Bach M, Maurer JP, Heinrich TS (2001) Assessing cortical motion-direction tuning from the population response in the motion-onset VEP. Investigative Ophthalmology & Visual Science 42, S321
Purpose. When the visual evoked response (VEP) to motion-onset is measured after motion adaptation, a global effect at all orientations and a specific effect at the adapting direction with a broad angle resolution is found. We set out to resolve orientation tuning, calculating cortical direction tuning from the VEP mass-response via a linear population model.
Methods. Stimuli were randomly distributed dots of 0.25° size, moving in 7 directions at a speed of 8.6°/s, or remaining stationary. A dot-lifetime of 93 ms adapted the global effect. Motion VEPs were recorded in 6 subjects at 3 scalp locations in a blocked baseline (=stationary)–adapted (motion to right)–recovery design. The analysis model assumed elementary motion detectors with a Gaussian angle-selectivity profile. The amount of individual adaptation was proportional to the overlap of any unit's direction profile with the adapting direction. Population response was calculated as the average of the convolution of all adaptation-weighted direction profiles with the test direction.
Results. We observed sizable adaptation effects in all subjects, without a global effect. The broad adaptation-induced 'notch' in the VEP-direction profile was well modeled by elementary motion detectors with a Gaussian profile of 79° full width at half height.
Conclusion. A limited dot-lifetime preadapts non-motion specific detectors, removing the global effect, isolating motion-specific VEP components. A simple linear population model with a ±40° FWHM angle selectivity fitted the VEP data well. This angular resolution compares well with single cell data from non-human primates.