The contribution of psychophysical spatial frequency channels to the discrimination of broadband contrast.

The design and function of the human visual system is thought to have been shaped by the environment and tasks humans have performed throughout evolution and experience. Thus, it is important to establish the association between the properties of natural scenes and the responses of the visual system to these features at a behaviourally relevant level (e.g., psychophysically). A relevant property of natural scenes - which human observers are sensitive to - is the slope of the orientation averaged Fourier amplitude spectrum (α). It characterizes the decrease in amplitude as a function of spatial frequency (1/fα), and has an average value of 1.0 (on logarithmic axes) in natural scenes. The process of detecting a change in α is thought to stem from the activity of one or more spatial frequency channels, but their exact contribution remains to be defined. The overarching goal of this dissertation was to assess the contribution of spatial frequency channels to the detection, and discrimination, of a change in α. We first set out to obtain psychophysical evidence that α discrimination thresholds were dependent on the spatial frequency content of noise stimuli with 1/f amplitude spectra. Our results showed that while all spatial frequency channels contribute to α discrimination, the channel(s) of most influence seem to correlate in peak spatial frequency to the dominant perceptual scale of the noise image. While the contribution of low spatial frequency channels discrimination of steep αs was evident from our data, the influence of higher spatial frequency channels to the discrimination of shallower αs was not. In an attempt to better isolate the role of high spatial frequency channels, we attempted to modulate their activity with trans-cranial Direct Current Stimulation (tDCS) and change α discrimination thresholds. However, while this technique is capable of modulating channels to alter contrast detection, we found tDCS to be an ill-suited technique to alter the response characteristics of channels under suprathreshold conditions (i.e., broadband noise discrimination). As a final effort to isolate the contribution of spatial frequency channels to the discrimination of broadband contrast, we used a classification image paradigm to understand how different spatial frequencies contribute to the identification of a change in α. Interestingly, this method revealed task dependent contributions of spatial frequency channels to the discrimination of α. The identification of α is specific to α, in that increments and decrements in contrast in specific spatial frequency bands signal for particular αs, while the identification of a change in α (i.e., discrimination) was not specific to α. Regardless of the reference α, observers used an increment in contrast in low spatial frequency bands and decrement of contrast in higher spatial frequency bands to identify the odd stimulus. Taken together, these findings demonstrate that the discrimination of α is unspecific to α. Observers to rely on differences in contrast between low and high spatial frequency bands to detect a change in α, but may not be particularly tuned to certain α values as has previously been argued.