CLUSTERING OF RESPONSE SELECTIVITY IN THE MEDIAL SUPERIOR TEMPORAL AREA OF EXTRASTRIATE CORTEX IN THE MACAQUE MONKEY

Ever since being described by Mountcastle (Mountcastle, 1957), columna r organization of sensory cortical areas has provided key leverage int o understanding the functional organization of neocortex. Columnar or clustered(1) organization of neurons sharing like properties is now kn own to be widespread, and probably universal in primary sensory areas. Visual cortex in primates consists of a primary area and a large numb er of secondary areas, which are organized in a manner both hierarchic al and parallel (Felleman & Van Essen, 1991; Young, 1993; Young et al. , 1995). One major component in the organization of extrastriate visua l cortex appears to be the division into dorsal and ventral ''streams' ' of processing (Ungerleider & Mishkin, 1982), each of which is organi zed hierarchically. Within each, columnar organization exists at early stages, but becomes less clear at higher levels. Columnar organizatio n has been described at the highest level of the ventral stream, infer otemporal cortex (IT, Saleem et al., 1993; Fujita & Fujita, 1996; Tana ka, 1996), but has not been well characterized at the higher levels of the dorsal stream. Hints of such organization are found in the litera ture (Saito et al., 1986; Lagae et al., 1994), but systematic measurem ents are needed. In this paper, I report the existence of clustered or ganization in the medial superior temporal area (MST) of the dorsal st ream, which is arguably the highest dominantly visual area on this pat hway. I have measured the selectivity of both single-and multiple-unit activity along oblique electrode penetrations through this area to th ree different kinds of optic flow stimuli, and find that nearby neuron s an more similar in their tuning than are more distant ones. This obs ervation documents the existence of some form of clustered organizatio n and supports the importance of this area in the processing of optic flow information.