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Winrich A. Freiwald, Doris Y. Tsao, Roger B. H. Tootell, Margaret S. Livingstone; Complex and dynamic receptive field structure in macaque cortical area V4d. Journal of Vision 2004;4(8):184. doi: 10.1167/4.8.184.
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Area V4 is an important intermediate stage in the ventral shape processing pathway. Receptive field structure of cells in early areas (V1 and V2) within this pathway can be characterized by reverse correlation techniques, while shape selectivity in higher areas (TEO and TE) has mainly been characterized by response profiles to sets of complex stimuli. Can coding in V4 be understood within the same conceptual framework as that in early cortical areas? To what extent can response selectivity for complex shapes be explained by RF substructure? Here we used a novel reverse correlation technique (Livingstone et al., Neuron 30, 2001, 781ff.) to analyze RF substructure of neurons in cortical area V4d in the fixating macaque monkey. Cells (n=133) were stimulated with a sparse noise stimulus consisting of pairs of small squares of same or opposite contrast flashed at 60 Hz at random positions within the RF. Reverse correlation of spikes to the absolute and relative position of the squares allowed the construction of a temporal sequence of first and second order RF maps. This technique revealed RF substructure for more than 90% of V4 neurons with varying degrees of orientation tuning (determined in separate experiments). Significant 1st and 2nd order structure, both contrast-dependent and independent, was repeatedly observed. Some of these interaction patterns and their temporal dynamics are indicative of new coding principles beyond what has so far been reported for V1 and V2. In separate experiments on a subset of cells, complex shape selectivity was characterized by another reverse correlation technique invoking the presentation of a rapid sequence of Cartesian and non-Cartesian stimuli (Gallant et al., Science 259, 1993, 100ff.). A comparison of RF substructure with shape selectivity revealed systematic relationships between the two, but indicates the necessity for inclusion of higher order terms as well.
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