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Processes in Biological Vision PROCESSES IN BIOLOGICAL VISION: including, ELECTROCHEMISTRY OF THE NEURON This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. James T. Fulton Vision Concepts [email protected] April 30, 2017 C o p y r i g h t 2 0 0 1 James T. Fulton Higher Level Perception Pt I 15- 1 [xxx Consolidate references to Tootel ] 15 Higher Level Perception– PART I Signal (Vector) Interpretation 1 This chapter has been divided into two PARTS because of its size and the rapid advances being made in the understanding of the operation of the brain. This PART I is concerned with our understanding of the operation of the brain, as reflected in the journal literature, up to the middle of the 1980's. PART I still includes considerable material not available in any other text published through the 1990's. PART I contains a description of the cortical portion of visual system and human brain current as of the 1998 time period and the processing of visuotopic (as opposed to abstract) signals. This description is sufficiently advanced, relative to the literature that it forms an important bridge leading to the more advanced material in PART II. PART II introduces the important results of the Brain Research community as they apply to an understanding of the complete visual system. It contains a further expansion on the figures found in Part I in order to describe our knowledge of the system circa 2002. The major new material deals with the critically important role of the thalamic reticular nucleus of the diencephalon portion of the “old brain.” Its focus is on the extraction of abstract signals from the visuotopic signals presented to the feature extraction engines, and the ultimate processing of those signals. 15.1 Introduction UPDATE re 15.6 Our understanding of the functional elements of the diencephalon and mesencephalon is maturing rapidly. This makes it difficult to keep this chapter current. A particular problem concerns the engines found in the space morphologically labeled the thalamus and superior colliculus. In some cases, the literature cited will use the term pretectum of the tectum for what is becoming known as the perigeniculate nucleus of the thalamus. Part II of this chapter will provide further details in this area. This Chapter will discuss higher level (both sub-cortical or old brain and neo-cortical) signal processing occurring in the CNS. This processing will be called signal manipulation for clarity. Of more importance, it will discuss the “context” in which imagery is manipulated and stored. The subject of morphology will not be addressed in detail but the anatomy and topology of the nervous system will be. As discussed in the introduction to PART D, stage 4 of the visual system includes both perceptual and cognitive functions. The literature of cognition is very large. However, prior to the development of MRI, PET and CAT imaging techniques, it has been a very immature field Although interesting concepts have been put forward for discussion, cognition remains poorly understood. At the perceptual level, more progress has been made, helped immensely recently by the development of the above imaging techniques. Spillmann & Werner have published an excellent compendium of the state of knowledge regarding the operation of the cerebral cortex, circa 1990, based primarily on psychophysics, clinical experience, anatomy of the cortex and some cytology2. Some differences in terminology and notation will be used in this work for reasons explained below. Much less is known about the function and capability of the cerebellum. Based on this work, its role appears to be of paramount importance in the efferent signal paths of vision. 1Released: April 30, 2017 2Spillmann, L. & Werner, J. (1990) Visual perception: the neurophysiological foundations. NY: Academic Press. [My Library] 2 Processes in Biological Vision More recently, an event of major importance has occurred. Semir Zeki of University College London, a prolific writer and leader in the investigation of the visual system of the brain, experienced an epiphany. His book of 19933 reflects his paradigm shift in thinking as do his articles subsequent to that date (see the preface, prolog and epilog). He apologizes for his long allegiance to the conventional wisdom of the last century and explains how his work subsequent to the early 1980's has forced this major change. The seeds of the change are seen in the series of papers he presented in block in 19784 where he notes the absence of an intersection between the vertical and horizontal meridians in V1 and the presence of signals in V4 that cannot be traced to V1. His epilogue in that book should not be overlooked. His new paradigm still suffers from the lack of a viable model of the visual system adequate for his purposes. Lacking such a model, he still supports the conventional concept of morphologists that “form dictates function.” However, his support is greatly diluted and continuing to fade (see his 1995 paper with Becker). His concept of the cause of the putative phenomena of color constancy is still inadequate. Lacking a model, he also continues to speak of the M and P pathways in a nebulous manner, exhibiting little appreciation of the multiple classes of signals being carried by these pathways. Zeki’s new concept of the brain is as a modular system without a hierarchal structure and with multiple input pathways from the sub-cortical areas. This work is in total agreement with his new concept. However, his lack of a model constrains his interpretation at the detailed level. A better model would also provide a clearer understanding of the transform between motion in object space and the waveform parameters in temporal space that must be converted back to a vector representative of said object space motion. In the following material, the data of Zeki is used extensively in support of this work. Opposition will be taken to many of his positions in his pre-1990 articles. However, it appears he no longer supports these older positions (See Ffytche, Guy & Zeki and Beckers & Zeki below). Most of the textbooks of the 1990's and 2000 have not incorporated Zeki’s new views. This is unfortunate. His views are being confirmed daily using the newer techniques of nuclear imaging and his additional technique of magnetic interference. Tootell & Hadjikhani have recently provided additional data on this question of the existence of a V45. In the magnetic interference technique, Zeki is using very high strength magnetic fields to disturb the electrical conductance of signals within the brain. This technique shares with his earlier degenerative techniques, involving lesions, an inadequate spatial specificity and no known direct relationship with the underlying mechanisms. It is also difficult to implement because of the folding of the cortex. Crick & Koch provided a provocative article entitled Consciousness and Neuroscience in 19986. It followed a paper that generated mild turmoil in the neuroscience community by stating the obvious7,8. They took the position that a subject “is not directly conscious of the features represented by the neural activity in primary visual cortex” (V1). The “extremely schematic diagram” of Figure 1 in the 1995 paper is obviously inadequate for their purposes. The 1998 paper includes a mini-review as well as a series of hypotheses. Their premises appear to be based on good logic, using a variety of anecdotal experimental results, but lack a sufficiently detailed model to avoid argument. Their task is complicated by the lack of sufficiently specific definitions of the terms used. While the discussion 3Zeki, S. (1993) A vision of the brain. London: Blackwell Scientific Publications 4Zeki, S. (1978) J. Physiol. (London), vol. 277: Three papers pp. 227 to 290 5Tootell, R. & Hadjikhani, N. (2001) Where is ‘Dorsal V4' in human visual cortex? Retinotopic, topographic and functional evidence Cerebral Cortex vol 11, pp 298-311 6Crick, F. & Koch, C. (1998) Consciousness and Neuroscience Cerebral Cortex vol. 8, pp 97-107 7Crick, F. & Koch, C. (1995) Are we aware of neural activity in primary visual cortex? Nature vol. 375, pp 121- 123 8Pollen, D. (1995) Cortical areas in visual awareness Nature vol. 377, pp 293-294 (reply on pp 294-295) Higher Level Perception Pt I 15- 3 between Crick & Koch, and others like Pollen, are applauded, their lack of precision prevents this author from joining the debate in its current form. The models provided in this Chapter go far toward framing the hypotheses of Crick & Koch, and several other recent authors, more precisely. More specifically, the hypotheses of Crick & Koch do not recognize the multiple signal paths through the brain related to visual signals. They rely upon a simple ladder concept attributed to Fuster. One leg of the ladder relates to afferent pathways and the other to efferent pathways. The approach is similar to that discussed in Section 11.1.4. The brain is much to complex to continue to be represented by such a simple framework. A more elaborate ladder will be presented in Section 15.2.2. This ladder, along with the top level schematic diagram of chordate vision in Section 15.2.3 provide a framework sufficient to discuss many of the terms used in the above articles. They also help provide a clearer separation between terms like perception and recognition. The concept of a zombie becomes much clearer using these models. As Crick & Hoch describe the zombie conceived by philosophers in their carefree way, a zombie “is supposed to act just as normal people do but to be completely unconscious.” By using the models of this chapter, additional differentiation can be introduced into what a zombie can and cannot do.
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