Abstract
A plaid is a combination of two gratings whose orientations are orthogonal to each other with the same or similar contrasts. We used plaid patterns as stimuli to investigate the mechanisms underlying the detection of a plaid to understand how the visual system combines information from orientation-selective channels. We used a masking paradigm in which an observer was required to detect a target (either a spiral or a plaid) superimposed on a pedestal. We measured the target threshold versus pedestal contrast (TvC) functions at 7 pedestal contrasts for various target-pedestal combinations with a temporal 2AFC paradigm and a staircase procedure. All TvC functions, except the one with an orthogonal spiral pedestal, showed a dipper shape, although the position of the dip and the slope varied across conditions. The result can be explained by a multiple-mechanism divisive inhibition model, which contains several orientation-selective mechanisms. The response of each mechanism is the excitation of a linear filter divided by a broadband inhibitory input. The threshold is determined by a nonlinear combination of the responses of those mechanisms. Alternative models with mechanism(s) specific for plaid did not provide a better description of the data. Thus, a plaid pattern is mediated by a combination of orientation-selective mechanisms. An early plaid-specific mechanism is not necessary for plaid detection.
Highlights
Much of our understanding of early visual processing comes from experiments using periodic patterns, such as sine-wave gratings or Gabor patches, as stimuli
We used plaid patterns as stimuli to investigate the mechanisms underlying the detection of a plaid to understand how the visual system combines information from orientation-selective channels
The reason for using such stimuli is to maximize the response of neurons in the primary visual cortex whose receptive fields contain elongated excitatory and inhibitory regions [1,2,3]; such a receptive field resembles a wavelet modulated along a specific orientation [4]
Summary
Much of our understanding of early visual processing comes from experiments using periodic patterns, such as sine-wave gratings or Gabor patches, as stimuli. The reason for using such stimuli is to maximize the response of neurons in the primary visual cortex whose receptive fields contain elongated excitatory and inhibitory regions [1,2,3]; such a receptive field resembles a wavelet modulated along a specific orientation [4]. An image is more complicated than a periodic pattern. A natural image contains numerous objects that are, in turn, composed of numerous image features. To comprehend human visual processing, it is crucial to understand how the visual system integrates individual image features into a coherent percept of an object.
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