CHAPTER_ 10

INTRODUCTION The pathway serving conscious visual perception includes the lateral geniculate nucleus (LGN) of the thalamus and the primary visual cortex. Information is funnelled through this geniculocortical pathway and is processed in parallel by neurons specialized for the analysis of different stimulus attribute. The striate cortex then feeds this information to different extrastriate cortical areas in the temporal and parietal lobes. THE RETINOFUGAL PROJECTION  The neural pathway leaving the eyes starting with the optic nerve. The Optic Nerve, Optic Chiasm, and Optic Tract  The ganglion cell axons leaving the retina pass through three structures before forming synapses in the brain stem (see Fig. 10.2): o Optic nerve o Optic chiasm o Optic tract  The optic nerves exit the left and right eyes at the optic disks and combine to form the optic chiasm. Here the axons originating in the nasal retinas cross from one side to the other and this is called decussation (i.e. the crossing of a fiber bundle from one side of the brain to the other). In this case the decussation is partial and following this crossing, the axons of the retinofugal projections form the optic tracts. Right and Left Visual Hemifields  Left visual hemifield: objects appearing to the left of the midline  Right visual hemifield: objects appearing to the right of the midline (see Fig. 10.3). The central portion of both visual hemifields is viewed by both retinas and this region of space is the binocular visual field. Objects in the binocular region of the left visual hemifield will be imaged on the nasal retina of the left eye and on the temporal region of the right eye.  Rule of Thumb: optic nerve fibers cross in the optic chiasm so that the left visual hemifield is” viewed” by the right hemisphere and the right visual hemifield is “viewed” by the left hemisphere. Targets of the Optic Tract A small number of optic tract axons peel off to form synaptic connections with cells in the hypothalamus and another 10% innervate the midbrain. Most innervate the lateral geniculate nucleus (LGN) of the dorsal thalamus and these neurons give rise to axons projecting to the primary visual cortex. This projection is called the optic radiation. Lesions in these areas will cause blindness. See Fig. 10.5:  A transaction of the left optic nerve would lead to blindness at the left eye only.  A transaction of the left optic tract would lead to blindness at the right visual field.  A midline transaction of the optic chiasm would affect only the fibers that cross the midline; blindness would result in the peripheral visual fields on both sides. Nonthalamic Targets of the Optic Tract Direct projections to part of the hypothalamus play an important role in synchronizing biological rhythms with the daily dark-light cycle.  Direct projections to part of the midbrain the pretectum control the size of the pupil and certain eye movements.  10% of the ganglion cells project to the superior colliculus and these projections are called the retinotectal projection. o In the superior colliculus, a patch of neurons activated by a point of light commands eye and head movements to bring the image of this point onto the fovea. o Involve in orienting the eyes in response to new stimuli in the visual periphery THE LATERAL GENICULATE NUCLEUS The right and left lateral geniculate nuclei are the major targets of the two optic tracts and each LGN appears to be arranged in six distinct layers of cells numbered 1 through 6 (see Fig. 10.7). The most ventral layer is layer 1. The layers of the LGN are stacked one on top of another and are bent around the optic tract. The LGN is the gateway to the visual cortex and to conscious visual perception. The Segregation of Input by Eye and Ganglion Cell Type The segregation of LGN neurons into layers suggests that different types of retinal information are being kept separate.  At the LGN, input from the two eyes is kept separate. o In the right LGN, the right eye (ipsilateral) axons synapse on LGN cells in layers 2, 3, and 5. The left eye (contralateral) axons synapse on cells in layers 1, 4, and 6 (see Fig. 10.8). Two ventral layers, 1 and 2, contain larger neurons than the other layers, 3 through 6. The ventral layers are thus called magnocellular LGN layers and the dorsal layers are called parvocellular LGN layers.  P-type ganglion cells project exclusively to the parvocellular LGN  M-type ganglion cells project entirely to the magnocellular LGN. Numerous tiny neurons also lie just ventral to each layer called the koniocellular layers. These layers receive input from nonM-nonP types of retinal ganglion cells and also project to visual cortex (see Fig. 10.9). Receptive Fields The visual receptive fields of LGN neurons are almost identical to those ganglion cells that feed them  Magnocellular LGN neurons have large center-surround receptive fields which respond to stimulation of their field centers with bursts of action potential. They are insensitive to differences in wavelength and are most similar to M-type ganglion cells.  Parvocellular LGN cells are most like P-type cells. They have small center-surround receptive fields and their responses are sustained increases in the frequency of action potentials. Many of these cells exhibit colour opponency.  Receptive fields of cells in the koniocellular layers are center-surround and have either light/dark or color opponency. Within all layers of the LGN, the neurons are activated by only one eye (i.e. monocular) and ON- center and OFF-center cells are intermixed. Nonretinal Inputs to the LGN The retina is not the main source of synaptic input to the LGN. The major input, constituting about 80% of the excitatory synapses comes from primary visual cortex. The LGN receives synaptic inputs from neurons in the brain stem whose activity is related to alertness and attentiveness (e.g. see a flash of light when you are startled in a dark room). This input does not directly evoke action potentials in LGN neurons but can powerfully modulate the magnitude of LGN responses to visual stimuli. ANATOMY OF THE STRIATE CORTEX LGN has a single major synaptic target: primary visual cortex. The primary visual cortex is Brodmann’s area 17 and is located in the occipital lobe of the primate brain (see Fig. 10.10). This area is also called V1 and striate cortex. Retinotopy It is an organization whereby neighbouring cells in the retina feed information to neighbouring places in their target structures (i.e. LGN and striate cortex) (see Fig. 10.11). There are three important points to remember about retinotopy:  The mapping of the visual field onto a retinotopically organized structure is often distorted because visual space is not sampled uniformly by the cells in the retina o For example there are more ganglion cells with receptive fields in or near the fovea than in the periphery. Thus the central few degrees of the visual field are overrepresented (i.e. magnified) in the retinotopic map.  A discrete point of light can activate many cells in the retina and often many more cells in the target structure, due to the overlap of receptive fields. Therefore the activity in striate cortex is a broad distribution with a peak at the corresponding retinotopic location.  Don’t be misled by the word “map.” It is NOT a map. Lamination of the Striate Cortex Striate cortex has neuronal cells bodies arranged into about a half-dozen layers named by Roman numerals VI, V, IV, III and II. Layer I, just under the pia mater, is devoid of neurons and consists almost entirely of axons and dendrites of cells in other layers (see Fig. 10.12). The striate cortex is about 2mm thick. In reality there are at least nine layers of neurons. However, to maintain Brodmann’s convention that neorcortex has six layers, three sublayers are combined into layer IV (i.e. IVA, IVB, IVC; IVC is further split into IVCα and IVCβ). The Cells of Different Layers See Fig. 10.13  Spiny stellate cells: small neurons with spine-covered dendrites that radiate out from the cell body and are primarily seen in the two tiers of layer IVC.  Pyramidal cells: located outside of the IVC layer. They are covered with spines and are characterized by a single thick apical dendrite that branches. Only pyramidal cells send axons out of striate cortex to form connections with other parts of the brain. Stellate cells make local connections only within the cortex. Inhibitory neurons (lack spines) are sprinkled in all cortical layers and form only local connections. Inputs and Outputs of the Striate Cortex LGN terminate in several different cortical layers with the largest number going to layer IVC.  Magnocellular LGN neurons project to layer IVCα  Parvocellular LGN neurons project to layer IVCβ  Koniocellular LGN axons bypass layer IV to make synapses in layers II and III. Ocular Dominance Columns Distribution of axon terminals relaying information is not continuous in layer IVC, but rather is split up into a series of equally spaced patches, each about 0.5mm wide. These patches are called ocular dominance columns. Innervation of Other Cortical Layers from Layer IVC Most intracortical connections extend perpendicular to the cortical surface along radial lines that run across layers, from white matter to layer I. Radial connections, thus, maintain the retinotopic organization established in layer IV.  A cell layer in layer VI, for example, received information from the same part of the retina as does a cell above it in layer IV (see Fig. 10.16a). Axons of some layer III pyramidal cells extend collateral branches that make ho