Coding of images of materials by macaque inferior temporal cortical neurons.

Objects vary not only in their shape but also in the material from which they are made. Knowledge of the material properties can contribute to object recognition as well as indicate properties of the object (e.g. ripeness of a fruit). We examined the coding of images of materials by single neurons of the macaque inferior temporal (IT) cortex, an area known to support object recognition and categorization. Stimuli were images of 12 real materials that were illuminated from three different directions. The material textures appeared within five different outline shapes. The majority of responsive IT neurons responded selectively to the material textures, and this selectivity was largely independent of their shape selectivity. The responses of the large majority of neurons were strongly affected by illumination direction. Despite the generally weak illumination-direction invariance of the responses, Support Vector Machines that used the neural responses as input were able to classify the materials across illumination direction better than by chance. A comparison between the responses to the original images and those to images with a random spectral phase, but matched power spectrum, indicated that the material texture selectivity did not depend merely on differences in the power spectrum but required phase information.

Task-related modulation in the monkey inferotemporal cortex.

The latencies of the neuronal responses from the inferotemporal cortical cells were analyzed in animals performing a visual fixation task and a recognition task with the same stimulus set. A consistent reduction in response latencies of about 10 ms was observed in favor of the recognition task. It was found that behavioral relevance reduces the latency in the inferotemporal cortex and it was concluded that behavioral significance accelerates information processing. This effect has not been described previously.

Achromatic shape processing in the inferotemporal cortex of the macaque.

The responses of single neurones in the inferior temporal cortex of awake macaque monkeys to chromatic and achromatic stimuli were investigated, with the aim of determining whether colour-independent processing occurs in this last unimodal area of the ventral visual pathway. There were no differences in the firing rate of the responses (responsiveness) or the selectivity of the inferior temporal neurons towards greyscale and coloured images. The latency of the responses was the same in the two conditions. These results stress the importance of the inferior temporal cortex in colour-independent object recognition.