Greg Detre
Wednesday, 24 October, 2001
Dr Iles, week 3
�The � mistake is to suppose that there are only two pathways emanating from V1� (Zeki, 1993). Discuss this comment in relation to parallel pathways in vision, especially the infero-temporal.
Describe and analyse the
contribution of different cortical areas to the visual processing of EITHER movement OR binocular stereopsis.
What functional significance can be attributed to
the presence of orientation selectivity in the primary visual cortex?
How are the visual pathways from retina to cortex
organised to provide high spatial acuity in primates?
Describe the relationship between the physiology and
perception of EITHER depth OR motion.
Describe the methods used for classifying and
investigating the receptive fields of simple and complex cells. What advances
in our understanding of the underlying neuronal circuitry have been made since
the original work of Hubel & Wiesel in the 1960s?
Discuss the neural control of eye movements that
mimic retinal image slip during head and body motion.
Evaluate the contribution of the cortical area V1 to
the processing of binocular information.
�The � mistake is to suppose that there are only two
pathways emanating from V1� (Zeki, 1993). Discuss this comment in relation to
parallel pathways in vision.
How is a moving visual target acquired and tracked
by human observers?
parallel processing - look at the IT side, and face recognition
how distinct are they anatomically - Young (Nature, 1992, pg 155)
accept the 2-stream model of vision as a useful simplifying model, but be careful
Livingstone + Hubel were wrong - inputs are not purely p- and m- - it's all mixed up
chapter 30 (+ surrounding), pp 441-464
From the first step of visual processing onwards, the neurons involved in different aspects of vision are distinct - discovery of molecular diversity among neurons can be exploited to study the organization and function of the primate visual system.
This has worked best so far in the early regions of the visual pathway (retina, LGN and V1) but there may be much to be learned of higher-order visual areas through the application of molecular biology to visual system function and organisation.
Type-specific markers have been found in the mammalian retina. There are two types:
a) Those that make sense, i.e. cells whose basic function is a direct function of the molecule, i.e. the photoreceptors stand out by their expression of photpigments or opsins
b) Many type-specific proteins or peptides make no apparent sense for the function of a neuron. Whereas much may be known of the role the molecule plays, nothing can be said about what any of them contributes to the unique functional properties of the relevant neuron type. Nevertheless, even neurochemical signatures whose function we don't understand can tell us about the density and distribution, morphological variation, synaptic organisation etc.
Neurochemically distinct bipolar cells are morphologically distinct.
Early reviews of the primate visual system suggest that the retina sends two sorts of signals to the LGN: a colour-opponent signal arising from P cells, and a broad-band, luminance contrast signal from a population of larger magnocellular cells. The discovery of several neurochemically distinct retinal circuits and of several retinogeniculate classes of ganglion cells indicates that other channels exist in the monkey visual system.
The koniocellular is the third LGN population, with generally very small somata. They can be viewed in isolation because they express two proteins that cells of the magnocellular and parvocellular layers do not. The K layers occupy regions ventral to each of the magnocellular and parvocellular layers in macaques. They are the most direction projections to the patches in layer III of V1 known as 'blobs'.
Blobs in V1 and thin stripes in V2 are viewed as a continuation of a
color system, interblobs in V1 and pale stripes in V2 as the formation of a
system dealing with stimulus shape, and a magnocellular-dominated system,
routed through layer IVB of V1 into thick stripes of V2, as part of a neural
mechanism for motion processing 48 . Other studies indicate, however, that the
physiological differ-ences between neurons in blobs and interblobs of V1 (Refs
49,50) and among neurons of thin stripes, thick stripes and pale stripes in V2
(Refs 51,52) are more limited. Arguments as to which view is closer to the
truth have revolved around methods and strategies, particularly as to which
visual stimuli are most appropriate for studying receptive fields of cortical
neurons.
From a viewpoint provided by neuronal chemistry and synaptic circuits, CO-rich compartments in V1 and V2 scarcely resemble CO-poor compartments, as afferent terminations 53 , intrinsic circuits 54 and efferent projections differ among them 55. Differences between blobs and interblobs extend to neurochemistry, as neurons expressing neuroactive substances such as glutamate and substance P, receptors such as GABA A and glutamate receptor subunits, cytoplasmic proteins such as calbindin, parvalbumin and microtubule asso-ciated protein-2 and cell-surface proteins such as the Cat-301 protein are either enriched in or confined to neurons in blobs or interblobs. So only if neurons sample equally from the two compartments would reported similarities in blob and interblob physiological properties be consistent with the substantive differences in their circuits and molecules. Unfortunately, the data on this point are conflicting, and, until this fundamental anatomical issue is more persuasively resolved, an agnostic point of view concerning the physiological distinctions between CO-rich and CO-poor compartments seems advisable.
Looking at the extrastriate cortex, it seems that functional precision arises out of anatomical homogeneity. Extrastriate areas exhibit only minor variations in cell sizes or densities ot mark the transition from one area to the next.
Extrastriate areas express similar neurochemical patterns as well as common cytoarchitecture. This is either because the science itself is too limited, or because it is solely in the afferents and projections that the homotypical areas of cerebral cortex differ.
pg 6 �
contrast sensitivity???
what difference fast/slow responding � spatial acuity (how???) vs speed of processing???
colour opponency??? different types???
how is there a blindspot � surely one eye covers for the other???
is the �magno� and �parvo� based on cell body size (rather than RF)???
what does �konio� mean???
how are the cortical layers usually arranged??? in V1??? why is layer 4 subdivided???
what does V3 do??? it�s part of the dorsal stream
did Zeki really think that about the P- and M- systems???
what does Zeki argue for???
type-specific markers
what are the S, M and L cones???