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Eyes right?
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Eyes right? Our eyes flick rapidly from place to place in the visual field around 170 000 times every day. As our eyes remain open during these saccadic eye movements, this means that the retinal image also rapidly moves on each occasion, and yet we do not seem to see this potentially disturbing sight. There are at least three possible explanations. Firstly, we might simply switch off vision during this time. This would, however, mean a loss of the continuity of vision on each occasion. Secondly, we could selectively switch off the detectors that signal motion during this time1. Whilst this theory has some strong support there is as yet no physiological evidence for such selective suppression in the action of single cells. A recent paper provides support for a third possibility – the rapidly changing retinal speeds that occur during the saccade do not allow any one particular detector to provide a motion signal2. Castet and Masson asked subjects to view a low spatial frequency grating that was moving so rapidly that it was invisible when the eyes were stationary. However, if the eyes flicked across the pattern in the same direction as the moving grating, the subject caught a glimpse of a grating during this saccade. The explanation for this is apparently simple. As the eyes accelerate during the saccade the effective speed on the retina of the rapidly moving grating reduces, and reaches a minimum at the peak velocity of the saccade. If the
effective retinal speed at this point falls within the normal range of sensitivity when the eyes are stationary, then the grating is seen, and will be seen as moving or stationary depending upon the relative speeds of the grating and the eye. So why don’t we see the grating during the rest of the saccade when its speed might also fall within the visible range? Because the eye is accelerating it never remains long enough at one speed for a signal to excite any one appropriate detector – it is only at the peak velocity that there is enough time for this to occur. Whilst these results and explanation seem plausible they still need to be reconciled with earlier findings that suggest selective suppression of signals related to motion1. The differences could lie in the orientation of the gratings viewed in these two studies. Clearly, a saccade that transverses many bars of a vertical grating produces very different retinal stimulation from one that moves along a single bar of a horizontal grating – perhaps the visual system has different tricks for these different situations? References 1 Burr, D.C. et al. (1994) Selective suppression of the magnocellular visual pathway during saccadic eye movements. Nature 371, 511–513 2 Castet, E. and Masson, G.S. (2000) Motion perception during saccadic eye movements. Nat. Neurosci. 3, 177–183
Population control? Visual neurophysiology has tended to focus on single-cell recordings. But, given that each visual area is made up of many thousands of neurons, it is important to understand how a population of cells responds to a stimulus. One view argues that the peak of the population activity corresponds to the percept. Instead of taking the usual approach of simultaneously recording from multiple neurons, Treue et al.1 instead recorded from individual neurons when stimuli that contained multiple components were presented. Specifically, they presented stimuli that contained two directions of motion. Such displays are perceived by human observers as containing two directions of motion whenever the directions differ by at least ten degrees. According to the ‘peak theory’ this should mean that the population response to a stimulus containing motion separated by at least ten degrees should contain two peaks. In experiments recording from direction-selective neurons in macaque area MT, Treue et al. found that peaks occur only when the two directions of motion are separated by a much greater angle –
about 90 degrees, although this depended on the directional tuning width of the neuron being recorded. Thus, they argue that the ‘peak theory’ is incorrect, and instead propose a novel scheme that suggests that the visual cortex uses the overall shape of the population response to determine what the percept is. This scheme predicted a series of perceptual metamers (e.g. a stimulus containing more than two directions of motion being interpreted as only two directions), which they confirmed psychophysically in human subjects. Although in this study the physiology was carried out in monkeys, and the psychophysics in humans, this is not likely to invalidate the results. Further research that combines the two methodologies in one species, and eventually extends the approach to other sensory domains, will reveal how generally this scheme is used by the perceptual systems of the brain.
Language rules Does language need rules? Investigations of the English past tense (e.g. Ref. 1) have provided some well-known evidence that the answer to this question is yes. Although irregular forms (e.g. ‘went’ or ‘broke’) might be represented as associative patterns, the range of properties of the regular past tense would seem to require the representation of a real rule. Hajiwara and colleagues haved turned to a new domain – Japanese nominalization – to provide further support that language needs rules2. Their study focused on two suffixes, ‘sa’ and ‘-mi’, which convert adjectives into nouns (comparable to the English suffixes ‘-ness’ as in bright–brightness, and ‘-th’ as in strong–strength). Using a simple ratings task with normal adults, they confirmed the linguistic analysis that ‘-sa’ is used in a rule-like way (e.g. it can apply to any type of adjective, and it can be applied freely to novel adjectives) while the use of ‘-mi’ is more restricted, consistent with its irregular status. In addition to finding this dissociation of linguistic uses, Hajiwara et al. also found a dissociation in brain damaged patients with the two suffixes. In a second experiment patients with Broca’s aphasia, Wernicke’s aphasia, transcortical aphasia and Gogi (wordmeaning) aphasia were asked to provide acceptability judgments for sentences containing the two suffixes. The results showed that the Broca’s aphasics were significantly worse than the other patients at applying the regular suffix ‘-sa’ to novel adjectives. The Gogi (wordmeaning) aphasics, by contrast, tended to over-use the regular ‘-sa’ on novel adjectives when all the other groups correctly chose ‘-mi’. Taken together with the judgments of the normal subjects, these results support the argument that regular and irregular forms are represented differently, and that the regular forms require rules. These results are significant in that they seem to imply a general principle of neural organization across languages, and apply to derivational as well as inflectional morphology. In combination with other techniques (e.g. fMRI studies) this work will add to our knowledge of the neural substrates of language processing.
References 1 Pinker, S. and Prince, A. (1991) Regular and irregular morphology and the psychological status of rules of grammar. Berkeley Linguist. Soc. 17, 230–251
Reference 1 Treue, S. et al. (2000) Seeing multiple directions
2 Hajiwara, H. et al. (1999) Neurolinguistic
of motion: physiology and psychophysics.
evidence for rule-based nominal suffixation.
Nat. Neurosci. 3, 270–276
Language 75, 739–763
169 Trends in Cognitive Sciences – Vol. 4, No. 5,
May 2000
Eyes right? Our eyes flick rapidly from place to place in the visual field around 170 000 times every day. As our eyes remain open during these saccadic eye movements, this means that the retinal image also rapidly moves on each occasion, and yet we do not seem to see this potentially disturbing sight. There are at least three possible explanations. Firstly, we might simply switch off vision during this time. This would, however, mean a loss of the continuity of vision on each occasion. Secondly, we could selectively switch off the detectors that signal motion during this time1. Whilst this theory has some strong support there is as yet no physiological evidence for such selective suppression in the action of single cells. A recent paper provides support for a third possibility – the rapidly changing retinal speeds that occur during the saccade do not allow any one particular detector to provide a motion signal2. Castet and Masson asked subjects to view a low spatial frequency grating that was moving so rapidly that it was invisible when the eyes were stationary. However, if the eyes flicked across the pattern in the same direction as the moving grating, the subject caught a glimpse of a grating during this saccade. The explanation for this is apparently simple. As the eyes accelerate during the saccade the effective speed on the retina of the rapidly moving grating reduces, and reaches a minimum at the peak velocity of the saccade. If the
effective retinal speed at this point falls within the normal range of sensitivity when the eyes are stationary, then the grating is seen, and will be seen as moving or stationary depending upon the relative speeds of the grating and the eye. So why don’t we see the grating during the rest of the saccade when its speed might also fall within the visible range? Because the eye is accelerating it never remains long enough at one speed for a signal to excite any one appropriate detector – it is only at the peak velocity that there is enough time for this to occur. Whilst these results and explanation seem plausible they still need to be reconciled with earlier findings that suggest selective suppression of signals related to motion1. The differences could lie in the orientation of the gratings viewed in these two studies. Clearly, a saccade that transverses many bars of a vertical grating produces very different retinal stimulation from one that moves along a single bar of a horizontal grating – perhaps the visual system has different tricks for these different situations? References 1 Burr, D.C. et al. (1994) Selective suppression of the magnocellular visual pathway during saccadic eye movements. Nature 371, 511–513 2 Castet, E. and Masson, G.S. (2000) Motion perception during saccadic eye movements. Nat. Neurosci. 3, 177–183
Population control? Visual neurophysiology has tended to focus on single-cell recordings. But, given that each visual area is made up of many thousands of neurons, it is important to understand how a population of cells responds to a stimulus. One view argues that the peak of the population activity corresponds to the percept. Instead of taking the usual approach of simultaneously recording from multiple neurons, Treue et al.1 instead recorded from individual neurons when stimuli that contained multiple components were presented. Specifically, they presented stimuli that contained two directions of motion. Such displays are perceived by human observers as containing two directions of motion whenever the directions differ by at least ten degrees. According to the ‘peak theory’ this should mean that the population response to a stimulus containing motion separated by at least ten degrees should contain two peaks. In experiments recording from direction-selective neurons in macaque area MT, Treue et al. found that peaks occur only when the two directions of motion are separated by a much greater angle –
about 90 degrees, although this depended on the directional tuning width of the neuron being recorded. Thus, they argue that the ‘peak theory’ is incorrect, and instead propose a novel scheme that suggests that the visual cortex uses the overall shape of the population response to determine what the percept is. This scheme predicted a series of perceptual metamers (e.g. a stimulus containing more than two directions of motion being interpreted as only two directions), which they confirmed psychophysically in human subjects. Although in this study the physiology was carried out in monkeys, and the psychophysics in humans, this is not likely to invalidate the results. Further research that combines the two methodologies in one species, and eventually extends the approach to other sensory domains, will reveal how generally this scheme is used by the perceptual systems of the brain.
Language rules Does language need rules? Investigations of the English past tense (e.g. Ref. 1) have provided some well-known evidence that the answer to this question is yes. Although irregular forms (e.g. ‘went’ or ‘broke’) might be represented as associative patterns, the range of properties of the regular past tense would seem to require the representation of a real rule. Hajiwara and colleagues haved turned to a new domain – Japanese nominalization – to provide further support that language needs rules2. Their study focused on two suffixes, ‘sa’ and ‘-mi’, which convert adjectives into nouns (comparable to the English suffixes ‘-ness’ as in bright–brightness, and ‘-th’ as in strong–strength). Using a simple ratings task with normal adults, they confirmed the linguistic analysis that ‘-sa’ is used in a rule-like way (e.g. it can apply to any type of adjective, and it can be applied freely to novel adjectives) while the use of ‘-mi’ is more restricted, consistent with its irregular status. In addition to finding this dissociation of linguistic uses, Hajiwara et al. also found a dissociation in brain damaged patients with the two suffixes. In a second experiment patients with Broca’s aphasia, Wernicke’s aphasia, transcortical aphasia and Gogi (wordmeaning) aphasia were asked to provide acceptability judgments for sentences containing the two suffixes. The results showed that the Broca’s aphasics were significantly worse than the other patients at applying the regular suffix ‘-sa’ to novel adjectives. The Gogi (wordmeaning) aphasics, by contrast, tended to over-use the regular ‘-sa’ on novel adjectives when all the other groups correctly chose ‘-mi’. Taken together with the judgments of the normal subjects, these results support the argument that regular and irregular forms are represented differently, and that the regular forms require rules. These results are significant in that they seem to imply a general principle of neural organization across languages, and apply to derivational as well as inflectional morphology. In combination with other techniques (e.g. fMRI studies) this work will add to our knowledge of the neural substrates of language processing.
References 1 Pinker, S. and Prince, A. (1991) Regular and irregular morphology and the psychological status of rules of grammar. Berkeley Linguist. Soc. 17, 230–251
Reference 1 Treue, S. et al. (2000) Seeing multiple directions
2 Hajiwara, H. et al. (1999) Neurolinguistic
of motion: physiology and psychophysics.
evidence for rule-based nominal suffixation.
Nat. Neurosci. 3, 270–276
Language 75, 739–763
169 Trends in Cognitive Sciences – Vol. 4, No. 5,
May 2000