This experiment was undertaken to clarify some conflicting explanations of phase effects on surround modulation of perceived contrast.
Ejima and Takahashi (1985) first reported that iso-orientation contrast suppression diminishes and sometimes changes to enhancement when the center and surround gratings are 180° out of phase. They explained this phase effect as a result of brightness induction due to local luminance contrast. The darkness of the dark bars and the brightness of the light bars of the center grating are enhanced by abutting opposite-polarity bars of the out-of-phase surround grating and produce an overall contrast enhancement that offsets contrast suppression. On the other hand,
Olzak and Laurinen (1999) reported that surround modulation of perceived contrast is affected by phase for sinusoidal gratings, but not for plaid gratings. They proposed a theory of multiple-stage gain-control processes in surround modulation of perceived contrast, in which surround modulation for simple sinusoidal gratings is a lower level phase-dependent process that “appears to operate only over spatially aligned pathways with similar phase or polarity tuning” and surround modulation for more complex plaid gratings is a higher level phase-independent process.
Xing and Heeger (2000) recently replicated a number of the previous experiments on surround modulation of perceived contrast using sinusoidal gratings as stimuli. To help their observers distinguish center and surround stimuli, they introduced a small center-surround gap, and surround modulation as they reported is unaffected by phase! These data appear to contradict
Olzak and Laurinen’s (1999) theory of phase-dependent first-order processing for sinusoidal gratings but favor
Ejima and Takahashi’s (1985) brightness induction explanation. The small center-surround gap diminishes local luminance (edge) contrast, which in turn diminishes brightness induction, but the contrast-suppression effect remains relatively unaffected.
We measured surround effects for iso-oriented sinusoidal gratings (8 cpd), in phase and out of phase, with the center-surround gap varying from 0 arcmin to 4 arcmin. The center and surround, when in phase, were clearly distinguishable at a gap of 4 arcmin. The center contrasts were 0.25 and 0.70, with the surround contrast constant at 0.40. The 0.25 center contrast was close to the 0.18 center contrast used by
Olzak and Laurinen (1999), and the 0.40 surround contrast was about the same as their highest surround contrast (0.39). This stimulus configuration with abutting center and surround was similar to some of
Olzak and Laurinen’s (1999) conditions, and with a 4-arcmin gap it approximated some of
Xing and Heeger’s (2000) conditions. The use of a 0.70 center contrast would further increase the local luminance contrast between the abutting out-of-phase center and surround gratings. If local brightness induction is responsible for the phase effects, higher local luminance contrast would lead to more enhancement, which could eventually enhance the perceived contrast of the center grating. Our results (
Figure 5) basically replicated all previous phase data and confirmed our predictions. At 0.25 center contrast, contrast suppression diminished when the abutting center and surround stimuli changed from in phase to out of phase. At 0.70 center contrast, suppression was reversed to enhancement. However, with a 4-arcmin gap, suppression was restored for out-of-phase stimuli regardless of the center contrast, and suppression for in-phase and out-of-phase stimuli was similar. These results clearly support
Ejima and Takahashi’s (1985) brightness induction explanation and argue against
Olzak and Laurinen’s (1999) first-order explanation of surround effects for sinusoidal grating stimuli. Surround effects on perceived contrast indeed are phase independent and appear to reflect second-stage visual processing.