| Understanding how auditory and visual stimuli interact with respect to spatial attention is an important piece toward an understanding of human attention processes more generally. Previous work has reported that both time-domain, namely the auditory-evoked contralateral positivity (ACOP), and frequency-domain, namely alpha power, aspects of the EEG over ventral-occipital cortex are modulated by the presentation of an auditory stimulus, suggestive of an automatic modulation of visual spatial attention in response to auditory stimuli. However, the extent of the automaticity of these effects has not been fully explored. In particular, it is unclear whether the modulation of visual spatial attention in response to an auditory stimulus is affected by an individual's belief of the location of auditory stimulation, or instead occurs as a purely bottom-up process, irrespective of top-down factors.;Experiment 1 sought to clarify this phenomenon via spatially ambiguous auditory stimuli, and a participant instruction manipulation. Twenty-four healthy adults performed an auditory detection task, in which they were asked to detect a pure tone embedded in noise. Within the task, the location of the auditory stimuli was unambiguous with respect to their lateral position in space, i.e. emanating from the left or right, but ambiguous with respect to their location in front of vs. behind the participants. The previously reported ACOP effect was replicated with respect to tone lateralization, but was not modulated by the front vs. back belief manipulation. The previously reported alpha power effect was also replicated with respect to tone lateralization, but was additionally modulated by the belief manipulation, such that time-frequency alpha suppression was greater when participants were instructed that the tones were being presented from in front of vs. back of their position.;Within Experiment 2, eighteen healthy adults performed a bimodal spatial judgment task, which required participants to make spatial judgments about the location of unimodal auditory, unimodal visual, or bimodal auditory-visual stimuli. Auditory stimuli were bursts of noise, while visual stimuli were low luminance ovals. Behaviorally, spatial judgments from Experiment 2 suggest that auditory and visual stimulus positions are not processed independently, with the influence of visual position on auditory response greater than the influence of auditory position of visual response. Physiologically, ACOP and alpha power effects in response to unimodal auditory stimuli that had been observed within Experiment 1 were replicated. However, while both unimodal visual and bimodal stimuli elicited alpha power lateralization, the ACOP effect was absent. Taken together with the results of Experiment 1, this result suggest that while the ACOP and alpha power modulation have a similar topography and time-course, they appear to reflect dissociable processes. |
