Sensory neuroscience seeks to understand how the brain encodes natural environments. However, neural coding has largely been studied using simplified stimuli. In order to assess whether the brain's coding strategy depends on the stimulus ensemble, we apply a new information-theoretic method that allows unbiased calculation of neural filters (receptive fields) from responses to natural scenes or other complex signals with strong multipoint correlations. In the cat primary visual cortex we compare responses to natural inputs with those to noise inputs matched for luminance and contrast. We find that neural filters adaptively change with the input ensemble so as to increase the information carried by the neural response about the filtered stimulus. Adaptation affects the spatial frequency composition of the filter, enhancing sensitivity to under-represented frequencies in agreement with optimal encoding arguments. Adaptation occurs over 40 s to many minutes, longer than most previously reported forms of adaptation.The neural circuits in the brain that underlie our behaviour are well suited for processing of real-world-or natural-stimuli. These neural circuits, especially at the higher stages of neural processing, may be largely or completely unresponsive to many artificial stimulus sets used to analyse the early stages of sensory processing and, more generally, for systems analysis. Thus, natural stimuli may be necessary to study higher-level neurons. Characterizing neural responses to natural stimuli at early or intermediate stages of neural processing, such as the primary visual cortex, is a necessary step for systematic studies of higher-level neurons. Neural responses are also known to be highly nonlinear 1-3 and adaptive 4-20 , making them difficult to predict across different stimulus sets 21 . Therefore, even early in visual processing, characterizations based on simplified stimuli may not be adequate to understand responses to the natural environment.For these reasons there has been a great deal of interest in studying neural responses to complex, natural stimuli (for example, see refs 1, 21-26 ). However, the relationship between coding of
Sensitivity to grammatical gender was investigated in 22 Russian-speaking aphasic patients, compared with young controls. Experiment 1 used a cued shadowing paradigm to assess gender priming (facilitation and/or inhibition of lexical access by a prenominal modifier with congruent, incongruent or neutral gender). Experiment 2 used a grammaticality judgment paradigm with similar stimuli. Normals showed significant interactions between gender and priming in Experiment 1 (facilitation for feminine and neuter nouns but not for masculines) and Experiment 2 (larger effects of context on feminine and neuter nouns) that we interpret as a Markedness Effect. Patients showed significant priming in Experiment 1 and above-chance accuracy in Experiment 2, but failed to show reduced effects for the least-marked masculine gender (the Markedness Effect) in either experiment. Context effects were not related to specific aphasic symptoms or subtypes in either experiment. However, canonical correlation revealed differential effects of specific aphasic symptoms on judgment accuracy (false alarms vs. misses). We conclude that knowledge of grammatical gender is spared in Russian aphasics, but gender processing is deviant. A possible model to account for these differences is discussed.
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