Figure 2.

Effect of Stimulus Contrast on Tuning

(A and B) Example units show the same spectrotemporal selectivity in the receptive fields (STRFs) estimated from the three contrast conditions. Red denotes components of the stimuli that excite the unit, blue denotes components that inhibit. As the color scale is uniform across the plots, it is clear that the dominant variation across the conditions lies in the magnitude of the drive to the unit: this is stronger (the cortex appears to “listen harder”) under low-contrast than under high-contrast stimulus conditions.

(C–E) Best frequency (C) does not vary systematically with stimulus contrast across units. Tuning bandwidth (D) shows a small, significant broadening at low contrast. In almost all units, the gain of the STRF (E) increases as contrast decreases. STRF properties under low-contrast stimulation are shown as blue circles, under medium contrast as green crosses. Filled dark blue circles and dark green pluses indicate data in these two same conditions from the awake recordings.

(F) The linear STRFs fitted from DRCs with different contrasts are sufficiently similar in tuning properties that, once adjusted for differences in STRF gain, they predict responses across stimulus conditions on average 96.5% as well as they do within their own conditions. Thus, most of the contrast dependence of STRFs is captured by a change in gain. Red crosses indicate data from the awake recordings. See also Table S3.