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Loes Koelewijn, Julie R. Dumont, Suresh D. Muthukumaraswamy, Anina N. Rich, Krish D. Singh; Orientation selectivity in primary visual cortex using MEG: an inverse oblique effect?. Journal of Vision 2010;10(7):928. doi: 10.1167/10.7.928.
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Orientation discrimination is much better for horizontal or vertical than for orientations with a 45-degree tilt. In partial support of these behavioural findings, some animal physiology studies show that a moderately larger number of neurons are tuned to cardinal than oblique orientations in primary visual cortex, and the former are more tightly tuned to their preferred orientation. A limited number of human neuroimaging studies also support this classic ‘oblique effect’, with the BOLD response localising the neural effect to V1, and EEG demonstrating both increased response magnitudes and reduced latencies. How is orientation selectivity reflected in the magnetoencephalography signal? The animal literature shows that GABAergic interneurons play a critical role in orientation selectivity. As these inhibitory interneurons are also important for stimulus-induced gamma oscillations, it is likely that responses in the gamma spectrum are influenced by orientation. We measured the evoked response, as well as the initial spike and sustained induced gamma response to maximum contrast, 3 cycle/degree, stationary black/white sine-wave circular grating patches of diameter 4 degrees. Three orientations (0, 45 and 90 degrees from vertical) were randomly chosen and presented in the lower left quadrant 2.5 degrees from fixation. Our results point towards a larger induced gamma response for oblique stimuli over cardinal ones in contralateral V1, reflected both in the initial spike as well as in the sustained response during stimulus presentation. The specific frequency of the peak response did not differ. Interestingly, in contrast to the EEG findings, we also found this ‘inverse oblique effect’ in the evoked response around 80ms post-stimulus. These results may suggest that V1 neurons have a more complex response tuning to non-preferred orientations. Alternatively, the results may be due to an oddball phenomenon, implicating that cardinals are grouped in perception, and that oblique stimuli attracted attention.
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