5Anatomy of the Visual System The EyesVergence Movementthe cooperative movement of the eyes, which ensures that the image of an object falls on identical portions of both retinasSaccadic Movement (suh kad ik)the rapid, jerky movement of the eyes used in scanning a visual scenePursuit Movementthe movement that the eyes make to maintain an image of a moving object on the fovea
6Anatomy of the Visual System The EyesAccommodationchanges in the thickness of the lens of the eye, accomplished by the ciliary muscles, that focus images of near or distant objects on the retinaRetinathe neural tissue and photoreceptive cells located on the inner surface of the posterior portion of the eye
7The 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.
8Another 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.
12Anatomy of the Visual System Connections between Eye and BrainDorsal 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 cortexMagnocellular Layerone 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 cortexParvocellular Layerone 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 cortexKoniocellular 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
17Coding of Visual Information in the Retina Coding of Light and DarkOver 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.
29Analysis of Visual Information: Role of the Striate Cortex Orientation and MovementSimple Cellan orientation-sensitive neuron in the striate cortex whose receptive field is organized in an opponent fashionComplex Cella 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 orientationHypercomplex Cella 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
31Although 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.
35In 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.)
38Two 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.)
39Analysis of Visual Information: Role of the Association Cortex Two Streams of Visual AnalysisThe 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.)
40Analysis of Visual Information: Role of the Association Cortex Two Streams of Visual AnalysisDorsal Streama 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 cortexVentral Streama system of interconnected regions of visual cortex involved in the perception of form, beginning with the striate cortex and ending with the inferior temporal cortexThe 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.)
42Analysis of Visual Information: Role of the Association Cortex Studies with HumansDamage 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 damageProsopagnosia (prah soh pag no zha)failure to recognize particular people by the sight of their facesA common symptom of visual agnosia is prosopagnosia: inability to recognize particular faces (prosopon is Greek for “face”).