Relationship between stimulus complexity and neuronal activity in the inferotemporal cortex of the macaque monkey.

The single-unit activity of 217 cells was recorded from the inferotemporal cortex (IT) of two awake macaque monkeys while they performed a fixation task. The stimuli were coloured geometrical shapes or coloured representations of natural or artificial objects. To determine whether the stimuli could be separated into groups on the basis on neuronal population behaviour, the responses to the images were analysed by factor analysis and cluster analysis. It was a common result of each analysis that, on the basis of neuronal responses, the stimulus set could be separated into two groups, despite the lack of difference in mean response rate to them. Similar groups were formed when only the first half of the responses was analysed. The results suggest a differential coding of the images of simple geometrical shapes and of the images of complex, real (photographic) objects. We found significant differences between the two stimulus groups in physical features, other than size or luminance. Our results suggest that the same neurone population might respond differently to simple and complex images in the first 150 ms of their responses. The differences might be attributed to "non-obvious" physical features of the stimuli, such as the amount of internal lines in the images, colourfulness and the length of perimeter of the stimuli.