What an amazing thing it is to see! Our brains are able to simultaneously process huge amounts of information taken in from our eyes in order to somehow give rise to conscious sight. Shape, edges, movement, depth, texture, and of course color must be properly integrated in order for us to experience what we know as vision. However, very few people can claim to have an in-depth knowledge of how our brains actually accomplish such a tremendous task. Neuroscientists studying the visual system have made it their lives’ work to understand how neural circuits in the visual system function in a way which allows us to perceive and interpret our surroundings.
One such scientist is Edward Callaway of the Salk Institute for Biological Studies. Callaway is a Fellow of The American Association for the Advancement of Science and of the American Academy of Arts and Sciences. He has spent his life using novel methods such as monosynaptic circuit tracing to understand the organization and function of the visual system. Recently, the Callaway lab published an article which revealed that color and orientation are coded in a way that differs from previously accepted models.
It was believed by some that color and orientation are retrieved separately from V1 and come to be represented by completely different cortical columns. These columns are thought to then project separately to higher order areas for additional processing. However, the Callaway lab demonstrates this is not the case.
Callaway and his team used a combinatorial approach of both drug-induced GCAMP6f expression, in vivo two-photon calcium imaging, and postmortem cytochrome oxidase staining to allow for the precise identification of cortical columns in relation to activity.
Fig. 1 In vivo GCaMp6f two-photon calcium imaging in primate V1.
(A) Schematic of experimental setup (see supplementary materials and methods). (B) Average fluorescence of one imaging region after presentation of colored drifting gratings. Four cells are indicated and their corresponding traces are shown in (C). Scale bar: 200 μm. (C) Sample fluorescence traces, indicated by the color of the stimulus to which they responded most strongly. Colored bars indicate the hue of the stimulus displayed at each time point. Adapted from “Color and orientation are jointly coded and spatially organized in primate primary visual cortex,” by E. Callaway, et al., 2019, Science, 1275-1279. Copyright  by American Association for the Advancement of Science.
Anesthetized primates were shown drifting grafts at 12 specific color hues on a grey background while scientists recorded from V1 neurons. Neuron activity was measured using GCAMP6f, which causes changes in cellular fluorescence during calcium events (neuronal spiking). Neurons were then given both orientation selective index scores and color preference index scores based on how often cells fired in response to being shown specific colors or directionally moving shapes.
Researchers discovered a significant correlation between orientation selective index and color preference index scores, indicating a relationship between orientation and color tuning in V1 cells. Additionally, their findings show that of all visually responsive cells analyzed, 11.6 percent of them were both strongly orientation tuned and had a strong color preference. This directly contradicts previous models which suggest a strict separation or color and orientation processing and has predicted no such cells should exist.
These findings show that shape and color are both mutually extracted and represented amongst V1 neurons, and changes the way neuroscientists must think about information processing in the visual system. New models based off these findings will account for the presence of the cells demonstrated through this complex set of experiments.
Callaway will be giving a talk titled, “Functional Organization for Color Appearance Mechanisms in Primary Visual Cortex” on Tuesday, March 31st at 4PM. To watch live stream, click the following link: ZOOM: https://uchealth.zoom.us/j/501283195
Melonie Vaughn is a first year PhD student in Neuroscience currently rotating at the Autism Center of Excellence with Dr. Karen Pierce.