Visual Perception

According to Kosslyn (1999), since the time of Aristotle it has been thought that visual perception and visual imagery share the same underlying mechanisms. Behavioral studies have demonstrated that when a person uses imagery, he or she experiences a deficit in perception and vice versa. That these two activities interfere with each other suggest that they are competing for the same neural apparatus. If both imagery and the perception of objects use the same neural apparatus, then under some circumstances a person should confuse imaged objects with seen objects. Johnson and Raye (1981) demonstrated this confusion in normal college students. Studies of brain-damaged individuals (Levine, Warach, & Farah, 1985) and functional imaging (using positron-emission tomography) provide further support for this hypothesis. For example, when asked to image objects, the individual's calcarine cortex (V1 or primary visual cortex) (see Figure 1.1) will show activation (see Kosslyn, 1999, for a review of this literature). Hence, to understand the brain mechanisms that might account for creative imagery, we might find it helpful to review briefly how stimuli are seen, perceived, comprehended, and stored by the brain.

Visual stimuli come into the eye and fall on the back portion of the eye, where the retina is. The retina contains special nerve cells

(ganglion cells) that become activated when light falls on them, and these ganglion cells send electrical messages along the optic nerve until they reach a relay station in the middle of the brain called the geniculate or optic thalamus. There are several classes of ganglion cells, including large cells that are called magnocellular and smaller cells that are called parvocellular. The larger magnocellular cells are able to process information more rapidly and hence are more important in visualizing motion. The parvocellular cells appear to have a higher spatial resolution and are better able to respond to spatial frequencies that define colors. These two types of neurons project to different regions of the optic thalamus. A second series of nerves sends messages from the optic thalamus to the occipital lobes. The part of the occipital cortex that receives these signals from the optic thalamus is called the calcarine or primary visual cortex, also called V1 or Brodmann's area 17 (see Figure 1.1). The cells in this region of the occipital lobe detect changes in brightness that occur in specific spatial locations on the retina. The detection of these brightness changes together with their relative location (spatial array) allows a person to develop what David Marr (1982) called a primal sketch. The edges of a visual stimulus usually have the greatest changes of intensity, and the intensity changes allow the viewer to separate the stimulus from the background.

The primary visual cortex (V1 or Brodmann's area 17) sends the information contained in this primal sketch to the visual association areas that surround the primary cortex. According to Fellerman and Van Essen (1991) there are at least 32 different visual association areas. Each of these areas processes a different aspect of incoming visual information. These areas are highly connected to each other, and Fellerman and Van Essen (1991) estimated that each of these 32 areas is probably connected to as many as 15 other areas. The primary visual cortex is spatially organized such that stimulus position in space is recorded spatially in the primary visual cortex, but as this information is progressively processed by visual association areas there is a loss of this point-to-point topographic mapping because the cells in regions of the visual association cortex are not all spatially organized. These visual association areas are important for the further processing of the visual stimulus.

On the ventral or bottom surface of the occipital lobes, there appears to be a region that is important in the processing of color. About 20 years ago, I was a visiting professor at the University of Iowa. Hanna and Antonio Damasio showed me a patient who had a stroke in the lower portion of his temporal-occipital region on the right side of his brain (see Figure 1.1). I asked this patient to stare at my nose and to tell me what happens to my pen when I move it from his right to his left. When I held the red pen in his right visual field, he said, "I see a red ballpoint pen." When I started to move the pen to the left, he said, "Now it's moving leftward." When the pen crossed his midline (midsagittal plane) into his left visual field, he said, "Now I can still see the pen but it has no color." He told me that everything on his left side was now in black and white. When I asked him to hold the pen, he had no difficulty in reaching and grasping the pen even when I moved it. Therefore, although this patient had an inability to see colors (achromotopsia), he could still see and recognize objects and estimate their size (as determined by how wide he opened his hand when he went to grasp the object), and he knew where in space this pen was located, including its movement. For those who would like to read more about this case and view the actual injury, you can find further details in the report by Damasio, Yamada, Damasio, Corbett, and McKee (1980). Functional imaging studies of the brain during the time normal participants are processing colors have provided converging evidence that this ventral-temporal-occipital region is important for color processing (Zeki et al., 199l).

In contrast to that patient who could not see color but could detect movement, Zihl, von Cramon, and Mai (1983) reported a patient who could see and recognize objects and colors but could not see movement (akinotopsia). Therefore, when this patient looked at something moving, he saw it as being frozen in space or as jumping from one portion of space to another. This patient had bilateral injury to the lateral posterior temporal-occipital areas of the brain, a region called V5 (Figure 4.1). Studies of monkeys and functional imaging studies in humans (Zeki et al., 1991) have provided converging evidence that this portion of the visual association cortex is important in perceiving visual motion.

At the beginning of the 20th century, Balint (1909) described a neurobehavioral syndrome that now bears his name. Patients with this syndrome have three neurological signs: optic ataxia, psychic paralysis of gaze, and simultanagnosia. This last sign, simultanagnosia, I describe later in this chapter. Patients with Balint's syndrome can fully perceive objects and recognize the meaning of objects. They can also see color and detect motion, but when they attempt to touch or grasp an object they have difficulty locating its spatial position and therefore their hands seem to wander through space in search of the object. This deficit in accurate reaching is called optic ataxia, and this disorder is thought to be related to an inability to visually compute where the object is positioned in viewer-centered space. Psychic paralysis of gaze is a similar defect of ocular movement. The most

Figure 4.1. Diagram showing area V5, in the lateral posterior temporal-occipital areas of the brain. This visual area is important for motion detection.

Figure 4.1. Diagram showing area V5, in the lateral posterior temporal-occipital areas of the brain. This visual area is important for motion detection.

sensitive portion of our retina is the fovea. When our attention is drawn to a stimulus, we rapidly move our eyes (saccade) such that the object of interest falls on the fovea. In patients with psychic paralysis of gaze, however, they cannot visually compute the spatial location of this object and they move their eyes to incorrect positions until, almost by chance, the object of interest falls on their fovea. Thus, psychic paralysis of gaze is similar to optic ataxia, except that the latter is a form of misreaching but the former is a form of mislooking. Both disorders, however, are related to a patient's inability to perceive and compute visual-spatial location. Patients with Balint's syndrome, which includes optic ataxia and psychic paralysis, have their lesions in the superior portion of the dorsal lateral parietal-occipital cortex, also called the dorsal stream (see Figure 1.1), and their problem in computing spatial location is related to damage to the parts of the visual association cortex that make these spatial computations.

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