Spatial frequency tuning of brightness polarity identification

Area of Science:

• Neuroscience
• Visual Perception
• Computational Neuroscience

Background:

• Primary visual cortex cells are known to encode luminance borders.
• Recent findings suggest these cells also encode surface brightness.
• The underlying mechanism for brightness encoding (extraclassical receptive field vs. filling-in) remains unclear and implies different spatial frequency tunings.

Purpose of the Study:

• To investigate the spatial frequency tuning of brightness and border encoding mechanisms in the visual cortex.
• To differentiate between a large extraclassical receptive field model and a brightness filling-in model for surface brightness perception.

Main Methods:

• A psychophysical masking paradigm was employed.
• Spatial frequency tuning functions were measured for identifying luminance polarity (bright/dark) and border orientation.
• Stimuli included oval and circular luminance patches of varying diameters (0.2-10 degrees).

Main Results:

• Nearly overlapping, narrow (1.5 octave) bandpass masking tuning functions were observed for both luminance polarity and border orientation tasks.
• Tuning functions were centered between 1.5-5.0 cycles per degree (c/deg).
• Stimulus size and shape had minimal impact on the observed spatial frequency tuning.

Conclusions:

• The results strongly support a brightness filling-in mechanism rather than a large extraclassical receptive field.
• Luminance border-activated cells appear to modulate activity in cells signaling surface brightness.
• The mechanism for brightness processing in the visual cortex is demonstrably spatial frequency selective.