is also present in ocular dominance. Even though the information of both eyes starts to be integrated after the LGN, the relative strength of the inputs from one eye is anyways dominant over the other. Therefore, an electrode penetrating perpendicularly will record the same ocular dominance, while an oblique electrode will record a shift in ocular dominance. Despite the columnar organization was at the beginning recognized as the basis of orientation selectivity and ocular dominance, further work has shown that others stimulus follow this organization such as colors. Within each column, there are blobs which “host” neurons with -​ poor orientation selectivity -​ a strong color preference; t -​ located mainly in the superficial layers -​ each blob corresponds to a color selectivity neuron. In V2, there is column organization for motion selectivity. It is divided into dark stripes and pale stripes. -​ dark stripes which can be divided in thick and thin stripes that host different kind of neurons: o​ thick stripes host neurons have selectivity for movement direction and binocular disparity, o​ thin stripes host neurons that are specialized for colors perception, they receive inputs from blobs of V1. -​ pale stripes host mostly orientation selective neutrons.  5.​ Functional organization of visual information from ≠ pathways There are different pathways originating at the level of the lateral geniculate nucleus (LGN in the thalamus), which is subdivided in several layers and conveys different types of information to the primary visual cortex. In fact, in the LGN inputs to the cortex are not only segregated for the different eyes but also for type of neurons. There are two main pathways: one that runs along the ventral part of the LGN and one along the medial part of it. The inputs are segregated in the different layers of the thalamocortical nucleus and are sent to specific infra-layers of the 4th layer; therefore, information always reaches layer 4, but at the level of different infra-layers. The pathway running ventrally to the LGN (originating at layers 1 and 2) is called magnocellular pathway (M channel). It receives information from M ganglion cells, which are characterized by some peculiar features: -​ Large receptive field -​ Faster axons -​ Transient response to visual stimuli -​ No color information -​ Project to IVCα - subcomponent Cα (C alpha) of the 4th layer. Whereas the more medial pathway from the LGN (originating at layers 3,4,5,6) is called parvocellular pathway (P channel). It receives information from P ganglion cells, having the following features: -​ Smaller receptive field -​ Slower axons -​ Sustained response to visual stimuli -​ Process color information thanks to specific organization of center and surrounds receptive fields driven by different type of cones with sensitivity for short-medium-long wavelength light -​ Project to IVCβ - subcomponent Cβ (C beta) of the 4th layer. Taking into consideration the lesions is always important in order to understand the causality of a phenotype; in other words, in order to understand if a specific activity is causally related to the behavior, lesion studies are really important. A selective lesion to the magnocellular pathway would lead to reduced activity to perceive rapidly changing stimuli, without effect on visual acuity and color perception. The clinical outcome of the lesion is related to the functional property of neurons involved in this pathway, meaning that functionally speaking the magnocellular pathway is really important for the high temporal resolution vision (location, speed, direction of moving objects). A selective lesion is at the level of the parvocellular pathway, the outcome will be impairment of the visual acuity and color perception, without effects on motion perception. Therefore, this pathway is mainly related to spatial resolution vision (detection of size, shape, colors of objects). Within the layers of LGN there are the so-called K-cell, which are involved in a third pathway: the Konicellular/K-cell pathway. The K-cells send information that reach the neurons in the Koniocellular layers (receiving from K ganglion cells), which are the sub-layers/intralaminar zones in the LGN. They receive inputs from fine-caliber retinal axons and project in a spread way to the superficial layers II and III of the cortex. Their functional role is not clear yet, but it seems that they are involved in color perception. 6.​ Intrinsic cortical circuits of V1 Magnocellular and parvocellular pathways from the LGN reach different sub-layers in layer IV. Afterwards, the information is sent to the superficial layers that in turn send horizontal information to other cortical areas and to the closest columns. Note that in the superficial layers the receptive fields are smaller with respect to the ones of the deeper layers; small receptive fields are involved in high resolution pattern recognition, whereas large receptive fields are selective for movement direction, so for tracking objects in space. The horizontal connections represent the minimal common part of the serial processing; their role is crucial as they allow important integrative functions among neurons in the different columns, encoding for different properties and different parts of the receptive fields. In fact, the visual cortex is organized visuotopically and therefore the horizontal connections allow target neurons to integrate information over a relatively larger area of the visual field. In conclusion, this long range of horizontal connections is important in assembling the components of the visual image into a unified concept. 7.​ Schematic representation of the visual module The visual module is repeated billions of times in the human cortex in order to cover all the visual receptive fields. It gives information about the colors, in fact each blob has a specific color dominance and direction selectivity. It represents the minimal part of the visual field that is useful to build the visual primitives. The visual module minimizes the distance required for neurons with similar functional properties to communicate with one another. Furthermore, it allows them to share inputs from discrete pathways that convey information about particular sensory attributes. This efficient connectivity economizes on the use of brain volume and maximizes processing speed. 8.​ Serial and parallel processing in visual pathways Serial processing goes from V1 to V2 and so on, whereas parallel processing occurs simultaneously in different subsets of fibers that process different sub-modalities  9.​ Extrastriate visual areas In a non-human primate, there are several visual areas, each provided with visuotopic and retinotopic organization, going from V1 to V4 and to the mediotemporal region (MT). -​ MT encodes for direction of moving edge, -​ V4 encodes for color of visual stimulus. These regions are present also at the level of the human brain, but in this case, there is an increased specialization and differentiation of the several areas, for example the visual area, extra-striate body area (selectively activated by body inputs). Human fMRI comparative studies have identified several areas involved in visual processing, based on behavioral/functional tasks. -​ MT sends information to the parietal lobe (forming the dorsal pathway), -​ V4 to the temporal lobe (giving rise to the ventral pathway). Visual areas are organized in two hierarchical pathways: ventral and dorsal, having different functions. -​ the dorsal pathway conveys visual information to guide movements (reaching movements, hand orientation movements…), -​ ventral pathway is important for object recognition. -​ intermediate level where the visual processing takes place (visual primitives). If there is a lesion at the level of MT, which is an area responsible for movements, it is possible to observe an impairment in its function. A famous example involves a patient who had difficulty in pouring tea into a cup because the fluid seemed frozen, and she/he couldn’t stop pouring at the right time. Whereas, if the lesion is at the level of V4, then cerebral achromatopsia can be observed and the patient has difficulty in perceiving colors. 10.​Ventral stream – high level visual processing High-level visual processing depends on integration between visual primitives (sensory representations) with semantic significance, such as that arising from short-term working memory and behavioral goals. In the temporal lobe “hot” information are encoded, meaning that they allow the integration between visual primitives; these are extracted from the primary visual cortex to be integrated with higher multi-sensory info so that the information can be organized from a conceptual point of view, becoming knowledge; for this reason, it is involved in object recognition. In the cartoon shown below, an example of high-level visual processing can be seen: the semantic content. For instance, a horse can be represented and linked to other images in different ways: taking into consideration the emotional valence, signals from other sensory modalities, through categorical or associative linking. The ventral stream and the role of the temporal lobe can be better understood looking at their anatomical connections. In particular, the inferotemporal lobe (IT) receives information from primary visual areas regarding the visual primitives and then can integrate them with regions located in the superior temporal lobe (STP) and in the pre-frontal cortex (PF). It can also receive information from the medial temporal lobe, in particular from the hippocampus (structure associated with different types of memory) that has a strong interaction with the limbic system, more specifically with a nucleus located within the amygdala. The amygdala activity can modulate both the activity of neurons in the hippocampus and in the temporal lobe in order to store visual information. All of these areas contribute to the properties of neurons that are located in the temporal lobe. A famous example is the flashbulb memory: when there is a distressing event, the activity of the amygdala allows to encode this info very fast leading to a visual memory that can be traumatic even for the rest of the life. The ventral stream is therefore a pathway which is significantly affected in subjects suffering from post-traumatic stress disorders. 11.​Lesion of the ventral stream – visual agnosia Eduard Albert Schafer is the scientist who found out that damage to the inferior temporal lobe impairs the ability to recognize visual objects leading to visual agnosia. The patient is able to extract visual primitives from objects but is unable to merge all this information in order to create an internal representation which is recognizable and usable. Lesions in the different anterior and posterior part of the inferotemporal cortex can lead to different types of agnosia. When the lesion is at the level of the most posterior part of the inferotemporal cortex, a condition known as apperceptive agnosia can be described. The patient is able to verbally identify the objects (circle, square, diamond, numbers…) but he is not able to reproduce them. If the lesion of the inferotemporal cortex is a bit more anterior, it is possible to talk about associative agnosia; this condition is characterized by an opposite outcome with respect to the other one, so the patient is able to reproduce the object but not to verbally identify it. 12.​Properties of neurons in the inferior temporal lobe – face recognition In the inferior temporal lobe, there are neurons that are selectively activated upon face recognition (study conducted on monkeys). The activation is not an on/off process, as it can be seen in the peri stimulus histogram (digitized version of a neuron): -​ in the first and third graphs there is a strong activation upon face recognition, -​ in the second one the neurons are not strongly activated because the stimulus is not clear. -​ If important parts of the face are hidden (for example the mouth in the 4th image or the eyes in the 5th one) or in the absence of color (image 6), the neurons are still activated as they recognize the presence of the face. -​ They can also respond to the face of members of other species, for example humans as depicted in the 7th image. -​ On the contrary, if another part of the body is shown, the neurons don’t fire or fire significantly less. Evidence from fMRI studies in non-human primates confirm the presence of these face recognition neurons in a specific region of the inferotemporal cortex. In human beings, a specific lesion of the inferior temporal lobe results in a specific form of associative agnosia known as prosopagnosia. Patients can recognize the face as a face, but they are unable to identify a particular face as belonging to a specific person. The Jennifer Aniston example is very famous; in this case the neurons of the affected patient selectively discharge when, among the faces shown during the experiment, a particular face is shown. 13.​Columnar organization in the inferior temporal lobe A sort of columnar organization is present also in such higher visual areas of the ventral stream. For example, an optical image study conducted on monkeys shows that, changing the color or shape of the stimulus (fire extinguisher is shown) causes different parts of the cortex discharge. Optical images of the surface of the anterior inferior temporal cortex illustrate regions selectively activated by the objects shown at the right (fire extinguishers). In this schematic depiction of the columnar structure of the inferior temporal cortex, the vertical axis represents cortical depth. According to this model each column includes neurons that represent a distinct complex pattern. Columns of neurons that represent variations of a pattern, such as the different faces or the different fire extinguishers, constitute a hypercolumn.  14.​Perceptual constancy in the inferior temporal lobe Limbic, paralimbic and prefrontal areas It is important to remember the connection of the inferior temporal lobe with the limbic, paralimbic and prefrontal areas. Invariant attributes of an object The ability to recognize objects as the same under different viewing conditions, despite the sometimes markedly different retinal images, is one of the most functionally important requirements of visual experience. The invariant attributes of an object are, for example, the spatial and chromatic relationships between image features or characteristic features. For example, in the image below a zebra can be seen under different points of view: the animal itself but also an image completely unrelated to the animal, but related to the football team. The same concept can be applied in the case of the devil. This is possible thanks to the activity of the inferotemporal cortex and its connection to other areas, including the prefrontal lobe. It’s important also to consider the concept of constancy. Size constancy Size constancy: an object placed at different distances from an observer is perceived as having the same size, even though the object produces images of different absolute size on the retina. However, human beings are not perceiving these changes in size of retinal representations. In fact neurons in the inferior temporal cortex (IT) discharge no matter what the distance of the object from the eyes is. These neurons don’t change their firing pattern with respect to changes of stimulus size at the retinal level. This concept is at the base of the principle of size constancy. Position constancy Position constancy: objects are recognized as the same regardless of their location in the visual field. In fact, neurons in the inferotemporal cortex (IT) don’t change their firing pattern when the position of the object is changed. Categorical perception Categorical perception, which is defined as the ability to distinguish objects of different categories even if very similar (example: distinguish among red apple, green apple and cherry). The categorical perception is a very peculiar feature characterizing the inferotemporal and prefrontal cortex. Earl Miller found out that recording at the level of the prefrontal cortex (which is in connection with the inferotemporal cortex), it is possible to discriminate, at different degrees, cats from dogs. He showed different pictures of cats and dogs, also mixing features belonging to one or another mammalian; when there is a shift in proportion (from 100% cat, to 80% cat, to 60% cat, to 60% dog) the neurons start to increase their firing rate with respect to the previous condition. This means that these neurons can modulate their activity based on their categorical information, even if this is very difficult to extract from the array (as in the example regarding apples and cherries). 15.​Link between visual information and memory Visual experience can be stored as memory and visual memory influences the processing of incoming visual information. In fact, the prefrontal cortex is really important for visual experience as it can modulate its activity thanks to the presence of a huge network. The prefrontal cortex itself can receive visual information (but not in a retinotopic or visuotopic way) and plays a pivotal role in visual memory. In the example below, the monkey was trained to look at the same object oriented at different angles. When the object starts moving and rotating towards the position which is easily recognized by the monkey, the firing rate of the neurons increases. The object has no meaning, as it is a fractal object, but was associated with a particular meaning during the training, for example through a reward mechanism. Another important aspect of prefrontal neurons, which are linked to inferotemporal neurons, is the activity they are performing in the delay period. In fact, visual memory is able to influence the processing of incoming visual information. In the experiment shown below, a monkey was trained to see the first stimulus (orange), then the stimulus disappears for a delay period of about 10 seconds and other 2 stimuli appear (orange, blue) and the monkey has to choose the stimulus that appeared before. In both inferotemporal and prefrontal neurons there is sustained neuronal activity during the delay period. This kind of activity is linked to the processes of memory and coding: the monkey was keeping in mind the color and the shape of the object to be selected in order to obtain the reward. Question time: if the experiment is performed on a baby monkey, then the outcome is going to be different. For example, considering studies performed on rats, it was possible to find out that the central nucleus of the amygdala in the early phase of life has corticospinal projections. This nucleus is important to code for emotional information, sending inputs to the spinal cord and resulting in movement. This kind of projection from the limbic system disappears in adult life (due to the non-use theory), when our behavior starts to be modulated based on other pathways, such as the rubrospinal system and other voluntary movement mechanisms. 16.​Dorsal stream The information from the visual area, in particular MT, goes to specific areas of the parietal lobe and through the parietal lobe can reach specific pre-motor and motor areas. Therefore, when considering the dorsal stream, we should not take into consideration a single area as a connection between different areas is present. The dorsal stream and the visual stream interact with each other during movement; however, the dorsal stream is more focused on using visual information to guide movements. There are lots of networks and sub-networks that have different functional properties. There is a connection between -​ inferior parietal lobe and lateral pre-motor area, -​ superior parietal lobe and dorsal pre-motor area. Connection between parietal and pre-motor area allows human beings to extract visual information regarding an object (physical information such as the shape, the material and also about the meaning of the object - the ventral stream sends information to the prefrontal cortex). In particular, a connection between a specific region of the parietal lobe (AIP) and F5 leads to the visuo-motor integration for hand-action. The overall activity in AIP is involved in extracting physical features of the object in order to be integrated in a proper hand-motor schema. Therefore, it is possible to easily plan a movement (for example, grab a bottle to drink some water) so that the hand is in an adequate position to grab the bottle, considering the physical features of the object and the intention. All this information is integrated from the visual primitives and will reach a network which includes the parietal and pre-motor area. A connection between VIP and F4 is able to integrate the visual information to somatosensory properties. These neurons are bimodal, visuotactile neurons present in the parietal lobe. They receive information from posterior visual areas (such as MT) and are able to integrate different sensory modalities. It is important in order to have space coding. In particular, this network is involved in encoding peripersonal space according to a body part-centered frame of reference and in transforming object locations into appropriate movements towards them. A connection between LIP and FEF is essential to integrate visual information that is useful to integrate eye movements and attentional focus. For example, if there’s a lesion at the level of the right parietal lobe or specific areas of the frontal lobe, we can observe a syndrome known as Neglect syndrome. These patients completely reject the space located on their left side; when asked to draw a clock, a home, a flower, the components present on the left side are missing. Another example, if the patients are asked to bisect a line, they will draw a perpendicular line on the right side. In conclusion, the visual field that is on the left side is completely neglected, ignored.