The contribution of magnocellular selective adaptation to spatial distance compression

Topographic maps early in visual processing preserve the spatial relations of visual stimuli but the metric relationships between these visual directions is not directly accessible. To investigate the magnocellular pathway's role in metric spatial vision, we employed an adaptation paradigm. Exposure to a 60 Hz flickering disc array (subjectively invisible) induced a systematic compression in the perceived distance between subsequently presented dot pairs. This compression was strongest when adaptation preferentially modulated low spatial frequency channels, consistent with the properties of transient channels tuned to low spatial and high temporal frequencies. Crucially, this compression was attenuated when the adaptor consisted of two cyan lattices rotating on a magenta background near isoluminance, as confirmed by a global motion direction discrimination task. The same pattern emerged when test dots were isoluminant with the background, ruling out test-adaptor similarity as a critical factor. Finally, an isoluminant red-green adaptor flickering on a yellow background induced compression at 3 Hz, but not at 60 Hz. This dissociation aligns with the known properties of magnocellular neurons, which are insesitive to high temporal frequency isoluminant red-green modulation, but can respond to slow isoluminant red-green modulations. These findings reveal a novel role of the magnocellular pathway in metric spatial vision.