Classification of real and imagined sounds in early visual cortex
Last modified: 2011-08-25
Abstract
Early visual cortex has been thought to be mainly involved in the detection of low-level visual features. Here we show that complex natural sounds can be decoded from early visual cortex activity, in the absence of visual stimulation and both when sounds are actually displayed and when they are merely imagined. Blindfolded subjects listened to three complex natural sounds (bird singing, people talking, traffic noise; Exp. 1) or received word cues (“forest”, “people”, ”traffic”; Exp 2) to imagine the associated scene. fMRI BOLD activation patterns from retinotopically defined early visual areas were fed into a multivariate pattern classification algorithm (a linear support vector machine). Actual sounds were discriminated above chance in V2 and V3 and imagined sounds were decoded in V1. Also cross-classification, i.e. training the classifier to real sounds and testing it to imagined sounds and vice versa, was successful. Two further experiments showed that an orthogonal working memory task does not interfere with sound classification in early visual cortex (Exp. 3), however, an orthogonal visuo-spatial imagery task does (Exp. 4). These results demonstrate that early visual cortex activity contains content-specific information from hearing and from imagery, challenging the view of a strict modality-specific function of early visual cortex.