Occipital+Lobes

**FUNCTIONAL OVERVIEW: ** The primary visual cortex is located predominantly within the medial walls of the cerebral hemispheres and is concerned with the elementary aspects of form perception.

From the primary area information is then relayed to the association areas, 18 and 19, where complex analysis including form recognition, position and analysis of depth take place. Visual information is next relayed to area 7 in the parietal lobe and to the inferior temporal lobule, where higher order analysis and multimodal processing occurs. Damage to the parietal-occipital borders may result in abnormalities involving depth and form perception as well as visual neglect. Destruction of the temporal-occipital regions can give rise to visual agnosias and an inability to recognize complex objects and faces. The occipital lobes also appear to be lateralized in regard to certain capabilities such as facial recognition. For example, desruction of the right occipital region is associated with prosopagnosia, and abnormal activity in this area is more likely to give rise to complex visual hallucinations. Simple and complex visual and central/foveal analysis is one of the main functions associated with the occipital lobe. Neurons respond to a number of modalities: vestibular, acoustic, visual, visceral, and somesthetic input. ASSOCIATED SYMPTOMOLOGY Homonymous Hemianopsia Quadrantanopsia Visual Hallucinations Cortical Blindness "Blind Sight" Denial of Blindness Visual Agnosia Visual Alexia Prosopagnosia Simultanagnosia Impaired Color Recognition

THE PRIMARY & ASSOCIATION VISUAL CORTEX Axons from the retina form the optic nerve, are organized so that visual information from the same points in visual space can be combined and rerouted at the otpic chiasm, and then directed so that all information arising from the left vs right half of the visual fields are directed to the right vs left half of the occipital lobes and visual cortex. That is, visual information arising in the nasal portion of the right eye, is combined with that from the lateral-temporal portion of the left eye (and vice versa), such that the primary visual area in the right hemisphere receives information from the left half of visual space. However, this information is first received in the lateral geniculate nucleus of the thalamus, each layer of which receives input from only one eye, the bulk of which is then relayed to the primary visual cortex. The Striate Cortex: Primary visual receiving area The association visual cortices V1: Primary Visual Cortex <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">V2: Prestriate Cortex  <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">V3: The Third Visual Complex  <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">V4: Visual Area Four  <span style="font-family: 'Times New Roman',serif; font-size: 11pt;"> V5: MT - Middle Temporal Region

<span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">PRE-CORTICAL VISUAL ANALYSIS <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">There is much processing of visual input prior to its reception in the occipital lobe and some visual signals appear to initially bypass the primary and association visual areas <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">-Initially: analysis in the receptor cells (rods and cones) within the retina <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">-Successive hierarchical stages of analysis as it is passed through the sequential cell layers of which the retina is composed; i.e. horizontal cells, bipolar cells, amacrine cells, ganglion cells. <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">-Then relayed via the optic nerve to the lateral geniculate nucleus of the thalamus, the pulvinar and superior colliculus, where yet further forms of analysis are performed

<span style="font-family: 'Times New Roman',serif; font-size: 11pt; line-height: 1.5;">From there visual stimuli are transmitted via the optic radiations to the primary visual receiving area, <span style="font-family: 'Times New Roman',serif; font-size: 11pt; line-height: 1.5;">the striate cortex as well as to surrounding areas; e.g. via the pulvinar and superior colliculus. These maintain a strict topographical relationship. Within the visual cortex immediately adjacent groups of neurons respond to visual information from neighboring regions within the retina. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">From the visual cortex this information is then transferred back to the lateral geniculate nucleus of the thalamus to the superior colliculus and to the association areas 18 and 19 as well as the middle temporal lobe where higher order processing occurs. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">Viewing or reading words: Left medial extrastriate visual cortex is activated Looking at pictures: right inferior occipital lobe becomes active <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">These visual association areas also transmit signals back to the primary area, such that highly processed information is passed back and forth, perhaps on a need to know basis. However, as noted, these visual areas are also capable of acting autonomously without pre- or post processing in the primary visual cortex such as via projections form the pulvinar to V5. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">A similar arrangement seems to characterized information reception and transfer in the auditory, somesthetic, and motor cortices as well. That is, information transferred from the thalamus to the neocortex is then transferred back to the thalamus as well as to the adjacent association areas which then transfer information back and forth on a need to know basis. In this manner a feedback loop is constructed so that information transmission from the thalamus to the cortex and within the neocortex, can be enhanced, diminished, or altered depending on neocortical requirements. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">Nevertheless, although different areas can act autonomously, in general, visual stimuli are received in the primary area and then relayed to the association areas18 and 19 (where further analyses take place) this information is transmitted back to area 17 and to the temporal and parietal lobe where poly modal associations are performed. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">NEOCORTICAL COLUMNAR ORGANIZATION <span style="font-family: 'Times New Roman',serif; font-size: 11pt; line-height: 1.5;">Throughout the striate cortex neurons with similar receptive properties are stacked in columns. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">One column of cells may respond to a certain visual orientation and the cells in the next column to an orientation of a slightly different angle. Moreover, columns exist for color, location, movement, etc. In addition, since certain cells respond predominantly to input from one eye, there are ocular (eye) dominance columns as well. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">A similar columnar arrangement in regard to somesthetic input is maintained in the parietal lobe. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">Within these the visual cortex is also organized so that they match and parallel input received within the retina, thus creating a retinotopic map. That is, adjacent cells in these columns receive input from adjacent cells in the retina. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">However, there is also almost a 50% overlap between columns, such that there are shared receptive fields, such that single cells can perceive more than one point in space--which in turn allows for a smooth transition which making eye and head movements. Indeed, neurons of a particular column, although communicating predominantly with those in the same column, also communicate with immediately adjacent columns. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">That is, information is analyzed vertically and horizontally. so as to create a series of superimposed mosaics of the visual word. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">SIMPLE, COMPLEX, LOWER- & HIGHER ORDER HYPERCOMPLEX FEATURE DETECTORS <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">The visual cortex is made up of a variety of cell types each of which is concerned with the analysis of different visual features including simple, complex, and (higher & lower order) hypercomplex cells which are distributed disproportionately throughout areas 17,18, 19. <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">-Initial analysis of incoming visual cortical input, most sensitive to slowly moving stimuli. <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">_Found predominantly within area 17 and in layer IVa,b,c,. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Some are sensitive to stimuli moving in one direction, whereas others may respond to stimuli moving in any direction. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Almost 95% of the neurons in area 17 are responsive to stimuli moving only in one direction, but not the direction of movement. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Responsive to the particular position and orientation a stimulus. For a cell to fire, a stimulus must assume a specific orientation and position. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Simple cells relay this processed information to the far more numerous complex cells which are found predominantly in layers II and III and V, -These interact and communicate with one another including with layer IV which receives thalamic input. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Each complex cell receives input from several simple cells. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-also concerned with orientation of the stimulus. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Probably involved in the earliest stages of actual form perception, i.e. the determination of the outline of an object. A considerable number of complex cells receive converging input from both eyes, the remainder being monocular. <span style="font-family: 'Times New Roman',serif; font-size: 15px;">-Found predominantly within area 18. **<span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">Hypercomplex cells ** <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-analysis of discontinuity <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-angles and corners  <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-movement  <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-position  <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-orientation. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Respond selectively to certain visual configurations and determine precise geometric form. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-(in conjuction with visual neurons in the temporal lobe) the first stages of visual closure are initiated. This in part requires that the functional activity of these cells be suppressed such that when presented with an incomplete figure these cells are overridden and the brain is able to "fill in the gaps" in stimuli perceived. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">-Found predominantly within area 19. **<span style="font-family: 'Times New Roman',serif; font-size: 11pt; line-height: 1.5;">Connections with the Temporal/Parietal Visual Areas ** <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">The visual association areas also maintain intimate relationships with the parietal visual regions (area 7) as well as the visual areas in the middle and inferior temporal lobes (ares 37) including V5 (the lateral occipital-temporal junction). The temporal visual areas are in turn reciprocally interconnected with area 7 and areas 17, 18, 19.
 * S<span style="font-family: 'Times New Roman',serif; font-size: 11pt;">imple cells **
 * Complex Cells**

<span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">Example: the inferior-medial temporal lobe is concerned with form perception and the analysis of emotional-motivational significance and transmits this information to area 7. Area 7 is involved in visual attention, visual fixation, and the analysis of distance, depth, and objects within grasping distance. Area V5 perceives motion. Through this temporal/occipital/parietal pathway, the individual can recognize and fixate on the object (temporal lobe), determine the speed at which it is traveling (V5) and reach out and grasp it (parietal lobe).

<span style="font-family: 'Times New Roman',serif; font-size: 11pt;">OTHER REGIONS FOR VISUAL FUNCTIONING
<span style="font-family: 'Times New Roman',serif; font-size: 11pt;">over half the primate neocortex is also concerned with visual functions. Much of thalamic-visual- retinal input is directed to the primary visual receptive areas in the occipital lobe (Broadmann's area 17) also transmitted directly to areas of temporal and parietal lobes, including signals related to rapid motion and object recognition

<span style="font-family: 'Times New Roman',serif; font-size: 11pt;">superior parietal lobe: receives peripheral and lower visual input via the optic radiations middle and inferior temporal lobes: receive foveal and upper visual field input via these fibers

<span style="font-family: 'Times New Roman',serif; font-size: 11pt;">Areas are functionally specialized to analyze different visual details such as color, motion, and object recognition. V5: can be independently activated by rapid visual motion, whereas parallel activation of area 17 (V1) is not always observed. This independent and semi-independent organization of the visual cortex (and other parts of the brain) and the fact that there are mutliple visual areas each of which are functionally specialized, has important implications regarding "blind sight" as well as other disconnection syndromes.

<span style="font-family: 'Times New Roman',serif; font-size: 11pt;">There are over 30 different visual areas that have been distinguished as based on functional specificity, chemistry, physiology, and cytoarchitexture. That so much of the human/primate neocortex is concerned with visual functioning is in part a reflection of our arboreal and semi-carnivorous ancestry. That is, early in our evolutionary history, the eyes migrated to the front of the head thereby providing for stereoscopic vision, and adaption which promoted life among the trees and the hunting and killing of small animals. Moreover, cortical layers I and VII (6b) are phylogentically ancient in organization and structure and resemble and may well be an extension of midbrain tissue (Marin-Padilla 1988a) which in turn is visually sensitive.

**<span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">MULTIPLE VISUAL REALITIES ** <span style="font-family: 'Times New Roman',serif; font-size: 11pt;">The brain is not only interactional, but is characterized by normal discontinuities where some areas do not always communicate together efficiently. This can give rise to multiple streams of conscious awareness which may not always correspond to reality. The same can be said even regarding those areas of the brain which are ostensibly concerned with the same visual stimulus, such as auditory or vision as is the case with V5 and areas 17, such that when these stimuli are processed, an internal reality which is slightly different from external reality may be produced.

<span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">This led to the concept of dynamic parallelism, by which, while both areas can be healthy and functional in a normal brain, which area gets activated first depends on the nature of the stimulus. <span style="color: #000000; font-family: 'Times New Roman',serif; font-size: 11pt;">Even within the visual system the various "perceptual systems are therefore different as are the processing systems" and "that there is no synchronizer in the brain capable of setting the results of the operations of the two processing-perceptual systems to time zero... that we thought was the hallmark of our visual experience.... the brain, therefore missychronizes and misbinds or rather more accurately, it binds the results of its processing-perceptual systems, not what happens in real time in the real world."