Tamás Tompa

The representation of Kanizsa illusory contours in the monkey inferior temporal cortex.

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.

A review on the inferior temporal cortex of the macaque.

In our review, we summarize recent advances in the research of the inferior temporal cortex (ITC) of the macaque monkey. This area of the cortex is known to have a crucial role in visual shape recognition and is regarded as being at the end stage of the so-called ventral visual pathway. In the last decade, several new findings appeared in the field without being integrated in a coherent view about the function, position, and operating principles of the area. During this decade, experimental techniques developed a great deal, and the way we look at the brain and brain function changed too. In this review, we try to integrate knowledge about the ITC to the changing view about the brain while outlining the work that has been done in the last decade.

Illusory shape representation in the monkey inferior temporal cortex

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.

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.

The interplay of holistic shape, local feature and color information in object categorization

Although it is widely accepted that colors facilitate object and scene recognition under various circumstances, several studies found no effects of color removal in tasks requiring categorization of briefly presented animals in natural scenes. In this study, three experiments were performed to test the assumption that the discrepancy between empirical data is related to variations of the available meaningful global information such as object shapes and contextual cues. Sixty-one individuals categorized chromatic and achromatic versions of intact and scrambled images containing either cars or birds. While color removal did not affect the classification of intact stimuli, the recognition of moderately scrambled achromatic images was more difficult. This effect was accompanied by amplitude modulations of occipital event-related potentials emerging from approximately 150ms post-stimulus. Our results indicate that colors facilitate stimulus classification, but this effect becomes prominent only in cases when holistic processing is not sufficient for stimulus recognition.