Society for Neuroscience 2006
Oct. 14-18, 2006 in Atlanta, Georgia

Clustering of color-selective neurons in macaque area V4: revisited with multiple single-unit recording

Yasuyo Kotake, Hiroshi Morimoto, Hiroshi Tamura, Ichiro Fujita
Grad. Sch. Frontier Biosciences, Osaka Univ.; CREST, JST, Japan

It is unclear how color-selective neurons are spatially arranged in area V4. Initial studies by Zeki (1973, 1980) reported that V4 neurons are clustered into columns and arranged orderly across the cortex according to their preferred color. A later study, however, found no evidence for this type of organization (Schein et al. 1982). These contradicting results were based on qualitative assessments of the degree of clustering. Here, we quantitatively assessed clustering of color-selective neurons in V4. Using multiple single-unit recording techniques, we recorded extracellular activities simultaneously from 2-12 nearby neurons in V4 of two anesthetized monkeys (Macaca fuscata). We examined their responses to 27 isoluminant color stimuli. A majority (72%) of the neurons selectively responded to a range of the colors (p < 0.05). We employed the color discrimination index (CDI), a measure characterizing a neuronfs stimulus discrimination ability normalized to the response variability across the trials. The CDI values were positively correlated between nearby neurons (n = 404, r = 0.38, p < 0.001), indicating that nearby neurons showed a similar degree of color tuning. We compared the similarity of color preferences for nearby color-selective neuron pairs (n = 202) with the correlation coefficient of their responses to the 27 colors. The correlation coefficients varied widely (?0.63 to 0.93), but the distribution was shifted to positive values (median, 0.32). The correlation coefficients of nearby pairs were greater than those of distant pairs (median, 0.11; p < 0.001). Nearby V4 neurons as a whole, weakly shared color preferences. To examine further the congruency of color preference among neurons in a local region, we measured the Euclidean distance on a MacLeod-Boynton chromaticity diagram between the optimal colors for nearby neurons, and calculated the standard deviation (SD) of the distances at each recording site. The distribution of the SDs was bimodal. The results suggest that V4 contains two types of local regions of color-selective neurons: one where nearby neurons share color preferences and one where color preferences are independent of neighboring neurons. Alternatively, clusters of V4 neurons with different color preferences may occasionally abut at singularity points, similar to orientation columns with different preferences that meet at pinwheel centers in V1.
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