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.

Humans are able to recognize objects when surface details, such as colour, texture and luminance gradients, are not available. By systematically eliminating colour, texture, shading, contrast and inner contours from given objects, we tested whether certain shape-selective inferior temporal cortex (IT) neurons of awake rhesus monkeys remain selective for these objects as the surface information is reduced. In psychophysical experiments, we established that the rhesus monkey can identify the shape of a coloured object largely independently of its surface characteristics and, to a lesser degree, of its inner contours. Shape selectivity of the neurons does not change when texture and shading are concealed. The responsiveness of the neurons is also affected by the removal of these surface attributes. The IT neurons were found to respond highly similarly to objects brighter or darker than their background. Selectivity for shape is preserved when the contrast is reversed. Deletion of the inner contours, outlining the main parts of the objects, did not affect the responses and selectivity of the IT neurons. These findings indicate that the IT can contribute to the invariant perception of objects having different surface details.

Perceived boundaries without physical differences between shape and background are called illusory contours (ICs). ICs and real contours (RCs) activate the early processing stages of the macaque visual pathway and the occipitotemporal areas of the human visual system in a similar way. However, it is not known how these contours are processed further in the highest visual areas. We tested how the responses of inferior temporal cortical (IT) neurons of macaque monkeys change in relationship to figures with RCs or ICs. The same set of figures [coloured pictures, ICs and silhouettes (SILs)] was presented to awake, fixating rhesus monkeys while the single-cell activity was recorded in the anterior part of the IT. Most of the neurons responsive to RCs were also responsive to the same shapes presented as ICs. The average net firing rates, however, were significantly lower for the illusory stimuli than for the stimuli in the RC conditions, and the latency of the responses was significantly longer for the ICs than for the RCs. The shape selectivity was found to be different for coloured stimuli and ICs, and similar for SILs and ICs, suggesting the invariance of selectivity to shapes having the same contour but lacking internal surface information. These results suggest different modes of processing of RCs and ICs in the IT, which might explain the differences in their perception.

Stimulus reduction is an effective way to study visual performance. Cues such as surface characteristics, colour and inner lines can be removed from stimuli, revealing how the change affects recognition and neural processing. An extreme reduction is the removal of the very stimulus, defining it with illusory lines. Perceived boundaries without physical differences between shape and background are called illusory (or subjective) contours. Illusory and real contours activate early stages of the macaque visual pathway in similar ways. However, data relating to the processing of illusory contours in higher visual areas are scarce. We recently reported how illusory contours based on abutting-line gratings affect neurones in the monkey inferotemporal cortex, an area essential for object and shape vision. We now present data on how inferotemporal cortical neurones of monkeys react to another type of shapes, the Kanizsa figures. A set of line drawings, silhouettes, their illusory contour-based counterparts, and control shapes have been presented to awake, fixating rhesus monkeys while single-cell activity was recorded in the anterior part of the inferotemporal cortex. Most of the recorded neurones were responsive and selective to shapes presented as illusory contours. Shape selectivity was proved to be different for line drawings and illusory contours, and also for silhouettes and illusory contours. Neuronal response latencies for Kanizsa figures were significantly longer than those for line drawings and silhouettes. These results reveal differences in processing for Kanizsa figures and shapes having real contours in the monkey inferotemporal cortex.

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