Psychohysical hallucinations of orientation and spatial frequency

Area of Science:

• Visual neuroscience
• Computational neuroscience
• Perceptual psychology

Background:

• Subjective visual phenomena, or hallucinations, can arise after adapting to visual stimuli.
• Previous work identified horizontal streaming (H) as a known aftereffect of grating adaptation.
• The existence and characteristics of other aftereffects, such as vertical (V) and diagonal (D) patterns, require further elucidation.

Purpose of the Study:

• To systematically investigate and categorize the distinct types of subjective visual patterns (aftereffects) generated by adapting to vertical sinusoidal gratings.
• To characterize the spatial frequency, orientation, contrast, and duration properties of these aftereffects.
• To propose a neural model of cortical organization that can account for the observed aftereffects.

Main Methods:

• Participants inspected vertical sinusoidal gratings under controlled conditions.
• Subjective visual patterns (H, V, D) were observed and characterized on a uniform test field.
• The influence of adapting spatial frequency and contrast on the aftereffects was systematically varied.

Main Results:

• Three distinct aftereffects were identified: horizontal streaming (H), vertical gratings (V) approximately 1-5 octaves above adapting frequency, and diagonal lines (D) at +/-40 degrees.
• The duration and strength of D increased with higher adapting spatial frequencies, while V declined.
• D and V increased with adapting contrast; H appeared only at the highest contrast, suggesting distinct underlying mechanisms.

Conclusions:

• The identified aftereffects (H, V, D) are functionally distinct, supporting a model of visual cortex organization.
• A proposed model involves modules with orientation-selective pattern channels and orthogonally direction-selective movement channels, with antagonism explaining H.
• Inhibition between modules tuned to different orientations/spatial frequencies explains D and V, suggesting a coarse Fourier transformation by cortical hypercolumns.