Article Abstract:
Experimental evidence exists that when people are presented with a series of letters, they process the information differently, depending on whether the letters are actual words, pseudowords (sequences of letters that obey English spelling rules, such as 'polt'), or nonwords (such as 'NLPFZ'). Thus, activation of different areas of the cortex (the outer layer of brain cells) is known to occur, depending on the semantic content of the visual stimuli. To study this issue in more detail, experiments were carried out using PET (positron emission tomography), an approach that allows cortical activation to be measured. Subjects (eight volunteers) were presented with four sets of stimuli: common nouns that are real words; pseudowords formed according to the rules of English; consonant strings; and strings of shapes that looked like letters, but were not. Seven PET scans were performed for each subject during the presentation of stimuli. Results showed that, while one region (the lateral extrastriate cortex) was activated for all four stimulus sets, the presentation of real words or pseudowords activated another processing area (left medial extrastriate area). This could be the result of the fact that pseudowords 'look like' real words, and, hence, are seen as a legitimate word, or because both stimuli are pronounceable, whereas consonant strings and shapes are not. A rationale for preferring the first explanation is presented. The fact that a region in the visual cortex responds similarly to real words and to pseudowords suggests that information specific to the English language is presented early in the visual processing process. Additional experiments showed that left frontal cortex is activated for real-word presentation, but not for pseudowords. This implies that the left frontal cortex has a role in processing semantic information. Overall, the data show that stimuli requiring semantic versus visual processing are, in fact, apparently processed in different brain regions. (Consumer Summary produced by Reliance Medical Information, Inc.)
User Contributions:
Comment about this article or add new information about this topic:
Article Abstract:
What it is in a scene that enables us to accurately characterize it has not been fully described, but the involvement of two visual subsystems has been postulated. One is a preattentive, more primitive, system that detects simple features of the image simultaneously. The second system inspects collections of features of the image in a serial, or sequential, fashion. More thorough description of these systems should take into account the fact that the visual system builds a two dimensional image out of a world that is, of course, three-dimensional. It is not known to what extent the hypothesized preattentive visual system is sensitive to certain attributes of three-dimensionality such as surface reflectance or viewer orientation. This was explored by asking subjects to search for a target item (a polygon) among a field of items shaded or rotated in various ways to mimic different illumination patterns of a real scene. Subjects responded by pressing a response key when they detected the target item. Results showed that the preattentive system responded fastest to aspects of the scene that would correspond to three-dimensionality, and when lighting came from a direction other than above the object. This suggests that even the most primitive level of visual processing is sensitive to more complex stimulus features than was formerly thought. Such an arrangement has implications for our understanding of how the brain interprets what the eye sees. (Consumer Summary produced by Reliance Medical Information, Inc.)
User Contributions:
Comment about this article or add new information about this topic:
Article Abstract:
The visual reflexes that improve vision during movement were evaluate in four rhesus monkeys. An experiment was conducted to examine the phenomenon whereby, when viewed from a moving vehicle, nearby objects appear to speed by while distant objects scarcely move at all. The monkeys were shown flashing lights while being pulled on sleds. Their compensatory eye movements were measured and analyzed. Two types of reflexes were studied, visual reflexes of the eyes and vestibular reflexes of the otolith organs in the base of the skull. Both visual and vestibular reflexes produced compensatory eye movements that stabilized the image reaching the retina in back of the eye, onto which images are projected. The amount of visual and vestibular response to the monkey's movement depended on the inverse of the distance between the monkey and the flashing light being observed. The inverse is a simple mathematical transformation, so the important conclusion was that both reflexes were distance-dependent. It is suggested that both reflexes respond to the same information about the environment by way of a shared pathway, and also cooperate to compensate for the observer's motion.
User Contributions:
Comment about this article or add new information about this topic: