Labeling the lines: A test of a six-mechanism model of chromatic detection.

Six linear chromatic mechanisms are sufficient to account for the pattern of threshold elevations produced by chromatic noise masking in the (L,M) plane of cone space (Shepard, Swanson, McCarthy, & Eskew, 2016). Here, we report results of asymmetric color matching of the threshold-level tests from that detection study and use those matches to test the detection model. We assume the mechanisms are univariant labeled lines (Rushton, 1972; Watson & Robson, 1981), implying that the chromaticities of physically different stimuli that are detected by a single mechanism should all be the same-they are postreceptoral metamers-but the chromaticities of two stimuli detected by different mechanisms should be different. The results show that color matches fall into six clusters in CIE (u',v') space (across all the noise conditions) and that these clusters correspond closely to the six mechanisms in the model. Most importantly, where the detection model determines that a given test angle is detected by different mechanisms under different noise conditions, the hue of that test angle changes in a consistent way. These color matches allow us to apply a color label to each of the mechanisms, confirm the six-mechanism model, and quantify the hue signaled by each mechanism.

Purkinje Cell Collaterals Enable Output Signals from the Cerebellar Cortex to Feed Back to Purkinje Cells and Interneurons.

Purkinje cells (PCs) provide the sole output from the cerebellar cortex. Although PCs are well characterized on many levels, surprisingly little is known about their axon collaterals and their target neurons within the cerebellar cortex. It has been proposed that PC collaterals transiently control circuit assembly in early development, but it is thought that PC-to-PC connections are subsequently pruned. Here, we find that all PCs have collaterals in young, juvenile, and adult mice. Collaterals are restricted to the parasagittal plane, and most synapses are located in close proximity to PCs. Using optogenetics and electrophysiology, we find that in juveniles and adults, PCs make synapses onto other PCs, molecular layer interneurons, and Lugaro cells, but not onto Golgi cells. These findings establish that PC output can feed back and regulate numerous circuit elements within the cerebellar cortex and is well suited to contribute to processing in parasagittal zones.