The visual system is divided into two separate pathways, which are specialized for functionally distinct tasks (Ungerleider & Mishkin 1982). The dorsal stream is associated with visually guided action, while the ventral pathway is involved in object recognition.
Physiology studies in monkeys have shown that activity of area LIP in the posterior parietal cortex (PPC) is related to motor intention and visual attention. Moreover, many LIP neurons exhibit shape-selectivity (Sereno & Maunsell 1998). This result indicates that area LIP integrates information from both pathways. In contrast, less is known about the crosstalk between both pathways in humans. Here, we mapped the human PPC and used fMRI adaptation to assess object selectivity in this region.
Six subjects participated in 6 scanning sessions. In session 1 and 2, we applied retinotopic mapping and a delayed saccade task in order to define areas in occipital and parietal cortex, respectively. In the following sessions, we investigated adaptation effects to different categories of objects (2D-, 3D-objects; line drawings). Furthermore, we examined geometric objects under different viewing conditions.
The delayed saccade paradigm revealed 4 topographically organized areas along the intraparietal sulcus (IPS): 2 areas in the posterior part (IPS1/2) and 2 in the anterior branch (IPS3/4). Only IPS1/2, but not IPS3/4 showed object adaptation effects. The signal was reduced when identical objects were presented repetitively compared to an equivalent number of non-repeated objects. Furthermore, adaptation remained when object size and viewpoint were changed.
The adaptation profiles found in IPS1/2 were similar to those in LOC, an area of the ventral stream thought to be critical for object recognition. Together, the similar response patterns in IPS1/2 and LOC point to object representations in both pathways that might be further used for object recognition in the ventral system and object-related action in the dorsal system.