Abstract
Typical psychophysical methods for evaluating human visual processing of spatial scale and orientation (at the striate cortex level) involve measuring threshold sensitivity for single sinusoidal gratings of different spatial frequencies (SFs) and/or orientations (Os). However, in the natural environment, the visual system typically processes spatial content that is broadband (energy at many SFs and Os) and is well above threshold. Thus, while such methods provide useful inferences regarding the sensitivity of small sub-populations of striate cortical cells at threshold, they do not provide information about how those sub-populations interact when presented high-contrast, broadband content. We therefore sought to investigate human visual processing of high-contrast broadband spatial structure at different Os with two 16-condition suprathreshold matching experiments. The tasks required participants to match the perceived strength of different amounts of oriented structure alone, or embedded in 1/f visual noise, to that of a standard stimulus. Stimuli were generated from broadband isotropic 1/f noise patterns by filtering their amplitude spectra to contain a test increment across a specified range of Os and SFs. The test increment's O and SF bandwidth was systematically varied from a single SF (16cpd) to a broadband (many Os and SFs) increment. Results showed the traditional oblique effect (worse sensitivity at the obliques) when a small range of Os and high SFs were incremented. A horizontal effect (worst sensitivity at horizontal and best at the obliques) was observed for broadband increments of ∼20° in orientation and 1-octave in frequency and larger. The current results are the first of a series of experiments designed to assess the extent of the interactions within and between different sub-populations of neurons in striate cortex in order to devise a striate normalization model to provide insight into visual processing of naturalistic content.
This work was supported by a grant from the Office of Naval Research (grant # N00014-03-1-0224) and from the Kentucky Space Grant Consortium (KSGC - NASA EPSCOR).