Inferior parietal lobule function in spatial perception and visuomotor integration

THE INFERIOR PARIETAL LOBULE (IPL) is located in the posterior aspect of the parietal lobe adjacent to the occipital lobe. It receives inputs from visual and somatosensory cortices and as a result has historically been considered an area important for the integration of these two modalities. This area is involved in higher cortical functions. Lesions here do not produce deficits in more primary aspects of vision (such as blindness) or in somatosensation (such as loss of tactile sensitivity); they produce deficits in more complex cortical functions including spatial perception and visuomotor integration. Recordings from neurons in this area demonstrate more complex response properties than those found in lower-order sensory areas and larger receptive fields, indicating a greater degree of integration of visual information. It differs from lower-order sensory areas in that it has strong connections with diverse cortical structures such as limbic cortical regions believed to be important for emotions and drives, ventral temporal lobe areas thought to play an important role in memory, and prefrontal cortical areas that may be involved in motor planning. Despite the complexity of the functions and connections of the IPL, much progress has been made recently in elucidating its role in cortical functioning. In particular, visual and oculomotor aspects of the IPL have lent themselves to careful experimental scrutiny; they are the subject of this chapter. Visual stimuli can be more precisely controlled than can somatosensory stimuli, and eye movements can be recorded simply and accurately, having many fewer degrees of freedom of movement than do limbs. A great deal is now known about the anatomy and, increasingly, the physiology of visual cortex. The direct and multiple connections between the IPL and extrastriate visual cortex provide avenues for elucidating the chain of processing events that result in the visual properties recorded in the IPL. Progress has been more difficult in gaining an understanding of the somatosensory functions of this area and the possible role the IPL may play in the integration of somatosensory and visual information. It is becoming clear that an important integration of incoming visual signals and oculomotor signals occurs in the IPL and possibly plays a critical role in perception of and motor functioning within external space. Recent experiments in subhuman primates have disclosed the presence of several visual cortical fields outside the primary (striate) visual cortex (for reviews see refs. 4,208,209). Many of these extrastriate fields, like striate cortex, contain a retinotopic representa-