FIG. 4.

Rare visual stimuli enhanced auditory responses more than frequently presented visual stimuli. A: average responses to repeated presentation of auditory-alone (black; open circles) and looming bimodal (grey; filled circles) stimuli. Responses represent averages across three successive stimulus presentations. Responses to both auditory-alone (R² = 0.21; P = 0.01; F = 6.64; regression of auditory response on stimulus presentation number) and bimodal stimuli (R² = 0.39; P = 0.0001; F = 16.61; regression of bimodal response on stimulus presentation number) declined with successive stimulus presentations. Error bars represent the bootstrap SE. B: the average bimodal response enhancement [(bimodal response − auditory response)/auditory response] induced by looming visual stimuli plotted as a function of the order of stimulus presentation. The percent enhancement of auditory responses decreased with repeated presentations of the looming stimulus [slope = −2.2; 95% CI = −4.0 to −0.5; R² = 0.38; P = 0.01; F = 7.93; regression of response enhancement on stimulus presentation number]. Error bars represent the bootstrap SE. C: rare stimuli within a stimulus block evoked stronger responses. Two identical visual stimuli moved across identical portions of the visual field but in opposite directions (1 to the left and the other to the right). Both of these visual stimuli were synchronized with an auditory stimulus at the best ITD and best ILD for the site. These 2 bimodal stimuli were randomly interleaved in 4 blocks of trials in which the ratios of leftward to rightward visual stimuli were: 20/80, 40/60, 60/40, or 80/20. Responses to leftward (×) and rightward (open circles) stimuli are plotted side by side for each case in which the stimulus was presented with a specific ratio. Stronger responses were observed with less frequently presented stimuli (R² = 0.26; F = 29.5; P < 0.00001; regression of response on stimulus probability). Error bars represent the bootstrap SE.