Orientation bandwidths are invariant across spatiotemporal frequency after isotropic components are removed.
It is well established that mammalian visual cortex possesses a large proportion of orientation-selective neurons. Attempts to measure the bandwidth of these mechanisms psychophysically have yielded highly variable results ( approximately 6 degrees -180 degrees ). Two stimulus factors have been proposed to account for this variability: spatial and temporal frequency; with several studies indicating broader bandwidths at low spatial and high temporal frequencies. We estimated orientation bandwidths using a classic overlay masking paradigm across a range of spatiotemporal frequencies (0.5, 2, and 8 c.p.d.; 1.6 and 12.5 Hz) with target and mask presented either monoptically or dichoptically. A standard three-parameter Gaussian model (amplitude and width, mean fixed at 0 degrees ) confirms that bandwidths generally increase at low spatial and high temporal frequencies. When incorporating an additional orientation-untuned (isotropic) amplitude component, however, we find that not only are the amplitudes of isotropic and orientation-tuned components highly dependent upon stimulus spatiotemporal frequency, but orientation bandwidths are highly invariant ( approximately 30 degrees half width half amplitude). These results suggest that previously reported spatiotemporally contingent bandwidth effects may have confounded bandwidth with isotropic (so-called cross-orientation) masking. Interestingly, the magnitudes of all monoptically derived parameter estimates were found to transfer dichoptically suggesting a cortical locus for both isotropic and orientation-tuned masking.
Cass
John
J
School of Psychology, University of Sydney, Australia. johnc@psych.usyd.edu.au
Stuit
Sjoerd
S
Bex
Peter
P
Alais
David
D
D000222:Adaptation, Physiological; D006801:Humans; D009474:Neurons / Q000502:physiology*; D010470:Perceptual Masking; D010775:Photic Stimulation; D011601:Psychophysics; D012684:Sensory Thresholds; D013028:Space Perception; D013997:Time Factors; D015348:Vision, Binocular; D015349:Vision, Monocular; D014793:Visual Cortex / Q000502:physiology*