Abstracts

Visual information is conveyed by three segregated subcortical pathways from the retina to the primary visual cortex, namely the magno, the parvo and koniocellular pathways. According to the extreme viewpoint, the magnocellular pathway carries motion information, while the other two pathways mediate color information. The parvocellular pathway has been shown to carry red/green signals via retinal L and M cones, the koniocellular pathway to mediate blue/yellow signals via S cones. Recent studies, however, have shown that motion signals can also be carried by the parvo and koniocellular pathways when motion is defined by color alone (isoluminant chromatic stimuli), but it is not clear whether red/green and blue/yellow motion signals remain segregated in extrastriate visual areas beyond V1 (or whether converging to a common cortical motion area, e.g. V5/MT).
A 33 year-old female patient suffering from medically intractable complex-partial seizures was investigated. Visual field testing revealed a left inferior homonymous paramacular quadrantanopsia. Her neurological examination was normal. High-resolution MRI showed a cavernous angioma in the right occipital lobe, partially affecting V1, V2 and V3. Subdural grid electrodes and laminar microelectrodes were implanted over the right occipital lobe, covering the lesion and the surrounding cortex. During presurgical seizure mapping, VEPs were recorded using 306-channel MEG and 64-channel scalp EEG. The same tests were repeated with intracranial contacts. The stimuli consisted of radial sinusoidal gratings defined along the red/green and blue/yellow color axis. These isoluminant stimuli were presented either stationary or in motion in separate trials. The patient had to fixate a central point throughout the task. At least 100 artifact-free trials were averaged per condition.
The MEG results showed a strong response to red/green chromatic motion, characterized by a peak at 150 ms, which was present over both the intact left and the lesioned right occipital cortex to the same extent. In contrast, the VEP response to isoluminant blue/yellow motion peaking at 200 ms post-stimulus was significantly reduced over the right as compared to the left hemisphere. MEG signals to stationary stimuli were not affected. The intracranial EEG recordings mirrored these MEG results.
This case report demonstrates impaired blue/yellow but intact red/green motion signals following a unilateral cortical lesion over V1 to V3. The differential influence of the cortical lesion on VEPs to blue/yellow and red/green motion stimuli illustrates that there is a segregation of the color pathways at the cortical level, beyond V1. V3 is a good candidate for chromatic motion processing along the blue/yellow axis, as it is considered to be the first area of the dorsal stream.
[Supported by: Swiss National Science Foundation, MIND Institute]