Figure 2.

Rhesus macaques perform a novel auditory feature-selective attention task. A, Subjects use a joystick to initiate trials and to respond. We present two sequential sounds after subjects initiate a trial. The first sound (S1) is always a broadband (9 octave wide) noise burst. The second sound (S2) could be any of the stimuli in the set (B). After S2 offset, the subject was required to make either a “yes” or a “no” response (to indicate target detection or rejection, respectively) with a vertical joystick movement (up or down). Correct responses were rewarded with liquid and incorrect responses were penalized with a timeout. B, Broadband noise burst (“unmodified sound”), as well as sounds that deviate from the unmodified sound along the temporal and/or spectral sound feature dimensions (x and y axes, respectively). The temporal feature is amplitude modulation (AM; sinusoidal modulation of the sound envelope). We parametrically increase the value of AM by increasing its depth. To introduce spectral variation, we change (decrease) the bandwidth of the unmodified sound (ΔBW). We parametrically increase the value of ΔBW by narrowing the width of the band-pass filter used to synthesize these sounds. Changes in spectral BW are schematized in B with increasing lightness relative to the unmodified sound. When subjects attend to AM, the sounds within the black box are targets; that is, AM depth > 0. Likewise, when subjects attend BW, all the sounds in the green box are targets: ΔBW >0. Here, and throughout the text, whenever feature values are collapsed across subjects, AM and ΔBW values are given as ranks. The exact values presented to each subject were determined separately for each subject and this process is described in detail in the main text. C, Performance of both subjects (+ for Monkey W and ○ for Monkey U) in each condition represented as regression coefficients that correspond to the influence of each feature on subjects' behavioral responses. An increased value of a given coefficient corresponds to an increased probability that the subject will report “yes” as the value of that feature increases. Subjects' behavioral responses are more strongly influenced by the target feature than the distractor feature, a hallmark of feature-selective attention.