Attentional effects on contrast discrimination in humans: evidence for both contrast gain and response gain

Abstract

In order to understand how attention affects visual processing, we investigated the degree to which attention effects can be accounted for by increases in the contrast gain of the contrast response function, CRF (represented by an increase in effective contrast) vs. increases in the response gain (represented by an overall amplification of response). To this end, we used a dual-task paradigm to compare psychophysical "threshold vs. pedestal contrast" (TvC) curves obtained under conditions of full- vs. poor-attention. The attention effect, defined as the ratio of thresholds for poor- vs. full-attention conditions, was roughly four-fold at a pedestal contrast of 0% (i.e., at detection threshold) and there was a significant decrease in attention effect with increasing pedestal contrast, from approximately ten-fold at the lowest non-zero pedestal contrast tested (0.25%) to three-fold at the highest pedestal contrast tested (64%). These findings are consistent with the existence of both contrast gain effects of attention (needed to account for the substantial attention effect at detection threshold and the decrease in attention effect with increasing pedestal contrast) as well as response gain effects of attention (needed to account for the fact that attention was beneficial across all pedestal contrasts-rather than harmful at some contrasts, as a pure contrast gain model would predict). The results of a model fitting Naka-Rushton CRF equations to the TvC data also support this conclusion. Here we found a two-fold increase in contrast gain and a five-fold increase in response gain in the CRF for the full-attention, as compared to the poor-attention, condition. Because pure contrast gain effects, on the order of two-fold, have been observed at early stages of visual processing (for example in areas V4 and MT), our psychophysical results suggest a hybrid model of attention; contrast gain control at an early stage of visual processing, followed by response gain control at a later stage.

A selective effect of parietal damage on letter identification in mixed case words

“…For example, a study by Huang and Dobkins (2005) showed reduced contrast sensitivity when participants were asked to engage in contrast discrimination under dual task conditions. These attentional effects were most pronounced close to the contrast threshold for successful detection of the visual stimuli, and the authors found the effects to primarily affect early areas in the visual system.…”

Attention and contrast differently affect contextual integration in an orientation discrimination task

“…We manipulated the allocation of voluntary attention using a single/dual task paradigm. This paradigm has been extensively used to investigate the involvement of attention in various visual processes as diverse as motion processing (Thiele et al 2002;Thornton et al 2002), stimulus localization (Adam et al 2008), contrast discrimination (Huang and Dobkins 2005;Alais et al 2006) and contextual modulation (Zenger et al 2000). Attention is a broad term covering a wide range of cognitive processes (Knudsen 2007).…”

“…Attending to low luminance contrast stimuli increases their apparent contrast (Carrasco et al 2004), and boosts neuronal responses to match responses elicited by higher contrast stimuli (Reynolds et al 2000;Martinez-Trujillo and Treue 2002;Williford and Maunsell 2006). Thus attending to a stimulus has been suggested to be equivalent to increasing its luminance contrast (Carrasco et al 2004;Huang and Dobkins 2005). The similarity between increasing contrast and attending to a stimulus has resulted in the "contrast gain model" of attention (Reynolds et al 2000).…”

Electrophysiological Events Related to Top-Down Contrast Sensitivity Control

“…In psychophysical studies, covert shifts of attention (also termed 'transient' attention) tend to decrease the increment stimulus threshold (the smallest contrast increment that can be reliably detected) for all baseline contrasts (Carrasco, Ling & Read, 2004;Huang & Dobkins, 2005;Ling & Carrasco, 2007), though it has also been suggested that this enhanced contrast sensitivity may be due to sensory interactions instead (Schneider, 2006). Likewise, studies using single-cell recordings have demonstrated that contrastdependent neuronal responses can also be enhanced by attention (Reynolds, Pasternak & Desimone, 2000;Martinez-Trujillo & Treue, 2002), regardless of whether attention is shifted exogenously or endogenously .…”