Response Suppression in V1 Agrees with Psychophysics of Surround Masking

Abstract

When a target stimulus is embedded in a high contrast surround, the target appears reduced in contrast and is harder to detect, and neural responses in visual cortex are suppressed. We used functional magnetic resonance imaging (fMRI) and psychophysics to quantitatively compare these physiological and perceptual effects. Observers performed a contrast discrimination task on a contrast-reversing sinusoidal target grating. The target was either presented in isolation or embedded in a high-contrast surround. While observers performed the task, we also measured fMRI responses as a function of target contrast, both with and without a surround. We found that the surround substantially increased the psychophysical thresholds while reducing fMRI responses. The two data sets were compared, on the basis of the assumption that a fixed response difference is required for correct discrimination, and we found that the psychophysics accounted for 96.5% of the variance in the measured V1 responses. The suppression in visual areas V2 and V3 was stronger, too strong to agree with psychophysics. The good quantitative agreement between psychophysical thresholds and V1 responses suggests V1 as a plausible candidate for mediating surround masking.

The effect of spatial attention on invisible stimuli

“…Accordingly, it has been demonstrated that the effect of spatial attention can be significantly increased when one of two stimuli, rather than a single stimulus, is attended (Lavie, 1995;Luck, Chelazzi, Hillyard, & Desimone, 1997). Furthermore, the contrast-decrement detection task of our study is known to strongly modulate attention (Braun, 1994;Zenger-Landolt & Heeger, 2003). The same argument can be made for Moradi et al's study, where participants were simply instructed to attend to a moving suppressor.…”

Contrast adaptation and representation in human early visual cortex

“…Previous block design imaging experiments (Kastner et al, 2004), which have compared contrast response functions measured with ascending versus descending contrasts, may not have been as sensitive to adaptation changes as event-related studies (this study and Engel and Furmanski, 2001). Changes in contrast response functions measured with BOLD imaging have been noted in surround suppression experiments (Zenger-Landolt and Heeger, 2003), sharing some similarities with the effect we find with adaptation. However, surround suppression induces more response-gain changes and is likely to be mediated by neurons with different receptive field locations.…”

“…In fact, our measurements of contrast sensitivity before and after adaptation, indicate that block design experiments (depending on the length of the block) could, in the limit, underestimate contrast sensitivity by more than three times. Accurate measurements of contrast sensitivity are particular important for studies that wish to relate BOLD measurements to behavior Zenger-Landolt and Heeger, 2003) or to level of neural activity (Heeger et al, 2000). Moreover, when comparing contrast sensitivity across areas (Avidan et al, 2002), differences in adaptation rates may result in misestimation of contrast sensitivities.…”

Population receptive field estimates in human visual cortex

“…Additionally, the pRF may be modeled using nonlinear models and sums and differences of Gaussians to test hypotheses about surround suppression. These suppressive effects have been demonstrated using fMRI (Dumoulin and Hess, 2006;Kastner et al, 2001;Williams et al, 2003;Zenger-Landolt and Heeger, 2003), and it has been shown that the strength and spatial extent of these suppressive effects increase in later visual areas (Kastner et al, 2001;Zenger-Landolt and Heeger, 2003). This increase may be related to increases in the RF sizes in these areas (Bles et al, 2006;Kastner et al, 2001).…”