Introduction and objectives: Achieving an understanding of the principles underlying categorisation is important for a wide range of psychological phenomena, and especially for emotion processing. In a previous study with a large non-clinical sample, we observed spontaneous categorisation of IAPS images according to their semantic content. In the present study, we explored whether this principle of categorisation is reflected at the neurophysiological level.

Methods: Eighteen patients suffering from pharmacologically intractable focal epilepsy (10 males) observed the same set of 120 IAPS images, and were asked subsequently to rate them on valence and arousal. The following semantic categories were contrasted on the basis of our previous results: Death, violence and suffering for negative valence; objects, erotica and portraits for neutral valence; and food, nature and social content for positive valence.

Results: Event-related potentials (ERPs) were modulated according to semantic content, as indexed by the latency (lat), length, and the magnitude of the peak amplitude difference. Importantly, this occurred independently of perceived valence and arousal. Such categorisation emerged throughout many brain structures.

Conclusions: Our findings provide strong evidence that semantic content is an important factor in the categorisation of non-verbal emotional stimuli, and pictures grouped together implicitly evoke distinct neurophysiological responses. This implies that an emotional response does indeed involve the extraction of meaning.