Anatomy of the Visual System 5 The Eyes Vergence Movement the cooperative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinas Saccadic Movement ( suh kad ik ) the rapid, jerky movement of the eyes used in scanning a visual scene Pursuit Movement the movement that the eyes make to maintain an image of a moving object on the fovea
Anatomy of the Visual System 6 The Eyes Accommodation changes in the thickness of the lens of the eye, accomplished by the ciliary muscles, that focus images of near or distant objects on the retina Retina the neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye
7 The human retina contains approximately 120 million rods and 6 million cones. The fovea, or central region of the retina—which mediates our most acute vision— contains only cones.
8 Another feature of the retina is the optic disk, where the axons conveying visual information gather together and leave the eye through the optic nerve. The optic disk produces a blind spot because no receptors are located there.
Anatomy of the Visual System 12 Connections between Eye and Brain Dorsal Lateral Geniculate Nucleus (LGN) a group of cell bodies within the lateral geniculate body of the thalamus; receives inputs from the retina and projects to the primary visual cortex Magnocellular Layer one of the inner two layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for the perception of form, movement, depth, and small differences in brightness to the primary visual cortex Parvocellular Layer one of the four outer layers of neurons in the dorsal lateral geniculate nucleus; transmits information necessary for perception of color and fine details to the primary visual cortex Koniocellular Sublayer ( koh nee oh sell yew lur ) one of the sublayers of neurons in the dorsal lateral geniculate nucleus found ventral to each of the magnocellular and parvocellular layers; transmits information from short-wavelength (“blue”) cones to the primary visual cortex
Coding of Visual Information in the Retina 17 Coding of Light and Dark Over seventy years ago, Hartline (1938) discovered that the frog retina contained three types of ganglion cells. ON cells responded with an excitatory burst when the retina was illuminated, OFF cells responded when the light was turned off, and ON/OFF cells responded briefly when the light went on and again when it went off. Kuffler (1952, 1953), recording from ganglion cells in the retina of the cat, discovered that their receptive field consists of a roughly circular center, surrounded by a ring. Stimulation of the center or surrounding fields had contrary effects: ON cells were excited by light falling in the central field ( center ) and were inhibited by light falling in the surrounding field ( surround ), whereas OFF cells responded in the opposite manner.
27 The striate cortex consists of six principal layers (and several sublayers), arranged in bands parallel to the surface.
Analysis of Visual Information: Role of the Striate Cortex 29 Orientation and Movement Simple Cell an orientation-sensitive neuron in the striate cortex whose receptive field is organized in an opponent fashion Complex Cell a neuron in the visual cortex that responds to the presence of a line segment with a particular orientation located within its receptive field, especially when the line moves perpendicularly to its orientation Hypercomplex Cell a neuron in the visual cortex that responds to the presence of a line segment with a particular orientation that ends at a particular point within the cell’s receptive field
31 Although the early studies by Hubel and Wiesel suggested that neurons in the primary visual cortex detected lines and edges, subsequent research found that they actually responded best to sine-wave gratings (De Valois, Albrecht, and Thorell, 1978). Figure 6.24 compares a sine-wave grating with a more familiar square-wave grating.
35 In addition, they will all have the same ocular dominance—that is, the same percentage of input from each of the eyes. If we move our electrode around the module, we will find that these two characteristics— orientation sensitivity and ocular dominance—vary systematically and are arranged at right angles to each other. (See Figure 6.29.)
38 Two Streams of Visual Analysis The hidden regions are shown in dark gray, while regions that are normally visible (the surfaces of gyri) are shown in light gray. Figure 6.32(e) shows an unrolling of the cortical surface caudal to the dotted red line and green lines in Figure 6.32(c) and 6.32(d). (See Figure 6.32.)
Analysis of Visual Information: Role of the Association Cortex 39 Two Streams of Visual Analysis The outputs of the striate cortex (area V1) are sent to area V2, a region of the extrastriate cortex just adjacent to V1. As we saw in Figure 6.28, a dye for cytochrome oxidase reveals blobs in V1 and three kinds of stripes in V2. Neurons in V1 blobs project to thin stripes, and neurons outside the blobs in V1 project to thick stripes and pale stripes. Thus, neurons in the thin stripes of V2 receive information concerning color, and those in the thick stripes and pale stripes receive information about orientation, spatial frequency, movement, and retinal disparity. (See Figure 6.33.)
Analysis of Visual Information: Role of the Association Cortex 40 Two Streams of Visual Analysis Dorsal Stream a system of interconnected regions of visual cortex involved in the perception of spatial location, beginning with the striate cortex and ending with the posterior parietal cortex Ventral Stream a system of interconnected regions of visual cortex involved in the perception of form, beginning with the striate cortex and ending with the inferior temporal cortex The primary behavioral function of the dorsal stream is to provide visual information that guides navigation and skilled movements directed toward objects, and that of the ventral stream is to provide visual information about the size, shape, color, and texture of objects (including, as we shall see, other people). (See Figure 6.34.)
Analysis of Visual Information: Role of the Association Cortex 42 Studies with Humans Damage to the human visual association cortex can cause a category of deficits known as visual agnosia. Agnosia (“failure to know”) refers to an inability to perceive or identify a stimulus by means of a particular sensory modality, even though its details can be detected by means of that modality and the person retains relatively normal intellectual capacity. Visual Agnosia ( ag no zha ) deficits in visual perception in the absence of blindness; caused by brain damage Prosopagnosia ( prah soh pag no zha ) failure to recognize particular people by the sight of their faces A common symptom of visual agnosia is prosopagnosia: inability to recognize particular faces ( prosopon is Greek for “face”).