Figure 4.

Dendrogram summarizing cluster analysis of the STRFs from the combined sample of 46 V1 and 96 V2 neurons. Lengths of horizontal lines quantify the difference between pairs of STRFs. The bars on the right identify neurons from V1 (white) and V2 (black). There are two significantly separated major clusters (clusters A and B, indicated by the pale red and blue regions; p < 0.001, randomization test; the p = 0.01 criterion is indicated by t). Cluster A contains 43% (41 of 96) of the V2 neurons in the sample and 78% (36 of 46) of the V1 neurons. Cluster A has two significant subclusters, A1 and A2 (p = 0.043; randomization test). Neurons in subcluster A1 have conventional excitatory tuning and minimal suppressive tuning, whereas neurons in subcluster A2 are weakly tuned for orientation. Together, neurons in cluster A have functional properties consistent with those reported previously in V1. Cluster B contains 57% (55 of 96) of the V2 neurons and only 22% (10 of 46) of the V1 neurons. Cluster B also has two significant (p < 0.001; randomization test) subclusters. Neurons in both subclusters have conventional excitatory tuning but also show strong suppressive tuning, which is not observed in V1. This suppressive tuning is strongest in subcluster B2.