1 Copyright © 2014 Elsevier Inc. All rights reserved. Chapter 19 Visual Network Moran Furman

2 Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 19.1 The Necker cube illusion. A two-dimensional drawing of a cube is normally perceived as a 3D structure appearing in one of two possible configurations. When observed continuously over tens of seconds, perception will typically alternate back and forth between the two 3D shapes.

3 Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 19.2 The retina. (A) Light enters the eye through the lens and forms an image on the retina, which lines the inner surface of the eye. (B) Magnified view of the retina in the region around the fovea. The fovea corresponds to the central fixation point, where visual acuity is maximal. The retina is a layered structure composed of neurons and glial cells. Rod and cone photoreceptors are located in the outermost layer of the retina, farthest from the lens. Therefore, light traverses the entire thickness of the retina to reach them, except in the fovea, where light reaches photoreceptors with minimal distortion. Bipolar, horizontal, and amacrine cells mediate signals from photoreceptors to retinal ganglion cells (RGCs). RGCs project their axons to the optic nerve and to the brain. (Source: Reproduced, with permission, from Blumenfeld H. Neuroanatomy through Clinical Cases. 2nd ed. Sinauer Associates, Inc.; 2010.)

4 Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 19.3 Center-surround receptive fields of retinal ganglion cells (RGCs). Schematic receptive field and response pattern of an on-center (A) and off-center (B) RGC. An on-center RGC responds with a train of action potentials to the presence of a light spot in the center of the receptive field, and it is inhibited by light in the surround. Conversely, off-center RGCs are excited by light in the surround but inhibited by light in the center of the receptive field. (Source: Reproduced, with permission, from Blumenfeld H. Neuroanatomy through Clinical Cases. 2nd ed. Sinauer Associates, Inc.; 2010.)

5 Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 19.4 Distributed hierarchical processing in the visual cortex of the macaque monkey. The visual system is organized in a parallel, interacting processing stream, organized in a semihierarchical structure. Area V1 (the primary visual cortex) projects directly and indirectly to a large network of extrastriate visual areas that specialize in processing specific aspects of the visual scene (see the text for details). RGCs: retinal ganglion cells; LGN: lateral geniculate nucleus; M: magnocellular pathway; P: parvocellular pathway. (Source: Reproduced, with permission, from Felleman DJ, Van Essen DC. Distributed hierarchical processing in the primate cerebral cortex. Cereb Cortex. 1991;1:1–47.)

6 Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 19.5 Ventral and dorsal cortical pathways. Overall organization of the ventral and dorsal cortical streams and their input pathways. Cortical areas downstream of V1 can be roughly organized into two processing streams. The ventral stream culminates in areas of the inferior temporal lobe, and it is involved primarily in object recognition and related perceptual functions. The dorsal stream culminates in association areas of the parietal lobe, and it is involved in spatial relationships among objects and visual guidance for motor actions. V1 receives visual input via the retino- geniculate pathway. A second visual pathway initiating in the retina conveys visual information to the parietal cortex via the superior-colliculus and thalamic pulvinar. In higher mammals, this colliculo-pulvinar-cortical pathway is less dominant than the retino-geniculate pathway, but it plays important roles in eye movements, spatial attention, and rapid motion processing. (Source: Reproduced, with permission, from Wandell BA, et al. Visual field maps in human cortex. Neuron. 2007;56:366–383.)

7 Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 19.6 Spatial stability during eye movements is achieved through integration of visual and motor signals. During eye movements, the visual system “compensates” for shifts in the location of the retinal image, to generate visual representations in externally based coordinates (“spatial stability”). Whereas early visual areas, such as V1, encode information in purely retinal coordinates, higher association areas of the cortex, such as the LIP in the parietal lobe and the FEF in the frontal lobe, adjust their responses during eye movements to compensate for the shift in the retinal image. Perception correlates with neural representations in higher association areas but does not correlate with retinal representations in the early visual areas. Eye movement compensation is achieved by integrating visual signals, and a motor signal that is a copy (“corollary discharge”) of the movement command is sent to the eyes (A, left). The superior colliculus, for instance, sends motor commands to generate eye movements, and at the same time it projects through the medial dorsal nucleus of the thalamus (MD) to the FEF (A, right). During saccadic eye movements, neurons in the FEF exhibit “shifting receptive fields” (B, C). To probe the responses properties of these neurons, visual stimuli are presented before and after an eye movement in their receptive field (RF), as well as in their “future field” (FF), which corresponds to the RF after the eye movement (B). Notably, shortly prior to the eye movement, these neurons become nonresponsive to stimuli in the RF but respond robustly to stimuli in the FF (C). (Source: Reproduced, with permission, from Wurtz RH. Neuronal mechanisms of visual stability. Vision Res. 2008;48:2070–2089.)