Abstract
Objective: The normalization model of attention (Reynolds & Heeger, SfN, 2008) has been proposed to reconcile disparate findings on the effects of attention on neuronal responses in visual cortex. According to the model, different forms of attentional modulation can occur depending on stimulus size and attention field size. We tested a key prediction of this model: a shift from response gain to contrast gain with smaller stimuli and larger attention field.
Methods: Observers performed a 2AFC orientation discrimination task on one of two slightly tilted Gabor stimuli, presented (30 ms) simultaneously in opposite hemifields (5° eccentricity). Exogenous attention was directed by a peripheral pre-cue, preceding stimulus onset by 100 ms. A response cue, at stimulus offset, indicated the target location. This yielded three cue conditions: valid (pre-cue matched to response cue), invalid (mismatched) and neutral (both locations pre-cued). Attention field size was enlarged by randomly varying stimulus locations (along arcs at 5° eccentricity), thus introducing spatial uncertainty. Stimulus size (σ=0.4° or 1.0°) and attention field size were yoked: smaller stimuli were combined with larger attention field and vice versa. Performance (d') was measured for each stimulus/attention field size, cue condition, and several contrasts.
Results: We assessed the effect of attention by fitting psychometric functions for the three cue conditions with two free parameters: asymptotic performance at high contrasts (max d') and the contrast yielding half-maximum performance (C50). For large stimuli and small attention field, attention altered only max d', as predicted by a change in response gain. For small stimuli and large attention field, attention altered only C50, as predicted by a change in contrast gain.
Conclusion: Exogenous attention has different effects on contrast response functions, depending on stimulus size and attention field size, supporting a key prediction of the normalization model of attention.
Support contributed by: NIMH R01-MH069880 to DJH, NIH R01-EY016200 to MC