Covert transient attention does not change the characteristics of a spatial frequency channel

Directing transient covert attention to the target location enhances sensitivity across the contrast sensitivity function (Carrasco, Penpeci-Talgar & Eckstein, 2001). Using critical band masking to characterize letter identification, Solomon and Pelli (1994) reported that although observers have access to many different bands of spatial frequencies to perform the task, a single spatial frequency channel mediates letter identification. In the present study, we used critical band masking to investigate whether covert attention affects the gain and/or the spatial frequency tuning of the channel mediating letter identification. We manipulated attention using either a peripheral (cue at target location), a neutral-central (cue at fixation) or neutral-distributed (one cue at each of 8 possible locations) cue. All three cues indicated the time of target onset but only the peripheral cue was informative regarding target location. The target letter (N, Z, or X; presented for 40 ms in low or high-pass noise with different cutoff frequencies) followed the cue at one of the 8 locations. Distracter letters (V's) occupied the remaining locations. We measured the energy threshold elevation (using the modified Quest staircase procedure; Watson & Pelli, 1983; King-Smith et al. 1994) at each of the low and high-pass cutoff noise frequencies. To derive frequency-dependent power gain of the inferred filter, we took the derivative of the threshold energy with respect to the cutoff frequency. The peripheral cue enhanced gain of the channel mediating the task compared to the conditions where the two different neutral cues preceded the display. Having established that attention enhances contrast sensitivity (Carrasco, Penpeci-Talgar & Eckstein, 2001), the present results indicate that this enhancement is a result of a change in the gain of the spatial frequency channel mediating the task.