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Head saccades
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Sketchpad capacity One of the most influential theories in cognitive psychology proposes that a working memory system is responsible for holding and manipulating information while performing a range of more complex tasks involving learning, reasoning and comprehending ~. The system is posited to involve a central executive that coordinates the operation of two, limited capacity, subsidiary slave systems, the phonological loop and the visuo-spatial sketchpad, which process speech-based and visuospatial information, respectively. While a considerable effort has been devoted to the characterization of the phonological loop, less attention has been focused on the visuo-spatial sketchpad. Luck and Vogel, in their recent article reported that the storage capacity of the visuo-spatial sketchpad is restricted to approximately four items of information at any one time z. In their basic
paradigm, subjects were exposed to a sample array containing 1-12 coloured squares on a VDU for a period of 100 ms. Following a delay period during which all the stimuli were removed from the screen for 900 ms, a test array was presented. Stimuli in the test arrays were either identical to the those observed in the sample array or they differed in the colour of one square. The subjects' task was to determine if the test array was the same or different from the sample. Performance, as measured by the accuracy of this judgement, was almost perfect if there were fewer than four squares in the array and declined as a function of the number of stimuli in the array thereafter. Luck and Vogel concluded that the subjects were able to retain the colours of approximately four items in working memory. In a series of control experiments they were able to demonstrate that the capacity for visual
information was similar when the stimuli were shapes with different orientations, and that it was not due to perceptual or encoding limitations imposed within the paradigm. The performance profile was similar when the objects were defined by a conjunction of four different features (colour, orientation, shape and the presence of a gap), suggesting that the working memory system stores integrated objects rather that individual features of objects. The authors suggest that these results indicate that the visual working memory system has a large capacity for the storage of individual features, providing those features are linked to a relatively small number of objects. These characteristics should inform future models of visual working memory.
References 1 Baddeley, A.D. (1986) Working Memory, Oxford 2 Luck,S.J.and Vogel, E.K.(1997)The capacity of visual working memory for features and conjunctionsNature 390, 2~/9-281
Face neurons and Head t h e p r e f r o n t a l cortex saccades A major issue in research into the functional organization of the prefrontal cortex is whether this area is organized with respect to the modality of the afferent input it receives. Thus, anatomical, lesion and functional neuroimaging data have been suggested to support a modalityspecific account of prefrontal cortex function in which more dorsal areas process spatial ('where') information and more ventral prefrontal areas are concerned with object ('what') processing. This view has been recently challenged ~. (5 Scalaidhe, Wilson and Goldman-Rakic now report the latest results from an electrophysiological approach to the study of this problem 2. Multiple areas of prefrontal cortex in three awake macaque monkeys were mapped for selective responses to pictures of faces. The face-selective neurons that were identified either continued to fire, or began to fire after stimulus offset. More than 95% of face-selective neurons were found in regions of the prefrontal cortex that also receive inputs from temporal lobe visual areas. All were
found within areas of the prefrontal cortex that have previously been suggested to encode object-specific information (inferior convexity, lateral orbital and arcuate sulcus) rather than spatial information (dorsolateral prefrontal cortex). Indeed, the authors noted that none of the 660 neurons recorded in dorsolateral areas (principle sulcus, superior prefrontal convexity) responded to face stimuli. These results, which were obtained using ecologically salient stimuli, are interpreted as providing further evidence in support of the authors' contention that the prefrontal cortex is functionally compartmentalized with respect to afferent inputs.
References 1 Petrides, M. (1996) Specializedsystemsfor the processing of mnemonic information within the primate frontal cortex Philos. Trans. R. Soc.London Ser. B 351, 1455-1462 2 6 5calaidhe, S.P.. Wilson, F.A.W. and Goldman-Rakic,P.S.(1997)Arealsegregation of face-processing neurons in prefrontal cortexScience278, 1135-1138
W o u l d y o u like to contribute to Monitor? The Monitor section of Trends in Cognitive Sciences provides a summary of recently published papers that may be of interest to scientists across the broad spectrum of cognitive sciences, A typical Monitor contribution has a short introduction that sets the scene, a description of the approach taken, the key results obtained, the conclusions reached and their implications. If you think that you could write all this within 200-250 clear and concise words then get in touch with the Editor ([email protected]), as we are always on the lookout for suitable writers. Regular contributors can expect to receive a free subscription to Trends in Cognitive Sciences.
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Copyright© 1997, ElsevierScienceLtd. All rights reserved.1364-6613/97/$17.00 Trends in C o g n i t i v e Sciences - Vol. 1, No. 9, December 1997
When reading this passage of text your eyes will scan the words on the page in a saccadic manner, with your eyes alternating between short fast movements and stable fixations. The eye movements will typically be between seven to nine characters long and will be mostly to the right, with the occasional leftwards saccade. Information will be extracted from the text during the fixation periods (200250 ms). At the end of a line there will be a large leftward saccade. Reading therefore requires the coordination of eye movements and visual information processing. But what would happen if you could not move your eyes? Gilchrist, Brown and Findlay report that a subject (AI) with an, apparently congenital, extraocular muscular fibrosis that prevented eye movement from birth has adopted a strategy to overcome this problem and this has facilitated the acquisition of an advanced educational standard. Subject Al's reading skills are supported by head movements that have a very similar pattern to the normal saccadic eye movements described above. Rather than adopting a smooth scanning strategy, AI has developed saccadic head movements to facilitate visual information processing. This result suggests that saccadic movements may therefore provide the optimum method for sampling visual material. Reference Gilchrist, LD., Brown, V. and Findlay, J.M. (1997) Saccades without eye movements Nature 390, 130-131
U~ ~ ~i!i~¸~ ~i ! %~i~!~
Sketchpad capacity One of the most influential theories in cognitive psychology proposes that a working memory system is responsible for holding and manipulating information while performing a range of more complex tasks involving learning, reasoning and comprehending ~. The system is posited to involve a central executive that coordinates the operation of two, limited capacity, subsidiary slave systems, the phonological loop and the visuo-spatial sketchpad, which process speech-based and visuospatial information, respectively. While a considerable effort has been devoted to the characterization of the phonological loop, less attention has been focused on the visuo-spatial sketchpad. Luck and Vogel, in their recent article reported that the storage capacity of the visuo-spatial sketchpad is restricted to approximately four items of information at any one time z. In their basic
paradigm, subjects were exposed to a sample array containing 1-12 coloured squares on a VDU for a period of 100 ms. Following a delay period during which all the stimuli were removed from the screen for 900 ms, a test array was presented. Stimuli in the test arrays were either identical to the those observed in the sample array or they differed in the colour of one square. The subjects' task was to determine if the test array was the same or different from the sample. Performance, as measured by the accuracy of this judgement, was almost perfect if there were fewer than four squares in the array and declined as a function of the number of stimuli in the array thereafter. Luck and Vogel concluded that the subjects were able to retain the colours of approximately four items in working memory. In a series of control experiments they were able to demonstrate that the capacity for visual
information was similar when the stimuli were shapes with different orientations, and that it was not due to perceptual or encoding limitations imposed within the paradigm. The performance profile was similar when the objects were defined by a conjunction of four different features (colour, orientation, shape and the presence of a gap), suggesting that the working memory system stores integrated objects rather that individual features of objects. The authors suggest that these results indicate that the visual working memory system has a large capacity for the storage of individual features, providing those features are linked to a relatively small number of objects. These characteristics should inform future models of visual working memory.
References 1 Baddeley, A.D. (1986) Working Memory, Oxford 2 Luck,S.J.and Vogel, E.K.(1997)The capacity of visual working memory for features and conjunctionsNature 390, 2~/9-281
Face neurons and Head t h e p r e f r o n t a l cortex saccades A major issue in research into the functional organization of the prefrontal cortex is whether this area is organized with respect to the modality of the afferent input it receives. Thus, anatomical, lesion and functional neuroimaging data have been suggested to support a modalityspecific account of prefrontal cortex function in which more dorsal areas process spatial ('where') information and more ventral prefrontal areas are concerned with object ('what') processing. This view has been recently challenged ~. (5 Scalaidhe, Wilson and Goldman-Rakic now report the latest results from an electrophysiological approach to the study of this problem 2. Multiple areas of prefrontal cortex in three awake macaque monkeys were mapped for selective responses to pictures of faces. The face-selective neurons that were identified either continued to fire, or began to fire after stimulus offset. More than 95% of face-selective neurons were found in regions of the prefrontal cortex that also receive inputs from temporal lobe visual areas. All were
found within areas of the prefrontal cortex that have previously been suggested to encode object-specific information (inferior convexity, lateral orbital and arcuate sulcus) rather than spatial information (dorsolateral prefrontal cortex). Indeed, the authors noted that none of the 660 neurons recorded in dorsolateral areas (principle sulcus, superior prefrontal convexity) responded to face stimuli. These results, which were obtained using ecologically salient stimuli, are interpreted as providing further evidence in support of the authors' contention that the prefrontal cortex is functionally compartmentalized with respect to afferent inputs.
References 1 Petrides, M. (1996) Specializedsystemsfor the processing of mnemonic information within the primate frontal cortex Philos. Trans. R. Soc.London Ser. B 351, 1455-1462 2 6 5calaidhe, S.P.. Wilson, F.A.W. and Goldman-Rakic,P.S.(1997)Arealsegregation of face-processing neurons in prefrontal cortexScience278, 1135-1138
W o u l d y o u like to contribute to Monitor? The Monitor section of Trends in Cognitive Sciences provides a summary of recently published papers that may be of interest to scientists across the broad spectrum of cognitive sciences, A typical Monitor contribution has a short introduction that sets the scene, a description of the approach taken, the key results obtained, the conclusions reached and their implications. If you think that you could write all this within 200-250 clear and concise words then get in touch with the Editor ([email protected]), as we are always on the lookout for suitable writers. Regular contributors can expect to receive a free subscription to Trends in Cognitive Sciences.
O
Copyright© 1997, ElsevierScienceLtd. All rights reserved.1364-6613/97/$17.00 Trends in C o g n i t i v e Sciences - Vol. 1, No. 9, December 1997
When reading this passage of text your eyes will scan the words on the page in a saccadic manner, with your eyes alternating between short fast movements and stable fixations. The eye movements will typically be between seven to nine characters long and will be mostly to the right, with the occasional leftwards saccade. Information will be extracted from the text during the fixation periods (200250 ms). At the end of a line there will be a large leftward saccade. Reading therefore requires the coordination of eye movements and visual information processing. But what would happen if you could not move your eyes? Gilchrist, Brown and Findlay report that a subject (AI) with an, apparently congenital, extraocular muscular fibrosis that prevented eye movement from birth has adopted a strategy to overcome this problem and this has facilitated the acquisition of an advanced educational standard. Subject Al's reading skills are supported by head movements that have a very similar pattern to the normal saccadic eye movements described above. Rather than adopting a smooth scanning strategy, AI has developed saccadic head movements to facilitate visual information processing. This result suggests that saccadic movements may therefore provide the optimum method for sampling visual material. Reference Gilchrist, LD., Brown, V. and Findlay, J.M. (1997) Saccades without eye movements Nature 390, 130-131